Risk Management Series
Design Guide for Improving Hospital Safety in Earthquakes, Floods, and High 
Winds
FEMA 577 / June 2007 

About the Cover
Olive View Hospital Replacement Fares Well In 1994 Quake
The new Olive View hospital building shown on the cover performed well during 
the 1994 Northridge earthquake. This quake, of the same magnitude as the 1971 
San Fernando Earthquake that nearly collapsed the original building, caused no 
serious damage. Built in 1970, the Medical Treatment and Care Building of the 
Olive View Hospital complex was designed to meet the earthquake provisions of 
building codes in place at that time. The hospital incurred heavy damage (at 
left) during the 1971 earthquake and was subsequently rebuilt to stricter design 
and construction standards.

[Photo]
Olive View Hospital After the 1971 Magnitude 6.7 San Fernando Earthquake
Photo Credit: E.V. Leyendecker, U.S. Geological Survey  





Providing Protection to People and Buildings

Any opinions, findings, conclusions, or recommendations expressed in this 
publication do not necessarily reflect the views of FEMA. Additionally, neither 
FEMA nor any of its employees makes any warranty, expressed or implied, or 
assumes any legal liability or responsibility for the accuracy, completeness, or 
usefulness of any information, product, or process included in this publication. 
Users of information from this publication assume all liability arising from 
such use.




FOREWORD AND ACKNOWLEDGMENTS

BACKGROUND

The United States is currently in the middle of the biggest hospital 
construction boom in more than 50 years. According to data from the U.S. Census 
Bureau, spending for construction of new hospitals and other medical facilities 
increased 65 percent between 2000 and 2006. New scientific and technological 
innovations, as well as advancements in medical practice and the organization of 
health care, demand a physical environment different from the hospitals of the 
past. This de­mand is being met by the increasing use of evidence-based design, 
which relies on a combination of scientifically proven research and the 
evaluation of completed projects to make design and construction decisions that 
improve the safety and functionality of hospital buildings. 

Architects and engineers now look at credible research related not just to 
structural and mechanical engineering, but also to clinical outcomes, behavioral 
science, the environment, and technology. New building designs are now seen as 
important components that can improve medical outcomes, patient safety, employee 
satisfaction, and even financial performance. The effective use of evidence-
based design requires continuous and timely updates of the information that 
affects hospital design. As part of this effort, the Federal Emergency 
Management Agency (FEMA) has developed this Design Guide to provide the 
designers of new hospitals and retrofits to existing ones with the latest 
information and research results on the best practices to reduce the risks from 
natural hazards. 

This publication is the latest addition to FEMA’s Risk Management Series, which 
provides guidelines for mitigating the risks associated with multiple hazards. 
The series emphasizes mitigation best practices for specific building uses and 
occupancies, such as schools, critical facilities, commer­cial buildings, and 
multi-family dwellings.

OBJECTIVE AND SCOPE

The objective of the “Design Guide for Improving Hospital Safety in Earthquakes, 
Floods, and High Winds” is to inform and assist design professionals, hospital 
administrators, and facility managers in implementing sound mitigation measures 
that will decrease the vulnerability of hospitals to disruptions caused by 
natural hazard events. The intent of the Design Guide is to provide its audience 
with state-of-the-art knowledge on the variety of vulnerabilities faced by 
hospitals exposed to earthquakes, flooding, and high-winds risks, as well as the 
best ways to mitigate the risk of damage and disruption of hospital operations 
caused by these events. 

The information presented in this publication provides an exhaustive review of 
mitigation measures and design solutions that can improve the safety of 
hospitals in natural hazard events. However, this publication is not 
intended to be a comprehensive mitigation design manual that the reader can use 
to develop actual plans and specifications. It is intended as an introduction to 
the fundamental principles of natural hazard risk reduction, with an emphasis on 
mitigation planning and the design of hospital buildings. The information 
presented here is intended to help design professionals, hospital 
administrators, and facility managers understand the broad aspects of risk 
reduction methods and strategies, and integrate them into hospital designs. 

ORGANIZATION AND CONTENT

The Design Guide is organized around three specific natural hazards: 
earthquakes, floods, and high winds. It comprises four main chapters.
Chapter 1 presents an overview of the principal considerations determining 
hospital design, from standard industry requirements to new developments that 
are transforming both hospital operations and organization of the physical 
environment. It highlights the known vulnerabilities of hospitals and the 
repercussions of damage caused by natural hazard events that frequently 
interfere with the operation of these facilities. The chapter concludes with a 
look at the multi-hazard approach to hospital design, and provides basic 
guidelines on the interaction between the re­sponses of building components to 
various natural hazard risks.

Chapter 2 examines potential earthquake damage to hospitals, and how these 
facilities can most efficiently improve their performance. The chapter opens 
with an introductory discussion on the nature and prob­ability of earthquakes, 
and procedures for determining seismic risk to specific locations. Typical 
seismic damages, and the possible resulting effects on building functions or 
risk to occupants, are described and related to the standard damage states 
currently used in performance-based earthquake engineering design. The chapter 
ends with a review of the best practices in seismic design and seismic retrofit 
of hospital facilities.

Chapter 3 discusses the nature of flood forces and their effects on buildings. 
It outlines the procedures for risk assessment and describes the current 
mitigation measures for reducing flood damage. It emphasizes the benefits of 
avoiding construction of new hospitals in high-risk areas, describes regulatory 
design requirements that help reduce the exposure of hospitals that must be 
located in flood hazard areas, and encourages the application of appropriate 
mitigation measures to existing hospitals at risk of flooding. 

Chapter 4 discusses the effects of wind forces on hospitals’ structural and 
nonstructural building components. By reviewing numerous examples of wind-
induced damage to these facilities, this chapter highlights the best mitigation 
practices for new hospital design and construction, and for the rehabilitation 
of existing facilities. It concentrates on the building components that are the 
most critical for maintaining uninterrupted operation of hospitals, and provides 
detailed guidelines for improving their design and construction.

At the end are Appendix A, which contains a list of acronyms, and Appendix B, 
which contains a glossary of terms that appear in the Design Guide.

ACKNOWLEDGMENTS

Principal Authors:
Christopher Arnold, Building Systems Development, Inc.
William B. Holmes, Rutherford and Chekene
Rebecca C. Quinn, RCQuinn Consulting, Inc.
Thomas L. Smith, TLSmith Consulting
Bogdan Srdanovic, URS Corporation
Wilbur Tussler, Consultant

Contributors:
Milagros Nanita Kennett, FEMA, Project Officer, Risk Management Series 
Publications
Eric Letvin, URS Corporation, Project Manager
Susan Patton, URS Corporation
Rob Fernandez, URS Corporation
Ivy Porpotage, URS Corporation
Wanda Rizer, design4impact

Project Advisory Panel:
Amar Chaker (AEI), American Society of Civil Engineers (ASCE) 
Ken Dickerman, AIA Academy of Architecture and Health 
Skip Gregory, Florida Agency for Health Care Administration
Mike Kuechenmeister, American Society of Healthcare Engineering
Jim Rossberg (SEI), American Society of Civil Engineers (ASCE) 
John Sullivan, Portland Cement Association
This manual will be revised periodically, and FEMA welcomes comments and 
suggestions for improvements in future editions. Please send your comments and 
suggestions via e-mail to: 
RiskManagementSeriesPubs@dhs.gov




TABLE OF CONTENTS

FOREWORD and Acknowledgments i
Background i
Objective and Scope ii
Organization and Contents ii
Acknowledgements iii

1 HOSPITAL DESIGN CONSIDERATIONS 1-1
1.1 Introduction 1-1
1.2 Health Care Industry 1-3
1.2.1 Ambulatory Care 1-3
1.2.2 Patient Volume 1-4
1.2.3 Aging Facilities 1-5
1.2.4 Healing Environments 1-5
1.2.5 Technological Advances 1-6
1.3 Hazard Mitigation 1-8
1.3.1 Assessing Risk 1-9
1.3.2 Evacuation Considerations 1-10
1.3.3 Potential Vulnerabilities 1-11
1.3.3.1 Structural Vulnerability 1-12
1.3.3.2 Nonstructural Vulnerability 1-12
1.3.3.3 Spatial and Other Organizational Vulnerabilities. 1-15
1.4 Hospital Design and Construction 1-17
1.4.1 Building Codes 1-18
1.5 Multi-Hazard Design Considerations. 1-20
1.5.1 The Need for a Multi-Hazard Approach 1-21
1.5.2 Multi-Hazard Design Matrix 1-22
1.6 References. 1-32

2 MAKING HOSPITALS SAFE FROM EARTHQUAKES 2-1
2.1 Introduction 2-1
2.1.1 The Nature and Probability of Earthquakes 2-1
2.1.2 Earthquake Effects 2-2
2.1.3 Measuring Earthquake Effects 2-3
2.1.3.1 Measuring Seismic Ground Motion 2-5
2.1.3.2 Measuring Potential for Liquefaction 2-7
2.1.3.3 Measuring Potential for Landslide2-7
2.1.3.4 Measuring Potential for Tsunami and Seiche 2-8
2.1.4 Earthquakes: A National Problem 2-8
2.2 Seismic Building Design 2-11
2.2.1 The Equivalent Lateral Force (ELF) Analysis Methodology 2-12
2.2.1.1 Acceleration 2-13
2.2.1.2 Amplification and Soil Type 2-14
2.2.1.3 Building Period 2-15
2.2.2 Critical Building Characteristics 2-16
2.2.2.1 Period and Resonance 2-16
2.2.2.2 Damping 2-16
2.2.2.3 Nonlinear Behavior 2-17
2.2.2.4 Ductility 2-17
2.2.2.5 Strength and Stiffness 2-18
2.2.2.6 Drift 2-19
2.2.2.7 Configuration: Size and Shape 2-20
2.2.2.8 Stress Concentrations 2-20
2.2.2.9 Torsional Forces 2-21
2.2.3 Specifications for Performance-Based Seismic Design 2-24
2.2.3.1 Performance Levels 2-24
2.2.3.2 New Developments in Performance-Based Design 2-27
2.3 Earthquake Damage to Hospitals 2-29
2.3.1 Types of Structural Damage 2-31
2.3.1.1 The Case of the Olive View Medical Center 2-32
2.3.2 Nonstructural Damage 2-35
2.3.2.1 The Case of New Olive View Medical Center 2-39
2.3.2.2 The Case of Kona Community Hospital, Hawaii 2-42
2.3.3 Consequences of Building Damage 2-48
2.3.4 Seismic Vulnerability of Hospitals 2-49
2.3.4.1 Seismic Vulnerability of Hospitals Based on Historical Performance in 
California. 2-50
2.3.4.2 Vulnerability Assessment of Hospital Buildings 2-52
2.3.4.3 Comparability of Hospital Buildings 2-53
2.4 Risk Reduction Measures 2-57
2.4.1 Site Selection Basics. 2-57
2.4.2 Seismic Design Basics 2-58
2.4.3 Structural Systems 2-59
2.4.3.1 Basic Types of Lateral Force Resisting Systems 2-59
2.4.3.2 Innovative Structural Systems 2-62
2.4.3.3 Structural Systems Selection 2-63
2.4.4 Nonstructural Components and Systems 2-64
2.4.4.1 Code Regulated Nonstructural Systems 2-65
2.4.4.2 Interstitial Space for Utility Installations 2-67
2.4.5 Mitigation Measures for New Buildings 2-68
2.4.5.1 The Case of Loma Linda Veterans Hospital 2-68
2.4.6 Mitigation Measures for Existing Buildings 2-72
2.4.6.1 Procedures and Design Strategies for Rehabilitation of Structural 
Systems 2-72
2.4.6.2 The Case of Naval Hospital Bremerton 2-73
2.4.6.3 Procedures and Design Strategies for Rehabilitation of Nonstructural 
Systems 2-77
2.4.6.4 Summary of Risk Reduction Measures for Existing 
Buildings 2-77
2.5 Checklist for Seismic Vulnerability of Hospitals 2-80
2.6 References and Sources of Additional Information 2-84

3 MAKING HOSPITALS SAFE FROM FLO0DING 3-1
3.1 General Design Considerations 3-1
3.1.1 The Nature of Flooding 3-1
3.1.2 Probability of Occurrence or Frequency 3-4
3.1.3 Flood Characteristics and Loads 3-7
3.1.3.1 Hydrostatic Loads 3-9
3.1.3.2 Hydrodynamic Loads 3-10
3.1.3.3 Debris Impact Loads 3-12
3.1.3.4 Erosion and Local Scour 3-13
3.1.4 Design Parameters 3-14
3.1.4.1 Flood Depth 3-14
3.1.4.2 Design Flood Elevation 3-17
3.1.4.3 Flood Velocity—Riverine. 3-17
3.1.4.4 Flood Velocity—Coastal 3-18
3.1.5 Flood Hazard Maps and Zones 3-19
3.1.5.1 NFIP Flood Maps 3-20
3.1.5.2 NFIP Flood Zones 3-21
3.1.5.3 Coastal A Zones 3-24
3.1.6 Floodplain Management Requirements and Building Codes 3-26
3.1.6.1 Overview of the NFIP 3-26
3.1.6.2 Summary of the NFIP Minimum Requirements 3-28
3.1.6.3 Executive Order 11988 and Critical Facilities 3-29
3.1.6.4 Scope of Model Building Codes and Standards 3-30
3.2 Hospitals Exposed to Flooding 3-33
3.2.1 Identifying Flood Hazards at Existing Hospitals 3-33
3.2.2 Vulnerability: What Flooding Can Do to Existing Hospitals 3-33
3.2.2.1 Site Damage 3-34
3.2.2.2 Structural Damage 3-36
3.2.2.3 Nonstructural Damage 3-38
3.2.2.4 Medical Equipment 3-41
3.2.2.5 Utility System Damage 3-41
3.2.2.6 Contents Damage 3-44
3.3 Requirements and Best Practices in Flood Hazard Areas 3-47
3.3.1 Evaluating Risk and Avoiding Flood Hazards 3-47
3.3.2 Benefits and Costs: Determining Acceptable Risk 3-48
3.3.3 Site Modifications 3-50
3.3.4 Elevation Considerations 3-53
3.3.4.1 The Case of Boulder Community Foothills Hospital, Boulder, Colorado 3-58
3.3.5 Dry Floodproofing Considerations 3-63
3.3.6 Flood-Resistant Materials 3-66
3.3.7 Access Roads 3-68
3.3.8 Utility Installations 3-69
3.3.9 Potable Water and Wastewater Systems 3-71
3.3.10 Storage Tank Installations 3-71
3.3.11 Accessory Structures 3-72
3.4 Reduction for Existing Hospitals 3-73
3.4.1 Introduction 3-73
3.4.2 Site Modifications 3-73
3.4.3 Additions 3-75
3.4.4 Repairs, Renovations, and Upgrades 3-77
3.4.5 Retrofit Dry Floodproofing 3-78
3.4.5.1 The Case of Pungo District Hospital, Belhaven, North Carolina 3-79
3.4.6 Utility Installations 3-84
3.4.7 Potable Water and Wastewater Systems. 3-86
3.4.8 Other Damage Reduction Measures 3-87
3.4.9 Emergency Measures 3-88
3.5 Checklist for Building Vulnerability of Flood-Prone 3-90
3.6 References and Sources of Additional Information 3-98

4 MAKING HOSPITALS SAFE FROM HIGH WIND 4-1
4.1 General Design Considerations 4-1
4.1.1 Nature of High Winds 4-3
4.1.2 Probability of Occurrence 4-3
4.1.3 Wind/Building Interactions 4-5
4.1.4 Building Codes 4-9
4.1.4.1 Scope of Building Codes 4-9
4.1.4.2 Effectiveness and Limitations of Building Codes 4-10
4.2 Hospitals Exposed to High Winds 4-13
4.2.1 Vulnerability: What High Winds Can Do to Hospitals 4-13
4.2.1.1 Types of Building Damage 4-13
4.2.1.2 Ramifications of Damage 4-16
4.2.1.3 The Case of West Florida Hospital, Pensacola, Florida 4-17
4.2.2 Evaluating Hospitals for Risk From High Winds 4-24
4.2.2.1 New Buildings 4-24
4.2.2.2 Existing Buildings 4-25
4.3 Requirements and Best Practices In High-Wind Regions 4-26
4.3.1 General Hospital Design Considerations 4-26
4.3.1.1 Site 4-28
4.3.1.2 Building Design 4-29
4.3.1.3 Construction Contract Administration 4-34
4.3.1.4 Post-Occupancy Inspections, Periodic Maintenance, Repair, and 
Replacement 4-35
4.3.1.5 Site and General Design Considerations in Hurricane-Prone Regions 4-36
4.3.2 Structural Systems 4-37
4.3.2.1 Structural Systems in Hurricane-Prone Regions 4-41
4.3.3 Building Envelope 4-42
4.3.3.1 Exterior Doors 4-42
4.3.3.2 Exterior Doors in Hurricane-Prone Regions 4-46
4.3.3.3 Windows and Skylights 4-47
4.3.3.4 Windows and Skylights in Hurricane-Prone Regions 4-49
4.3.3.5 Non-Load-Bearing Walls, Wall Coverings, and Soffits 4-52
4.3.3.6 Non-Load-Bearing Walls, Wall Coverings, and Soffits in Hurricane-Prone 
Regions 4-64
4.3.3.7 Roof Systems 4-65
4.3.3.8 Roof Systems in Hurricane-Prone Regions 4-74
4.3.3.9 The Case of DeSoto Memorial Hospital, Arcadia, Florida 4-81
4.3.4 Nonstructural Systems and Equipment 4-85
4.3.4.1 Exterior-Mounted Mechanical Equipment 4-86
4.3.4.2 Nonstructural Systems and Mechanical Equipment in Hurricane-Prone 
Regions 4-94
4.3.4.3 Exterior-Mounted Electrical and Communications Equipment4-94
4.3.4.4Lightning Protection Systems (LPS) in Hurricane-Prone Regions4-97
4.3.4.5 The Case of Martin Memorial Medical Center, Stuart, Florida 4-102
4.3.5 Municipal Utilities in Hurricane-prone Regions 4-104
4.3.5.1 Electrical Power 4-104
4.3.5.2 Water Service 4-105
4.3.5.3 Sewer Service 4-106
4.3.6 Post-design Considerations in Hurricane-Prone Regions 4-106
4.3.6.1 Construction Contract Administration 4-106
4.3.6.2 Periodic Inspections, Maintenance, and Repair 4-106
4.4 Remedial Work on Existing Facilities 4-107
4.4.1 Structural Systems 4-109
4.4.2 Building Envelope 4-111
4.4.2.1 Sectional and Rolling Doors 4-111
4.4.2.2 Windows and Skylights 4-112
4.4.2.3 Roof Coverings 4-115
4.4.3 Exterior-Mounted Equipment 4-117
4.4.3.1 Antenna (Communications Mast) 4-117
4.4.3.2 Lightning Protection Systems 4-118
4.4.4 The Case of Baptist Hospital, Pensacola, 4-118
4.5 Best Practices in Tornado-Prone Regions 4-122
4.5.1 The case of Kiowa County Memorial Hospital, Greensburg, Kansas 4-126
4.5.2 The case of Sumter Regional Hospital, Americus, Georgia 4-135
4.6 Checklist for Building Vulnerability of Hospitals Exposed to High Winds 4-
141
4.7 References and Sources of Additional Information 4-147

APPENDICES
Appendix A Acronyms A-1
Appendix B Glossary of Terms B-1




CHAPTER 1 - HOSPITAL DESIGN CONSIDERATIONS

1.1 INTRODUCTION

Most Americans are accustomed to receiving sophisticated and prompt medical 
attention after an injury or a medical problem occurs, anytime and anywhere in 
the country, without traveling great distances. Such expectations are even 
greater during mass emergencies that require immediate care for a large number 
of casualties. In circumstances in which hospital operations are disrupted or 
completely disabled, the adverse effects of such disasters can be quickly 
compounded, frequently with catastrophic results. A recent report from the 
Congressional Research Service (CRS), Hurricane Katrina: the Public Health and 
Medical Response, examined the performance of the public health system during 
this devastating event. According to the CRS report, Hurricane Katrina pushed 
some of the most critical health care delivery systems to their limits, for the 
first time in recent memory (Lister, 2005). Therefore, the importance of 
uninterrupted hospital operations and ready access to, and availability of, 
immediate medical care cannot be exaggerated.

The intent of this publication is to provide guidelines for planning, design, 
and construction of new hospitals and rehabilitation of existing ones, for the 
purpose of improving their performance during, and in the immediate aftermath 
of, seismic, flooding, and high-wind events. It is im­portant to acknowledge 
that there are no universal design guidelines to protect buildings from all such 
events. Different natural phenomena present different challenges, and each 
hazard requires a different approach and a different set of recommendations. 
When communities face more than one of these hazards, the design team must 
select the mitigation measures most appropriate for achieving the desired 
performance level, regardless of the immediate cause for the potential losses.

For instance, flooding is a more site-specific hazard than others. The preferred 
approach for new facilities is to select a site that is not subject to flooding. 
When that is not feasible, site modifications or other site-specific building 
design features that mitigate anticipated flood hazard will reduce the potential 
for damage. When it comes to seismic and high-wind events, however, in addition 
to carefully selecting the site, it is necessary to design the buildings to be 
resistant to the variety of forces associated with these phenomena. The 
protection against seismic forces requires that both structural and non-
structural building components have sufficient resistance.  For high winds, 
protection efforts focus mainly on the exposed building components and systems.

This chapter addresses the general issues that influence the operations and 
hospital building designs. Typically, the design of hospital facilities is 
driven by their function and the type of services they provide to the community. 
These services are constantly evolving in response to trends in the health care 
industry and changing expectations of health care customers. Some of these 
health care trends have been the logical result of advances in medical science 
and technology, while the others, driven by social and economic conditions, 
represent new approaches to management of medical care. Additionally, hospital 
design has been greatly influenced by the recognition that physical environment 
has a measurable influence on human well-being. A growing body of evidence has 
been accumulated that shows how appropriate hospital designs can create the 
healing environments that improve patient treatment outcomes and patient care in 
general. Increasingly, hospital designers are expected to use this new evidence-
based design approach when designing new hospitals.

In order to design effective medical facilities for the future, designers must 
be familiar with the latest industry developments, building requirements 
stemming from these trends, and the latest research findings on the impact of 
building designs on hospital operations, staff and patient morale, and patient 
care. It is the purpose of this Design Guide to add to these general 
considerations the issues of buildings’ resistance to natural hazards and 
recommended hazard mitigation measures for risk reduction.



1.2 HEALTH CARE INDUSTRY

1.2.1 Ambulatory Care

In the last 30 years or so, the health care industry has increasingly been 
moving toward greater emphasis on ambulatory care. The increasing availability 
of procedures that can be successfully completed without an overnight stay in 
the hospital has led to a proliferation of free­standing ambulatory care 
centers. Many of these centers are performing sophisticated surgeries and 
complicated diagnostic procedures. Frequently, these centers are not affiliated, 
or are only loosely affiliated with, other hospitals in the community. 

The emphasis on the ambulatory care had a profound effect on the healthcare 
industry, leading to the reduction in the number of hospital beds and, in many 
cases, closing of hospitals because of the reduced de­mand for overnight stays. 
At the same time, hospitals had to increase their own role in ambulatory care to 
remain competitive. As the freestanding ambulatory facilities took an ever-
increasing market share, many hospi­tals had to downsize, and in some cases, 
scale back even their surgical capacity. In many respects, this development has 
diminished the capacity of medical facilities to care for the casualties in the 
event of a disaster, because most of the freestanding ambulatory care centers 
are not suitable for post-disaster emergency care. There are several reasons for 
this:

- They do not have dedicated emergency departments or adequate facilities and 
equipment to deal with trauma patients.
- They are not available or staffed on a 24-hour, 7 days-a-week basis.
- They are not adequately equipped with emergency communications systems.
- The staff is not experienced or well trained to care for the types of patients 
and injuries expected in post-disaster emergencies.
- These facilities are not considered essential and are sometimes built to a 
lower building code standard than hospitals, which makes them more vulnerable to 
disruption resulting from building damage.

On the other hand, hospital-based ambulatory care (outpatient) departments can 
easily be converted for post-disaster care during an emergency, assuming the 
hospital itself remains operational. Hospital-based ambulatory surgeries are 
often contiguous to surgery for inpatients. Clinic space can be used for triage 
or emergency treatment rooms, and the ambulatory diagnostic equipment is 
suitable for use in an emergency. The key to having hospital-based ambulatory 
capacity available in the aftermath of major disasters is thoughtful planning, 
so that the emergency department can remain the command center, coordinating 
other areas of the hospital where post-disaster patients might be transferred 
and treated.

1.2.2 Patient Volume

Hospital emergency rooms have become the primary source of medical care for 
millions of people. Unlike other medical facilities, hospitals are required to 
treat anyone who walks in, or is brought in, irrespective of their insurance 
status or ability to pay. This trend puts an enormous pres­sure on emergency 
departments, not only because it increases the patient load, but also because it 
expands their functions beyond the treatment of emergency and trauma cases.

As a result, many hospitals have enlarged and better equipped their emer­gency 
departments to accommodate the ever-increasing patient load, which had a 
positive influence on their capacity to deal with disaster-re­lated emergencies. 
Additionally, hospital emergency departments are well trained in triage that 
involves prioritization of cases according to the level of medical urgency. 
Patients who are most in need of immediate treat­ment are treated first, while 
the others who can wait without harm are treated later. Emergency department 
staff members also go through ex­tensive disaster drills, and in most cases are 
well trained to respond to mass emergencies.

Despite current hospital construction boom, the trend towards reduced inpatient 
capacity in the past decades has adversely affected hospitals’ readiness for 
emergencies. In 1990, the national average was 372 hospital beds per 100,000 
people, whereas in 2004 there were only 275 beds. The number of hospitals in the 
country [footnote 1] dropped from 5,384 in 1990 to 4,919 in 2004. This has 
reduced the number of hospital beds in many communities to the point where some 
rural communities were left without any acute inpatient capacity. 

Begin Footnote 1. Data from Trendwatch Chartbook 2006, by the American Hospital 
Association and the Lewin Group. End footnote.

As a result of hospital closures and inpatient capacity reduction, occupancy 
rates in the remaining hospitals have increased. The increase means that most 
hospitals are operating at near-maximum inpatient capacity on a regular basis, 
making it difficult to accommodate a potential influx of casualties in a 
disaster emergency. Some reports, especially those that analyzed the response to 
Hurricane Katrina, singled out the limited surge capacity for health care 
emergencies as one of the most pressing problems of the Nation’s health care 
system (see Lister, 2005).

1.2.3 Aging Facilities

Renovation and replacement of health facilities have been at record highs in 
recent years. Still, a majority of hospital buildings throughout the country are 
of considerable age. A great many of them were built in the 1950s and 1960s, 
particularly in rural areas, with Federal funding assistance provided by the 
Hill-Burton Act passed by Congress following World War II. In the 1960s and 
1970s, this grant program was also used for replacement and renovation of urban 
hospitals. As a consequence, a large number of hospitals were built in a short 
period of time. Many of these hospitals are now nearing the end of their useful 
life. Even with periodic renovations, there are limits to the value of continued 
investment in older facilities to be used for acute care.

Hospitals are constantly renovating, whether they are just adding elec­trical 
outlets or communications cables, or engaging in more complex projects that 
involve moving functions and building additions to the existing structures. This 
kind of change is a result of many factors: changing personnel, new 
technologies, and competitive pressure. Some changes, however, may affect the 
use of spaces or facilities originally planned and built for emergency 
operations. For instance, renovation may inadver­tently upset bracing for piping 
and communications conduits, making them more vulnerable to hazards like 
earthquakes or high winds. Similarly, functional reorganization of a hospital 
that makes some critical functions and services more accessible by placing them 
on the ground floor increases the risks from flooding to these facilities.

1.2.4 Healing Environments

Since the advent of the hospital “birthing unit” in the late 1970s, the health 
industry has shown an increasing interest in the ability of the physical 
environment to contribute to healing. The growing evidence indicates that a 
pleasant and comfortable environment reduces stress and provides a sense of 
well-being, both of which are important preconditions for successful recovery. 
Additionally, patient satisfaction with hospital services today extends beyond 
medical care and encompasses a whole hospital experience. To be able to compete 
successfully with the increasing number of outpatient and other alternative 
healthcare providers, hospitals have started to pay greater attention to 
providing a “hospitable” and “healing environment,” in addition to medical 
expertise, new technology, and advanced procedures. 

This trend manifests itself in building designs that introduce the spirit of 
nature into the hospital environment: more natural light, views of nature and 
direct access to the outdoors from many more areas of the hospital, and 
increased use of courtyards and gardens. Hospital gardens have been found to 
provide not only the restorative and calming nature views, but also help reduce 
stress by providing opportunities for escape from clinical settings, and by 
fostering greater social interaction among patients and staff. The social aspect 
is particularly important for patients who might feel isolated in a sterile 
hospital environment without the support of their families and friends. 
Considering the significance of social support for patients’ successful 
recovery, hospitals are planning for greater involvement of the family in the 
care of their patients by requiring single-bed rooms for all newly built acute 
care hospitals. Additionally, hospitals are providing more public spaces that 
facilitate social interaction, such as lounges, atria, and interior streets with 
shops and restaurants that were not part of the traditional hospital 
environment.

The advent of new hospital architecture, especially the new physical 
arrangements designed to assist in healing have, in many respects, increased the 
exposure of hospital buildings to natural hazards. The emphasis on natural 
light, the use of single-patient rooms, and a greater variety of public spaces 
usually produces complex building designs with greater exterior perimeter and a 
greater number of doors and windows that frequently increase building’s 
vulnerability to wind and windborne debris damage. 

1.2.5 Technological Advances

New and emerging technologies ranging from new electronic devices, such as 
nanoscale sensors, to new wireless communication networks are rapidly changing 
patient treatment practices as well as the organization of hospitals. These 
innovative medical technologies help empower the physicians, nurses, and 
patients with tools that enable faster diagnosis and better treatment of 
diseases. Most diagnostic practices and procedural functions in clinics and 
surgeries are dependent on modern instruments, tools, and laboratory equipment. 
Today’s hospitals are in the process of replacing all analog-based instruments 
with digital ones. New hospital organization is now based on an IT architecture 
that maximizes the flow of diagnostic and monitoring data from these instruments 
through new communications networks and makes them available to hospital staff 
in making treatment decisions.

Digitized X-rays and other images are being transmitted electronically to all 
parts of the hospital and to doctors’ offices. Clinical laboratory test results, 
prescriptions, and most forms of medical data are now instantly available to 
medical practitioners. There is a national movement to adopt a universal 
electronic medical record, which would make patient’s medical history and other 
information almost instantly available to the treating physician. 

These technological innovations substantially changed not only the medical 
practice, but also turned this new electronic and IT infrastructure into an 
essential and indispensable backbone of all hospital operations. Therein lays 
the grave vulnerability of hospitals in cases where this sensitive 
infrastructure can be disrupted, as was seen in many hospitals affected by 
Hurricane Katrina. When a disaster, such as Hurricane Katrina, disables or 
impairs the functioning of electronic equipment and data transmission systems, 
or even the hospital voice communications systems, the ability of the medical 
staff to care for their patients is significantly reduced.

Emergency power systems, therefore, become the critical component for 
maintaining hospital’s functions. The dependence on electrical power generators 
is increasing as the hospitals rely more and more on energy-intensive equipment 
and procedures. Even if the hospital is designed to continue to care for 
patients after a disaster, its ability to function is limited by the emergency 
power supply capacity, the extent of coverage by emergency power systems, and 
the ability to remain operational for extended periods of time.

[Begin text box]
A U.S. Department of Energy (DOE) 2002 report ranked healthcare facilities 
second only to food-service facilities in the intensity of energy use, defined 
as the amount of energy consumed per square foot of space.
[End text box]



1.3 HAZARD MITIGATION

Mitigation is defined as any sustained action taken to reduce or eliminate long-
term risk to life and property from hazard events. The goal is to save lives and 
reduce property damage in ways that are cost-effective and environmentally 
sound. Hazard mitigation measures should be integrated into the process of 
planning and design because they reduce casualties and damage resulting from 
building failures during hazard events. The effects of a disaster on a hospital, 
however, are never restricted to the physical damage or the distress among the 
staff and patients as a result of such damage. Consequences frequently include 
partial or total loss of the ability to provide services and meet the demand for 
health care when it is most needed. Incorporating mitigation measures in the 
design of hospitals is therefore especially important because they minimize the 
disruption of hospital operations and protect the uninterrupted provision of 
critical health services. 

Advances in building science and technology, and changes in design philosophy 
and quality assurance techniques for the construction and maintenance of medical 
infrastructure, now make it possible to limit the damage during seismic, 
flooding, and high-wind events. The advent of performance-based design allows 
the use of different levels of protection for different types of infrastructure 
and operations that frequently exceed the minimum requirements of currently 
applicable codes. However, it is not always possible to achieve the protection 
levels one might desire, owing to a variety of factors. Natural or technical 
barriers may exist, or the funding may be insufficient. Even though financial 
resources may be limited, and other circumstances may impose technical barriers 
to the fulfillment of performance objectives, a detailed assessment is still 
required in order to ensure the optimal utilization of available resources. 

The starting point for such an assessment should be a general review of the 
existing hospital network in the area — its operational characteristics, 
geographical distribution, the degree to which it is able to meet health care 
needs and expectations, the epidemiological and demographic profile of the 
population served, and the natural hazards that threaten the provision of 
medical services. The effective functional capacity of all existing hospitals 
should be taken into account, considering as fully as possible all factual 
information on the natural hazards they face and their current level of 
vulnerability. Once the actual characteristics of this network and the potential 
hazards have been identified, and the need to build a new hospital in a specific 
location has been established, it is still necessary to define the role that the 
new facility will play, both in normal times and during emergencies of various 
kinds and intensities. Based on all this information, the level of overall 
functional performance should be set for the new facility. The process of 
determination of the performance level must address the questions of the 
importance of continued and uninterrupted operation of the facility, as well as 
the feasibility and cost-effectiveness of such a performance objective. 

All this will be influenced by the characteristics of the site, the specifics of 
the infrastructure to be built, and the basic services it can realistically be 
expected to provide based on different disaster scenarios. In considering 
disaster mitigation, the goal should be to provide the community with access to 
health care in a reasonable period of time, within reasonable travel distances, 
and to have essential services available to treat patients who sustained 
injuries as a result of the disaster. At the same time, a hospital needs to 
continue to care for their pre-disaster patients and ensure that no harm comes 
to them. 

Much of the procedure for a new building described above can apply to hazard 
mitigation in an existing building as well, with obvious limitations.

1.3.1 Assessing Risk

Beyond the building codes in existence at the time a hospital is designed or 
slated for renovation, the leadership of the facility and the design consultants 
must address key questions to establish the adequacy of the building’s 
performance in the event of a disaster. Hospitals are under enormous financial 
pressure. Any funds invested in making a hospital facility safer for patients 
and staff, more resistant to damage, or capable of continued operations in a 
post-disaster situation must consider the following questions:

- What are the vulnerabilities of the site or existing building to natural 
hazards?
- What are the anticipated frequencies of hazard events?
- What level of loss/damage/injury/death, if any, is acceptable?
- What might be the financial impact of extended downtime on the institution?
- What is the impact to the community if the hospital cannot maintain operations 
in the aftermath of a disaster?

It is not possible to protect against every conceivable event, or to be 100 
percent safe and free of damage in a major disaster. The level of acceptable 
risk must be decided on an individual facility basis by those responsible for 
the institution and its mission.

1.3.2 Evacuation Considerations

In anticipation of high winds or flooding, timely evacuation of some or all of 
the hospital patients to facilities out of the disaster area may sometimes be a 
prudent choice for patient welfare. The risks of transferring acutely ill 
patients must be taken into consideration, as pointed out by the General 
Accounting Office (GAO) report to Congress dated February 16, 2006, titled, 
Disaster Preparedness: Preliminary Observations on the Evacuation of Hospitals 
and Nursing Homes Due to Hurricanes. It stated: “Administrators consider several 
issues when deciding to evacuate or to shelter in place, including the 
availability of adequate resources to shelter in place, the risks to patients in 
deciding when to evacuate, the availability of transportation to move patients 
and of receiving facilities to accept patients, and the destruction of the 
facility’s or community’s infrastructure.”

Many patients have limited mobility and some are on critical life support, 
oxygen or other medical gasses, ventilators, or IV pumps. Moving these patients 
to evacuate the hospital is difficult and requires highly trained staff.

In each geographical area, acute care facility managers must evaluate the likely 
time that they would need to hold patients, how many additional patients might 
arrive seeking care, and what services would be needed and for what period of 
time. In the case of Hurricane Katrina, some hospitals evacuated their Neonatal 
Intensive Care Unit, bariatric patients, and dialysis patients before the storm 
landfall, which proved both prudent and beneficial. For hospitals planning to 
retain their dialysis patients, it is essential to assure a constant supply of 
electricity to power the equipment needed for these patients.

Most hospitals plan to “shelter in place” and weather the storms, rather than 
evacuate. In order to do this, they must take care of their existing patients, 
many of whom are critically ill, and in addition, be prepared to accommodate the 
casualties as well as the increased number of outpatients. In order to 
accomplish this, there are a wide variety of services that must remain 
functional. Often municipal utility services will be cut off during a disaster, 
so alternative power, water, and waste disposal services need to be provided 
onsite whenever possible. Communication systems are often cut off, so redundancy 
is a key factor in maintaining links to the outside world as well as internal 
communications within the hospital. FEMA publications 361 and 543 both address 
the specific needs of structures to be used as disaster shelters.  Sheltering in 
place can be challenging, but in most cases it is the preferred option for most 
acutely ill patients. 

If there is a plan to evacuate patients, or a probability that patients will 
need to be evacuated, regional planning with other hospitals in the area and 
coordination of resources is essential. Agreements and appropriate provisions 
need to be put in place so that space and staff are available to accommodate 
evacuated patients. The State of Florida has put in place an evacuation tracking 
system for all evacuated patients. This might be a useful model for other States 
to follow.

1.3.3 Potential Vulnerabilities

Hospitals provide services that are essential for protecting and safeguarding 
the health and well-being of a community. The continued provision of these 
services is even more critical during and in the immediate aftermath of 
disasters. Considering the complexity of hospital operations, even the smallest 
breakdown in one of its building or equipment systems can cause serious 
disruption of hospital functions. This makes the hospitals extremely vulnerable 
to a variety of natural hazards. 

Hospitals usually have high levels of occupancy, with patients, staff, and many 
visitors present 24 hours a day. Many patients require constant attention, and 
in many cases continuous specialized care and the use of sophisticated medical 
instruments or other equipment. Hospital operations also depend on a steady 
supply of medical and other types of material, as well as public services or 
lifelines. In addition, hospital vulnerability is aggravated by the presence of 
hazardous substances that may be spilled or released in a hazard event. 

Given the importance of hospital services for response and recovery following 
emergencies, and the need for uninterrupted operation of these facilities, 
hospital administrators and designers must consider all aspects of their 
vulnerability. Three main aspects of hospital vulnerability must be taken into 
account:

- Structural
- Nonstructural
- Organizational

1.3.3.1 Structural Vulnerability

Structural vulnerability is related to potential damage to structural components 
of a building. They include foundations, bearing walls, columns and beams, 
staircases, floors and roof decks, or other types of structural components that 
help support the building. The level of vulnerability of these components 
depends on the following factors:

- The level to which the design of the structural system has addressed the 
hazard forces
- The quality of building materials, construction, and maintenance
- The architectural and structural form or configuration of a building

The aspects of adequate design and construction in most hazard-prone areas are 
regulated by building codes and other regulations. The main purpose of these 
regulations is to protect the safety of occupants. They are usually prescriptive 
in nature, i.e. they establish minimum requirements that are occasionally 
updated based on newly acquired knowledge. The building regulations alone, 
however, cannot guarantee uninterrupted operation of a hospital, because a great 
many other factors affect hospital functions. 

1.3.3.2 Nonstructural Vulnerability

The experience of hospital evacuations and other types of disruption during 
recent hazard events (many of which are described in greater detail in later 
chapters) has heightened the awareness that hospital functions could be 
seriously impaired or interrupted, even when the facilities did not sustain 
significant structural damage. The effects of damage to nonstructural building 
components and equipment, as well as the effects of breakdowns in public 
services (lifelines), transportation, re-supply, or other organizational aspects 
of hospital operations, can be as disruptive, and as dangerous for the safety of 
patients, as any structural damage. 

Architectural Components

Nonstructural vulnerabilities that can affect hospital functions and the safety 
of occupants include the potential failures of architectural components, both on 
the exterior and the interior of buildings. 
Damage to roof coverings, facades, or windows can make way for water penetration 
that can damage sensitive equipment and shut down many hospital functions. When 
roofing material is disturbed by wind, the roof may start to leak and the 
moisture can knock out vital equipment, disrupt patient care, and penetrate 
walls and other concealed spaces, allowing mold to build up over time. Window 
breakage resulting from high winds, earthquakes, and even flooding frequently 
requires patient evacuation from affected areas. Patients in critical care and 
acute care units are particularly vulnerable because the move separates them 
from medical gas outlets, monitors, lighting, and other essential support 
services.

Non-load bearing and partition walls and ceilings, for instance, are rarely 
designed and constructed to the same standards of hazard resistance as the 
structural elements. Collapse of these components has caused a number of 
evacuations and closures of hospitals following a hazard event. 

Installations

Hospitals are extremely complex building systems that depend on an extensive 
network of mechanical, electrical, and piping installations. The air and 
ventilation system is one of the most important ones because it is responsible 
for maintaining an appropriate environment in different parts of the hospital. 
Isolation rooms usually have negative pressure so that harmful airborne 
organisms do not migrate outside the patient’s room and infect others. Likewise, 
wards housing patients with immune system deficiencies require a positive 
pressure differential, so that harmful organisms do not enter the patient room 
and needlessly infect them. The malfunction in any one part of this ventilation 
system could create a risk of infection to patients and staff. This system is 
extremely vulnerable to disruption as a result of indirect building damage. 
Winds habitually overturn improperly attached roof-mounted ventilation and air-
conditioning equipment, while the ductwork is very susceptible to collapse in 
earthquakes. Additionally, strong winds may change the airflow from ventilation 
exhaust outlets, potentially causing harmful discharges from patient care areas 
and the clinical laboratory to be sucked back into the fresh air intakes. 
Airborne debris from windstorms could quickly clog the air filtration systems, 
making them inoperable or impaired. 

Hospitals depend on several essential piping systems. Medical gasses are among 
the most important, along with water, steam, and fire sprinkler systems. 
Physicians and nurses depend on oxygen and other gasses required for patient 
care. Unless properly secured and braced, these installations can be easily 
dislodged or broken, causing dangerous leakage and potential additional damage. 

In floods and earthquakes particularly, sewers are apt to overflow, back up, or 
break down. Waste disposal is essential for any hospital, because when the 
toilets back up, or sterilizers, dishwashers, and other automated cleaning 
equipment cannot be discharged, patient care is immediately affected. Retention 
ponds or holding tanks coupled with backflow and diversion valves can be 
employed to solve this problem; however, in many hospitals, this issue has not 
been adequately addressed. 

Elevator service is vulnerable not only to power outage, but also to direct 
damage to elevator installations. Wind and windborne debris can damage elevator 
penthouses, opening a path for water penetration that can disable elevator 
motors and controls, as has happened in numerous hurricanes in recent years. In 
the event of an earthquake, elevator shafts and other equipment can be damaged 
or dislodged, effectively shutting down the building. Flooding of elevator pits 
was a common problem during Hurricane Katrina, and responsible for the loss of 
elevator service.

The emergency power supply system is probably the most critical element in this 
group. Together with fuel supply and storage facilities, this system enables all 
the other hospital installations and equipment that have not sustained direct 
physical damage to function normally in any disaster. However, uninterrupted 
operation of a hospital during a power outage is possible only if adequate 
electrical wiring is installed in all the areas that require uninterrupted power 
supply. Since extra wiring and additional circuits for emergency power increase 
the initial construction costs of the building, the decision on the emergency 
power coverage requires a thorough evaluation of the relative vulnerability of 
various functions to power outage. As patients become more critically ill and 
the nature of diagnosis and treatment becomes more dependent on computers, 
monitors, and other electrical equipment, the need for emergency power will 
continue to grow.

The experience of Hurricane Katrina has demonstrated the need for emergency 
power coverage even for services that typically have not been regarded as 
critical, such as climate control and air-conditioning systems. Extreme heat 
caused a number of hospitals to evacuate their patients and staff when the 
conditions became unbearable.

Equipment and Furnishings

There are many types of internal hazards that might occur as the result of a 
disaster. In the past, bottles in clinical laboratories have fallen and started 
fires. Earthquakes have catapulted filing cabinet drawers and ventilators across 
rooms at high speed, with the potential of causing considerable injury to 
personnel. Any wheeled equipment is vulnerable to displacement and has the 
potential to cause injury. 

Electronic communication systems

Hospitals use and depend on many types of communication systems. For 
communications with emergency vehicles or first response agencies, hospitals 
depend on radio equipment that is frequently mounted on roofs and exposed to 
high winds and windborne debris impact. Satellite dishes, communication masts, 
antennae, and other equipment can be blown off the roof or be severely damaged, 
leaving the hospital without this vital service at a critical time. 

1.3.3.3 Spatial and Other Organizational Vulnerabilities

Most hospitals have disaster mitigation or emergency operation plans, but not 
all of them provide organizational alternatives in the event of disruption of 
the normal movement of staff, patients, equipment, and supplies that 
characterizes everyday hospital operations. The critical nature and 
interdependence of these processes represent a separate category of 
vulnerabilities that need careful attention. Spatial distribution of hospital 
functions and their inter-relationship determines the extent hospital operations 
are affected when normal movement and communication of people, materials, and 
waste are disrupted. The disruption by natural hazard events of administrative 
services such as contracting, procurement, maintenance, as well as allocation of 
resources, can impair hospital functions almost as much as any physical damage.

Just-in-time delivery: Many hospitals have currently eliminated, or greatly 
reduced, onsite storage for linen, supplies, food, and other materials essential 
to normal operations. Any prolonged isolation or blockage of streets serving the 
hospital could lead to a need to ration supplies and triage patients for 
treatment, due to the limited supplies stored on site. During Hurricane Katrina, 
many hospitals were isolated by floodwaters for 5 or more days and, in many 
cases, could not replenish critical supplies, which in some instances 
contributed to the decision to evacuate the facility. 

Evacuation: Evacuation of patients is a measure of last resort, but occasionally 
necessary, especially in extreme situations. Many different conditions or 
vulnerabilities mentioned above can cause the evacuation of a hospital, but the 
process of evacuation itself can also be vulnerable to disruption that can 
seriously aggravate the health and safety of patients. Frequently, a flood, 
earthquake, or a windstorm can cause blockage of access roads, cut ting off a 
hospital from normal evacuation routes, as happened during Hurricane Katrina.  
Surface escape routes were under water and unusable, and even air evacuation was 
impaired because many ground level helicopter landing pads were under water. 
Elevated helipads located on roof tops or elevated parking structures proved 
invaluable features in this type of an emergency. The spatial relationship to 
the hospital building was another aspect that greatly influenced the evacuation 
and reduced the risk of aggravating patients’ condition. Helipads physically 
connected to the hospital were most useful, because patients could be 
transported directly and very rapidly from the upper levels of the hospital to 
the helipad without interference from other hospital functions. 




1.4 HOSPITAL DESIGN AND CONSTRUCTION

Permanent high occupancy and the need for uninterrupted operation are the most 
important characteristics of hospital facilities. They determine most of the 
building design requirements and pose the greatest challenge in the design of 
mitigation measures. Contemporary hospitals must accommodate both critically ill 
patients and a high volume of ambulatory patients. Length of stay for inpatients 
may be as short as one day, but usually averages around 5 to 6 days in most 
hospitals. Acute care patients often have visitors on a daily basis, while 
emergency departments are routinely crowded with patients and their families, 
particularly at peak times during the day.

It is not uncommon that some building designs that are otherwise suitable for 
the complex requirements of hospital operations can impair these operations in 
emergency conditions. This is particularly true of many older hospitals that 
were not designed to maintain their performance level in all conditions. Older 
emergency departments are generally not large enough and are often overcrowded. 
Many of the older hospitals would not have been designed to adjust their 
operations and their physical space to the conditions of mass post-disaster 
care.

Similarly, larger hospitals typically have greater flexibility to cope with the 
emergencies and large numbers of casualties than smaller hospitals.  This, 
however, can be a liability, especially in dense urban areas where hospitals 
buildings are frequently 10 or more stories high. Large, tall hospital 
buildings, with greater than usual floor-to-floor height, are almost completely 
dependent on elevators for vertical communication, which exposes them to serious 
disruption in case of electrical or mechanical failures common during hazard 
events. Such difficulties are further compounded if an evacuation is necessary. 
When the elevators are rendered inoperable, the patients must be carried up or 
down long stairwells, which can be an overwhelming task for the staff of any 
large hospital. 

Since the development of effective ventilation systems, most hospitals were 
designed as “thick” buildings, where many areas do not have natural light and 
depend on mechanical ventilation to be usable. Generally, the larger the 
hospital, the more functions and areas depend on mechanical ventilation and 
artificial light. This is another important aspect of hospital vulnerability in 
situations where normal power supply is disrupted. Hospital closures and 
evacuations caused by nonfunctioning air-conditioning systems in the wake of 
Hurricane Katrina stand as stark examples of the need to protect these systems 
much more effectively. 

Hospitals usually do not occupy just one building. In most cases a hospital is 
located on campus that comprises a number of different buildings, each housing a 
separate function. In addition to an acute care hospital, which might be 
composed of several wings of varying ages, there might also be a separate power 
plant, medical office building, ambulatory surgery and procedures building, 
behavioral health building, fitness center, dialysis center, or cancer center. 
Since all of these buildings have a different type and level of occupancy, from 
the perspective of patient safety and that of uninterrupted hospital operations, 
they do not need the same level of disaster-resistant construction. 

1.4.1 Building Codes

Most States have adopted one of the model building codes, frequently with 
modifications and local additions. Building codes address minimum requirements 
for building resistance to major hazards based on historical experience. Recent 
disaster experience, however, indicates that current code requirements are not 
always adequate, especially not for essential facilities such as hospitals. To 
make things worse, many existing hospitals were built to older codes that 
frequently did not have any provisions for protection against natural hazards. 

Most essential facilities require special attention, in addition to compliance 
with building code requirements, in order to be able to sustain their operations 
after a major disaster. Some States, like Florida or California, for example, 
have amended their codes to address the need for adequate protection of 
hospitals and other critical facilities from prevailing local hazards. 
California has adopted legislation for seismic design based on the principle 
that hospitals should be able to function at least at a basic level after an 
earthquake of moderate to large magnitude. This new standard has resulted in the 
closure of many hospitals that could not comply with the new requirements in a 
cost-effective manner. The implementation of this standard was significant 
because it established the new criterion for post-disaster functionality that 
should serve as a model for hazard-prone regions. This and similar standards 
have expanded the narrow, prescriptive nature of most building codes, by 
defining the performance goals that hospitals must achieve. This Design Guide 
fully supports the trend toward performance-based codes for design and 
construction of hospitals.

In addition to local building codes, various organizations and agencies, like 
the U.S. Department of Veterans Affairs (VA), have developed their internal 
building design and construction regulations to address the three major hazards: 
flood, wind and earthquake. The VA standard “Natural Disasters Non-Structural 
Resistive Design (formerly CD-54)” together with this publication, is a valuable 
resource for information on new hospital construction and renovation of existing 
hospitals.




1.5 MULTI-HAZARD DESIGN CONSIDERATIONS

A comprehensive hazard risk reduction design strategy that considers all the 
risks to which a facility may be subject is an evolving concept that is still in 
its infancy. Multi-hazard design is an approach that aims to integrate risk 
reduction with the building design process, rather than pursuing a traditional 
tendency towards fragmented risk reduction efforts. 

Chapters 2, 3, and 4 outline the characteristics of the three natural hazards 
that are the subject of this publication, in terms of their geographical 
locations, intensity, and frequency. In addition, methods used to mitigate the 
risks and issues relating to building codes and regulations are also discussed. 
However, each hazard is discussed separately, without reference to the others. 
Many building locations are vulnerable to more than one hazard, requiring the 
application of appropriate design solutions that would mitigate each relevant 
hazard.

This section looks at the interaction between various building design features 
and mitigation measures used to protect buildings against specific hazards, by 
comparing their effects for each individual hazard. The similarities and 
differences in the ways that hazards affect buildings, and how to guard against 
them, demand an integrated approach to building design that would be resistant 
to natural hazards. This, in turn, must be pursued as part of a larger, 
integrated approach to the whole building design process.

Of the many hazards that can endanger a hospital and impair the services it 
provides to a community, fire is the most prevalent. Every hospital is at risk 
from fire, which makes this hazard much more pervasive than any of the natural 
hazards noted above. However, fire protection measures have been present in 
building codes for a long time, in the form of approved materials, fire-
resistant assemblies, exiting requirements, the minimum number and capacity of 
emergency exit routes, and many other specifications. For that reason, fire 
hazards are not addressed in this publication as a stand-alone hazard. However, 
the mitigation measures used to protect the buildings against high winds, 
floods, and earthquakes may interact favorably or unfavorably with the need for 
fire protection. For this reason, fire is included as a hazard in the table of 
system interactions at the end of this chapter.

1.5.1 The Need for a Multi-hazard Approach 

The need to embrace a multi-hazard approach when designing or retrofitting a 
hospital is essential for their protection, especially when they are located in 
areas that are exposed to a variety of natural and manmade hazards. A multi-
hazard approach can help identify potentially conflicting effects of certain 
mitigation measures and help to avoid aggravating the vulnerability of many 
hospital building components and systems. A comprehensive evaluation and 
application of hazard mitigation in building design serves to improve the 
overall effectiveness of mitigation measures that protect the continuity of 
hospital functions and operations. The importance of this practice has become 
increasingly evident following the catastrophic disasters that have occurred in 
the recent past.

The aim should be to anticipate and coordinate how the building and its systems 
interact, how mitigation of the risk from one hazard can influence the 
building’s vulnerability to others, and how undesirable conditions and conflicts 
may be avoided or resolved. Through the application of a multi-hazard and multi-
disciplinary approach, cost savings, efficiency, and better performance can be 
achieved in programming and planning new buildings and retrofitting existing 
ones.

[Begin text box]
A multi-hazard risk reduction approach requires a multidisciplinary design team. 
This ensures that the project design benefits from an appropriate professional 
expertise and a thorough discussion of project issues from start to finish.
[End text box]

The design team should be able to take an all-hazard viewpoint, and understand 
how the structure and systems interact under extreme conditions imposed by 
natural hazards. Thus, an important aspect of multi-hazard design should be to 
investigate the extent to which the methods used for mitigation of one hazard 
may reinforce or conflict with design elements necessary for protection against 
other possible hazards. When the design methods reinforce one another, the costs 
of multi-hazard design may be reduced and the performance improved, but where 
they conflict, costs may be increased in order to satisfy the requirements for 
resistance to all relevant hazards. 

1.5.2 Multi-Hazard Design Matrix

The Multi-hazard Design System Interaction Matrix highlights the interaction 
between a particular hazard and a building design component or system. Table 1-1 
presents this system interaction matrix in a graphic form by adding small 
illustrations for each site or building characteristic listed. For each entry 
the matrix provides a description; a sample illustration; positive, negative, or 
neutral characterization of the interaction; and an explanation of the nature of 
interaction.

The explanations are general statements intended more to provoke thought and 
further analysis towards design integration, rather than to provide definite 
restrictions or recommendations. It is possible to overcome conflicts by sound, 
coordinated design between the consultants, starting at the inception of design. 
General cautions, such as the relationship between building weight and seismic 
forces, for example, are intended only as reminders of basic physical facts. 

In order to facilitate comparison between hazards, the following convention has 
been used in Table 1-1.
 A check mark indicates a desirable condition or beneficial interaction between 
the designated component/system and a given hazard. An x indicates an 
undesirable condition or the increased vulnerability of a designated 
component/system to a given hazard. A 0 indicates little or no significant 
interaction between the designated component/system and a given hazard

Begin Table 1-1: Multi-hazard Design Matrix

System ID: 1 SITE
1A Site and Building Characteristics: Site-specific and building specific all-
hazard analysis.
1A HAZARDS:
Seismic: check
Flood: check
Wind: check
Fire: check
Explanation of Interaction: Beneficial for all hazards.

1B Site and Building Characteristics: Two or more means of access to the site
1B HAZARDS:
Seismic: check
Flood: check
Wind: check
Fire: check
Explanation of Interaction: Beneficial for all hazards.

1C Site and Building Characteristics: Site modification to elevate building on 
engineered fill.
1C HAZARDS:
Seismic: x
Flood: check
Wind: 0
Fire: 0
Explanation of Interaction: Highly beneficial for flood, needs very careful site 
engineering for earthquakes. Not significant for fire. Probably not significant 
for 
wind but depends on topography.

System ID: 2 ARCHITECTURAL
2A CONFIGURATION

2A-1 Site and Building Characteristics: Reentrant corner plan forms
2A-1 HAZARDS:
Seismic: x
Flood: 0
Wind: x
Fire: 0
Explanation of Interaction: May cause stress concentrations and torsional forces 
in earthquakes, and contribute to localized high-wind pressures.

2A-2 Site and Building Characteristics: Enclosed courtyard plan forms
2A-2 HAZARDS:
Seismic: x
Flood: 0
Wind: 0
Fire: 0
Explanation of Interaction: May cause stress concentrations and torsional forces 
in earthquakes.

2A-3 Site and Building Characteristics: Very irregular three-dimensional 
building forms
2A-3 HAZARDS:
Seismic: x
Flood: 0
Wind: x
Fire: x
Explanation of Interaction: May create indirect load paths, stress 
concentrations, and torsional forces in earthquakes. May contribute to localized 
high wind pressures, and aggravate evacuation during fire emergencies.

2A-4 Site and Building Characteristics: Large roof overhangs
2A-4 HAZARDS:
Seismic: x
Flood: 0
Wind: x
Fire: 0
Explanation of Interaction: Vulnerable to vertical earthquake and wind forces, 
needs careful engineering.

2B CEILINGS
2B-1 Site and Building Characteristics: Hung ceilings
2B-1 HAZARDS:
Seismic: check
Flood: 0
Wind: 0
Fire: check
Explanation of Interaction: If properly attached to structural components using 
diagonal braces, reduce damage from earthquakes. 

2C PARTITIONS

2C-1 Site and Building Characteristics: Unreinforced CMU or hollow clay tile, 
used as partitions or infill between structural framing 
2C-1 HAZARDS:
Seismic: x
Flood: x
Wind: x
Fire: check
Explanation of Interaction: High vulnerability to seismic and wind forces, but 
desirable against fire if not in seismic zone. If exposed, vulnerable to flood 
forces.

2C-2 Site and Building Characteristics: Non-rigid (ductile) connections for 
attachment of interior non-load-bearing walls to structures including extra-high 
and -heavy gypsum board walls 
2C-2 HAZARDS:
Seismic: check
Flood: 0
Wind: 0
Fire: x 
Explanation of Interaction: Beneficial for earthquakes but gaps between 
components may threaten fire resistance. Not significant for flood and wind.

2C-3 Site and Building Characteristics: Gypsum wall board partitions
2C-3 HAZARDS:
Seismic: check
Flood: x
Wind: 0
Fire: check 
Explanation of Interaction: Gypsum partitions properly braced to structure 
beneficial in seismic zones. Susceptible to flood damage, but good for fire if 
proper resistance is specified. Not significant for wind.

2D OTHER ELEMENTS
2D-1 Site and Building Characteristics: Tile roofs
2D-1 HAZARDS:
Seismic: x
Flood: 0
Wind: x
Fire: check 
Explanation of Interaction: Undesirable in seismic zones unless properly 
attached. On light structures, may cause poor seismic response. 

2D-2 Site and Building Characteristics: Parapets
2D-2 HAZARDS:
Seismic: check
Flood: 0
Wind: 0
Fire: check 
Explanation of Interaction: Properly engineered parapet beneficial in seismic 
zones, but unbraced URM very dangerous in earthquake and wind. High parapets (> 
3ft.) beneficial for wind. May assist in reducing fire spread to adjacent 
buildings. 

System ID: 3 STRUCTURAL SYSTEM

3-1 Site and Building Characteristics: Base isolation and/or energy dissipating 
dampers
3-1 HAZARDS:
Seismic: check
Flood: x
Wind: 0
Fire: 0 
Explanation of Interaction: Beneficial for earthquake, but base isolation in 
basement should be dry floodproofed to reduce vulnerability to flood damage. 
Not significant for wind and fire.

3-2 Site and Building Characteristics: Wood frame structure, used for small 
hospitals and ancillary and service buildings
3-2 HAZARDS:
Seismic: check
Flood: x
Wind: 0
Fire: 0 
Explanation of Interaction: Light weight beneficial in seismic zones provided 
adequate connections and shear walls are used.
Lightness and lack of moisture resistance a disadvantage in floods. 

3-3 Site and Building Characteristics: Heavy structure with concrete floors, 
reinforced concrete moment frame, or frame with reinforced concrete or masonry 
shear walls.
3-3 HAZARDS:
Seismic: check
Flood: check
Wind: check
Fire: check
Explanation of Interaction: Although weight increases seismic forces it is not a 
design problem. Requires special non-ductile detailing for large building 
frames. Generally beneficial for all other hazards.

3-4 Site and Building Characteristics: Reinforced concrete or reinforced CMU 
structural walls with concrete floors and roof deck
3-4 HAZARDS:
Seismic: check
Flood: check
Wind: check
Fire: check
Explanation of Interaction: Very beneficial for wind, good performance for 
earthquake, flood, and fire when correctly designed and constructed.

3-5 Site and Building Characteristics: Steel structural frame
HAZARDS:
Seismic: check
Flood: check
Wind: check
Fire: x
Explanation of Interaction: Lighter than concrete, needs properly detailed 
moment frame, steel braces, or shear walls in seismic and high-wind zones. Good 
in flood with proper detailing, especially for elevated structure. Vulnerable to 
fire.

3-6 Site and Building Characteristics: Unreinforced masonry load-bearing walls
3-6 HAZARDS:
Seismic: x
Flood: x
Wind: x
Fire: 0
Explanation of Interaction: Very poor performance in earthquakes and high winds. 
Undesirable for all hazards because of possibility of collapse.

3-7 Site and Building Characteristics: Steel or concrete frame structure with 
open first floor
3-7 HAZARDS:
Seismic: 0
Flood: check
Wind: 0
Fire: 0
Explanation of Interaction: Very beneficial for flood. Requires careful design 
for earthquake, wind, and fire.

3-8 Site and Building Characteristics: Indirect vertical load path 
3-8 HAZARDS:
Seismic: x
Flood: 0
Wind: x
Fire: x
Explanation of Interaction: Undesirable for seismic and wind hazards because 
poor structural integrity increases likelihood of collapse. Fire may further 
weaken structure.

3-9 Site and Building Characteristics: Large seismic separation joints in 
structure
3-9 HAZARDS:
Seismic: check
Flood: 0
Wind: x
Fire: x
Explanation of Interaction: Improves seismic response, but creates possible path 
for toxic gases during fire. (Cause of deaths in Las Vegas MGM Grand fire.) 
Needs careful protection against wind-driven rain.

3-10 Site and Building Characteristics: Ductile detailing of steel and RC 
structure and connections
3-10 HAZARDS:
Seismic: check
Flood: 0
Wind: check
Fire: 0
Explanation of Interaction: Provides better nonlinear response and a structure 
that is more resistant to collapse.

3-11 Site and Building Characteristics: Reinforced concrete or reinforced CMU 
around exit ways and exit stairs
3-11 HAZARDS:
Seismic: x
Flood: 0
Wind: check
Fire: check
Explanation of Interaction: Properly designed, will preserve evacuation routes 
in event of fire. May create torsional response and stress concentrations in 
earthquakes unless isolated. If fully encloses staircases beneficial as wind 
shelter.

System ID: 4 BUILDING ENVELOPE
4A EXTERIOR WALL CLADDING
4A-1 Site and Building Characteristics: Brick veneer on exterior walls
4A-1HAZARDS:
Seismic: x
Flood: x
Wind: x
Fire: 0
Explanation of Interaction: In earthquakes, winds, and floods material may 
detach and cause costly damage and injury. Careful detailing and quality control 
necessary for good performance.

4A-2 Site and Building Characteristics: Lightweight insulated cladding and EIFS
4A-2 HAZARDS:
Seismic: check
Flood: 0
Wind: x
Fire: 0
Explanation of Interaction: Light weight reduces structural response in 
earthquakes, but needs very careful engineering and application to prevent 
leakage and detachment in winds. Vulnerable to windborne debris impact. Not 
significant in floods or fire.

4A-3 Site and Building Characteristics: Precast concrete panels
4A-3 HAZARDS:
Seismic: x
Flood: 0
Wind: check
Fire: 0
Explanation of Interaction: Requires special detailing with ductile connections 
to structure in high seismic zones. Good for winds if well attached and joints 
are protected against wind-driven rain.

4B GLAZING
4B-1 Site and Building Characteristics: Metal/glass curtain wall
4B-1 HAZARDS:
Seismic: check
Flood: 0
Wind: x
Fire: x
Explanation of Interaction: Light weight reduces seismic forces, but needs 
special design and installation to prevent failure in earthquakes. Fire can 
spread upward behind curtain wall if not properly fireproofed. Vulnerable to 
windborne debris.

4B-2 Site and Building Characteristics: Impact-resistant glazing
4B-2 HAZARDS:
Seismic: 0
Flood: 0
Wind: check
Fire: x
Explanation of Interaction: Can cause problems during fire rescue operations, 
limiting smoke ventilation and access. Good against wind-borne debris but not 
significant for earthquake or flood.

System ID: 5 UTILITIES
5-1 Site and Building Characteristics: Anchorage/bracing of system components
5-1 HAZARDS:
Seismic: check
Flood: check
Wind: check
Fire: 0
Explanation of Interaction: Essential for earthquake and wind (especially 
exterior mounted), beneficial for flood, not significant for fire.

5-2 Site and Building Characteristics: Location of system components above flood 
level
5-2 HAZARDS:
Seismic: x
Flood: check
Wind: 0
Fire: 0
Explanation of Interaction: Very beneficial for flood, if in upper floors may be 
subject to greater forces in earthquake, not significant for wind or fire.

System ID: 6 MECHANICAL

6-1 Site and Building Characteristics: Anchorage/bracing of system components
6-1 HAZARDS:
Seismic: check
Flood: check
Wind: check
Fire: 0
Explanation of Interaction: Essential for earthquake and wind (exterior-
mounted), beneficial for flood.

6-2 Site and Building Characteristics: Vibration-isolated equipment designed for 
seismic and wind forces: snubbers prevent equipment from falling off isolators
6-2 HAZARDS:
Seismic: check
Flood: 0
Wind: 0
Fire: 0
Explanation of Interaction: Very beneficial for earthquake, not significant for 
flood or fire. If not designed to resist uplift inadequate for wind.

6-3 Site and Building Characteristics: Anchorage of rooftop equipment
6-3 HAZARDS:
Seismic: check
Flood: 0
Wind: check
Fire: 0
Explanation of Interaction: Very beneficial for wind and earthquake (with 
seismic designed isolators where necessary), not significant for floods and 
fire.

System ID: 7 PLUMBING

7-1 Site and Building Characteristics: Anchorage/bracing of system components
7-1 HAZARDS:
Seismic: check
Flood: 0
Wind: check
Fire: check
Explanation of Interaction: Essential for earthquake and wind (for exterior-
mounted systems), beneficial for fire.

System ID: 8 ELECTRICAL AND COMMUNICATIONS SYSTEM
8-1 Site and Building Characteristics: Anchorage/bracing of system components
8-1 HAZARDS:
Seismic: check
Flood: 0
Wind: check
Fire: check
Explanation of Interaction: Essential for earthquake and wind (for exterior-
mounted systems), beneficial for fire.

8-2 Site and Building Characteristics: Emergency power supply adequate for 
essential services and equipment securely braced
8-2 HAZARDS:
Seismic: check
Flood: check
Wind: check
Fire: check
Explanation of Interaction: Essential for seismic, flood, wind, and fire.
[End table]




1.6 REFERENCES

S. A. Lister, “Hurricane Katrina: The Public Health and Medical Response,” 
Congressional Research Service, 21 September 2005.

US Department of Energy (DOE). 2002. Buildings for the 21st Century. Office of 
Building Technology, State and Community Programs Newsletter. Washington, DC. 
DOE/GO-102002-1518. Winter 2002.

World Health Organization, 2006. Health Facility Seismic Vulnerability 
evaluation. WHO Regional Office for Europe, Copenhagen, Denmark.






CHAPTER 2 MAKING HOSPITALS SAFE FROM EARTHQUAKES

2.1 INTRODUCTION

This chapter examines potential earthquake damage to hospitals and how these 
facilities can most efficiently improve their expected performance. An 
explanation of the nature and probability of earthquakes is provided, together 
with procedures for determining the approximate earthquake threat to specific 
locations. Typical seismic damages and the possible resulting effects on 
building function or risk to occupants are described and related to standard 
damage states currently used in performance-based earthquake engineering design.

The enhanced performance normally expected from hospitals is discussed, 
specifically as it relates to protection of inpatients and provision of care for 
the injured and other outpatients. Since many older buildings may not have been 
designed for seismic forces, particularly for enhanced performance, estimation 
of the actual expected performance is critical for adequate emergency planning.

The case studies in this chapter illustrate the performance of hospital 
buildings in earthquakes, and look at the enhancements made in the existing 
seismic protection of the structural and nonstructural systems to improve 
performance. The chapter ends with a review of best practices in seismic design 
and seismic retrofit of hospital facilities.

2.1.1 The Nature and Probability of Earthquakes

Although earthquakes cannot be prevented, modern science and engineering provide 
tools that can be used to reduce their effects. Firstly, science can now 
identify where earthquakes are likely to occur, at what magnitude, and determine 
the relative likelihood of a range of ground shaking levels. This information is 
readily available to architects, engineers, code writers, planners, and to the 
general public. Secondly, seismic researchers and structural engineers with 
experience in seismic design have sufficient understanding of the effects of 
earthquake shaking on buildings to create designs that will be safe for various 
intensities of shaking. Modern building codes incorporate all of this 
information and require buildings to have seismic designs appropriate for each 
region.

However, earthquakes are complex phenomena, and the exact nature of ground 
shaking, and a building’s response to that shaking, are still shrouded in 
considerable uncertainty. The primary intent of the seismic provisions of 
building codes is to provide buildings that will be safe in the expected 
earthquake. Current buildings designed to modern codes are extremely unlikely to 
sustain serious structural damage or partial collapse in a design earthquake. 
However, subtle changes in shaking from site to site, the wide range of building 
types and configurations, and the variation in skill and thoroughness with which 
any one building is designed and constructed can result in a wide range of 
damage levels in any given earthquake. Perhaps more importantly, many older 
hospital buildings were designed and built without seismic design features, or 
at best outdated ones. These buildings cannot be expected to perform well enough 
to serve their intended roles after an earthquake event. Lastly, it is now well 
known that the nonstructural systems of essential buildings are extremely 
important in maintaining post-earthquake functionality. Until very recently 
these systems, in general, have not been designed and installed with adequate 
seismic protection.

2.1.2 Earthquake Effects

Fractures and movements within the earth’s crust generate earthquake ground 
motion by sending waves through the rocks and soil outward from the source. Most 
commonly, these sources are known faults, defined as cracks or weakened planes 
in the earth’s crust most likely to “break” as a result of global tectonic 
movements. The propagation of the waves through the crust produces movement of 
the earth’s surface. Any one location on the surface will move in every 
direction simultaneously, back and forth, side to side, and up and down, 
creating the shaking effect that is both strange and frightening. The shaking 
effect, or seismic ground motion, is felt in all directions from the epicenter—
the location where the fracture started—and diminishes with distance from the 
epicenter. Buildings, bridges, transmission towers, and other structures 
supported by, and attached to, the ground will also be shaken. If the intensity 
of shaking is high, most structures will sustain some damage. The criteria used 
to determine the capacity of the ground motion to inflict damage on the built 
environment are somewhat intuitive: large displacements of the ground (3 feet 
versus 3 inches), rapid changes in the movement (measured in units of 
acceleration), or the duration of shaking.

Although seismic ground motion is most often identified with earthquakes, it is 
not the only phenomenon that causes damage. Earthquakes involve movements of 
large portions of the earth’s crust, and the resulting shaking can produce other 
geologic hazards:

Surface Fault Rupture affects a small strip at the ground when the movement on a 
fault deep within the earth breaks through to the surface. The relative 
displacement of the ground on each side of the rupture may be several feet or 
more, and structures straddling this zone are likely to be severely damaged. 

Liquefaction occurs when the behavior of loose granular soils and sand in the 
presence of water changes temporarily from that of a solid to that of a liquid 
material when subjected to ground shaking. This condition occurs mainly at sites 
located near rivers, lakes, and bays.

Landslides occur when the top layers of soil and rock slip on sloping ground, 
triggered by earthquake ground motion.

Tsunamis are earthquake-caused wave movements in the ocean that travel at high 
speed and may result in large coastal waves of 30 feet or more. They are 
sometimes, and incorrectly, called tidal waves.

Seiches are waves similar to tsunamis. They can be triggered by earthquakes and 
generated by sloshing in closed lakes or bays; they have the potential to cause 
serious damage, although such occurrences are very rare.

2.1.3 Measuring Earthquake Effects

Earthquakes vary in many respects, but the resulting shaking depends mainly on 
the magnitude of the earthquake and the distance from the epicenter. The 
potential risk to manmade structures is determined on the basis of frequency of 
occurrence of earthquakes at a given site, and measurements of a number of 
physical characteristics of ground shaking. The following section discusses the 
measurements used for this purpose, and their role as damage parameters.

Perhaps the most familiar measure of earthquakes is the Richter Magnitude, 
devised by Professor Charles Richter of the California Institute of Technology 
in 1935. Richter’s scale is based on the maximum amplitude of certain seismic 
waves recorded on a standard seismograph at a distance of 100 kilometers (km) 
from the earthquake epicenter. Because the instruments are unlikely to be 
exactly 100 km from the source, Richter devised a method to allow for the 
diminishing of wave amplitude with increased distance. The Richter scale is 
logarithmic, and each unit of magnitude indicates a ten-fold increase in wave 
amplitude. The energy level is multiplied approximately by 31 for a unit 
increase in Ritcher magnitude scale. The scale is open-ended, but a magnitude of 
about 9.5 represents the largest earthquake scientists now expect within the 
current understanding of movement in the earth’s crust.

Among scientists, Richter Magnitude has been replaced by Moment Magnitude, a 
similar measure of energy that is based on the physical characteristics of the 
fault rupture, which is a more useful measure for large events. The Moment 
Magnitude scale has been set to produce values similar to the Richter scale, and 
for damaging earthquakes, values are normally in the 5.5 to 8.0 range, although 
magnitudes over 9.0 also occur.

The level of damage is often measured by intensity scales, and the most common 
scale used in the United States is the Modified Mercalli Intensity (MMI) scale, 
reported in Roman Numerals from I to XII. MMI is often incorrectly used to 
measure the size of an earthquake. In fact, the MMI is assigned to small areas, 
like zip codes, based on the local damage to structures or movements of soil. 
Many MMIs can be associated with a single earthquake because the shaking, and 
therefore the damage, diminishes as the distance to the epicenter increases. 
Although the MMI is useful for the purpose of comparing damage from one event to 
another (particularly events for which little or no instrumental measurements 
are available), it is very subjective, and scientists and engineers prefer 
instrumental measurements of the ground shaking to measure intensity.

[Begin text box]
It is important to understand that magnitude is not a measure of damage, but a 
physical characteristic of an earthquake. An earthquake with magnitude 6.7 that 
occurs in a remote area may cause no damage to manmade structures, but one with 
the same magnitude can cause considerable damage if it occurs close to an urban 
area.
[End text box]

Scientists and engineers need measures of the damaging characteristics of 
earthquakes to compare the inherent risk at different locations, and to develop 
design solutions to limit damage to acceptable levels. The universal 
characteristic of earthquakes, and the one that can be measured most precisely, 
is the ground motion. Extensive networks of instruments are now employed on the 
ground and in manmade structures to record continuously the motions during an 
earthquake. The ever-growing database of earthquake recordings can be analyzed 
in various ways to develop appropriate measures of intensity that best predict 
potential damage to buildings and other structures, and the possibility of 
liquefaction and landslides. Tsunamis and seiches are normally not caused by the 
traveling seismic waves, but by large, single movements of land under water as 
part of the fault movement or resulting large landslides.

2.1.3.1 Measuring Seismic Ground Motion

Acceleration is a measure of velocity changes over time, and is commonly 
experienced when our heads snap back when a car starts off rapidly. Similarly, 
acceleration causes a building to “snap back” as a result of sudden large ground 
movement. This movement within the building or other structure, which becomes 
very complex when caused by 20 seconds or more of ongoing, rapidly changing 
accelerations, is what causes direct shaking damage. Consequently, it is common 
to describe earthquake motion using the largest acceleration that occurred 
during the event, or peak ground acceleration (PGA).

Although PGA is useful as a simple way to measure and compare ground motions, it 
is not the most comprehensive one. From studies of ground motions and structural 
responses to ground motions, engineers and researchers have developed parameters 
that consider the characteristics of the entire motion, rather than the one 
instant when the PGA occurs. This characterization of the ground motion is 
called a response spectrum and measures the extent of shaking different 
structures will experience, based on their natural period of vibration, when 
subjected to a given ground motion (see section 2.2.2.1). The maximum response 
to a specific ground motion of a structure with a given period is called the 
spectral ordinate. The full response spectrum simply represents the suite of 
spectral ordinates for a wide range of structures—from periods of about 0.2 
seconds (short, very stiff buildings) to periods of about 4.0 seconds (tall, 
very flexible buildings).

Response spectra can be calculated for the entire database of recorded ground 
motions, and trends analyzed. For example, it has been determined that the 
response spectrum for most earthquake shaking has a similar shape, and that this 
shape has subtle changes based on the soil on which it was recorded. Figure 2-1 
shows the typical shape of earthquake ground motion spectra and the variations 
that will be caused, on average, by different site soils. Like PGA, higher 
spectral ordinates typically mean more intense and potentially damaging motions. 
By studying the location of potential earthquake sources and the probability of 
them generating an earthquake in any given time period, scientists can develop a 
response spectrum for earthquake shaking likely to occur at that site. This has 
been done by the U.S. Geological Survey (USGS) for the entire United States, and 
is the basis for seismic design requirements in building codes. This information 
is presented on maps. For simplicity, only two spectral ordinates are mapped, 
for periods of 0.2 second and 1.0 second. Examples of these maps are shown in 
Figure 2-2. Rules in the building codes allow engineers to calculate the 
appropriate spectral ordinate for all periods, as shown in Figure 2-1, based on 
the mapped values. Site classes in the figure are also defined in the building 
codes, Class A being hard rock, and Class E being a very soft site with 
potential soil failure.

[Begin Figures]
Figure 2-1: Graph showing representative shapes of building code (or 
design)response spectra for different soils. 

Figure 2-2: Two examples of national seismic hazard maps. One shows 1.0 spectral 
acceleration (%g) with 2% probability of exceedance in 50 years, and the second 
shows 0.2% spectral acceleration (%g) with 2% probability of exceedance in 50 
years. [end figures]

2.1.3.2 Measuring Potential for Liquefaction

Soils that are loose, not well graded, and saturated with water are prone to 
liquefaction. These conditions often occur near waterways, but not always. In 
addition to the soil type, the probability of liquefaction also depends on the 
depth from the surface to the layer, and the intensity of ground motion. 
Further, the results of liquefaction can vary from a small, uniform settlement 
across a site, to loss of foundation bearing, resulting in extreme settlement 
and horizontal movement of tens of feet (called lateral spreading). Lastly, the 
risk of liquefaction is directly dependent on the earthquake risk. Due to this 
complex set of conditions, damage potential from liquefaction is difficult to 
map. For all but the smallest projects, many building jurisdictions in seismic 
areas require identification of liquefaction potential in the geotechnical 
report, particularly in areas of known potential vulnerability. On sites where 
liquefaction is more than a remote possibility, the likely results of 
liquefaction at the ground surface or at the building foundations will also be 
estimated. Small settlements may be tolerated without mitigation. Larger 
potential settlements can be prevented by site remediation measures, if 
economically justified. In some cases of potential massive liquefaction and 
lateral spreading, using the site for structures may not be cost effective. 
Officials in some regions of high seismicity have developed maps of local areas 
that are potentially susceptible to liquefaction and require site-specific 
investigation.

2.1.3.3 Measuring Potential for Landslide

The shaking from earthquakes can also cause landslides, depending on the slope, 
type, and configuration of soil stratum. Landslides can cause damage to 
improvements built within the slide area or near the top of the slide, ranging 
from complete destruction to distortion from relatively small vertical or 
lateral movements. Sites can also be threatened by landslides occurring uphill, 
sometimes completely offsite and quite a distance away. Similar to liquefaction, 
accurate probability of land sliding is difficult to map on a regional or 
national scale, and this threat is normally identified in site-specific geologic 
hazard studies. Also similar to liquefaction, the largest portion of the risk 
may be a triggering event. In some cases, it is possible and cost effective to 
stabilize small areas at risk of potential landslides. Stabilizing larger areas 
at risk of landslides may not be feasible. Some regions of high seismicity have 
developed maps of the areas susceptible to landslides based on average slopes, 
geologic soil types, and the past history of sliding. Sites within these 
susceptible zones require site-specific investigation.

2.1.3.4 Measuring Potential for Tsunami and Seiche

Researchers have studied tsunamis and seiches for many years, but the tragic 
tsunami in the Indian Ocean in December 2004 highlighted the need for better 
measurement of the threat in terms of magnitude and location. Obviously, only 
sites near large bodies of water are susceptible, and normally at elevations 50 
feet or less above the water surface, although bays and narrow canyons can 
amplify the wave height. Although similar to storm surge, the height and the 
potential velocity of a tsunami wave represent a separate risk and must be 
mapped separately. In addition to dependence on local conditions, quantification 
of the risk from tsunamis and seiches is made more difficult because not every 
earthquake generates such a wave. Studies are required that consider the 
individual characteristics of the site and the facility, to establish the risk 
and identify possible mitigating measures.

2.1.4 Earthquakes: A National Problem

Most people now know that although most frequent in California and Alaska, 
earthquakes are not restricted to just a few areas in the United States. In 
fact, two of the greatest earthquakes in U.S. history occurred not in 
California, but near New Madrid, MO, in 1811 and 1812. In the International 
Building Code (IBC), the most common model building code in use in the United 
States and its territories, buildings on sites with a low enough seismic risk 
that specific design for seismic forces is not required are classified as 
Seismic Design Category (SDC) A. As shown in Figure 2-3, 37 of 50 States have 
regions with sufficient seismic risk to require designs more stringent than SDC 
A. The likelihood of a damaging earthquake occurring west of the Rocky 
Mountains—and particularly in California, Alaska, Oregon, Washington, and Utah—
is much greater than it is in the East, Midwest, or South. However, the New 
Madrid and Charleston, SC, regions are subject to potentially more severe 
earthquakes, although with a lower probability, than most regions of the western 
United States. According to the IBC design maps and the USGS hazard maps upon 
which they are based, other locations should also plan for intermediate ground 
motions.

[Begin figure]
Figure 2-3: Map showing states with seismic risk. [End figure]

[Begin text box]
The U.S. Congress recognized earthquakes as a national problem by passing 
legislation authorizing the National Earthquake Hazards Reduction Program 
(NEHRP) in 1977. NEHRP has since supported numerous research and hazard 
mitigation efforts. 
[End text box]

Records show that some seismic zones in the United States experience potentially 
damaging earthquakes approximately every 50 to 70 years, while other areas have 
“recurrence intervals” for the same size earthquake of about 200 to 400 years. 
These frequencies of occurrence are simply statistical probabilities, and one or 
several earthquakes could occur in a much shorter than average period. With 
current knowledge, there is no practical alternative for those responsible for 
healthcare facilities located in earthquake-prone regions but to assume that the 
design earthquake, specified in the building code for the local area, could 
occur at any time, and that appropriate planning for that event should be 
undertaken.

Moderate and even very large earthquakes are inevitable, although very 
infrequent, in some areas of normally low seismicity. Consequently, in these 
regions, most buildings in the past were not designed to deal with an earthquake 
threat; they are extremely vulnerable. In other places, however, the earthquake 
threat is quite familiar. Medical facilities in many areas of California and 
Alaska will be shaken by an earthquake, perhaps two or three times a year, and 
some level of “earthquake-resistant” design has been accepted as a way of life 
since the early 20th century.

Nationally, the areas where earthquakes are likely to occur have been 
identified, and scientists have a broad statistical knowledge of the potential 
magnitude of these earthquakes and the likelihood of their occurrence. However, 
it is not yet possible to predict the near-term occurrence of a damaging 
earthquake. Therefore, it makes sense to take the minimum precautionary measures 
and conform to local seismic building code requirements for new buildings. U.S. 
seismic building code provisions focus on requiring the minimum measures 
necessary to prevent building collapse, because most lives are lost in 
earthquakes as a result of building collapse. The code provisions for essential 
buildings intended to remain functional after a major earthquake have not yet 
been thoroughly tested.

[Begin text box]
If a healthcare facility or community desires to obtain more detailed 
information on the seismic hazard than is shown on the code maps, or if the 
location does not enforce a seismic code but there is concern about seismicity, 
the USGS Web page at www.USGS.gov, Earthquake Hazards Program, is an excellent 
resource. The USGS provides more detailed earthquake hazard maps for general 
regions such as the Western, Central, and Eastern United States. Local building 
or planning departments, fire departments, or other local emergency management 
agencies should be consulted for the availability of mapping for liquefaction, 
landslide, tsunami, and seiche. For even more localized information, the USGS 
provides seismicity information for any location in the United States on the 
basis of latitude and longitude or zip code. This information can be obtained by 
referring to the Seismic Hazard listings on the USGS Web page, and opening 
“Hazards by Latitude and Longitude,” or “Hazards by Zip Code.” These listings 
show information on the expected maximum shaking that is estimated for the 
location. The information and terminology are quite technical, and may need to 
be interpreted by qualified staff at the responsible local code office, a 
structural engineer, or other knowledgeable seismic professional.
[End text box]

2.2 SEISMIC BUILDING DESIGN

Seismic design is highly developed, complex, and strictly regulated by codes and 
standards. Seismic codes present criteria for the design and construction of new 
structures subject to earthquake ground motions in order to minimize the hazard 
to life and to improve the capability of essential facilities to function after 
an earthquake. To these ends, current building codes provide the minimum 
requirements necessary for reasonable and prudent life safety. 

More basic information about seismic design of buildings can be found in FEMA 
454, Designing for Earthquakes (FEMA, 2007)

[Begin text box]
Building design codes for cities, States, or other jurisdictions throughout the 
United States are typically based on the adoption, sometimes with more 
restrictive local modification, of a model building code. Up until the mid-
1990s, there were three primary model building code organizations: Building 
Officials and Code Administrators International, Inc. (BOCA), International 
Conference of Building Officials (ICBO), and Southern Building Code Congress 
International, Inc. (SBCCI). In 1994, these three organizations united to found 
the International Code Council (ICC), a nonprofit organization dedicated to 
developing a single set of comprehensive and coordinated national model 
construction codes. The first code published by ICC was the 2000 International 
Building Code (IBC; ICC, 2000) and was based on the NERHP Provisions. The IBC 
now references ASCE 7, Minimum Design Loads for Buildings and Other Structures 
(ASCE, 2005) for its seismic provisions. Some jurisdictions in the country may 
still be using the Uniform Building Code (UBC) seismic provisions (its final 
update was in 1997), while most have adopted or are preparing to adopt the IBC. 
In this document, code references are to the IBC Code and to its seismic 
standard, ASCE 7.
[End text box]

Seismic code requirements cover:
- A methodology for establishing the design ground motion at any site based on 
seismicity and soil type
- Procedures for the seismic analysis of the building structure and key 
nonstructural components and systems
- Some detailed design requirements for materials, systems, and components
- Definitions of irregular building configurations and limitations on their use
- Building height limitations related to structural type and level of seismicity

2.2.1 The Equivalent Lateral Force (ELF) analysis Methodology

Of the 96 equations in the IBC, the Equivalent Lateral Force (ELF) equation is 
the key element in the most-used code methodology for determining seismic 
forces. This force is termed the equivalent force because it represents, in 
greatly simplified and reduced form, the complex to-and-fro, multidirectional 
earthquake forces with a single static force applied at the base of the 
building. Once this force is determined, all the structural components of the 
building (walls, beams, columns, etc.) can be analyzed through other code-
prescribed procedures to determine what proportion of this force must be 
assigned to each of them. This general methodology is characteristic of all 
seismic codes throughout the world. 

The ELF equation is derived from Newton’s Second Law of Motion, which defines 
inertial force as the product of mass and acceleration. The ELF equation 
replaces Newton’s acceleration with an acceleration coefficient that 
incorporates some of the other factors necessary to represent more accurately 
the acceleration of the mass of the building, which is generally higher than the 
ground acceleration. To determine this coefficient, the code provides another 
equation and additional coefficients that encompass most of the characteristics 
that affect the building’s seismic performance. The ELF procedure is used for 
the great majority of buildings. Buildings of unusual form, or with other 
special features or site conditions, may be required to use more complex 
analytical methods.

Hospitals are classified in the building code as Occupancy Category IV — 
“essential for post-earthquake response and recovery” — and therefore have 
special design requirements intended to improve performance. Designers should 
use 50 percent greater earthquake forces for design of Category IV buildings 
than for normal buildings, which will provide an additional safety factor and 
reduce potential structural damage. In addition, design rules for Category IV 
buildings allow less movement between floors during earthquake shaking, reducing 
nonstructural damage to windows, walls, stairways, and elevators. Lastly, these 
buildings are required to incorporate more complete and stronger anchorage and 
bracing of nonstructural components and systems than normal buildings.

2.2.1.1 Acceleration

The most common and widespread cause of earthquake damage is ground shaking 
caused by the seismic waves that radiate out from the focus of the earthquake. 
The waves begin like ripples in a still pond when a pebble is thrown into it, 
but rapidly become more complex. There are four main wave types, of which “body” 
waves, within the earth, are most important for seismic design purposes. First 
to arrive at a given site is the P or Primary wave: this wave successively 
pushes and pulls the ground along the wave front as it moves forward. The effect 
is felt as a sharp punch—it feels as if a truck has hit the building. The P wave 
is followed by S, the secondary or shear wave, which is a lateral motion, back 
and forth, but perpendicular to the wave front.

The nature of the waves and their interactions are such that actual movement of 
the ground will be random: predominantly horizontal, often with considerable 
directional emphasis and sometimes with a considerable vertical component. 
Because of the random nature of the shaking, structures must be designed on the 
assumption that earthquake forces will come from all directions in very rapid 
succession, often fractions of a second apart.

The inertial forces inside the building, generated by ground shaking, depend on 
the building’s mass and acceleration [footnote 1]. The seismic code provides 22 
maps that provide values for spectral acceleration (the acceleration to be 
experienced by structures of different periods). These values, with some 
additional operations, are inserted into the ELF equation and provide the 
acceleration value and eventually the base shear value for the structure. 

[Begin footnote 1]
1. Acceleration is the change of velocity (or speed) in a certain direction over 
time, and is a function of the earthquake characteristics: acceleration is 
measured in “g,” which is the acceleration of a falling body due to gravity. 
[End footnote] 

[Begin figures]
Figure 2-4 shows an example of a portion of map from the IBC, showing contour 
lines of spectral acceleration. The numbers are the acceleration values to be 
used in the equation, based on the project location.

Figure 2-4 is a portion of an earthquake ground motion map used in the seismic 
code. [End figures]

2.2.1.2 Amplification and Soil Type

As seismic vibrations propagate towards the earth’s surface, they may be 
amplified depending on the intensity of the shaking, the nature of the rock and, 
above all, by the surface soil type and depth. Earthquake shaking tends to be 
more severe on soft ground than in stiff soil or rock, which produces greater 
building damage in areas of soft soils. This amplification is most pronounced 
for shaking at longer periods and may not be significant at short periods. 
Studies after the 1989 Loma Prieta Earthquake showed that shaking in the soft 
ground was 2.5 to 3.5 times that of shaking in rock.

The ELF equation deals with soil amplification by introducing a soil type 
coefficient in the process of determining the acceleration coefficient. The code 
defines six soil types, ranging from hard rock to very soft soil, and provides 
varying coefficients that relate soil type to building period, because the 
amplification is also modified by building period.

2.2.1.3 Building Period

All objects have a natural or fundamental period. This is the rate at which they 
will vibrate if they are given a horizontal push. When a building begins to 
vibrate as a result of ground motion, it will tend to sway back and forth at its 
natural period (Figure 2-5).

[Begin figure]
Figure 2-5 is an illustration showing that each object has its own fundamental 
period at which it will vibrate. [End figure]

More complex structures will oscillate at several different periods, the longest 
one (greatest amount of time to complete one cycle) often being called the 
fundamental period. The fundamental periods of structures vary from about 0.05 
second for a piece of equipment anchored to the ground to about 0.10 second for 
a one-story building. Taller buildings between 10 to 20 stories will oscillate 
in the fundamental mode at periods of between 1 and 2 seconds. The building 
height is normally the main determinant of building period (Figure 2-6), 
although more technically, the period is based on the mass and stiffness 
characteristics of the structure.

[Begin figure] Figure 2-6 is an illustration showing the period (in seconds) and 
building height. Height is the main building determinant. [End figure]

Acceleration within the building is influenced by its period, and diminishes as 
the period increases (the motion changes from abrupt shocks to a gentler 
swaying) as explained in Section 2.1.3.1.

2.2.2 Critical Building Characteristics 

2.2.2.1 Period and Resonance

As described in Section 2.1.3.1, the natural period of a building measures the 
time it takes to vibrate one full cycle. This basic characteristic of the 
structure will determine to a large degree how a building responds to earthquake 
ground shaking. Short, stiff buildings will have a short period and will shake 
with sharp, jerky movements (high accelerations), which will tend to cause 
contents such as equipment or furniture to slide around and possibly overturn. 
Taller, more flexible buildings will have longer periods and “shake” slower and 
smoother, but with larger “to and fro” movement than short buildings. The larger 
movements may create more relative displacement between floors and cause damage 
to walls, stairs, and elevators connected to multiple floors. In rare cases, 
buildings with periods over about 1.5 seconds on soft sites may match the 
vibration patterns of the site and resonate, causing large amplifications of the 
motions within the buildings.

2.2.2.2 Damping

A pendulum — or a child’s swing — is a very effective oscillator, and will 
continue to swing for many minutes after a push, although the extent of the 
swinging, or amplitude, will gradually diminish. Buildings and other objects do 
not oscillate as effectively because the vibration is damped, or reduced. The 
extent of damping in a building depends on the structural system, materials of 
construction; how the structural components are connected; and on the type and 
quantity of architectural elements such as partitions, ceilings, and exterior 
walls. A high level of damping, in which the vibration of the building will 
rapidly diminish, is a desirable feature.

2.2.2.3 Nonlinear Behavior

It is generally not cost-effective to design buildings to be completely 
undamaged in strong earthquake motion. Building codes require designers to base 
their designs on forces that are not as great as the shaking can generate, on 
the assumption that the building’s structure will deform and absorb part of the 
energy, thus limiting the forces that can be generated. These severe 
deformations represent “nonlinear behavior” and structural damage, ranging from 
minor (that can be left alone) to more serious (that will require repair). The 
building code has been “tuned” over the last 5 or 6 decades by adjusting code 
requirements according to the results of detailed observations of the behavior 
of buildings. Nonlinear deformations are expected in hospitals and other 
critical buildings, but to a lesser extent than normal buildings. The intent is 
to minimize structural damage to enable the buildings to remain occupied after 
the shaking.

Some materials and structural systems can accept nonlinear behavior better than 
others and are thus considered superior seismic systems.

2.2.2.4 Ductility

Ductility is the characteristic of certain materials — steel in particular — 
that fail only after considerable distortion or deformation has occurred. This 
is why it is more difficult to break a metal spoon by bending it than one made 
of plastic. The metal object will remain whole, though distorted, while the 
plastic spoon will break suddenly without warning (Figure 2-7). This property of 
materials is used to ensure that a building may adequately resist more than its 
design ground motion.

[Begin figure] 
Figure 2-7 is an illustration showing ductility. Ductility is the characteristic 
of materials such as steel that fail only after considerable deformation has 
occurred. Nonductile materials (like poorly reinforced concrete) fail without 
warning in a brittle manner. [End figure]

The deformation of metal, even in the spoon, absorbs energy and defers absolute 
failure of the structure. Brittle materials, such as unreinforced masonry or 
ceramic tile, fail suddenly with a minimum of distortion. Ductility is an 
important characteristic of the structural system. Thus, buildings with 
appropriate seismic designs are either designed so that the materials and 
connections will distort but not break, in case they are subjected to forces 
higher than those required by the code, or they are designed for very large 
forces. Some structural materials, like masonry and concrete, are brittle on 
their own, but when properly combined with steel reinforcing bars, can exhibit 
high ductility. This characteristic of the structural system is also considered 
in the ELF methodology.

2.2.2.5 Strength and Stiffness

Strength and stiffness are the two of most important seismic characteristics of 
any structure. Two structural beams may be equally strong (or safe) in 
supporting a load, but may vary in their stiffness — the extent to which they 
bend or deflect in doing so. Stiffness is a material property but is also 
dependent on shape. For vertical forces this is usually the only aspect of 
stiffness that is of concern. When floor joists are designed for a house, for 
instance, their deflection rather than strength is what often dictates their 
size. Typically, an unacceptable amount of deflection will occur well before the 
members are stressed to the point at which they break. 

In seismic design, there is another very important aspect to stiffness. The 
problem of determining the overall lateral force on the building by multiplying 
its weight by its acceleration has already been discussed. But how is this force 
distributed among the various structural members so that the engineer can design 
each one appropriately? Relative stiffness enters into this issue because the 
applied forces are “attracted to” and concentrated at the stiffer elements of 
the structure—in engineering terms, the forces are distributed in proportion to 
the stiffness of the resisting elements. Mathematically, the stiffness of a 
structural member varies approximately as the cube of its length: thus one 
column that is half the length of another will be eight times stiffer (23) and 
will be subject to eight times the horizontal load of the long column. This 
concept has serious implications for structures with lateral members of varying 
lengths, and in designing such a structure the engineer tries to equalize the 
stiffness of the resisting elements so that no one member or small group of 
members takes a disproportionate amount of the overall load (Figure 2-8).

[Begin figure] 
Figure 2-8 is an illustration showing the short column problem. The stiffness of 
a column varies approximately as a cube of its length. The short column is half 
the height but takes 8 times the load of the long column. [End figure]

Short columns represent a problem that emphasizes the need for good structural 
seismic design. Columns in this category may not even be part of the lateral 
force resisting system. Nevertheless, if the structural and nonstructural 
components create such a condition, these columns are likely to be severely 
damaged during strong ground shaking. 

2.2.2.6 Drift

Drift is the term used in seismic design to describe the horizontal deflection 
of structural members in response to seismic forces. In the seismic code, limits 
are set on the amount of drift permitted. This is done to ensure that the 
structure will not be designed to be so flexible, even if structurally sound, 
that its nonstructural components will be unacceptably damaged. Drift is limited 
on a story basis. The allowable story drift is limited to floor-to-floor height 
times 0.010 (1 percent of the floor height) for essential buildings and 0.015 
(1.5 percent of the floor height) if the nonstructural components have been 
designed to accommodate drift. A story drift of 0.010 is equivalent to a 
deflection of 1-1/2 inches for a floor-to-floor height of 12 feet 6 inches 
(Figure 2-9).

[Begin figure] 
Figure 2-9 is an illustration showing allowable story drift. For most buildings, 
drift is limited to: floor-to floor height x 0.010 to 0.020. A story drift of 
0.010 = deflection of one and a half inches for a height of 12 feet 6 inches. 
[End figure]

2.2.2.7 Configuration: Size and Shape

Experience has shown that the architectural form of a building has a major 
influence on its performance during ground motion. This influence is the result 
of the three-dimensional interaction of all the structural systems and all 
architectural components when subjected to earthquake forces. For certain 
architectural forms, the response of the building can become very complex and 
the earthquake forces can become concentrated and distributed in undesirable 
ways. The term building configuration is used in seismic design to determine the 
architectural form of the building.

The kinds of unusual conditions that warrant concern are a result of early 
architectural decisions that determine the configuration of the building. In 
making these decisions, the architect plays a major role in determining the 
seismic performance of the building and can make it easy or difficult for an 
engineer to develop an efficient and cost-effective structural design. For 
seismic design purposes, configuration can be defined as building size and 
shape; the size and location of the structural elements; and the nature, size, 
and location of nonstructural elements that may affect structural performance. 
The latter include such elements as heavy and/or stiff nonstructural walls, 
staircases and elevator shafts, exterior wall panels, and heavy equipment items.

The seismic significance of the building configuration is that it determines 
both the way forces are distributed throughout the structure and the relative 
magnitude of those forces. Seismic codes distinguish between regular and 
irregular configurations, and it is the latter that may have a detrimental 
influence on the effectiveness of the seismic engineering and on building 
seismic performance. Configuration irregularity results in two main undesirable 
conditions — stress concentrations and torsional forces.

2.2.2.8 Stress Concentrations

Irregularities tend to create abrupt changes in strength or stiffness that may 
concentrate forces in an undesirable way. Stress concentration means that an 
undue proportion of the overall forces is concentrated at one or a few points of 
the structure, such as a particular set of beams, columns, or walls. Those few 
members may fail, and by chain reaction bring down the whole building. Stress 
concentration can also be created by vertical irregularity. The most serious 
condition of vertical irregularity occurs when a building has a soft or weak 
story, usually the ground floor, which is significantly weaker or more flexible 
than those above. This design creates a major stress concentration at the points 
of discontinuity, and in extreme cases may lead to collapse unless adequate 
design is provided for such points. Figure 2-10 shows some types of soft story 
design, and Figure 2-11 shows the collapse mechanism that is created.

The severe damage to Olive View Hospital in 1971, described in Section 2.3.1.1, 
was largely the result of a soft first story design. Such soft or weak stories 
are not permitted in current seismic designs. 

[Begin figure] Figure 2-10 is an illustration showing types of soft and weak 
story structures. Stress concentrations: the most serious conditions of vertical 
irregularity is the soft or weak story, in which one story, usually the first 
with taller, fewer columns, is significantly weaker or more flexible than the 
stories above. [End figure]

2.2.2.9 Torsional Forces

Irregularities in building configuration may produce torsional forces, which 
complicates the analysis of building resistance. Torsion is created by a lack of 
balance between the location of the resisting elements and the arrangement of 
the building mass. Engineers refer to this as eccentricity between the center of 
mass and the center of resistance, which tends to make the building rotate 
around the latter and create torsion within the resisting elements.

[Begin text box]
The IBC lists a dozen conditions of irregularity (six horizontal and six 
vertical) for which special design requirements apply. These special 
requirements either restrict the level of irregularity, amplify forces to 
account for it, or require more sophisticated analysis. A severe soft first 
story is specifically prohibited, although it is often encountered in existing 
buildings.
[End text box]

As explained in Section 2.2.1.1, the weight of the floors, walls, and roof 
contributes to the main lateral forces exerted on the structure through the 
center of mass, usually the geometric center of the floor plan. If the 
resistance provided by the building components is exerted through this same 
point (the center of resistance), then there is no torsion and balance is 
maintained. As shown in Figure 2-12, conditions of eccentricity — when the 
centers of mass and resistance are offset — produce torsional forces.

[Begin figure] 
Figure 2-12 is an illustration showing torsion. [End figure]

One building configuration that is most likely to produce torsion features re-
entrant corners (buildings shaped in plan like an “L” or a “T,” for example). 
The wings of such buildings tend to twist and result in torsion and stress 
concentration at the “notch” where the wings meet, also called a re-entrant 
corner (Figure 2-13).

[Begin figure] 
Figure 2-13 is an illustration showing the re-entrant corner building. [End 
figure]

Buildings that have large variations in their perimeter resistance on different 
facades of the building also tend to produce torsion. Such variations often 
occur when some facades have large areas of glazing while the others have solid 
walls.

Irregular configurations generally arise because of functional planning and 
programming requirements, or sometimes because of the architect’s or owner’s 
desire to create an original or striking architectural form.

Hospitals often have irregular and complicated configurations as a result of 
their functional complexity. Broadly speaking, smaller hospitals are usually 
planned in one or two stories with horizontal-planned layouts; large hospitals 
often have a vertical tower for patient rooms elevated above horizontally 
planned floors for the diagnostic, treatment, and administrative services. 
Emergency services are generally placed at the ground floor level, with direct 
access for emergency vehicles. However, new developments in hospital design (see 
Section 1.2.4) represent a radical departure from this traditional hospital 
morphology. The new designs are based on decentralization of functions, and the 
introduction of natural environment into hospital buildings. New hospital 
buildings have even more complex configurations consisting of fragmented blocks 
interspersed with courtyards and gardens, where different blocks frequently have 
not only different shapes, but different structural systems as well.

The structural design for a hospital, however, should still focus on reducing 
configuration irregularities to the greatest extent possible and ensuring direct 
load paths. Framing systems need careful design to provide the great variety of 
spatial types necessary without introducing localized irregularities (Figure 2-
14). 

[Begin 2-14] 
Figure 2-14is an illustration showing the complex footprint of a large community 
hospital. There are 2 shaded areas which represent open courtyards. [End figure]

2.2.3 Specifications for Performance-Based Seismic Design

Beginning with the 1989 Loma Prieta earthquake in the San Francisco Bay Area, 
the importance of the consequences of damage, other than endangering life 
safety, has been increasingly recognized, not only in hospitals and other 
critical facilities, but in all buildings. A major effort to develop guidelines 
for seismic rehabilitation of buildings was funded by FEMA in 1992, and 
published as FEMA 273 (1997). Subsequently, this guideline was improved and 
republished as FEMA 356 (2000), and in 2007 was made a standard by the American 
Society of Civil Engineers (ASCE 41).

2.2.3.1 Performance Levels

As a result of the high cost of retrofit and the growing interest in 
understanding the various performance levels of buildings in earthquakes, FEMA 
273 described a variety of seismic performances for both structural and 
nonstructural systems that could be targeted in design. These performances were 
summarized in a matrix (see Table 2-1) that allowed specification of a given 
performance level by combining the desired structural performance with a desired 
nonstructural performance. Four overall performances levels from that table were 
highlighted as discussed below. These performance levels were developed to be 
applicable to any building with any occupancy, as appropriate.

[Begin table] 
Table 2-1: Combinations of Structural and Nonstructural Seismic Performance

Nonstructural Performance Levels: N-A Operational
Structural Performance Levels and Ranges:
S-1 Immediate Occupancy: Operational 1A
S-2 Damage Control Range: 2-A
S-3 Life Safety: Not recommended
S-4 Limited Safety Range: Not recommended
S-5 Collapse Prevention: Not recommended
S-6 Not Considered: Not recommended

Nonstructural Performance Levels: N-B Immediate Occupancy
Structural Performance Levels and Ranges:
S-1 Immediate Occupancy: Immediate Occupancy 1B
S-2 Damage Control Range: 2-B
S-3 Life Safety: 3-B
S-4 Limited Safety Range: Not recommended
S-5 Collapse Prevention: Not recommended
S-6 Not Considered: Not recommended

Nonstructural Performance Levels: N-C Life Safety
Structural Performance Levels and Ranges:
S-1 Immediate Occupancy: 1-C
S-2 Damage Control Range: 2-C
S-3 Life Safety: Life Safety 3C
S-4 Limited Safety Range: 4-C
S-5 Collapse Prevention: 5-C
S-6 Not Considered: 6-C

Nonstructural Performance Levels: N-D Hazards Reduced
Structural Performance Levels and Ranges:
S-1 Immediate Occupancy: Not recommended
S-2 Damage Control Range: 2-D
S-3 Life Safety: 3-D
S-4 Limited Safety Range: 4-D
S-5 Collapse Prevention: 5-D
S-6 Not Considered: 6-D

Nonstructural Performance Levels: N-E Not Considered
Structural Performance Levels and Ranges:
S-1 Immediate Occupancy: Not recommended
S-2 Damage Control Range: Not recommended
S-3 Life Safety: Not recommended
S-4 Limited Safety Range: 4-E	
S-5 Collapse Prevention: Collapse Prevention 5E
S-6 Not Considered: No rehabilitation
[End table]

Operational Building Performance Level (1-A)
Buildings meeting this target building performance level are expected to sustain 
minimal or no damage to their structural and nonstructural components. The 
building would be able to continue its normal operations, possibly with only 
slight adjustments, mainly with respect to power, water, and other utilities 
that may need to be provided from emergency sources. 

Under low levels of earthquake ground motion, most hospitals should be able to 
meet or exceed this target building performance level. However, it would not be 
cost-effective to design buildings for this target building performance level 
under very rare, intense ground shaking, except for buildings that offer unique 
services or that contain exceptionally hazardous material.

Full functionality is normally considered difficult to achieve in the immediate 
aftermath of strong earthquake shaking. Offsite issues, such as staff 
availability and potential loss of utilities that are not under the control of 
the facility, may impair operations. In addition, relatively minor onsite damage 
to key components can significantly affect overall functionality. A single 
anchorage failure of the emergency generator, or a leak in one of the many 
pressurized water systems, can significantly disrupt hospital operations. 

Immediate Occupancy Building Performance Level (1-B)
Buildings meeting this target building performance level are expected to sustain 
minimal damage to their structural elements and only minor damage to their 
nonstructural components. While it would be safe to reoccupy a building meeting 
this target building performance level immediately following a major earthquake, 
nonstructural systems may not function, either because of the lack of electrical 
power or damage to fragile equipment. Therefore, although immediate occupancy is 
possible, it may be necessary to perform some cleanup and repair and await the 
restoration of utility services before the building can function in a normal 
mode. The risk of casualties at this target building performance level is very 
low.

Many building owners may wish to achieve this level of performance when the 
building is subjected to moderate earthquake ground motion. In addition, some 
owners may desire such performance for very important buildings under severe 
earthquake ground shaking. This level provides most of the protection obtained 
under the Operational Building Performance Level, without the cost of providing 
full standby utilities and performing rigorous seismic qualification of 
equipment performance.

Immediate Occupancy is more realistic than the Operational performance level for 
most buildings, and at a minimum, should be the goal of all new hospital 
buildings. However, since even the smallest disruption of non-structural systems 
may be too detrimental for continued operation of a hospital, the owners and 
designers should consider a higher level of protection for critical hospital 
functions. For instance, it is recommended that provisions be made for 
independent operation of critical utilities for a minimum of 4 days. Critical 
utilities usually include electric power; water; the sanitary sewer; and, 
depending on the local weather conditions, fuel for heating and cooling.

Life Safety Building Performance Level (3-C)

Buildings meeting this performance level may experience extensive damage to 
structural and nonstructural components. Repairs may be required before re-
occupancy of the building is allowed, although in some cases the repair may not 
be deemed cost-effective. The risk of casualties in buildings meeting this 
target building performance level is low.

This target building performance level entails somewhat more extensive damage 
than anticipated for new buildings that have been properly designed and 
constructed for seismic resistance. 

The Life Safety level should prevent significant casualties among able-bodied 
hospital occupants, but may not protect bed-ridden patients. In these 
circumstances, life safety level of protection is not appropriate for new 
hospitals.

Collapse Prevention Building Performance Level (5-E)
Although buildings meeting this target building performance level may pose a 
significant hazard to life safety resulting from failure of nonstructural 
components, significant loss of life may be avoided by preventing collapse of 
the entire building. Many buildings meeting this performance level may, however, 
be complete economic losses.

This level has been sometimes selected as the basis for mandatory seismic 
rehabilitation ordinances enacted by municipalities, as it results in mitigation 
of the most severe life-safety hazards at the lowest cost. Collapse Prevention 
is intended to prevent only the most egregious structural failures, and includes 
no consideration for continued occupancy and functionality of a hospital, the 
economics of damage repair, or damage to nonstructural components. 

2.2.3.2 New Developments in Performance-Based Design

Although developed for use in the process of seismic rehabilitation of older 
buildings, the aforementioned damage descriptions have filled a void and have 
become an interim standard for describing seismic performance of both new and 
existing buildings.

The goal of performance-based earthquake engineering has thus become the 
development of methods to predict the expected losses adequately, measured by 
the risk of casualties, the cost of damage repair, and the length of building 
downtime. These losses are to be calculated on a cumulative and probabilistic 
basis, allowing communication with stakeholders based on losses in a given 
scenario earthquake, the losses due to a probabilistically determined event, or 
the average annual losses over a given time period.

Since the publication of FEMA 273, performance-based earthquake engineering has 
continued to develop, particularly through research performed at the Pacific 
Earthquake Engineering Research Center, one of three major earthquake research 
centers funded by the National Science Foundation, and through the FEMA funded 
project, Next Generation Performance-Based Seismic Design Guidelines, FEMA 445 
(2006). When this work is completed, the global performance states used by FEMA 
356 will be redefined better to reflect current knowledge and to communicate the 
potential losses to stakeholders more effectively.

[Begin text box]
The example of California shows how earthquake damage affects legislation. The 
1971 San Fernando earthquake was particularly damaging to hospital buildings, 
most notably the Olive View Medical Center, a brand new facility that was 
damaged so badly that it was eventually demolished. Based on similar experiences 
with schools, the legislature passed the Hospital Seismic Safety Act (HSSA) in 
1972. The intent of the law was both to protect acute care patients and to 
provide post-earthquake medical care. The law was patterned after the Field Act 
covering schools in California, specifying the same State review agency, and 
stipulating design by specially experienced and approved “Structural Engineers.” 
It covered new buildings only and provided for a “Building Safety Board” of 
industry design professionals and facility experts, appointed by the Director of 
Health Services, to advise the State on implementation of requirements.

The law and regulations included four main considerations:
- Geologic hazard studies for sites

- Structural design forces in excess of those used for “normal” buildings 
(initially a “K-factor” of 3.0; later, an importance factor, I, of 1.5)

- Specific design requirements for nonstructural elements

- Strict review of design and inspection of construction

Surprisingly, only 23 years after the San Fernando earthquake, another damaging 
event occurred in almost the same spot. In January of 1994, the Northridge 
earthquake produced very large ground motions in the San Fernando Valley just 
north of Los Angeles. Just as the San Fernando event, the Northridge earthquake 
had a profound effect on hospital design in California. Although there were no 
failures in hospitals comparable to the Olive View disaster, several hospitals 
required evacuation as a result of failures of both structural and nonstructural 
systems. These high-profile evacuations once again put the hospital building 
inventory in the spotlight. Analysis and comparison of the performance of 
buildings in Northridge built before and after the HSSA clearly indicated its 
effectiveness. This analysis also indicated that further improvements were 
needed in the performance of nonstructural systems.

Senate Bill 1953, which introduced a plan to bring all pre-Act hospital 
buildings into compliance with the HSSA by the year 2030, was signed into law by 
the governor of California in September, 1994. Standards and regulations needed 
to implement the law included:

- Definition of structural vulnerabilities and evaluation standards

- Definition of nonstructural vulnerabilities and evaluation standards

- Standards for retrofit

- Building evaluations and facility compliance plans shall be submitted to the 
Office of Statewide Health Planning and Development (OSHPD) by January 1, 2001; 
Facility owners, 60 days after approval by OSHPD, shall submit building 
performance categories to local emergency service agencies and use the 
performance information to improve emergency training, response, and recovery 
plans
- Hospital buildings with a high risk of collapse cannot be used for acute care 
purposes after January 1, 2008. These buildings must be retrofit (to a “life 
safe” performance), demolished, or abandoned for acute care use by that date
- High-risk nonstructural systems (pre- and post-Act) shall be mitigated in 
accordance with priorities and timelines to be set in regulation by OSHPD, in 
consultation with the Hospital Building Safety Board
- All facilities shall be in substantial compliance with the intent of the HSSA 
by January 1, 2030
[End text box]




2.3 EARTHQUAKE DAMAGE TO HOSPITALS

Although earthquakes damage most manmade structures in similar ways, to 
understand the true consequences of damage to buildings with special occupancies 
and functions requires a much more detailed and accurate damage description than 
may be needed for other buildings. The effects of earthquake damage on hospital 
operations and the safety of occupants have been described below based on the 
experiences of hospitals in the United States and around the world.

Historically, buildings have been engineered to provide adequate life safety to 
occupants and passers-by from earthquake hazards. For most buildings, life 
safety is primarily threatened by building collapse or the debris falling into 
the street and neighboring buildings. A higher level of performance is required 
to address the life safety issues of hospitals, since patients often have 
limited mobility and are dependant on caregivers or specialized medical 
equipment. 

Figure 2-15 shows typical building components and systems present in hospital 
facilities. The structural elements consist of the foundations, columns, beams 
and slabs, walls, and braces that hold the building up against vertical gravity 
forces and horizontal wind and earthquake forces. The nonstructural elements 
include building service systems, such as electricity and lighting, heating and 
cooling, plumbing, and interior architectural systems, such as ceilings, floors, 
partitions, and other interior components. The building envelope includes the 
systems that separate the interior spaces from the exterior, both structural and 
nonstructural. It includes exterior walls and cladding, roof systems, doors and 
windows, and floors or slabs that separate the building interior from the 
ground. Contents and equipment are completely dependent on the type of occupancy 
and the function of the space, and range from items such as furniture 
encountered in a lobby or a waiting room, to highly technical equipment commonly 
present in treatment rooms. In addition, laboratories, pharmacies, bulk storage 
areas, and large central energy plants have highly specialized and frequently 
very sensitive equipment.

[Begin figure]
Figure 2-15A is an illustration showing the structural and nonstructural 
elements of a buildings structure. [End figure]

[Begin figure]
Figure 2-15B is an illustration showing the structural and nonstructural 
elements of a buildings nonstructural components. These include roofing and 
insulation, air conditioning units, ducts, lighting, windows partitions, 
exterior wall panels, ceiling furniture, and sprinkler piping. [End figure]

In general, both the building service systems and the contents of hospitals rank 
among the most complex and expensive of any building type. Furthermore, both the 
structural system and most of the nonstructural systems are required to perform 
without interruption after an earthquake to enable adequate functionality. 

2.3.1 Types of Structural Damage

When the ground shakes in an earthquake, the shaking is transferred to the 
building, potentially causing structural damage. The damage can consist of 
cracks in structural walls, bent or broken braces, or damage to columns and 
beams. Damage can range from minor (a few cracks), to major (parts of the 
structure rendered ineffective and potentially unsafe), to complete collapse. 
See Figures 2-16, 2-17, 2-18, and 2-19 for examples of structural damage.

[Begin figures]
Figure 2-16 is a photo showing how this concrete building suffered severe damage 
to the columns at the second floor level. It was deemed unsafe by the local 
jurisdiction and later demolished.

Figure 2-17 is a photo showing a steel frame building with a post-earthquake 
“lean” to the right, seen particularly at the first floor. Severe damage was 
found in its beam column joints and it was later demolished.

Figure 2-18 is a photo showing severe damage to a poorly reinforced masonry wall 
on a steam plant.

Figure 2-19 is a photo showing damage to an exterior concrete wall. This 
hospital building was evacuated. 
[End figures]

2.3.1.1 The Case of the Olive View Medical Center

The Olive View Hospital in the northern San Fernando Valley, owned by Los 
Angeles County in Southern California, was severely damaged on February 9, 1971, 
when the San Fernando earthquake damaged the almost-new facility so severely 
that it was later demolished. Over 500 patients were evacuated immediately after 
this event. 

The 850-bed Olive View Hospital campus comprised of over 30 buildings of 
various ages, but most notably featured a complex of buildings completed in 
1970, only months before the 1971 San Fernando earthquake. These “new” buildings 
included the five-story Medical Care Facility, the two-story Psychiatric Unit, 
the Heat and Refrigeration Plant, and several other smaller ancillary buildings 
such as a warehouse, assembly building, walkway canopy, and ambulance canopy. 
The 1970 buildings were all made of reinforced concrete and designed under the 
1965 Los Angeles Building Code, which included seismic provisions. However, 
neither of these buildings had any special seismic protection features. In fact, 
the poor performance of this facility was one of the prime reasons for passage 
of California’s Hospital Seismic Safety Act (HSSA). The shaking experienced in 
the 1971 San Fernando earthquake was extreme; however, structural performance of 
these buildings was worse than the engineering community had expected. 
Subsequent investigations indicated that the buildings technically met the 
requirements of the code, but included features that made them particularly 
vulnerable to earthquake damage. Seismic codes were subsequently refined to 
prevent this type of vulnerability in future buildings.

The damage to Olive View Hospital buildings was nearly catastrophic. The first 
story of the medical treatment and care unit was over 15 inches out of plumb and 
near collapse (Figure 2-20). Three of the four exterior stair towers pulled away 
from the main building or collapsed completely, rendering them useless for 
egress (Figure 2-21). The Ambulance Canopy collapsed onto the parked ambulances 
and destroyed them. The first story of the Psychiatric Unit collapsed, but all 
the occupants were on the second floor at the time.

Almost immediately after the event, the patients in the Psychiatric Unit began 
assembling in a parking lot adjacent to their facility. The need to evacuate was 
obvious, and the second floor wards were only feet from the ground after the 
first floor collapse (see Figure 2-22). Controlling and tracking these patients 
was nearly impossible, particularly in the first few hours. Within 5 hours, 
evacuation was underway in the main building using interior stairwells. The 
building had no power and therefore no elevators or lights. The nurses evacuated 
their own units, ambulatory patients first, and, when sufficient assistance was 
available, non-ambulatory patients. By that time, a network of ambulances and 
helicopters had been set up for transfer to other facilities (Arnold, 1983; 
Lew, 1971; NOAA, 1973).

Subsequent analysis of the effects of this earthquake on the hospital noted, as 
particularly troublesome, the lack of functioning communications, either 
internal or external, the lack of an effective evacuation plan or identified 
assembly area, and the lack of any control or tracking of medical records. 

[Begin figures] 
Figure 2-20 is an aerial photo of the 1971 Olive View Hospital. Source NOAA 
1973.

Figure 2-21 is a photo showing the collapsed stair tower in main building of 
Olive View Hospital.

Figure 2-22 is a photo showing the completely collapsed first level (not 
visible) of Psychiatric Unit. 
[End figures]

2.3.2 Nonstructural Damage

When the ground shakes the structure, the structure shakes everything that is in 
it or on it, including the building envelope and components of the interior 
nonstructural systems. This shaking can damage most components directly. 
Indirect damage, resulting from structural deformation between floors, can cause 
damage to all the systems connected to these structural components.

Damage to architectural systems consists of broken windows and cracked exterior 
walls and interior partitions. In extreme cases, exterior walls and partitions 
topple completely. Ceilings are also vulnerable to damage and can break into 
small pieces or fall to the floor (see Figure 2-23).

Damage to the building service systems can consist of sliding or overturning of 
equipment like boilers, generators, and fans, or swaying and possible fracture 
of mechanical ducts, pipes, and electrical conduit (see Figures 2-24, 2-25, and 
2-26).

[Begin figures] 
Figure 2-23 is a photo showing a damaged exit corridor – dark and barely 
passable 

Figure 2-24 is a photo showing vibration isolation bearing assemblies on 
mechanical equipment collapsed due to seismic shaking – such movement breaks 
pipe or electrical connections to the equipment

Figure 2-25 is a photo showing a heavy transformer that moved several feet, 
breaking all connections

Figure 2-26 is a photo showing a large chiller almost tipped over.

Figure 2-27 is a photo showing damaged radiology equipment that not only will 
not function, but has become a life-safety risk. 
[end figures]

Because of the wide variation of contents and equipment in a typical hospital, 
the type of damage experienced by these systems varies widely. For example, 
medical equipment, such as operating tables and lights, radiation and x-ray 
units, sterilizers, and patient monitors, is often heavy and not well anchored 
to the structure (see Figure 2-27). Offices and storage rooms, such as the areas 
used to store critical supplies, medicine, medical records, chemicals, and fuel, 
can also be severely damaged by shaking (see Figures 2-28, 2-29, and 2-30).

[Begin figures]
Figure 2-28 is a photo showing chaos in a storage area similar to central 
storage or medical records.

Figure 2-29 is a photo showing jumbled contents of a typical office

Figure 2-30 is a photo showing overturned tank similar to medical gas storage
[End figures]

2.3.2.1 The Case of New Olive View Medical Center

Nearly 15 years after the 1971 earthquake event that destroyed the Olive View 
Medical Center (see Section 2.3.1.1), a new hospital was opened on the same site 
in 1986 (see Figure 2-31). Built even stronger than required under the HSSA of 
1972, the building has no basement and features a seismic system of concrete 
shear walls and steel plate shear panels. The building was also equipped with 
instruments to record its response in future earthquakes. 

[Begin figure]
Figure 2-31 is an Aerial view of “new” Olive View Hospital (1986) [end figure] 

Early in the morning of February 17, 1994, a Magnitude 6.7 earthquake occurred 
on a little known fault not generally considered dangerous, located near 
Northridge, about 10 miles from the hospital. The recorders captured 
exceptionally high accelerations of 0.82 g at the first floor and 1.7 g at the 
roof. The structure suffered little or no structural damage, but its stiffness 
and strength contributed to the transfer of unprecedented accelerations through 
the building, in some cases overwhelming the seismic anchorage and bracing 
provided for the building’s nonstructural systems. Some of these components and 
systems were not considered sufficiently vulnerable to require special bracing.

Damage included the following (see Figures 2-25, 2-26, and 2-27):

- Shifting, and in some cases, failure, of anchorages of equipment at the roof 
level, where accelerations were the highest. This movement broke one or more 
chiller water lines, causing flooding in the top floors (see Figure 2-32).

[Begin figure]
Figure 2-32 is a photo showing anchor bolts stretched by large seismic 
accelerations at roof level [end figure] 

- Excessive movement and failure of wall-mounted television brackets, causing 
some television units to fall.

- Extensive damage to suspended panelized ceilings, some exacerbated by leaks 
from water pipes from above.

- Excessive movement and interaction between gypsum board and other fire-rated 
ceilings and sprinkler lines, causing additional leaks.

- Movement at the copper tube reheat coils in the ceiling spaces, almost 
universally at the third through the sixth floor, causing leaks (see Figures 2-
33 and 2-34).

[Begin figures] 
Figure 2-33 is a photo showing damage to ceiling and fixtures from sprinkler 
breaks
Figure 2-34 is a photo showing detail of water damage to ceiling 
[end figures]

- Damage to some equipment anchorage in the Central Plant and at the bulk oxygen 
tank.

- Damage to elevators, seven of which were temporarily out of service. Four had 
sustained severe damage as a result of derailed and bent counterweight frames. 

Right after the earthquake event, administrators planned a partial evacuation of 
79 patients, but by the afternoon, they decided to transfer all 377 patients to 
other facilities, despite the dangers associated with such a move. The 
evacuation was prompted primarily by water damage and lasted 41 hours (EERI, 
1995; LACDHS, 1994; URS, 1996).

Despite vastly improved structural performance and compliance with the 
requirements for anchorage and bracing of nonstructural components and systems 
compared to the conditions in 1971, the hospital’s operations were severely 
compromised. It was not ready to accept local casualties, but actually increased 
the load of neighboring facilities by requiring evacuation. The unexpectedly 
poor performance was caused by extraordinary ground motions, probably made more 
damaging to nonstructural systems and contents by the very stiff and strong 
structure of the hospital.

2.3.2.2 The Case of Kona Community Hospital, Hawaii 

The Kona Community Hospital (KCH) is located in Kealakekua, Kona, on the central 
west coast of the Big Island of Hawaii. It was originally a county hospital but 
became part of the State-owned Hawaii Health Systems Corporation in 1996. KCH is 
a full service medical center and is the primary health care facility serving 
West Hawaii. The facility has 33 acute medical-surgical beds, a 9-bed intensive 
care unit, 7 obstetric beds, an 11-bed behavioral health unit, and a 34-bed 
skilled nursing/long term care wing.

The KCH campus includes several buildings, but the primary medical facility 
occupies a three-story L-shaped building that consists of the original 60,500-
square-foot block built in 1972, and an 18,300-square foot addition built in 
1989. Both structures are concrete, rectangular in plan, and each forms one leg 
of the L shape. The site slopes east to west creating two stories above grade 
and one basement floor on the east, and three stories above grade on the west 
face. The lateral force (wind and earthquake) resisting system of the original 
building consists of concrete pier shear walls that are part of the exterior 
wall and concrete walls around the elevators and stairs. The lateral force 
resisting system of the addition is a ductile concrete frame, which consists of 
the beams and columns rigidly tied together in a manner that resists lateral 
motion. The addition is notable in that the two bays of the western end are open 
at the ground floor and serve as a drive-through to the back of the campus and 
as an ambulance entrance. 

Seismic Characteristics of the Facility

A technical evaluation of the facility, performed in 1993 by the Hawaii State 
Earthquake Advisory Board, identified several seismic deficiencies. It was found 
that the layout of the lateral-force-resisting shear walls in the original 1972 
building and the outdated pattern of column reinforcement were of the type that 
previously contributed to unacceptable levels of damage in similar buildings. 
Conditions that presented potential seismic deficiencies were also found in the 
1989 addition, including:

- The eccentric location of the stair and elevator core, which could potentially 
create torsional response 
- Inadequate connection of the floor slabs to the core, which could cause 
moderate damage 
- The potential for nonstructural plaster cracks in the upper floors,
- The potential for local damage at the connection between new and original 
wings during shaking 

As part of the evaluation, the torsion issue (related to the stair and elevator) 
was checked and found not to represent a significant problem. No specific 
recommendations related to the other problems were included in the evaluation. 
The structural evaluation concluded with a recommendation to retain a local 
structural engineer to review the seismic adequacy of KCH in more detail. The 
evaluation also covered the seismic protection of nonstructural systems and 
equipment, but included no specific recommendations for KCH.

The evaluation categorized nonstructural components and systems as:

- Systems and elements which are essential to hospital operations and without 
which the hospital cannot function (Essential) 

- Nonessential elements whose failure could compromise hospital operations 
(Nonessential) 

In fact, when considering seismic preparedness of hospital facilities, there is 
little consensus about the types of nonstructural systems that should be 
considered essential. These classifications vary from facility to facility and 
are closely tied to elements of the emergency response plan. For example, 
unbraced, suspended panelized ceiling systems typically used in hospitals on the 
island were noted as a deficiency, but consistent with standard seismic 
evaluation procedures for hospitals, these systems were classified as 
nonessential. However, as described below, damage to these ceilings proved 
critical when a real event struck.

Most of the larger equipment categorized as “essential” in the evaluation, 
including the emergency generator, the bulk oxygen tank, the chiller, and the 
rooftop cooling tower were seismically anchored and continued to function during 
the earthquake.

Damage

The Hawaii Earthquake of October 15, 2006, had a magnitude 6.7 and was centered 
about 35 miles north of the KCH. It was followed by a second shock of magnitude 
6.0. The shaking caused moderate damage over much of the Big Island and was felt 
as far away as Oahu. Shaking on the island of Hawaii, as recorded on several 
instruments installed by the USGS, featured relatively high accelerations, but 
the energy was restricted to very high frequency (short period), which proved 
damaging to stiff, brittle structures. Several unreinforced masonry buildings 
suffered severe damage, many ungrouted stone masonry fences and walls partially 
collapsed, and landslides and rockslides were common, which is consistent with 
this kind of motion. 

The Special Services Building at KCH (seen in Figure 2-35 behind the small 
single-story building on the left) contained such an instrument. The response 
spectrum for one component of motion is shown in Figure 2-36 in blue. Note the 
rapid decline of spectral acceleration values for very short periods, much less 
than 0.5 seconds. The orange curve shows a spectral shape that might be expected 
in association with such high ground accelerations, and is included in building 
codes. High values of spectral acceleration between the periods of 0.5 seconds 
and 2.0 seconds, as shown in this standard curve, are usually associated with 
much greater building damage. 

[Begin figures]
Figure 2-35 is a photo showing Kona Community Hospital. Addition (1989) 
supported on isolated columns in the foreground. The original building (1972) is 
in the left background. 

Figure 2-36 is a graph showing response spectra for one of the orthogonal 
components recorded at the site of the KCH. The blue line is for the recorded 
motion. The orange line represents the spectral shape more commonly expected and 
contained in building codes.(Graph courtesy of www.Cosmos-eg.org) 
[end figures]

The high acceleration motion caused the unbraced, suspended panelized ceilings 
(see Figure 2-37), found almost everywhere at KCH, to strike partitions that 
demarcated rooms and corridors. The partitions were constructed of steel studs 
and gypsum board, and the studs run from the structural floor to the underside 
of the structure above, making them very strong and stiff. The impact of the 
panelized ceilings caused the lightweight support tees either to buckle or pull 
off of a typical perimeter trim angle (Figure 2-38). In the absence of vertical 
support wire, the tees collapsed, allowing the ceiling tiles to fall. Many of 
the florescent light fixtures, also supported on the tees, became dangerously 
unstable, though few fell to the floor. This type of damage was concentrated at 
the perimeter of rooms, adjacent to the partition walls, but it was not 
completely limited to these locations (see Figures 2-39). When ceiling tiles 
became dislodged, decades of accumulated dust on the top of the tiles came down, 
adding to the general disarray and threatening the health of patients and other 
occupants. 

[Begin figures]
Figure 2-37 is an illustration showing a typical suspended panelized ceiling 
system showing seismic bracing (according to the 1993 seismic evaluation report, 
this system was not used in the hospitals reviewed at that time

Figure 2-38 is a photo showing typical damage at the perimeter of a room from 
loss of support from the partition-mounted perimeter angles to the ceiling 
system tees or from contact with the ceiling system and the partitions.

Figure 2-39 is a photo showing ceiling damage in the operating room. The round 
operating lights on the right were not damaged. 
[end figures]

Almost simultaneously with the first shock, the power went out. The emergency 
generator was seismically protected and provided power for the emergency 
circuits. Eight minutes after the main shock, about the time some semblance of 
order was being restored, the second shock hit. With equipment and contents as 
well as ceiling tiles and metal tees littering the floor or hanging precariously 
over patients’ beds, a decision was made to evacuate the most disrupted areas.

At the time of the earthquake KCH had 69 patients, including 31 in the long-term 
care unit. These 31 long-term care patients were taken to a local hotel; 27 
patients were discharged; 5 were airlifted to an acute care facility in Hilo, 
and one was transported to a local long-term care facility. The remaining five 
patients were more difficult to move and were placed in usable rooms in the 
obstetrics or the intensive care unit (ICU) on site.

The potential torsional response of the building addition, identified in the 
1993 evaluation, proved prophetic, as the column-supported west end attempted to 
rotate around the stiffly supported east end. As shown in Figure 2-40, the two 
beam connections tending to restrain the motion spalled. Similarly, in the 
partially enclosed penthouse, the embedded connection of a steel beam spanning 
between two exterior concrete walls pulled out due to a stiffness 
incompatibility. This damage was not considered serious enough for the structure 
and did not affect the occupancy status.

[Begin figure]
Figure 2-40 is two photos showing spalling at beam connections. [end figure]

Exterior and interior nonstructural plaster or stucco walls had many cracks, 
especially around some door frames. This damage was costly, but did not affect 
operations.

Emergency generators and medical gas storage were seismically anchored and 
survived the tremor undamaged. The communications were disrupted for the first 
hour or so, because the telephone main switching equipment, which was not 
anchored, failed. The radio-repeater mast on the roof fell over and dislodged 
the coaxial cable, limiting the use of that system. The hospital has 
subsequently decided that their radio system does not have sufficient channels 
for emergency use and intends to upgrade the system. Ham radio was also 
available on the site but was not used. 

The hydraulic elevators, which are not as susceptible to damage as the traction 
elevators used in taller buildings, sustained no major damage but were not 
functional as a result of power outage. Elevators can be very valuable after 
earthquakes for moving bed-ridden patients, provided special seismic anchorage 
and controls are installed and emergency power is available. The pressurized 
water systems (including sprinklers, domestic, and chilled water), considered by 
many the most likely to break or leak and disrupt operations, suffered no 
damage.

The experience of KCH highlighted the vulnerability of unbraced suspended 
panelized ceilings, which are often given a low retrofit priority because they 
are not considered a serious life safety hazard. It was also thought that the 
benefits of bracing do not warrant the extreme disruption that such a retrofit 
usually causes for hospital operations. It is very likely that lessons from Kona 
Community Hospital’s experience will help change this view.

2.3.3 Consequences of Building Damage 

The uninterrupted operation of hospitals is crucial in the aftermath of an 
earthquake, because of a potentially large number of casualties. Damage to these 
facilities and a possible need for an evacuation not only hampers the emergency 
response but can also compound the disaster by adding casualties. The most 
obvious risk to life safety is serious structural failure of a hospital, similar 
to the experience of the original Olive View hospital. In the aftermath of an 
earthquake the local building authority typically determines which buildings are 
unsafe for use based on the level of damage. For this purpose, a simple 
“tagging” procedure has been developed in California that uses colored placards 
or “tags” affixed to buildings that show that the building has been inspected 
and indicate the level of safety. The evacuation is inevitable when the hospital 
buildings are “red tagged,” i.e. when the building is in imminent danger of 
collapse in an aftershock. In such circumstances, a hospital becomes unavailable 
for emergency services and the staff can only provide medical care in parking 
lots or other ancillary facilities, as has happened in the past.

[Begin text box]
TAGGING
A red tag indicates UNSAFE: Extreme hazard, may collapse. Imminent danger of 
collapse from an aftershock. Unsafe for occupancy or entry, except by 
authorities.
A yellow tag indicates LIMITED ENTRY: Dangerous condition believed to be 
present. Entry by owner permitted only for emergency purposes and only at own 
risk. No usage on a continuous basis. Entry by public not permitted. Possible 
major aftershock hazard.
A green tag indicates INSPECTED: No apparent hazard found, although repairs may 
be required. Original lateral load capacity not significantly decreased. No 
restriction on use or occupancy.
[End text box]

Even in cases where hospitals have avoided major damage, their operations may be 
sufficiently disrupted to require complete evacuation, which can be very 
dangerous for many patients. Failures in nonstructural systems, such as broken 
pipes that cause extensive flooding, failed emergency generators, lack of water, 
or general chaos created by contents that have been thrown about have all 
created conditions that have forced hospital administrators or local 
jurisdictions to order an evacuation. Minor nonstructural damage, especially if 
essential equipment or other contents are affected, can still cause considerable 
disruption in hospital operation, even if global evacuation is not deemed 
necessary.

Finally, almost all hospitals have financial constraints. The cost of repairs 
and/or the partial loss of capacity will affect the financial well-being of any 
facility and must be considered as a significant potential consequence of 
earthquake damage.

2.3.4 Seismic Vulnerability of Hospitals 

Seismic vulnerability of a hospital facility is a measure of the damage a 
building is likely to experience when subjected to ground shaking of a specified 
intensity. The response of a structure to ground shaking is very complex and 
depends on a number of interrelated parameters that are often very difficult, if 
not impossible, to predict precisely. These include: the exact character of the 
ground shaking the building will experience; the extent to which the structure 
will respond to the ground shaking; the strength of the materials in the 
building; the quality of construction, the condition of individual structural 
elements and of the whole structure; the interaction between structural and 
nonstructural elements; and the live load in the building at the time of the 
earthquake.

Frequently, seismic activity causes insignificant damage to the structure of a 
hospital, yet its operations are impaired or disrupted because of damage to 
nonstructural elements. Even a low-magnitude seismic event can affect vital 
functions of a hospital and cause its evacuation and closure. This was evident 
in some recent earthquakes, whereby structures designed in accordance with 
modern seismic resistance criteria performed well, while the poor performance of 
the nonstructural elements caused serious disruption of hospital operations. 

A variety of methods for assessing seismic vulnerability of buildings exist that 
differ in cost and precision. The type of method to be used depends on the 
objective of the assessment and the availability of data and appropriate 
technology. Typically, quicker and less sophisticated methods, like the commonly 
used rapid visual screening, are used for larger areas and large number of 
buildings. They often form the first phase of a multiphase procedure for 
identifying hazardous buildings, which must then be analyzed in more detail to 
determine upgrading strategies. Detailed assessment procedures use computer 
models and various forms of engineering analysis that are time consuming and 
expensive and require a high degree of analytical expertise to obtain reliable 
results. Consequently, they are used for detailed verification of the safety of 
structural and nonstructural elements, including proposals for specific 
mitigation measures.

A simple preliminary vulnerability assessment of existing hospitals can be 
performed using the results of the historical study of hospital performance in a 
variety of seismic events. This method is described below in greater detail.

2.3.4.1 Seismic Vulnerability of Hospitals Based on Historical Performance in 
California

A recently completed study on “Seismic Vulnerability of Hospitals Based on 
Historical Performance in California,” (Holmes and Burkett, 2006) analyzed the 
historical record of losses to hospitals damaged in major California earthquakes 
since 1971. The data base contained two hundred eighteen cases, each 
representing a hospital (potentially one or more buildings) that experienced 
earthquake ground shaking in the earthquakes of San Fernando (1971), Imperial 
Valley (1979), Coalinga (1983), Whittier (1987), Loma Prieta (1989), Sierra 
Madre (1991), and Northridge (1994). Damage reports varied from brief, one-
paragraph summaries to elaborate narratives of the damage patterns and the 
consequences. Evacuations or shut-downs of facilities were always noted. These 
descriptions were used to categorize hospital damage into one of the structural 
and nonstructural performance categories shown in Table 2-2.

[Begin table]
Table 2-2: The description of performance categories in terms of structural and 
nonstructural building damage

Performance category (damage level): None

Performance category (damage level): Minor
Type of Structural Damage: Minor structural damage (light concrete cracking, 
etc.)
Type of Nonstructural damage: Minor damage to nonstructural components or 
systems (plaster cracking, ceiling damage, some equipment shaken off supports)

Performance category (damage level): Affecting hospital operations
Type of Structural Damage: Damage requiring immediate evacuation due to 
dangerous conditions or concern for collapse in an aftershock
Type of Nonstructural damage: Nonstructural damage that prevents full 
functioning of the hospital (loss of emergency generator, local pipe breaks and 
causes flooding, computer system down)

Performance category (damage level): At least temporary closure
Type of Structural Damage: Closure could be temporary or permanent
Type of Nonstructural damage: Temporary Closure based on major nonstructural 
damage (long-term power or water outage, extensive ceiling and light fixture 
damage, major flooding)
[End table]

The study related the recorded levels of damage experienced by hospitals and 
ground motion data for each seismic event, determined on the basis of recorded 
ground motion at the site during the earthquake and USGS data. The intensity of 
ground motion was represented by spectral acceleration at short periods (SDS), 
measured in units of the acceleration of gravity “g” in order to match building 
code information that can be obtained locally (see Section 2.2.2.1). In this 
study, the spectral acceleration at short periods for each case in the 
historical record was labeled SHS, which stands for historical short period 
spectral acceleration. By matching spectral acceleration data with performance 
categories, a relationship was established between the damage (and the 
consequences of damage) and the ground motion intensity that caused it. All 
buildings were divided into two groups according to the level and quality of 
seismic design and construction. The adoption of the HSSA in 1972 was used as a 
divide between pre-Act buildings and post-Act buildings. The results of this 
study presented the differences in performance of hospital buildings in 
California in graphic form on Figures 2-41 for pre-Act buildings and in Figure 
2-42 for post-Act buildings. 

The measure of ground motion intensity is different in the two figures. For pre-
Act buildings, SHS represents accelerations recorded at the time, while post-Act 
building performance was categorized according to ground motion expressed as the 
percentage of SDS typically used in California for seismic design in the post-
Act period. Post-Act buildings in the database were designed for the higher 
seismic zones of California, typically with an SDS of about 1.0. To use this 
data to estimate potential damage to newer buildings in other parts of the 
country that have incorporated thorough seismic design with other values of SDS, 
it is necessary to normalize the data to an SDS of 1.0. The ground motion in 
Figure 2-42 is thus represented as a percentage of the SDS used to design the 
building. “SHS = 0.25–0.50 of SDS” shows expected damage to a relatively new 
building when it is shaken with an intensity of 25 to 50 percent of its code 
design, as measured by SDS. Since damage occurs even at levels considerably 
below the code earthquake, the data is still very useful for planning purposes. 
The probability of occurrence of shaking different from the code level can be 
estimated by local seismologists or engineers.

This study, based on past damages, provided a clearer picture of the 
vulnerability of hospitals and established a benchmark for vulnerability 
assessments of all the existing hospitals. Among other things, the analysis 
indicated that the threshold for potentially significant seismic damage 
coincided with the current, lower-bound ground motion intensity requiring 
seismic design for new buildings (SDS greater than 0.167 g). The pre-Act charts 
indicated a slight possibility of significant structural damage, but a strong 
possibility of nonstructural damage at low shaking levels. It should be noted 
however, that structural damage requiring building closure has been recorded 
even at the SDS level just above 0.4 g. The analysis of the post-Act data 
indicated that hospital buildings built in accordance with the 2000 IBC (or 
later edition) are unlikely to suffer serious structural damage for events up 
to, and including, the intensity of the code earthquake. However, as previously 
discussed, new hospital buildings may not perform as well as indicated on the 
post-Act charts, since most regions of the country do not have as strict design 
and construction codes and code enforcement as California. Nonstructural damage, 
which can reduce hospital effectiveness or even cause evacuation, remains a 
significant vulnerability, even in new buildings.

2.3.4.2 Vulnerability Assessment of Hospital Buildings

Although the above-mentioned study was based on data from California, where the 
ground motions have slightly different characteristics than in other parts of 
the country, the vulnerability to damage for certain building types at given 
levels of ground motion is comparable to any location. Prior to obtaining the 
detailed site-specific seismic evaluations, the expected damage from a given 
ground motion can be estimated on the basis of historical data. The results of 
the study, therefore, can be used effectively to make preliminary assessments of 
the vulnerability of hospitals in any seismic region of the country. While this 
type of analysis will not take the place of a formal seismic building evaluation 
performed by experienced design professionals, it can be very helpful in raising 
the awareness of potential earthquake risks, in determining whether a more 
detailed study of vulnerability is justified, and whether to incorporate more 
realistic damage projections in a disaster plan or emergency exercise.

For example, a hypothetical hospital near Charleston, SC, may determine from the 
local building code that the seismic spectral acceleration value for short 
periods at the site is 0.8 g. If one or more of their buildings are 20 or more 
years old and were designed without seismic provisions or with out-of-date 
seismic provisions, the pre-Act columns of Figure 2-41 should be consulted. The 
chart on Figure 2-41 for the range of SDS between 0.6-0.8 g shows that, unless 
building-specific studies are done to prove otherwise, it should be assumed that 
a code earthquake will cause sufficient damage to hinder the full operations 
(about a 33 percent chance) or require closure of a building (about a 13 percent 
chance). Perhaps more importantly, the right column of Figure 2-41 indicates a 
high probability that hospital operations will be interrupted by nonstructural 
damage. In fact, for such a facility, even very low shaking levels of SDS 0.2 g 
to 0.4 g could be expected to cause nonstructural damage resulting in 
significant disruption of hospital operations.

In contrast, a brand new hospital in northern Utah, on a site with SDS of about 
0.7 g, is unlikely to suffer significant structural damage (as shown on the 
post-Act charts, left column of Figure 2-42). However, if such a hospital was 
not constructed according to the best design and construction standards, it can 
be expected to suffer a more significant nonstructural damage than the post-Act 
data shown in Figure 2-42. A prudent disaster plan would in such a case consider 
the possibility that nonstructural damage could cause a temporary closure.

2.3.4.3 Comparability of Hospital Buildings

The structural performance of hospital buildings built anywhere in the country 
without any seismic design provisions is most likely comparable to the 
performance of pre-Act buildings as shown on Figure 2-41. All buildings in the 
pre-Act group were either constructed with no seismic design, or with the 
archaic seismic design rules of the 1960s or earlier. Most of these buildings 
were constructed with concrete, steel, reinforced masonry, or wood construction. 
Unreinforced masonry buildings are rare in California because most of these 
buildings have either been abandoned or retrofitted. However, it should be 
assumed that older buildings of unreinforced masonry bearing wall construction, 
that are still common in other parts of the country, would perform at the low 
end of the ranges recorded for pre-Act buildings. 

Buildings with seismic designs completed in the 1970s and 1980s can be expected 
to perform somewhere between pre- and post-Act levels, but probably closer to 
the pre-Act data. Buildings classified as post-Act are exceptionally strong, 
because the designs were thoroughly checked and the construction monitored in 
detail. Due to the State-of-California-mandated scrutiny given design and 
construction, there are few hospital buildings outside of that area that would 
be equivalent to the post-Act category. However, the structural performance of 
hospitals built since the early 1990s that incorporated full seismic design, 
including an importance factor of 1.5, can be compared to the post-Act category. 

The vulnerability assessment of existing hospitals with respect to nonstructural 
building components is different from the assessment of potential structural 
damage. Unless a significant emphasis was placed on the design and installation 
of nonstructural anchorage and bracing, and unless the construction was 
monitored and inspected regularly, the potential for damage to ceilings, 
partitions, pipes, ducts, equipment, and other nonstructural systems should be 
estimated using the pre-Act column and charts. Since it is unlikely that 
nonstructural systems have been adequately protected in areas outside of 
California, it is recommended to assume that substantial damage to these systems 
would be a certainty in moderate to high levels of shaking. Even with the 
extraordinary measures taken in California to assure thorough seismic protection 
of nonstructural systems, the level of nonstructural damage recorded indicates 
that the potential disruption of hospital operations can happen at shaking of 
about 50 percent SDS. The shaking above 80 percent SDS would most likely result 
in nonstructural damage that would require immediate closure and total 
evacuation of the hospital.

This type of vulnerability analysis should be used for preliminary assessments 
or for the purpose of awareness training or emergency planning. It cannot 
replace a formal seismic building evaluation performed by experienced design 
professionals. The type and the extent of seismic risk exposure for hospitals 
can only be identified by expert analysis.

[Begin figures] 
Figure 2-41: Charts showing performance categories for pre-Act buildings for 
various ground motions 

Figure 2-42: Charts showing performance categories for post-Act buildings for 
various ground motions
[end figure]



2.4 RISK REDUCTION MEASURES

This section outlines some of the basic approaches and techniques of mitigating 
earthquake hazards. Since the theory and practice of seismic design are well 
established and have largely been incorporated into the current model building 
codes, this section starts with the review of basic steps in the process of 
planning and design of hospitals. It also highlights some of the well-
established seismic design and construction techniques and specialized building 
systems designed specifically to address the seismic forces. Although the 
general principles of design are similar for new and existing hospitals, this 
section highlights the differences in code requirements and overall project 
delivery processes that reflect the opportunities and constraints of seismic 
design for new and existing buildings. 

2.4.1 Site Selection Basics

The first priority for owners of existing hospitals and planners and designers 
of new facilities is to understand the seismic hazard risk. The location and the 
physical characteristics of the site determine the extent of seismic hazards, 
which include the potential intensity of ground shaking, the possibility of 
liquefaction, earthquake induced landslide, or more rarely, the potential of a 
tsunami or seiche. If ground motion is the only or predominant hazard, site 
seismicity can be determined from the local building code by a local structural 
engineer or a building department official. More detailed information can be 
obtained from the USGS Web site, which provides seismological information for 
any zip code in the nation. When hazards in addition to ground motion exist, 
seismic experts are needed to determine the probability and extent of these 
risks and possible mitigation techniques.

The selection of hospital sites is generally based more on factors associated 
with availability of land, proximity to service area, cost, convenience of 
access for patients, visitors, and staff, or general demographic concerns than 
on the exposure of these sites to natural hazards, particularly earthquakes. 
Careful site selection, however, is a critical first step in risk reduction, 
because the potential ground motion from a single earthquake may vary 
considerably depending on the nature of the soil and the distance of the site 
from known earthquake faults. A large medical center that is developing a plan 
for new multi-building facility should include comprehensive analysis of the 
site characteristics and its exposure to natural hazard as an important factor 
in evaluating alternative sites. 

- Consider seismic constraints in site selection. Although it would be very rare 
for a hospital district to make a site selection decision based solely on 
seismic risk, moving a hospital even a few miles in some cases can make a big 
difference to its exposure to seismic hazard. An example is locating a hospital 
5 to 10 miles away from a major earthquake fault, rather than locating it within 
1 mile of the fault.

- Locate the building on a soil type that reduces the risk. Local soil profiles 
can be highly variable, especially near water, on sloped surfaces, or close to 
faults. In an extreme case, siting on poor soils can lead to damage from 
liquefaction, land sliding, or lateral spreading of the soil. 

- Since hospitals should be designed to performance-based criteria that include 
minimum disruption and continued operation, the location of the site within the 
region may play a critical role in achieving the required performance. The 
definition of “site” becomes the region within which the facility is located, 
and assessments should be made with respect to its access to materials, 
personnel, and utilities, as well as its position in the regional transportation 
grid. 

In most cases, however, it is probable that a site with optimal characteristics 
(other than seismic considerations) will be selected and that seismic issues 
will be mitigated as part of planning and design of the facility. A proposed 
construction site located directly over a fault is probably the only location 
characteristic that would lead to rejection of an otherwise suitable site. 

2.4.2 Seismic Design Basics

Minimum standards and criteria for seismic design are defined in the seismic 
section of building codes. The codes provide maps that show whether the location 
is subject to earthquakes and, if so, the probability of occurrence, expressed 
by varying levels of seismic forces for which a building must be designed. The 
seismic provisions in building codes are adopted by State or local authorities, 
so it is possible for a seismically-prone region to be exempt from seismic 
building code regulations if the local community decides that the adoption of 
such provisions is not required. Hospital board officials and designers should 
not ignore seismic design requirements irrespective of whether local communities 
have adopted seismic code regulations or not.

Budgeting extra costs for seismic design is a difficult issue, because although 
the risk may appear to be minimal, the effects could be catastrophic if a 
significant event were to occur. The very fact that such an event is rare means 
that the community may have no history of designing for earthquakes, and the 
building stock in general would be especially vulnerable. Hospital buildings are 
an important community resource (along with other essential buildings, such as 
schools and fire and police stations) that must be protected as much as 
possible.

2.4.3 Structural Systems

Health care facilities occupy buildings of different sizes, configurations, and 
structural systems. Additionally, hospitals differ in the level of services they 
provide. Some hospitals are distinguished by high occupancies, while others 
emphasize outpatient services. The mixture of functions and services is such 
that hospitals frequently require building systems that can accommodate very 
diverse functions. These can vary from acute care with many diagnostic, 
laboratory, and treatment areas requiring high-tech facilities and services to 
support functions such as laundry, food service, receiving, storage, and 
distribution. 

Smaller healthcare facilities may encompass one or more of these functions, such 
as predominantly longer residential care, or specialized treatment such as 
physical rehabilitation or dialysis. This functional variety may influence some 
structural choices, but the structure, as in all buildings, plays a primary role 
in providing a safe and secure support for the facility activities. Since 
continued operation is the preferred performance objective, structural design 
that goes beyond life safety standards is necessary, which requires special 
attention.

2.4.3.1 Basic Types of Lateral Force Resisting Systems

Figure 2-43 shows the basic types of structural lateral force resisting systems. 
These systems compose the three basic alternatives for providing lateral 
resistance: shear walls, braced frames, and moment-resistant frames. Each of 
these has specific characteristics, such as stiffness, relationship to spatial 
requirements, and cost-effectiveness that must be evaluated for each project. 
Diaphragms connect horizontal and vertical elements and transfer the seismic 
loads to the lateral force resisting system. This concept is shown in Figure 2-
43. Structural material alternatives—steel, reinforced concrete, reinforced 
concrete masonry, and wood, provide further options that must be evaluated. 
Figures 2-44, 2-45, and 2-46 show the three systems and materials that can be 
used to resist seismic forces in more detail. 

[Begin figures]
Figure 2-43 is an illustration showing basic types of lateral force resisting 
systems. Source: BSSC: Presentations to the Architectural Community, 2001, Chris 
Arnold and Tony Alexander

Figure 2-44 is an illustration showing a reinforced concrete shear wall 
structure

Figure 2-45 is an illustration showing a steel braced frame structure

Figure 2-46 is an illustration showing a steel moment resistant frame
[End figures]

Many variations of these types are possible, together with other structural 
types such as wood frame, light steel, reinforced masonry, and tilt-up 
reinforced concrete. The seismic codes expand these three basic systems into six 
categories by the addition of two categories of dual systems (composite steel 
and reinforced concrete) and inverted pendulum systems (such as cantilevered 
water tanks). 

For each of the three basic systems, four coefficients and factors are provided, 
of which the most unique is the response modification, or “R” factor. This is a 
coefficient related to the overall behavior and ductility of the structural 
system. R has values from 1.25 to 8, and is a divisor that modifies the base 
shear value obtained in the ELF procedure by reducing the design forces. The 
higher the number assigned to R, the greater the reduction. Thus, the highly 
ductile and better-understood moment resisting frame has a value of 8, while an 
unreinforced masonry structure has a value of 1.5. Using the R reduction is 
justified because experience and research confirm the expected performance of 
these systems and their ability to accept overloads.

2.4.3.2 Innovative Structural Systems

In recent years, a number of new approaches to the seismic protection of 
buildings have been developed that are now seeing increased use. These systems 
depend on modifying either the seismic loads that are transmitted from the 
ground to the building, or the building response. In both instances, the 
strategy is to let the building “ride with the punch,” rather than relying on 
resistance alone to protect the building.

Seismic isolation, generally referred to as base isolation, is a design concept 
that reduces the earthquake motions in the building superstructure by isolating 
the building from ground motions. This is accomplished by supporting the 
building on bearings that greatly reduce the transmission of ground motion (see 
Figure 2-47). Both the structure and the nonstructural systems are subjected to 
reduced shaking levels, so the system is well suited for essential facilities, 
like hospitals that need to remain functional following an earthquake. Many 
emergency management centers and a few hospitals in the United States have 
employed this technique. It has also been used by private industry on buildings 
of high importance, and as a retrofit technique, mostly for significant historic 
buildings.

[Begin figure]
Figure 2-24 is an illustration showing seismic isolation. [End figure]

Passive energy dissipation is another concept in which the earthquake forces in 
the building are reduced by the introduction of devices that are designed to 
dissipate the earthquake energy in a controlled manner using friction, 
hydraulics, or deformation of material specially placed for this purpose (see 
Figure 2-52). These devices usually take the form of a bracing system that 
connects the vertical structural members together. These devices also increase 
the damping of the structure and reduce the drift. The Bremerton Naval Hospital 
provides an example of successful rehabilitation that used these devices (see 
Section 2.4.6.2).

2.4.3.3 Structural Systems Selection

The seismic code prescribes the analysis procedures for a number of different 
types of structural systems. The engineer, however, must choose the system that 
is appropriate for the type of building and its use. Structural systems vary 
greatly in their performance attributes, even though they may all meet the 
requirements of the seismic code if correctly designed and constructed. 

The critical initial step in selecting a structural system is to establish 
performance goals for the project. Because hospitals are classified as essential 
buildings, the seismic code requirements imply a requirement for a certain level 
of performance. An informed performance-based design procedure is necessary. 
This procedure should involve the owner, the full design team, and any other 
stakeholders. The process should cover all steps, from determining performance 
goals to the detailed design and construction of the project. 

On the technical side, the most important measure of good earthquake-resistant 
design is the effect on the structure after the earth has stopped shaking. With 
little building damage, there is a good chance that the building will be 
functional and repair costs will be low. With significant damage, it is unlikely 
that the building would be functional and repair or replacement costs would be 
high. The measure of success in seismic design of a hospital is the extent to 
which a building can avoid significant damage and retain a reasonable level of 
functionality without the need to evacuate patients. The behavior of each 
structural system and building configuration differs with earthquake ground 
motions, soil types, duration of strong shaking, etc., but past observations of 
damage can inform the decisions about the most appropriate systems.

The best performing structural systems will do the following:

- Possess stable cyclic behavior. Will not be prone to sudden structural failure 
or collapse

- Control lateral drift. Will keep drift (lateral distortion between floors) to 
reasonable dimensions to reduce damage to nonstructural components, such as 
glazing, partitions, and cladding

- Dissipate seismic energy without failing. Absorb the earthquake energy in a 
controlled manner without causing structural members to fail 

- Create a good chance of functionality and a low post-earthquake repair cost

2.4.4 Nonstructural Components and Systems

For a long time, seismic building codes focused exclusively on the structural 
components of building. Although this focus still remains dominant, experience 
in recent earthquakes has shown that damage to nonstructural components and 
systems is also of great concern. Continued hospital operation is increasingly 
dependent on nonstructural components and systems, including medical and 
building equipment. Hospital operations also depend on specialized services, 
some of which involve storing of hazardous substances, such as pharmaceuticals, 
toxic chemicals, oxygen, and other gases, that must be protected against 
spilling. Distribution systems for hazardous gases must be well supported and 
braced. Unlike most buildings, hospitals require a very extensive plumbing 
network to supply water throughout the building, and an adequate piping network 
to supply water for fire sprinklers, which increases the risk of secondary water 
damage in case of failure of these systems during earthquakes.

In a typical hospital, not only do the nonstructural components play a major 
role in operations, they also account for large share of the cost. Typically, 
for a medium-size hospital, the structure accounts for around 15 percent of the 
total cost, and the nonstructural components for the remaining 85 percent. Of 
the latter, the mechanical, electrical, and plumbing systems alone account for 
approximately 35 percent of the total building cost.

Even though the building structure may be relatively undamaged after an 
earthquake, excessive structural motion and drift may cause damage to ceilings, 
partitions, light fixtures, service piping, and exterior walls and glazing. In 
addition, storage units, medical equipment, and filing cabinets may topple and 
cause injuries if not properly anchored or braced. Excessive drift and motion 
may also lead to damage to rooftop equipment, and localized damage to water 
systems and fire suppression piping and sprinklers. Heavy equipment, such as 
shop machinery, kilns, and heavy mechanical and electrical equipment, may also 
be displaced and become non-functional. 

2.4.4.1 Code Regulated Nonstructural Systems

The seismic code categorizes nonstructural components as architectural 
components or mechanical and electrical components. Many of the hospital 
contents, such as furnishings and specialized equipment, which may be critical 
to hospital function, are not subject to regulation.

Table 2-3 shows the list of nonstructural components and systems that are 
subject to code regulation. 

[Begin table]
Table 2-3: Code Regulated Architectural, Mechanical, and Electrical Components 

Architectural Components: 
Interior Nonstructural Partitions 
 Plain unreinforced masonry 
 Other walls and partitions 
Cantilever Elements, Unbraced
 Parapets 
 Chimneys and stacks 
Cantilever Elements, Braced 
 Parapets 
 Chimneys and stacks
Exterior Nonstructural Wall Elements 
 Wall elements 
 Body of panel connections 
 Fasteners of connecting systems 
Veneer 
 Limited deformability 
 Low deformability 
Penthouses, Not Part of Main Structure 
Ceilings 
Cabinets 
 Storage cabinets and lab equipment 
Access Floors
 Special access floors 
 All others 
Appendages and Ornamentation
Signs and Billboards
Other Rigid Components
 High deformability elements
 Limited deformability elements
 Low deformability elements
Other Flexible Components
 High deformability elements
 Limited deformability elements
 Low deformability elements

Mechanical and Electrical Components: 
General Mechanical
 Boilers and furnaces
 Pressure vessels
 Stacks
 Cantilevered chimneys
 Other
Manufacturing and Process Machinery
 General
 Conveyors (non-personnel)
Piping Systems 
 High deformability
 Limited deformability
 Low deformability
HVAC System Equipment
 Vibration isolated
 Non-vibration isolated
 Mounted in-line with ductwork
 Other
Elevator Components 
Escalator Components
Trussed Towers (Free-standing or Guyed)
 Special access floors
All others
Lighting Fixtures
[End table]

Figure 2-48 is an example of a cable tray with a braced support system designed 
in conformance with the seismic code.

[Begin figure]
Figure 2-48 is an illustration showing seismically braced cable tray support 
[end figure]

2.4.4.2 Interstitial Space for Utility Installations

Developments in medical technology frequently require hospitals to add or re-
route utility and medical services infrastructure and add electrical capacity to 
various parts of the building, often with consequent disruption of the 
operations and difficulties caused by limited available space above the ceiling. 
In response to these problems, a number of hospitals have been designed with 
interstitial service space. In some designs, the interstitial floor is a 
nonstructural floor hung from the structural floor above (Figure 2-49).

[Begin figure] 
Figure 2-29 is an illustration showing an interstitial floor system hung from 
the floor above. [End figure]

The interstitial space concept provided a full floor above the functional space 
where service personnel can work to add or modify installations without 
disturbing the space below, or disrupting hospital operations. The main 
structural effect of an interstitial floor is to increase the floor-to-floor 
height of the occupied floors and the building as a whole, with some increase in 
the amount of structural material. Any of the common steel and concrete frame 
systems can be used to provide the interstitial floor arrangement. The 
interstitial space concept was used in the Loma Linda Veterans Hospital (see 
Section 2.4.5.1)

2.4.5 Mitigation Measures for New Buildings

If mitigation measures are instituted in the planning stage, a high level of 
seismic protection can be achieved in a new building with the following steps:

- Assure that the latest model building code is used, using the appropriate 
importance factor.

- Include provisions in the design to allow the facility to function on its own 
for 4 days. This would require storage of sufficient fuel for emergency standby 
power supply system, reserve water supply, and provisions for waste water 
storage. Assure that access to the site will not be impaired by earthquake 
damage.

- Although seismic isolation should be considered for all buildings that are 
intended to be functional after an earthquake, this measure should be considered 
as a realistic alternative only for sites with SDS over 0.8 g.

- Consider a peer review of the structural design, particularly in jurisdictions 
that do not require a thorough plan check to obtain a building permit. A peer 
review is essential if innovative structural systems, such as isolation or 
structural dampers, are used.

- Assure that the seismic design for nonstructural elements and systems 
regulated by the building code are the responsibility of the design team. A list 
of such responsibilities is given in FEMA 356. One method of gaining such 
assurance is to create the position of a Nonstructural Seismic Coordinator 
(NSC), whose responsibility is to review the specifications, drawings, or other 
methods proposed to attain adequate seismic anchoring and bracing of all 
systems. Further, the NSC will follow the submittals and construction processes 
to assure appropriate implementation.

- Put in place a comprehensive system to monitor construction quality and to 
track significant change orders that might imply subtle reductions in structural 
or nonstructural seismic performance.

2.4.5.1 The Case of Loma Linda Veterans Hospital

The 500-bed Veterans Administration Hospital in Loma Linda, CA, opened in 
September 1977. Designed as a replacement facility for the Veterans 
Administration Hospital lost in the 1971 San Fernando earthquake, it was built 
in the area of extremely high seismicity, and represents an interesting case of 
careful seismic design.

The project was remarkable because of the extent to which the configuration of a 
large and complex building was influenced by seismic design concerns. At the 
same time, the project provides a lesson in showing that early recognition of 
seismic design determinants by the whole design team, and a serious 
interdisciplinary approach from the inception of design, can enable requirements 
of both seismic design and hospital planning and economy to be achieved with 
equal effectiveness.

The San Bernardino Valley is seismically very active, and the final site 
selected for the new hospital had 11 known active faults within a 65-mile 
radius, including the San Jacinto fault and two segments of the San Andreas 
fault. The potentially active Loma Linda fault was also believed to be located 
near the site, and after intensive studies, it was concluded that the most 
likely location of the fault was 200 to 400 feet southwest of the site, but that 
surface rupture at the site was not likely. The risk of soil amplification was 
significant.

The consultants recommended that design earthquakes of magnitude 8+ and duration 
of 35–40 seconds on the San Andreas fault, and magnitude 6.5–7.25 and duration 
of 20 seconds on the San Jacinto fault, should be consid­ered. The building 
structure was to be designed for a peak acceleration of 0.5 g, while the 
essential and potentially damaging nonstructural components were to be designed 
for an acceleration of 2.0 g. 

Some of the design force determinants evaluated were dependent on a building 
configuration concept. 

1. Site geometry: The large 40-acre site enabled the designers to consider a 
freestanding building unconstrained by site geometry. The site area was 
sufficiently large to allow consideration of a relatively low, horizontally 
planned building.

2. Design Program: Research studies on hospital organization and plan­ning had 
established some general benefits of horizontal planning—defined as plans in 
which clinical and diagnostic areas are placed on the same floor as nursing 
areas, rather than being concentrated into a base structure with a vertical 
connection to the bed-related functions. Experience in vertically planned 
hospitals had indicated some problems in ensuring adequate circulation, since 
the concentration of vertical circulation into a single tower tended to result 
in over- or under-capacity, depending on the time of the day. There were also 
indications of a general preference by staff for horizontal movement over 
vertical, and an indication that a reduction of vertical circulation for 
severely ill patients would be desirable.

3. Aesthetics: The design of hospitals tends to be dominated by the solu­tion of 
very complex planning, service, and equipment problems, and appearance tends to 
be a secondary concern. The city of Loma Linda was anxious that the setting of 
the hospital should be “park-like.” In response to this desire, and to the 
generally small scale of the immediate site surroundings, the image of a low, 
nonassertive building, placed toward the center of the site, seemed appropriate. 
Although very large in size, the building’s relatively low height and the 
considerable size of the site was to help reduce the visual effect of the 
building on the neighboring community

4. Building system: The Loma Linda Hospital was intended as a demonstration of 
the Veterans Administration Hospital Building System, which had been developed 
over a period of several years by the same team that was respon­sible for the 
hospital design. The building system consisted of a carefully conceived set of 
design concepts intended to rationalize and organize the preliminary hospital 
design.

The structural design comprised moderate-span simple post and beam shallow floor 
framing system, large floor-to-floor heights, and lateral force resistance 
elements concentrated in the service tower at the end of each of the service 
modules. The planning and aesthetic requirements of a low, deep plan building 
coincided well with a stiff seismic design that would minimize story drift; 
consequent architectural, mechanical, electrical, and contents damage; and loss 
of operational capability. In addition, the low, stiff building would have a 
shorter period and possibly a lower response than the projected response spectra 
peaks of 0.3 and 0.8 seconds from the two nearby faults. The only way of moving 
the building response well away from the ground response would be to develop a 
flexible high-rise structure that would be undesirable from all the other 
viewpoints considered.

The chosen configuration was the simplest of all those studied: a simple block, 
almost square in plan, with no basement and with a symmetrical pattern of four 
courtyards within the block. The courtyards were relatively small. The plan had 
an even distribution of shear walls throughout, running uninterrupted from roof 
to foundation and having direct continuity in plan with the structural framing 
members.

The planning and circulation of the building were carefully related to shear 
wall layout to achieve minimum shear wall penetration with clearly defined, 
highly accessible public and departmental planning. The eight service towers 
(four at each end) provided a location for major shear walls. Each tower 
pro­vided two shear walls in the east-west direction and one in the north-south 
direction. 

The general lateral resisting system used concrete shear walls and a ductile 
moment-resistant “backup” frame. The stiff primary shear wall system was 
designed for a high force level, so that the structure will tend to have low 
lateral deflections for the design earthquakes described earlier. The calculated 
maximum story-to-story drift was well within presently accepted desirable ranges 
for hospitals.

The backup frame was intended to form a stable and reliable backup system for 
lateral force resistance and redistribution of forces should one or more of the 
shear walls become seriously damaged. The chances of the backup frame being 
forced to work to its full capacity were considered small, but considering the 
size and importance of the facility, and the uncertainties of estimating the 
nature of ground motion, the possibility could not be ignored.

Shear walls were always placed at the perimeter of service modules to mini­mize 
interference. Interior girders were dropped below the beams to minimize 
interference with plumbing service and to allow beam continuity across the 
module. As a result of the service organization, all beams and girders are free 
of penetrations. These framing characteristics were the product of the research 
study that determined the system’s design.

The shear walls are collocated with frame lines. The advantages of this 
arrangement were:

- Beams or girders were always parallel and lined up with the walls, to serve as 
lateral force collectors.

- The continuation of these members through the wall allowed direct transfer of 
forces from the diaphragm to the wall.

- The columns at the end of walls formed the required ductile flange members.

- Frame members were in the correct position to provide vertical support for 
shear wall dead loads.

Complete calculations and designs were completed for all nonstructural 
components and systems. The bracing of utility distribution systems, ceilings, 
partitions, and lights was made easier and less expensive by the presence of the 
interstitial floor system shown in Figure 2-49.

2.4.6 Mitigation Measures for Existing Buildings

Engineering of structural and nonstructural risk reduction measures is similar 
for new and existing hospitals. New hospital design offers the possibility to 
minimize the risk by selecting a site subject to less ground motion, with better 
soil conditions, or located farther from a fault. It can be designed with the 
most appropriate structural system, using known and tested materials and a good 
building configuration. These possibilities are not available when retrofitting 
an existing hospital. The existing building may have been designed to an 
obsolete seismic code or no code at all, its materials may be questionable, or 
the building configuration and structural system may be inappropriate. 
Therefore, protecting an existing hospital must start with a detailed evaluation 
of its vulnerability, because seismic retrofitting is both disruptive and 
expensive, and should not be implemented without careful study.

2.4.6.1 Procedures and Design Strategies for Rehabilitation of Structural 
Systems

Additions to an existing hospital must meet all the code requirements for a new 
building. With the exception of California, there are currently no seismic codes 
that apply to the retrofit of existing hospitals. Typically, the standards to be 
applied are derived from the code for new buildings and negotiated with the 
applicable building department. It is generally recognized that it is difficult, 
or almost impossible, to bring an existing structure to full compliance with a 
current code, and so some compromises have to be made. There is, however, no 
general agreement as to how the code for new buildings should be applied to the 
retrofit design of existing ones.

FEMA has developed many such documents and several have been adopted by ASCE as 
standards suitable for adoption by building codes (see ASCE 31 and ASCE 41). The 
planning process for retrofits should begin with an evaluation procedure, such 
as Tier 1 of the ASCE 31 Seismic Evaluation of Existing Buildings (ASCE, 2003). 
If the evaluation results require a retrofit of an existing building, ASCE 41, 
Seismic Rehabilitation of Buildings (ASCE, 2007) is the authoritative source 
document and can be used to help a hospital design team select seismic 
protection criteria. The architect and engineer can also use the document for 
the design and analysis of the seismic rehabilitation project. 

ASCE 41 provides methods and design criteria for several different levels of 
seismic performance. The document also recommends a thorough and systematic 
procedure for performance-based seismic design, intended to produce a design 
responsive to the owner’s level of acceptable risk and available resources. This 
process starts with a requirement to select specific performance goals 
(Rehabilitation Objectives) as a basis for design. In this way, users can 
directly determine the effect of different performance goals on the design 
requirements, including their complexity and cost. See Section 2.2.3 for a 
further description of performance-based seismic design.

Typical basic design concepts for improving the structural seismic performance 
of an existing hospital include:

- Modifying and improving local components or materials, such as beam/column 
connections. This involves retrofitting connections and strengthening structural 
members by such methods as adding, reinforcing, or replacing them with new 
components.

- Adding new lateral force resisting elements, such as shear walls or braced 
frames. 

- Removing or reducing configuration irregularities. This involves providing 
seismic separations in irregular configurations, or adding shear walls or 
bracing to reduce torsional effects. Mass can also be removed by removing 
stories.

- Modifying the basic seismic response of a structure by adding dampers or 
installing seismic isolation systems.

2.4.6.2 The Case of Naval Hospital Bremerton

Naval Hospital Bremerton, located in Bremerton, WA, not only serves military 
personnel and their families in the area—up to 60,000 people—but could also be 
called on to serve more than 250,000 people on Washington’s Kitsap Peninsula 
after a major earthquake. The facility is spread over 40 acres, and includes 20 
buildings, some of which are 70 years old. Being one of only two major hospitals 
in the region, the Navy was concerned about its response capability to moderate-
to-strong seismic ground shaking expected at the site (Wilson, 2005).

To obtain a broad understanding of the vulnerability of the facility and the 
relative risk among the many buildings, a rapid visual screening was completed 
by the Navy’s consultant, Reid Middleton, using FEMA 154 (FEMA, 2002), which 
considers structural type, basic seismic characteristics, building use, and 
occupancy load. This relatively modest effort provided an overview of the 
seismic risk at the campus, as well as a preliminary relative ranking among 
buildings. It made it obvious that the main hospital building, a nine-story, 
250,000-square-foot building that housed most of the essential medical 
functions, should be a priority (see Figure 2-50).

[Begin figure]
Figure 2-50 is a photo of the main building at Naval Hospital Bremerton. Source: 
Reid Middleton, Inc. [End figure]

The building is a steel moment frame, constructed in the 1960s, that employed 
welded beam-to-column connections, now known to be prone to cracking when 
subjected to large seismic deformations. A more detailed structural analysis and 
evaluation was performed using FEMA 356 (FEMA, 2000). It was found that although 
the frame does not pose a significant threat of collapse, the building’s ability 
to function after an earthquake could be severely limited by damage to 
structural joints, and by nonstructural damage. 

In February 2001, while the Navy and the design team were considering their 
options, the Magnitude 6.8 Nisqually earthquake struck the area. The earthquake 
occurred deep below the surface and approximately 30 miles from the site. The 
ground motions at the site were below the design shaking for the site, but 
nevertheless caused significant nonstructural damage — particularly in the upper 
floors (see Figure 2-51). Such nonstructural damage always causes concern about 
the integrity of the structure—and often requires destructive exploration to 
verify. Based on preliminary structural inspections, aided by recordings of the 
response from instruments in the building, the Navy decided to stay in the 
building while more extensive exploration and analysis were performed. It took 
several months before nonstructural repairs were completed and the building 
returned to full use. Subsequent structural review indicated no structural 
damage.

[Begin figure]
Figure 2-51 is photo of the disruption in the upper floors at Naval Hospital 
Bremerton from the Nisqually earthquake. Source: Reid Middleton, Inc. [End 
figure]

The data on the response of the building to the Nisqually earthquake provided an 
opportunity to simulate, by computer analysis, a design event. This simulation 
revealed that shaking at the design level could cause unacceptable damage, both 
to the beam-column joints and to nonstructural systems. The team found that 
traditional, code-based retrofits, such as local modification of the 1,550 beam-
column connections, or introduction of braced frames to meet current standards, 
were too costly and disruptive. Solutions were investigated using the more 
finely tuned, performance-based design methodologies of FEMA 356, and a more 
innovative scheme, based on the introduction of supplemental seismic dampers, 
appeared feasible.

Several styles of dampers are available for use in buildings, including those 
employing controlled yielding of steel elements, those based on friction 
(similar to a brake shoe), and more sophisticated hydraulic dampers (similar to 
a shock absorber). Dampers are normally placed in a building in a configuration 
similar to diagonal braces, and work against the movement between floors (see 
Figure 2-52). This action reduces inter-story drift and, to a lesser extent, 
reduces floor accelerations that cause sliding and overturning of equipment and 
contents. Although dampers should be evenly distributed throughout the building, 
they allow greater flexibility in placement than braced frames or other standard 
lateral force resisting systems. The flexibility in placement of the dampers 
means that critical locations, those that would reduce usefulness of the 
intended space or cause operational shutdowns during installation, can be 
avoided. Using performance-based design concepts, the team set the target 
structural performance levels for Immediate Occupancy (see Section 2.2.3.1) for 
the design earthquake shaking, and Collapse Prevention for a reasonable estimate 
of the worst shaking expected at the site. Structural analysis indicated that 
adding a total of 88 fluid viscous dampers, spread through the facility, would 
deliver the desired performance within the bounds of the current analysis and 
prediction capability. The cost of the structural retrofit was projected to be 
less than 10 percent of the replacement cost, a value that would yield a 
favorable life cycle cost (from damage avoided) on most sites in the country.

[Begin figure]
Figure 2-52 is a photo of the installation of dampers at the Naval Hospital 
Bremerton. The configuration is similar to steel braced frames. [End figure]

In parallel with the retrofit design, the team developed an inexpensive and 
innovative program (called REACH—Rapid Evaluation and Assessment Checklist) to 
improve the speed and effectiveness of future building safety evaluation 
following a seismic event. For many structural types, the seismic structural 
damage is covered and not obvious; concern about the extent of structural damage 
can sometimes delay decisions about building safety, or even cause an unneeded 
evacuation. The building-specific REACH program incorporates real-time data from 
the building’s seismograph network. Threshold values for acceleration and 
displacement are pre-determined by the design team, and actual building motion 
is then compared with these threshold values to allow more accurate and informed 
decision making regarding the ability of the building to sustain the safe 
delivery of medical services after earthquakes. The REACH checklist contains 
additional information, such as a description of the structural system, 
structural drawings, recommended structural inspection sites, and other 
information of use to the facility engineer and building inspectors. The REACH 
documents are stored in the facility department’s disaster response locker, and 
are reviewed by facility staff during routine emergency drills.

Recognizing the significant effect that damage to nonstructural systems can have 
on post earthquake functionality, the facility has begun retrofit of 
nonstructural elements, including bracing of acoustic tile ceiling and light 
grids, mechanical system piping, as well as cabinetry and furniture. Future 
analysis and reinforcement of bracing for critical equipment (such as radiology 
and laboratory) is planned.

2.4.6.3 Procedures and Design Strategies for Rehabilitation of Nonstructural 
Systems

Complete, nonstructural seismic rehabilitation of an existing hospital in 
operation is disruptive and very expensive. However, it is relatively easy to 
incorporate seismic bracing and anchoring during ongoing renovation or 
rehabilitation work. A more active and aggressive program requires developing 
databases of components and systems, and developing a process for prioritizing. 
Priorities can be set by considering importance to life safety, importance to 
overall functionality, associated cost and disruption, component vulnerability, 
or by cost-benefit considerations (see FEMA 274 for more information). 

Components commonly found to be of high priority because of their importance, 
high level of vulnerability, and relatively low cost include anchorage of 
standby generators, medical gas storage, pressurized piping, and communications 
systems. Although not normally considered a dangerous or high-priority system, 
damage to lightweight suspended panelized ceiling and light systems can disrupt 
hospital operations, as happened in Hawaii where such damage forced the KCH to 
evacuate (see Section 2.3.2.2). It is important that the key vulnerabilities of 
each facility be identified and considered in emergency planning and mitigation 
programs.

2.4.6.4 Summary of Risk Reduction Measures for Existing Buildings

Achieving cost-effective improvements in seismic performance of existing 
facilities is far more complex than improving expected performance for proposed 
new buildings in the planning and design stage. First, and most obvious, it is 
always far less expensive to include relatively small changes in a new design to 
create seismically resistant structural and nonstructural systems than it is to 
retrofit — or sometimes replace — existing systems. The complexity and expense 
of retrofitting is exacerbated when such work is not done in conjunction with 
complete renovation — that is, if the building has to remain mostly occupied and 
operational. 

Following the recommended seismic evaluations, careful analysis is needed to 
identify significant life safety risks from potential structural collapse; to 
identify and achieve short-term, high benefit-cost mitigation measures; and to 
plan for longer-term overall mitigation. The following steps are recommended:

- Engage a structural engineer experienced in seismic evaluation and design to 
perform a seismic structural evaluation of existing buildings on the campus that 
contribute to the hospital function. The primary purpose of such an analysis is 
to quickly identify buildings that may be seriously damaged or even collapse in 
a code-level earthquake. A secondary purpose is to gain an understanding of the 
probable performance of the structural and nonstructural systems of each 
building by using the data charted in Figures 2-41 and 2-42.

- Engage an architect, mechanical and electrical engineer, and structural 
engineer, as needed, to evaluate the probable seismic performance of 
nonstructural components and systems.

- Update the emergency response plan, considering the results of the seismic 
evaluations.

- If significant life safety risks are identified from review of either 
structural or nonstructural systems, make plans to minimize occupancy of the 
building, replace the building, or retrofit to an acceptable level of 
performance.

- In most cases, shaking of less than code intensity, which may cause minor or 
no structural damage, can cause serious nonstructural damage. The most 
vulnerable elements that can affect the functions of the hospital have been 
identified from past earthquakes. They are the emergency generator, the bulk 
oxygen storage tank, the internal and external emergency communication systems, 
and the patient elevators. These elements, other than the elevators, normally 
can be anchored and braced against seismic damage rather inexpensively and 
quickly. The elevators may require extensive retrofit to assure operation after 
strong shaking. However, to assure safe patient relocation immediately after an 
event, it is recommended that one patient elevator serving each floor be 
retrofitted to at least the capacity of the structural system and to current 
standards for essential facilities. Automatic seismic switches that demobilize 
elevators at low shaking levels should be used with caution, as the switch may 
defeat the purpose of the strengthened elevator.

- Mechanical equipment on vibration isolators that are not designed for seismic 
forces are extremely vulnerable to seismic damage. This equipment should be 
identified and fitted with appropriate anti-seismic isolators, or seismic 
snubbers, as soon as possible.

- Nonstructural elements affected by or exposed by normal renovation should be 
upgraded to current standards.

- The expected seismic structural performance of all buildings on the campus 
should be considered as part of master planning. Planning should also consider 
opportunities to provide 4 days of self reliance, (See Section 2.4.5).

- Vulnerable medical buildings that can lose full functionality after a code 
earthquake should be studied for retrofit or replacement. Improvements in 
seismic structural performance can often be combined with major renovations. 
Adjacent additions can sometimes be made sufficiently strong to buttress an 
existing building.

- Incremental seismic rehabilitation, as described in FEMA 396 (2003), should 
also be considered for applicability. 

- An emergency plan that considers the care of the patients and staff of the 
facility, as well as the surrounding community, should be kept up to date and 
should include a realistic estimation of the seismic performance of the 
structural and nonstructural systems in each building, and on the site in 
general.

- The Hospital Seismic Safety Evaluation Checklist (Table 2.4) should be 
applied.




2.5 CHECKLIST FOR SEISMIC VULNERABILITY OF HOSPITALS

The Checklist for Seismic Vulnerability of Hospitals (Table 2-4) is a tool that 
can be used to help assess site-specific seismic hazards and building 
vulnerability. The checklist is useful during site selection, preliminary design 
of a new building, or when considering rehabilitation of an existing facility. 
In addition to examining building design issues that affect vulnerability, the 
checklist also helps users to examine the functionality of the crit­ical and 
emergency systems upon which most hospitals depend. The checklist is organized 
into separate sections, so that each section can be assigned to a subject expert 
for greater accu­racy of the examination. The results should be integrated into 
a master vulnerability assessment to guide the design process and the choice of 
appropriate mitigation measures.

[Begin table]
Table 2-4: Hospital Seismic Safety Evaluation Checklist

Site Condition
Vulnerability Sections: Is there an active fault on or adjacent to the site?
Guidance: If suspected, site-specific geologic investigations should be 
performed. Consult local building department, State geologist, local university, 
or local geotechnical expert.

Vulnerability Sections: Does the site consist of soft, stiff, or dense soil or 
rock?
Guidance: If the presence of softer soil that can lead to force amplification or 
liquefaction is suspected, site-specific geologic investigations should be 
performed

Vulnerability Sections: Are post-earthquake site egress and access secured?
Guidance: Alternative routes—unlikely to be blocked by falling buildings, power 
lines, etc. —are desirable.

Vulnerability Sections: Are utility and communications lifelines vulnerable to 
disruption and failure?
Guidance: Security of the entire utility and communications network is the 
issue: the facility may be affected by offsite failures.

Vulnerability Sections: Are there alternate or backup sources for vital 
utilities such as water and power?
Guidance: Redundant systems increase the probability of the hospital remaining 
functional after an event.

Architectural

Vulnerability Sections: Is the architectural/structural configuration irregular?
Guidance: Irregular vertical and horizontal configurations, such as set backs, 
open first stories, or L- or T-shaped plans, may lead to significant stress 
concentrations.

Vulnerability Sections: Is the building cladding attached to structural frames 
so that it can accommodate drift?
Guidance: Frames are flexible, and cladding must be detailed to accommodate 
calculated drifts and deformations. If waterproofing of these systems is 
compromised, rain following an earthquake could cause parts of the building to 
be closed.

Vulnerability Sections: Are heavy veneers, such as brick or stone, securely 
attached to the structural walls?
Guidance: Shear wall structures are very stiff and carry large earthquake 
forces; heavy attachments must be securely attached.

Vulnerability Sections: Are glazing and other panels attached so that they can 
accommodate drift?
Guidance: Glazing must be installed with sufficient bite and adequate space 
between glass and metal to accommodate drift

Vulnerability Sections: Are light, suspended grid ceilings and lights braced and 
correctly attached at walls?
Guidance: Suspended ceilings, if not braced, easily distort (particularly in 
light and flexible frame structures), thus causing ceiling panels to fall out.

Vulnerability Sections: Are heavy plaster suspended ceilings securely supported 
and braced?
Guidance: Heavy lath and plaster ceilings in older facilities are very dangerous 
if poorly supported.

Vulnerability Sections: Are partitions that terminate at a hung ceiling braced 
to the structure above?
Guidance: Partitions need support for out-of-plane forces, and attachment to a 
suspended ceiling grid only is inadequate.

Vulnerability Sections: Are masonry or hollow tile partitions reinforced, 
particularly those surrounding exit stairs?
Guidance: Heavy partitions attract strong earthquake forces because of their 
stiffness and mass, and are prone to damage. They are particularly dangerous 
around stairs and exit ways.

Vulnerability Sections: Are parapets and other appendages securely braced and 
attached to the building structure?
Unreinforced masonry parapets are especially vulnerable. Brace items such as 
cornices, signs, and large antennas.

Structural System
Vulnerability Sections: When was the existing structure designed?
Guidance: Buildings with no, or outdated, seismic design are unlikely to perform 
adequately in strong shaking. Verify that the Importance Factor was used in 
design.

Vulnerability Sections: Has the local seismic zoning changed significantly since 
the building was designed?
Guidance: Local expectation of shaking intensity can change as scientific 
knowledge increases

Vulnerability Sections: Is there a continuous load path from all components of 
the building to the foundation?
Guidance: A continuous load path assures that the structure will act together as 
a whole when shaken. Connections from walls to floors and roofs should also form 
part of this load path.

Vulnerability Sections: Is all load-bearing structural masonry reinforced 
according to code?
Guidance: Older unreinforced masonry has proven very vulnerable in strong 
shaking.

Vulnerability Sections: Are horizontal diaphragms correctly designed and 
constructed with necessary chords and collectors?
Guidance: Large diaphragm openings and the edges of diaphragms need careful 
design to ensure forces are properly transmitted to walls and frames.

Nonstructural Systems
Vulnerability Sections: Are there backups for critical municipal utilities?
Guidance: Municipal utilities such as water, power, and gas, are often disrupted 
in strong shaking. Onsite backups should provide 48 hours of use.

Vulnerability Sections: Are ducts, piping, conduit, fire alarm wiring, and 
communication systems that pass through seismic joints provided with flexible 
connections?
Guidance: Differential movement between sections of the building can cause 
breakage and leaks in pipes and ducts if no provision is made for movement. If 
walls at joint are firewalls, penetrations should be fireproofed.

Vulnerability Sections: Is heavy mechanical equipment adequately secured?
Guidance: Heavy equipment may slide and break utility connections.

Vulnerability Sections: Are vibration isolators for vibrating equipment designed 
for seismic forces?
Guidance: Equipment may jump off very loose isolators and may break restraints 
designed for wind only.

Vulnerability Sections: Is the piping properly braced and provided with 
expansion joints?
Guidance: See Section 2.4.4.

Vulnerability Sections: Is ductwork properly supported and braced?
Guidance: See Section 2.4.4.

Vulnerability Sections: Are boilers and other tanks securely braced?
Guidance: Gas heaters or tanks with flammable or hazardous materials must be 
secured against toppling or sliding.

Vulnerability Sections: Are plumbing lines adequately supported and braced?
Guidance: Leaks in pressure pipes can cause damage over a large area. Protection 
of joints is especially important. See Section 2.4.4.

Vulnerability Sections: Is fire protection piping correctly installed and 
braced?
Guidance: See Section 2.4.4.


Vulnerability Sections: Is heavy electrical equipment adequately secured?
Guidance: Switch gear and transformers are heavy and sliding or movement failure 
can shut down the electrical system. See Section 2.4.4.

Vulnerability Sections: Is emergency generator and associated equipment secured 
against movement?
Guidance: The generator, muffler, batteries, day tank, and other electrical 
equipment may be necessary for emergency operation. See Section 2.4.4.

Vulnerability Sections: Are suspended lighting fixtures securely attached, 
braced, or designed to sway safely?
Guidance: Older suspended lighting fixtures have performed badly in earthquakes, 
and are an injury hazard. See Section 2.4.4.

Vulnerability Sections: Are light fixtures supported in an integrated ceiling, 
braced, and provided with safety wires?
Guidance: Light fixtures within a grid often fall when the grid is distorted, 
unless the fixtures are secured with safety wires. See Section 2.4.4.

Vulnerability Sections: Are the elevator cars, counterweights, and equipment 
anchored for seismic forces?
Guidance: Elevators are important for patient movement, particularly in an 
emergency. After strong shaking, elevators and shafts should be checked for 
safety before use. See Section 2.4.4.

Vulnerability Sections: Is at least one elevator in each wing connected to the 
emergency power system?
Guidance: Even if properly anchored and undamaged, the elevator needs power to 
enable vertical patient movement. See 2.4.4.

Vulnerability Sections: Is the bulk oxygen tank and associated equipment 
secured?
Guidance: The legs, anchorage, and foundations of large tanks need to be checked 
for adequacy.

Vulnerability Sections: Is nitrogen storage secured?
Guidance: Loose tanks may fall and break connections.

Vulnerability Sections: Are small natural gas lines to laboratories or small 
equipment vulnerable?
Guidance: Incompatibility of large and small lines and equipment movement can 
cause dangerous leaks.

Vulnerability Sections: Is the fire alarm system connected to a secondary power 
supply?
Guidance: This is also necessary to support daily operational needs, including 
lighting, heating, communications, etc.

Vulnerability Sections: Is significant fire alarm equipment secured against 
movement?
Guidance: Equipment can slide or topple, breaking connections. See FEMA 74.

Communications and IT Systems
Vulnerability Sections: Are communications components, including antennas, 
adequately braced and supported?
Guidance: Post-event communications are vital for post-earthquake operations. 
See FEMA 74.

Vulnerability Sections: Are plans in place for emergency communication systems, 
both within the facility and to outside facilities?
Guidance: Planning must consider likely post-earthquake conditions at the site 
and offsite.
[End table]




2.6 REFERENCES AND SOURCES OF ADDITIONAL INFORMATION

Note: FEMA publications may be obtained at no cost by calling (800) 480-2520, 
faxing a request to (301) 497-6378, or downloading from the library/publications 
section online at http://www.fema.gov. 

ASCE, 2003, Seismic Evaluation of Existing Buildings, ASCE Standard ASCE-31, 
American Society of Civil Engineers, Reston, VA.

ASCE, 2005, Minimum Design Loads for Buildings and Other Structures, ASCE 
Standard ASCE-7-05, American Society of Civil Engineers, Reston, VA.

ASCE, 2007, Seismic Rehabilitation of Buildings, ASCE Standard ASCE-41, Reston, 
VA.

Arnold, Christopher, and Michael Durkin, 1983, Hospitals and the San Fernando 
Earthquake, August 1983, Building Systems Development, Inc., Palo Alto, CA.

Behr, R.A., 2004, “Design of Architectural Glazing to Resist Earthquakes,” 
Proceeding, 13th World Conference on Earthquake Engineering, Vancouver, BC, 
August 2004.

Earthquake Engineering Research Institute (EERI), John F. Hall, Technical 
Editor, 1995, “Chapter 11, Nonstructural Damage, Northridge Earthquake of 
January 17, 1994 Reconnaissance Report,” Volume 1, Earthquake Spectra, 
Supplement C to Volume 11, April 1995, Earthquake Engineering Research 
Institute, Oakland, CA.

Federal Emergency Management Agency (FEMA), 1997a, NEHRP Guidelines for the 
Seismic Rehabilitation of Buildings, FEMA 273, Federal Emergency Management 
Agency, Washington, DC. 

FEMA, 1997b, NEHRP Commentary on the Guidelines for the Seismic Rehabilitation 
of Buildings, FEMA 274, Federal Emergency Management Agency, Washington, DC. 

FEMA, 2000, Prestandard and Commentary for the Seismic Rehabilitation of 
Buildings, FEMA 356, Federal Emergency Management Agency, Washington, DC. 

FEMA, 2002, Rapid Visual Screening of Buildings for Potential Seismic Hazards, A 
Handbook, FEMA 154, Federal Emergency Management Agency, Washington, DC. 

FEMA, 2003, Incremental Seismic Rehabilitation of Hospital Buildings, FEMA 396, 
Federal Emergency Management Agency, Washington, DC. 

FEMA, 2007, Designing for Earthquakes, A Manual for Architects, FEMA 454, 
Federal Emergency Management Agency, Washington, DC. 

Holmes, William T., 2002, “Background and History of the Seismic Hospital 
Program in California,” Proceedings, 7th U.S. National Conference on Earthquake 
Engineering, July 21-25, 2005, Boston, MA, Earthquake Engineering Research 
Institute, Oakland, CA.

Holmes, William T. and Lawrence Burkett, 2006, “Seismic Vulnerability of 
Hospitals Based on Historical Performance in California,” Proceedings, 8th U. S. 
National Conference on Earthquake Engineering, April 18-22, 2006, San Francisco, 
CA, Earthquake Engineering Research Institute, Oakland, CA.

Lew, H. S., E. V. Leyendecker, and R. D. Dikkers, 1971, Engineering Aspects of 
the 1971 San Fernando Earthquake, Building Science Series 40, U. S. Department 
of Commerce, National Bureau of Standards, Washington, DC.

Los Angeles County, Department of Health Services (LACDHS), 1994, Report to the 
California Hospital Building Safety Board’s Special Forum on Damage to Health 
Facilities in the January 17, 1994 Northridge Earthquake, Minutes of the 
California State Hospital Building Safety Board, April 12, 1994, Office of 
Statewide Health Planning and Development, Sacramento, CA.

National Oceanic and Atmospheric Administration (NOAA), Leonard Murphy, 
Scientific Coordinator, 1973, San Fernando, California Earthquake of February 9, 
Volume 1, Part A, U.S. Department of Commerce, NOAA, Washington, DC.

Precast/Prestressed Concrete Institute (PCI), 1989, Architectural Precast 
Concrete, 2nd. Edition, PCI, Chicago, IL.

URS Consultants (URS), 1996, Northridge Earthquake Study of Seismic Design 
Factors for Hospital Non-Structural Components, Office of Statewide Health 
Planning and Development, Sacramento, CA.

Wang, M.L., 1987, “Cladding Performance on a Full-Scale Test Frame,” Earthquake 
Spectra, EERI, Vol. 3, No. 1, pp. 119-173, February 1987.

Wilson, Douglas, Kent Russell, Stephen Stanek, and David Swanson, 2005, “Strong 
Medicine” pp 26-29, Modern Steel Construction, Volume 45, Number 2, American 
Institute of Steel Construction, Chicago, IL.

Whole Building Design Guide, Seismic Design of the Building Envelope, National 
Institute of Building Sciences, Washington, DC, 2006.






CHAPTER 3 – MAKING HOSPITALS SAFE FROM FLOODING

3.1 GENERAL DESIGN CONSIDERATIONS

This chapter introduces the physical nature and mechanics of floods and explains 
how flood probabilities are determined and how flood hazard areas are 
identified. It describes the types of flood damage that can result when 
hospitals are located in flood hazard areas or are affected by flooding. A 
series of requirements and best practices are introduced that facility owners, 
planners, and designers should consider for reducing the risks from flooding to 
new hospitals and to existing facilities located in areas prone to flooding.

This chapter demonstrates why avoidance of flood hazard areas is the most 
effective way to minimize the life-safety risk to patients, staff, and the 
citizens who rely on these facilities, as well as to minimize the potential for 
damage to buildings and other elements of hospitals. When an existing facility 
is ex­posed to flooding, or if a new facility is proposed to be located in a 
flood hazard area, steps need to be taken to minimize the risks. A well-planned, 
designed, constructed, and maintained hospital should be able to withstand 
damage and remain functional after a flooding event, even one of low 
probability. 

3.1.1 The Nature of Flooding

Flooding is the most common natural hazard in the United States, affecting more 
than 20,000 local jurisdictions and representing more than 70 percent of 
Presidential disaster declarations. Several evaluations have estimated that 7 to 
10 percent of the Nation’s land area is subject to flooding. Some communities 
have very little flood risk; others lie entirely within the floodplain.

Flooding is a natural process that may occur in a variety of forms: long-
duration flooding along rivers that drain large watersheds; flash floods that 
send a devastating wall of water down a mountain canyon; and coastal flooding 
that accompanies high tides and onshore winds, hurricanes, and nor’easters. When 
this natural process does not affect human activity, flooding is not a problem. 
In fact, many species of plants and animals that live adjacent to bodies of 
water are adapted to a regimen of periodic flooding. 

Flooding is only considered a problem when human development is located in areas 
prone to flooding. Such development exposes people to potentially life-
threatening situations and makes property vulnerable to serious damage or 
destruction. It also can disrupt the natural surface flow, redirecting water 
onto lands not normally subject to flooding. 

Flooding along waterways normally occurs as a result of excessive rainfall or 
snowmelt that exceeds the capacity of channels. Flooding along shorelines is 
usually a result of coastal storms that generate storm surges or waves above 
normal tidal fluctuations. Factors that can affect the frequency and severity of 
flooding and the resulting damage include:

- Channel obstructions caused by fallen trees, accumulated debris, and ice jams
- Channel obstructions caused by road and rail crossings where the bridge or 
culvert openings are insufficient to convey floodwaters

- Erosion of shorelines and stream banks, often with episodic collapse of large 
areas of land

- Deposition of sediment that settles out of floodwaters or is carried inland by 
wave action

- Increased upland development of impervious surfaces and manmade drainage 
improvements that increase runoff volumes

- Land subsidence, which increases flood depths

- Failure of dams (resulting from seismic activity, lack of maintenance, flows 
that exceed the design, or destructive acts), which may suddenly and 
unexpectedly release large volumes of water

- Failure of levees (associated with flows that exceed the design, weakening by 
seismic activity, lack of maintenance, or destructive acts), which may result in 
sudden flooding of areas behind levees

- Failure of seawalls, revetments, bulkheads, or similar coastal structures, 
which can lead to rapid erosion and increased flooding and wave damage during 
storms

Each type of flooding has characteristics that represent important aspects of 
the hazard. These characteristics should be considered in the selection of 
hospital sites, the design of new facilities, and the expansion or 
rehabilitation of existing flood-prone facilities.

Riverine flooding results from the accumulation of runoff from rainfall or 
snowmelt, such that the volume of water exceeds the capacity of waterway 
channels and spreads out over the adjacent land. Riverine flooding flows 
downstream under the force of gravity. Its depth, duration, and velocity are 
functions of many factors, including watershed size and slope, degree of 
upstream development, soil types and nature of vegetation, topography, and 
characteristics of storms (or depth of snowpack and rate of melting). Figure 3-1 
illustrates a cross-section of a generic riverine floodplain.

[Begin figure]
Figure 3-1 is an illustration showing the riverine floodplain. 
[end figure]

Coastal flooding is experienced along the Atlantic, Gulf, and Pacific coasts, 
and the Great Lakes. Coastal flooding is influenced by storm surges associated 
with tropical cyclonic weather systems (hurricanes, tropical storms, tropical 
depressions, typhoons), extratropical systems (nor’easters and other large low-
pressure systems), seiches and tsunamis (surges induced by seismic activity). 
Coastal flooding is characterized by wind-driven waves which also may affect 
areas along the Great Lakes shorelines; winds blowing across the broad expanses 
of water generate waves that can rival those experienced along ocean shorelines. 
Some Great Lakes shorelines experience coastal erosion, in part because the 
erosion is associated with fluctuations in water levels. Figure 3-2 is a 
schematic of a generic coastal floodplain.

[Begin figure]
Figure 3-2 is an illustration showing the floodplain along an open coast. 
[end figure]

3.1.2 Probability of Occurrence or Frequency

The probability of occurrence, or frequency, is a statement of the likelihood 
that an event of a certain magnitude will occur in a given period of time. For 
many decades, floodplain management has been based on the flood that has a 1 
percent chance of occurring in any given year, commonly called the “100-year 
flood.” For certain critical actions and decisions, such as planning or 
constructing hospitals and emergency operations centers, the basis of risk 
decisions should be the flood that has a 0.2 percent probability of occurring in 
any given year, commonly called the “500-year flood.” In most locations, the 
benefits of added protection to the 500-year level are greater than the added 
costs. 

The term “100-year flood” is often misunderstood because it conveys the 
impression that a flood of that magnitude will occur only once every 100 years. 
Actually, the 1-percent-annual-chance flood has one chance in 100 of occurring 
in any given year. The fact that a 1-percent-annual-chance flood is experienced 
at a specific location does not alter the probability that a flood of the same 
or greater magnitude could occur at the same location in the next year, or even 
multiple times in a single year. As the length of time considered increases, so 
does the probability that a flood of a specific magnitude or greater will occur. 
For example, Figure 3-3 illustrates the probability that a 100-year flood will 
occur is 26 percent in a 30-year period. And during a 70-year period (the 
potential useful life of many buildings), the probability increases to 50 
percent. Similarly, a 500-year flood has a 0.2-percent probability of being 
equaled or exceeded in any given year, a 6 percent probability of occurrence in 
a 30-year period, and an 18 percent probability of occurrence during a 70-year 
period. 

[Begin figure]
Figure 3-3 is a graph showing probability and magnitude. 
SOURCE: U.S. Geological Survey, Guidelines for Determining Flood Flow Frequency, 
Bulletin 17B (Appendix D). 
[end figure]

The assigned frequency of a flood (e.g., 100-year) is independent of the number 
of years between actual occurrences. Hurricane Camille hit the Mississippi coast 
in 1969 with storm surge flooding that far exceeded previous events, and 
Hurricane Katrina affected much the same area. Although just 36 years apart, 
both storms produced flood levels that were significantly higher than the 
predicted 100-year flood. Similarly, the Mississippi River flooded large areas 
in Missouri in 1993 with flooding that exceeded the predicted 100-year flood 
levels. Just two years later, many of the same areas were flooded again. 

Regardless of the flood selected for design purposes (the “design flood”), the 
designer must determine specific characteristics associated with that flood. 
Determining a flood with a specific probability of occurrence is done in a 
multi-step process that typically involves using computer models that are in the 
public domain. If a sufficiently long record of flood information exists, the 
design flood may be determined by applying statistical tools to the data. 
Alternatively, water resource engineers sometimes apply computer models to 
simulate different rainfall events over watersheds, to predict how much water 
will run off and accumulate in channels. Other computer models are used to 
characterize the flow of water down the watershed and predict how high the 
floodwaters will rise. 

Flood frequency analyses are performed using historical records, and the results 
are influenced by the length of the record. Such analyses do not account for 
recent changes to the land (upland development or subsidence) or future changes 
(additional development, greater subsidence, or climatic variations).

For coastal areas, both historical storms and simulated storm surge models can 
be used to predict the probability that floodwaters will rise to a certain level 
and be accompanied by waves of certain heights. Many coastal storms will produce 
storm surge flooding that, depending on local topography, may extend inland 
significantly farther than anticipated for a 1-percent-annual-chance flood. 
Statistically, such extreme storm surges occur less frequently than the 1-
percent or 0.2-percent-annual-chance floods, but their consequences can be 
catastrophic.

The Saffir-Simpson Hurricane Scale categorizes hurricanes based on sustained 
wind speeds (see Chapter 4). Storm surge, though suggested by the Scale, is not 
always well correlated with the storm category because other factors influence 
surge elevations, notably forward speed of the storm, tide cycle, offshore 
bathymetry, and land topography.

Planners and designers should research the relationship between flood levels for 
different frequency events, including extreme events, especially in hurricane-
prone communities. The difference in flood levels may be extreme in some 
situations, depending on local conditions and the source of flooding. In other 
areas flood levels of lower probability floods might not be much higher than a 
1-percent-annual-chance flood.

The National Flood Insurance Program (NFIP) is a Federal program that encourages 
communities to regulate flood hazard areas and, in return, offers property 
owners insurance protection against losses from flooding (see Sections 3.1.6.1 
and 3.1.6.2). The NFIP uses the 1-percent-annual-chance flood as the basis for 
flood hazard maps, for setting insurance rates, and for application of 
regulations in order to minimize future flood damage. The 1-percent-annual-
chance flood is also used as the standard for examination of older buildings to 
determine the measures to apply in order to reduce future damage.

Satisfying the minimum requirements of the NFIP does not provide adequate 
protection for hospitals that need to be functional even after low-probability 
events. Nearly every year, a very low probability flood occurs somewhere in the 
United States, often with catastrophic consequences. Therefore, for planning and 
design of hospitals, use of a lower probability flood (at least the 500-year) is 
strongly recommended (and may be required by some States and local 
jurisdictions). As noted in Section 3.1.6.3, the 500-year level of protection is 
required if Federal funds are involved in constructing facilities that are vital 
for emergency response and rapid recovery, including hospitals, emergency 
operations centers, emergency shelters, and other buildings that support vital 
services. This reinforces the importance of protecting both the functionality 
and financial investment in a hospital with stricter standards than those 
applied to other buildings.

3.1.3 Flood Characteristics and Loads

A number of factors associated with riverine and coastal flooding are important 
in the selection of sites for hospitals, in site design, and in the 
determination of flood loads which must be considered as part of architectural 
and engineering design. 

Depth: The most apparent characteristic of any flood is the depth of the water. 
Depending on many factors, such as the shape of a river valley or the presence 
of obstructing bridges, riverine flooding may rise just a few feet or tens of 
feet above normal levels. The depth of coastal flooding is influenced by such 
factors as the tidal cycle, the duration of the storm, the elevation of the 
land, offshore bathymetry, and the presence of waves. Depth is a critical factor 
in building design because the hydrostatic forces on a vertical surface (such as 
a foundation wall) are directly related to depth, and because costs associated 
with protecting buildings from flooding increase with depth. Under certain 
conditions, hurricanes can produce storm surge flooding that is 20 to 30 feet 
above mean sea level or, in extreme cases along the Gulf Coast, as much as 35 
feet above mean sea level.

Duration: Duration is the measure of how long the water remains above normal 
levels. The duration of riverine flooding is primarily a function of watershed 
size and the longitudinal slope of the valley (which influences how fast water 
drains away). Small watersheds are more likely to be “flashy,” a characteristic 
that refers to the rapidity with which floodwaters rise and fall. Areas adjacent 
to large rivers may be flooded for weeks or months. Most coastal flooding is 
influenced by the normal tidal cycle, as well as how fast coastal storms move 
through the region. Areas subject to coastal flooding can experience long 
periods of flooding where drainage is poor or slow as a result of topography or 
the presence of flood control structures. For example, water may be trapped in 
depressions in the land or behind a floodwall or levee with inadequate drainage. 
More commonly, coastal flooding is of shorter duration, on the order of 12 to 24 
hours, especially if storms move rapidly. Flooding of large lakes, including 
those behind dams, can be of very long duration because the large volume of 
water takes longer to drain. For building design, duration is important because 
it affects access, building usability, and saturation and stability of soils and 
building materials. Information about flood duration is sometimes available as 
part of a flood study, or could be developed by a qualified engineer.

Local drainage problems create ponding and local flooding that is often not 
directly associated with a body of water such as a creek or river. Although such 
flooding is relatively shallow and not characterized by high velocity flows, 
considerable damage may result. Areas with poor drainage frequently experience 
repetitive damage. Some local drainage problems are exacerbated by old or 
undersized drainage system infrastructure. Flooding caused by drainage problems 
typically occurs as sheetflow or along waterways with small drainage areas. This 
type of flooding is often not mapped or regulated.

Velocity: The velocity of floodwaters ranges from extremely high (associated 
with flash floods or storm surge) to very low or nearly stagnant (in backwater 
areas and expansive floodplains). Velocity is important in site planning because 
of the potential for erosion. In structural design, velocity is a factor in 
determining the hydrodynamic loads and impact loads. Even shallow, high-velocity 
water can threaten the lives of pedestrians and motorists. Accurate estimates of 
velocities are difficult to make, although information about mean velocities may 
be found in some floodplain studies.

Wave action: Waves contribute to erosion and scour, and also contribute 
significantly to design loads on buildings. The magnitude of wave forces can be 
10 to more than 100 times greater than wind and other design loads, and thus may 
control many design parameters. Waves must be accounted for in site planning 
along coastal shorelines, in flood hazard areas that are inland of open coasts, 
and other areas where waves occur, including areas with sufficient fetch that 
winds can generate waves (such as lakes and expansive riverine floodplains). 
Waves on top of storm surges may be as much as 50 percent higher than the 
stillwater depth of the surge. 

Impacts from debris and ice: Floating debris and ice contribute to the loads 
that must be accounted for in structural design. The methods and models used to 
predict and delineate flood hazard areas do not specifically incorporate the 
effects of debris. Thus, there are few sources to determine the potential 
effects of debris impact loads, other than past observations and judgment. 

Erosion and scour: In coastal areas, erosion refers to the lowering of the 
ground surface as a result of a flood event, or the gradual recession of a 
shoreline as a result of long-term coastal processes. Along riverine waterways, 
erosion refers to undermining of channel banks, lateral movement of the channel, 
or cutting of new channels. Scour refers to a localized lowering of the ground 
surface due to the interaction of currents and/or waves with structural 
elements, such as pilings. Soil characteristics influence an area’s 
susceptibility to scour. Erosion and scour may affect the stability of 
foundations and earthen-filled areas, and may cause extensive site damage. 

3.1.3.1 Hydrostatic Loads

Hydrostatic loads occur when water comes into contact with a building or 
building component, both above and below the ground level. They act as lateral 
pressure or vertical pressure (buoyancy). Hydrostatic loads on inclined or 
irregular surfaces may be resolved into lateral and vertical loads based on the 
surface geometry and the distribution of hydrostatic pressure. 

Lateral hydrostatic loads are a direct function of water depth (see Figure 3-4). 
These loads can cause severe deflection or displacement of buildings or building 
components if there is a substantial difference in water levels on opposite 
sides of the component (or inside and outside of the building). Hydrostatic 
loads are balanced on foundation elements of elevated buildings, such as piers 
and columns, because the element is surrounded by water. If not oriented 
parallel to the flow of water, shearwalls may experience hydrostatic loads due 
to a difference of water depth on either side of the wall. To reduce excessive 
pressure from standing water, floodplain management requirements in A Zones call 
for openings in walls that enclose areas below the flood elevation (see 
description of continuous perimeter wall foundation in Section 3.4.4).

[Begin figure]
Figure 3-4 is an illustration showing hydrostatic loads on buildings 
[end figure]

Buoyancy force resulting from the displacement of water is also of concern, 
especially for dry floodproofed buildings and aboveground and underground tanks. 
Buoyancy force is resisted by the dead load of the building or the weight of the 
tank. When determining buoyancy force, the weight of occupants or other live 
loads (such as the contents of a tank) should not be considered. If the building 
or tank does not weigh enough “empty,” then additional stabilizing measures need 
to be taken to avoid flotation. This becomes a significant consideration for 
designs intended to dry floodproof a building. Buoyancy force is slightly larger 
in saltwater, because saltwater weighs slightly more than fresh water.

3.1.3.2 Hydrodynamic Loads

Water flowing around a building or a structural element that extends below the 
flood level imposes hydrodynamic loads. The loads, which are a function of flow 
velocity and structure geometry, include frontal impact on the upstream face, 
drag along the sides, and suction on the downstream side (see Figure 3-5). Ways 
to determine or estimate flood velocities are described in Section 3.1.4.3 and 
Section 3.1.4.4.

[Begin figure]
Figure 3-5 is an illustration showing hydrodynamic loads on a building or 
building element. 
[end figure]

The most common computation methods for hydrodynamic loads are outlined in the 
design standard ASCE 7, Minimum Design Loads for Build­ings and Other 
Structures, produced by the American Society of Civil Engineers’ Structural 
Engineers Institute (ASCE/SEI, 2005). Those methods assume that the flood 
velocity is constant (i.e., steady state flow) and that the dynamic load imposed 
by floodwaters moving at less than 10 feet per second can be converted to an 
equivalent hydro­static load. This conversion is accomplished by adding an 
equivalent surcharge depth to the depth of water on the upstream side. The 
equivalent surcharge depth is a function of the velocity. Loads imposed by 
floodwaters with velocities greater than 10 feet per second cannot be converted 
to equivalent hydro­static loads. Instead, they must be determined according to 
the principles of fluid mechanics or hydraulic models. 

Hydrodynamic loads become important when flow reaches moderate velocities of 5 
feet per second. The components of hydrodynamic loads are laterally imposed, 
caused by the impact of the mass of water against the building, and drag forces 
along the wetted surfaces. Drag coefficients for common building elements, such 
as columns and piers, can be found in a number of sources. ASCE 7 recommends 
values for a variety of conditions. 

Wave loads are another important component of hydrodynamic loads. As described 
in ASCE 7, “design and construction of buildings and other structures subject to 
wave loads shall account for the following loads: waves breaking on any portion 
of the building or structure; uplift forces caused by shoaling waves beneath a 
building or structure, or portion thereof; wave runup striking any portion of 
the building or structure; wave-induced drag and inertia forces; and wave-
induced scour at the base of a building or structure, or its foundation.” 
Wave forces striking buildings and building elements can range from 10 to more 
than 100 times wind or other forces. Forces of this magnitude can be 
substantial, even when acting over the relatively small surface area of the 
supporting structure of elevated buildings. Post-storm damage inspections show 
that breaking wave loads overwhelm virtually all wood-frame and unreinforced 
masonry walls below the wave crest elevation. Only engineered or massive 
structural elements are capable of consistently withstanding breaking wave 
loads. 

The magnitude of wave forces is the rationale behind the floodplain management 
requirement for the bottom of the lowest horizontal structural member to be at 
or above the design flood elevation in environments where high-velocity wave 
action from storms or seismic sources is possible (called V Zones, also referred 
to as Coastal High Hazard Areas). In V Zones, breaking wave heights or wave 
runup depths are predicted to be 3 feet or higher. Because breaking waves as 
small as 1.5 feet in height can impose considerable loads, there is a growing 
awareness of the value of accounting for waves in areas immediately landward of 
V Zones, which are referred to as “Coastal A Zones” (see Section 3.1.5.3). 

Of the variety of wave forces described in ASCE 7—breaking waves, uplift, wave 
runup, wave-induced drag and inertia, and scour—breaking waves constitute the 
greatest hazard. Designers should therefore use breaking wave forces as the 
basis of the design load. Computation of breaking wave loads depends on the 
determination of wave height. For further information on how wave heights can be 
estimated, see Section 3.1.4.1. Designers should refer to ASCE 7 for detailed 
discussion and computation procedures for determining breaking wave loads.

Breaking wave loads on vertical walls or supporting structural members reach a 
maximum when the direction of wave approach is perpendicular to the wall. It is 
common to assume that the direction of approach will be perpendicular to the 
shoreline, in which case the orientation of the wall to the shoreline will 
influence the direction of approach used in load calculations. ASCE 7 provides a 
method for reducing breaking wave loads on vertical walls for waves that 
approach a building from a direction other than straight on. 

Breaking wave forces are much higher than typical wind pressures, even wind 
pressures that occur during a hurricane or typhoon. However, the duration of 
individual loads is brief, with peak pressures probably occurring within 0.1 to 
0.3 seconds after the wave breaks. Structures are to be designed for repetitive 
impact loads that occur over the duration of a storm. Some storms may last just 
a few hours, as hurricanes move through the area, or several days, as during 
some winter coastal storms (nor’easters) that affect the Mid-Atlantic and 
northeastern States. 

3.1.3.3 Debris Impact Loads

Debris impact loads on a building or building element are caused by objects 
carried by moving water. Objects commonly carried by floodwaters include trees, 
dislodged tanks, and remnants of manmade structures such as docks and buildings. 
Extreme impact loads result from less common sources, such as shipping 
containers, boats, and barges. The magnitude of these loads is very difficult to 
predict, yet some reasonable allowance should be made during the design process. 

Impact loads are influenced by the location of the building in the potential 
debris stream. The potential for debris impacts is significant if a building is 
located immediately adjacent to, or downstream from, other buildings, among 
closely spaced buildings, or downstream from large floatable objects. While 
these conditions may be observable in coastal areas, it is more difficult to 
estimate the potential for debris in riverine flood hazard areas. Any riverine 
waterway, whether a large river or smaller urban stream, can carry large 
quantities of debris, especially uprooted trees and trash. 

The basic equation for estimating the magnitude of impact loads depends on 
several variables that must be selected by the designer. These variables include 
several coefficients, building or building element stiffness, debris weight, 
debris velocity, and duration of impact. The latter three variables, described 
in more detail in ASCE 7, are briefly described below.

Debris weight: Debris weight is one of the more difficult variables to estimate. 
Unless otherwise indicated by field conditions, ASCE 7 recommends using an 
average object weight of 1,000 pounds. This weight corresponds to a 30-foot long 
log that is 1 foot in diameter, which is relatively small compared to large 
trees that may be uprooted during a flood. In coastal areas, expected debris 
weights depend on the nature of the debris. In the Pacific Northwest, large 
trees and logs are common, with weights in excess of 4,000 pounds. In areas 
where piers and pilings are likely to become debris, 1,000 pounds is reasonable. 
In areas where most debris is likely to result from building damage (failed 
decks, steps, failed walls, propane tanks), the average debris weight may be 
less than 500 pounds.

Debris velocity: The velocity of the debris when it strikes a building depends 
on the nature of the debris and the velocity of floodwaters. For the impact load 
computation, the velocity of the water-borne object is assumed to be the same as 
the flood velocity. Although this assumption is reasonable for smaller objects, 
it is considered conservative for large objects.

Debris impact duration: Duration of impact is the elapsed time during which the 
impact load acts on the building or building element. The duration of impact is 
influenced primarily by the natural frequency [footnote 1] of the building or 
element, which is a function of the building’s stiffness. Stiffness is 
determined by the properties of the material, the number of supporting members 
(columns or piles), the height of the building above the ground, and the height 
at which the element is struck. Despite all the variables that may influence 
duration of impact, early assumptions suggested 1-second duration. A review of 
results from several laboratory tests that measured impacts yielded much briefer 
periods, and ASCE 7 currently recommends the duration of 0.03 second. 

[Begin footnote 1] Natural frequency is the frequency at which an object will 
vibrate freely when set in motion. [end footnote]

3.1.3.4 Erosion and Local Scour 

Strictly speaking, erosion and scour are not loads; however, they must be 
considered during site evaluation and load calculations because they increase 
the local flood depth, which in turn influences load calculations. 

Erosion may occur in riverine and coastal flood hazard areas. In coastal areas, 
storms can erode or completely remove sand dunes, which act as barriers to 
flooding and damaging waves. Erosion may also lower the ground surface or cause 
a short-term or long-term recession of the shoreline. In areas subject to 
gradual erosion of the ground surface, additional foundation embedment depth can 
mitigate the effects. However, where waterways are prone to changing channels 
and where shoreline erosion is significant, engineered solutions are unlikely to 
be effective. Avoidance of sites in areas subject to active erosion usually is 
the safest and most cost-effective course of action.

Local scour results from turbulence at the ground level around foundation 
elements. Scour occurs in both riverine and coastal flood hazard areas, 
especially in areas with erodible soils. Determining potential scour is critical 
in the design of foundations, to ensure that the bearing capacity or anchoring 
resistance of the soil around posts, piles, piers, columns, footings, or walls 
is not compromised. Scour determinations require knowledge of the flood depth, 
velocity, waves, soil characteristics, and foundation type. 

At some locations, soil at or below the ground surface can be resistant to local 
scour, and calculated scour depths based on unconsolidated surface soils below 
will be excessive. In instances where the designer believes the underlying soil 
at a site will be scour-resistant, the assistance of a geotechnical engineer or 
geologist should be sought to verify that assumption. 

3.1.4 Design Parameters

Flood hazards and characteristics of flooding must be identified to evaluate the 
impact of site development and to determine the design parameters necessary to 
calculate flood loads, to design floodproofing measures, and to identify and 
prioritize retrofit measures for existing hospitals. Table 3-3 in Section 3.6 
outlines a series of questions to facilitate this objective.

3.1.4.1 Flood Depth 

Flood depth is the most important factor required to compute flood loads because 
almost every other flood load calculation depends directly or indirectly on this 
factor. The first step in determining flood depth at a specific site is to 
identify the flood that is specified by governing authorities’ regulations. The 
most common flood used for design is the “base flood” (see Section 3.1.4.2). The 
second step is to determine the expected elevation of the ground at the site. 
This expected ground elevation must account for any erosion, scour, subsidence, 
or other ground eroding condition that occurs over time. Flood depth is computed 
by subtracting the ground elevation from the flood elevation. Since these data 
usually are obtained from different sources, it is important to determine 
whether they are based on the same datum. If not, standard corrections must be 
applied.

In riverine areas, the flood elevations shown on flood hazard maps rarely 
account for waves. Fast moving water usually has an undulating surface that is 
referred to as “standing waves,” which do not break as do waves in coastal 
areas. Standing waves may rise higher than the flood elevation specified on maps 
used for regulatory purposes, thus increasing flood depth. This increase should 
be taken into account when determining flood loads by increasing the flood depth 
used for design purposes. 

In coastal areas, the flood elevations shown on FEMA flood maps account for 
stillwater flooding plus local wave effects, including wave heights, wave runup, 
or wave overtopping over vertical walls. As shown in Figure 3-6, subtracting the 
ground elevation from the FEMA flood map elevation will provide a flood depth 
comprised of the stillwater component and the predicted wave contribution. 

For design purposes, it is important to know that wave forces on buildings cause 
the most damage. FEMA has identified V Zones (velocity zones) on coastal flood 
maps, where wave heights or wave runup depths are predicted to be 3 feet or 
greater (see Section 3.1.5.2). However, post-disaster assessments and laboratory 
studies have shown that waves as small as 1.5 feet in height can also cause 
significant damage. While FEMA flood maps do not specifically designate flood 
hazard areas subject to 1.5- to 3-foot waves, referred to as “Coastal A Zones” 
(see Section 3.1.5.3). It is important to consider these smaller waves and their 
potential damaging effects on buildings. 

Figure 3-6 illustrates the two main principles that are used to estimate wave 
heights at a particular site. Equations for wave height are based on the concept 
that waves are depth-limited, that is, waves propagating into shallow water will 
break when the wave height reaches a certain proportion of the underlying 
stillwater depth. For modeling wave heights during the base flood, FEMA utilizes 
the proportion determined by the National Academy of Sciences (1977):  the total 
wave height will reach a maximum of 78 percent of stillwater depth before 
breaking. At any given site, this proportion may be reduced because of 
obstructions between open water and the site, such as dense stands of vegetation 
or unelevated buildings. In V Zones, 3-foot waves can be supported in only 4 
feet of stillwater and the smaller “Coastal A Zone” waves of 1.5 feet can be 
supported in only 2 feet of stillwater. The second principle is that the wave 
height extends from the trough, which is below the stillwater elevation to the 
crest ,which is above the stillwater elevation, and is equal to 55 percent of 
this stillwater depth. 

[Begin figure]
Figure 3-6 is a definition sketch showing coastal wave height and stillwater 
depth. 
[end figure]

Using these two principles, some general rules of thumb are available to 
estimate wave heights. If the only information available is the base flood depth 
(i.e., the depth calculated using the FEMA flood map elevation minus the ground 
elevation), assume that flood depths between 3 and 6 feet can have an added 
wave-height component between 1.5 and 3 feet, while flood depths of 6 feet or 
more will likely have wave heights in excess of 3 feet. If only the stillwater 
flood depth is known (from an alternative surge map or other data source), the 
maximum flood depth (including wave height) will be approximately 1.5 times the 
stillwater depth. 

In any area with erodible soils, whether coastal or inland site, designers need 
to consider the effects of erosion where floodwaters lower the ground surface or 
cause local scour around foundation elements. The flood depth determined using 
flood elevation and ground elevation should be increased to account for changes 
in conditions during a flood event. Not only does lowering the ground surface 
effectively result in deeper water against the foundation, it may also remove 
supporting soil from the foundation, which must be accounted for in the 
foundation design. 

[Begin text box]
Waves and storm-induced erosion are most common in coastal areas. However, wide 
rivers and lakes may experience wind-driven waves and erodible soils are found 
throughout the United States. For more information about waves and erosion, 
refer to FEMA 55, Coastal Construction Manual. 
[End text box]

3.1.4.2 Design Flood Elevation 

The design flood elevation (DFE) establishes the minimum level of flood 
protection that must be provided. The DFE, as used in the model building codes, 
is defined as either the base flood elevation (BFE) determined by the NFIP and 
shown on FIRMs, or the elevation of a design flood designated by the community, 
whichever is higher. The DFE will always be at least as high as the BFE. 
Communities may use a design flood that is higher than the base flood for a 
number of reasons. For example, a design flood may be used to account for future 
upland development, to recognize a historic flood, or to incorporate a factor of 
safety, known as freeboard. 

[Begin text box]
“Freeboard” is a factor of safety usually expressed in feet above a flood level. 
Freeboard compensates for the many unknown factors that could contribute to 
flood heights, such as wave action, constricting bridge openings, and the 
hydrological effect of urbanization of the watershed. A freeboard of 1 to 3 feet 
is often applied to hospitals.
[End text box]

Facility owners, planners, and designers should check with the appropriate 
regulatory authority to determine the minimum flood elevation to be used in site 
planning and building design. Although the NFIP minimum is the BFE, State or 
local regulations commonly cite the 0.2-percent-annual-chance flood (500-year 
flood) as the de­sign requirement for hospitals, or the regulations may call for 
added freeboard above the minimum flood elevation. Even if there is no specific 
requirement to use the 0.2-percent-annual-chance flood for siting and design 
purposes, it is strongly recommended that decisionmakers take into consideration 
the flood conditions associated with this lower probability event.

If significant flood events have occurred since the effective date of the FIRM, 
these events may change the statistical analyses, which might prompt an update 
of the flood maps and produce revised elevations for the 1-percent-annual-chance 
flood. Hospital owners, planners, and designers should contact community 
officials to determine whether there have been any significant flood events or 
other changes that may affect flood hazards since the effective date of the 
FIRM. The best available information should be used at all times. 

[Begin text box]
After Hurricane Katrina in 2005, FEMA expedited development of Flood Recovery 
Maps and Advisory Base Flood Elevations for the Mississippi coast; the new maps 
were delivered less than 3 months after the storm. 

In 2004, after widespread wildfires in California changed runoff 
characteristics, FEMA developed recovery maps to show increased riverine flood 
hazards.
[End text box]

3.1.4.3 Flood Velocity—Riverine

There are few sources of information that are readily available for estimating 
flood velocities at specific locations along riverine bodies of water. If a 
riverine source has been studied using detailed hydraulic methods, some 
information may be available in summary form in published studies. Studies 
prepared for the NFIP contain tables of data for waterways for which floodways 
were delineated (see Section 3.1.5.2). For specified cross-sections along the 
waterway, the Floodway Data Table includes a mean velocity expressed in feet per 
second. This value is the average of all velocities across the floodway. 
Generally, velocities in the flood fringe (landward of the floodway) will be 
lower than in the floodway. 

For waterways without detailed studies, methods that are commonly used in civil 
engineering for estimating open channel flow velocities can be applied. 

3.1.4.4 Flood Velocity—Coastal

Estimating flood velocities in coastal flood hazard areas involves considerable 
uncertainty and there is little reliable historical information or measurements 
from actual coastal flood events. In this context, velocity does not refer to 
the motion associated with breaking waves, but the speed of the mass movement of 
floodwater over an area.

The direction and velocity of floodwaters can vary significantly throughout a 
coastal flood event. Floodwaters can approach a site from one direction as a 
storm approaches, then shift to another direction (or through several 
directions) as the storm moves through the area. Floodwaters can inundate some 
low-lying coastal sites from both the front (e.g., ocean) and the back (e.g., 
bay, sound, or river). In a similar manner, at any given site, flow velocities 
can vary from close to zero to very high. For these reasons, when determining 
flood loads for building design, velocities should be estimated conservatively, 
and it should be assumed that floodwaters can approach from the most critical 
direction. 

Despite the uncertainties, there are methods to approximate coastal flood 
velocities. One common method is based on the stillwater depth (flood depth 
without waves). Designers should consider the topography, the distance from the 
source of flooding, and the proximity to other buildings and obstructions before 
selecting the flood velocity for design. Those factors can direct and confine 
floodwaters, with a resulting acceleration of velocities. This increase in 
velocities is described as the “expected upper bound.” The “expected lower 
bound” velocities are experienced in areas where those factors are not expected 
to influence the direction and velocity of floodwaters. 

[Begin text box]
Upper bound velocities caused by Hurricane Katrina along the Mississippi coast, 
where storm surge depths neared 25 feet deep (with waves, total flood depths 
approached 35 feet), have been estimated at nearly 30 feet per second (20 miles 
per hour).
[End text box]

Figure 3-7 shows the general relationship between velocity and stillwater depth. 
For design purposes, actual flood velocities are assumed to lie between the 
upper and lower bounds. Conservative designs will use the upper bound 
velocities.

[Begin figure]
Figure 3-7 is a graph showing the velocity as a function of stillwater flood 
depth 
[end figure]

3.1.5 Flood Hazard Maps and Zones

Flood hazard maps identify areas of the landscape that are subject to flooding, 
usually flooding by the 1-percent-annual-chance flood. Maps prepared by the NFIP 
are the minimum basis of State and local floodplain regulatory programs. Some 
States and communities have prepared maps of a floodplain based on the 
assumption that the upper watershed area is fully developed according to 
existing zoning. Some communities base their regulations on a flood of record or 
a historically significant flood that exceeds the base flood shown on the NFIP 
maps.

The flood hazard maps used by the appropriate regulatory authority should be 
consulted during planning and site selection, site design, and architectural and 
engineering design (whether for the design of new buildings or rehabilitation of 
existing buildings). Regardless of the flood hazard data required for regulatory 
purposes, additional research should be conducted on past major floods and other 
factors that could lead to more severe flooding.

3.1.5.1 NFIP Flood Maps

The NFIP produces Flood Insurance Rate Maps (FIRMs) for more than 20,000 
communities nationwide. FIRMs are prepared for each local jurisdiction that has 
been determined to have some degree of flood risk. The current effective maps 
are typically available for viewing in community planning or permit offices. 
[footnote 2] It is important to use the most recent flood hazard map when 
deter­mining site-specific flood hazard characteristics. Although many FIRMs are 
more than 15 years old, often one or more panels or portions of a map panel have 
been revised and republished. Communities must adopt revised maps to continue 
participating in the NFIP. 

[Begin footnote 2] Flood maps may also be viewed at FEMA’s Map Service Center at 
http://msc.fema.gov. For a fee, copies may be ordered online or by calling (800) 
358-9616. The Flood Insurance Study (FIS) and engineering analyses used to 
determine the flood hazard area may be ordered through the FEMA Web site.

[Begin text box]
It is important to note that the number of revised and updated FIRMs is 
increasing rapidly. During the last few years FEMA, in partnership with many 
States and communities, has been implementing an initiative to modernize and 
update all maps that are determined to be out of date. The modernization process 
may involve an examination of flood experience in the period since the original 
flood studies were prepared, use of more detailed topographic and base maps, re-
computation of flood discharges and flood heights, and re-delineation of flood 
hazard area boundaries.
[End text box]

Some FIRMs do not show the 0.2-percent-annual-chance flood hazard area (500-year 
floodplain), and many FIRMs do not provide detailed information about predicted 
flood elevations along every body of water, especially smaller streams and 
tributaries. Determining the 500-year flood is especially difficult when records 
of past flood events are limited. When existing data are insufficient, 
additional statistical methods and engineering analyses are necessary to 
determine the flood-prone areas and the appropriate characteristics of flooding 
required for site layout and building design. If a proposed hospital site or 
existing hospital is affected by flooding, a site-specific topographic survey is 
critical to delineate the land that is below the flood elevation used for 
planning purposes. If detailed flood elevation information is not available, a 
floodplain study may be required to identify the important flood characteristics 
and data required for sound design. However, having flood hazard areas 
delineated on a map conveys a degree of precision that may be misleading. Flood 
maps have a number of limitations that should be taken into consideration, 
especially during site selection and building design. Some of the well-known 
limitations are:

- Flood hazard areas are approximations based on probabilities; the flood 
elevations shown and the areas delineated should not be taken as absolutes, in 
part because they are based on numerical approximations of the real world. 

- For the most part, floodplains along smaller streams and drainage areas (less 
than 1 square mile) are not shown.

- Especially for older maps, the topography used to delineate the flood boundary 
may have had contour intervals of 5, 10, or even 20 feet, which significantly 
affects the precision with which the boundary is determined. The actual 
elevation of the ground relative to the flood elevation is critical, as opposed 
to whether an area is shown as being in or out of the mapped flood hazard area. 

- Maps are based on the data available at the time they were prepared, and, 
therefore, do not account for subsequent upland development that increases 
rainfall-runoff, which may increase flooding. 

- The scale of the maps may impede precise determinations (many older maps are 1 
inch = 2,000 feet).

- The land surface of the floodplain may have been altered by modifications 
after the maps were prepared, including fills, excavations, or levees.

- Local conditions are not reflected, especially conditions that change 
regularly, such as stream bank erosion and shoreline erosion.

- Areas exposed to very low probability flooding are not shown, such as flooding 
from extreme hurricane storm surges, extreme riverine flooding, dam failures, or 
overtopping or failure of levees.

[Begin text box]
In communities along the Gulf and Atlantic coasts, facility owners, planners, 
and designers should check with emergency management offices for maps that 
estimate storm surge flooding from hurricanes. Local planning or engineering 
offices may have post-disaster advisory flood maps and documentation of past 
storm surge events. The FIRMs and regulatory design flood elevations (DFEs) do 
not reflect low probability/high magnitude flooding that may result from a 
hurricane making landfall at a specific location. 

Be aware that most storm surge maps report stillwater flood elevations only; 
local wave heights or wave runup are seldom included. If necessary, local wave 
effects should be estimated and added to the stillwater elevation when 
determining flood depths for design purposes (see Section 3.1.4.1).
[End text box]

3.1.5.2 NFIP Flood Zones

The flood hazard maps prepared by the NFIP show different flood zones to 
delineate different floodplain characteristics (see Figures 3-8, 3-9, and 3-10). 
The flood zones shown on the NFIP maps, and some other designations, are 
described below.

A Zones: Also called “unnumbered A Zones” or “approximate A Zones,” this 
designation is used for flood hazard areas where engineering studies have not 
been performed to develop detailed flood elevations. BFEs are not provided. 
Additional engineering analyses and site-specific assessments usually are 
required to determine the DFE.

[Begin text box]
“Base flood elevation” (BFE) is the elevation above a datum to which floodwaters 
are predicted to rise during the 1-percent-annual-chance flood (also called the 
“base flood” or the 100-year flood).
[End text box]

AE Zones or A1-A30 Zones: Also called “numbered A Zones,” these designations are 
used for flood hazard areas where engineering analyses have produced detailed 
flood elevations and boundaries for the base flood (1-percent-annual-chance 
flood). BFEs are provided. For riverine waterways with these zones, FISs include 
longitudinal profiles showing water surface elevations for different frequency 
flood events.

Floodways: The floodway includes the waterway channel and adjacent land areas 
that must be reserved in order to convey the discharge of the base flood without 
cumulatively increasing the water surface elevation above a designated height. 
Floodways are designated for most waterways that have AE Zones or numbered A 
Zones. FISs include data on floodway widths and mean floodway velocities. 

AO and AH Zones: These zones include areas of shallow flooding and are generally 
shown where the flood depth averages from 1 to 3 feet, where a clearly defined 
channel does not exist, where the path of flooding is unpredictable, and where 
velocity flow may be evident. These zones are characterized by ponding or 
sheetflow. BFEs may be provided for AH Zones; flood depths may be specified in 
AO Zones.

Shaded X (or B) Zones: This zone shows areas of the 500-year flood (0.2-percent-
annual-chance flood), or areas protected by flood control levees. This zone is 
not shown on many NFIP maps, and its absence does not imply that flooding of 
this frequency will not occur. 

Unshaded X (or C) Zones: These zones are all land areas not mapped as flood 
hazard areas that are outside of the floodplain that is designated for the 
purposes of regulating development pursuant to the NFIP. These zones may still 
be subject to small stream flooding and flooding from local drainage problems.

V Zones (V, VE, and V1-V30):Also known as Coastal High Hazard Areas or special 
flood hazard areas subject to high-velocity wave action. V Zones are relatively 
narrow areas along open coastlines and some large lake shores that are subject 
to high-velocity wave action from storms or seismic sources. V Zones extend from 
offshore to the inland limit of a primary frontal dune, or to an inland limit 
where the predicted breaking wave height or wave runup depth drops below 3 feet.

[Begin figures]
Figure 3-8 is a map showing riverine flood hazard zones 

Figure 3-9 is a map showing coastal flood hazard zones

Figure 3-10 is a map showing a sample digital FIRM format used for modernized 
maps
[End figures]

3.1.5.3 Coastal A Zones 

As shown in Figure 3-9, coastal floodplains can be subdivided into A Zones and, 
where Primary Frontal Dunes occur or wave heights or runup depths exceed 3 feet, 
V Zones. NFIP maps do not currently differentiate which portions of the A Zone 
will experience wave heights between 1.5 and 3 feet, which are capable of 
causing structural damage to buildings. These areas of special concern, called 
Coastal A Zones, can be identified through assessment of coastal flood hazard 
data (see Figure 3-11). 

Coastal A Zones are present where two conditions exist:where the expected 
stillwater flood depth is sufficient to support breaking waves 1.5 to 3 feet 
high, and where such waves can actually occur. The first condition occurs where 
stillwater depths (vertical distance between the stillwater elevation and the 
ground) are more than 2 feet deep. The second condition occurs where there are 
few obstructions between the shoreline and the site. In these areas, the 
principal sources of flooding are tides, storm surges, seiches, or tsunamis, not 
riverine flooding.

[Begin text box]
The current editions of the model building codes refer to ASCE 7 and ASCE 24; 
both design standards include requirements for Coastal A Zones.
[End text box]

The stillwater depth requirement is necessary, but is not sufficient by itself 
to warrant designation as a Coastal A Zone. This is because obstructions in the 
area may block wind (limiting the initial growth of waves) or cause friction 
that attenuates wave energy. Obstructions can include buildings, locally high 
ground, and dense, continuous stands of vegetation (trees, shrubs, etc.). 
Designers should determine whether Coastal A Zone conditions are likely to occur 
at a hospital site because of the anticipated wave action and loads. This 
determination is based on an examination of the site and its surroundings, the 
actual surveyed ground elevations, and the estimated wave heights (calculated 
using predicted stillwater elevations found in the FIS or derived from 
elevations shown on the FEMA flood map; see Section 3.1.4.1).

[Begin figure]
Figure 3-11 is an illustration showing flood hazard zones in coastal areas 
[end figure]

When a decision is made to build a hospital in a Coastal A Zone, the 
characteristics of the site and the nature of the flood hazards must be examined 
prior to making important design decisions. Field observations and laboratory 
research have determined that flooding with breaking waves between 1.5 and 3 
feet high pro­duces more damage than flooding of similar depths without waves. 
Therefore, ASCE 24, Flood Resistant Design and Construction, produced by the 
American Society of Civil Engineers’ Structural Engineers Institute (ASCE/SEI, 
2005) specifically requires application of the NFIP’s V Zone design requirements 
in Coastal A Zones. The designers are advised to pay special attention to two 
additional considerations:

- Debris loads may be significant in Coastal A Zones landward of V Zones where 
damaged buildings, piers, and boardwalks can produce battering debris. Damage 
caused by debris can be minimized if foundations are designed to account for 
debris impact loads.

- Especially in high-wind regions, designers must pay special attention to the 
entire roof-to-foundation load path when designing and specifying connections. 
To meet V Zone requirements, designs for buildings in Coastal A Zones should 
account for simultaneous wind and flood forces. Corrosion-resistant connections 
are especially important for the long-term integrity of the structure.

3.1.6 Floodplain Management Requirements and Building Codes 

The NFIP is the basis for the minimum requirements included in model building 
codes and standards for design and construction methods to resist flood damage. 
The original authorizing legislation for the NFIP is the National Flood 
Insurance Act of 1968 (42 U.S.C. 4001 et seq.). In that act, Congress expressly 
found that “a program of flood insurance can promote the public interest by 
encouraging sound land use by minimizing exposure of property to flood losses…” 

The most convincing evidence of the effectiveness of the NFIP minimum 
requirements is found in flood insurance claim payment statistics. Buildings 
that pre-date the NFIP requirements are, by and large, not constructed to resist 
flood damage. Buildings that post-date the NFIP (i.e., those that were 
constructed after a community joined the program and began applying the minimum 
requirements) are designed to resist flood damage. The NFIP reports that 
aggregate loss data indicate that buildings that meet the minimum requirements 
experience 70 percent less damage than buildings that pre-date the NFIP. There 
is ample evidence that buildings designed to exceed the minimum requirements are 
even less likely to sustain damage. 

3.1.6.1 Overview of the NFIP

The NFIP is based on the premise that the Federal government will make flood 
insurance available in communities that agree to recognize and incorporate flood 
hazards in land use and development decisions. In some States and communities, 
this is achieved by guiding development to areas with a lower risk. When 
decisions result in development within flood hazard areas, application of the 
criteria set forth in Federal regulation 44 CFR Part 60.3 are intended to 
minimize exposure and flood-related damage. State and local governments are 
responsible for applying the provisions of the NFIP through the regulatory 
permitting processes. At the Federal level, the NFIP is managed by FEMA and has 
three main elements:

- Hazard identification and mapping, under which engineering studies are 
conducted and flood maps are prepared in partnership with States and 
communities. These maps delineate areas that are predicted to be subject to 
flooding under certain conditions. 

- Floodplain management criteria for development establish the minimum 
requirements to be applied to development within mapped flood hazard areas. The 
intent is to recognize flood hazards in the entire land development process. 

- Flood insurance, which provides some financial protection for property owners 
to cover flood-related damage to buildings and contents.

Federal flood insurance is intended to shift some of the costs of flood 
disasters away from the taxpayer by providing property owners an alternative to 
disaster assistance and disaster loans. Disaster assistance provides limited 
funding for repair and cleanup, and is available only after the President signs 
a major disaster declaration for the area. NFIP flood insurance claims are paid 
any time damage from a qualifying flood event [footnote 3] occurs, regardless of 
whether a major disaster is declared. Community officials should be aware that 
public buildings may be subject to a mandated reduction in disaster assistance 
payments if the building is in a mapped flood hazard area and is not covered by 
flood insurance. 

[Begin footnote 3] For the purpose of adjusting claims for flood damage, the 
NFIP defines a flood as “a general and temporary condition of partial or 
complete inundation of two or more acres of normally dry land area or of two or 
more properties (at least one of which is the policyholder’s property) from: 
overflow of inland or tidal waters; unusual and rapid accumulation or runoff of 
surface waters from any source; mudflow; or collapse or subsidence of land along 
the shore of a lake or similar body of water as a result of erosion or 
undermining caused by waves or currents of water exceeding anticipated cyclical 
levels that result in a flood as defined above.” [end footnote]

[Begin text box]
“Substantial damage” is damage of any origin sustained by a structure whereby 
the cost of restoring the structure to its condition before the damage would 
equal or exceed 50 percent of the market value of the structure before the 
damage occurred. 

“Substantial improvement” is any repair, reconstruction, rehabilitation, 
addition, or improvement of a building, the cost of which exceeds 50 percent of 
the market value of the building before the improvement or repair is started 
(certain historic structures may be excluded).
[End text box]

Another important objective of the NFIP is to break the cycle of flood damage. 
Many buildings have been flooded, repaired or rebuilt, and flooded again. Before 
the NFIP, in some parts of the country this cycle was repeated every couple of 
years, with reconstruction taking place in the same flood-prone areas, using the 
same construction techniques that did not adequately resist flood damage. NFIP 
provisions guide development to lower-risk areas by requiring compliance with 
performance measures to minimize exposure of new buildings and buildings that 
undergo major renovation or expansion (called “substantial improvement” or 
repair of “substantial damage”). This achieves the long-term objective of 
building disaster-resistant communities. 

3.1.6.2 Summary of the NFIP Minimum Requirements 

The performance requirements of the NFIP are set forth in Federal regulation 44 
CFR Part 60. The requirements apply to all development, which the NFIP broadly 
defines to include buildings and structures, site work, roads and bridges, and 
other activities. Buildings must be designed and constructed to resist flood 
damage, which is primarily achieved through elevation (or floodproofing). 
Additional specific requirements apply to existing development, especially 
existing buildings. Existing buildings that are proposed for substantial 
improvement, including restoration following substantial damage, are subject to 
the regulations. 

Although the NFIP regulations primarily focus on how to build structures, one of 
the long-term objectives of the program is to guide development to less 
hazardous locations. Preparing flood hazard maps and making the information 
available to the public is fundamental in satisfying that objective. With that 
information, people can make informed decisions about where to build, how to use 
site design to minimize exposure to flooding, and how to design buildings that 
will resist flood damage.

The NFIP’s broad performance standards for site work in flood hazard areas 
include the following requirements.

- Building sites shall be reasonably safe from flooding.

- Adequate site drainage shall be provided to reduce exposure to flooding. 

- New and replacement sanitary sewage systems shall be designed to minimize or 
eliminate infiltration of floodwaters into the systems and discharges from the 
systems into floodwaters.

- Development in floodways shall be prohibited, unless engineering analyses show 
that there will be no increases in flood levels. 

The NFIP’s broad performance standards for new buildings proposed for flood 
hazard areas (and substantial improvement of existing flood-prone buildings) 
include the following requirements.

- Buildings shall be designed and adequately anchored to prevent flotation, 
collapse, or lateral movement resulting from hydrodynamic and hydrostatic loads, 
including the effects of buoyancy.

- Building materials used below the DFE shall be resistant to flood damage.

- Buildings shall be constructed by methods and practices that minimize flood 
damage (primarily by elevating to or above the base flood level, or by specially 
designed and certified floodproofing measures).

- Buildings shall be constructed with electrical, heating, ventilation, 
plumbing, and air conditioning equipment and other service facilities that are 
designed and/or located so as to prevent water from entering or accumulating 
within the components.

Owners, planners, and designers should determine if there are any applicable 
State-specific requirements for floodplain development. Some States require that 
local jurisdictions apply standards that exceed the minimum requirements of the 
NFIP. In particular, some States require that hospitals be located outside of 
the floodplain (including the 500-year floodplain) or they are to be designed 
and constructed to resist conditions associated with the 500-year flood. Some 
States have regulations that impose other higher standards, while some States 
have direct permitting authority over certain types of construction or certain 
types of applicants. 

[Begin text box]
States often use governors’ executive orders to influence State-constructed and 
State-funded critical facilities, requiring location outside of the 500-year 
floodplain where feasible, or protection to the 500-year flood level if avoiding 
the floodplain is not practical. In 2004, a review of State and local floodplain 
management programs determined that Alabama, Illinois, Michigan, New York, North 
Carolina, Ohio, and Virginia have requirements for critical facilities (ASFPM 
2004).
[End text box]

As participants in the NFIP, States are required to ensure that development not 
subject to local regulations, such as the development of State-owned properties, 
satisfies the same performance requirements. If hospitals are exempt from local 
permits, this may be accomplished through a State permit, a governor’s executive 
order, or other mechanisms that apply to entities not subject to local 
authorities.

3.1.6.3 Executive Order 11988 and Critical Facilities 

When Federal funding is provided for the planning, design, and construction of 
new critical facilities (including hospitals), or for the repair of existing 
critical facilities that are located within the 500-year floodplain, the funding 
agency is required to address additional considerations. Executive Order 11988, 
Floodplain Management, requires Federal agencies to apply a decisionmaking 
process to avoid, to the extent possible, the long- and short-term adverse 
impacts associated with the occupancy and modification of floodplains, and to 
avoid the direct or indirect support of floodplain development whenever there is 
a practicable alternative. If there is no practicable alternative, the Federal 
agency must take steps to minimize any adverse impacts to life, property, and 
the natural and beneficial functions of floodplains. 

The executive order establishes the BFE as the minimum standard for all Federal 
agencies. Implementation guidance specifically addresses “critical actions,” 
which are described as those actions for which even a slight chance of flooding 
would be too great. The construction or repair of critical facilities, such as 
hospitals and clinics, fire stations, emergency operations centers, and 
facilities for storage of hazardous wastes or storage of critical records, are 
examples of critical actions. 

After determining that a site is in a mapped flood hazard area, and after giving 
public notice, the Federal funding agency is required to identify and evaluate 
practicable alternatives to locating a hospital in a 500-year floodplain. If the 
Federal agency has determined that the only practicable alternative is to 
proceed, then the impacts of the proposed action must be identified. If the 
identified impacts are harmful to people, property, and the natural and 
beneficial functions of the floodplain, the Federal agency is required to 
minimize the adverse effects on the floodplain and the funded activity.

[Begin text box]
FEMA’s eight-step decisionmaking process for complying with Executive Order 
11988 must be applied before Federal disaster assistance is used to repair, 
rehabilitate, or reconstruct damaged existing critical facilities in the 500-
year floodplain.
[end text box]

Having identified the impacts of the proposed action and the methods to minimize 
these impacts, the Federal agency is required to re-evaluate the proposed 
action. The re-evaluation must consider whether the action is still feasible, 
whether the action can be modified to relocate the facility or eliminate or 
reduce identified impacts, or if a “no action” alternative should be chosen. If 
the finding results in a determination that there is no practicable alternative 
to locating a critical facility in the floodplain, or otherwise affecting the 
floodplain, then a statement of findings and a public explanation must be 
provided. 

3.1.6.4 Scope of Model Building Codes and Standards 

The International Building Code (IBC) and the Building Construction and Safety 
Code (NFPA 5000) were the first model codes to include comprehensive provisions 
that addressed flood hazards. Both codes are consistent with the minimum 
provisions of the NFIP that pertain to the de­sign and construction of 
buildings. The NFIP requirements that pertain to site development, floodways, 
coastal setback lines, ero­sion-prone areas, and other environmental constraints 
are found in other local ordinances. The codes require designers to identify and 
design for anticipated environmental loads and load com­binations, including 
wind, seismic, snow, and flood loads, as well as the soil conditions.

The IBC and NFPA 5000 incorporate, by reference, a number of standards that are 
developed through a formal or accredited consensus process. The best known is 
ASCE 7. The model building codes require that applicable loads be accounted for 
in the building design. The designer must identify the pertinent, site-specific 
characteristics and then use ASCE 7 to determine the specific loads and load 
combinations. In effect, it is similar to a local floodplain ordinance that 
requires determination of the environmental condition (in/out of the mapped 
flood hazard area, DFE/depth of water), and then specifies certain conditions 
that must be met during design and construction. The 1998 edition of ASCE 7 was 
the first version of the standard to include flood loads explicitly, including 
hydrostatic loads, hydrodynamic loads (velocity and waves), and debris impact 
loads. 

The IBC and NFPA 5000 also incorporate, by reference, a standard that was first 
published by ASCE in 1998 and revised in 2005: ASCE 24. Developed through a 
consensus process, ASCE 24 addresses specific topics pertinent to designing 
buildings in flood hazard areas, including floodways, coastal high-hazard areas, 
and other high-risk flood hazard areas such as alluvial fans, flash flood areas, 
mudslide areas, erosion-prone areas, and high-velocity areas. 

[Begin text box]
ASCE 7-05 outlines methods to determine design loads and load combinations in 
flood hazard areas, including hydrostatic loads, hydrodynamic loads, wave loads, 
and debris impact loads. In order to compute the loads and load combinations the 
designer must identify site-specific characteristics, including flood depths, 
velocities, waves, and the likelihood that debris impacts need to be considered. 
ASCE 24-05 addresses design requirements for structures in coastal high-hazard 
areas (V Zones) and Coastal A Zones.
[End text box]

ASCE 7 and the model building codes classify structures based on occupancy into 
four categories, each with different requirements. The same categories are used 
in ASCE 24 and different flood-resistant requirements apply to the different 
categories. Table 3-1 summarizes the elevation requirements of ASCE 24 that 
exceed the NFIP minimum requirements for the hospitals and health care 
facilities addressed by this manual (Category III or Category IV structures). 

Although most State and local building codes are based on the International Code 
Series produced by the International Code Council, jurisdictions often adopt 
specific amendments. For example, the State of Florida adopted requirements that 
are specific to nursing homes, new hospitals, and additions, alterations, or 
renovations to existing hospitals and all detached outpatient facilities. Such 
facilities are required to be “located above the 100-year flood plain or 
hurricane Category 3 (Saffir-Simpson scale) hurricane surge inundation 
elevation, whichever requires the highest elevation.”

[Begin table]
Table 3-1: ASCE/SEI 24-05 provisions related to the elevation of hospitals

Elevation of Lowest Floor or Bottom of Lowest Horizontal Structural Member
A Zone: elevation of lowest floor
Category III: BFE +1 ft or DFE, whichever is higher
Category IV: BFE +2 ft or DFE, whichever is higher

V Zone and Coastal A Zone: where the lowest horizontal structural member is 
parallel to direction of wave approach
Category III: BFE +1 ft or DFE, whichever is higher
Category IV: BFE +1 ft or DFE, whichever is higher

V Zone and Coastal A Zone: where the lowest horizontal structural member is 
perpendicular to direction of wave approach
Category III: BFE +2 ft or DFE, whichever is higher
Category IV: BFE +2 ft or DFE, whichever is higher

Elevation Below which Flood-Damage-Resistant Materials Shall be Used
A Zone
Category III: BFE +1 ft or DFE, whichever is higher
Category IV: BFE +2 ft or DFE, whichever is higher

V Zone and Coastal A Zone: where the lowest horizontal structural member is 
parallel to direction of wave approach
Category III: BFE +2 ft or DFE, whichever is higher
Category IV: BFE +2 ft or DFE, whichever is higher

V Zone and Coastal A Zone: where the lowest horizontal structural member is 
perpendicular to direction of wave approach
Category III: BFE +3 ft or DFE, whichever is higher
Category IV: BFE +3 ft or DFE, whichever is higher

Minimum Elevation of Utilities and Equipment
A Zone
Category III: BFE +1 ft or DFE, whichever is higher
Category IV: BFE +2 ft or DFE, whichever is higher

V Zone and Coastal A Zone: where the lowest horizontal structural member is 
parallel to direction of wave approach
Category III: BFE +2 ft or DFE, whichever is higher
Category IV: BFE +2 ft or DFE, whichever is higher

V Zone and Coastal A Zone: where the lowest horizontal structural member is 
perpendicular to direction of wave approach
Category III: BFE +3 ft or DFE, whichever is higher
Category IV: BFE +3 ft or DFE, whichever is higher

Dry Floodproofing
Category III: A Zone: elevation to which dry floodproofing extends
BFE +1 ft or DFE, whichever is higher
Category IV: BFE +2 ft or DFE, whichever is higher

Category III: V Zone and Coastal A Zone: dry floodproofing not allowed
Not applicable
Category IV: Not applicable
[End table



3.2 HOSPITALS EXPOSED TO FLOODING 

3.2.1 Identifying Flood Hazards at Existing Hospitals

Facility owners, planners, and designers of hospitals should investigate site-
specific flood hazards and character­istics as part of site selection, guiding 
the location of a new hospital and other improvements on a site. This same 
investigation should be undertaken when examining existing hospitals and when 
planning improvements or rehabilitation work. The best available infor­mation 
should be examined, including flood hazard maps, records of historical flooding, 
storm surge maps, and advice from local experts and others who can evaluate 
flood risks. Table 3-3 in Section 3.5 outlines questions that should be answered 
prior to initiating site layout and de­sign work.

3.2.2 Vulnerability: What Flooding Can Do to Existing Hospitals

Existing flood-prone hospitals are susceptible to damage, the nature and 
severity of which is a function of site-specific flood characteristics. Damage 
may include: site damage, structural and nonstructural building damage, 
destruction or impairment of utility service equipment, and loss of contents. 

Regardless of the nature and severity of damage, flooded hospitals typically are 
not functional while cleanup and repairs are undertaken. The length of closure, 
and thus the impact on the ability of the facility to become operational, 
depends on the severity of the damage and lingering health hazards. Sometimes 
repairs are put on hold pending a decision on whether a hospital should be 
rebuilt at the flood-prone site. When damage is substantial, rehabilitation or 
reconstruction is allowed only if compliance with flood-resistant design 
requirements is achieved (see Section 3.1.6.2).

3.2.2.1 Site Damage

The degree of site damage associated with flooding is a function of several 
variables related to the characteristics of the flood, as well as the site 
itself.

Erosion and scour: All parts of a site that are subject to flooding by fast-
moving water could experience erosion, and local scour could occur around any 
permanent obstructions to flow. Graded areas, filled areas, and cut or fill 
slopes are especially susceptible. Stream and channel bank erosion, and erosion 
of coastal shorelines, are natural phenomena that may, over time, threaten site 
improvements and buildings (see Figure 3-12). 

[Begin figure]
Figure 3-12 is a photo showing riverbank erosion of the Genesee River during 
Hurricane Agnes flooding in 1972 eventually led to collapse of this wing of the 
Jones Memorial Hospital, Wellsville, NY. Source: Dick Neal 
[end figure]

Debris and sediment removal: Even when buildings are not subject to water 
damage, floods can deposit large quantities of debris and sediment that can 
damage a site and be expensive to remove. 

Landscaping: Grass, trees, and plants suffer after floods, especially long-
duration flooding that prevents oxygen uptake, and coastal flooding that 
stresses plants that are not salt-tolerant. Fast-moving floodwaters and waves 
also can uproot plants and trees.

Fences: Some types of fences that are relatively solid can significantly 
restrict the free flow of floodwaters and trap floating debris. Fences can be 
damaged or knocked down by the pressure of flowing water, or by the buildup of 
debris that may result in significant loads. 

Accessory structures: Accessory structures can sustain both structural and 
nonstructural damage. In some locations, such structures can be designed and 
built using techniques that minimize damage potential, without requiring 
elevation above the DFE. 

Access roads: Access roads that extend across flood-prone areas may be damaged 
by erosion, washout of drainage culverts, failure of fill and bedding materials, 
and loss of road surface (see Figure 3-13). Road damage could prevent 
uninterrupted access to a facility and thus impair its functionality. 

[Begin figure]
Figure 3-12 is a photo showing how flooding caused the failure of this road bed. 
Source U.S. Army Corps of Engineers 
[end figure]

Parking lots and parking garages: Paved parking lots may be damaged by failure 
of bedding materials and loss of driving surface. Vehicles left in parking lots 
and parking garages could also be damaged. Most large parking garages are 
engineered structures that can be designed to allow for the flow of water.

Helicopter landing pads: Helicopters landing pads that are flooded are not 
serviceable when access is critical. Hospitals on flood-prone sites should have 
rooftop landing pads. 

Signage: Signage on ground that is subject to flooding may be damaged. Loss of 
signage can impair ready access, especially by those unfamiliar with the 
facility or on large medical campuses. 

[Begin text box]
Damage to other site elements such as water supply, sewer lines, underground and 
aboveground tanks, and emergency power generators, is discussed in Section 
3.2.2.5. 
[End text box]

Stormwater management facilities and site drainage: Site improvements such as 
swales and stormwater basins may be eroded, filled with sediments, or clogged by 
debris.

3.2.2.2 Structural Damage

Structural damage includes all damage to the load-bearing portions of a 
building. Damage to other components of buildings is described below, including 
nonstructural components (Section 3.2.2.3), medical equipment (Section 3.2.2.4), 
utility service equipment (Section 3.2.2.5), and contents (Section 3.2.2.6). 

Depth: The hydrostatic load against a wall or foundation is directly related to 
the depth of water. Standard stud and siding, or unreinforced brick veneer 
walls, may collapse under hydrostatic loads associated with relatively shallow 
water. Reinforced masonry walls perform better than unreinforced masonry walls 
(see Figure 3-14), although an engineering analysis is required to determine 
performance. Walls and floors of below-grade areas (basements) are particularly 
susceptible to damage by buoyancy force. When soils are saturated, pressures 
against below-grade walls are a function of the total depth of water, including 
the depth below-grade and the weight of the saturated soils.

[Begin figure]
Figure 3-14 is a photo showing the interior unreinforced masonry walls of the 
Port Sulphur High School in Louisiana were damaged by hydrostatic loads 
associated with Hurricane Katrina’s storm surge (2005).
[End figure]

Buoyancy and uplift: If below-grade areas are essentially watertight, buoyancy 
or uplift forces can float a building out of the ground or rupture concrete 
slabs-on-grade (see Figure 315). Buildings that are not adequately anchored can 
be floated or pushed off foundations. Although rare for large and heavy 
buildings, this is a concern for smaller structures. Buoyancy is a significant 
concern for underground and aboveground tanks, especially those used for 
emergency generator fuel and bulk oxygen.

[Begin figure]
Figure 3-15 is a photo showing a concrete slab ruptured by hydrostatic pressure 
(buoyancy) induced by the floodwaters of Hurricane Katrina (2005). 
[End figure]

Duration: By itself, saturation is unlikely to result in significant structural 
damage to masonry construction. Saturation of soils, a consequence of long 
duration flooding, increases pressure on below-grade foundation walls. 

Velocity, wave action, and debris impacts: Each of these components of dynamic 
loads can result in structural damage if buildings are not designed to resist 
overturning, repetitive pounding by waves, or short-duration impact loads 
generated by floating debris (see Figure 3-16).

[Begin figure]
Figure 3-16 is a photo showing how the South Cameron Memorial Hospital, Cameron, 
LA, was damaged by debris carried by Hurricane Rita’s Storm surge (2005) 
[End figure]

Erosion and scour: Structural damage is associated with foundation failure when 
erosion or scour results in partial or complete removal of supporting soil (see 
Figure 3-17). Erosion of slopes, especially unprotected slopes, can lead to 
slope failures and loss of foundation supporting soil.

[Begin figure]
Figure 3-17 is a photo showing how local scour undermined the footing of this 
exterior stair tower (Hurricane Ivan, 2004) 
[End figure]

3.2.2.3 Nonstructural Damage 

Many flood-prone buildings are exposed to floodwaters that are not fast moving, 
or that may be relatively shallow and not result in structural damage. Simple 
inundation and saturation of the building and finish materials can result in 
significant and costly damage, including long-term health complications 
associated with mold. Floodwaters often are contaminated with chemicals, 
petroleum products, and sewage. Under such circumstances, recovery generally 
involves removal of nonstructural materials and finishes because cleanup and 
decontamination are expensive and time-consuming (see Figure 3-18). Damage to 
contents is discussed in Section 3.2.2.6. 

[Begin figure]
Figure 3-18 is a photo of drying out the ground floor at Hancock Medical Center 
(Hurricane Katrina, 2005) 
[End figure]

Saturation damage can vary as a function of the duration of exposure. Some 
materials are not recoverable even after very brief inundation, while others 
remain serviceable if in contact with water for only a few hours. Use of water-
resistant materials will help to minimize saturation damage and reduce the costs 
of cleanup and restoration to service. (For more information, see FEMA Technical 
Bulletin FIA-TB-2, Flood-Resistant Materials Requirements.)

Wall finishes: Painted concrete and concrete masonry walls usually resist water 
damage, provided the type of paint used can be readily cleaned. Tiled walls may 
resist water damage depending on the type of adhesive and foundation (gypsum 
board substrate and wood-framed walls with tile typically do not remain stable). 

Flooring: Many hospitals have durable floors that resist water damage. Ground 
floors are often slab-on-grade and finished with tile or sheet products. 
Flooring adhesives in use since the early 1990s likely are latex-based and tend 
to break down when saturated (see Figure 3-19). Most carpeting, even the indoor-
outdoor kind, is difficult to clean. 

[Begin figure]
Figure 3-19 is a photo of floor damage at West Jefferson Medical Center 
(Hurricane Katrina, 2005) 
[End figure]

Wall and wood components: When soaked for long periods of time, some materials 
change composition or shape. Most types of wood swell when wet and, if dried too 
quickly, will crack, split, or warp. Plywood can delaminate and wood door and 
window frames may swell and become unstable. Gypsum wallboard, wood composition 
panels, other wall materials, and wood cabinetry not intended for wet locations 
can fall apart (see Figure 3-18). The longer these materials are wet, the more 
moisture, sediment, and pollutants they absorb. Some materials, such as the 
paper facing on gypsum wallboard, “wick” standing water, resulting in damage 
above the actual high-water line (see Figure 3-20). 

[Begin figure]
Figure 3-20 is a photo of the test of the effects of flooding on materials 
showed that water damage and mold growth extended above the water line. Source: 
Oak Ridge National Laboratory. 
[End figure]

Metal components: Metal structural components are unlikely to be permanently 
damaged by short-term inundation. However, hollow metal partitions are 
particularly susceptible when in contact with water because they cannot be 
thoroughly dried and cleaned. Depending on the degree of corrosion protection on 
the metal, repetitive flooding by saline coastal waters may contribute to long-
term corrosion.

Metal connectors and fasteners: Depending on the composition of the metal, 
repetitive flooding, especially by saline coastal waters, may contribute to 
long-term corrosion. Connectors and fasteners are integral to the structural 
stability of buildings; therefore, failure caused by accelerated corrosion would 
jeopardize the building. 

3.2.2.4 Medical Equipment

Large medical equipment that is permanently installed usually is considered to 
be part of the building rather than contents. The nature and sensitivity of most 
medical equipment suggests that post-flood cleaning to restore functionality may 
not be feasible. This limits options for existing hospitals that use such 
equipment in areas that will be exposed to flooding, because temporary 
relocation of the equipment cannot be part of an emergency response plan. 

3.2.2.5 Utility System Damage

Utility system service equipment that is exposed to flooding is vulnerable to 
damage. Damage may result in a total loss, or may require substantial cleaning 
and restoration efforts. The degree of damage varies somewhat as a function of 
the characteristics of flooding. Certain types of equipment and installation 
measures will help minimize damage and reduce the costs of cleanup and 
restoration to service.

Displacement of equipment and appliances: Installation below the flood level 
exposes equipment and appliances to flood forces, including drag resulting from 
flowing water and buoyancy. Gas-fired appliances are particularly dangerous: 
flotation can separate appliances from gas sources, resulting in fires and 
explosive situations. Displaced equipment may dislodge lines from fuel oil 
tanks, contributing to the threat of fire and causing water pollution and 
environmental damage.

Elevators: If located in areas subject to flooding, elevator component equipment 
and controls will be damaged, and movement between floors will be impaired. In 
hospitals, maintaining elevator function is important, especially if services 
have to be consolidated to upper floors after a flood.

Corrosion: Corrosion related to inundation of equipment and appliances may not 
be apparent immediately, but can increase maintenance demand and shorten the 
useful life of some equipment and appliances. 

Electrical systems and components: Electrical systems and components, and 
electrical controls of heating, ventilation and air-conditioning systems, are 
subject to damage simply by getting wet, even for short durations. Unless 
specifically designed for wet locations, switches and other electrical 
components can short out due to deposits of sediment, or otherwise not function, 
even when allowed to dry before operation. Wiring and components that have been 
submerged may be functional, although generally it is more cost-effective to 
discard flooded outlets, switches, and other less-expensive components than to 
attempt thorough cleaning.

Communications infrastructure: Critical communications infrastructure, such as 
control panels and wiring for warning systems, 911 systems, and regular 
telephone and wireless networks, are most susceptible to failure during 
emergencies if located in below-grade basements. 

Specialized piping: Unprotected piping for medical gas supply systems may be 
damaged and threaten care that depends on an uninterrupted supply of oxygen and 
other gasses for the treatment of patients.

Ductwork damage: Ductwork is subject to two flood-related problems. Flood forces 
can displace ductwork, and saturated insulation can overload support straps, 
causing failure.

Mold and dust: Furnaces, air handlers, and ductwork that have been submerged 
must be thoroughly cleaned and sanitized. Otherwise, damp conditions contribute 
to the growth of mold and accumulated sediment can be circulated throughout the 
hospital, causing respiratory problems. Fiberglass batt or cellulose insulation 
that has been submerged cannot be sanitized and must be replaced. In sensitive 
environments, ductwork should be replaced rather than cleaned.

Gas-fired systems: Water-borne sediment can impair safe functioning of jets and 
controls in gas-fired furnaces and water heaters, necessitating professional 
cleaning and inspection prior to restoration of service. Control equipment 
(valves, electrical switches, relays, temperature sensors, circuit breakers, and 
fuses) that have been submerged may pose an explosion and fire hazard and should 
be replaced.

Emergency power generators: Generators that are installed at-grade are 
susceptible to inundation and will be out of service after a flood (see Figure 
3-21). Even if fuel tanks are located above flood level, truck access for 
refueling would be impaired if the site is flooded for any length of time.

[Begin figure]
Figure 21 is a photo showing that although it was anchored and not displaced by 
floodwaters, this generator was out of service after being submerged (Hurricane 
Katrina, 2005) 
[end figure]

Tanks (underground): Underground storage tanks are subjected to significant 
buoyant forces and can be displaced, especially when long-duration flooding 
occurs. Computations of stability should be based on the assumption that the 
tank is empty in order to maximize safety. Tank inlets, fill openings, and vents 
should be above the DFE, or designed to prevent the inflow of floodwaters or 
outflow of tank contents during flood conditions.

Tanks (aboveground): Permanently installed aboveground storage tanks are subject 
to buoyant forces and displacement caused by moving water. Standard strapping of 
propane tanks may be inadequate for the anticipated loads. Tank inlets, fill 
openings, and vents should be above the DFE, or designed to prevent the inflow 
of floodwaters or outflow of tank contents during flood conditions. Even 
temporary storage of tanks can be problematic (see Figure 3-22).

[Begin figure]
Figure 22 is a photo showing how oxygen tanks stored outside of the Hancock 
Medical Center were dislodged by flooding (Hurricane Katrina, 2005) 
[end figure]

Public Utility Service: Damage to public utility service (potable water supply 
and wastewater collection) can affect operations and may cause damage to 
hospitals:

- Potable water supply systems may become contaminated if distribution lines or 
treatment facilities are damaged, or if wellheads are submerged.

- During heavy rains, sewers back up from infiltration and inflow of stormwater 
into the sewer lines and manholes, cross connections between storm and sanitary 
sewers, and flooded wastewater treatment plants. Sewer backup into a hospital 
poses a major health hazard. Even when the water has receded, exposed building 
components, finish materials, and contents are contaminated, and usually must be 
removed because adequate cleaning is difficult, if not impossible. 

3.2.2.6 Contents Damage

Hospitals contain high-value equipment and contents that can be damaged and 
unrecoverable when exposed to flooding. For the purpose of this discussion, the 
term “contents” includes items such as furniture, appliances, computers, 
laboratory equipment and materials, records, and specialized moveable machinery. 
The following types of contents often are total losses after flooding.

Furniture: Porous woods become saturated and swollen, and joints may separate. 
Furniture with coverings or pads generally cannot be restored. Metal furniture 
is difficult to thoroughly dry and clean, is subject to corrosion, and typically 
is discarded (see Figure 3-23). Some wood furniture may be recoverable after 
brief inundation.

[Begin figure]
Figure 3-23 is a photo showing that the interior of the Hancock Medical Center 
required extensive cleanup following flooding (Hurricane Katrina, 2005) 
[end figure]

Computers: Flood-damaged computers and peripheral equipment cannot be restored 
after inundation, although special recovery procedures may be able to recover 
information on hard drives.

Communications equipment: Even though some communications equipment may be able 
to be restored with appropriate cleaning, the loss of functionality would 
seriously impair the ability of the facility to provide critical services 
immediately after a flood. Equipment with printed circuit boards generally 
cannot be restored.

Medical records and office files: Valuable records may be lost if flooded. 
Although expensive, some recovery of computerized and paper records may be 
possible with special procedures (see Figure 3-24).

[Begin figure]
Figure 3-24 is a photo showing medical records saturated by floodwaters 
(Hurricane Katrina, 2005) 
[end figure]

Health care equipment and laboratory materials: Most medical and health care 
equipment cannot be cleaned and restored to safe functioning, and would need to 
be replaced. Depending on the nature of laboratory materials and chemicals, 
complete disposal or special cleanup procedures may be required.

Kitchen equipment and goods: Floodwaters can dislodge appliances that can float 
and damage other equipment. Stainless steel equipment generally has cleanable 
surfaces that can be disinfected and restored to service. Because of 
contamination, all food stuffs must be discarded. 

Vehicles associated with hospitals: If left in flood-prone areas, vehicles must 
be replaced or cleaned to be serviceable, and may not be functional and 
available for service immediately after a flood.



3.3 REQUIREMENTS AND BEST PRACTICES IN FLOOD HAZARD AREAS

3.3.1 Evaluating Risk and Avoiding Flood Hazards

Flood hazards are very site-specific. When a flood hazard map is prepared, lines 
drawn on the map appear to define the hazard area precisely. Land that is on one 
side of the line is “in” the mapped flood hazard area, while the other side of 
the line is “out.” Although the delineation may be an approximation, having 
hazard areas shown on a map facilitates avoiding such areas to the maximum 
extent practical. If such areas are unavoidable, facility owners should 
carefully evaluate all of the benefits and all of the costs in order to 
determine long-term acceptable risks, and to develop appropriate plans for 
design and construction of new facilities.

Even in communities with expansive floodplains, it should be possible to avoid 
locating new hospitals in floodways and coastal areas subject to significant 
waves (V Zones).

Section 3.2 describes the damage sustained by existing buildings exposed to 
flood hazards, including site damage, structural and nonstructural building 
damage, destruction or impairment of service equipment, and loss of contents. 
These types of damage, along with loss of function, are avoided if hospitals are 
located away from flood hazard areas. Damage is reduced and the ability to 
sustain function is increased when hospitals that must be located in flood 
hazard areas are built to exceed the minimum requirements.

[Begin text box]
Construction in V Zones is required to meet certain design and construction 
requirements that are different than those required in A Zones. This chapter 
will identify these differences. 
[End text box]

Flood hazard areas designated as “V Zones” on FIRMs are relatively narrow areas 
along open coasts and lake shores where the base flood conditions are expected 
to produce 3-foot or higher waves. V Zones, sometimes called coastal high-hazard 
areas or special flood hazard areas subject to high-velocity wave action, are 
found on the Pacific, Gulf, and Atlantic coasts, and around the Great Lakes. 
Every effort should be made to locate hospitals outside of V Zones, because the 
destructive nature of waves makes it difficult to design a building to be fully 
functional during and after a flood event. This is particularly true in coastal 
areas subject to hurricane surge flooding. 

3.3.2 Benefits and Costs: Determining Acceptable Risk

Many decisions made with respect to hospitals are, in part, based on a 
determination of acceptable risk. Risk includes the potential losses associated 
with a hazard. Ideally, risks can be defined in terms of expected probability 
and frequency of the hazard occurring, the people and property exposed, and the 
potential consequences. Choosing a site that is affected by flooding is a 
decision to accept some degree of risk. Although the flood-prone land may have a 
lower initial cost, the incremental costs of construction, plus the likely 
increased costs of maintenance, repair, and replacement, may be significant. 
Another cost of locating a hospital in a flood-prone area is related to access 
problems if streets and access roads are impassable. The building may be 
elevated and protected, but if access is restricted periodically, then the use 
of the facility is affected (see Figure 3-25).

[Begin figure]
Figure 3-25 is a photo showing how Hurricane Katrina’s floodwaters surrounded 
most hospitals in New Orleans, complicating access for evacuation, as well as 
limiting treatment options for residents. 
[end figure]

In communities with expansive flood hazard areas, there may be no practical 
alternatives to using a flood-prone site. In these situations, an evaluation of 
acceptable risk should lead to selection of design measures that exceed the 
minimum requirements to mitigate the impacts of flooding. 

[Begin text box]
Extreme hurricane storm surge flooding may be a very low-probability event, but 
the flood depths and wave heights may be much more severe than the conditions of 
the base flood shown on the FIRMs. The potential impacts on a hospital must be 
carefully considered in order to make an informed decision regarding acceptable 
risk and potential damage. If possible, it is always best to avoid locating 
hospitals in areas subject to extreme storm surge flooding. 
[End text box]

The building owner and the design team can influence the degree of risk (e.g., 
the frequency and severity of flooding that may affect the site). They control 
it through the selection of the site design and the building design measures. 
Fundamentally, this process is a balancing of the benefits of an acceptable 
level of disaster resistance with the costs of achieving that degree of 
protection. With respect to mitigation of future hazard events:

- Benefits are characterized and measured as future damages avoided if the 
mitigation measures (including avoiding flood hazard areas) are implemented.

- Costs are the costs associated with implementing measures to eliminate or 
reduce exposure to hazards. 

Section 3.2 describes typical damage and losses sustained by buildings exposed 
to flooding. Direct damage includes damage to physical property, including the 
site, the building, building materials, utilities, and building contents. 
Indirect damage that is not listed includes health hazards, loss of 
functionality, emergency response, evacuation, expenses associated with 
relocating services to another building during repairs, and loss of revenue. 

Benefits other than avoided physical damage are difficult to measure. They are 
associated with future damage that does not occur because of the mitigation 
activity, cleanup that is not required because of the mitigation activity, 
service that is not interrupted because flooding does not affect normal 
operations, and revenue that is not lost. In addition, benefits accrue over long 
periods of time, thus making it more difficult to make a direct comparison of 
the benefits with the up-front costs of mitigation. Mitigation costs can be more 
readily expressed in terms of the higher costs of a flood-free site, or the 
initial capital costs of work designed to resist flood damage. Thus, without 
full accounting of both benefits and costs, decisionmakers may not be able to 
make fully informed decisions. Some questions that should be answered include:

- If the site is flood-prone and the building is out of the flood hazard area or 
is elevated on fill, what are the average annual cleanup costs associated with 
removal of sand, mud, and debris deposited by floods of varying frequencies?

- If the facility building is elevated by means other than fill, will periodic 
inundation of the exposed foundation elements cause higher average annual 
maintenance costs?

- If the facility is protected with floodproofing measures, what are the costs 
of annual inspection, periodic maintenance and replacement of materials, and 
staff training and drills?

- If the hospital meets only the minimum elevation requirements, what are the 
average annual damages and cleanup costs over the anticipated useful life of the 
building, including the occurrence of floods that exceed the design flood 

- How do long-term costs associated with periodic inundation compare to up-front 
costs of selecting a different site or building to a higher level of protection?

- If a site outside of the flood hazard area is available but less than optimal 
in terms of access by the community, are the trade-offs acceptable?

- If the facility is located in a hurricane-prone community, how should the 
facility design account for low-probability, but high-impact, storm surge 
flooding?

- If access to the facility is periodically restricted by flooding, especially 
long-duration flooding, what are the resulting cost effects? How often would an 
alternate location need to be provided to continue normal operations?

3.3.3 Site Modifications

When sites being considered for hospitals are prone to flooding, planners and 
designers may want to evaluate the feasibility of certain site modifications in 
order to provide an increased level of protection to buildings. The evaluations 
involve engineering analyses to determine whether the desired level of 
protection is cost-effective, and whether the proposed site modifications alter 
the floodplain in ways that could increase flooding. The effectiveness of 
typical site modifications and their ramifications must be examined for each 
specific site. 

Earthen fill: Fill can be placed in the flood hazard area to elevate an entire 
site above the DFE. If the fill is placed and compacted to be stable during the 
rise and fall of floodwaters, and if the fill is protected from erosion, then 
modifying a site with fill to elevate a facility is preferred over other methods 
of elevation. Not only will buildings be less exposed to flood forces, but, 
under some circumstances (such as long duration floods), hospitals may be able 
to continue to function. Whether nonstructural fill is placed solely to modify 
the site, or structural fill is placed to elevate buildings, placement of fill 
can change flooding characteristics, including increased flooding on other 
properties. Engineering analyses can be conducted to determine whether 
eliminating floodplain storage by filling will change the direction of the flow 
of water, create higher flow velocities, or increase the water surface elevation 
in other parts of the floodplain.

[Begin text box]
Site modifications are not appropriate in floodways along riverine waterways, 
where obstructions to flows can increase flood elevations. Engineering analyses 
are required to determine the impact of such modifications.
[End text box]

In Coastal A Zones, back bays, and along the banks of wide rivers where wave 
action is anticipated, fill is a less-effective site modification method because 
wave action may erode the fill, and adequate armoring or other, protection 
methods can be expensive. 

In V Zones, structural fill is not allowed as a method of elevating buildings. 
Beachfront areas with sand dunes pose special problems. Manmade alterations of 
sand dunes are not allowed unless analyses indicate that such modifications will 
not increase potential flood damage. 

Excavation: Excavation on a given parcel of land alone rarely results in 
significant alteration of the floodplain. Excavation that modifies a site is 
more commonly used in conjunction with fill in order to offset or compensate for 
the adverse impacts of fill.

Earthen levee: A levee is a specially designed barrier that modifies the 
floodplain by keeping the water away from certain areas (see Figure 3-26). 
Levees are significant structures that require detailed, site-specific 
geotechnical investigations; engineering analyses to identify whether flooding 
will be made worse on other properties; structural and site design to suit 
existing constraints; design of interior drainage (on the land side); and long-
term commitment for maintenance, inspection, and repairs. It is important to 
remember that areas behind levees are protected only up to a certain design 
flood level—once overtopped or breeched, most levees fail and catastrophic 
flooding results. Levees that protect hospitals and other critical facilities 
usually are designed for at least the 0.2-percent-annual-chance flood (500-year) 
and have freeboard to increase the factor of safety. Depending on the site 
layout and duration of flooding, access for vehicles can be problematic. Low 
levees can be designed with road access; higher levees can be designed with 
vehicle access points that require special closures when flooding is predicted.

[Begin figure]
Figure 3-26 is a schematic of typical earthen levee and permanent floodwall 
[end figure]

Floodwall: Floodwalls are similar to levees in that they provide protection to 
certain areas (see Figure 3-26). Failure or overtopping of a floodwall can 
result in catastrophic flooding. A floodwall is a significant structure designed 
to hold back water of a certain depth based on the design flood for the site. 
Generally, floodwalls are most effective in areas with relatively shallow 
flooding and minimal wave action. As with levees, designs must accommodate 
interior drainage on the land side, and maintenance and operations are critical 
for adequate performance. Floodwalls that protect buildings that provide 
essential services usually are designed for the 0.2-percent-annual-chance flood 
(500-year) and have freeboard to increase the factor of safety. If a protected 
facility is intended to remain operational during long-duration flooding, 
vehicle access to the site and pedestrian access to the building are required.

3.3.4 Elevation Considerations 

The selection of the appropriate method of elevating a hospital in a special 
flood hazard area depends on many factors, including type of flood zone, costs, 
level of safety and property protection determined as acceptable risk, and 
others. Another consideration is the elevation of the lowest floor relative to 
the flood elevation. Table 3-1 in Section 3.1.6.4 summarizes the elevation 
requirements in ASCE 24. Given the importance of hospitals, elevation of the 
lowest floor to or above the 0.2 percent-annual-chance flood (500-year) 
elevation is crucial. Various methods used to elevate buildings in flood hazard 
areas are described below. 

[Begin text box]
“Lowest floor” is the floor of the lowest enclosed area (including the 
basement). An unfinished or flood-resistant enclosure, usable solely for parking 
of vehicles, building access, or storage in an area other than a basement, is 
not the lowest floor, provided the enclosure is built in compliance with 
applicable requirements. 
[End text box]

In A Zones, the minimum requirement is that the lowest floor (including the 
basement) must be at or above the DFE (plus freeboard, if desired or required). 
For building elevation methods other than fill, the area under elevated 
buildings in A Zones may be used only for limited purposes: parking, building 
access, and limited storage (crawlspaces are treated as enclosures, see below). 
Owners and designers are cautioned that enclosures below the DFE are exposed to 
flooding and the contents will be damaged or destroyed by floodwaters. The walls 
surrounding an enclosure must have flood openings that are intended to equalize 
interior and exterior water levels in changing flood conditions, to prevent 
differential hydrostatic pressures leading to structural damage. The enclosed 
area must not contain utilities and equipment (including ductwork) below the 
required elevation. 

In a V Zone, the minimum requirement is that the elevation of the bottom of the 
lowest horizontal structural member of the lowest floor (including basement) 
must be at or above the DFE (plus freeboard, where required). Given the 
importance of hospitals, elevation to or above the 0.2-percent-annual-chance 
flood (500-year) elevation is appropriate and strongly recommended. The V Zone 
requirements are recommended in Coastal A Zones.

[Begin text box]
Communities that participate in the NFIP require that a registered professional 
engineer or architect develop or review the structural design, specifications, 
and plans of buildings in V Zones, and certify that the design and methods of 
construction to be used are in accordance with accepted standards of practice. 
The standards of practice require that the foundation and structure attached 
thereto is anchored to resist flotation, collapse, and lateral movement caused 
by wind and water loads acting simultaneously on all building components. Water 
loading values are those associated with the base flood conditions, and wind 
loading values are those required by applicable State or local building codes 
and standards. 
[End text box]

The area under elevated buildings in V Zones may be used only for parking, 
building access, and limited storage. The areas may be open or enclosed by 
lattice walls or screening. If areas are enclosed by solid walls, the walls must 
be specifically designed to break away under certain flood loads to allow the 
free passage of floodwaters under the building. Breakaway walls are non-load-
bearing walls, i.e., they do not provide structural support for the building. 
They must be designed and constructed to collapse under the pressure of 
floodwaters in such a way that the supporting foundation system and the 
structure are not affected. 

Coastal communities along the Atlantic and Gulf coasts are subject to storm 
surge flooding generated by hurricanes and tropical storms. Depending on a 
number of variables, storm surge flood depths may significantly exceed the BFE. 
In addition, waves are likely to be higher than predicted for the base flood, 
and will occur in areas where significant wave action during the base flood is 
not expected. Application of the minimum requirements related to elevation of 
the lowest floor and foundation design does not result in flood resistance for 
such extreme conditions. Foundations for hospitals in areas subject to storm 
surge should be designed to elevate the building so that the lowest horizontal 
structural members are higher than the minimum required elevation. Additional 
elevation not only reduces damage that results from lower probability events, 
but the cost of Federal flood insurance is usually lower. Designers and owners 
should plan to use the lowest elevated floor for non-critical uses that, even if 
exposed to flooding more severe than the design flood, will not impair critical 
functioning during post-flood recovery.

Storm surge flooding and waves can cause scour and erosion, even at locations 
that are some distance from the shoreline. Foundation designs for hospitals in 
coastal communities should account for some erosion and local scour of 
supporting soil during low-probability surge events. Storm surge flooding can 
also produce large quantities of floating debris, even at locations that are 
some distance from the shoreline. Debris can damage nonstructural building 
components and, in some cases of prolonged battering, can lead to structural 
failure. Foundation designs for hospitals in coastal communities should account 
for debris loads. This is especially important where damage to other buildings 
in the area may generate additional debris, thereby increasing the loads. 

Notes on continuous load path: In coastal communities and other areas exposed to 
high winds, designers should pay special attention to the entire roof-to-
foundation load path when designing and specifying connections. Connections must 
be capable of withstanding simultaneous wind and flood forces. Poorly connected 
buildings may fail or float off foundations when floodwaters and waves are 
higher than the design flood elevation. Corrosion-resistant connections are 
critical for the long-term integrity of the structure, and should be inspected 
and maintained regularly. 

Slab-on-grade foundation on structural fill: This is considered to be the safest 
method to elevate a building in many flood hazard areas, except those where 
waves and high velocity flows may cause erosion. Consequently, this foundation 
type is not allowed in V Zones. Structural fill can be placed so that even if 
water rises up to the DFE, the building (see Figure 3-27) and building access 
would still be protected from flooding. The fill must be designed to minimize 
adverse impacts, such as increasing flood elevations on adjacent properties, 
increasing erosive velocities, and causing local drainage problems. To ensure 
stability, especially as floodwaters recede and the soils drain, fill must be 
designed for the anticipated water depths and duration. A geotechnical engineer 
or soil scientist may need to examine underlying soils to determine if the 
bearing capacity is sufficient to carry the added weight of fill, or if 
consolidation over time may occur. In addition, the effects of long-term 
compaction of the fill should be considered, and may prompt additional elevation 
as a factor of safety. The horizontal extent of the fill, away from the 
foundation, should be designed to facilitate access by emergency vehicles, with 
a minimum 25-foot width recommended. Engineered concrete slabs supported by 
piers should have sufficient resistance to erosion and scour if designed for 
anticipated flood conditions. Designers are cautioned to avoid excavating a 
basement into fill without added structural protection (and certification that 
the design meets the requirements for dry floodproofing), due to the potential 
for significant hydrostatic loads and uplift on basement floors.

[Begin text box]
Communities may require a registered design professional to certify that 
buildings elevated on fill are reasonably safe from flooding. The FEMA NFIP 
Technical Bulletin 10-01 discusses criteria for this certification.
[End text box]

[Begin figure]
Figure 3-27 is a photo showing how structural fill was placed to elevate the 
Henrietta Johnson Medical Center above the shallow flood hazard area in 
Wilmington, DE. 
[end figure]

Stem wall foundations: Stem wall foundations have a continuous perimeter grade 
beam, or perimeter foundation wall, that is backfilled with compacted earth to 
the underside of the concrete floor slab (see Figure 3-28). This foundation type 
is not allowed in V Zones. Stem wall foundations are designed to come in contact 
with floodwaters on the exterior. They are more stable than perimeter wall 
foundations with crawlspaces, but could experience structural damage if 
undermined by local scour and erosion. Designs must account for anticipated 
debris and ice impacts, and incorporate methods and materials to minimize impact 
damage.

[Begin figure]
Figure 3-28 is an illustration showing a typical stem wall foundation 
[end figure]

Columns or shear wall foundations (open foundations): Open foundations consist 
of vertical load-bearing members (columns, piers, pilings, and shear walls) 
without solid walls connecting the vertical members. Open foundations minimize 
changes to the floodplain and local drainage patterns, and the area under the 
building can be used for parking or other uses (see Figure 3-29). The design of 
the vertical members must also account for hydrodynamic loads and debris and ice 
impact loads. Flood loads on shear walls are reduced if they are oriented 
parallel to the anticipated direction of flow. If erodible soils are present and 
local scour is likely, both conditions must be accounted for in determining 
embedment depth. Depending on the total height of the elevated facility, the 
design may need to take into consideration the increased exposure to wind and 
uplift, particularly where loads are expected from breaking waves. 

[Begin figure]
Figure 3-29 is a photo showing how Tampa General Hospital had its new Emergency 
Department wing designed to be elevated on columns, well above hurricane storm 
surge flooding elevations. Source: Tampa General Hospital [end figure]

In V Zones, buildings must be elevated using open foundations, which consist of 
vertical load-bearing members (columns, piers, pilings, and shear walls) without 
solid walls connecting the vertical members. The design of the vertical members 
must also account for hydrodynamic loads and debris impact loads. Flood loads on 
shear walls are reduced if the walls are oriented parallel to the anticipated 
direction of flow. Erodible soils may be present and local scour may occur; both 
must be accounted for in designs by extending the load-bearing members and 
foundation elements well below the expected scour depth. 

Continuous perimeter walls (enclosed foundations with crawlspace): Unlike stem 
wall foundations, continuous perimeter walls enclose an open area or crawlspace 
(see Figure 3-30). The perimeter walls must have flood openings, also called 
vents) that are intended to equalize interior and exterior water levels 
automatically during periods of rising and falling flood levels, to prevent 
differential hydrostatic pressures that could lead to structural damage. Flood 
openings may be engineered and certified for the required performance, or they 
must meet prescriptive requirements (notably, the opening must provide at least 
1 square inch of net open area for each square foot of area enclosed). Perimeter 
wall design must also account for hydrodynamic loads, and debris and ice impact 
loads. Enclosed crawlspaces must not contain utilities or equipment (including 
ductwork) below the required elevation. Designers must provide adequate 
underfloor ventilation and subsurface drainage to minimize moisture problems 
after flooding. This foundation type is not allowed in V Zones.

[Begin figure]
Figure 3-30 is an illustration showing typical crawlspace with flood openings 
[end figure]

Pier supports for manufactured and portable units: Manufactured buildings and 
portable units must be elevated above the DFE (plus freeboard, if required). 
Pier supports must account for hydrodynamic loads and debris and ice impact 
loads, and units must be anchored to resist wind loads. Although written 
specifically for manufactured housing units, FEMA 85, Manufactured Home 
Installation in Flood Hazard Areas, has useful information that is applicable to 
portable units.

3.3.4.1 The Case of Boulder Community Foothills Hospital, Boulder, Colorado 

Located on the east side of the City of Boulder, Colorado, the Boulder Community 
Foothills Hospital (BCFH) is framed by the Flatirons, the first of the Rocky 
Mountains rising steeply above the Front Range plains to the east. The new 
facility, completed in 2003 but not fully occupied until 2004, is an expansion 
of the existing Boulder Community Hospital located in the older part of the 
city. 

The master site plan for complete development of the site is shown in Figure 3-
31. The primary building on the site incorporates the hospital and medical 
building. It consists of a main reception area linking two large wings. The 
hospital wing is cast-in-place concrete with a steel frame roof. The medical 
building wing is steel frame, fireproofed with concrete floor slabs. Exteriors 
are brick veneer on metal studs. The two wings have three floors above-grade and 
one floor below-grade. The original designs complied with the 1997 Uniform 
Building Code for the core and shell; clinical spaces and interior designs 
comply with the 2000 International Building Code. Other buildings on the campus 
include the Table Mesa Medical Building, a free-standing parking garage, and the 
utility plant building. The Cancer Care Center and another parking garage are 
under construction. 


[Begin figure]Figure 3-31 is an illustration showing the master site plan of the 
Boulder Community Foothills Hospital, Boulder, CO. Source Civitas, Inc. and OZ 
Architecture [end figure]

BCFH is licensed for a maximum patient capacity of 54 and has approximately 475 
employees, of which about 120 are non-medical. Between the two wings, 220,000 
square feet of space is available for patient care (including in-patient rooms, 
clinics, operating rooms, and the emergency room), and laboratory and 
administrative uses (including reception, waiting areas, and offices). 

In the early planning of the hospital, a search for suitable sites revealed that 
few vacant parcels of sufficient size (16-17 acres) were available within the 
city limits. However, a 39-acre parcel in the county was just over the city 
boundary and could be annexed into the city. The site was entirely within the 
floodplain of Boulder Creek and a tributary, Bear Canyon Creek, which come 
together on the property. Two reservoirs are located upstream: Barker Dam on 
Boulder Creek and Grosse Reservoir on Bear Canyon Creek. 

Design of the hospital and the site began while negotiations for the annexation 
were underway. As part of the conditions of annexation, the city required that 
the design of the hospital meet the standards of the city’s building code and 
floodplain management ordinance, which include some provisions that are more 
restrictive than those required by the county. The requirements resulted in 
several measures intended to provide a higher level of protection against flood 
hazards than is required for buildings that do not provide critical services. 

The 17 acres of the site that were needed for the campus, entirely outside of 
the designated floodway, were proposed to be filled to an elevation of one-foot 
above the 100-year flood elevation. The remaining 22 acres were placed in 
conservation easement. Engineering analyses were performed to demonstrate that 
no increase in flood elevations would result. The city required approval of a 
Conditional Letter of Map Revision from FEMA prior to approving the plans. The 
fill was required to be compacted to 95 percent of the maximum density 
obtainable with the Standard Proctor Test method. The earthen fill is gently 
sloped to natural grade and various landscaping elements and retaining walls 
provide a pleasing transition (Figure 3-32). 

[Begin figure]
Figure 3-32 is a photo showing Boulder Community Foothills Hospital main 
entrance, with landscaping and retaining walls for portions of site filled to 
the 500-year flood elevation (Boulder Creek is behind photographer). [end 
figure]

In the BCFH main building, the first floors are used for reception, cafe, 
ambulatory services, the emergency room, and a six-patient Intensive Care Unit. 
All patient rooms are on the second and third floors of the three wings, which 
are built off a single-story connecting reception area. In addition to a 
separate parking garage structure, some parking is provided in a one-level, 
below-grade parking garage. The three patient wings have below-grade floors that 
are used for offices, laundry, laboratories, materials management, and building 
service and equipment. To provide natural light, a portion of these below-grade 
floors are surrounded by a moat (Figure 3-33). Because of anticipated high 
groundwater and the fact that the below-grade areas are constructed into fill 
that is subject to saturation during flooding, all below-grade areas are 
designed and certified as floodproofed spaces. Floodproofing extends 2 feet 
above the 100-year flood elevation, and 1 foot above the 500-year flood 
elevation.

[Begin figure]
Figure 3-33 is a photo showing how below-grade office spaces provide natural 
light by construction of reinforced moat designed to prove flood protection for 
a foot above the 500-year flood elevation. [end figure]

Although the risk of flooding is very low, hospital personnel have identified 
low points where floodwaters that rise higher than the predicted 500-year flood 
could begin to affect the facility. The lowest point of entry is the ramp to the 
parking garage, which is more than a foot higher than the 500-year water surface 
elevation (Figure 3-34). A supply of sandbags is kept onsite for placement 
across the ramp. The next most susceptible location is the air handler, although 
its ground elevation is somewhat higher than the 500-year flood level (Figure 3-
35).

[Begin figures]
Figure 3-34 is a photo showing the ramp entrance to below-grade parking garage. 
Note retaining walls on either side; ramp crests at about 16-inches above the 
500-year flood elevation.

Figure 3-35 is a photo showing the upper floors provide patient care. The top of 
the retaining wall and the air handler are approximately one-foot above the 500-
year flood elevation.
[end figures]

The City of Boulder has experienced severe flooding of Boulder Creek on numerous 
occasions, and is widely known for its efforts to clear portions of the 
floodplain for use as a greenway and public open space. Prompted by concern 
about how effectively the city could respond to serious flooding, in early 2006 
the city developed a scenario that involved catastrophic flooding, bridge 
failures, and numerous flooded buildings and neighborhoods. The drill was 
organized with partners throughout the area, including the Boulder Community 
Foothills Hospital and other health care facilities. 

BCFH, linked to area-wide warnings through NOAA weather radios and the county’s 
emergency management office, had an emergency action plan, but the participation 
in the city’s flood drill resulted in a number of improvements in communications 
and protocols for ensuring patient safety. 

[Begin text box]
The original downtown Boulder Community Hospital, built in the 1940s, is 
partially affected by the 500-year floodplain of Goose Creek. The hospital has 
implemented measures to reduce flooding. The side of the building that is 
susceptible to high water from Goose Creek has two doors that are protected by 
swinging panels permanently mounted in the retaining wall (Figure 3-41). The 
panels are designed with gaskets to create a seal intended to keep water out 
when they are deployed. After consideration of the potential for damage and 
options for protection, a decision was made to leave air-handling equipment at 
grade in the area that is predicted to flood. The equipment provides extra 
ventilation for office space, a function that would not be impaired if the 
equipment was damaged and not functional for a short period of time.

Although not associated with overflow of the creek during wet weather, 
subsurface drainage off the mountains flows through an abandoned sewer that runs 
under the building and often overflows through a manhole in the center of a 
courtyard. To protect the building, sandbags are stockpiled onsite and deployed 
at the two doors that lead from the courtyard into the building.
[end text box]

3.3.5 Dry Floodproofing Considerations 

Dry floodproofing involves a combination of design and special features that are 
intended both to prevent water infiltration and resist flood forces. According 
to the NFIP regulations, non­residential buildings and nonresidential portions 
of mixed-use buildings in A Zones may be dry floodproofed. Areas used for living 
and sleeping purposes in health care facil­ities may not be dry floodproofed 
because of risks to occupants. Although floodproofing of the nonresidential 
spaces is allowed, careful consideration must be given to the possible risk to 
occupants and additional physical damage. Dry floodproofing is not allowed in V 
Zones. 

[Begin text box]
Communities that participate in the NFIP require that a registered professional 
engineer or architect develop or review the structural design, specifications, 
and plans, and certify that the dry floodproofing design and methods of 
construction to be used are in accordance with accepted standards of practice. 
The standards of practice require that the building, together with attendant 
utility and sanitary facilities, be designed so that it is watertight, with 
walls substantially impermeable to the passage of water and with structural 
components having the capability of resisting hydrostatic and hydrodynamic loads 
and effects of buoyancy associated with the design flood event.
[End text box]

Dry floodproofing typically involves structural reinforcement so that exterior 
walls are sufficiently robust to withstand the loads described in Section 3.1.3 
(hydrostatic loads, hydrodynamic loads, wave loads, and debris impact loads). 
Exterior walls must also be designed to prevent infiltration and seepage of 
water, whether through the wall or the openings, including the places where 
utility lines penetrate the envelope. Floodproofed buildings constructed on 
permeable soils require additional design attention, because they are also 
susceptible to hydrostatic pressure from below (buoyancy). An alternative to 
reinforcement of the structure’s walls involves the installation of a permanent 
floodwall that is slightly offset from the exterior of the structure, but 
designed to be integral to the foundation. 

All flood protection measures are designed for certain flood conditions. 
Considering the possibility that the design conditions can be exceeded (i.e., 
water can rise higher than the protective structures) a dry floodproofed 
building may, in such circumstances, sustain catastrophic damage. As a general 
rule, dry floodproofing is a poor choice for new hospitals when avoidance of the 
floodplain or elevation methods to raise the building above the flood level can 
be applied. Floodproofing may be acceptable for retrofitting existing buildings 
under certain circumstances (see Section 3.4.5). 

[Begin text box]
Although dry floodproofing of facilities in Coastal A Zones is allowed by the 
NFIP, designs that comply with the IBC must take into consideration the 
additional forces associated with wave impacts, which may make dry floodproofing 
a less feasible alternative.  
 [end text box]

A number of dry floodproofing limitations and requirements are specified in ASCE 
24:

- Dry floodproofing is limited to areas where flood velocities at the site are 
less than or equal to 5 feet per second.

- If human intervention is required to deploy measures to protect doors and 
windows, the flood warning time shall be a minimum of 12 hours unless the 
community operates a flood warning system and implements a notification 
procedure that provides sufficient time to undertake these measures.

- At least one door satisfying building code requirements for an exit door or 
primary means of escape must be provided above the level of protection.

- An emergency plan, approved by the community and posted in at least two 
conspicuous locations, is required in floodproofed buildings; the plan is 
intended to specify the location of panels and hardware, methods of 
installation, conditions that activate deployment, a schedule for routine 
maintenance of any aspect that may deteriorate over time, and periodic practices 
and drills. 

Windows and doors that are below the flood level used for dry floodproofing 
design present significant potential failure points. They must be specially 
designed units (see Figure 3-36) or be fitted with gasketed, mountable panels 
that are designed for the anticipated flood conditions and loads. Generally 
speaking, it is difficult to protect window and door openings from water more 
than a few feet deep. The framing and connections must be specifically designed 
for these protective measures, or water pressure may cause window and door 
frames to separate from the building. 

[Begin figure]
Figure 3-36 is a photo of permanent watertight doors designed for deep water
Source: Presray Corporation [end figure]

[Begin text box]
The following documents provide additional information about floodproofing: 
Flood Resistant Design and Construction (ASCE 24-05), Flood Proofing: How to 
Evaluate Your Options (USACE, 1993), Flood Proofing Regulations (USACE, 1995), 
Floodproofing Non-Residential Structures (FEMA 102), Non-Residential 
Floodproofing – Requirements and Certification (FIA-TB-3), Flood Proofing 
Systems & Techniques (USACE, 1984). 
[End text box]

Dry floodproofing is required to extend to 1 or 2 feet above the DFE (see Table 
3-1). For the purpose of obtaining NFIP flood insurance, the floodproofing must 
extend at least 1 foot above the BFE, or the premiums will be very high. A 
higher level of protection is recommended. 

Floodproofing techniques are considered to be permanent measures if they are 
always in place and do not require any specific human intervening action to be 
effective. Use of contingent floodproofing measures that require installation or 
activation, such as window shields or inflatable barriers, may significantly 
reduce the certainty that floodproofing will be effective. Rigorous adherence to 
a periodic maintenance plan is critical to ensure proper functioning. The 
facility must have a formal, written plan, and the people responsible for 
implementing the measures must be informed and trained. These measures also 
depend on the timeliness and credibility of the warning. In addition, 
floodproofing devices often rely on flexible seals that require periodic 
maintenance and that, over time, may deteriorate and become ineffective. 
Therefore, a maintenance plan must be developed and a rigorous annual inspection 
and training must be conducted. 

Safety of occupants is a significant concern with dry floodproofed buildings, 
because failure or overtopping of the floodproofing barriers is likely to cause 
catastrophic structural damage. When human intervention is required for 
deploying of barriers, those responsible for implementing the measures remain at 
risk, even if a credible warning system is in place, because of the many 
uncertainties associated with predicting the onset of flood conditions. 

[Begin text box]
Dry floodproofed hospitals must not be considered completely safe for occupancy 
during periods of high water; floodproofing measures are intended only to reduce 
physical damage. 
[end text box]

3.3.6 Flood-Resistant Materials 

All structural materials, nonstructural materials, and connectors that are used 
below certain elevations (see Table 3-1) are to be flood-resistant. Flood-
resistant materials have sufficient strength, rigidity, and durability to 
adequately resist flood loads and damage due to saturation. They are building 
materials that are capable of withstanding direct and prolonged contact with 
floodwaters without sustaining any damage that requires more than cosmetic 
repair. As defined in ASCE 24, the term “prolonged contact” means partial or 
total inundation by floodwaters for 72 hours for non-coastal areas (fresh water) 
or 12 hours for coastal areas. 

In general, materials that are exposed to floodwaters are to be capable of 
resisting damage, deterioration, corrosion, or decay. Typical construction 
materials range from highly resistant to not at all resistant to water damage. 
FEMA NFIP Technical Bulletin FIA-TB-2 contains tables with building materials, 
classified based on flood resistance (Table 3-2).

[Begin text box]
FEMA NFIP Technical Bulletin FIA-TB-2, Flood-Resistant Materials Requirements, 
provides some additional information. Many types of materials and application 
products are classified by degrees of resistance to flood damage. 
[end text box]

[Begin table]
Table 3-2: Classes of Flood-Resistant Materials

NFIP Acceptable
Class 5
Class Description: Highly resistant to floodwater damage. Materials in this 
class are permitted for partially enclosed or outside uses with essentially 
unmitigated flood exposure.

Class 4
Class Description: Resistant to floodwater damage. Materials in this class may 
be exposed to and/or submerged in floodwaters in interior spaces and do not 
require special waterproofing protection.

NFIP Not Acceptable
Class 3
Class Description: Resistant to clean water damage. Materials in this class may 
be submerged in clean water during periods of intentional flooding.

Class 2
Class Description: Not resistant to water damage. Materials in this class 
require essentially dry spaces that may be subject to water vapor and slight 
seepage.

Class 1
Class Description: Not resistant to water damage. Materials in this class 
require dry conditions.

Source: From U.S. Army Corps of Engineers, FloodProofing Regulations (1995).
[End table

In coastal areas, airborne salt aerosols and inundation with saline water 
increase the potential for corrosion of some metals. Structural steel and other 
metal components that are exposed to corrosive environments should be stainless 
steel or hot-dipped galvanized after fabrication.

In areas away from the coast, exposed structural steel should be primed, coated, 
plated, or otherwise protected against corrosion. Secondary components such as 
angles, bars, straps, and anchoring devices, as well as other metal components 
(plates, connectors, screws, bolts, nails angles, bars, straps, and the like) 
should be stainless steel or hot-dipped galvanized after fabrication. 

Concrete and masonry that are designed and constructed in compliance with 
applicable standards are generally considered to be flood-resistant. However, 
masonry facings are undesirable finishes unless extra anchoring is added to 
prevent separation (see Figure 3-37). Wood and timber members exposed to 
floodwaters should be naturally decay-resistant species, or pressure treated 
with appropriate preservatives. 

[Begin figure]
Figure 3-37 is a photo of brick facing separated from masonry wall (Hurricane 
Katrina, 2005). [end figure]

3.3.7 Access Roads 

Roads and entrances leading to hospitals should be designed to provide safe 
access at all times, to minimize impacts on flood hazard areas, to minimize 
damage to the road itself, and to minimize exposing vehicles to dangerous 
situations. Even if the hospital is elevated and protected from flood damage, 
when access is impaired, functionality is also impaired. Planners and designers 
should take the following factors into consideration.

Safety factors: Although a hospital’s access road off the primary surface street 
may not be required to carry regular traffic like other streets, a flood-prone 
road always presents a degree of risk to public safety. To minimize those risks, 
some State or local regulatory authorities require that access roads be designed 
so that the driving surface is at the DFE, or no more than 1 to 2 feet below the 
DFE. At a minimum, a hospital’s access road should be at least as high as the 
adjacent public road, so that the same level of access is provided during 
conditions of flooding. To maximize evacuation safety, two separate access roads 
to different feeder roads are recommended. In some circumstances, especially 
long-duration flooding where a hospital is built on fill, access roads designed 
to be above flood levels would help the hospital to continue its operations. 

Floodplain impacts: Engineering analyses may be required to determine the 
effects on flood elevations and flow patterns if large volumes of fill are 
required to elevate a road to minimize or eliminate flooding above the driving 
surface.

Drainage structure and road surface design: The placement of multiple drainage 
culverts, even if not needed for local drainage, can facilitate the passage of 
floodwaters and minimize the potential for a road embankment to act as a dam. 
Alternatively, an access road can be designed with a low section over which high 
water can flow without causing damage. Embankments should be designed to remain 
stable during high water and as waters recede. They should be sloped and 
protected to resist erosion and scour. Similarly, the surface and shoulders of 
roads that are intended to flood should be designed to resist erosion. The 
increased resistance to erosion may be accomplished by increasing the thickness 
of the road base.

3.3.8 Utility Installations 

Utilities associated with new hospitals in flood hazard areas must be protected 
either by elevation or special designs and installation measures. Utilities 
subject to this provision include all systems, equipment, and fixtures, 
including mechanical, electrical, plumbing, heating, ventilating, and air 
conditioning. Potable water systems (wellheads and distribution lines) and 
wastewater collection lines are addressed in Section 3.3.9. 

[Begin text box]
For more information on utility installations, see Protecting Building Utilities 
from Flood Damage: Principles and Practices for the Design and Construction of 
Resistant Building Utility Systems (FEMA 348).
[End text box]

Utility systems and equipment are best protected when elevated above the DFE 
(plus freeboard, if required). Equipment that is required for emergency 
functioning during or immediately after an event, such as emergency generators 
and fuel tanks, is best installed well above the DFE. In some cases, equipment 
can be located inside protective floodproofed enclosures, although it must be 
recognized that if flooding exceeded the design level of the enclosure, the 
equipment would be adversely affected (see Figure 3-38). De­signers should pay 
particular attention to underfloor utilities and ductwork to ensure that they 
are properly elevated. Plumbing conduits, water supply lines, gas lines, and 
electric cables that must extend below the DFE should be located, anchored, and 
protected to resist the effects of flooding. Equipment that is outside of 
elevated building also must be elevated:

- In A Zones, equipment may be affixed to raised support structures or mounted 
on platforms that are attached to or cantilevered from the primary structure. 

- In V Zones and Coastal A Zones, equipment may be affixed to raised support 
structures designed for the flood conditions (waves, debris impact, erosion, and 
scour) or mounted on platforms that are attached to or cantilevered from the 
primary structure. If an enclosure is constructed under the elevated building, 
the designer must take care that utilities and attendant equipment are not 
mounted on or do not pass through walls that are intended to break away. 

Although it is difficult to achieve, the model building codes and NFIP 
regulations provide an alternative that allows equipment to be located below the 
DFE. This alternative requires that such equipment be designed, constructed, and 
installed to prevent floodwaters from entering or accumulating within the 
components during flood events. 

Figure 3-38 is a photo of an equipment room with watertight door
SOURCE: Presray Corporation [end figure]

3.3.9 Potable Water and Wastewater Systems

New installations of potable water systems and wastewater collection systems are 
required to resist flood damage, including damage associated with infiltration 
of floodwaters and discharge of effluent. Health concerns arise when water 
supply systems are exposed to floodwaters. Contamination from flooded sewage 
systems poses additional health and environmental risks. Onsite water supply 
wellheads should be located on land elevated from the surrounding landscape to 
allow contaminated surface water and runoff to drain away. Well casings should 
extend above the DFE, and casings should be sealed with a tight-fitting, 
floodproof, and vermin-proof well cap. The space between the well casing and the 
side of the well must be sealed to minimize infiltration and contamination by 
surface waters. 

Sewer collection lines should be located and designed to avoid infiltration and 
backup due to rising floodwaters. Devices designed to prevent backup are 
available and are recommended to provide an added measure of protection. 

Onsite sewage systems usually are not used as the primary sewage disposal 
systems for new hospitals. However, owners, planners and designers should 
consider a backup onsite system if a facility’s functionality can be impaired 
when the public system is affected by flooding. Local or State health 
departments may impose constraints that limit or prevent locating septic fields 
in floodplain soils or within a mapped flood hazard area. If allowed, septic 
fields should be located on the highest available ground to minimize inundation 
and damage by floodwaters. An alternative to a septic field is installation of a 
holding tank that is sized to contain wastewater for a period of time, perhaps a 
few days, when the municipal system is out of service.

3.3.10 Storage Tank Installations 

Aboveground and underground storage tanks located in flood hazard areas must be 
designed to resist flotation, collapse, and lateral movement. ASCE 24 specifies 
that aboveground tanks be elevated or constructed, installed, and anchored to 
resist at least 1.5 times the potential buoyant and other flood forces under 
design flood conditions, assuming the tanks are empty. Similarly, underground 
tanks are to be anchored to resist at least 1.5 times the potential buoyant 
forces under design flood conditions, assuming the tanks are empty. In all 
cases, designers are cautioned to address hydrodynamic loads and debris impact 
loads that may affect tanks that are exposed to floodwaters. Vents and fill 
openings or cleanouts should be elevated above the DFE or designed to prevent 
the inflow of floodwaters or outflow of the contents of tanks. 

3.3.11 Accessory Structures 

Depending on the type of accessory structures, full compliance with floodplain 
management regulations is appropriate and may be required. For example, 
mechanical buildings, storage buildings, and buildings used for ancillary 
purposes, such as medical offices and therapy clinics, are not considered to be 
accessory in nature and must be elevated and protected to the same standards as 
other buildings. 

Some minor accessory structures need not fully comply, but may be “wet 
floodproofed” using techniques that allow them to flood while minimizing damage. 
Accessory structures must be anchored to resist flotation, collapse, and lateral 
movement. Flood-resistant materials must be used and utilities must be elevated 
above the DFE (plus freeboard, if required). In A Zone flood hazard areas, 
openings in walls must be provided to allow the free inflow and outflow of 
floodwaters to minimize the hydrostatic loads that can cause structural damage. 
Because wet floodproofed accessory buildings are designed to flood, hospital 
staff must be aware that contents will be damaged. 



3.4 RISK REDUCTION FOR EXISTING HOSPITALS

3.4.1 Introduction

Section 3.2 describes the type of damage that can be sustained by hospitals that 
already are located in flood hazard areas. The vulnerability of these facilities 
can be reduced, if they can be made more resistant to flood damage. 
Decisionmakers may take such action when flood hazards are identified and there 
is a desire to undertake risk reduction measures proactively. Interest may be 
prompted by a flood or by the requirement to address flood resistance as part of 
proposed substantial improvement or an addition. Some questions and guidance 
intended to help identify building characteristics of importance when 
considering risk reduction measures for existing facilities are included in the 
checklist in Section 3.6. 

Work on existing buildings and sites is subject to codes and regulations, and 
the appropriate regulatory authority with jurisdiction should be consulted. With 
respect to reducing flood risks, work generally falls into the categories 
described in the following subsections.

[Begin text box]
Owners and operators of public and not-for-profit hospitals should be aware of 
the importance of flood insurance coverage for facilities that are located in 
the flood hazard areas shown on NFIP maps. If not insured for flood peril, the 
amount of flood insurance that should have been in place will be deducted from 
any Federal disaster assistance payment that would otherwise have been made 
available. A particular facility may have to absorb up to $1 million in un-
reimbursable flood losses per building, because the NFIP offers $500,000 in 
building coverage and $500,000 in contents coverage for nonresidential buildings 
(coverage limits as of early 2006). 
[end text box]

3.4.2 Site Modifications

Modifying the site of an existing facility that is subject to flooding requires 
careful examination by an experienced professional engineer. Determining the 
suitability of a specific measure requires a complex evaluation of many factors, 
including the nature of flooding and the nature of the site. The first part of 
Table 3-3 in Section 3.5 identifies elements that influence the choice of 
mitigation measures applicable to existing sites. Some flood characteristics may 
make it infeasible to apply site modification measures to existing facilities 
(e.g., depths greater than 3 to 4 feet, very high velocities, insufficient 
warning because of flash flooding or rapid rate of rise, and very long 
duration). In Coastal A Zones, wave conditions must be accounted for in design 
of site modifications. Such modifications are not allowed in V Zones. 

A common problem with all site modifications is the matter of access. Depending 
on the topography of the site, construction of barriers to floodwaters may 
require special access points. Access points may be protected with manually 
installed stop-logs or designed gates that drop in, slide, or float into place. 
Whether activated by automatic systems or manually operated, access protection 
requires sufficient warning time.

Other significant constraining factors include poor soils and insufficient land 
area, which can make site modifications either infeasible or very costly. For 
any type of barrier, rainfall that collects on the dry side must be accounted 
for in the design, whether through adequately sized stormwater storage basins 
set aside for this purpose, or by providing large-capacity pumps to move 
collected drainage to the water side of the barrier.

Each of these site modification measures described below has limitations, 
including the fact that floods larger than the design flood will exceed the 
level of protection.

Regrading the site (berm): Regrading of the site, or the construction of an 
earthen berm, may provide adequate protection for situations in which a facility 
is exposed to relatively shallow flooding, and sufficient land area is 
available.

Earthen levee: Earthen levees are engineered structures that are designed to 
keep water away from certain areas and buildings. Hydraulic analyses and 
geotechnical investigations are required to determine their feasibility and 
effectiveness. The use of earthen levees to protect existing facilities is 
constrained by the availability of land (levees have a large “footprint” and 
require large land areas), cost (including availability of suitable fill 
material and long-term maintenance), and access difficulties. Locating levees 
and floodwalls within a designated floodway is generally not allowed. Rapid 
onset flooding makes it impractical to design a flood levee with access points 
that require installation of a closure system. Additionally, high velocity flows 
can cause erosion and reduce the stability of earthen levees. 

Permanent floodwall: Floodwalls are freestanding, permanent engineered 
structures designed to prevent encroachment of floodwaters. Typically, a 
floodwall is located some distance from a building, so that structural 
modification of the existing building is not required. Depending on the 
topography of the site, floodwalls may protect only the low side (in which case 
they must “tie” into high ground) or completely surround a site (which may 
affect access because special closure structures are required and must be 
installed before the onset of flooding, see Figure 3-39).

[Begin figure]
Figure 3-39 is a photo of a masonry floodwall with multiple engineered openings 
in Fargo, ND during flooding in 2001.
SOURCE: Flood Central America, LLC [end figure]

Mobilized floodwall: This category of flood protection measures includes fully 
engineered flood protection structures that have permanent features (foundation 
and vertical supports) and features that require human intervention when a flood 
is predicted (horizontal components called planks or stop-logs). Mobilized 
floodwalls have been used to protect entire sites, or to tie into permanent 
floodwalls or high ground. Because of the manpower and time required for proper 
placement, these measures are better suited to conditions that allow long 
warning times.

3.4.3 Additions 

Model building codes generally treat additions as new construction, and require 
hospital additions in flood hazard areas to be elevated or dry floodproofed to 
minimize exposure to flooding. However, full compliance with the code and NFIP 
requirements is only required if an addition is a substantial improvement (i.e., 
the cost of the addition plus all other costs associated with the work equal or 
exceed 50 percent of the market value of the building, see Section 3.1.6.1 and 
Section 3.1.6.2). Designers are cautioned that even the existing buildings may 
be required to comply with the flood-resistant provisions of the code or local 
ordinances, if the addition is structurally connected to the existing building 
and is determined to be a substantial improvement. 

[Begin text box]
For more information on additions and substantial improvements, see Answers to 
Questions About Substantially Damaged Buildings (FEMA 213).
[End text box]

Section 3.3.4 outlines foundation methods used to elevate buildings that also 
are applicable to additions. Elevation of an addition on fill may not be 
feasible unless structural fill can be placed adjacent to the existing building. 
Utility service equipment for additions must meet the requirements for new 
installations (see Section 3.3.7).

If an evaluation determines that dry floodproofing is appropriate, additions may 
be floodproofed (see Section 3.3.5). To provide adequate protection for the 
addition, floodproofing must be applied to all exterior walls and the wall 
adjoining the existing building. Openings, including doors between the addition 
and existing building, must also be protected.

With respect to code compliance and designing additions to resist flood damage, 
one of the more significant issues to be considered is ease of access. If the 
lowest floor of the existing facility is below the DFE, steps, ramps, or 
elevators will be required for the transition to the new addition (See Figure 3-
40). Some jurisdictions may contemplate allowing variances to the requirement 
for elevation, because alternative means of access are available, such as ramps 
and elevators. Under the regulations of the NFIP and FEMA guidance, it is not 
considered appropriate to grant such a variance. 

[Begin figure]
Figure 3-40 is a photo of how Tampa General Hospital solved the problem of 
access to the new elevated Emergency Department by designing a vehicle ramp. 
Visitors and ambulatory patients take elevators from the ground floor. 
Source: Tampa General Hospital [end figure]

3.4.4 Repairs, Renovations, and Upgrades

Every hospital considered for upgrades and renovations, or being repaired after 
substantial damage from any cause, must be examined for structural integrity and 
stability to determine compatibility with structural modifications that may be 
required to achieve acceptable performance. When an existing facility is located 
in a flood hazard area, that examination should include consideration of 
measures to improve resistance to flood damage and to reduce risks. 

The model building codes and the regulations of the NFIP require that work 
constituting “substantial improvement” of an existing building be in compliance 
with the flood-resistant provisions of the code. Non-substantial improvements 
should take into account measures to reduce future flood damage, such as those 
described in Section 3.3, emergency measures (see Section 3.4.9), and wet 
floodproofing measures that allow water to enter the building to avoid 
structural damage. 

[Begin text box]
Additional information on rehabilitation of existing buildings is provided in: 
Flood Proofing: How to Evaluate Your Options (USACE, 1993), Floodproofing Non-
Residential Structures (FEMA 102), Floodproofing—Requirements and Certification 
(FIA-TB-3), and Engineering Principles and Practices for Retrofitting Flood-
prone Buildings (FEMA 259). Although written primarily for homes, this last 
reference contains very detailed checklists and worksheets that can be modified. 
They also provide some guidance for evaluating the costs and benefits of various 
measures.
[End text box]

Compliance with flood-resistant provisions means that the existing building must 
be elevated or dry floodproofed. Both options can be difficult for existing 
hospitals, given the typical use, size, and complexity of some of these 
buildings. Retrofit dry floodproofing (described in Section 3.4.5) is generally 
limited to water depths of 3 feet or less, provided an assessment by a qualified 
design professional determines that the building is capable of resisting the 
anticipated loads, or can be modified to provide that level of performance. 

Elevating an existing building presents an entirely different set of challenges 
and also requires detailed structural engineering analyses. It involves the same 
equipment and methods used to move other types of buildings; expert building 
movers have successfully moved large, heavy, and complex buildings, sometimes by 
segmenting them. A building that is elevated in-place must meet the same 
performance standards set for new construction.

3.4.5 Retrofit Dry Floodproofing 

Modification of an existing building may be required or desired in order to 
address exposure to design flood conditions. Modifications that may be 
considered include construction of a reinforced supplementary wall, measures to 
counter buoyancy (especially if there is below-grade space), installation of 
special watertight door and window barriers (see Figure 3-41), and providing 
watertight seals around the points of entry of utility lines. The details of 
structural investigations and structural design of such protection measures are 
beyond the scope of this manual. 

[Begin figure]
Figure 3-42 is a photo showing how Boulder Community Hospital, Boulder, CO, 
installed this permanently mounted floodgate in a low floodwall; the floodgate 
swings to the left to keep water away from the mechanical equipment room. [end 
figure]

[Begin text box]
“Dry floodproofing” refers to measures and methods to render a building envelope 
substantially impermeable to floodwater. 
[End text box]

Because of the tremendous flood loads that may be exerted on a building not 
originally designed to keep water out, detailed structural engineering 
evaluations are required to determine whether an existing building can be dry 
floodproofed. The following elements must be examined:

- The strength of the structural system

- Whether non-load bearing walls can resist anticipated flood loads; secondary 
walls can be constructed immediately adjacent to existing walls, with a 
waterproof membrane, to provide adequate strength

- The effects of hydrostatic pressures on the walls and floors of below-grade 
areas

- Effective means to install watertight doors and windows, or mountable panels

- Protection where utilities enter the building

- Methods to address seepage, especially where long-duration flooding is 
anticipated

- Whether there is sufficient time for deployment of measures that require human 
intervention, given the availability of official warnings of predicted flood 
conditions

Application of waterproofing products or membranes directly to exterior walls 
may minimize infiltration of water; although there are concerns with durability 
and limitations on use (this measure is most effective for shallow, short-
duration flooding). Some protection can be achieved using emergency measures 
that are not designed to be integral to the building (see Section 3.4.9). 

3.4.5.1 The Case of Pungo District Hospital, Belhaven, North Carolina

The Pungo District Hospital has served the waterfront town of Belhaven, North 
Carolina, and the surrounding area for nearly 60 years (Figure 3-42). The 
facility is only about 100 feet back from the waters of a canal and Pantego 
Creek, a tidal tributary to the Pungo River in Beaufort County. As with the rest 
of coastal North Carolina, Beaufort County has seen more than its share of 
hurricanes. Notable named storms that affected the area in just the past decade 
include Hurricanes Fran (1996), Dennis (1999), Floyd (1999), and Isabel (2003).

[Begin figure]
Figure 3-42 is a photo Pungo District Hospital is situated adjacent to a canal 
and Pantego Creek, a tidal tributary to the Pungo River in eastern North 
Carolina. [end figure]

With about 20,000 residents in its service area, Pungo District Hospital offers 
49 beds for acute care, transitional care, intensive care, and ventilation care 
services. Outpatient clinics and programs include a cardiology clinic, 
nephrology clinic, pulmonary clinic, EKG/EEG, home health, sleep apnea program, 
speech therapy, laboratory medicine, imaging services, cardio-pulmonary 
services, nutritional counseling, and patient education. The 175 employees 
(full- and part-time) include 108 medical and 67 non-medical staff. The facility 
encompasses a total of 57,000 square feet, of which 84 percent is used for 
patient care and the remainder is used for administrative and other support 
services.

The original one-story hospital building was built on piers in 1949, in the 
traditional crawlspace style that typifies older buildings in areas with high 
groundwater and humid conditions. A number of one-story additions were 
constructed in the 1960s and 1970s. The most recent work, started in 1997, 
consisted of two additions that expanded the facility and renovation of a large 
portion of the hospital. With the exception of the mechanical room, the 
additions were built to match the original floor elevation using masonry block 
stemwall foundations (perimeter walls with slab-on-structural fill). The floor 
of the mechanical room is approximately 18-inches lower than the main floor 
elevation, but does not extend below-grade. 

The Town of Belhaven has participated in the NFIP since the mid-1970s, 
administering an ordinance that requires new buildings and substantially 
improved buildings to comply with certain requirements to reduce exposure to 
flood hazards. The predicted BFE along the waterfront is 8-feet above mean sea 
level. Worst-case hurricane surge flooding is likely to rise even higher. 
Observations in the past decade indicate that flooding may last from 12 to 18 
hours, largely as a function of the path of a hurricane and tidal cycle. 

The ground elevation around the hospital is about 4.5-feet above mean sea level, 
indicating the site would experience 3.5-feet of flooding during the base flood. 
The floor of the main building is nearly 2.5-feet above-grade; thus, the 100-
year flood would reach a level approximately 1-foot above the floor. Because the 
floor of the mechanical room is lower than in the main building, the 100-year 
flood would flood the room with about 3-feet of water.

In 1997, the City of Belhaven determined that the proposed expansions and other 
work in the main building would be a substantial improvement (the cost of the 
proposed work exceeded 50 percent of the market value of the building). This 
determination triggered compliance with the city’s building code and floodplain 
management ordinance. Two alternatives were available to bring the building into 
compliance: elevating the existing building and additions nearly 18 inches (so 
the floor level would be at the flood level) or retrofitting the building by dry 
floodproofing. To address this requirement, the hospital hired an engineering 
company to examine the existing building and the proposed new addition, and to 
design appropriate floodproofing measures. 

The engineer’s examination determined that the exterior walls of the existing 
building were sufficiently strong to resist the flood loads anticipated during 
the 100-year flood conditions, provided the entrances and other openings through 
the exterior wall were sealed to prevent entry of water. The dry floodproofing 
measures proposed entailed construction of low concrete walls in certain areas 
and installation of specially fabricated frames and metal panels for some doors, 
floodwall openings, and crawlspace ventilation openings (Figures 3-43 and 3-44). 
The top of the concrete wall and the tops of the panels were set about 2 feet 
higher than the BFE, providing 2 feet of freeboard as a factor of safety. To 
provide additional protection to the new addition, a rubber membrane was 
installed between the brick facing and the block wall. The total cost of the 
floodproofing measures was $125,000 (1997 dollars). 

[Begin figures]
Figure 3-43 is a photo showing how a special floodproofing panel is manually 
installed in this concrete floodwall to keep floodwater away from the rear 
courtyard and patient rooms. 

Figure 3-44 is a photo showing how the ventilation openings for the original 
building’s crawlspace are protected with floodproofing panels that are bolted in 
place when flooding is predicted.
[End figures}

The frames that hold the floodproofing panels are permanently-mounted and sealed 
against the building to prevent seepage. The frames are designed with an 
aluminum cover to keep the channels free of dirt and debris. The metal panels 
that fit into the channels have rubber gaskets that are inspected each year as 
part of the hospital’s routine maintenance program. The manufacturer recently 
advised replacement of the gaskets, as the 10-year anniversary of installation 
nears.

A total of 15 panels of different sizes are stored onsite (Figure 3-45). 
Although the panels can be handled by two people (Figure 3-46), the hospital has 
an on-call agreement with a local rental company to use a forklift to facilitate 
moving them into place. All panels can be installed in about 3 hours.

[Begin figures]
Figure 3-45 is a photo showing how large floodproofing panels are stored at the 
hospital, ready for installation when coastal flooding is predicted.

Figure 3-46 is a photo showing how most of the panels can be installed by hand. 
Note the protective metal strip leaning against the building on the left; 
normally this strip covers the horizontal channel into which the panel is 
inserted. 
[End figures]

Pungo District Hospital fully recognizes its vulnerability to coastal storms and 
the importance of protecting its patients as well as providing services after a 
major event. The facilities services director monitors weather throughout the 
hurricane season, coordinating with the county emergency services and town 
officials responsible for issuing evacuation notices. 

The detailed evacuation plan was triggered as Hurricane Isabel approached in 
September 2003. In less than 4 hours all patients, staff, and supplies were 
relocated safely to the Beaufort Memorial Hospital, about 30 miles inland. Total 
shutdown of the building, including installation of the floodproofing panels, 
took about 12 hours. An emergency generator, located inside the protected area, 
was activated in order to run four sump pumps to handle groundwater that seeps 
into the crawlspace of the original building. Isabel’s storm surge produced the 
highest flooding on record in Belhaven, yet the hospital weathered the storm 
without damage.

Reoccupation of the hospital began when the town cleared the streets and all 
systems of the building were brought back online. Municipal water and sewer 
services were not interrupted by flooding.

Despite decades of weathering hurricanes, the Pungo District Hospital has not 
sustained major wind damage. The absence of trees or other buildings between the 
hospital and the water prevents major wind-borne debris damage. Water-borne 
debris is deposited around the facility every time high water crests the 
bulkhead that lines creek. One corner of the parking lot, which extends nearly 
to the bulkhead, sustained damage as Hurricane Isabel’s rising water lifted and 
displaced the asphalt (Figure 3-47). 

[Begin figure]
Figure 3-47 is a photo showing how high water and waves eroded the slope between 
the parking lot and the bulkhead and shifted a portion of the asphalt. [End 
figure]

3.4.6 Utility Installations 

Some features of utility systems in existing hospitals prone to flooding may 
need to be modified to reduce damage. The effectiveness of such measures depends 
not only on the nature of the flooding, but the type of service and the degree 
of exposure. Table 3-3 in Section 3.5 lists some questions to help facility 
planners and designers examine risk reduction measures. 

Even if a facility is unlikely to sustain extensive structural damage from 
flooding, significant recovery costs and interruption of operations may result 
if utility systems are damaged. The damage reduction measures described below 
can be applied, whether undertaken as part of large-scale retrofits of existing 
buildings or as separate projects.

Relocate from below-grade areas: The most vulnerable utility installations are 
those located below grade, and the most effective protection measure is to 
relocate them to higher floors or platforms that are at least 2 feet above the 
DFE. The complexity of rerouting pipes, conduits, ductwork, electrical service, 
lines, and connections will depend on building- and site-specific factors.

[Begin text box]
Additional guidance on improving the flood resistance of utility installations 
in existing buildings is found in FEMA 348, Protecting Building Utilities From 
Flood Damage: Principles and Practices for the Design and Construction of Flood 
Resistant Building Utility Systems.
[End text box]

Elevate components: Whether located inside or outside of the building, some 
components of utility systems can be elevated-in-place on platforms, including 
electric transformers, communication switch boxes, water heaters, air-
conditioning compressors, generators, furnaces, boilers, and heat pumps (see 
Figure 3-48). 

[Begin figure]
Figure 3-48 is a photo showing an elevated utility box [end figure]

Anchor tanks and raise openings: Existing tanks can be elevated or anchored, as 
described in Section 3.4.10. If anchored below the DFE, tank inlets, vents, fill 
pipes, and openings should be elevated above the DFE, or fitted with covers 
designed to prevent the inflow of floodwaters or outflow of the tank’s contents.

Protect components: If utility components cannot be elevated, it may be feasible 
to construct watertight enclosures, or enclosures with watertight seals that 
require human intervention to install when flooding is predicted.

Elevate control equipment: Control panels, gas meters, and electrical panels can 
be elevated, even if the equipment they service cannot be protected.

Separate electrical controls: Where areas within an existing facility are flood-
prone, separation of control panels and electrical feeders will facilitate 
shutdown before floodwaters arrive, and help protect workers during cleanup.

Protect against electrical surges: Current fluctuations and service 
interruptions are common in areas affected by flooding. Equipment and sensitive 
electrical components can be protected by installing surge protection and 
uninterruptible power supplies.

Connections for portable generators: Prewired portable generator connections 
allow for quick, failure-free connection and disconnection of the generators 
when needed for continued functionality.

3.4.7 Potable Water and Wastewater Systems

All plumbing fixtures connected to the potable water system may become weak 
points in the system if they allow floodwaters to contaminate the system. 
Relocating the fixtures and services that require plumbing to elevated floors 
and removing the fixtures that are below the DFE provides protection. Wellheads 
can be sealed with watertight casings or protected within sealed enclosures. 

Wastewater system components become sources of contamination during floods. 
Rising floodwaters may force untreated sewage to backup through toilets. 
Specially designed devices that prevent back-flow can be installed, or restrooms 
below the DFE can be provided with overhead piping that may require specially 
designed pumps to operate properly. Septic tanks can be sealed and anchored. 

3.4.8 Other Damage Reduction Measures 

A number of steps can be taken to make existing facilities in flood hazard areas 
more resistant to flood damage, which also facilitates rapid recovery, cleanup, 
and timely return to normalcy. Whether these measures are applicable to a 
specific facility depends, in part, on the characteristics of the flood hazard 
and the characteristics of the building itself. Facility planners and designers 
should consider the following:

- Rehabilitate and retrofit the building envelope with openings specifically 
designed to allow floodwaters to flow in and out to minimize hydrostatic 
pressure on walls (called wet floodproofing). Although it allows water to enter 
the building, this measure minimizes the likelihood of major structural damage. 
Walls that enclose interior spaces would also be retrofitted with openings.

- Replace interior walls that have cavities with flood-resistant construction or 
removable panels to facilitate cleanup and drying.

- Abandon the use of below-grade areas (basements) and fill them in to prevent 
structural damage.

- Permanently relocate high-value or sensitive functions that are often found on 
the ground floor of hospitals (e.g., offices, records, libraries, and computer 
laboratories) to higher floors or elevated additions.

- Install backflow devices in sewer lines.

- Preplan actions to move high-value contents from the lower floors to higher 
floors when a flood warning is issued.

- Replace wall, flooring, and finish materials with flood-resistant materials. 
Concrete floors with a sealed, polished, or terrazzo finish have few maintenance 
requirements, but tend to be slippery when wet.

- Use epoxy or other impervious paints on concrete and other permeable surfaces 
to minimize contamination. 

- Install separate electric circuits and ground fault interrupter circuit 
protection in areas that will flood. Emergency measures should be provided so 
that electrical service can be shut down to avoid electrocution hazards.

- Relocate chemicals to storage areas not subject to flooding.

3.4.9 Emergency Measures

Emergency response to flooding is outside the scope of this manual. However, it 
is appropriate to examine feasible emergency measures that may provide some 
protection. The following discussion pertains only to emergency measures that 
have been used to reduce flood damage to older buildings that are already 
located in flood hazard areas. They may not provide protection to occupants and 
they can experience a high frequency of failure depending on human factors 
related to deployment. These measures do not achieve compliance with building 
and life safety codes for new construction. 

Emergency barriers are measures of “last resort,” and should be used only when a 
credible flood warning with adequate lead-time is available and dependable. 
These measures have varying degrees of success, depending on the available 
manpower, skill required, long-term maintenance of materials and equipment, 
suitability for site-specific flood conditions, and having sufficient advanced 
warning. Complete evacuation of protected buildings is appropriate, as these 
measures should not be considered adequate protection for occupants. 

Sandbag walls: Unless emergency placement is planned well in advance or under 
the direction of trained personnel, most sandbag barriers are not constructed in 
accordance with proper practices, leading to leakage and failures. Because of 
the intensive work effort and length of time required for protection even from 
relatively shallow water, sandbag walls are not a reliable protection measure. 
To be effective, sandbags and sand should be stockpiled and checked regularly to 
ensure that sandbags have not deteriorated. Sandbags have some other drawbacks, 
including high disposal costs and their tendency to absorb pollutants from 
contaminated floodwaters, which necessitates disposal as hazardous waste.

Water-filled barriers: A number of vendors make water-filled barriers that can 
be assembled with relative ease, depending on the source of water for filling. 
The barriers must be specifically sized for the site. Training and annual drills 
are important so that personnel know how to place and deploy the barriers. 
Proper storage, including cleaning after deployment, is necessary to protect the 
materials over long periods of time.

Panels for doors: For shallow and short-duration flooding, panels of sturdy 
material can be made to fit doorways to minimize the entry of floodwaters, 
although failure is common (see Figure 3-49). Effectiveness is increased 
significantly if a flexible gasket or sealant is provided, and the mounting 
hardware is designed to apply even pressure. Personnel must know where the 
materials are stored and be trained in their deployment. A number of vendors 
make special doors for permanent installation and drop-in panels or barriers 
that are designed to be watertight.

[Begin figure]
Figure 3-49 is a photo showing flooding at Hancock Medical Center during 
Hurricane Katrina. Source: Hancock Medical Center [end figure]



3.5 CHECKLIST FOR BUILDING VULNERABILITY OF FLOOD-PRONE HOSPITALS

The Checklist for Building Vulnerability of Flood-Prone Hospitals (Table 3-3) is 
a tool that can be used to help assess site-specific flood hazards and building 
vulnerability. The checklist is useful during site selection, preliminary design 
of a new building, or when considering rehabilitation of an existing facility. 
In addition to examining building design issues that affect vulnerability, the 
checklist also helps users to examine the functionality of the crit­ical and 
emergency systems upon which most hospitals depend. The checklist is organized 
into separate sections, so that each section can be assigned to a subject expert 
for greater accu­racy of the examination. The results should be integrated into 
a master vulnerability assessment to guide the design process and the choice of 
appropriate mitigation measures.

[Begin table]
Table 3-3: Checklist for Building Vulnerability of Flood-Prone Hospitals

Vulnerability Section: Site Condition

Vulnerability: Is the site located near a body of water (with or without a 
mapped flood hazard area)? 
Is the site in a flood hazard area shown on the community’s map (FIRM or other 
adopted map)? If so, what is the flood zone?
Is the site affected by a regulatory floodway?

Guidance: All bodies of water are subject to flooding, but not all have been 
designated as a floodplain on FIRMs. 
Flood hazard maps usually are available for review in local planning and permit 
offices. Electronic versions of the FIRMs may be available online at 
www.fema.gov. Paper maps may be ordered by calling (800) 358-9616.
Development in floodways, where floodwaters typically are faster and deeper, 
must be supported by engineering analyses that demonstrate no rise in flood 
levels

Vulnerability: Is the site located in a storm surge inundation zone (or tsunami 
inundation area)?

Guidance: In coastal communities, even sites at some distance inland from the 
shoreline may be exposed to extreme storm surge flooding. Storm surge maps may 
be available at State or local emergency management offices.

Vulnerability: What is the DFE (or does an analysis have to be done to determine 
the DFE)? What is the minimum protection level required by regulatory 
authorities?
Does the FIS or other study have information about the 500-year flood hazard 
area?
Has FEMA issued post-disaster advisory flood elevations and maps?
What are the expected depths of flooding at the site (determined using flood 
elevations and ground elevations)?

Guidance: Reference the FIS for flood profiles and data tables. Site-specific 
analyses should be performed by qualified engineers. 
Check with regulatory authorities to determine the required level of protection.
If a major flood event has affected the community, FEMA may have issued new 
flood hazard information, especially if areas not shown on the FIRMs have been 
affected. Sometimes these maps are adopted and replace the FIRMs; sometimes the 
new data are advisory only.

Vulnerability: Has the site been affected by past flood events? What is the 
flood of record? 

Guidance: Records of actual flooding augment studies that predict flooding, 
especially if historic events resulted in deeper or more widespread flooding. 
Information may be available from local planning, emergency management, and 
public works agencies, or State agencies, the U.S. Army Corps of Engineers, or 
the Natural Resources Conservation Service.
The flood of record is often a lower probability event (with higher flood 
elevations) than the 100-year flood.

Vulnerability: What is the expected velocity of floodwaters on the site?

Guidance: Velocity is a factor in computing loads associated with hydrodynamic 
forces, including drag on building surfaces. Approximations of velocity may be 
interpolated from data in the FIS Floodway Data Table if the waterway was 
studied using detailed methods, application of approximation methods based on 
continuity, local observations and sources, or site-specific studies.

Vulnerability: Are waves expected to affect the site?

Guidance: Waves can exert considerable dynamic forces on buildings and 
contribute to erosion and scour. Wind-driven waves occur in areas subject to 
coastal flooding and where unobstructed winds affect wide floodplains (large 
lakes and major rivers). Standing waves may occur in riverine floodplains where 
high velocities are present.

Vulnerability: Is there information on how quickly floodwaters may affect the 
site?
What is the expected duration of flooding?

Guidance: Warning time is a key factor in the safe and orderly evacuation of 
critical facilities. Certain protective measures may require adequate warning so 
that actions can be taken by skilled personnel.
Duration has bearing on the stability of earthen fills, access to a site and 
emergency response, and durability of materials that come into contact with 
water. Records of actual flooding are the best indicator of duration as most 
floodplain analyses do not examine duration. 

Vulnerability: Is there a history of flood-related debris problems or erosion on 
the site?

Guidance: Site design should account for deposition of debris and sediment, as 
well as the potential for erosion-related movement of the shoreline or waterway. 
Buildings exposed to debris impact or undermining by scour and erosion should be 
designed to account for these conditions. 

Vulnerability: Is the site within an area predicted to flood if a levee or 
floodwall fails or is overtopped? 
Is the site in an area predicted to be inundated if an upstream dam were to 
fail?

Guidance: Flood protection works may be distant from sites and not readily 
observable. Although a low probability event, failure or overtopping can cause 
unexpected and catastrophic damage because the protected lands are not regulated 
as flood hazard areas.
The effects of an upstream dam failure are not shown on the FIRMs or most flood 
hazard maps prepared locally. Although dam failure generally is considered an 
unlikely event, the potential threat should be evaluated due to the catastrophic 
consequences. (Note: owners of certain dams should have emergency action plans 
geared toward notification and evacuation of vulnerable populations and critical 
facilities.)

Vulnerability: Does the surrounding topography contribute to the flooding at the 
site? Is there a history of local surface drainage problems due to inadequate 
site drainage?

Guidance: If areas with poor local drainage and frequent flooding cannot be 
avoided, filling, regrading, and installation of storm drainage facilities may 
be required.

Vulnerability: Given the nature of anticipated flooding and soils, is scour 
around and under the foundation likely?

Guidance: Scour-prone sites should be avoided, in part due to likely long-term 
maintenance requirements. Flooding that is high velocity or accompanied by waves 
is more likely to cause scour, especially on fills, or where local soils are 
unconsolidated and subject to erosion.

Vulnerability: Has water from other sources entered the building (i.e., high 
groundwater, water main breaks, sewer backup, etc.)? Is there a history of water 
intrusion through floor slabs or well-floor connections? Are there underground 
utility systems or areaways that can contribute to basement flooding? Are there 
stormwater sewer manholes upslope of window areas or openings that allow local 
drainage to enter the basement/lower floor areas? 

Guidance: These questions pertain to existing facilities that may be impaired by 
water from sources other than the primary source of flooding. The entire 
building envelope, including below-grade areas, should be examined to identify 
potential water damage.

Vulnerability: Is at least one access road to the site/building passable during 
flood events? 
Are at-grade parking lots located in flood-prone areas?
Are below-grade parking areas susceptible to flooding?

Guidance: Access is increasingly important as the duration of flooding 
increases. For the safety of occupants, most critical facilities should not be 
occupied during flood events.
Areas where vehicles could be affected should have signage to warn users of the 
risk. Emergency response plans should include notification of car owners.

Vulnerability Section: Architectural

Vulnerability: Are any critical building functions occupying space that is below 
the elevation of the 500-year flood or the Design Flood Elevation?
Can critical functions be relocated to upper levels that are above predicted 
flood elevations?
If critical functions cannot be relocated, is floodproofing feasible?
If critical functions must continue during a flood event, have power, supplies, 
and access issues been addressed?

Guidance: New critical facilities built in flood hazard areas should not have 
any functions occupying flood-prone spaces (other than parking, building access, 
and limited storage). 
Existing facilities in floodplains should be examined carefully to identify the 
best options for protecting functionality and the structure itself.

Vulnerability: Have critical contents (files, computers, servers, equipment, 
research, and data) been located on levels of the facility above the flood 
elevations? 
Are critical records maintained offsite?

Guidance: For existing facilities that are already located in flood hazard 
areas, the nature of the facility may require continued use of flood-prone 
space. However, the potential for flooding should be recognized and steps taken 
to minimize loss of expensive equipment and irreplaceable data. If critical 
contents cannot be permanently located on higher floors, a flood response plan 
should take into account the time and attention needed to move such contents 
safely

Vulnerability Section: Structural Systems

Vulnerability: What is the construction type and the foundation type and what is 
the load bearing capacity?
Has the foundation been designed to resist hydrostatic and hydrodynamic flood 
loads?

Guidance: If siting in a floodplain is unavoidable, new facilities are to be 
designed to account for all loads and load combinations, including flood loads.

Vulnerability: If the building has below-grade areas (basements), are the lower 
floor slabs subject to cracking and uplift?

Guidance: Below-grade spaces and their contents are most vulnerable to flooding 
and local drainage problems. Rapid pump out of below-grade spaces can unbalance 
forces if the surrounding soil is saturated, leading to structural failure. If 
below-grade spaces are intended to be dry floodproofed, the design must account 
for buoyant forces.

Building spaces below the design flood level can be dry floodproofed, although 
it must be recognized that higher flood levels will overtop the protection 
measures and may result in severe damage. Dry floodproofing creates large 
unbalanced forces that can jeopardize walls and foundations that are not 
designed to resist the hydrostatic and hydrodynamic loads.

Vulnerability: Are any portions of the building below the Design Flood 
Elevation?
Has the building been damaged in previous floods?

Guidance: For existing buildings, it is important to determine which portions 
are vulnerable in order to evaluate floodproofing options. If flood depths are 
expected to exceed 2 or 3 feet, dry floodproofing may not be feasible. 
Alternatives include modifying the use of flood-prone areas.

Vulnerability: If the building is elevated on a crawlspace or on an open 
foundation, are there any enclosed areas?

Guidance: New buildings may have enclosures below the flood elevation. provided 
the use of the enclosures is limited (crawlspace, parking, building access, and 
limited storage). In addition, the enclosures must have flood openings to 
automatically allow for inflow and outflow of floodwaters to minimize 
differential hydrostatic pressure. 
Existing buildings that are elevated and have enclosures below the flood 
elevation can be retrofit with flood openings.

Vulnerability: For an existing building with high-value uses below the flood 
elevation, is the building suitable for elevation-in-place, or can it be 
relocated to higher ground? 

Guidance: Elevating a building provides better protection than dry 
floodproofing. Depending on the type and soundness of the foundation, even large 
buildings can be elevated on a new foundation or moved to a site outside of the 
floodplain.

Vulnerability Section: Building Envelope

Vulnerability: Are there existing floodproofing measures in place below the 
expected flood elevation? What is the nature of these measures and what 
condition are they in? Is there an annual inspection and maintenance plan?
Is there an “action plan” to implement floodproofing measures when flooding is 
predicted? Do the building operators/occupants know what to do when a flood 
warning is issued?

Guidance: Floodproofing measures are only as good as the design and their 
condition, especially if many years have passed since initial installation. 
Floodproofing measures that require human intervention are entirely dependent on 
the adequacy of advance warning, and the availability and ability of personnel 
to properly install the measures. 

Vulnerability: Are For existing buildings, what types of openings penetrate the 
building envelope below the 500-year flood elevation or the DFE (doors, windows, 
cracks, vent openings, plumbing fixtures, floor drains, etc.)?

Guidance: For dry floodproofing to be effective, every opening must be 
identified and measures taken to permanently seal or to prepare special barriers 
to resist infiltration. Sewage backflow can enter through unprotected plumbing 
fixtures.

Vulnerability: Are flood-resistant materials used for structural and 
nonstructural components and finishes below the 500-year elevation or the DFE?

Guidance: Flood-resistant materials are capable of withstanding direct and 
prolonged contact with floodwaters without sustaining damage that requires more 
than cosmetic repair. Contact is considered to be prolonged if it is 72 hours or 
longer in freshwater flooding areas, or 12 hours or longer in areas subject to 
coastal flooding.

Vulnerability Section: Utility Systems

Vulnerability: Is the potable water supply for the facility protected from 
flooding? If served by a well, is the wellhead protected? 

Guidance: Operators of critical facilities that depend on fresh water for 
continued functionality should learn about the vulnerability of the local water 
supply system, and the system’s plans for recovery of service in the event of a 
flood.

Vulnerability: Is the wastewater service for the building protected from 
flooding? Are any manholes below the DFE? Is infiltration of floodwaters into 
sewer lines a problem? If the site is served by an onsite system that is located 
in a flood-prone area, have backflow valves been installed?

Guidance: Most waste lines exit buildings at the lowest elevation. Even 
buildings that are outside of the floodplain can be affected by sewage backups 
during floods. 

Vulnerability: Are there any aboveground or underground tanks on the site in 
flood hazard areas? Are they installed and anchored to resist flotation during 
the design flood? Are tank openings and vents elevated above the 500-year 
elevation or the DFE, or otherwise protected to prevent entry of floodwater or 
exit of product during a flood event?

Guidance: Dislodged tanks become floating debris that pose special hazards 
during recovery. Lost product causes environmental damage. Functionality may be 
impaired if tanks for heating fuel, propane, or fuel for emergency generators 
are lost or damaged. 

Vulnerability Section: Mechanical Systems

Vulnerability: Are air handlers, HVAC systems, ductwork, and other mechanical 
equipment and systems located above the 500-year elevation or the DFE? Are the 
vents and inlets located above flood level, or sealed to prevent entry of 
floodwater?

Guidance: In existing buildings, utility equipment that is critical for 
functionality should be relocated to higher floors or into elevated additions. 

Vulnerability Section: Plumbing and Gas Systems

Vulnerability: Are plumbing fixtures and gas-fired equipment (meters, pilot 
light devices/burners, etc.) located above the 500-year elevation or the DFE?

Guidance: In existing buildings, utility equipment that is critical for 
functionality should be relocated to higher floors or into elevated additions.

Vulnerability: Is plumbing and gas piping that extends below flood levels 
installed to minimize damage?

Guidance: Piping that is exposed could be impacted by debris.

Vulnerability Section: Electrical Systems

Vulnerability: Are electrical systems, including backup power generators, 
panels, and primary service equipment, located above the 500-year elevation or 
the DFE? 
Are pieces of electrical stand-by equipment and generators equipped with 
circuits to turn off power?
Are the switches and wiring required for safety (minimal lighting, door openers) 
located below the flood level designed for use in damp locations?

Guidance: In existing buildings, utility equipment that is critical for 
functionality should be relocated to higher floors or into elevated additions.

Vulnerability Section: Fire Alarm Systems

Vulnerability: Is the fire alarm system located above the 500-year elevation or 
the DFE?

Guidance: In existing buildings, utility equipment that is critical for 
functionality should be relocated to higher floors or into elevated additions.

Vulnerability Section: Communications and IT Systems

Are the communication/IT systems located above the 500-year elevation or the 
DFE? 
[End table]



3.6 REFERENCES AND SOURCES OF ADDITIONAL INFORMATION

Note: FEMA publications may be obtained at no cost by calling (800) 480-2520, 
faxing a request to (301) 497-6378, or downloading from the library/publications 
section online at http://www.fema.gov. 

American Society of Civil Engineers (ASCE), Structural Engineering Institute, 
2005a, Flood Resistant Design and Construction, ASCE/SEI 24-05, Reston, VA

American Society of Civil Engineers (ASCE), Structural Engineering Institute, 
2005b, Minimum Design Loads for Buildings and Other Structures, ASCE/SEI 7-05, 
Reston, VA.

Association of State Floodplain Managers, Inc. (ASFM), 2004, Floodplain 
Management 2003: State and Local Programs, Madison, WI.

Federal Emergency Management Agency (FEMA), 1986, Floodproofing Non-Residential 
Structures, FEMA 102, Washington, DC, May 1986.

Federal Emergency Management Agency (FEMA), 1991, Answers to Questions about 
Substantially Damaged Buildings, FEMA 213, Washington, DC, May 1991.

Federal Emergency Management Agency (FEMA), 1995, Engineering Principles and 
Practices for Retrofitting Flood-prone Residential Buildings, FEMA 259, 
Washington, DC, January 1995.

Federal Emergency Management Agency (FEMA), 1999, Protecting Building Utilities 
From Flood Damage: Principles and Practices for the Design and Construction of 
Flood Resistant Building Utility Systems, FEMA 348, Washington, DC, November 
1999. 

Federal Emergency Management Agency (FEMA), 2000, Coastal Construction Manual, 
FEMA 55 (3rd Edition), Washington, DC.

Federal Emergency Management Agency (FEMA), 2004, Answers to Questions about the 
National Flood Insurance Program, FEMA F-084, Washington, DC, May 2004. 

Federal Emergency Management Agency (FEMA), NFIP Technical Bulletins:

- User’s Guide to Technical Bulletins, FIA-TB-0, April 1993. 

- Openings in Foundation Walls, FIA-TB-1, April 1993.

- Flood-Resistant Materials Requirements, FIA-TB-2, April 1993.

- Non-Residential Floodproofing—Requirements and Certification, FIA-TB-3, April 
1993. 

- Elevator Installation, FIA-TB-4, April 1993. 

- Free-of-Obstruction Requirements, FIA-TB-5, April 1993. 

- Below-Grade Parking Requirements, FIA-TB-6, April 1993.

- Wet Floodproofing Requirements, FIA-TB-7, December 1993.

- Corrosion Protection for Metal Connections in Coastal Areas, FIA-TB-8, 1996.

- Design and Construction Guidance for Breakaway Walls Below Elevated Coastal 
Buildings, FIA-TB-9, 1999.

- Ensuring That Structures Built on Fill In or Near Special Flood Hazard Areas 
Are Reasonably Safe From Flooding, FIA-TB-10, 2001. 

- Crawlspace Construction for Buildings Located in Special Flood Hazard Areas, 
FIA-TB-11, 2001.

International Code Council, Inc. (ICC), 2006a, ICC Performance Code for 
Buildings and Facilities, Country Club Hills, IL.

International Code Council, Inc. (ICC), 2006b, International Building Code, 
Country Club Hills, IL.

International Code Council, Inc. (ICC) and Federal Emergency Management Agency 
(FEMA), 2006, Reducing Flood Losses Through the International Codes, Meeting the 
Requirements of the National Flood Insurance Program (2006 I-Codes), Country 
Club Hills, IL.

National Academy of Sciences, 1977, Methodology for Calculating Wave Action 
Effects Associated with Storm Surges. Washington, DC.

National Fire Protection Association (NFPA), 2006, Building Construction and 
Safety Code (NFPA 5000), Quincy, MA. 

U.S. Army Corps of Engineers, 1993, National Flood Proofing Committee, Flood 
Proofing – How To Evaluate Your Options, Washington, DC, July 1993.

U.S. Army Corps of Engineers, 1995, Flood Proofing Regulations, EP 1165-2-314, 
Washington, DC. 

U.S. Army Corps of Engineers, 1996, Flood Proofing Programs, Techniques and 
References, Washington, DC.

U.S. Army Corps of Engineers, 1998, Flood Proofing Performance—Successes & 
Failures, Washington, DC.

Organizations and Agencies:
Federal Emergency Management Agency: FEMA’s regional offices can be contacted 
for advice and guidance on NFIP mapping and regulations (www.fema.gov). 

NFIP State Coordinating offices help local governments to meet their floodplain 
management obligations, and may provide technical advice to others; the offices 
are listed by the Association of State Floodplain Managers, Inc., 
(www.floods.org/stcoor.htm). 

State agencies that coordinate state funding and or administer regulations may 
have state-specific requirements for hospitals.

U.S. Army Corps of Engineers: District offices offer Flood Plain Management 
Services, (www.usace.army.mil/inet/functions/cw/). 






CHAPTER 4 – MAKING HOSPITALS SAFE FROM HIGH WIND

4.1 GENERAL DESIGN CONSIDERATIONS

Wind with sufficient speed to cause damage to weak hospitals can occur anywhere 
in the United States and its territories. [footnote 1] Even a well-designed, 
constructed, and maintained hospital may be damaged by a wind event much 
stronger than one the building was designed for. However, except for tornado 
damage, this scenario is a rare occurrence. Rather, most damage occurs because 
various building elements have limited wind resistance due to inadequate design, 
poor installation, or material deterioration. Although the magnitude and 
frequency of strong windstorms vary by locale, all hospitals should be designed, 
constructed, and maintained to minimize wind damage (other than that associated 
with tornadoes — see Section 4.5). 

[Begin footnote 1] The U.S. territories include American Samoa, Guam, Northern 
Mariana Islands, Puerto Rico, and the U.S. Virgin Islands. ASCE 7 provides basic 
wind speed criteria for all but Northern Mariana Islands. [end footnote]

This chapter discusses structural, building envelope, and nonstructural building 
systems, and illustrates various types of wind-induced damage that affect them. 
It also presents six case studies. Numerous examples of best practices 
pertaining to new and existing hospitals are presented as recommended design 
guidelines. Incorporating those practices applicable to specific projects will 
result in greater wind-resistance reliability and will, therefore, decrease 
expenditures for repair of wind-damaged facilities, provide enhanced protection 
for occupants, and avoid disruption of critical services.

The recommendations presented in this manual are based on field observation 
research conducted on 25 hospitals that were struck by hurricanes. [footnote 2] 
The recommendations are also based on numerous investigations of other types of 
critical facilities and other types of buildings exposed to hurricanes and 
tornadoes, and on literature review. Some of the 25 hospitals were exposed to 
extremely high wind speeds, while others experienced moderate speeds. 
Approximately 88 percent of the 25 hospitals experienced roof covering damage 
(many of which also experienced damage to rooftop equipment), and windows were 
broken on approximately 50 percent of them. Because of wind damage and 
subsequent water leakage, one of the hospitals was totally evacuated after a 
hurricane (Figure 4-1). Another hospital was also evacuated after a hurricane, 
but evacuation was prompted by flooding. Five other hospitals were partially 
evacuated after the storm because of interior water damage. None of the main 
hospital buildings on these 25 campuses experienced structural failure, although 
a few auxiliary buildings did collapse.

[Begin footnote 2] The research on the 25 hospitals was conducted by a team from 
Texas Tech University (Hurricane Hugo, Charleston, South Carolina, 1989), a team 
under the auspices of the Wind Engineering Research Council—now known as the 
American Association for Wind Engineering (Hurricane Andrew, South Florida, 
1992), and teams deployed by FEMA (Hurricane Marilyn, U.S. Virgin Islands, 1995; 
Typhoon Paka, Guam, 1997; Hurricane Charley, Port Charlotte, Florida, 2004; 
Hurricane Frances, east coast of Florida, 2004, Hurricane Ivan, Pensacola, 
Florida, 2004; and Hurricane Katrina, Louisiana and Mississippi, 2005). [end 
footnote]

Figure 4-1 is a photo showing how Deering Hospital was evacuated after Hurricane 
Andrew due to water infiltration caused by roof covering, window, and door 
damage. [end figure]

[Begin text box]
The 200-bed Deering Hospital opened shortly before Hurricane Andrew struck south 
Florida in 1992. Aggregate from the hospital’s built-up roofs broke several 
windows, the roof covering was blown off in some areas (Figure 4-9), and the 
entrance doors at the emergency room were blown away. Because of extensive 
interior water damage, the entire hospital was evacuated after the storm and 
remained closed for 9 months.
[End text box]

4.1.1 Nature of High Winds

A variety of windstorm types occur in different areas of the United States. The 
characteristics of the types of storms that can affect the site should be 
considered by the design team. The primary storm types are straight-line winds, 
down-slope winds, thunderstorms, downbursts, northeasters (nor'easters), 
hurricanes, and tornadoes. For information on these storm types, refer to 
Section 3.1.1 in FEMA 543, Design Guide for Improving Critical Facility Safety 
from Flooding and High Winds. [footnote 3]

[Begin footnote 3] Available at the FEMA Web site. See 
www.fema.gov/library/viewRecord.do?id=2441 [end footnote]


Of all the storm types, hurricanes have the greatest potential for devastating a 
large geographical area and, hence, affect the greatest number of people. See 
Figure 4-2 for hurricane-prone regions. 

Figure 4-2 is a map of hurricane-prone regions and special wind regions
Source: Adapted from ASCE 7-05 [end figure]

4.1.2 PROBABILITY OF OCCURRENCE

Via the importance factor [footnote 4], ASCE 7 requires Category III and IV 
buildings to be designed for higher wind loads than Category I and II buildings. 
Hence, hospitals designed in accordance with ASCE 7 have greater resistance to 
stronger, rarer storms. When designing a hospital, design professionals should 
consider the following types of winds.

[Begin footnote 4] The importance factor accounts for the degree of hazard to 
human life and damage to property. Importance factors are given in ASCE 7. [end 
footnote]

Routine winds: In many locations, winds with low to moderate speeds occur daily. 
Damage is not expected to occur during these events.

[Begin text box]
Missile damage is very common during hurricanes and tornadoes. Missiles can 
puncture roof coverings, many types of exterior walls, and glazing. The IBC does 
not address missile-induced damage, except for glazing in wind-borne debris 
regions. (Wind-borne debris regions are limited to portions of hurricane-prone 
regions.) In hurricane-prone regions, significant missile-induced building 
damage should be expected, even during design level hurricane events, unless 
special enhancements are incorporated into the building’s design (discussed in 
Section 4.3).
[End text box]

Stronger winds: At a given site, stronger winds (i.e., winds with a speed in the 
range of 70 to 80 mph peak gust, measured at 33 feet in Exposure C—refer to 
Section 4.1.3) may occur from several times a year to only once a year or even 
less frequently. This is the threshold at which damage normally begins to occur 
to building elements that have limited wind resistance due to problems 
associated with inadequate design, insufficient strength, poor installation, or 
material deterioration.

Design level winds: Hospitals exposed to design level events and events that are 
somewhat in excess of design level should experience little, if any, damage. 
Actual storm history, however, has shown that design level storms frequently 
cause extensive building envelope damage. Structural damage also occurs, but 
less frequently. Damage incurred in design level events is typically associated 
with inadequate design, poor installation, or material deterioration. The 
exceptions are wind-driven water infiltration and wind-borne debris (missiles) 
damage. Water infiltration is discussed in Sections 4.3.3.1, 4.3.3.3, and 
4.3.3.5. 

[Begin text box]
ASCE 7, Minimum Design Loads for Buildings and Other Structures, provides 
guidance for determining wind loads on buildings. The IBC and NFPA 5000 refer to 
ASCE 7 for wind load determination.
[End text box]

Tornadoes: Although more than 1,200 tornadoes typically occur each year in the 
United States, the probability of a tornado occurring at any given location is 
quite small. The probability of occurrence is a function of location. As 
described in Section 4.5, only a few areas of the country frequently experience 
tornadoes, and tornadoes are very rare in the west. The Oklahoma City area is 
the most active location, with 112 recorded tornadoes between 1890 and 2003 
(www.spc.noaa.gov/faq/tornado/#History). 

Well-designed, constructed, and maintained hospitals should experience little if 
any damage from weak tornadoes, except for window breakage. However, weak 
tornadoes often cause building envelope damage because of wind-resistance 
deficiencies. Most hospitals experience significant damage if they are in the 
path of a strong or violent tornado because they typically are not designed for 
this type of storm. See Section 4.5 for recommendations pertaining to tornadoes. 

4.1.3 Wind/Building Interactions

When wind interacts with a building, both positive and negative (i.e., suction) 
pressures occur simultaneously. Hospitals must have sufficient strength to 
resist the applied loads from these pressures to prevent wind-induced building 
failure. Loads exerted on the building envelope are transferred to the 
structural system, where in turn they must be transferred through the foundation 
into the ground. The magnitude of the pressures is a function of the following 
primary factors: exposure, basic wind speed, topography, building height, 
internal pressure, and building shape. For general information on these factors, 
refer to Section 3.1.3 in FEMA 543, Design Guide for Improving Critical Facility 
Safety from Flooding and High Winds. A description of key issues follows. 

Wind Speed: In the ASCE 7 formula for determining wind pressures, the basic wind 
speed is squared. Therefore, as the wind speed increases, the pressures are 
exponentially increased, as illustrated in Figure 4-3. This figure also 
illustrates the relative difference in pressures exerted on the main wind-force 
resisting system (MWFRS) and the components and cladding (C&C) elements.

Figure 4-3 is a chart showing wind pressure as a function of wind speed

[Begin text box]
The MWFRS is an assemblage of structural elements assigned to provide support 
and stability for the overall structure. The system generally receives wind 
loading from more than one surface. The C&C are elements of the building 
envelope that do not qualify as part of the main wind-force resisting system.
[End text box]

Building shape: The highest uplift pressures occur at roof corners because of 
building aerodynamics (i.e., the interaction between the wind and the building). 
The roof perimeter has a somewhat lower load compared to the corners, and the 
field of the roof has still lower loads. Exterior walls typically have lower 
loads than the roof. The ends (edges) of walls have higher suction loads than 
the portion of wall between the ends. However, when the wall is loaded with 
positive pressure, the entire wall is uniformly loaded. Figure 4-4 illustrates 
these aerodynamic influences. The negative values shown in Figure 4-4 indicate 
suction pressure acting upward from the roof surface and outward from the wall 
surface. Positive values indicate positive pressure acting inward on the wall 
surface. 

[Begin figure]
Figure 4-4 is an illustration of relative roof uplift pressures as a function of 
roof geometry, roof slope, and location on roof, and relative positive and 
negative wall pressures as a function of location along the wall. 
[end figure]

Aerodynamic influences are accounted for by using external pressure coefficients 
in load calculations. The value of the coefficient is a function of the location 
on the building (e.g., roof corner or field of roof) and building shape as 
discussed below. Positive coefficients represent a positive (inward-acting) 
pressure, and negative coefficients represent negative (outward-acting 
[suction]) pressure. External pressure coefficients for MWFRS and C&C are listed 
in ASCE 7.

Building shape affects the value of pressure coefficients and, therefore, the 
loads applied to the various building surfaces. For example, the uplift loads on 
a low-slope roof are larger than the loads on a gable or hip roof. The steeper 
the slope, the lower the uplift load. Pressure coefficients for monoslope (shed) 
roofs, sawtooth roofs, and domes are all different from those for low-slope and 
gable/hip roofs.

Building irregularities, such as re-entrant corners, bay window projections, a 
stair tower projecting out from the main wall, dormers, and chimneys can cause 
localized turbulence. Turbulence causes wind speed-up, which increases the wind 
loads in the vicinity of the building irregularity, as shown in Figures 4-5 and 
4-6. Figure 4-5 shows the aggregate ballast on a hospital’s single-ply membrane 
roof blown away at the re-entrant corner and in the vicinity of the corners of 
the wall projections at the window bays. The irregular wall surface created 
turbulence, which led to wind speed-up and loss of aggregate in the turbulent 
flow areas.

Figure 4-6 shows a stair tower at a hospital that caused turbulence resulting in 
wind speed-up. The speed-up increased the suction pressure on the base flashing 
along the parapet behind the stair tower. The built-up roof’s base flashing was 
pulled out from underneath the coping because its attachment was insufficient to 
resist the suction pressure. The base flashing failure propagated and caused a 
large area of the roof membrane to lift and peel. Some of the wall covering on 
the stair tower was also blown away. Had the stair tower not existed, the built-
up roof would likely not have been damaged. To avoid damage in the vicinity of 
building irregularities, attention needs to be given to the attachment of 
building elements located in turbulent flow areas. 

To avoid the roof membrane damage shown in Figure 4-6, it would be prudent to 
use corner uplift loads in lieu of perimeter uplift loads in the vicinity of the 
stair tower, as illustrated in Figure 4-7. Wind load increases due to building 
irregularities can be identified by wind tunnel studies; however, wind tunnel 
studies are rarely performed for hospitals. Therefore, identification of wind 
load increases due to building irregularities will normally be based on the 
designer’s professional judgment. Usually load increases will only need to be 
applied to the building envelope, and not to the MWFRS.

[Begin figures]
Figure 4-5 is a photo of aggregate blow-off associated with building 
irregularities. Hurricane Hugo (South Carolina, 1989)

Figure 4-6 is a photo the irregularity created by the stair tower (covered with 
a metal roof) caused turbulence resulting in wind speed-up and roof damage. 
Hurricane Andrew (Florida, 1992)

Figure 4-7 is a plan view of a portion of the building in Figure 4-6 showing the 
use of a corner uplift zone in lieu of a perimeter uplift zone on the low-slope 
roof in the vicinity of the stair tower
[End figures]

[Begin text box]
Information pertaining to load calculations is presented in Section 4.3.1.2. For 
further general information on the nature of wind and wind-building 
interactions, see Buildings at Risk: Wind Design Basics for Practicing 
Architects, American Institute of Architects, 1997.
[End text box]

4.1.4 Building Codes

The IBC is the most extensively used model code. However, in some jurisdictions 
NFPA 5000 may be used. In other jurisdictions, one of the earlier model building 
codes, or a specially written State or local building code, may be used. The 
specific scope and/or effectiveness and limitations of these other building 
codes will be somewhat different from those of the IBC. It is incumbent upon the 
design professionals to be aware of the specific code (including the edition of 
the code and local amendments) that has been adopted by the authority having 
jurisdiction over the location of the hospital.

4.1.4.1 Scope of Building Codes

With respect to wind performance, the scope of the model building codes has 
greatly expanded since the mid-1980s. Some of the most significant improvements 
are discussed below.

Recognition of increased uplift loads at the roof perimeter and corners: Prior 
to the 1982 edition of the Standard Building Code (SBC), Uniform Building Code 
(UBC), and the 1987 edition of the National Building Code (NBC), these model 
codes did not account for the increased uplift at the roof perimeter and 
corners. Therefore, hospitals designed in accordance with earlier editions of 
these codes are very susceptible to blow-off of the roof deck and/or roof 
covering.

Adoption of ASCE 7 for design wind loads: Although the SBC, UBC, and NBC 
permitted use of ASCE 7, the 2000 edition of the IBC was the first model code to 
require ASCE 7 for determining wind design loads on all buildings. ASCE 7 has 
been more reflective of the current state of the knowledge than the earlier 
model codes, and use of this procedure typically has resulted in higher design 
loads. 

Roof coverings: Several performance and prescriptive requirements pertaining to 
wind resistance of roof coverings have been incorporated into the model codes. 
The majority of these additional provisions were added after Hurricanes Hugo 
(1989) and Andrew (1992). Poor performance of roof coverings was widespread in 
both of those storms. Prior to the 1991 edition of the SBC and UBC, and the 1990 
edition of the NBC, these model codes were essentially silent on roof covering 
wind loads and test methods for determining uplift resistance. Code improvements 
continued to be made through the 2006 edition of the IBC, which added a 
provision that prohibits aggregate roof surfaces in hurricane-prone regions.

[Begin text box]
ASCE 7 requires impact-resistant glazing in wind-borne debris regions within 
hurricane-prone regions. Impact-resistant glazing can either be laminated glass, 
polycarbonate, or shutters tested in accordance with standards specified in ASCE 
7. The wind-borne debris load criteria were developed to minimize property 
damage and to improve building performance. The criteria were not developed for 
occupant protection. Where occupant protection is a specific criterion, the more 
conservative wind-borne debris criteria given in FEMA 361, Design and 
Construction Guidance for Community Shelters is recommended.
[End text box]

Glazing protection: The 2000 edition of the IBC was the first model code to 
address wind-borne debris requirements for glazing in buildings located in 
hurricane-prone regions (via reference to the 1998 edition of ASCE 7). The 1995 
edition of ASCE 7 was the first edition to address wind-borne debris 
requirements.

Parapets and rooftop equipment: The 2003 edition of the IBC was the first model 
code to address wind loads on parapets and rooftop equipment (via reference to 
the 2002 edition of ASCE 7, which was the first edition of ASCE 7 to address 
these elements).

4.1.4.2 Effectiveness and Limitations of Building Codes

A key element of an effective building code is for a community to have an 
effective building department. Building safety depends on more than the codes 
and the standards they reference. Building safety results when trained 
professionals have the resources and ongoing support they need to stay on top of 
the latest advancements in building safety. An effective building safety system 
provides uniform code interpretations, product evaluations, and professional 
development and certification for inspectors and plan reviewers. Local building 
departments play an important role in helping to ensure buildings are designed 
and constructed in accordance with the applicable building codes. Meaningful 
plan review and inspection by the building department are particularly important 
for hospitals.

General limitations to building codes include the following:

- Because codes are adopted and enforced on the local or State level, the 
authority having jurisdiction has the power to eliminate or modify wind-related 
provisions of a model code, or write its own code instead. In places where 
important wind-related provisions of the current model code are not adopted and 
enforced, hospitals are more susceptible to wind damage. Additionally, a 
significant time lag often exists between the time a model code is updated and 
the time it is implemented by the authority having jurisdiction. Buildings 
designed to the minimum requirements of an outdated code are, therefore, not 
taking advantage of the current state of the knowledge. These buildings are 
prone to poorer wind performance compared to buildings designed according to the 
current model code.

- Adopting the current model code alone does not ensure good wind performance. 
The code is a minimum that should be used by knowledgeable design professionals 
in conjunction with their training, skills, professional judgment, and the best 
practices presented in this manual. To achieve good wind performance, in 
addition to good design, the construction work must be effectively executed, and 
the building must be adequately maintained and repaired.

- Hospitals need to perform at a higher level than required by codes and 
standards. 

IBC 2006: The 2006 edition of the IBC is believed to be a relatively effective 
code, provided that it is properly followed and enforced. Some limitations of 
the 2006 IBC have, however, been identified:

- With respect to hurricanes, the IBC provisions pertaining to building 
envelopes and rooftop equipment do not adequately address the special needs of 
hospitals. For example:(1) they do not account for water infiltration due to 
puncture of the roof membrane by missiles; (2) they do not adequately address 
the vulnerabilities of brittle roof coverings (such as tile) to missile-induced 
damage and subsequent progressive failure; (3) they do not adequately address 
occupant protection with respect to missiles; (4) they do not adequately address 
protection of equipment in elevator penthouses; and (5) they do not account for 
interruption of water service or prolonged interruption of electrical power. All 
of these elements are of extreme importance for hospitals, which need to remain 
operational before, during, and after a disaster. Guidance to overcome these 
shortcomings is given in Section 4.3 and 4.4.

- The 2000, 2003, and 2006 IBC rely on several referenced standards and test 
methods developed or updated in the 1990s. Prior to adoption, most of these 
standards and test methods had not been validated by actual building performance 
during design level wind events. The hurricanes of 2004 and 2005 provided an 
opportunity to evaluate the actual performance of buildings designed and 
constructed to the minimum provisions of the IBC. Building performance 
evaluations conducted by FEMA revealed the need for further enhancements to the 
2006 IBC pertaining to some of the test methods used to assess wind and wind-
driven rain resistance of building envelope components. For example, there is no 
test method to assess wind resistance of gutters. Further, the test method to 
evaluate the resistance of windows to wind-driven rain is inadequate for high 
wind events. However, before testing limitations can be overcome, research needs 
to be conducted, new test methods need to be developed, and some existing test 
methods need to be modified.

- Except to the extent covered by reference to ASCE 7, the 2006 IBC does not 
address the requirement for continuity, redundancy, or energy-dissipating 
capability (ductility) to limit the effects of local collapse, and to prevent or 
minimize progressive collapse after the loss of one or two primary structural 
members, such as a column. Chapter 1 of ASCE 7 addresses general structural 
integrity, and the Chapter 1 Commentary provides some guidance on this issue.

- The 2006 IBC does not account for tornadoes; therefore, except for weak 
tornadoes, it is ineffective for this type of storm. Guidance to overcome this 
shortcoming is given in Section 4.5.




4.2 HOSPITALS EXPOSED TO HIGH WINDS

4.2.1 Vulnerability: What High Winds Can Do to Hospitals

This section provides an overview of the common types of wind damage and their 
ramifications.

4.2.1.1 Types of Building Damage

When damaged by wind, hospitals typically experience a variety of building 
component damage. For example, at the hospital shown in Figure 4-8, the roof 
covering was severely damaged, windows were broken, and rooftop equipment was 
blown away. The subsequent water infiltration required that most of the hospital 
be evacuated. The most common types of damage are discussed below in descending 
order of frequency. 

Figure 4-8 is a photo of a field military hospital in tents set up to replace 
evacuated hospital in U.S. Virgin Islands following Hurricane Marilyn (1995) 
[end figure]

Roof: Roof covering damage (including rooftop mechanical, electrical, and 
communications equipment) is the most common type of wind damage, as illustrated 
by Figure 4-9. In addition to blowoff of the roof membrane (yellow arrow), 
ductwork blew away (red circle), a gooseneck was blown over (red arrow), and 
wall panels at an equipment enclosure were blown off (blue arrow). The cast-in-
place concrete deck kept most of the water from entering the hospital. 

[Begin figure]
Figure 4-9 is a photo of a damaged roof membrane and rooftop equipment. Typhoon 
Paka (1997). 
[end figure]

Glazing: Exterior glazing damage is very common during hurricanes and tornadoes, 
but is less common during other storms. The glass shown in Figure 4-10 was 
broken by the aggregate from a built-up roof. The inner panes had several impact 
craters. In several of the adjacent windows, both the outer and inner panes were 
broken. The aggregate flew more than 245 feet (the estimated wind speed was 104 
mph, measured at 33 feet in Exposure C).

[Begin figure]
Figure 4-10 is a photo showing how the outer window panes were broken by 
aggregate from a built-up roof. Hurricane Hugo (South Carolina, 1989) 
[end photo]

Wall coverings, soffits, and large doors: Exterior wall covering, soffit, and 
large door damage is common during hurricanes and tornadoes, but is less common 
during other storms. Wall covering damage is shown at the hospital complex 
described in the West Florida Hospital case study, Section 4.1.3.

Wall collapse: Collapse of non-load-bearing exterior walls is common during 
hurricanes and tornadoes, but is less common during other storms. At the 
hospital shown in Figure 4-11, a portion of the non-load-bearing wall collapsed. 
Several windows were also broken by aggregate ballast blown from the hospital’s 
roof (see Figure 4-5).

[Begin figure]
Figure 4-11 is a photo showing the collapse of non-load-bearing wall (red 
circle) and broken glazing from roof aggregate (red arrow). Hurricane Hugo 
(South Carolina, 1989) 
[end figure]

Structural system: Structural damage (e.g., roof deck blow-off, blow-off or 
collapse of the roof structure, collapse of exterior bearing walls, or collapse 
of the entire building or major portions thereof) is the principal type of 
damage that occurs during strong and violent tornadoes (see Figure 4-12).

[Begin figure]
Figure 4-12 is a photo showing how this building in Northern Illinois was 
heavily damaged by a strong tornado in 1990. 
[end figure]

4.2.1.2 Ramifications of Damage

The ramifications of building component damage on hospitals are described below.

Property damage: Property damage requires repairing/replacing the damaged 
components (or replacing the entire facility), and may require 
repairing/replacing interior building components, furniture, and other 
equipment, and mold remediation. As illustrated by Figures 4-1 and 4-8, even 
when damage to the building envelope is limited, such as blow-off of a portion 
of the roof covering or broken glazing, substantial water damage frequently 
occurs because heavy rains often accompany strong winds (particularly in the 
case of thunderstorms, tropical storms, hurricanes, and tornadoes). 

Wind-borne debris such as roof aggregate, gutters, rooftop equipment, and siding 
blown from buildings can damage vehicles and other buildings in the vicinity. 
Debris can travel well over 300 feet in high-wind events.

[Begin text box]
Modest wind speeds can drive rain into exterior walls. Unless adequate 
provisions are taken to account for water infiltration (see Sections 4.3.3.1 – 
4.3.3.6), damaging corrosion, dry rot, and mold can occur within walls.
[End text box]

Ancillary buildings (such as storage buildings) adjacent to hospitals are also 
vulnerable to damage. Although loss of these buildings may not be crippling to 
the operation of the hospital, debris from ancillary buildings may strike and 
damage the hospital. 

Injury or death: Although infrequent, hospital occupants or people outside 
hospitals may be injured and killed if struck by collapsed building components 
(such as exterior masonry walls or the roof structure) or wind-borne debris. The 
greatest risk of injury or death is during strong hurricanes and strong/violent 
tornadoes. If a hospital, or a portion of a hospital, needs to be evacuated due 
to wind-related damage, patients may be exposed to risk of injury or death 
during their relocation.

Interrupted use: Depending on the magnitude of wind and water damage, it can 
take days, months, or more than a year to repair the damage or replace a 
facility. In addition to the costs associated with repairing/replacing the 
damage, other social and financial costs can be even more significant. The 
repercussions related to interrupted use of hospitals can include lack of 
medical care, and the costs to rent temporary facilities. These additional costs 
can be quite substantial.

[Begin text box]
Although people are not usually outside during hurricanes, it is not uncommon 
for people to seek medical care during a storm. Missiles, such as roof aggregate 
or tile shedding from a hospital, could injure or kill people before they have a 
chance to enter the building.
[End text box]

4.2.1.3 The Case of West Florida Hospital, Pensacola, Florida

The case of West Florida Hospital illustrates a variety of building performance 
problems. The 531-bed West Florida Healthcare facility (also called the 
Pavillion) includes the 400-bed acute tertiary West Florida Hospital, the 58-bed 
Rehabilitation Institute, and a 73-bed behavioral health facility. The Pavillion 
is located north of downtown Pensacola, approximately 3 miles west of Escambia 
Bay and 7 miles north of Pensacola Bay. West Florida was struck by Hurricane 
Ivan in 2004. The estimated peak gust wind speed at this site was 105 to 115 
mph. [footnote 6] The design wind speed in the 2005 edition of ASCE 7 for this 
location is 135 mph. 

[Begin footnote 6] The 105 to 1115 mph speeds were estimated for Exposure C. 
[end footnote]

The West Florida Hospital (J on Figure 4-13) and several of the other buildings 
on the campus experienced a variety of damages during the storm. The roof 
membrane was punctured in several places by windborne missiles and by damaged 
rooftop equipment (see Figures 4-14 to 4-16). 


[Begin figure]Figure 4-13 is an illustration of the West Florida Hospital. 
[end figure]

Exterior insulation finish system (EIFS) blew off the hospital and caused 
significant glass breakage in the MOB (Figure 4-17) and the walkway connecting 
the hospital to the MOB. Some of the lower-level windows may have been broken by 
wind-blown aggregate ballast from the roof over the dialysis unit and urgent 
care facility. In addition, some window frames were reportedly blown out. These 
failures were likely caused by the development of high internal pressure after 
windows on windward surfaces were broken by missiles, combined with suction 
pressure on the exterior surface of windows on the leeward side of the building. 
Glass damage to the MOB, and subsequent wind and water damage to the interior, 
resulted in closure of several offices. 

In addition to the window breakage, EIFS blew off the elevator enclosure, the 
stair tower, and the spandrels. The single-ply roof membrane was damaged and the 
Lightning Protection System (LPS) on the MOB was also displaced. 

The hospital originally had exposed concrete walls. However, in a subsequent 
refurbishing, the walls were faced with EIFS. Steel hat channels were installed 
over the concrete, followed by gypsum board, insulation, and synthetic stucco. 
In areas where the EIFS blew off, the gypsum board typically pulled over the 
screw heads and blew away (Figure 4-23). The screws and hat channels were 
moderately corroded. Although the corrosion could have eventually caused loss of 
the EIFS, it did not play a role in this failure.

With loss of the EIFS wall covering, wind-driven rain destroyed the elevator 
control equipment (see Figure 4-22). Water damage to the elevator control 
equipment resulted in failure of the MOB stair tower elevator. As a result, 
several people were trapped in the MOB stair tower elevator shown in Figure 4-18 
during the hurricane. Fortunately, the MOB had another bank of elevators in the 
core of the building that was not damaged, so vertical transportation was still 
possible, although handicapped by the loss of the stair tower elevator. At the 
MOB stair tower, some of the gypsum board on the interior side of the studs 
collapsed into the stairway, thus trapping a maintenance worker who had gone to 
the mechanical penthouse during the hurricane. 

Glass shards from the MOB punctured the ballasted single-ply membrane over the 
regional dialysis unit and urgent care facility (item L on Figure 4-13). 
Although the roof membrane had been punctured in numerous areas (Figure 4-20), 
the concrete deck (concrete topping over metal decking) over the dialysis 
unit and urgent care facility acted as an secondary line of protection against 
water leakage and was effective in minimizing water infiltration into the 
facility, thereby minimizing interrupted use of these facilities. By quickly 
performing emergency roof repairs and cleaning up the interior, the dialysis 
unit was non-operational for only 1 day. 

At the cancer treatment facility (H on Figure 4-13), asphalt shingles were blown 
from the roof hips and some eave edge metal lifted. Additionally, sewage backed 
up at this facility because of power loss to a lift station. Sewage backup was 
cleaned up quickly and the facility was non-operational for only 1 day.

At the imaging center (F on Figure 4-13), there were some broken windows and a 
fan cowling blew away. Some large parking lot light fixtures also collapsed 
because the bottoms of the tubes were severely corroded (Figure 4-21).

Communications outside of the hospital were lost about an hour after the arrival 
of high winds because of damage to the communications antenna; the LPS was also 
displaced (Figure 4-15). A canopy at the loading dock was blown away, which 
caused difficulties in materials handling.

Because of rapid emergency response by construction and clean-up crews, the 
hospital and other facilities on campus remained functional. However, the damage 
was very costly and created many hardships for hospital staff. 

[Begin figures]
Figure 4-14 is a photo showing numerous repairs where the modified bitumen roof 
membrane was punctured by missiles. Water from the punctured membrane entered 
the surgical suite. 

Figure 4-15 is a photo showing damaged rooftop equipment, collapsed antenna 
(circled), and displaced LPS

Figure 4-16 is a photo showing damaged rooftop equipment. Although some of this 
damage may have been caused by wind pressure, some of it was caused by missiles. 
Note the open ducts (red arrows).

Figure 4-17 is a photo showing broken windows in the MOB. Wood studs and gypsum 
board had been temporarily installed after the hurricane to prevent patients 
from inadvertently falling out of the MOB. 

Figure 4-18 is a photo showing broken windows in the connecting walkway between 
the hospital (right) and MOB (left). Also note the broken windows and loss of 
EIFS (including the gypsum board on both sides of the studs) at the elevator 
enclosure (blue arrow).

Figure 4-19 is a photo showing EIFS debris blown off the hospital building (Item 
J on Figure 4-13) in the background (red square) broke numerous windows in the 
MOB (item G on figure 4-13) in the foreground. 

Figure 4-20 is a photo looking down at the one-story roof to the right of the 
MOB in Figure 4-19. The small dark areas are locations where emergency patches 
had been placed to repair punctures from falling glass shards. (Note: At the 
time the photo was taken, the ballast had been repositioned into rows in 
preparation for removal)

Figure 4-21 is a photo of a collapsed light fixtures caused by severe corrosion 
(see inset). The cancer treatment facility is beyond to the left.

Figure 4-22 is a photo showing the only remaining portion of the exterior wall 
surrounding the elevator penthouse on the MOB was the steel studs. 

Figure 4-23 is a photo of close-up of the damaged EIFS at the hospital. In this 
area most of the insulation and gypsum board was blown from the steel furring 
channels.
[End figures]

4.2.2 Evaluating Hospitals for Risk from High Winds

This section describes the process of hazard risk assessment. Although no formal 
methodology for risk assessment has been adopted, prior experience provides a 
sufficient knowledge base upon which a set of guidelines can be structured into 
a recommended procedure for risk assessment of hospitals. The procedures 
presented below establish guidelines for evaluating the risk to new and existing 
buildings from windstorms other than tornadoes. These evaluations will allow 
development of a vulnerability assessment that can be used along with the site’s 
wind regime to assess the risk to hospitals.

In the case of tornadoes, neither the IBC nor ASCE 7 requires buildings 
(including hospitals) to be designed to resist tornado forces, nor are occupant 
shelters required in buildings located in tornado-prone regions. Constructing 
tornado-resistant hospitals is extremely expensive because of the extremely high 
pressures and missile impact loads that tornadoes can generate. Therefore, when 
consideration is voluntarily given to tornado design, the emphasis is typically 
on occupant protection, which is achieved by “hardening” portions of a hospital 
for use as safe havens. FEMA 361 includes a comprehensive risk assessment 
procedure that designers can use to assist building owners in determining 
whether a tornado shelter should be included as part of a new hospital. See 
Section 4.5 for recommendations pertaining to hospitals in tornado-prone 
regions.

4.2.2.1 New Buildings

When designing new hospitals, a two-step procedure is recommended for evaluating 
the risk from windstorms (other than tornadoes).

Step 1: Determine the basic wind speed from ASCE 7. As the basic wind speed 
increases beyond 90 mph, the risk of damage increases. Design, construction, and 
maintenance enhancements are recommended to compensate for the increased risk of 
damage (see Section 4.3).

Step 2: For hospitals in hurricane-prone regions, refer to the design, 
construction, and maintenance enhancements recommended in Sections 4.3.1.5, 
4.3.2.1, 4.3.3.2, 4.3.3.4, 4.3.3.6, 4.3.3.8, 4.3.4.2, 4.3.4.4, 4.3.5, and 4.3.6. 

For hospitals in remote areas outside of hurricane-prone regions, it is 
recommended that robust design measures be considered to minimize the potential 
for disruption resulting from wind damage. Because of their remote location, 
disruption of hospitals could severely affect patients. Some of the 
recommendations in the sections pertaining to hurricane-prone regions may 
therefore be prudent.

4.2.2.2 Existing Buildings

The resistance of existing buildings is a function of their original design and 
construction, various additions or modifications, and the condition of building 
components (which may have weakened due to deterioration or fatigue). For 
existing buildings, a two-step procedure is also recommended.

Step 1: Calculate the wind loads on the building using the current edition of 
ASCE 7, and compare these loads with the loads for which the building was 
originally designed. The original design loads may be noted on the contract 
drawings. If not, determine what building code or standard was used to develop 
the original design loads, and calculate the loads using that code or standard. 
If the original design loads are significantly lower than current loads, 
upgrading the load resistance of the building envelope and/or structure should 
be considered. An alternative to comparing current loads with original design 
loads is to evaluate the resistance of the existing facility as a function of 
the current loads to determine what elements are highly overstressed.

Step 2: Perform a field investigation to evaluate the primary building envelope 
elements, rooftop equipment, and structural system elements, to determine if the 
facility was generally constructed as indicated on the original contract 
drawings. As part of the investigation, the primary elements should be checked 
for deterioration. Load path continuity should also be checked.

If the results of either step indicate the need for remedial work, see Section 
4.4.




4.3 REQUIREMENTS AND BEST PRACTICES IN HIGH-WIND REGIONS 

4.3.1 General Hospital Design Considerations

The performance of hospitals in past wind storms indicates that the most 
frequent and the most significant factor in the disruption of the operations of 
these facilities has been the failure of nonstructural building components. 
While acknowledging the importance of the structural systems, Chapter 4 
emphasizes the building envelope components and the nonstructural systems. 
According to National Institute of Building Sciences (NIBS), the building 
envelope includes the below-grade basement walls and foundation and floor slab 
(although these are generally considered part of the building’s structural 
system). The envelope includes everything that separates the interior of a 
building from the outdoor environment, including the connection of all the 
nonstructural elements to the building structure. The nonstructural systems 
include all mechanical, electrical, electronic, communications, and lightning 
protection systems. Historically, damage to roof coverings and rooftop equipment 
has been the leading cause of building performance problems during windstorms. 
Special consideration should be given to the problem of water infiltration 
through failed building envelope components, which can cause severe disruptions 
in the functioning of hospitals. 

The key to enhanced wind performance is paying sufficient attention to all 
phases of the construction process (including site selection, design, and 
construction) and to post-occupancy maintenance and repair. 

Hospital Design Considerations in Hurricane-Prone Regions

Following the general design and construction recommendations, this manual 
presents recommendations specific to hospitals located in hurricane-prone 
regions. These recommendations are additional to the ones presented for 
hospitals located outside of hurricane-prone regions, and in many cases 
supersede those recommendations. Hospitals located in hurricane-prone regions 
require special design and construction attention because of the unique 
characteristics of this type of windstorm. Hurricanes can bring very high winds 
that last for many hours, which can lead to material fatigue failures. The 
variability of wind direction increases the probability that the wind will 
approach the building at the most critical angle. Hurricanes also generate a 
large amount of wind-borne debris, which can damage various building components 
and cause injury and death. 

Hospitals in hurricane-prone regions require special attention because they 
normally have vulnerable occupants (patients) at the time of a hurricane, and 
afterwards, many injured people seek medical care. Significant damage to a 
hospital can put patients at risk and jeopardize delivery of care to those 
seeking treatment. In order to ensure continuity of service during and after 
hurricanes, the design, construction, and maintenance of hospitals should be 
very robust to provide sufficient resiliency to withstand the effects of 
hurricanes.

[Begin text box]
Because of advanced warning of impending land fall, with the exception of 
Hurricane Katrina (Louisiana and Mississippi, 2005), the death toll from 
hurricanes in the U.S. has been extremely low for the last several decades. 
However, large numbers of people are often injured and seek care at hospitals. 
Blunt-force trauma injuries caused by wind-borne debris, falling trees, 
collapsed ceilings, or partial building collapse occur during hurricanes. But 
most of the hurricane-related injuries typically occur in the days afterward. 
These injuries are typically due to chainsaw accidents, stepping on nails, 
lacerations incurred while removing debris, vehicle accidents at intersections 
that no longer have functional traffic lights, people falling off roofs as they 
attempt to make emergency repairs, and carbon monoxide poisoning or electrical 
shock from improper use of emergency generators. Therefore, at a time when many 
hospitals in an area may be functionally impaired or no longer capable of 
providing service due to building damage (Figures 4-1 and 4-8), hospital staffs 
are faced with a higher than normal number of people seeking treatment. Before 
arrival of a hurricane, hospitals also often receive an influx of women in their 
third trimester of pregnancy, so that they will already be at the hospital in 
case they go into labor during the storm or shortly thereafter, when getting to 
the hospital could be hazardous or impossible.
[End text box]

Full or partial evacuation of a hospital prior to, during, or after a hurricane 
is time consuming, expensive, and for some patients, potentially life 
threatening. Water infiltration that could damage electrical equipment or 
medical supplies, or inhibit the use of critical areas (such as operating rooms 
and nursing floors) needs to be prevented. The emergency and standby power 
systems need to remain operational and be adequately sized to power all needed 
circuits, including the HVAC system. Provisions are needed for water and sewer 
service in the event of loss of municipal services, and antenna towers need to 
be strong enough to resist the wind. 

4.3.1.1 Site 

When selecting land for a hospital, sites located in Exposure D (see ASCE 7 for 
exposure definitions) should be avoided if possible. Selecting a site in 
Exposure C or preferably in Exposure B would decrease the wind loads. Also, 
where possible, avoid selecting sites located on an escarpment or the upper half 
of a hill, where the abrupt change in the topography would result in increased 
wind loads. [footnote 7]

[Begin footnote 7] When selecting a site on an escarpment or the upper half of a 
hill is necessary, the ASCE 7 design procedure accounts for wind speed-up 
associated with this abrupt change in topography. [end footnote]

Trees with trunks larger than 6 inches in diameter, poles (e.g., light fixture 
poles, flagpoles, and power poles), or towers (e.g., electrical transmission and 
large communication towers) should not be placed near the building. Falling 
trees, poles, and towers can severely damage a hospital and injure the occupants 
(see Figure 4-24). Large trees can crash through pre-engineered metal buildings 
and wood frame construction. Falling trees can also rupture roof membranes and 
break windows.

[Begin figure]
Figure 4-24 is a photo of the roof membrane on this hospital’s materials 
management facility was ruptured by falling trees. Hurricane Ivan (Florida, 
2004) 
[end figure]


Street signage should be designed to resist the design wind loads so that 
toppled signs do not block access roads or become wind-blown debris. AASHTO LTS-
4-M (amended by LTS-4-12 2001 and 2003, respectively) provides guidance for 
determining wind loads on highway signs. 

Providing at least two means of site egress is prudent for all hospitals, but is 
particularly important for hospitals in hurricane-prone regions. If one route 
becomes blocked by trees or other debris, or by floodwaters, the other access 
route may still be available.

4.3.1.2 Building Design 

Good wind performance depends on good design (including details and 
specifications), materials, installation, maintenance, and repair. A significant 
shortcoming in any of these five elements could jeopardize the performance of a 
hospital against wind. Design, however, is the key element to achieving good 
performance of a building against wind damage. Design inadequacies frequently 
cannot be compensated for with other elements. Good design, however, can 
compensate for other inadequacies to some extent. The following steps should be 
included in the design process for hospitals.

[Begin text box]
In the past, design professionals seldom performed load calculations on the 
building envelope (i.e., roof and wall coverings, doors, windows, and skylights) 
and rooftop equipment. These building components are the ones that have failed 
the most during past wind events. In large part they failed because of the lack 
of proper load determination and inappropriate design of these elements. It is 
imperative that design professionals determine the loads for the building 
envelope and rooftop equipment, and design them to accommodate such loads.
[End text box]

Step 1: Calculate Loads 

Calculate loads on the MWFRS, the building envelope, and rooftop equipment in 
accordance with ASCE 7 or the local building code, whichever procedure results 
in the highest loads. In calculating wind loads, design professionals should 
consider the following items.

Importance factor: The effect of using a 1.15 importance factor versus 1 is that 
the design loads for the MWFRS and C&C are increased by 15 percent. The 
importance factor for hospitals is required to be 1.15. However, some buildings 
on a hospital campus, such as medical office buildings that are integrally 
connected to the hospital and various types of non-emergency treatment 
facilities (such as storage, cancer treatment, physical therapy, and dialysis), 
are not specifically required by ASCE 7 to be designed with a 1.15 factor. This 
manual recommends a value of 1.15 for all facilities on a hospital campus.

[Begin text box]
Uplift loads on roof assemblies can also be determined from FM Global (FMG) Data 
Sheets. If the hospital is FMG insured, and the FMG-derived loads are higher 
than those derived from ASCE 7 or the building code, the FMG loads should 
govern. However, if the ASCE 7 or code-derived loads are higher than those from 
FMG, the ASCE 7 or code-derived loads should govern (whichever procedure results 
in the highest loads).
[End text box]

Wind directionality factor: The ASCE 7 wind load calculation procedure 
incorporates a wind directionality factor (kd). The directionality factor 
accounts for the reduced probability of maximum winds coming from any given 
direction. By applying the prescribed value of 0.85, the loads are reduced by 15 
percent. Because hurricane winds can come from any direction, and because of the 
historically poor performance of building envelopes and rooftop equipment, this 
manual recommends a more conservative approach for hospitals in hurricane-prone 
regions. A directionality factor of 1.0 is recommended for the building envelope 
and rooftop equipment (a load increase over what is required by ASCE 7). For the 
MWFRS, a directionality factor of 0.85 is recommended (hence, no change for 
MWFRS).

Step 2: Determine Load Resistance

When using allowable stress design, after loads have been determined, it is 
necessary to determine a reasonable safety factor in order to select the minimum 
required load resistance. For building envelope systems, a minimum safety factor 
of 2 is recommended. For anchoring exterior-mounted mechanical, electrical, and 
communications equipment (such as satellite dishes), a minimum safety factor of 
3 is recommended. When using strength design, load combinations and load factors 
specified in ASCE 7 are used.

[Begin text box]
When using allowable stress design, a safety factor is applied to account for 
reasonable variations in material strengths, construction workmanship, and 
conditions when the actual wind speed somewhat exceeds the design wind speed. 
For design purposes, the ultimate resistance an assembly achieves in testing is 
reduced by the safety factor. For example, if a roof assembly resisted an uplift 
pressure of 100 pounds per square foot (psf), after applying a safety factor of 
2, the assembly would be suitable where the design load was 50 psf or less. 
Conversely, if the design load is known, multiplying it by the safety factor 
equals the minimum required test pressure (e.g., 50 psf design load multiplied 
by a safety factor of 2 equals a minimum required test pressure of 100 psf).
[End text box]

ASCE 7 provides criteria for combining wind loads with other types of loads 
(such as dead and flood loads) using allowable stress design.

For structural members and cladding elements where strength design can be used, 
load resistance can be determined by calculations. For other elements where 
allowable stress design is used (such as most types of roof coverings), load 
resistance is primarily obtained from system testing.

The load resistance criteria need to be provided in contract documents. For 
structural elements, the designer of record typically accounts for load 
resistance by indicating the material, size, spacing, and connection of the 
elements. For nonstructural elements, such as roof coverings or windows, the 
load and safety factor can be specified. In this case, the specifications should 
require the contractor’s submittals to demonstrate that the system will meet the 
load resistance criteria. This performance specification approach is necessary 
if, at the time of the design, it is unknown who will manufacture the system.

Regardless of which approach is used, it is important that the designer of 
record ensure that it can be demonstrated, via calculations or tests, that the 
structure, building envelope, and nonstructural systems (exterior-mounted 
mechanical, electrical, and communications equipment) have sufficient strength 
to resist design wind loads.

Step 3: Detailed Design

It is vital to design, detail, and specify the structural system, building 
envelope, and exterior-mounted mechanical, electrical, and communications 
equipment to meet the factored design loads (based on appropriate analytical or 
test methods). It is also vital to respond to the risk assessment criteria 
discussed in Section 4.2.2, as appropriate.

[Begin text box]
Connections are a key aspect of load path continuity between various structural 
and nonstructural building elements. In a window, for example, the glass must be 
strong enough to resist the wind pressure and must be adequately anchored to the 
window frame, the frame adequately anchored to the wall, the wall to the 
foundation, and the foundation to the ground. As loads increase, greater load 
capacity must be developed in the connections.
[End text box]

As part of the detailed design effort, load path continuity should be clearly 
indicated in the contract documents via illustration of connection details. Load 
paths need to accommodate design uplift, racking, and overturning loads. Load 
path continuity obviously applies to MWFRS elements, but it also applies to 
building envelope elements. Figure 4-25 shows a load path discontinuity between 
a piece of HVAC equipment and its equipment stand. The equipment on this new 
building blew away because it was resting on vibration isolators that provided 
lateral resistance, but no uplift resistance (also see Figure 4-92).

[Begin figure]
Figure 4-25 is a photo of emporary coverings placed over two large openings in 
the roof that were left after the ductwork blew away. Hurricane Katrina 
(Mississippi, 2005)
[end figure]

Figure 4-26 illustrates the load path concept. Members are sized to accommodate 
the design loads. Connections are designed to transfer uplift loads applied to 
the roof, and the positive and negative loads applied to the exterior bearing 
walls, down to the foundation and into the ground. The roof covering (and wall 
covering, if there is one) is also part of the load path. To avoid blow-off, the 
nonstructural elements must also be adequately attached to the structure.

[Begin figure]
Figure 4-26 Illustration of load path continuity 
[end figure]

As part of the detailed design process, special consideration should be given to 
the durability of materials and water infiltration.

Durability: Because some locales have very aggressive atmospheric corrosion 
(such as areas near oceans), special attention needs to be given to the 
specification of adequate protection for ferrous metals, or to specify 
alternative metals such as stainless steel. FEMA Technical Bulletin, Corrosion 
Protection for Metal Connectors in Coastal Areas (FIA-TB-8, 1996), contains 
information on corrosion protection. Attention also needs to be given to dry rot 
avoidance, for example, by specifying preservative-treated wood or developing 
details that avoid excessive moisture accumulation. Appendix J of the Coastal 
Construction Manual, (FEMA 55, 2000) presents information on wood durability. 

[Begin text box]
Further information on the rain-screen principle can be found in the National 
Institute of Building Sciences’ Building Envelope Design Guide 
(www.wbdg.org/design/envelope.php).
[End text box]

Durable materials are particularly important for components that are 
inaccessible and cannot be inspected regularly (such as fasteners used to attach 
roof insulation). Special attention also needs to be given to details. For 
example, details that do not allow water to stand at connections or sills are 
preferred. Without special attention to material selection and details, the 
demands on maintenance and repair will be increased, along with the likelihood 
of failure of components during high winds.

Water infiltration (rain): Although prevention of building collapse and major 
building damage is the primary goal of wind-resistant design, consideration 
should also be given to minimizing water damage and subsequent development of 
mold from the penetration of wind-driven rain. To the extent possible, non-load-
bearing walls and door and window frames should be designed in accordance with 
rain-screen principles. With this approach, it is assumed that some water will 
penetrate past the face of the building envelope. The water is intercepted in an 
air-pressure equalized cavity that provides drainage from the cavity to the 
outer surface of the building. See Sections 4.3.3.1 and 4.3.3.5, and Figure 4-45 
for further discussion and an example. 

[Begin text box]
Coastal environments are conducive to metal corrosion, especially in buildings 
within 3,000 feet of the ocean. Most jurisdictions require metal building 
hardware to be hot-dipped galvanized or stainless steel. Some local codes 
require protective coatings that are thicker than typical “off-the-shelf” 
products. For example, a G90 zinc coating (0.75 mil on each face) may be 
required. Other recommendations include the following:

- Use hot-dipped galvanized or stainless steel hardware. Reinforcing steel 
should be fully protected from corrosion by the surrounding material (masonry, 
mortar, grout, or concrete). Use galvanized or epoxy-coated reinforcing steel in 
situations where the potential for corrosion is high.

- Avoid joining dissimilar metals, especially those with high galvanic 
potential.

- Avoid using certain wood preservatives in direct contact with galvanized 
metal. Verify that wood treatment is suitable for use with galvanized metal, or 
use stainless steel.

- Metal-plate-connected trusses should not be exposed to the elements. Truss 
joints near vent openings are more susceptible to corrosion and may require 
increased corrosion protection.

Note: Although more resistant than other metals, stainless steel is still 
subject to corrosion. 
[End text box]

In conjunction with the rain-screen principle, it is desirable to avoid using 
sealant as the first or only line of defense against water infiltration. When 
sealant joints are exposed, obtaining long-lasting watertight performance is 
difficult because of the complexities of sealant joint design and installation 
(see Figure 4-45, which shows the sealant protected by a removable stop).

Step 4: Peer Review

If the design team’s wind expertise and experience is limited, wind design input 
and/or peer review should be sought from a qualified individual. The design 
input or peer review could be arranged for the entire building, or for specific 
components, such as the roof or glazing systems, that are critical and beyond 
the 
design team’s expertise. 

Regardless of the design team’s expertise and experience, peer review should be 
considered when a hospital:

- Is located in an area where the basic wind speed is greater than 90 mph (peak 
gust).

- Will incorporate a tornado shelter.

4.3.1.3 Construction Contract Administration

After a suitable design is complete, the design team should endeavor to ensure 
that the design intent is achieved during construction. The key elements of 
construction contract administration are submittal reviews and field 
observations, as discussed below.

Submittal reviews: The specifications need to stipulate the submittal 
requirements. This includes specifying what systems require submittals (e.g., 
windows) and test data (where appropriate). Each submittal should demonstrate 
the development of a load path through the system and into its supporting 
element. For example, a window submittal should show that the glazing has 
sufficient strength, its attachment to the frame is adequate, and the attachment 
of the frame to the wall is adequate.

During submittal review, it is important for the designer of record to be 
diligent in ensuring that all required documents are submitted and that they 
include the necessary information. The submittal information needs to be 
thoroughly checked to ensure its validity. For example, if an approved method 
used to demonstrate compliance with the design load has been altered or 
incorrectly applied, the test data should be rejected, unless the contractor can 
demonstrate the test method was suitable. Similarly, if a new test method has 
been developed by a manufacturer or the contractor, the contractor should 
demonstrate its suitability.

Field observations: It is recommended that the design team analyze the design to 
determine which elements are critical to ensuring high-wind performance. The 
analysis should include the structural system and exterior-mounted electrical 
equipment, but it should focus on the building envelope and exterior-mounted 
mechanical and communications equipment. After determining the list of critical 
elements to be observed, observation frequency and the need for special 
inspections by an inspection firm should be determined. Observation frequency 
and the need for special inspections will depend on the magnitude of the results 
of the risk assessment described in Section 4.2.2, complexity of the facility, 
and the competency of the general contractor, subcontractors, and suppliers.

4.3.1.4 Post-Occupancy Inspections, Periodic Maintenance, Repair, and 
Replacement

The design team should advise the building owner of the importance of periodic 
inspections, maintenance, and timely repair. It is important for the building 
owner to understand that a facility’s wind resistance will degrade over time due 
to exposure to weather unless it is regularly maintained and repaired. The goal 
should be to repair or replace items before they fail in a storm. This approach 
is less expensive than waiting for failure and then repairing the failed 
components and consequential damage. 

The building envelope and exterior-mounted equipment should be inspected once a 
year by persons knowledgeable of the systems/materials they are inspecting. 
Items that require maintenance, repair, or replacement should be documented and 
scheduled for work. For example, the deterioration of glazing is often 
overlooked. After several years of exposure, scratches and chips can become 
extensive enough to weaken the glazing. Also, if an engineered film was surface-
applied to glazing for wind-borne debris protection, the film should be 
periodically inspected and replaced before it is no longer effective.

A special inspection is recommended following unusually high winds (such as a 
thunderstorm with wind speeds of 70 mph peak gust or greater). The purpose of 
the inspection is to assess whether the storm caused damage that needs to be 
repaired to maintain building strength and integrity. In addition to inspecting 
for obvious signs of damage, the inspector should determine if cracks or other 
openings have developed that may allow water infiltration, which could lead to 
corrosion or dry rot of concealed components.

4.3.1.5 Site and General Design Considerations in Hurricane-Prone Regions

Via ASCE 7, the 2006 edition of the IBC has only one special wind-related 
provision pertaining to hospitals in hurricane-prone regions. It pertains to 
glazing protection within wind-borne debris regions (as defined in ASCE 7). This 
single additional requirement does not provide adequate protection for occupants 
of a hospital during a hurricane, nor does it ensure a hospital will remain 
functional during and after a hurricane. A hospital may comply with IBC but 
still remain vulnerable to water and missile penetration through the roof or 
walls. To provide occupant protection, the exterior walls and the roof must be 
designed and constructed to resist wind-borne debris as discussed in Sections 
4.3.2.1, 4.3.3.2, 4.3.3.4, 4.3.3.6, and 4.3.3.8. The following recommendations 
are made regarding siting:

- Locate poles, towers, and trees with trunks larger than 6 inches in diameter 
away from primary site access roads so that they do not block access to, or hit, 
the facility if toppled.

- Determine if existing buildings within 1,500 feet of the new facility have 
aggregate surfaced roofs. If roofs with aggregate surfacing are present, it is 
recommended that the aggregate be removed to prevent it from striking the new 
facility. Aggregate removal may necessitate reroofing or other remedial work in 
order to maintain the roof’s fire or wind resistance.

- In cases where multiple buildings are occupied during a storm, it is 
recommended that enclosed walkways be designed to connect the buildings. The 
enclosed walkways (above- or below-grade) are particularly important for 
protecting people moving between buildings during a hurricane (e.g., to retrieve 
equipment or supplies) or for situations when it is necessary to evacuate 
occupants from one building to another during a hurricane (see Figure 4-27).

[Begin figure]
Figure 4-27 is a photo showing how open walkways do not provide protection from 
wind-borne debris. (Hurricane Katrina, Mississippi)
[end figure]

4.3.2 Structural Systems 

Based on post-storm damage evaluations, with the exception of strong and violent 
tornado events, the structural systems (i.e., MWFRS and structural components 
such as roof decking) of hospitals have typically performed quite well during 
design wind events. There have, however, been notable exceptions; in these 
cases, the most common problem has been blow-off of the roof deck, but instances 
of collapse have also been documented (Figure 4-34). The structural problems 
have primarily been caused by lack of an adequate load path, with connection 
failure being a common occurrence. Problems have also been caused by workmanship 
errors (commonly associated with steel decks attached by puddle welds), and 
limited uplift resistance of deck connections in roof perimeters and corners 
(due to lack of code-required enhancement in older editions of the model codes).

With the exception of strong and violent tornado events, structural systems 
designed and constructed in accordance with the IBC should typically offer 
adequate wind resistance, provided attention was given to load path continuity 
and to the durability of building materials (with respect to corrosion and 
termites). However, the greatest reliability is offered by cast-in-place 
concrete. There are no known reports of any cast-in-place concrete buildings 
experiencing a significant structural problem during wind events, including the 
strongest hurricanes (Category 5) and tornadoes (F5). 

The following design parameters are recommended for structural systems:

- If a pre-engineered metal building is being contemplated, special steps should 
be taken to ensure the structure has more redundancy than is typically the case 
with pre-engineered buildings. Steps should be taken to ensure the structure is 
not vulnerable to progressive collapse in the event a primary bent (steel moment 
frame) is compromised or bracing components fail.

- Exterior load-bearing walls of masonry or precast concrete should be designed 
to have sufficient strength to resist external and internal loading when 
analyzed as C&C. CMU walls should have vertical and horizontal reinforcing and 
grout to resist wind loads. The connections of precast concrete wall panels 
should be designed to have sufficient strength to resist wind loads.

- For roof decks, concrete, steel, plywood, or oriented strand board (OSB) is 
recommended. 

- For steel roof decks, it is recommended that a screw attachment be specified, 
rather than puddle welds or powder-driven pins. Screws are more reliable and 
much less susceptible to workmanship problems. Figure 4-28 shows decking that 
was attached with puddle welds. At most of the welds, there was only superficial 
bonding of the metal deck to the joist, as illustrated by this example. Only a 
small portion of the deck near the center of the weld area (as delineated by the 
circle) was well fused to the joist. Figures 4-29 and 4-30 show problems with 
acoustical decking attached with powder-driven pins. The pin shown on the left 
of Figure 4-30 is properly seated. However, the pin at the right did not 
penetrate far enough into the steel joist below. 

[Begin figures]
Figure 4-28 is a photo of a view looking down at the top of a steel joist after 
the metal decking blew away. Only a small portion of the deck was well fused to 
the joist (circled area). Tornado (Oklahoma, 1999)

Figure 4-29 is a photo looking down at a sidelap of a deck attached with powder-
driven pins. The washer at the top pin blew through the deck.

Figure 4-30 is a photo of a view looking along a sidelap of a deck attached with 
powder-driven pins. The right pin does not provide adequate uplift and shear 
resistance.
[End figures]

- For attaching wood-sheathed roof decks, screws, ring-shank, or screw-shank 
nails are recommended in the corner regions of the roof. Where the basic wind 
speed is greater than 90 mph, these types of fasteners are also recommended for 
the perimeter regions of the roof.

- For precast concrete decks it is recommended that the deck connections be 
designed to resist the design uplift loads because the deck dead load itself is 
often insufficient to resist the uplift. The deck in Figure 4-31 had bolts to 
provide uplift resistance; however, anchor plates and nuts had not been 
installed. Without the anchor plates, the dead load of the deck was insufficient 
to resist the wind uplift load.

[Begin figure]
Figure 4-31 is a photo of portions of this waffled precast concrete roof deck 
were blown off. Typhoon Paka (Guam, 1997)
[end figure]

- For precast Tee decks, it is recommended that the reinforcing be designed to 
accommodate the uplift loads in addition to the gravity loads. Otherwise, large 
uplift forces can cause member failure due to the Tee’s own pre-stress forces 
after the uplift load exceeds the dead load of the Tee. This type of failure 
occurred at one of the roof panels shown in Figure 4-32, where a panel lifted 
because of the combined effects of wind uplift and pre-tension. Also, because 
the connections between the roof and wall panels provided very little uplift 
load resistance, several other roof and wall panels collapsed.

[Begin figure]
Figure 4-32 is a photo of a Twin-Tee roof panel lifted as a result of the 
combined effects of wind uplift and pre-tension. Tornado (Missouri, May 2003)
[end figure]

- For buildings that have mechanically attached single-ply or modified bitumen 
membranes, designers should refer to the decking recommendations presented in 
the Wind Design Guide for Mechanically Attached Flexible Membrane Roofs, B1049 
(National Research Council of Canada, 2005).

[Begin text box]
ASCE 7-05 provides pressure coefficients for open canopies of various slopes 
(referred to as “free roofs” in ASCE 7). The free roof figures for MWFRS in ASCE 
7-05 (Figures 6-18A to 6-18D) include two load cases, Case A and Case B. While 
there is no discussion describing the two load cases, they pertain to 
fluctuating loads and are intended to represent upper and lower limits of 
instantaneous wind pressures. Loads for both cases must be calculated to 
determine the critical loads. Figures 6-18A to 6-18C are for a wind direction 
normal to the ridge. For wind direction parallel to the ridge, use Figure 6-18D 
in ASCE 7-05.
[End text box]

- If an FMG-rated roof assembly is specified, the roof deck also needs to comply 
with the FMG criteria.

- Walkway and entrance canopies are often damaged during high winds (see Figure 
4-33). Wind-borne debris from damaged canopies can damage nearby buildings and 
injure people, hence these elements should also receive design and construction 
attention. 

[Begin figure]
Figure 4-33 is a photo of the destroyed walkway canopy in front of this building 
became wind-borne debris. Hurricane Ivan (Florida, 2004) 
[end figure]

4.3.2.1 Structural Systems in Hurricane-Prone Regions

Because of the exceptionally good wind performance and wind-borne debris 
resistance that reinforced cast-in-place concrete structures offer, a reinforced 
concrete roof deck and reinforced concrete or reinforced and fully grouted CMU 
exterior walls are recommended as follows:

Roof deck: A minimum 4-inch-thick, cast-in-place reinforced concrete deck is the 
preferred deck. Other recommended decks are minimum 4-inch-thick structural 
concrete topping over steel decking, and precast concrete with an additional 
minimum 4-inch structural concrete topping.

[Begin text box]
If precast concrete is used for the roof or wall structure, the connections 
should be carefully designed, detailed, and constructed.
[End text box]

If these recommendations are not followed for hospitals located in areas where 
the basic wind speed is 100 mph or greater, it is recommended that the roof 
assembly be able to resist complete penetration of the deck by the “D” missile 
specified in ASTM E 1996 (2005) (see text box in Section 4.3.3.2). 

Exterior load-bearing walls: A minimum 6-inch-thick cast-in-place concrete wall 
reinforced with #4 rebars at 12 inches on center each way is the preferred wall. 
Other recommended walls are a minimum 8-inch-thick fully grouted CMU reinforced 
vertically with #4 rebars at 16 inches on center, and precast concrete that is a 
minimum 6-inches-thick and reinforced equivalent to the recommendations for 
cast-in-place walls.

4.3.3 Building Envelope 

The following section highlights the design considerations for building envelope 
components that have historically sustained the greatest and most frequent 
damage in high winds.

The design considerations for building envelope components of hospitals in 
hurricane-prone regions include a number of additional recommendations. The 
principal concern that must be addressed is the additional risk from wind-borne 
debris and water leakage. Design considerations specific to hurricane-prone 
regions are discussed in Sections 4.3.3.2, 4.3.3.4, 4.3.3.6, and 4.3.3.8.

4.3.3.1 Exterior Doors

This section addresses primary and secondary egress doors, sectional (garage) 
doors, and rolling doors. Although blow-off of personnel doors is uncommon, it 
can cause serious problems (see Figure 4-34). Blown-off doors allow entrance of 
rain and tumbling doors can damage buildings and cause injuries. 

Blown off sectional and rolling doors are quite common. These failures are 
typically caused by the use of door and track assemblies that have insufficient 
wind resistance, or by inadequate attachment of the tracks or nailers to the 
wall (see Figure 4-35). 

[Begin figures]
Figure 4-34 is a photo of a door on a hospital penthouse blown off its hinges 
during Hurricane Katrina (Mississippi, 2005)

Figure 4-35 is a photo showing how this new rolling door failed because the CMU 
spalled at the door frame’s expansion bolts, which were too close to the end of 
the CMU. Hurricane Charley (Florida, 2004)
[End figures]

[Begin text box]
For further general information on doors, see “Fenestration Systems” in the 
National Institute of Building Sciences’ Building Envelope Design Guide 
(www.wbdg.org/design/envelope.php).
[End text box]

Loads and Resistance

The IBC requires that the door assembly (i.e., door, hardware, frame, and frame 
attachment to the wall) be of sufficient strength to resist the positive and 
negative design wind pressure. Design professionals should require that doors 
comply with wind load testing in accordance with ASTM E 1233. Design 
professionals should also specify the attachment of the door frame to the wall 
(e.g., type, size, spacing, and edge distance of frame fasteners). For sectional 
and rolling doors attached to wood nailers, design professionals should also 
specify the attachment of the nailer to the wall.

[Begin text box]
For design guidance on attachment of door frames, see Technical Data Sheet #161, 
Connecting Garage Door Jambs to Building Framing, published by the Door & Access 
Systems Manufacturers Association, 2003 (available at www.dasma.com).
[End text box]

Water Infiltration
Heavy rain that accompanies high winds (e.g., thunderstorms, tropical storms, 
and hurricanes) can cause significant wind-driven water infiltration problems. 
The magnitude of the problem increases with the wind speed. Leakage can occur 
between the door and its frame, the frame and the wall, and between the 
threshold and the door. When wind speeds approach 120 mph, some leakage should 
be anticipated because of the very high wind pressures and numerous 
opportunities for leakage path development. 

[Begin text box]
Where corrosion is problematic, anodized aluminum or galvanized doors and 
frames, and stainless steel frame anchors and hardware are recommended. 
[End text box]

The following recommendations should be considered to minimize infiltration 
around exterior doors. 

Vestibule: Adding a vestibule allows both the inner and outer doors to be 
equipped with weatherstripping. The vestibule can be designed with water-
resistant finishes (e.g., concrete or tile) and the floor can be equipped with a 
drain. In addition, installing exterior threshold trench drains can be helpful 
(openings must be small enough to avoid trapping high-heeled shoes). Note that 
trench drains do not eliminate the problem, since water can still penetrate at 
door edges.

[Begin text box]
For primary swinging entry/exit doors, exit door hardware is recommended to 
minimize the possibility of the doors being pulled open by wind suction. Exit 
hardware with top and bottom rods is more secure than exit hardware that latches 
at the jamb.
[End text box]

Door swing: Out-swinging doors have weatherstripping on the interior side of the 
door, where it is less susceptible to degradation, which is an advantage when 
compared to in-swinging doors. Some interlocking weatherstripping assemblies are 
available for out-swinging doors.

The successful integration of the door frame and the wall is a special challenge 
when designing doors. See Section 4.3.3.3 for discussion of this juncture. 
ASTM E 2112 provides information pertaining to the installation of doors, 
including the use of sill pan flashings with end dams and rear legs (see Figure 
4-36). It is recommended that designers use ASTM E 2112 as a design resource.
Weatherstripping

Figure 4-36 is an illustration of a door sill pan flashing with end dams, rear 
leg, and turned-down front leg [end figure]

A variety of pre-manufactured weatherstripping components is available, 
including drips, door shoes and bottoms, thresholds, and jamb/head 
weatherstripping. 

Drips: These are intended to shed water away from the opening between the frame 
and the door head, and the opening between the door bottom and the threshold 
(see Figures 4-37 and 4-38). Alternatively, a door sweep can be specified (see 
Figure 4-38). For high-traffic doors, periodic replacement of the neoprene 
components will be necessary.

[Begin figures]
Figure 4-37 is an illustration of a drip at door head and drip with hook at head

Figure 4-38 is an illustration of a door shoe with drip and vinyl seal (left). 
Neoprene door bottom sweep (right)
[End figures]

Door shoes and bottoms: These are intended to minimize the gap between the door 
and the threshold. Figure 4-38 illustrates a door shoe that incorporates a drip. 
Figure 4-39 illustrates an automatic door bottom. Door bottoms can be surface-
mounted or mortised. For high-traffic doors, periodic replacement of the 
neoprene components will be necessary.

[Begin figure]
Figure 4-39 is an illustration of an automatic door bottom 
[end figure]

Thresholds: These are available to suit a variety of conditions. Thresholds with 
high (e.g., 1-inch) vertical offsets offer enhanced resistance to wind-driven 
water infiltration. However, the offset is limited where the thresholds are 
required to comply with the Americans with Disabilities Act (ADA), or at high-
traffic doors. At other doors, high offsets are preferred. 

Thresholds can be interlocked with the door (see Figure 4-40), or thresholds can 
have a stop and seal (see). In some instances, the threshold is set directly on 
the floor. Where this is appropriate, setting the threshold in butyl sealant is 
recommended to avoid water infiltration between the threshold and the floor. In 
other instances, the threshold is set on a pan flashing (as previously discussed 
in this section). If the threshold has weep holes, specify that the weep holes 
not be obstructed during construction (see Figure 4-40).

[Begin figures]
Figure 4-40 is an illustration of an interlocking threshold with drain pan

Figure 4-41 is an illustration of a threshold with stop and seal
[End figures]

Adjustable jamb/head weatherstripping: This type of weatherstripping is 
recommended because the wide sponge neoprene offers good contact with the door 
(see Figure 4-42). The adjustment feature also helps to ensure good contact, 
provided the proper adjustment is maintained.

[Begin figure]
Figure 4-42 is an illustration of an adjustable jamb/head weatherstripping
[End figure]

Meeting stile: At the meeting stile of pairs of doors, an overlapping astragal 
weatherstripping offers greater protection than weatherstripping that does not 
overlap. 

4.3.3.2 Exterior Doors in Hurricane-Prone Regions

Although the ASCE-7 wind-borne debris provisions only apply to glazing within a 
portion of hurricane-prone regions, it is recommended that all hospitals located 
where the basic wind speed is 100 mph or greater comply with the following 
recommendations:

- To minimize the potential for missiles penetrating exterior doors and striking 
people inside the facility, it is recommended that doors (with and without 
glazing) be designed to resist the “E” missile load specified in ASTM E 1996. 
The doors should be tested in accordance with ASTM E 1886 (2005). The test 
assembly should include the door, door frame, and hardware. 

[Begin text box]
ASTM E 1996 specifies five missile categories, A through E. The missiles are of 
various weights and fired at various velocities during testing. Building type 
(critical or non-critical) and basic wind speed determine the missiles required 
for testing. Of the five missiles, the E missile has the greatest momentum. 
Missile E is required for critical facilities located where the basic wind speed 
is greater than or equal to 130 mph. Missile D is permitted where the basic wind 
speed is less than130 mph. FEMA 361 also specifies a missile for shelters. The 
shelter missile has much greater momentum than the D and E missiles, as shown 
below:

Missile: ASTM E 1996—D
Missile Weight: 9 pound 2x4 lumber
Impact Speed: 50 feet per second (34 mph)
Momentum: 14 lbf -s*

Missile: ASTM E 1996—E
Missile Weight: 9 pound 2x4 lumber
Impact Speed: 80 feet per second(55 mph)
Momentum: 22 lbf -s*

Missile: FEMA 361 (Shelter Missile)
Missile Weight: 15 pound 2x4 lumber
Impact Speed: 47 feet per second (100 mph)
Momentum: 68 lbf -s*

*lb sub-f -s = pounds force per second
[End text box]

4.3.3.3 Windows and Skylights

This section addresses general design considerations for exterior windows and 
skylights. For additional information on windows and skylights located in 
hurricane-prone regions, see Section 4.3.3.4, and for those in tornado-prone 
regions, see Section 4.5.

[Begin text box]
For further general information on windows, see the National Institute of 
Building Sciences’ Building Envelope Design Guide 
(www.wbdg.org/design/envelope.php).
[End text box]

Loads and Resistance

The IBC requires that windows, curtain walls, and skylight assemblies (i.e., the 
glazing, frame, and frame attachment to the wall or roof) have sufficient 
strength to resist the positive and negative design wind pressure (see Figure 4-
43). Design professionals should specify that these assemblies comply with wind 
load testing in accordance with ASTM E 1233. It is important to specify an 
adequate load path and to check its continuity during submittal review.

[Begin figure]
Figure 4-43 is a photo of two complete windows, including frames, blew out as a 
result of an inadequate number of fasteners. Typhoon Paka (Guam, 1997)
[end figure]

Where water infiltration protection is particularly demanding and important, it 
is recommended that onsite water infiltration testing in accordance with ASTM E 
1105 be specified.

Water Infiltration 

Heavy rain accompanied by high winds can cause wind-driven water infiltration 
problems. The magnitude of the problem increases with the wind speed. Leakage 
can occur at the glazing/frame interface, the frame itself, or between the frame 
and wall. When the basic wind speed is greater than 120 mph, because of the very 
high design wind pressures and numerous opportunities for leakage path 
development, some leakage should be anticipated when the design wind speed 
conditions are approached.

The successful integration of windows and curtain walls into exterior walls is a 
challenge in protecting against water infiltration. To the extent possible when 
detailing the interface between the wall and the window or curtain wall units, 
designers should rely on sealants as the secondary line of defense against water 
infiltration, rather than making the sealant the primary protection. If a 
sealant joint is the first line of defense, a second line of defense should be 
designed to intercept and drain water that drives past the sealant joint.

[Begin text box]
The maximum test pressure used in the current ASTM test standard for evaluating 
resistance of window units to wind-driven rain is well below design wind 
pressures. Therefore, units that demonstrate adequate wind-driven rain 
resistance during testing may experience leakage during actual wind events.
[End text box]

When designing joints between walls and windows and curtain wall units, consider 
the shape of the sealant joint (i.e., a square joint is typically preferred) and 
the type of sealant to be specified. The sealant joint should be designed to 
enable the sealant to bond on only two opposing surfaces (i.e., a backer rod or 
bond-breaker tape should be specified). Butyl is recommended as a sealant for 
concealed joints, and polyurethane for exposed joints. During installation, 
cleanliness of the sealant substrate is important (particularly if polyurethane 
or silicone sealants are specified), as is the tooling of the sealant. ASTM E 
2112 provides guidance on the design of sealant joints, as well as other 
information pertaining to the installation of windows, including the use of sill 
pan flashings with end dams and rear legs (see Figure 4-44). Windows that do not 
have nailing flanges should typically be installed over a pan flashing. It is 
recommended that designers use ASTM E 2112 as a design resource. 

Sealant joints can be protected with a removable stop, as illustrated in Figure 
4-45. The stop protects the sealant from direct exposure to the weather and 
reduces the possibility of wind-driven rain penetration.

[Begin figures]
Figure 4-44 Illustrative view of a typical window sill pan flashing with end 
dams and rear legs Source: ASTM E 2112 

Figure 4-45 Illustration showing how protecting sealant retards weathering and 
reduces the exposure to wind-driven rain.
[End figures]

4.3.3.4 Windows and Skylights in Hurricane-Prone Regions 

Exterior glazing that is not impact-resistant (such as laminated glass or 
polycarbonate) or protected by shutters is extremely susceptible to breaking if 
struck by wind-borne debris. Even small, low-momentum missiles can easily break 
glazing that is not protected (see Figures 4-46 and 4-47). At the hospital shown 
in Figure 4-46, approximately 400 windows were broken. Most of the breakage was 
caused by wind-blown aggregate from the hospital’s aggregate ballasted single-
ply membrane roofs, and aggregate from built-up roofs. At the hospital shown in 
Figure 4-47, several of the skylight’s tempered glass outer panes were broken by 
wind-blown aggregate from the hospital’s aggregate ballasted single-ply 
membrane. The inner laminated glass panes were not broken. Note that some of the 
copings were also blown off (blue arrow).Some of the glazing may have been 
damaged by wind-blown copings. At the grey hospital on the other side of the 
street, much of the roof membrane was blown away (yellow arrow).

With broken windows, a substantial amount of water can be blown into a building, 
and the internal air pressure can be greatly increased, which may damage the 
interior partitions and ceilings. 

[Begin figures]
Figure 4-46 is a photo of plywood panels (black continuous bands) installed 
after the glass spandrel panels were broken by roof aggregate. [footnote 9] 
Hurricane Katrina (Mississippi, 2005) 

Figure 4-47 is a photo showing how the outer glass panes of the skylight were 
broken by roof aggregate (red arrow). Hurricane Hugo (South Carolina, 1989)
[End figures] 

[begin footnote 9] Glass spandrel panels are opaque glass. They are placed in 
curtain walls to conceal the area between the ceiling and the floor above. [end 
footnote

In order to minimize interior damage, the IBC, through ASCE 7, prescribes that 
exterior glazing in wind-borne debris regions be impact-resistant, or be 
protected with an impact-resistant covering (shutters). For Category III and IV 
buildings in areas with a basic wind speed of 130 mph or greater, the glazing is 
required to resist a larger momentum test missile than would Category II 
buildings and Category III and IV buildings in areas with wind speeds of less 
than 130 mph.

ASCE 7 refers to ASTM E 1996 for missile loads and to ASTM E 1886 for the test 
method to be used to demonstrate compliance with the E 1996 load criteria. In 
addition to testing impact resistance, the window unit is subjected to pressure 
cycling after test missile impact to evaluate whether the window can still 
resist wind loads. If wind-borne debris glazing protection is provided by 
shutters, the glazing is still required by ASCE 7 to meet the positive and 
negative design air pressures.

Although the ASCE 7 wind-borne debris provisions only apply to glazing within a 
portion of hurricane-prone regions, it is recommended that all hospitals located 
where the basic wind speed is 100 mph or greater comply with the following 
recommendations:

- To minimize the potential for missiles penetrating exterior glazing and 
injuring people, it is recommended that exterior glazing up to 60 feet above 
grade be designed to resist the test Missile E load specified in ASTM E 1996 
(see text box in Section 4.3.3.2). In addition, if roofs with aggregate 
surfacing are present within 1,500 feet of the facility, glazing above 60 feet 
should be designed to resist the test Missile A load specified in ASTM E 1996. 
The height of the protected glazing should extend a minimum of 30 feet above the 
aggregate surfaced roof per ASCE 7. 

Because large missiles are generally flying at lower elevations, glazing that is 
more than 60 feet above grade and meets the test Missile A load should be 
sufficient. However, if the facility is within a few hundred feet of another 
building that may create debris, such as EIFS, tiles, or rooftop equipment, it 
is 
recommended that the test Missile E load be specified instead of the Missile A 
for the upper-level glazing.

- For those facilities where glazing resistant to bomb blasts is desired, the 
windows and glazed doors can be designed to accommodate wind pressure, missile 
loads, and blast pressure. However, the window and door units need to be tested 
for missile loads and cyclic air pressure, as well as for blast. A unit that 
meets blast criteria will not necessarily meet the E 1996 and E 1886 criteria, 
and vice versa. 

[Begin text box]
For further information on designing glazing to resist blast, see the “Blast 
Safety” resource pages of the National Institute of Building Sciences’ Building 
Envelope Design Guide (www.wbdg.org/design/envelope.php).
[End text box]

With the advent of building codes requiring glazing protection in wind-borne 
debris regions, a variety of shutter designs have entered the market. Shutters 
typically have a lower initial cost than laminated glass. However, unless the 
shutter is permanently anchored to the building (e.g., an accordion shutter), 
storage space will be needed. Also, when a hurricane is forecast, costs will be 
incurred each time shutters are installed and removed. The cost and difficulty 
of shutter deployment and demobilization on upper-level glazing may be avoided 
by using motorized shutters, although laminated glass may be a more economical 
solution. For further information on shutters, see Section 4.4.2.2.

4.3.3.5 Non-Load-Bearing Walls, Wall Coverings, and Soffits

[Begin text box]
For further general information on non-load-bearing walls and wall coverings, 
see the National Institute of Building Sciences’ Building Envelope Design Guide 
(www.wbdg.org/design/envelope.php).
[End text box]

This section addresses exterior non-load-bearing walls, exterior wall coverings, 
and soffits, as well as the underside of elevated floors, and provides guidance 
for interior non-load-bearing masonry walls. See Section 4.4.3.6 for additional 
information pertaining to hospitals located in hurricane-prone regions, and 
Section 4.5 for additional information pertaining to hospitals located in 
tornado-prone regions. 

[Begin text box]
To ensure the continuity of elevator service, elevator penthouse walls must 
possess adequate wind and water resistance. If the walls blow away or water 
leaks through the wall system, the elevator controls and/or motors can be 
destroyed. Loss of elevators may critically affect facility operations (see 
Figures 4-22 and 4-48). The restoration of elevator service can take weeks, even 
with expedited work.
[End text box]

[Begin figure]
Figure 4-48 is a photo showing how the wall covering blew off the penthouse at 
this hospital complex, allowing rainwater to destroy the elevator controls. 
Hurricane Ivan (Florida, 2004)
[end figure]

Loads and Resistance 

The IBC requires that soffits, exterior non-load-bearing walls, and wall 
coverings have sufficient strength to resist the positive and negative design 
wind pressures.

Soffits: Depending on the wind direction, soffits can experience either positive 
or negative pressure. Besides the cost of repairing the damaged soffits, wind-
borne soffit debris can cause property damage and injuries (see Figures 4-49 and 
4-50). Failed soffits may also provide a convenient path for wind-driven rain to 
enter the building. Storm-damage research has shown that water blown into attic 
spaces after the loss of soffits can cause significant damage and the collapse 
of ceilings. Even in instances where soffits remain in place, water can 
penetrate through soffit vents and cause damage. At this time, there are no 
known specific test standards or design guidelines to help design wind- and 
water-resistant soffits and soffit vents.

[Begin text box]
Where corrosion is a problem, stainless steel fasteners are recommended for wall 
and soffit systems. For other components (e.g., furring, blocking, struts, and 
hangers), nonferrous components (such as wood), stainless steel, or steel with a 
minimum of G-90 hot-dipped galvanized coating are recommended. Additionally, 
access panels are recommended so components within soffit cavities can be 
periodically inspected for corrosion or dry rot.
[End text box]

[Begin figures]
Figure 4-49 is a photo showing how this suspended metal soffit was not designed 
for upward-acting wind pressure. Typhoon Paka (Guam, 1997)

Figure 4-50 is a photo showing Hospital canopy damage. Hurricane Katrina 
(Louisiana, 2005) 
[end figures]

Exterior non-load-bearing masonry walls: Particular care should be given to the 
design and construction of exterior non-load-bearing masonry walls. Although 
these walls are not intended to carry gravity loads, they should be designed to 
resist the external and internal loading for components and cladding in order to 
avoid collapse. When these types of walls collapse, they represent a severe risk 
to life because of their great weight. 

Interior non-load-bearing masonry walls: Special consideration should also be 
given to interior non-load-bearing masonry walls. Although these walls are not 
required by building codes to be designed to resist wind loads, if the exterior 
glazing is broken, or the exterior doors are blown away, the interior walls 
could be subjected to significant load as the building rapidly becomes fully 
pressurized. To avoid casualties, it is recommended that interior non-load-
bearing masonry walls adjacent to occupied areas be designed to accommodate 
loads exerted by a design wind event, using the partially enclosed pressure 
coefficient (see Figure 4-51). By doing so, wall collapse may be prevented if 
the building envelope is breached. This recommendation is applicable to 
hospitals located in areas with a basic wind speed greater than 120 mph, and to 
hospitals in tornado-prone regions that do not have shelter space designed in 
accordance with FEMA 361.

[Begin figure]
Figure 4-51 is a photo of red arrows showing the original location of a CMU wall 
that nearly collapsed following a rolling door failure. Hurricane Charley 
(Florida, 2004)
[end figure]

Wall Coverings

There are a variety of exterior wall coverings. Brick veneer, exterior 
insulation finish systems (EIFS), stucco, metal wall panels, and aluminum and 
vinyl siding have often exhibited poor wind performance. Veneers (such as 
ceramic tile and stucco) over concrete, stone veneer, and cement-fiber panels 
and siding have also blown off. Wood siding and panels rarely blow off. Although 
tilt-up precast walls have failed during wind storms, precast wall panels 
attached to steel or concrete framed buildings typically offer excellent wind 
performance.

Brick veneer: [footnote 10] Brick veneer is frequently blown off walls during 
high winds. When brick veneer fails, wind-driven water can enter and damage 
buildings, and building occupants can be vulnerable to injury from wind-borne 
debris (particularly if the walls are sheathed with plastic foam insulation or 
wood fiberboard in lieu of wood panels). Pedestrians in the vicinity of damaged 
walls can also be vulnerable to injury from falling bricks (see Figure 4-52). 
Common failure modes include tie (anchor) fastener pull-out (see Figure 4-53), 
failure of masons to embed ties into the mortar, poor bonding between ties and 
mortar, a mortar of poor quality, and tie corrosion.

[Begin footnote 10] The brick veneer discussion is from Attachment of Brick 
Veneer in High-Wind Regions—Hurricane Katrina Recovery Advisory (FEMA, December 
2005). 

[Begin figures]
Figure 4-52 is a photo showing how the brick veneer failure on this building was 
attributed to tie corrosion. Hurricane Ivan (Florida, 2004)

Figure 4-53 is a photo showing how this tie remained embedded in the mortar 
joint while the smooth-shank nail pulled from the stud.
[end figures]

Ties are often installed before brick laying begins. When this is done, ties are 
often improperly placed above or below the mortar joints. When misaligned, the 
ties must be angled up or down to be embedded into the mortar joints (Figure 4-
54). Misalignment not only reduces the embedment depth, but also reduces the 
effectiveness of the ties, because wind forces do not act in parallel direction 
to the ties.

[Begin figure]
Figure 4-54 is an illustration of how misalignment of the tie reduces the 
embedment and promotes veneer failure.
[end figure]

Corrugated ties typically used in residential veneer construction provide little 
resistance to compressive loads induced by positive and negative pressure. The 
use of compression struts would likely be beneficial, but off-the-shelf devices 
do not currently exist. Two-piece adjustable ties (Figure 4-55) provide 
significantly greater compressive strength than corrugated ties.

[Begin figure]
Figure 4-55 is an illustration of examples of two-piece adjustable ties 
[end figure]

The following Brick Industry Association (BIA) technical notes provide guidance 
on brick veneer: Technical Notes 28: Anchored Brick Veneer, Wood Frame 
Construction (2002); Technical Notes 28B:Brick Veneer/Steel Stud Walls (2005); 
and Technical Notes 44B:Wall Ties (2003) (available online at www.bia.org). 
These technical notes provide attachment recommendations; however, they are not 
specific for high-wind regions. To enhance wind performance of brick veneer, the 
following are recommended:

- Calculate wind loads and determine tie spacing in accordance with the latest 
edition of the Building Code Requirements for Masonry Structures, ACI 530/ASCE 
5/TMS 402 (ACI 530, 2005). A stud spacing of 16 inches on center is recommended 
so that ties can be anchored at this spacing.

- Ring-shank nails are recommended in lieu of smooth-shank nails for wood studs. 
A minimum embedment of 2 inches is suggested.

- For use with wood studs, two-piece adjustable ties are recommended. However, 
where corrugated steel ties are used, they should be 22-gauge minimum, 7/8-inch 
wide by 6-inch long, and comply with ASTM A 1008, with a zinc coating complying 
with ASTM A 153 Class B2. For ties used with steel studs, see BIA Technical 
Notes 28B—Brick Veneer/Steel Stud Walls. Stainless steel ties should be used for 
both wood and steel studs in areas within 3,000 feet of the coast.

- Install ties as the brick is laid so that the ties are properly aligned with 
the mortar joints.

- Locate ties within 8 inches of door and window openings, and within 12 inches 
of the top of veneer sections.

- Although corrugated ties are not recommended, if they are used, bend the ties 
at a 90-degree angle at the nail head to minimize tie flexing when the ties 
cycle between tension and compression loads (Figure 4-56).

- Embed a minimum of 1½ inches into the bed joint, with a minimum mortar cover 
of 5/8-inch to the outside face of the wall (Figure 4-57). 

[Begin figures]
Figure 4-56 is an illustration showing bend ties at nail heads

Figure 4-57 is an illustration showing tie embedment 
[end figures]

To avoid water leaking into the building, it is important that weep holes be 
adequately spaced and not be blocked during brick installation, and that 
through-wall flashings be properly designed and installed. When the base of the 
brick veneer occurs near grade, the grade should be designed so that it occurs 
several inches below the weeps so that drainage from the weeps is not impeded. 
Also, landscaping should be kept clear of weeps so that vegetation growth does 
not cause blockage of weeps. At the hospital shown in Figure 4-58, water leaked 
into the building along the base of many of the brick veneer walls. When high 
winds accompany heavy rain, a substantial amount of water can be blown into the 
wall cavity. 

[Begin figure]
Figure 4-58 is a photo showing how water leaked inside along the base of the 
brick veneer walls (red arrow). Hurricane Katrina (Louisiana, 2005) 
[end figure] 

EIFS: Figure 4-59 shows typical EIFS assemblies. Figure 4-48 and several figures 
in Section 4.2.1.3 show EIFS blow-off. In these cases, the molded expanded 
polystyrene (MEPS) was attached to gypsum board, which in turn was attached to 
metal studs or hat channels. The gypsum board detached from the studs/hat 
channels, which is a common EIFS failure mode. When the gypsum board on the 
exterior side of the studs is blown away, it is common for gypsum board on the 
interior side to also be blown off. The opening allows the building to become 
fully pressurized and allows the entrance of wind-driven rain. Other common 
types of failure include wall framing failure, separation of the MEPS from its 
substrate, and separation of the synthetic stucco from the MEPS. 

[Begin figure]
Figure 4-59 is an illustration showing typical EIFS assemblies
[end figure] 

At the hospital shown in Figure 4-60, the EIFS was applied over a concrete wall. 
The MEPS debonded from the concrete. In general, a concrete substrate prevents 
wind and water from entering a building, but if the EIFS debonds from the 
concrete, EIFS debris can break unprotected glazing. Glazing damage can be very 
devastating, as shown and discussed in Section 4.2.1.3.

[Begin figure]
Figure 4-60 is a photo showing EIFS blown off a cast-in-place concrete wall. 
Note the damaged rooftop ductwork. Hurricane Ivan (Florida, 2004)
[end figure] 

Reliable wind performance of EIFS is very demanding on the designer and 
installer, as well as the maintenance of EIFS and associated sealant joints in 
order to minimize the reduction of EIFS’ wind resistance due to water 
infiltration. It is strongly recommended that EIFS be designed with a drainage 
system that allows for the dissipation of water leaks. For further information 
on EIFS performance during high winds and design guidance, see FEMA 489 and 549.

Another issue associated with EIFS is the potential for judgment errors. EIFS 
applied over studs is sometimes mistaken for a concrete wall, which may lead 
people to seek shelter behind it. However, instead of being protected by several 
inches of concrete, only two layers of gypsum board (i.e., one layer on each 
side of the studs) and a layer of MEPS separate the occupants from the impact of 
wind-borne debris that can easily penetrate such a wall and cause injury.

Stucco over studs: Wind performance of traditional stucco walls is similar to 
the performance of EIFS, as shown in Figure 4-61. In several areas the metal 
stud system failed; in other areas the gypsum sheathing blew off the studs; and 
in other areas, the metal lath blew off the gypsum sheathing. The failure shown 
in Figure 4-61 illustrates the importance of designing and constructing wall 
framing (including attachment of stud tracks to the building and attachment of 
the studs to the tracks) to resist the design wind loads.

[Begin figure]
Figure 4-61 is a photo showing how the stucco wall failure was caused by 
inadequate attachment between the stud tracks and the building’s structure. 
Hurricane Ivan (Florida, 2004)
[end figure]

Metal wall panels: Wind performance of metal wall panels is highly variable. 
Performance depends on the strength of the specified panel (which is a function 
of material and thickness, panel profile, panel width, and whether the panel is 
a composite) and the adequacy of the attachment (which can be by either 
concealed clips or exposed fasteners). Excessive spacing between clips/fasteners 
is the most common problem. Clip/fastener spacing should be specified, along 
with the specific type and size of fastener. Figure 4-62 illustrates metal wall 
panel problems. At this building, the metal panels were attached with concealed 
fasteners. The panels unlatched at the standing seams. In addition to generating 
wind-borne debris, loss of panels allowed wind-driven rain to enter the 
building. Water entry was facilitated by lack of a moisture barrier and solid 
sheathing behind the metal panels (as discussed below).

[Begin figure]
Figure 4-62 is a photo showing how the loss of metal wall panels allowed a 
substantial amount of wind-driven rain to penetrate this building. Hurricane 
Ivan (Florida, 2004)  
[end figure]

To minimize water infiltration at metal wall panel joints, it is recommended 
that sealant tape be specified at sidelaps when the basic wind speed is in 
excess of 90 mph. However, endlaps should be left unsealed so that moisture 
behind the panels can be wicked away. Endlaps should be a minimum of 3 inches (4 
inches where the basic wind speed is greater than 120 mph) to avoid wind-driven 
rain infiltration. At the base of the wall, a 3-inch (4-inch) flashing should 
also be detailed, or the panels should be detailed to overlap with the slab or 
other components by a minimum of 3 inches (4 inches).

Vinyl siding: Vinyl siding blow-off is typically caused by nails spaced too far 
apart and/or the use of vinyl siding that has inadequate wind resistance. Vinyl 
siding is available with enhanced wind resistance features, such as an enhanced 
nailing hem, greater interlocking area, and greater thickness. 

[Begin text box]
The Vinyl Siding Institute (VSI) sponsors a Certified Installer Program that 
recognizes individuals with at least 1 year of experience who can demonstrate 
proper vinyl siding application. If vinyl siding is specified, design 
professionals should consider specifying that the siding contractor be a VSI-
certified installer. For further information on this program, see 
www.vinylsiding.org.
[End text box]

Secondary line of protection: Almost all wall coverings permit the passage of 
some water past the exterior surface of the covering, particularly when the rain 
is wind-driven. For this reason, most wall coverings should be considered water-
shedding, rather than waterproofing coverings. To avoid moisture-related 
problems, it is recommended that a secondary line of protection with a moisture 
barrier (such as housewrap or asphalt-saturated felt) and flashings around door 
and window openings be provided. Designers should specify that horizontal laps 
of the moisture barrier be installed so that water is allowed to drain from the 
wall (i.e., the top sheet should lap over the bottom sheet so that water running 
down the sheets remains on their outer surface). The bottom of the moisture 
barrier needs to be designed to allow drainage. Had the metal wall panels shown 
in Figure 4-62 been applied over a moisture barrier and sheathing, the amount of 
water entering the building would have likely been eliminated or greatly 
reduced. 

In areas that experience frequent wind-driven rain, incorporating a rain screen 
design, by installing vertical furring strips between the moisture barrier and 
siding materials, will facilitate drainage of water from the space between the 
moisture barrier and backside of the siding. In areas that frequently experience 
strong winds, enhanced flashing is recommended. Enhancements include use of 
flashings that have extra-long flanges, and the use of sealant and tapes. 
Flashing design should recognize that wind-driven water could be pushed up 
vertically. The height to which water can be pushed increases with wind speed. 
Water can also migrate vertically and horizontally by capillary action between 
layers of materials (e.g., between a flashing flange and housewrap). Use of a 
rain screen design, in conjunction with enhanced flashing design, is recommended 
in areas that frequently experience wind-driven rain or strong winds. It is 
recommended that designers attempt to determine what type of flashing details 
have successfully been used in the area where the facility will be constructed.

Underside of Elevated Floors

If sheathing is applied to the underside of joists or trusses elevated on piles 
(e.g., to protect insulation installed between the joists/trusses), its 
attachment should be specified in order to avoid blow-off. Stainless steel or 
hot-dip galvanized nails or screws are recommended. Since ASCE 7 does not 
provide guidance for load determination, professional judgment in specifying 
attachment is needed.

4.3.3.6 Non-Load-Bearing Walls, Wall Coverings, and Soffits in Hurricane-Prone 
Regions 

In order to achieve enhanced missile resistance of non-load-bearing exterior 
walls, the wall types discussed in Section 4.3.2.1 (i.e., reinforced concrete, 
or reinforced and fully grouted CMU) are recommended. 

To minimize long-term problems with exterior wall coverings and soffits, it is 
recommended that they be avoided to the maximum extent possible. Exposed or 
painted reinforced concrete or CMU offers greater reliability (i.e., they have 
no coverings that can blow off and become wind-borne debris). 

For all hospitals located where the basic wind speed is 100 mph or greater that 
are not constructed using reinforced concrete or reinforced and fully grouted 
CMU (as is recommended in this manual), it is recommended that the wall system 
selected be sufficient to resist complete penetration of the wall by the “E” 
missile specified in ASTM E 1996. 

For interior non-load-bearing masonry walls in hospitals located where the basic 
wind speed is greater than 120 mph, see the recommendations given in Section 
4.3.3.5.

4.3.3.7 Roof Systems

[Begin text box]
For further general information on roof systems, see the National Institute of 
Building Sciences’ Building Envelope Design Guide 
(www.wbdg.org/design/envelope.php).
[End text box]

Because roof covering damage has historically been the most frequent and the 
costliest type of wind damage, special attention needs to be given to roof 
system design. See Section 4.3.3.8 for additional information pertaining to 
hospitals located in hurricane-prone regions, and Section 4.5 for hospitals 
located in tornado-prone regions.

Code Requirements 

The IBC requires the load resistance of the roof assembly to be evaluated by one 
of the test methods listed in IBC’s Chapter 15. Design professionals are 
cautioned that designs that deviate from the tested assembly (either with 
material substitutions or change in thickness or arrangement) may adversely 
affect the wind performance of the assembly. The IBC does not specify a minimum 
safety factor. However, for the roof system, a safety factor of 2 is 
recommended. To apply the safety factor, divide the test load by 2 to determine 
the allowable design load. Conversely, multiply the design load by 2 to 
determine the minimum required test resistance.

For structural metal panel systems, the IBC requires test methods UL 580 or ASTM 
E 1592. It is recommended that design professionals specify use of E 1592, 
because it gives a better representation of the system’s uplift performance 
capability. 

[Begin text box]
The roof of the elevator penthouse must possess adequate wind and water 
resistance to ensure continuity of elevator service. It is recommended that a 
secondary roof membrane, as discussed in Section 4.3.3.8, be specified over the 
elevator penthouse roof deck. 
[End text box]

Load Resistance 

Specifying the load resistance is commonly done by specifying a Factory Mutual 
Research (FMR) rating, such as FM 1-75. The first number (1) indicates that the 
roof assembly passed the FMR tests for a Class 1 fire rating. The second number 
(75) indicates the uplift resistance in pounds per square foot (psf) that the 
assembly achieved during testing. With a safety factor of two this assembly 
would be suitable for a maximum design uplift load of 37.5 psf.

The highest uplift load occurs at the roof corners because of building 
aerodynamics as discussed in Section 4.1.3. The perimeter has a somewhat lower 
load, while the field of the roof has the lowest load. FMG Property Loss 
Prevention Data Sheets are formatted so that a roof assembly can be selected for 
the field of the roof. For the perimeter and corner areas, FMG Data Sheet 1-29 
provides three options:1) use the FMG Approval Guide listing if it includes a 
perimeter and corner fastening method; 2) use a roof system with the appropriate 
FMG Approval rating in the field, perimeter, and corner, in accordance with 
Table 1 in FMG Data Sheet 1-29; or 3) use prescriptive recommendations given in 
FMG Data Sheet 1-29. 

When perimeter and corner uplift resistance values are based on a prescriptive 
method rather than testing, the field assembly is adjusted to meet the higher 
loads in the perimeter and corners by increasing the number of fasteners or 
decreasing the spacing of adhesive ribbons by a required amount. However, this 
assumes that the failure is the result of the fastener pulling out from the 
deck, or that the failure is in the vicinity of the fastener plate, which may 
not be the case. Also, the increased number of fasteners required by FMG may not 
be sufficient to comply with the perimeter and corner loads derived from the 
building code. Therefore, if FMG resistance data are specified, it is prudent 
for the design professional to specify the resistance for each zone of the roof 
separately. Using the example cited above, if the field of the roof is specified 
as 1-75, the perimeter would be specified as 1-130 and the corner would be 
specified as 1-190. 

If the roof system is fully adhered, it is not possible to increase the uplift 
resistance in the perimeter and corners. Therefore, for fully adhered systems, 
the uplift resistance requirement should be based on the corner load rather than 
the field load.

[Begin text box]
FM Global (FMG) is the name of the Factory Mutual Insurance Company and its 
affiliates. One of FMG’s affiliates, Factory Mutual Research (FMR) provides 
testing services, produces documents that can be used by designers and 
contractors, and develops test standards for construction products and systems. 
FMR evaluates roofing materials and systems for resistance to fire, wind, hail, 
water, foot traffic and corrosion. Roof assemblies and components are evaluated 
to establish acceptable levels of performance. Some documents and activities are 
under the auspices of FMG and others are under FMR.
[End text box]

Roof System Performance 

Storm-damage research has shown that sprayed polyurethane foam (SPF) and liquid-
applied roof systems are very reliable high-wind performers. If the substrate to 
which the SPF or liquid-applied membrane is applied does not lift, it is highly 
unlikely that these systems will blow off. Both systems are also more resistant 
to leakage after missile impact damage than most other systems. Built-up roofs 
(BURs) and modified bitumen systems have also demonstrated good wind performance 
provided the edge flashing/coping does not fail (which happens frequently). The 
exception is aggregate surfacing, which is prone to blow-off (see Figures 4-10 
and 4-81). Modified bitumen applied to a concrete deck has demonstrated 
excellent resistance to progressive peeling after blow-off of the metal edge 
flashing. Metal panel performance is highly variable. Some systems are very 
wind-resistant, while others are quite vulnerable. 

Of the single-ply attachment methods, the paver-ballasted and fully adhered 
methods are the least problematic. Systems with aggregate ballast are prone to 
blow-off, unless care is taken in specifying the size of aggregate and the 
parapet height (see Figures 4-5, 4-46, and 4-47). The performance of protected 
membrane roofs (PMRs) with a factory-applied cementitious coating over 
insulation boards is highly variable. When these boards are installed over a 
loose-laid membrane, it is critical that an air retarder be incorporated to 
prevent the membrane from ballooning and disengaging the boards. ANSI/SPRI RP-4 
(which is referenced in the IBC) provides wind guidance for ballasted systems 
using aggregate, pavers, and cementitious-coated boards. 

The National Research Council of Canada, Institute for Research in 
Construction’s Wind Design Guide for Mechanically Attached Flexible Membrane 
Roofs (B1049, 2005) provides recommendations related to mechanically attached 
single-ply and modified bituminous systems. B1049 is a comprehensive wind design 
guide that includes discussion on air retarders. Air retarders can be effective 
in reducing membrane flutter, in addition to being beneficial for use in 
ballasted single-ply systems. When a mechanically attached system is specified, 
careful coordination with the structural engineer in selecting deck type and 
thickness is important. 

If a steel deck is selected, it is critical to specify that the membrane 
fasteners be attached in rows perpendicular to the steel flanges to avoid 
overstressing the attachment of the deck to the deck support structure. At the 
building shown in Figure 4-63, the fastener rows of the mechanically attached 
single-ply membrane ran parallel to the top flange of the steel deck. The deck 
fasteners were overstressed and a portion of the deck blew off and the membrane 
progressively tore. At another building, shown in Figure 4-64, the membrane 
fastener rows also ran parallel to the top flange of the steel deck. When 
membrane fasteners run parallel to the flange, the flange with membrane 
fasteners essentially carries the entire uplift load because of the deck’s 
inability to transfer any significant load to adjacent flanges. Hence, at the 
joists shown in Figure 4-64, the deck fasteners on either side of the flange 
with the membrane fasteners are the only connections to the joist that are 
carrying substantial uplift load.

[Begin figures]
Figure 4-63 is a photo of how the orientation of the membrane fastener rows led 
to blow-off of the steel deck. Hurricane Marilyn (U.S. Virgin Islands, 1995)

Figure 4-64 is a photo of a view of the underside of a steel deck showing the 
mechanically attached single-ply membrane fastener rows running parallel to, 
instead of across, the top flange of the deck. 
[End figures]

For metal panel roof systems, the following are recommended:

- When clip or panel fasteners are attached to nailers, detail the connection of 
the nailer to the nailer support (including the detail of where nailers are 
spliced over a support). 

- When clip or panel fasteners are loaded in withdrawal (tension), screws are 
recommended in lieu of nails. 

- For concealed clips over a solid substrate, it is recommended that chalk lines 
be specified so that the clips are correctly spaced.

- When the basic wind speed is 110 mph or greater, it is recommended that two 
clips be used along the eaves, ridges, and hips.

- For copper panel roofs in areas with a basic wind speed greater than 90 mph, 
it is recommended that Type 316 stainless steel clips and stainless steel screws 
be used in lieu of copper clips.

- Close spacing of fasteners is recommended at hip and ridge flashings (e.g., 
spacing in the range of 3 to 6 inches on center, commensurate with the design 
wind loads.)

Edge Flashings and Copings

Roof membrane blow-off is almost always a result of lifting and peeling of the 
metal edge flashing or coping, which serves to clamp down the membrane at the 
roof edge. Therefore, it is important for the design professional to carefully 
consider the design of metal edge flashings, copings, and the nailers to which 
they are attached. The metal edge flashing on the modified bitumen membrane roof 
shown in Figure 4-65 was installed underneath the membrane, rather than on top 
of it, and then stripped in. In this location, the edge flashing was unable to 
clamp the membrane down. At one area, the membrane was not sealed to the 
flashing. An ink pen was inserted into the opening prior to photographing to 
demonstrate how wind could catch the opening and lift and peel the membrane. 

[Begin figure]
Figure 4-65 is a photo of how an ink pen shows an opening that the wind can 
catch, and cause lifting and peeling of the membrane. 
[End figure]

ANSI/SPRI ES-1, Wind Design Standard for Edge Systems Used in Low Slope Roofing 
Systems (2003) provides general design guidance including a methodology for 
determining the outward-acting load on the vertical flange of the 
flashing/coping (ASCE 7 does not provide this guidance). ANSI/SPRI ES-1 is 
referenced in the IBC. ANSI/SPRI ES-1 also includes test methods for assessing 
flashing/coping resistance. This manual recommends a minimum safety factor of 3 
for edge flashings, copings, and nailers for hospitals. For FMG-insured 
facilities, FMR-approved flashing should be used and FM Data Sheet 1-49 should 
also be consulted. 

The traditional edge flashing/coping attachment method relies on concealed 
cleats that can deform under wind load and lead to disengagement of the 
flashing/coping (see Figure 4-66) and subsequent lifting and peeling of the roof 
membrane. When a vertical flange disengages and lifts up, the edge flashing and 
membrane are very susceptible to failure. Normally, when a flange lifts the 
failure continues to propagate and the metal edge flashing and roof membrane 
blows off.

[Begin figure]
Figure 4-66 is a photo showing that the metal edge flashing on this hospital 
disengaged from the continuous cleat and the vertical flange lifted. Hurricane 
Hugo (South Carolina, 1989)
[End figure] 

Storm-damage research has revealed that, in lieu of cleat attachment, the use of 
exposed fasteners to attach the vertical flanges of copings and edge flashings 
has been found to be a very effective and reliable attachment method. The coping 
shown in Figure 4-67 was attached with 1/4-inch diameter stainless steel 
concrete spikes at 12 inches on center. When the fastener is placed in wood, #12 
stainless steel screws with stainless steel washers are recommended. The 
fasteners should be more closely spaced in the corner areas (the spacing will 
depend upon the design wind loads). ANSI/SPRI ES-1 provides guidance on fastener 
spacing and thickness of the coping and edge flashing.

[Begin figure]
Figure 4-67 is a photo showing that both vertical faces of the coping were 
attached with exposed fasteners instead of concealed cleats. Typhoon Paka (Guam, 
1997)
[End figure]

Gutters 

Storm-damage research has shown that gutters are seldom constructed to resist 
wind loads (see Figure 4-68). When a gutter lifts, it typically causes the edge 
flashing that laps into the gutter to lift as well. Frequently, this results in 
a progressive lifting and peeling of the roof membrane. The membrane blow-off 
shown in Figure 4-69 was initiated by gutter uplift. The gutter was similar to 
that shown in Figure 4-68. The membrane blow-off caused significant interior 
water damage.

[Begin figures]
Figure 4-68 is a photo showing that this gutter, supported by a type of bracket 
that provides no significant uplift resistance, failed when wind lifted it, 
together with the metal edge flashing that lapped into the gutter. Hurricane 
Francis (Florida, 2004)

Figure 4-69 is a photo showing that the original modified bitumen membrane was 
blown away after the gutter lifted in the area shown by the red arrow (the black 
membrane is a temporary roof). Hurricane Francis (Florida, 2004)
[End figures] 

Special design attention needs to be given to attaching gutters to prevent 
uplift, particularly for those in excess of 6 inches in width. Currently, there 
are no standards pertaining to gutter wind resistance. It is recommended that 
the designer calculate the uplift load on gutters using the overhang coefficient 
from ASCE 7. There are two approaches to resist gutter uplift.

- Gravity-support brackets can be designed to resist uplift loads. In these 
cases, in addition to being attached at its top, the bracket should also be 
attached at its low end to the wall. The gutter also needs to be designed so it 
is attached securely to the bracket in a way that will effectively transfer the 
gutter uplift load to the bracket. Bracket spacing will depend on the gravity 
and uplift load, the bracket’s strength, and the strength of connections between 
the gutter/bracket and the bracket/wall. With this option, the bracket’s top 
will typically be attached to a wood nailer, and that fastener will be designed 
to carry the gravity load. The bracket’s lower connection will resist the 
rotational force induced by gutter uplift. Because brackets are usually spaced 
close together to carry the gravity load, developing adequate connection 
strength at the lower fastener is generally not difficult. 

- The other option is to use gravity-support brackets only to resist gravity 
loads, and use separate sheet-metal straps at 45-degree angles to the wall to 
resist uplift loads. Strap spacing will depend on the gutter uplift load and 
strength of the connections between the gutter/strap and the strap/wall. Note 
that FMG Data Sheet 1-49 recommends placing straps 10 feet apart. However, at 
that spacing with wide gutters, fastener loads induced by uplift are quite high. 
When straps are spaced at 10 feet, it can be difficult to achieve sufficiently 
strong uplift connections.

When designing a bracket’s lower connection to a wall or a strap’s connection to 
a wall, designers should determine appropriate screw pull-out values. With this 
option, a minimum of two screws at each end of a strap is recommended. At a 
wall, screws should be placed side by side, rather than vertically aligned, so 
the strap load is carried equally by the two fasteners. When fasteners are 
vertically aligned, most of the load is carried by the top fastener.

Since the uplift load in the corners is much higher than the load between the 
corners, enhanced attachment is needed in corner areas regardless of the option 
chosen. ASCE 7 provides guidance about determining a corner area’s length.

Parapet Base Flashings 

Information on loads for parapet base flashings was first introduced in the 2002 
edition of ASCE 7. The loads on base flashings are greater than the loads on the 
roof covering if the parapet’s exterior side is air-permeable. When base 
flashing is fully adhered, it has sufficient wind resistance in most cases. 
However, when base flashing is mechanically fastened, typical fastening patterns 
may be inadequate, depending on design wind conditions (see Figure 4-70). 
Therefore, it is imperative that the base flashing loads be calculated, and 
attachments designed to accommodate these loads. It is also important for 
designers to specify the attachment spacing in parapet corner regions to 
differentiate them from the regions between corners.

[Begin figure]
Figure 4-70 is a photo showing that if mechanically attached base flashings have 
an insufficient number of fasteners, the base flashing can be blown away. 
Hurricane Andrew (Florida, 1992) 
[End figure]

When the roof membrane is specified to be adhered, it is recommended that fully 
adhered base flashings be specified in lieu of mechanically attached base 
flashings. Otherwise, if the base flashing is mechanically attached, ballooning 
of the base flashing during high winds can lead to lifting and progressive 
peeling of the roof membrane.

Steep-Slope Roof Coverings 

For a discussion of wind performance of asphalt shingle and tile roof coverings, 
see FEMA 488 (2005), 489 (2005), and 549 (2006). For recommendations pertaining 
to asphalt shingles and tiles, see Fact Sheets 19, 20, and 21 in FEMA 499 
(2005).

4.3.3.8 Roof Systems in Hurricane-Prone Regions

The following types of roof systems are recommended for hospitals in hurricane-
prone regions, because they are more likely to avoid water infiltration if the 
roof is hit by wind-borne debris, and also because these systems are less likely 
to become sources of wind-borne debris:

- In tropical climates where insulation is not needed above the roof deck, 
specify either liquid-applied membrane over cast-in-place concrete deck, or 
modified bitumen membrane torched directly to primed cast-in-place concrete 
deck.

- Install a secondary membrane over a concrete deck (if another type of deck is 
specified, a cover board may be needed over the deck). Seal the secondary 
membrane at perimeters and penetrations. Specify rigid insulation over the 
secondary membrane. Where the basic wind speed is up to 110 mph, a minimum 2-
inch thick layer of insulation is recommended. Where the speed is between 110 
and 130 mph, a total minimum thickness of 3 inches is recommended (installed in 
two layers). Where the speed is greater than 130 mph, a total minimum thickness 
of 4 inches is recommended (installed in two layers). A layer of 5/8-inch thick 
glass mat gypsum roof board is recommended over the insulation, followed by a 
modified bitumen membrane. A modified bitumen membrane is recommended for the 
primary membrane because of its somewhat enhanced resistance to puncture by 
small missiles compared with other types of roof membranes.

- When fully adhering boards to concrete decks, it is recommend that a planar 
flatness of a maximum of 1/4-inch variation over an 10 foot length (when 
measured by a straightedge) be specified. Prior to installation of the roof 
insulation, it is recommended that the planar flatness be checked with a 
straightedge. If the deck is outside of the 1/4-inch variation, it is 
recommended that the high spots be ground or the low spots be suitably filled.

- The purpose of the insulation and gypsum roof board is to absorb missile 
energy. If the primary membrane is punctured or blown off during a storm, the 
secondary membrane should provide watertight protection unless the roof is hit 
with missiles of very high momentum that penetrate the insulation and secondary 
membrane. Figure 4-72 illustrates the merit of specifying a secondary membrane. 
The copper roof blew off the hospital’s intensive care unit (ICU). Patients and 
staff were frightened by the loud noise generated by the metal panels as they 
banged around during the hurricane. Fortunately there was a very robust 
underlayment (a built-up membrane) that remained in place. Since only minor 
leakage occurred, the ICU continued to function. 

[Begin text box]
When fully adhering insulation boards, it is recommended that the boards be no 
larger than 4 feet by 4 feet. It is also recommended that the board thickness 
not exceed 2 inches ( 11/2 inches is preferable). Use of small thin boards makes 
it easier for the contractor to conform the boards to the substrate. At the 
hospital shown in Figure 4-71, 4 foot by 8 foot insulation boards were set in 
hot 
asphalt over a concrete deck. A few of the boards detached from the deck. The 
boards may have initiated the membrane blow off, or the membrane blow off may 
have been initiated by lifting and peeling of the metal edge flashing, in which 
case, loss of the insulation boards was a secondary failure. 
[End text box]

Figure 4-71 is a photo of how blown off insulation may have initiated blow off 
of the roof membrane. Hurricane Ivan (Florida, 2004) [end figure]

[Begin figure]
Figure 4-72 is a photo showing that the secondary membrane prevented leakage 
into the ICU after the copper roof blew off. Hurricane Andrew (Florida, 1992) 
[End figure]

- For an SPF roof system over a concrete deck, where the basic wind speed is 
less than 130 mph, it is recommended that the foam be a minimum of 3 inches 
thick to avoid missile penetration through the entire layer of foam. Where the 
speed is greater than 130 mph, a 4-inch minimum thickness is recommended. It is 
also recommended that the SPF be coated, rather than protected with an aggregate 
surfacing.

- For a PMR, it is recommended that pavers weighing a minimum of 22 psf be 
specified. In addition, base flashings should be protected with metal (such as 
shown in Figure 4-79) to provide debris protection. Parapets with a 3-foot 
minimum height (or higher if so indicated by ANSI/SPRI RP-4, 2002) are 
recommended at roof edges. This manual recommends that PMRs not be used for 
hospitals in hurricane-prone regions where the basic wind speed exceeds 130 mph. 

- For structural metal roofs, it is recommended that a roof deck be specified, 
rather than attaching the panels directly to purlins as is commonly done with 
pre-engineered metal buildings. If panels blow off buildings without roof 
decking, wind-borne debris and rain are free to enter the building. 

Structural standing seam metal roof panels with concealed clips and mechanically 
seamed ribs spaced at 12 inches on center are recommended. If the panels are 
installed over a concrete deck, a modified bitumen secondary membrane is 
recommended if the deck has a slope less than 1/2:12. If the panels are 
installed over a steel deck or wood sheathing, a modified bitumen secondary 
membrane (over a suitable cover board when over steel decking) is recommend, 
followed by rigid insulation and metal panels. Where the basic wind speed is up 
to 110 mph, a minimum 2-inch thick layer of insulation is recommended. Where the 
speed is between 110 and 130 mph, a total minimum thickness of 3 inches is 
recommended. Where the speed is greater than 130 mph, a total minimum thickness 
of 4 inches is recommended. Although some clips are designed to bear on 
insulation, it is recommended that the panels be attached to wood nailers 
attached to the deck, because nailers provide a more stable foundation for the 
clips. 

If the metal panels are blown off or punctured during a hurricane, the secondary 
membrane should provide watertight protection unless the roof is hit with 
missiles of very high momentum. At the roof shown in Figure 4-73, the structural 
standing seam panel clips bore on rigid insulation over a steel deck. Had a 
secondary membrane been installed over the steel deck, the membrane would have 
likely prevented significant interior water damage and facility disruption.
Figure 4-73 is a photo showing significant interior water damage and facility 
interruption occurred after the standing seam roof blew off. Hurricane Marilyn 
(U.S. Virgin Islands, 1995)

[Begin figure]
Figure 4-73 is a photo showing significant interior water damage and facility 
interruption occurred after the standing seam roof blew off. Hurricane Marilyn 
(U.S. Virgin Islands, 1995)
 [End figure] 

- Based on field performance of architectural metal panels in hurricane-prone 
regions, exposed fastener panels are recommended in lieu of architectural panels 
with concealed clips. For panel fasteners, stainless steel screws are 
recommended. A secondary membrane protected with insulation is recommended, as 
discussed above for structural standing seam systems. 

In order to avoid the possibility of roofing components blowing off and striking 
people arriving at a hospital during a storm, the following roof systems are not 
recommended: aggregate surfacings, either on BUR, single-plies or SPF; 
lightweight concrete pavers; cementitious-coated insulation boards; slate; and 
tile (see Figure 4-74). Even when slates and tiles are properly attached to 
resist wind loads, their brittleness makes them vulnerable to breakage as a 
result of wind-borne debris impact. The tile and slate fragments can be blown 
off the roof, and fragments can damage other parts of the roof, causing a 
cascading failure. 

[Begin figure]
Figure 4-74 is a photo of how brittle roof coverings, like slate and tile, can 
be broken by missiles, and tile debris can break other tiles. Hurricane Charley 
(Florida, 2004)
[End figure] 

Mechanically attached and air-pressure equalized single-ply membrane systems are 
susceptible to massive progressive failure after missile impact, and are 
therefore not recommended for hospitals in hurricane-prone regions. At the 
building shown in Figure 4-75, a missile struck the fully adhered low-sloped 
roof and slid into the steep-sloped reinforced mechanically attached single-ply 
membrane in the vicinity of the red arrow. A large area of the mechanically 
attached membrane was blown away as a result of progressive membrane tearing. 
Fully adhered single-ply membranes are very vulnerable to missile puncture and 
are not recommended unless they are ballasted with pavers. At the hospital shown 
in Figure 4-76, several missiles, including exhaust fans and copings struck the 
roof. 

[Begin figures]
Figure 4-75 is a photo showing how mechanically attached single-ply membrane 
progressively tore after being cut by wind-borne debris. Hurricane Andrew 
(Florida, 1992)

Figure 4-76 is a photo showing how this fully adhered single-ply roof membrane 
was struck by a variety of missiles. Hurricane Marilyn (U.S. Virgin Islands, 
1995) 
[End figures] 

Edge flashings and copings: If cleats are used for attachment, it is recommended 
that a “peel-stop” bar be placed over the roof membrane near the edge 
flashing/coping, as illustrated in Figure 4-77. The purpose of the bar is to 
provide secondary protection against membrane lifting and peeling in the event 
that edge flashing/coping fails. A robust bar specifically made for bar-over 
mechanically attached single-ply systems is recommended. The bar needs to be 
very well anchored to the parapet or the deck. Depending on design wind loads, 
spacing between 4 and 12 inches on center is recommended. A gap of a few inches 
should be left between each bar to allow for water flow across the membrane. 
After the bar is attached, it is stripped over with a stripping ply.

[Begin figure]
Figure 4-77 is an illustration showing a continuous peel-stop bar over the 
membrane may prevent a catastrophic progressive failure if the edge flashing or 
coping is blown off. (Modified from FEMA 55, 2000)
[End figure] 

Walkway pads: Roof walkway pads are frequently blown off during hurricanes 
(Figures 4-78 and 4-82). Pad blow off does not usually damage the roof membrane. 
However, wind-borne pad debris can damage other building components and injure 
people. Currently there is no test standard to evaluate uplift resistance of 
walkway pads. Walkway pads are therefore not recommended in hurricane-prone 
regions.

[Begin figure]
Figure 4-78 is a photo showing how several rubber walkway pads were blown off 
the single-ply membrane roof on this hospital. Hurricane Katrina (Mississippi, 
2005)
[End figure] 

Parapets: For low-sloped roofs, minimum 3-foot high parapets are recommended. 
With parapets of this height or greater, the uplift load in the corner region is 
substantially reduced (ASCE 7 permits treating the corner zone as a perimeter 
zone). Also, a high parapet (as shown in Figures 4-96) may intercept wind-borne 
debris and keep it from blowing off the roof and damaging other building 
components or injuring people. To protect base flashings from wind-borne debris 
damage and subsequent water leakage, it is recommended that metal panels on 
furring strips be installed over the base flashing (Figure 4-79). Exposed 
stainless steel screws are recommended for attaching the panels to the furring 
strips, because using exposed fasteners is more reliable than using concealed 
fasteners or clips (as were used for the failed panels shown in Figure 4-62). 

[Begin figure]
Figure 4-79 is a photo of base flashing protected by metal panels attached with 
exposed screws. Hurricane Katrina (Mississippi, 2005)
[End figure]

4.3.3.9 The Case of DeSoto Memorial Hospital, Arcadia, Florida

The case of DeSoto Memorial Hospital illustrates damage caused by aggregate-
surfaced roofs. The 82-bed hospital is located just off Florida Highway 17 in 
Arcadia, approximately 30 miles east of Florida’s gulf coast. The hospital was 
constructed in 1964, though the current emergency room (ER), ICU, and third 
floor patient rooms were added in 1984. A separate pre-engineered storage 
building was constructed in 1979.

The facility was struck by Hurricane Charley in 2004, with an estimated peak 
gust wind speed between 125 to 140 mph. [footnote 11] Since the design wind 
speed in the 2005 edition of ASCE 7 for this location is 110 mph, the estimated 
speeds at this site were above current design conditions. Also, even with the 
1.15 importance factor, the actual wind pressures were above the design 
pressures.

[Begin footnote 11] The 125 to 140 mph speeds were estimated for Exposure C.

The hospital sustained damage to windows, rooftop equipment, and a freestanding 
storage building on the campus. Thirty-three windows were broken, including 
patient room windows and windows at three of the eight ICU rooms (Figures 4-80 
and 4-81). Windows were also broken in many vehicles in the parking lot. The 
majority of the glass breakage was caused by aggregate blown from the hospital’s 
built-up roofs. Some of the glass breakage may have been due to blown off 
gutters and walkway pads from the hospital’s roof (Figure 4-82) or other 
missiles such as tree limbs; blown off gutters can be high-momentum missiles 
that can travel a substantial distance (see Figure 4-81). 

As a result of the window breakage the entire ICU was evacuated during the 
hurricane and closed for about 2 weeks before repairs were completed and the 
unit reoccupied. Some patients were moved to lower floors; the elevator could 
not be used so the patients were either carried down or slid down the stairs on 
mattresses. Fortunately, no one was injured during the evacuation.

A portion of the roof covering was blown off and the satellite dish was nearly 
blown off too. The LPS was displaced in a few areas (see Figure 4-83). As a 
result of the damage to the roof covering and to rooftop equipment, water leaked 
into the building in several areas, which caused the closing of the operating 
room (OR), sterile processing, portions of the lab, and numerous offices. The OR 
was temporarily relocated to the Caesarian section (C-section) room. It was 
about a month before the OR was repaired and reoccupied. 

The metal storage building that contained the hospital’s supplies, maintenance 
shop, environmental services, and shipping and receiving collapsed, and its loss 
was significant for the hospital operations. Almost all of the tools and stock 
materials for repairs were lost (Figure 4-84). Tents were set up after the 
hurricane to provide storage. Because of subsequent storms in the next several 
weeks (including two hurricanes), the stored items had to be moved in and out of 
the tents on several occasions. 

Municipal power and water were lost during the hurricane. The hospital ran on 
its generators for about 5 days before power was restored. Municipal water 
service was out for about 2 weeks, but fortunately the hospital had a secondary 
well for potable water, so water service was not interrupted. 

Access to the hospital was not hampered by fallen trees or by flooding. However, 
some staff and emergency medical service (EMS) personnel were unable to get to 
the hospital quickly after the storm because of downed trees or floodwaters in 
their neighborhoods. Landline telephone service was not lost, but paging and 
cell phone services were. The homes of many staff members were no longer 
habitable after the storm, so tents were set up on the hospital campus to house 
staff and volunteers that came to provide assistance. This in turn required 
additional security services and shower and laundry facilities.

The hospital did not have a contingency plan to cope with the damages, and 
therefore, did not have pre-arranged contracts with contractors to perform 
inspections and emergency repairs. Fortunately, there were no problems finding 
contractors quickly after the storm, although the building materials needed for 
repairs were in short supply. 

[Begin figures]
Figure 4-80 is a photo of the second floor beyond the canopy houses the ICU. 
Several windows along the two ICU walls that are visible were broken. 

Figure 4-81 is a photo of a view of the ICU from the third floor roof. The 
gutters (red arrow) are from the back side of the third floor roof.

Figure 4-82 is a photo of a view of the back side of the third floor roof where 
the gutter and an asphalt plank walkway pad were blown away. The loose aggregate 
surfacing was also blown away.

Figure 4-83 is a photo showing how this satellite dish was held down only with 
CMU. Note the displaced LPS at the corner (circled).
 
Figure 4-84 is a photo showing how this pre-engineered storage building 
contained the hospital supplies and maintenance shop. 
[End figures]

4.3.4 Nonstructural Systems and Equipment

Nonstructural systems and equipment include all components that are not part of 
the structural system or building envelope. Exterior-mounted mechanical 
equipment (e.g., exhaust fans, HVAC units, relief air hoods, rooftop ductwork, 
and boiler stacks), electrical equipment (e.g., light fixtures and lightning 
protection systems), and communications equipment (e.g., antennae and satellite 
dishes) are often damaged during high winds. Damaged equipment can impair the 
operation of the facility, the equipment can detach and become wind-borne 
missiles, and water can enter the facility where equipment was displaced (see 
Figure 4-85). The most common problems typically relate to inadequate equipment 
anchorage, inadequate strength of the equipment itself, and corrosion.

[Begin figure]
Figure 4-85 is a photo showing how this gooseneck was attached with only two 
small screws. A substantial amount of water was able to enter the building 
during Hurricane Francis. (Florida, 2004)
 
Exterior-mounted equipment is especially vulnerable to hurricane-induced damage, 
and special attention should be paid to positioning and mounting of these 
components in hurricane-prone regions. Specific information pertaining to 
hospitals located in hurricane-prone regions is presented for each of the 
nonstructural component sections below.

4.3.4.1 Exterior-Mounted Mechanical Equipment

This section discusses loads and attachment methods, as well as the problems of 
corrosion and water infiltration.

Loads and Attachment Methods [footnote 12]

[Begin footnote 12] Discussion is based on: Attachment of Rooftop Equipment in 
High-Wind Regions - Hurricane Katrina Recovery Advisory (May 2006, revised July 
2006) [end footnote]

Information on loads on rooftop equipment was first introduced in the 2002 
edition of ASCE 7. For guidance on load calculations, see “Calculating Wind 
Loads and Anchorage Requirements for Rooftop Equipment” (ASHRAE, 2006). A 
minimum safety factor of 3 is recommended for hospitals. Loads and resistance 
should also be calculated for heavy pieces of equipment since the dead load of 
the equipment is often inadequate to resist the design wind load. The 30' x 10' 
x 8' 18,000-pound HVAC unit shown in Figure 4-86 was attached to its curb with 
16 straps (one screw per strap). Although the wind speeds were estimated to be 
only 85 to 95 miles per hour (peak gust), the HVAC unit blew off the medical 
office building. The inset at Figure 4-86 shows the curb upon which the unit was 
attached. A substantial amount of water entered the building at the curb 
openings before the temporary tarp was placed.

[Begin text box]
Mechanical penetrations through the elevator penthouse roof and walls must 
possess adequate wind and water resistance to ensure continuity of elevator 
service (see Section 4.3.3.5). In addition to paying special attention to 
equipment attachment, air intakes and exhausts should be designed and 
constructed to prevent wind-driven water from entering the penthouse.
[End text box]

[Begin figure]
Figure 4-86 is a photo showing that although this 18,000-pound HVAC unit was 
attached to its curb with 16 straps, it blew off during Hurricane Ivan. 
(Florida, 2004)
[end figure]
 
To anchor fans, small HVAC units, and relief air hoods, the minimum attachment 
schedule provided in Table 4-1 is recommended. The attachment of the curb to the 
roof deck also needs to be designed and constructed to resist the design loads. 
The cast-in-place concrete curb shown in Figure 4-87 was cold-cast over a 
concrete 
roof deck. Dowels were not installed between the deck and the curb, hence a weak 
connection occurred.

[Begin figure]
Figure 4-87 is a photo showing how the gooseneck on this hospital remained 
attached to the curb, but the curb detached from the deck. Typhoon Paka (Guam, 
1997)
[End figure]

[Begin table]
Table 4-1: Number of #12 Screws for Base Case Attachment of Rooftop Equipment

Case No: 1
Curb Size and Equipment Type: 12” x 12” Curb with Gooseneck Relief Air Hood
Equipment Attachment: Hood Screwed to Curb 
Fastener Factor for Each Side of Curb or Flange: 1.6

Case No: 2
Curb Size and Equipment Type: 12” x 12” Gooseneck Relief Air Hood with Flange 
Equipment Attachment: Flange Screwed to 22 Gauge Steel Roof Deck 
Fastener Factor for Each Side of Curb or Flange: 2.8

Case No: 3
Curb Size and Equipment Type: 12” x 12” Gooseneck Relief Air Hood with Flange
Equipment Attachment: Flange Screwed to 15/32” OSB Roof Deck
Fastener Factor for Each Side of Curb or Flange: 2.9

Case No: 4
Curb Size and Equipment Type: 24” x 24” Curb with Gooseneck Relief Air Hood
Equipment Attachment: Hood Screwed to Curb 
Fastener Factor for Each Side of Curb or Flange: 4.6

Case No: 5
Curb Size and Equipment Type: 24” x 24” Gooseneck Relief Air Hood with Flange
Equipment Attachment: Flange Screwed to 22 Gauge Steel Roof Deck 
Fastener Factor for Each Side of Curb or Flange: 8.1

Case No: 6
Curb Size and Equipment Type: 24” x 24” Gooseneck Relief Air Hood with Flange

Equipment Attachment: Flange Screwed to 15/32” OSB Roof Deck
Fastener Factor for Each Side of Curb or Flange: 8.2

Case No: 7
Curb Size and Equipment Type: 24” x 24” Curb with Exhaust Fan
Equipment Attachment: Fan Screwed to Curb
Fastener Factor for Each Side of Curb or Flange: 2.5

Case No: 8
Curb Size and Equipment Type: 36” x 36” Curb with Exhaust Fan
Equipment Attachment: Fan Screwed to Curb
Fastener Factor for Each Side of Curb or Flange: 3.3

Case No: 9
Curb Size and Equipment Type: 5’-9” x 3’- 8” Curb with 2’- 8” high HVAC Unit
Equipment Attachment: HVAC Unit Screwed to Curb
Fastener Factor for Each Side of Curb or Flange: 4.5*

Case No: 10
Curb Size and Equipment Type: 5’-9”x 3’- 8” Curb with 2’- 8” high Relief Air 
Hood
Equipment Attachment: Hood Screwed to Curb
Fastener Factor for Each Side of Curb or Flange: 35.6*

Notes to Table 4-1: 
1. The loads are based on ASCE 7-05. The resistance includes equipment weight.

2. The Base Case for the tabulated numbers of #12 screws (or ¼ pan-head screws 
for flange-attachment) is a 90-mph basic wind speed, 1.15 importance factor, 30’ 
building height, Exposure C, using a safety factor of 3. 

3. For other basic wind speeds, multiply the tabulated number of #12 screws by _ 
to determine the required number of #12 screws (or ¼ pan-head screws) required 
for the desired basic wind speed, VD (mph). 

4. For other roof heights up to 200’, multiply the tabulated number of #12 
screws by (1.00 + 0.003 [h - 30]) to determine the required number of #12 screws 
or ¼ pan-head screws for buildings between 30’ and 200’.
- Example A: 24” x 24” exhaust fan screwed to curb (table row 7), Base Case 
conditions (see Note 1): 2.5 screws per side; therefore, round up and specify 3 
screws per side.
- Example B: 24” x 24” exhaust fan screwed to curb (table row 7), Base Case 
conditions, except 120 mph: 1202 x 1 ÷ 902 = 1.78 x 2.5 screws per side = 4.44 
screws per side; therefore, round down and specify 4 screws per side.
- Example C: 24” x 24” exhaust fan screwed to curb (table row 7), Base Case 
conditions, except 150’ roof height: 1.00 + 0.003 (150’ - 30’) = 1.00 + 0.36 = 
1.36 x 2.5 screws per side = 3.4 screws per side; therefore, round down and 
specify 3 screws per side.

* This factor only applies to the long sides. At the short sides, use the 
fastener spacing used at the long sides.
[End table

Fan cowling attachment: Fans are frequently blown off their curbs because they 
are poorly attached. When fans are well attached, the cowlings frequently blow 
off (see Figure 4-88). Blown off cowlings can tear roof membranes and break 
glazing. Unless the fan manufacturer specifically engineered the cowling 
attachment to resist the design wind load, cable tie-downs (see Figure 4-89) are 
recommended to avoid cowling blow-off. For fan cowlings less than 4 feet in 
diameter, one half-inch diameter stainless steel cables are recommended. For 
larger 
cowlings, use 3/16-inch diameter cables. When the basic wind speed is 120 mph or 
less, specify two cables. Where the basic wind speed is greater than 120 mph, 
specify four cables. To minimize leakage potential at the anchor point, it is 
recommended that the cables be adequately anchored to the equipment curb (rather 
than anchored to the roof deck). The attachment of the curb itself also needs to 
be designed and specified. 

[Begin text box]
To avoid corrosion-induced failure (Figure 4-21), it is recommended that 
exterior-mounted mechanical, electrical, and communications equipment be made of 
nonferrous metals, stainless steel, or steel with minimum G-90 hot-dip 
galvanized coating for the equipment body, stands, anchors, and fasteners. When 
equipment with enhanced corrosion protection is not available, the designer 
should advise the building owner that periodic equipment maintenance and 
inspection is particularly important to avoid advanced corrosion and subsequent 
equipment damage during a windstorm.
[End text box]

[Begin figures]
Figure 4-88 is a photo showing how cowlings blew off two of the three fans. Note 
also the loose lightning protection system conductors and missing walkway pad 
(red arrow). Hurricane Charley (Florida, 2004) 

Figure 4-89 is a photo showing how cables were attached to prevent the cowling 
from blowing off. Typhoon Paka (Guam, 1997)
[end figures]

Ductwork: To avoid wind and wind-borne debris damage to rooftop ductwork, it is 
recommended that ductwork not be installed on the roof (see Figures 4-16, 4-60, 
and 4-124). If ductwork is installed on the roof, it is recommended that the 
ducts’ gauge and the method of attachment be able to resist the design wind 
loads. 

Condenser attachment: In lieu of placing rooftop-mounted condensers on wood 
sleepers resting on the roof (see Figure 4-90), it is recommended that 
condensers be anchored to equipment stands. The attachment of the stand to the 
roof deck also needs to be designed to resist the design loads. In addition to 
anchoring the base of the condenser to the stand, two metal straps with two 
side-by-side #12 screws or bolts with proper end and edge distances at each 
strap end are recommended when the basic wind speed is greater than 90 mph (see 
Figure 4-91).

[Begin figure]
Figure 4-90 is a photo showing a sleeper-mounted condensers displaced by high 
winds. Hurricane Katrina (Mississippi, 2005) 

Figure 4-91 is a photo showing how this condenser had supplemental attachment 
straps (see red arrows). Typhoon Paka (Guam, 1997)
[end figure]

[Begin text box]
Three publications pertaining to seismic restraint of equipment provide general 
information on fasteners and edge distances: 
- Seismic Restraints for Mechanical Equipment (FEMA 412, 2002) 
- Installing Seismic Restraints for Electrical Equipment (FEMA 413, 2004)
- Installing Seismic Restraints for Duct and Pipe (FEMA 414, 2004) 
[End text box]

Vibration isolators: If vibration isolators are used to mount equipment, only 
those able to resist design uplift loads should be specified and installed, or 
an alternative means to accommodate uplift resistance should be provided (see 
Figure 4-92). 

[Begin figure]
Figure 4-92 is a photo showing the failure of vibration isolators that provided 
lateral resistance but no uplift resistance caused equipment damage. A damaged 
vibration isolator is shown in the inset. Hurricane Katrina (Mississippi, 2005)
[end figure]

Boiler and exhaust stack attachment: To avoid wind damage to boiler and exhaust 
stacks, wind loads on stacks should be calculated and guy-wires should be 
designed and constructed to resist the loads. Toppled stacks, as shown at the 
hospital in Figure 4-93, can allow water to enter the building at the stack 
penetration, damage the roof membrane, and become wind-borne debris. The 
designer should advise the building owner that guy-wires should be inspected 
annually to ensure they are taut.

[Begin figure]
Figure 4-93 Three of the five stacks that did not have guy-wires were blown 
down. Hurricane Marilyn (U.S. Virgin Islands, 1995) 
[end figure]

Access panel attachment: Equipment access panels frequently blow off. To 
minimize this, job-site modifications, such as attaching hasps and locking 
devices like carabiners, are recommended. The modification details need to be 
customized. Detailed design may be needed after the equipment has been delivered 
to the job site. Modification details should be approved by the equipment 
manufacturer.

Equipment screens: Screens around rooftop equipment are frequently blown away 
(see Figure 4-94). Screens should be designed to resist the wind load derived 
from ASCE 7. Since the effect of screens on equipment wind loads is unknown, the 
equipment attachment behind the screens should be designed to resist the design 
load. 

[Begin figure]
Figure 4-94 Equipment screen panels, such as these blown away at a hospital, can 
break glazing, puncture roof membranes, and cause injury. Hurricane Ivan 
(Florida, 2004)
[end figure]

Water Infiltration

During high winds, wind-driven rain can be driven through air intakes and 
exhausts unless special measures are taken. Louvers should be designed and 
constructed to prevent leakage between the louver and wall. The louver itself 
should be designed to avoid water being driven past the louver. However, it is 
difficult to prevent infiltration during very high winds. Designing sumps with 
drains that will intercept water driving past louvers or air intakes should be 
considered. ASHRAE 62.1 (2004) provides some information on rain and snow 
intrusion. The Standard 62.1 User’s Manual provides additional information, 
including examples and illustrations of various designs.

4.3.4.2 Nonstructural Systems and Mechanical Equipment in Hurricane-Prone 
Regions

Elevators: Where interruption of elevator service would significantly disrupt 
facility operations, it is recommended that elevators be placed in separate 
locations within the building and be served by separate elevator penthouses. 
This is recommended, irrespective of the elevator penthouse enhancements 
recommended in Sections 4.3.3.5, 4.3.3.7, and 4.3.4.1, because of the greater 
likelihood that at least one of the elevators will remain operational and 
therefore allow the facility to function as intended, as discussed in Section 
4.2.1.3.

Mechanical Penthouses: By placing equipment in mechanical penthouses rather than 
leaving them exposed on the roof, equipment can be shielded from high-wind loads 
and wind-borne debris. Although screens (such as shown in Figure 4-94) could be 
designed and constructed to protect equipment from horizontally-flying debris, 
they are not effective in protecting equipment from missiles that have an 
angular trajectory. It is therefore recommended that mechanical equipment be 
placed inside mechanical penthouses. The penthouse itself should be designed and 
constructed in accordance with the recommendations given in Sections 4.3.2.1, 
4.3.3.6, and 4.3.3.8.

4.3.4.3 Exterior-Mounted Electrical and Communications Equipment

Damage to exterior-mounted electrical equipment is infrequent, mostly because of 
its small size (e.g., disconnect switches). Exceptions include communication 
towers, surveillance cameras, electrical service masts, satellite dishes, and 
lightning protection systems. The damage is typically caused by inadequate 
mounting as a result of failure to perform wind load calculations and anchorage 
design. Damage is also sometimes caused by corrosion (see Figure 4-21 and text 
box in Section 4.3.4.1 regarding corrosion).

Communication towers and poles: ANSI/C2 provides guidance for determining wind 
loads on power distribution and transmission poles and towers. AASHTO LTS-4-M 
(amended by LTS-4-12 2001 and 2003, respectively) provides guidance for 
determining wind loads on light fixture poles (standards).

Both ASCE 7 and ANSI/TIA-222-G contain wind load provisions for communication 
towers (structures). The IBC allows the use of either approach. The ASCE wind 
load provisions are generally consistent with those contained in ANSI/TIA-222-G. 
ASCE 7, however, contains provisions for dynamically sensitive towers that are 
not present in the ANSI/TIA standard. ANSI/TIA classifies towers according to 
their use (Class I, Class II, and Class III). This manual recommends that towers 
(including antennae) that are mounted on, located near, or serve hospitals be 
designed as Class III structures.

Collapse of both large and small communication towers at hospitals is quite 
common during high-wind events (see Figures 4-15 and 4-95). These failures often 
result in complete loss of communication capabilities. In addition to the 
disruption of communications, collapsed towers can puncture roof membranes and 
allow water leakage into the hospital, unless the roof system incorporated a 
secondary membrane (as discussed in Section 4.3.3.8). At the tower shown in 
Figure 4-95, the anchor bolts were pulled out of the deck, which resulted in a 
progressive peeling of the fully adhered single-ply roof membrane. Tower 
collapse can also injure or kill people. 

[Begin figure]
Figure 4-95 is a photo showing how the collapse of the antenna tower caused 
progressive peeling of the roof membrane. Also note that the exhaust fan blew 
off the curb, but the high parapet kept it from blowing off the roof. Hurricane 
Andrew (Florida, 1992) 
[end figure]

See Sections 4.3.1.1 and 4.3.1.5 regarding site considerations for light fixture 
poles, power poles, and electrical and communications towers.

Electrical service masts: Service mast failure is typically caused by collapse 
of overhead power lines, which can be avoided by using underground service. 
Where overhead service is provided, it is recommended that the service mast not 
penetrate the roof. Otherwise, a downed service line could pull on the mast and 
rupture the roof membrane. 

Satellite dishes: For the satellite dish shown in Figure 4-96, the dish mast was 
anchored to a large metal pan that rested on the roof membrane. CMU was placed 
on the pan to provide overturning resistance. This anchorage method should only 
be used where calculations demonstrate that it provides sufficient resistance. 
In this case the wind approached the satellite dish in such a way that it 
experienced very little wind pressure. In hurricane-prone regions, use of this 
anchorage method is not recommended (see Figures 4-83 and 4-97). 

[Begin figures]
Figures 4-96 is a photo of a common anchoring method for satellite dish. 
Hurricane Ivan (Florida, 2004)

Figure 4-97 is a photo showing how a satellite dish anchored similarly to that 
shown in Figure 4-96 was blown off this five-story building. Hurricane Charley 
(Florida, 2004) 
[end figures]

Lightning protection systems: For attachment of building lightning protection 
systems higher than 100 feet above grade, and for buildings located where the 
basic wind speed is in excess of 90 mph, see the following section on attaching 
LPS in hurricane-prone regions.

4.3.4.4 Lightning Protection Systems (LPS) in Hurricane-Prone Regions

Lightning protection systems frequently become disconnected from rooftops during 
hurricanes. Displaced LPS components can puncture and tear roof coverings, thus 
allowing water to leak into buildings (see Figures 4-98 and 4-99). Prolonged and 
repeated slashing of the roof membrane by loose conductors (“cables”) and 
puncturing by air terminals (“lightning rods”) can result in lifting and peeling 
of the membrane. Also, when displaced, the LPS is no longer capable of providing 
lightning protection in the vicinity of the displaced conductors and air 
terminals. 

[Begin figures]
Figure 4-98 is a photo showing an air terminal debonded from the hospital’s 
roof. Displaced air terminals can puncture tough membranes, such as this 
modified bitumen membrane. Hurricane Ivan (Florida, 2004)

Figure 4-99 is a photo showing a view of an end of a conductor at a hospital 
that became disconnected. Hurricane Katrina (Mississippi, 2005) 
[end figures]

Lightning protection standards such as NFPA 780 and UL 96A provide inadequate 
guidance for attaching LPS to rooftops in hurricane-prone regions, as are those 
recommendations typically provided by LPS and roofing material manufacturers. 
LPS conductors are typically attached to the roof at 3-foot intervals. The 
conductors are flexible, and when they are exposed to high winds, the conductors 
exert dynamic loads on the conductor connectors (“clips”). Guidance for 
calculating the dynamic loads does not exist. LPS conductor connectors typically 
have prongs to anchor the conductor. When the connector is well-attached to the 
roof surface, during high winds the conductor frequently bends back the 
malleable connector prongs (see Figure 4-100). Conductor connectors have also 
debonded from roof surfaces during high winds. Based on observations after 
Hurricane Katrina and other hurricanes, it is apparent that pronged conductor 
connectors typically have not provided reliable attachment.

[Begin figure]
Figure 4-100 is a photo showing how the conductor deformed the prongs under wind 
pressure, and pulled away from the connector. Hurricane Katrina (Mississippi, 
2005)
[end figure]

To enhance the wind performance of LPS, the following are recommended: [footnote 
13]

[Begin footnote 13] Discussion is based on Rooftop Attachment of Lightning 
Protection Systems in High-Wind Regions—Hurricane Katrina Recovery Advisory (May 
2006, Revised July 2006).[end footnote]

Parapet attachment: When the parapet is 12-inches high or greater, it is 
recommended that the air terminal base plates and conductor connectors be 
mechanically attached with #12 screws that have minimum 1¼-inch embedment into 
the inside face of the parapet nailer and are properly sealed for watertight 
protection. Instead of conductor connectors that have prongs, it is recommended 
that mechanically attached looped connectors be installed (see Figure 4-101). 

[Begin figure]
Figure 4-101 is a photo showing how this conductor was attached to the coping 
with a looped connector. Hurricane Katrina (Mississippi, 2005)
[end figure]

Attachment to built-up, modified bitumen, and single-ply membranes: For built-up 
and modified bitumen membranes, attach the air terminal base plates with asphalt 
roof cement. For single-ply membranes, attach the air terminal base plates with 
pourable sealer (of the type recommended by the membrane manufacturer). 

In lieu of attaching conductors with conductor connectors, it is recommended 
that conductors be attached with strips of membrane installed by the roofing 
contractor. For built-up and modified bitumen membranes, use strips of modified 
bitumen cap sheet, approximately 9 inches wide at a minimum. If strips are 
torch-applied, avoid overheating the conductors. For single-ply membranes, use 
self-adhering flashing strips, approximately 9 inches wide at a minimum. Start 
the strips approximately 3 inches from either side of the air terminal base 
plates. Use strips that are approximately 3 feet long, separated by a gap of 
approximately 3 inches (see Figure 4-102).

[Begin figure]
Figure 4-102 is plan showing conductor attachment 
[end figure]

As an option to securing the conductors with stripping plies, conductor 
connectors that do not rely on prongs could be used (such as the one shown in 
Figure 4-103). However, the magnitude of the dynamic loads induced by the 
conductor is unknown, and there is a lack of data on the resistance provided by 
adhesively-attached connectors. For this reason, attachment with stripping plies 
is the preferred option, because the plies shield the conductor from the wind. 
If adhesive-applied conductor connectors are used, it is recommended that they 
be spaced more closely than the 3-foot spacing required by NFPA 780 and UL 96A. 
Depending on wind loads, a spacing of 6 to 12 inches on center may be needed in 
the corner regions of the roof, with a spacing of 12 to 18 inches on center at 
roof perimeters (see ASCE 7 for the size of corner regions).

[Begin figure]
Figure 4-103 is a illustration of adhesively attached conductor connector that 
does not use prongs
[end figure]

Mechanically attached single-ply membranes: It is recommended that conductors be 
placed parallel to, and within 8 inches of, membrane fastener rows. Where the 
conductor falls between or is perpendicular to membrane fastener rows, install 
an additional row of membrane fasteners where the conductor will be located, and 
install a membrane cover-strip over the membrane fasteners. Place the conductor 
over the cover-strip and secure the conductor as recommended above.

By following the above recommendations, additional rows of membrane fasteners 
(beyond those needed to attach the membrane) may be needed to accommodate the 
layout of the conductors. The additional membrane fasteners and cover-strip 
should be coordinated with, and installed by, the roofing contractor.

Standing seam metal roofs: It is recommended that pre-manufactured, mechanically 
attached clips that are commonly used to attach various items to roof panels be 
used. After anchoring the clips to the panel ribs, the air terminal base plates 
and conductor connectors are anchored to the panel clips. In lieu of conductor 
connectors that have prongs, it is recommended that mechanically attached looped 
connectors be installed (see Figure 4-101). 

[Begin text box]
It is recommended that the building designer advise the building owner to have 
the LPS inspected each spring, to verify that connectors are still attached to 
the roof surface, that they still engage the conductors, and that the splice 
connectors are still secure. Inspections are also recommended after high-wind 
events.
[End text box]

Conductor splice connectors: In lieu of pronged splice connectors (see Figure 4-
104), bolted splice connectors are recommended because they provide a more 
reliable connection (see Figure 4-105). It is recommended that strips of 
flashing membrane (as recommended above) be placed approximately 3 inches from 
either side of the splice connector to minimize conductor movement and to avoid 
the possibility of the conductors becoming disconnected. To allow for 
observation during maintenance inspections, do not cover the connectors.

[Begin figures]
Figure 4-104 is a photo showing how if conductors detach from the roof, they are 
likely to pull out from pronged splice connectors. Hurricane Charley (Florida, 
2004) 

Figure 4-105 is a photo showing how bolted splice connectors are recommended to 
prevent free ends of connectors from being whipped around by wind. Hurricane 
Katrina (Mississippi, 2005)
[end figures]

4.3.4.5 The Case of Martin Memorial Medical Center, Stuart, Florida

The case of Martin Memorial Hospital illustrates the importance of elevator 
penthouse envelopes. Martin Memorial Medical Center is a 244-bed facility 
located on the south bank of the St. Lucie River in Stuart, Florida. The 
original hospital building, opened in 1939, is still in use but not for patient 
care. Currently the main hospital building is the six-story North Tower 
constructed in the early 1970s. MOBs and a cancer treatment facility are also 
located on the hospital campus. In 2004 the facility was struck by Hurricane 
Frances, and about 3 weeks later by Hurricane Jeanne.  The estimated peak gust 
wind speed at this site during Hurricane Frances was 100 mph. [footnote 14] The 
design wind speed in the 2005 edition of ASCE 7 for this location is 140 mph.

The hospital sustained damage to the elevator penthouse, roof covering, and 
roof-mounted equipment. Many of the metal panels on the elevator penthouse of 
the North Tower that were blown off during Hurricane Frances (Figure 4-106) tore 
the roof membrane on the tower roof  as well as on lower roofs. Mechanical 
equipment was damaged on the tower roof (Figure 4-107) and on lower roofs. The 
LPS was displaced on the tower roof (Figure 4-108) and on lower roofs. Unlike 
the windows on the first floor that were protected with shutters, the upper-
level windows were not protected. However, none of them were broken, although a 
significant amount of water leaked through many of these windows. The second 
hurricane (Jeanne) caused additional water infiltration and interrupted 
reconstruction work.

[Begin figures]
Figure 4-106 is a photo showing a view of the reconstruction of the damaged 
walls at the elevator penthouse

Figure 4-107 is a photo of damaged HVAC equipment. Work is underway on the 
elevator penthouse beyond. 

Figure 4-108 is a photo of displaced LPS
[end figures] 

[Begin footnote 14] The 100 mph speed was estimated for exposure C. [end 
footnote]

Loss of the elevator penthouse panels allowed water infiltration into the 
elevator equipment room, which destroyed the control equipment. Water also 
leaked into the nursing floors, which made it necessary to evacuate the 
patients. 
Because of significant interior water damage and lack of vertical 
transportation, many patients had to be evacuated by helicopter. 

As dramatically illustrated at this hospital, water infiltration and the lack of 
elevator service can take portions of the hospital offline for several weeks. 
Rather than simply replace the elevator penthouse walls, an engineer was 
retained to design a more wind-resistant wall covering system. The new design 
for the elevator penthouse wall system was developed and new elevator control 
equipment was installed, bringing the 5th floor back online about 4 weeks after 
the first hurricane struck. The remaining floors (2, 3, 4, and 6) were brought 
back online at a rate of about one floor every 2 weeks after the 5th floor was 
reopened. It cost $3,733,233 to repair the North Tower. In addition to this 
expenditure, the hospital lost a significant amount of patient revenue. 

Electrical service was interrupted for 36 hours (generators were used during 
that time), though there was no disruption of site access or water, sewer, and 
communications services. The hospital had an existing contingency plan, which 
was helpful during the response to these hurricanes. For instance, the hospital 
had contractors on site the day after the hurricane struck to perform emergency 
repairs; some of these contractors were under a pre-arranged contract with the 
hospital. The contingency plan was updated and modified based on experiences 
with these two hurricanes. 

4.3.5 Municipal Utilities in Hurricane-Prone Regions

Hurricanes typically disrupt municipal electrical service, and often they 
disrupt telephone (both cellular and land line), water, and sewer services. 
These disruptions may last from several days to several weeks. Electrical power 
disruptions can be caused by damage to power generation stations and by damaged 
lines, such as major transmission lines and secondary feeders. Water disruptions 
can be caused by damage to water treatment or well facilities, lack of power for 
pumps or treatment facilities, or by broken water lines caused by uprooted 
trees. Sewer disruptions can be caused by damage to treatment facilities, lack 
of power for treatment facilities or lift stations, or broken sewer lines. Phone 
disruptions can be caused by damage at switching facilities and collapse of 
towers. Hospitals should be designed to prevent the disruption of services 
arising from prolonged loss of municipal services.

4.3.5.1 Electrical Power 

It is recommended that buildings on hospital campuses that will be occupied 
during a hurricane, or will be needed within the first few weeks afterwards, be 
equipped with one or more emergency generators. In addition to providing 
emergency generators, it is recommended that one or more additional standby 
generators be considered, because continued availability of electrical power is 
vital. The purpose of providing the standby generators is to power those 
circuits that are not powered by the emergency generators. With both emergency 
and standby generators, the entire facility will be completely backed up. It is 
recommended that the emergency generator and standby generator systems be 
electrically connected via manual transfer switches to allow for 
interconnectivity in the event of emergency generator failure. The standby 
circuits can be disconnected from the standby generators, and the emergency 
circuits can be manually added. The emergency generators should be rated for 
prime power (continuous operation). 

Running generators for extended time periods frequently results in equipment 
failure. Thus, provisions for back-up generation capacity are important, because 
the municipal power system may be out of service for many days or even weeks. 
Therefore, it is recommended that an exterior box for single pole cable cam 
locking connectors be provided, so that a portable generator can be connected to 
the facility. With a cam locking box, if one or more of the emergency or standby 
generators malfunction, a portable generator can be brought to the facility and 
quickly connected. Back-up portable generators should be viewed as a third 
source of power (i.e., they should not replace standby generators), because it 
may take several days to get a back-up portable generator to the site. 

Generators should be placed inside wind-borne debris resistant buildings (see 
recommendations in Sections 4.3.2.1, 4.3.3.2, 4.3.3.6 and 4.3.3.8) so that they 
are not susceptible to damage from debris or tree fall. Locating generators 
outdoors or inside weak enclosures (see Figure 4-138) is not recommended. 

It is recommended that wall louvers for generators be capable of resisting the 
test Missile E load specified in ASTM E 1996. Alternatively, wall louvers can be 
protected with a debris-resistant screen wall so that wind-borne debris is 
unable to penetrate the louvers and damage the generators. If a screen wall is 
used, it should be designed to allow adequate air flow to the generator in order 
to avoid overheating the generator. 

It is recommended that sufficient onsite fuel storage be provided to allow all 
of the facility’s emergency and standby generators to operate at full capacity 
for a minimum of 96 hours (4 days).[footnote 15] If at any time it appears that 
refueling won't occur within 96 hours, provision should be made to shut off part 
or all the standby circuits in order to provide longer operation of the 
emergency circuits. For remote facilities or situations where it is believed 
that refueling may not occur within 96 hours, the onsite fuel storage capacity 
should be increased as deemed appropriate. It is recommended that fuel storage 
tanks, piping, and pumps be placed inside wind-borne debris resistant buildings, 
or underground. If the site is susceptible to flooding, refer to Chapter 3 
recommendations.

[Begin text box]
It is recommended that a minimum of 96 hours (4 days) of onsite fuel storage be 
provided for boilers. Storage tanks, piping, and pumps should be located within 
wind-borne debris resistant buildings or be placed underground (if site is 
susceptible to flooding, refer to Chapter 3).
[End text box]

[Begin footnote 15] The 96-hour fuel supply is based in part on the Department 
of Veterans Affairs criteria. [end footnote]

4.3.5.2 Water Service 

It is recommended that hospitals be provided with an independent water supply — 
a well or onsite water storage. If water is needed for cooling towers, the 
independent water supply should be sized to accommodate the system. It is 
recommended that the well or onsite storage be capable of providing an adequate 
water supply for fire sprinklers. Alternatively, it is recommended that the 
building designer should advise the building owner to implement a continual 
fire-watch and provide additional fire extinguishers until the municipal water 
service is restored. It is recommended that the well or onsite water storage be 
capable of providing a minimum of 100 gallons of potable water per day per 
patient bed for four days (the 100 gallons includes water for cooling towers). 
[footnote 16]

[Begin footnote 16] The 96-hour fuel supply is based in part on the Department 
of Veterans Affairs criteria. [end footnote]

It is recommended that pumps for wells or onsite storage be connected to an 
emergency power circuit, that a valve be provided on the municipal service line, 
and that onsite water treatment capability be provided where appropriate.

[Begin text box]
It is recommended that onsite storage of medical gases be sized to provide a 
minimum of 96 hours (4 days) of service.
[End text box]

4.3.5.3 Sewer Service 

It is recommended that hospitals be provided with an alternative means of waste 
disposal, such as a temporary storage tank that can be pumped out by a local 
contractor. It is also recommended that back-flow preventors be provided.

4.3.6 Post-Design Considerations in Hurricane-Prone Regions

In addition to adequate design, proper attention must be given to construction, 
post-occupancy inspections, and maintenance. 

4.3.6.1 Construction Contract Administration 

It is important for owners of hospitals in hurricane-prone regions to obtain the 
services of a professional contractor who will execute the work described in the 
contract documents in a diligent and technically proficient manner. The 
frequency of field observations and extent of special inspections and testing 
should be greater than those employed on hospitals that are not in hurricane-
prone regions.

4.3.6.2 Periodic Inspections, Maintenance, and Repair 

The recommendations given in Section 4.3.1.4 for post-occupancy and post-storm 
inspections, maintenance, and repair are crucial for hospitals in hurricane-
prone regions. Failure of a building component that was not maintained properly, 
repaired, or replaced, can present a considerable risk of injury or death to 
occupants, and the continued operation of the facility can be jeopardized. 




4.4 REMEDIAL WORK ON EXISTING FACILITIES

Many existing hospitals need to strengthen their structural or building envelope 
components. The reasons for this are the deterioration that has occurred over 
time, or inadequate facility strength to resist current design level winds. It 
is recommended that building owners have a vulnerability assessment performed by 
a qualified architectural and engineering team. A vulnerability assessment 
should be performed for all facilities older than 5 years. An assessment is 
recommended for all facilities located in areas where the basic wind speed is 
greater than 90 mph (even if the facility is younger than 5 years – see 
Figure 4-109). It is particularly important to perform vulnerability assessments 
on hospitals located in hurricane-prone and tornado-prone regions.

[Begin figure]
Figure 4-109 is a photo showing how the roof on this 5-year-old hospital blew 
off. Water leaked into the patient floor below. The floor was taken out of 
service for more than a month. Hurricane Katrina (Mississippi, 2005) 
[end figure]

Components that typically make buildings constructed before the early 1990s 
vulnerable to high winds are weak non-load-bearing masonry walls, poorly 
connected precast concrete panels, long-span roof structures with limited uplift 
resistance, inadequately connected roof decks, weak glass curtain walls, 
building envelope, and exterior-mounted equipment. Although the technical 
solutions to these problems are not difficult, the cost of the remedial work is 
typically quite high. If funds are not available for strengthening or 
replacement, it is important to minimize the risk of injury and death by 
evacuating areas adjacent to weak non-load-bearing walls, weak glass curtain 
walls, and areas below long-span roof structures when winds above 60 mph are 
forecast. 

[Begin text box]
Although it is unlikely, a hospital may occupy a building that was originally 
intended for another use. Buildings that were not designed for a critical 
occupancy were likely designed with a 1.0 rather than a 1.15 importance factor, 
and hence are not as wind-resistant as needed. It is particularly important to 
perform a vulnerability assessment if a hospital is located in a building not 
originally designed for a critical occupancy, especially if the hospital is 
located in a hurricane- or tornado-prone region.
[End text box]

As a result of building code changes and heightened awareness, some of the 
common building vulnerabilities have generally been eliminated for facilities 
constructed in the mid-1990s or later. Components that typically remain 
vulnerable to high winds are the building envelope and exterior-mounted 
mechanical, electrical, and communications equipment. Many failures can be 
averted by identifying weaknesses and correcting them. 

By performing a vulnerability assessment, items that need to be strengthened or 
replaced can be identified and prioritized. A proactive approach in mitigating 
weaknesses can save significant sums of money and decrease disruption or total 
breakdown in hospital operations after a storm. For example, a vulnerability 
assessment on a hospital such as that shown in Figure 4-110 may identify 
weakness of the roof membrane and/or rooftop equipment. Replacing weak 
components before a hurricane is much cheaper than replacing them and repairing 
consequential damages after a storm, and proactive work avoids the loss of use 
while repairs are made.

[Begin text box]
Before beginning remedial work, it is necessary to understand all significant 
aspects of the vulnerability of a hospital with respect to wind and wind-driven 
rain. If funds are not available to correct all identified deficiencies, the 
work should be systematically prioritized so that the items of greatest need are 
first corrected. Mitigation efforts can be very ineffective if they do not 
address all items that are likely to fail.
[End text box]

[Begin figure]
Figure 4-110 The roof membrane and some of the rooftop equipment blew off. 
Although the deck was cast-in-place concrete, water leaked into the patient 
floor below. Hurricane Charley (Florida, 2004)
[end figure]

A comprehensive guide for remedial work on existing facilities is beyond the 
scope of this manual. However, the following are examples of mitigation measures 
that are often applicable. 

4.4.1 Structural Systems

As discussed in Section 4.1.4.1, roof decks on many facilities designed prior to 
the 1982 edition of the SBC and UBC and the 1987 edition of the NBC are very 
susceptible to failure. Poorly attached decks that are not upgraded are 
susceptible to blow-off, as shown in Figures 4-111 and 4-132. Decks constructed 
of cementitious wood-fiber, gypsum, and lightweight insulating concrete over 
form boards were commonly used on buildings built in the 1950s and 1960s. In 
that era, these types of decks, as well as precast concrete decks, typically had 
very limited uplift resistance due to weak connections to the support structure. 
Steel deck attachment is frequently not adequate because of an inadequate number 
of welds, or welds of poor quality. Older buildings with overhangs are 
particularly susceptible to blow-off, as shown in Figure 4-112, because older 
codes provided inadequate uplift criteria.

[Begin figures]
Figures 4-111 is a photo showing how the built-up roof blew off after one of the 
cementitious wood-fiber deck panels detached from the joists. Hurricane Katrina 
(Mississippi, 2005) 

Figure 4-112 is a photo showing how the cementitious wood-fiber deck panels 
detached from the joists along the overhangs and caused the built-up membranes 
to lift and peel. Hurricane Katrina (Mississippi, 2005)
[end figures]

A vulnerability assessment of the roof deck should include evaluating the 
existing deck attachment, spot checking the structural integrity of the deck 
(including the underside, if possible), and evaluating the integrity of the 
beams/joists. If the deck attachment is significantly overstressed under current 
design wind conditions or the deck integrity is compromised, the deck should be 
replaced or strengthened as needed. The evaluation should be conducted by an 
investigator experienced with the type of deck used on the building. 

If a low-slope roof is converted to a steep-slope roof, the new support 
structure should be engineered and constructed to resist the wind loads and 
avoid the kind of damage shown in Figure 4-113.

[Begin figure]
Figure 4-113 is a photo showing how the steel truss superstructure installed as 
part of a steep-slope conversion blew away because of inadequate attachment. 
Hurricane Marilyn (U.S. Virgin Islands, 1995)
[end figure]

4.4.2 Building Envelope 

The following recommendations apply to building envelope components of existing 
hospitals.

4.4.2.1 Sectional and Rolling Doors

Sectional and rolling doors (e.g., at hospital loading docks and ambulance 
garages), installed in older buildings before attention was given to the wind 
resistance of these elements, are very susceptible to being blown away. Although 
weak doors can be retrofitted, it is difficult to ensure that the door, door 
tracks, and connections between the door and tracks are sufficient. It is 
therefore recommended that weak doors and tracks be replaced with new assemblies 
that have been tested to meet the factored design wind loads. As part of the 
replacement work, nailers between the tracks and building structure should 
either be replaced, or their attachment should be strengthened.

If a facility has more than one sectional or rolling door, all doors should be 
replaced, rather than just replacing one of the doors. The building shown in 
Figure 4-114 had six sectional doors. One door had been replaced before a 
hurricane. It performed very well, but three of the older doors were blown away 
and two of the older doors remained in place but had some wind damage.

[Begin figure]
Figure 4-114 is a photo showing how the new door in the center performed very 
well, but the older doors on either side of it were blown away. Hurricane 
Charley (Florida, 2004)
[end figure]

4.4.2.2 Windows and Skylights

Windows in older facilities may possess inadequate resistance to wind pressure. 
Window failures are typically caused by wind-borne debris, however, glazing or 
window frames may fail as a result of wind pressure (see Figure 4-115). Failure 
can be caused by inadequate resistance of the glazing, inadequate anchorage of 
the glazing to the frame, failure of the frame itself, or inadequate attachment 
of the frame to the wall. For older windows that are too weak to resist the 
current design pressures, window assembly replacement is recommended. Some older 
window assemblies have sufficient strength to resist the design pressure, but 
are inadequate to resist wind-driven rain. If the lack of water resistance is 
due to worn glazing gaskets or sealants, replacing the gaskets or sealant may be 
viable. In other situations, replacing the existing assemblies with new, higher-
performance assemblies may be necessary.

[Begin figure]
Figure 4-115 is a photo showing how wind pressure caused the window frames on 
the upper floor to fail (red arrow). Hurricane Katrina (Mississippi, 2005)
[end figure]

It is recommended that all non-impact-resistant, exterior glazing located in 
hurricane-prone regions (with a basic wind speed of 100 mph or greater) be 
replaced with impact-resistant glazing or be protected with shutters, as 
discussed in Section 4.3.3.4. Shutters are typically a more economical approach 
for existing facilities. There are a variety of shutter types, all illustrated 
by Figures 4-116 to 4-118. Accordion shutters are permanently attached to the 
wall (Figure 4-116). When a hurricane is forecast, the shutters are pulled 
together and latched into place. Panel shutters (Figure 4-117) are made of metal 
or polycarbonate. When a hurricane is forecast, the shutters are taken from 
storage and inserted into metal tracks that are permanently mounted to the wall 
above and below the window frame. The panels are locked into the frame with wing 
nuts or clips. Track designs that have permanently mounted studs for the nuts 
have been shown to be more reliable than track designs using studs that slide 
into the track. A disadvantage of panel shutters is the need for storage space. 
Roll-down shutters (Figure 4-118) can be motorized or pulled down manually. 
Figure 4-118 illustrates the benefits of shuttering. Two of the unprotected 
window units experienced glass breakage and the third window unit blew in. 

[Begin figures]
Figure 4-116 is a photo showing how this building has accordion shutters. 
Hurricane Ivan (Florida, 2004)

Figure 4-117 is a photo that illustrates a metal panel shutter. Hurricane 
Georges (Puerto Rico, 1998) 

Figure 4-118 is a photo showing how the lower window assembly was protected with 
a motorized shutter. Hurricane Ivan (Florida, 2004)
[end figures]

Deploying accordion or panel shutters a few stories above grade is expensive. 
Although motorized shutters have greater initial cost, their operational cost 
should be lower. Other options for providing missile protection on upper levels 
include replacing the existing assemblies with laminated glass assemblies, or 
installing permanent impact resistant screens. Engineered films are also 
available for application to the interior of the glass. The film needs to be 
anchored to the frame, and the frame needs to be adequately anchored to the 
wall. The film degrades over time and requires replacement (approximately every 
decade). Use of laminated glass or shutters is recommended in lieu of engineered 
films.

4.4.2.3 Roof Coverings

For roofs with weak metal edge flashing or coping attachment, face-attachment of 
the edge flashing/coping (as shown in Figure 4-67) is a cost-effective approach 
to greatly improve the wind-resistance of the roof system. 

The vulnerability assessment of roofs ballasted with aggregate, pavers, or 
cementitious-coated insulation boards, should determine whether the ballast 
complies with ASNI/SPRI RP-4. Corrective action is recommended for non-
compliant roof coverings. It is recommended that roof coverings with aggregate 
surfacing, lightweight pavers, or cementitious-coated insulation boards on 
buildings located in hurricane-prone regions be replaced to avoid blow-off 
(Figures 4-5, 4-46, and 4-47). 

When planning the replacement of a roof covering, it is recommended that all 
existing roof covering be removed down to the deck rather than simply re-
covering the roof. Tearing off the covering provides an opportunity to evaluate 
the structural integrity of the deck and correct deck attachment and other 
problems. For example, if a roof deck was deteriorated due to roof leakage (see 
Figure 4-119), the deterioration would likely not be identified if the roof was 
simply re-covered. By tearing off down to the deck, deteriorated decking like 
that shown in Figure 4-119 can be found and replaced. In addition, it is 
recommended that the attachment of the wood nailers at the top of parapets and 
roof edges be evaluated and strengthened where needed, to avoid blow-off and 
progressive lifting and peeling of the new roof membrane (see Figure 4-126). 

[Begin figure]
Figure 4-119 is a photo showing how the built-up roof was blown off after a few 
of the rotted wood planks detached from the joists. Hurricane Katrina 
(Mississippi, 2005)
[end figure]

If the roof has a parapet, it is recommended that the inside of the parapet be 
properly prepared to receive the new base flashing. In many instances, it is 
prudent to re-skin the parapet with sheathing to provide a suitable substrate. 
Base flashing should not be applied directly to brick parapets because they have 
irregular surfaces that inhibit good bonding of the base flashing to the brick 
(see Figure 4-120). Also, if moisture drives into the wall from the exterior 
side of the parapet with base flashing attached directly to brick, the base 
flashing can inhibit drying of the wall. Therefore, rather than totally sealing 
the parapet with membrane base flashing, the upper portion of the brick can be 
protected by metal panels (as shown in Figure 4-79), which permit drying of the 
brick.

[Begin figure]
Figure 4-120 is a photo showing hoe failed base flashing adhered directly to the 
brick parapet. Hurricane Katrina (Louisiana, 2005) 
[end figure]

If the parapet is constructed of masonry, it is recommended that its wind 
resistance be evaluated and strengthened if found to be inadequate. The masonry 
parapet shown in Figure 4-139 fell onto the roof. Had it fallen in the other 
direction, it would have blocked the entry and would have had the potential to 
cause injury.

4.4.3 Exterior-Mounted Equipment

Exterior-mounted equipment on existing hospitals should be carefully examined 
and evaluated.

4.4.3.1 Antenna (Communications Mast)

Antenna collapse is very common. Besides loss of communications, collapsed masts 
can puncture roof membranes or cause other building damage as shown in Figure 4-
121. This case also demonstrates the benefits of a high parapet. Although the 
roof still experienced high winds that blew off this penthouse door, the parapet 
prevented the door from blowing off the roof (red arrow in Figure 4-121).

[Begin figure]
Figure 4-121 is a photo showing how the antenna at this hospital collapsed and 
was whipped back and forth across the roof membrane. Hurricane Andrew (Florida, 
1992) 
[Begin figure]

In hurricane-prone regions, it is recommended that antennae strength be 
evaluated as part of the vulnerability assessment. Chapter 15 of ANSI/TIA-222-G 
provides guidance on the structural evaluation of existing towers. Appendix J of 
that standard contains checklists for maintenance and condition assessments. 
Additional bracing, guy-wires, or tower strengthening or replacement may be 
needed.

[Begin text box]
Fastening rooftop equipment to curbs, as discussed in Section 4.3.4.1, is a 
cost-effective approach to minimize wind-induced problems.
[End text box]

4.4.3.2 Lightning Protection Systems

Adhesively attached conductor connectors and pronged splice connectors typically 
have not provided reliable attachment during hurricanes. To provide more 
reliable attachment for LPS located in hurricane-prone regions where the basic 
wind speed is 100 mph or greater, or on hospitals more than 100 feet above 
grade, it is recommended that attachment modifications based on the guidance 
given in Section 4.3.4.4 be used. 

4.4.4 The Case of Baptist Hospital, Pensacola, Florida

The case of Baptist Hospital illustrates the challenges faced by older 
facilities. Baptist Hospital is a 492-bed tertiary care hospital located in 
downtown Pensacola on West Moreno St. approximately 2 miles north of Pensacola 
Bay. This hospital campus, which dates back to the 1950s, includes the hospital 
itself, a psychiatric hospital, and MOBs. 

This facility was also struck by Hurricane Ivan. The estimated wind speed and 
design wind speed at this hospital are the same as the case study presented in 
Section 4.2.1.3. Figure 4-122 shows the site plan and Figure 4-123 is a general 
view from the northwest (looking southeast).

[Begin figure]
Figure 4-122 is a site plan

Figure 4-123 is a photo showing a general view.
[end figure]

Water entered the hospital at damaged rooftop equipment (Figure 4-124), and at 
areas where the roof membrane blew off (Figures 4-125 and 4-126) and where it 
was punctured. The roof failure shown in Figure 4-126 was caused by an 
inadequately attached edge nailer anchored to the brick wall. Failure of the 
nailer caused a progressive lifting and peeling of the roof membrane. Gutters 
and downspouts were blown off and a few windows were broken. The elevator 
penthouse roof was damaged at the psychiatric hospital (Figure 4-127) and the 
MOBs (Figure 4-125).

[Begin figures]
Figure 4-124 is a photo showing how this hospital had a substantial amount of 
rooftop ductwork. Ductwork and fan units were damaged in several locations (see 
inset). Some of the windows in this area were also broken. Note the missing 
downspout (yellow arrow). 

Figure 4-125 is a photo showing how the roof covering blew off – an emergency 
roof covering had been installed (yellow arrow). Note the damaged MOB penthouse 
beyond (red arrow).

Figure 4-126 is a photo showing how the roof covering blew off – an emergency 
roof covering had been installed (blue arrow). The failure was caused by the 
inadequately attached nailer (see inset). The leaning mast at the right is a 
ladder (yellow arrow), with an extension for communications and an anemometer 
for the nearby heliport. 

Figure 4-127 is a photo showing how an emergency roof covering had been 
installed over the elevator penthouse at the psychiatric hospital.
[end figures]



4.5 BEST PRACTICES IN TORNADO-PRONE REGIONS

Strong and violent tornadoes may reach wind speeds substantially greater than 
those recorded in the strongest hurricanes. The wind pressures that these 
tornadoes can exert on a building are tremendous, and far exceed the minimum 
pressures derived from building codes. 

Strong and violent tornadoes can generate very powerful missiles. Experience 
shows that large and heavy objects, including vehicles, can be hurled into 
buildings at high speeds. The missile sticking out of the roof in the foreground 
of Figure 4-128 is a double 2-inch by 6-inch wood member. The portion sticking 
out of the roof is 13 feet long. It penetrated a ballasted ethylene propylene 
diene monomer (EPDM) membrane, approximately 3 inches of polyisocyanurate roof 
insulation, and the steel roof deck. The missile lying on the roof just beyond 
is a 2-inch by 10-inch by 16-foot long wood member. 

[Begin figure]
Figure 4-128 is a photo showing how a violent tornado showered the roof with 
missiles. (Oklahoma, 1999)
[end figure]

Besides the case studies presented in Sections 4.5.1 and 4.5.2, there is little 
documentation regarding tornado-induced damage to hospitals. Most of the damage 
reports on critical facilities pertain to schools because schools are the most 
prevalent type of critical facilities and, therefore, are more likely to be 
struck. A 1978 report prepared for the Veterans Administration [footnote 17] 
identified four hospitals that were struck by tornadoes between 1973 and 1976. 
Table 4-2 (taken from that report) further illustrates the effects tornados can 
have on hospitals. 

[begin footnote 17]
A Study of Building Damage Caused by Wind Forces, McDonald, J.R. and Lea, P.A, 
Institute for Disaster Research, Texas Tech University, 1978. [end footnote]


[Begin Table]
Table 4-2: Examples of Ramifications of Tornado Damage at Four Hospitals

Location and Building Characteristics: Mountain View, Missouri (St. Francis 
Hospital). One-story steel frame with non-load bearing masonry exterior walls.
Tornado Characteristics: The tornado crossed over one end of the hospital. 
Damage: Metal roof decking was blown off, some windows were broken, and rooftop 
mechanical equipment was displaced.
Ramifications of Damage: Patients were moved to undamaged areas of the hospital.

Location and Building Characteristics: Omaha, Nebraska (Bishop Bergen Mercy 
Hospital). Five-story reinforced concrete frame. 
Tornado Characteristics: Maximum wind speed estimated at 200 mph. Proximity to 
hospital not documented.
Damage: Windows were broken, and rooftop mechanical equipment was damaged and 
displaced. Communications and electrical power were lost (emergency generators 
provided power). 
Ramifications of Damage: A few minor cuts; “double walled corridors” provided 
protection for patients and staff. Some incoming emergency room patients 
(injured elsewhere in the city) were rerouted to other hospitals. Loss of 
communications hampered the rerouting.

Location and Building Characteristics: Omaha, Nebraska (Bishop Bergen Mercy 
Hospital – Ambulatory Care Unit). One-story load bearing CMU walls with steel 
joists. 
Tornado Characteristics: See above.
Damage: The building was a total loss due to wall and roof collapse.
Ramifications of Damage: Patients were evacuated to the first floor of the main 
hospital when the tornado watch was issued.

Location and Building Characteristics: Corsicana, Texas (Navarro County Memorial 
Hospital). Five-story reinforced concrete frame with masonry non-load bearing 
walls in some areas and glass curtain walls. 
Tornado Characteristics: The tornado was very weak.
Damage: Many windows were broken by aggregate from the hospital’s built-up 
roofs. Intake duct work in the penthouse collapsed.
Ramifications of Damage: Two people in the parking lot received minor injuries 
from roof aggregate. Electrical power was lost for 2 hours (emergency generators 
provided power).

Location and Building Characteristics: Monahans, Texas (Ward Memorial Hospital). 
One-story load bearing CMU walls with steel joists. Some areas had metal roof 
deck and others had gypsum deck.
Tornado Characteristics: The tornado passed directly over the hospital, with 
maximum wind speed estimated at 150 mph. 
Damage: The roof structure was blown away on a portion of the building (the bond 
beam pulled away from the wall). Many windows were broken. Rooftop mechanical 
equipment was damaged.
[end table]

[Begin text box]
In this manual, the term “tornado-prone regions” refers to those areas of the 
United States where the number of recorded F3, F4, and F5 tornadoes per 3,700 
square miles is 6 or greater per year (see Figure 4-129). However, an owner of a 
hospital may decide to use other frequency values (e.g., 1 or greater, 16 or 
greater, or greater than 25) in defining whether the hospital is in a tornado-
prone area. In this manual, tornado shelters are recommended for all hospitals 
in tornado-prone regions.
Where the frequency value is 1 or greater, and the hospital does not have a 
tornado shelter, the best available refuge areas should be identified, as 
discussed at the end of this Section.
[End text box]

[Begin figure]
Figure 4-129 is a map showing Frequency of recorded F3, F4, and F5 tornadoes 
(1950-1998)
[end figure]

For hospitals located in tornado-prone regions (as defined in the text box), the 
following are recommended:

- Incorporate a shelter within the facility to provide occupant protection. For 
shelter design, FEMA 361 criteria are recommended. 

- For interior non-load-bearing masonry walls, see the recommendations given in 
4.3.3.5. 

- Brick veneer, aggregate roof surfacing, roof pavers, slate, and tile cannot be 
effectively anchored to prevent them from becoming missiles if a strong or 
violent tornado passes near a building with these components. To reduce the 
potential number of missiles, and hence reduce the potential for building damage 
and injury to people, it is recommended that these materials not be specified 
for hospitals in tornado-prone regions. 

- To minimize disruption from nearby weak tornadoes and from strong and violent 
tornados that are on the periphery of a hospital, the following are recommended:

1) For the roof deck, exterior walls, and doors, follow the recommendations 
given in Sections 4.3.2.1, 4.3.3.2, and 4.3.3.6. 

2) For exterior glazing, specify laminated glass window assemblies that are 
designed to resist the test Missile E load specified in ASTM E 1996, and are 
tested in accordance with ASTM E 1886. Note that missile loads used for 
designing tornado shelters significantly exceed the missile loads used for 
designing glazing protection in hurricane-prone regions. Missiles from a strong 
or violent tornado passing near the facility could penetrate the laminated 
glazing and result in injury or interior damage. Therefore, to increase occupant 
safety, even when laminated glass is specified, the facility should also 
incorporate a shelter as recommended above.

[Begin text box]
It is recommended that hospitals have a National Oceanic and Atmospheric 
Administration (NOAA) weather radio, so that they will be aware of tornado 
watches and warnings. It is also recommended that hospitals have a plan to 
distribute notice of watches and warnings received via the radio to hospital 
staff.
[End text box]

Existing Hospitals without Tornado Shelters 

Where the number of recorded F3, F4, and F5 tornadoes per 3,700 square miles is 
one or greater (see Figure 4-129), the best available refuge areas should be 
identified if the hospital does not have a tornado shelter. FEMA 431, Tornado 
Protection, Selecting Refuge Areas in Buildings provides useful information for 
building owners, architects, and engineers who perform evaluations of existing 
facilities.

To minimize casualties in hospitals, it is very important that the best 
available refuge areas be identified by a qualified architect or engineer. 
[footnote 18] Once identified, those areas need to be clearly marked so that 
occupants can reach the refuge areas without delay. Building occupants should 
not wait for the arrival of a tornado to try to find the best available refuge 
area in a particular facility; by that time, it will be too late. If refuge 
areas have not been identified beforehand, occupants will take cover wherever 
they can, frequently in very dangerous places. Corridors, as shown in Figure 4-
142, sometimes provide protection, but they can also be death traps. 

[Begin footnote 18] It should be understood that the occupants of a “best 
available refuge area” are still vulnerable to death and injury if the refuge 
area was not specifically designed as a tornado shelter. [end footnote]

Retrofitting a shelter space inside an existing hospital can be very expensive. 
An economical alternative is an addition that can function as a shelter as well 
as serve another purpose. This approach works well for smaller facilities. For 
very large facilities, constructing two or more shelter additions should be 
considered in order to reduce the time it takes to reach the shelter (often 
there is ample warning time, but sometimes an approaching tornado is not noticed 
until a few minutes before it strikes). This is particularly important for 
hospitals because of the difficulty of accommodating patients with different 
medical needs. 

4.5.1 The case of Kiowa County Memorial Hospital, Greensburg, Kansas

The case of Kiowa County Memorial Hospital illustrates damage that is indicative 
of smaller, older facilities that are struck by strong tornadoes. The hospital 
is a 28-bed, one-story facility. The original hospital comprised three separate 
buildings (two of these are shown in Figure 4-131): a patient wing, a kitchen 
facility, and an equipment building, and was constructed in 1950. Additions were 
built in 1965 and 1982. A separate ambulance garage was built prior to 1979, 
and a separate pre-engineered metal storage building was added in 2006. The 
original buildings had precast twin-tee roof structures and at the time of the 
storm, they had aggregate ballasted single-ply membrane roof systems. The 
additions all had different structural systems. The 1965 patient wing addition 
had a concrete topping slab over metal form deck over steel roof joists. The 
1982 addition, which housed the emergency room, semi-intensive care, operating 
room, MRI, lab, medical records, and business offices, had a plywood roof deck 
over wood joists. The ambulance garage had a precast twin-tee roof structure. 

[Begin figure]
Figure 4-131 is a photo of the building housing the generator is shown by the 
yellow arrow. The kitchen facility is shown by the blue arrow. The red arrow 
shows the collapsed storage building. 
[end figure]

Except for the storage building, the majority of the exterior walls were brick 
veneer over unreinforced CMU. A finished basement was built under portions of 
the 1965 and 1982 additions (the two basements were not interconnected). 

Figure 4-130 shows the site plan.

[Begin figure]
Figure 4-130 is site plan
[end figure]

The hospital was struck by a tornado in 2007. The National Weather Service rated 
a portion of the track as an EF5 (with an estimated peak gust speed in excess of 
200 mph). At the hospital site, the damage was indicative of an EF3 (with a 
speed between 136 and 165 mph). The 2005 edition of ASCE 7 lists the design wind 
speed for this location as 90 mph. Therefore, the speeds at this site were well 
above current design conditions.

[Begin text box]
Using the Damage Indicators in the Enhanced Fujita Scale (EF-Scale) for the main 
portion of the hospital, the Degree of Damage (DOD) indicated an expected wind 
speed during the tornado of 142 mph (with lower and upper bounds ranging from 
119 to 163 mph). Failure analysis of the precast twin-tee that blew off the 
hospital indicated that a speed of approximately 147 mph was needed to blow off 
the tee, and a speed of approximately 193 mph was needed to toss it the 80 feet 
that it traveled.
The DOD at the pre-engineered storage building indicated an expected wind speed 
of 155 mph (with lower and upper bounds ranging from 132 to 178 mph). 
Thus, the DOD of the hospital, the DOD of the storage building and the failure 
analysis of tee blow off indicate essentially the same estimated speed (142, 
147, and 155 mph). The upper bound values of the DODs (163 and 178 mph) are 
lower than the 193 mph speed that was calculated to have tossed the tee. Based 
on the EF-Scale DODs, the 193 mph speed appears to be high.
[End text box]

One of the precast twin-tees from the original building blew off and flew 
approximately 80 feet (Figures 4-132 and 4-133). Where the tees rested on the 
bearing wall, steel bearing plates were embedded in the tee’s beams, and bearing 
plates were embedded on top of the wall. At the wall in the foreground of Figure 
4-132, the bearing plates had not been welded together. Hence, at that end of 
the tee, no uplift resistance was provided other than the tee’s dead load. At 
the opposite bearing wall, the bearing plates were welded, but the bolts 
anchoring the plates to the wall failed in tension as the tee lifted. Both of 
the bearing plates had two small anchor bolts (about 3/8-inch diameter).

[Begin figure]
Figure 4-132 is a photo of the view of the end of the patient wing where the 
twin-tee blew off. Note the missing window. 

Figure 4-133 is a photo showing how the missing tee shown in Figure 4-132 landed 
about 80 feet away (red oval). The red arrows show tees that were blown from the 
ambulance garage. Note the missing wood roof structure on the 1982 addition 
(yellow arrow). 
[end figures]

The hospital complex was struck with a very large number of missiles. Virtually 
all of the hospital’s exterior glazing was broken. Figure 4-134 shows a damaged 
door at the kitchen facility. A piece of built-up roof (BUR) membrane struck the 
right door. Although the doors were out-swinging, the missile pushed the door 
inward. The lower right hinge was broken, and the right side of the door frame 
buckled and pulled from the rough opening. The laminated glass in the door was 
broken, but remained in place.


Figure 4-134 is a photo showing how the BUR missile (red arrow) struck the right 
door. Missiles also punctured the siding and wood sheathing (red oval).
[end figure]

Aggregate from the ballasted single-ply membrane roofs broke several windows 
(Figure 4-135), and pieces of the large aggregate (11/2-inches in diameter, 
nominal) were found inside the building. Windows were also broken by 2x wood 
framing (Figure 4-136).

[Begin figures]
Figure 4-135 All six panes of glass were broken. The craters shown in the right 
center pane and at the vehicle windshield were caused by the large aggregate 
blown from the ballasted single-ply membranes.

Figure 4-136 is a photo showing how a large missile (2x framing) penetrated this 
patient room. Note the debris on the bed in the inset (the arrow shows the same 
2x missile).
[end figures]

Figure 4-137 illustrates an opening through the entire exterior wall.

[Begin figure]
Figure 4-137 is a photo showing how a missile impact created an opening in the 
brick veneer and unreinforced CMU.
[end figure]

The building shown in Figure 4-131 (yellow arrow) and in Figure 4-138 housed the 
emergency generator. The sectional door collapsed and allowed wind-borne debris 
to strike the generator. All window panes in the wall adjacent to the sectional 
door were broken (see inset at Figure 4-138). The wall louver adjacent to the 
window, which served the generator room, escaped damage. 

[Begin figure]
Figure 4-138 is a photo showing how the sectional door (red arrow) at the 
generator building was blown in and windows were broken by debris. 
[end figure]

There was no apparent structural damage to the 1965 addition. However, the 
unreinforced brick/CMU parapet shown in Figure 4-139 fell onto the roof, rather 
than in the other direction, where it would have blocked the entry and possibly 
could have caused injuries. Virtually all of the exterior windows were broken 
(see Figures 4-135 and 4-136). The roof insulation and aggregate-ballasted 
single-ply roof membrane were blown off. The wood roof structure blew off the 
majority of the 1982 addition (Figures 4-133 and 4-140), and virtually all of 
the exterior windows were broken. 

[Begin figures]
Figure 4-139 is a photo showing a collapsed unreinforced masonry parapet. Note 
the broken windows. 

Figure 4-140 is a photo showing the loss of the wood roof structure at the 1982 
addition. 
[end figures]

The precast twin-tees on the ambulance garage blew off and landed against the 
wall of the 1982 addition (Figures 4-133 and inset at 4-141). The garage door 
and virtually all of the exterior unreinforced brick/CMU bearing walls collapsed 
(Figure 4-141). Where the tees rested on the bearing wall, steel bearing plates 
were embedded in the tee’s beams. However, bearing plates had not been embedded 
on top of the support walls. Rather, where the tee’s beams rested on the wall, a 
piece of roof membrane had been installed between the beams and the wall. Hence, 
at the ends of the tees, no uplift resistance was provided other than the tee’s 
dead load. 

[Begin figure]
Figure 4-141 is a photo showing a view of the collapsed ambulance garage. The 
twin-tees (red arrows) flew to the left, as shown in the inset.
[end figure]

There were 20 patients and 10 staff in the facility when the tornado struck 
around 9:45 p.m. Fortunately, the staff was aware of the tornado warning that 
was issued by the National Weather Service. Patients and staff took refuge in 
the basement of the 1965 addition per the hospital’s tornado plan. Because of 
the extensive structural and non-structural damage, it was necessary to 
completely evacuate the hospital after the storm. Evacuation was completed by 
around 2:45 a.m. (about 5 hours after the tornado). None of the occupants were 
injured during the storm or evacuation. The ambulance shown in Figure 4-141 was 
not usable because of missile damage. Even though some portions of the facility 
could be salvaged, significant demolition and reconstruction will be necessary.

The loss of life and avoidance of injuries was attributed to three factors. 1) A 
tornado warning was issued by the National Weather Service about 20 minutes 
before the tornado struck, and the hospital’s staff was aware of the warning. 2) 
The hospital had a tornado evacuation plan and there was sufficient time to 
execute the plan. 3) Although it is believed that the basement was not 
specifically designed as a tornado shelter, it provided a safe area of refuge 
for patients and staff. For hospitals located in tornado-prone regions, the 
experience from this tornado demonstrates the importance of having a pre-
identified, best available refuge area (or preferably a FEMA 361-compliant 
shelter). Although small interior rooms and corridors sometimes provide adequate 
protection during tornadoes, unless specifically designed as a tornado shelter, 
they often provide inadequate protection, as shown in Figure 4-142.

[Begin figure]
Figure 4-142 is a photo showing a view of a main corridor in the 1982 addition  
[end figure]

[Begin text box]
It was determined that with minimal work, the basement under the 1965 addition 
could be used as an interim, best available refuge area for the city. In the 
weeks and months following the storm, several hundred people would be in the 
city performing demolition, salvaging personal items, and conducting 
reconstruction. Much of this work would occur during the time of year when 
tornado activity is high. Hence, although severely damaged, a portion of the 
hospital continued to serve the community by providing an interim refuge area.
[End text box]

The damage investigation of this facility validates several of the 
recommendations provided in Section 4.5, summarized below:

- Incorporate a tornado shelter within the facility.

- Don't use aggregate roof surfacing.

- Use roof decks, exterior walls, and doors as recommended in sections 4.3.2.1, 
4.3.3.2, and 4.3.3.6.

- Use laminated glass window assemblies that are designed to resist the test 
Missile E.

4.5.2 The case of Sumter Regional Hospital, Americus, Georgia 

The previous case study reported on a smaller, older hospital that was struck by 
a tornado. The case of Sumter Regional Hospital illustrates the performance of a 
much larger and newer hospital. This hospital is a 143-bed, four-story facility 
built in 1953, and expanded in 1975, 1983, and 1999. The 1999 addition had a 
cast-in-place concrete roof deck. All of the other buildings had steel roof 
decks. Roof coverings included single-ply membranes (exposed and aggregate-
ballasted) and aggregate surfaced built-up. Exterior walls included brick veneer 
(over unreinforced CMU and over steel studs) and EIFS over steel studs.  

[Begin figure]
Figure 4-143 is an aerial view of the facility after it was struck by the 
tornado. The red arrow indicates the general direction of the tornado. The blue 
arrow shows the 1953 building, and the yellow arrow shows the 1999 building. The 
1975 and 1983 additions are adjacent to and behind the 1953 and 1999 buildings. 
Courtesy of the National Weather Service. 
[end figure]

The hospital was struck by a tornado in 2007. The National Weather Service rated 
a portion of the track as an EF3 (with an estimated peak gust speed between 136 
and 165 mph). At the hospital site, the damage was indicative of an EF2 (with a 
speed between 111 and 135 mph). The 2005 edition of ASCE 7 lists the design wind 
speed for this location as 90 mph. Therefore, the speeds at this site were well 
above current design conditions.

[Begin text box]
Using the Damage Indicators in the Enhanced Fujita Scale (EF-Scale), the Degree 
of Damage (DOD) at the hospital indicated an expected wind speed during the 
tornado of 131 mph (with lower and upper bounds ranging from 110 to 152 mph).
[End text box]

As the tornado approached the southwest side of the hospital, numerous tree 
branches from trees in front of the hospital were thrown against the building. 
Missiles broke virtually all of the glass in the southwest walls (Figures 4-144 
and 4-145), and missiles penetrated the EIFS and landed inside the building. 
Roof decking and steel joists were blown off of portions of the 1953 building, 
and the roof membrane was blown off of several different areas of the facility. 

The right (curved) portion of the building in Figure 4-144 is the 1999 addition. 
This portion of the first floor housed waiting and exam rooms. Offices were on 
the second floor, and medical offices were on the third floor. Figures 4-144 and 
4-146 also show part of the 1953 building. Offices were on the first floor, and 
patient rooms were on the second and third floors. 

[Begin figures]
Figure 4-144 is a photo showing the 1999 addition on the right, and the 1953 
building to the left. Virtually all of the glass on these facades was broken.

Figure 4-145 is a photo showing how one window frame was blown away (red 
circle). The EIFS was penetrated by many missiles, and in some areas, the entire 
wall (except the studs) was blown away (arrows).

Figure 4-146 is a photo showing how several of the window frames were blown 
away, along with some of the brick veneer (1999 addition).
[end figures]

Broken glass showered the rooms along the exterior walls. The breach of the 
building envelope allowed strong winds to enter the rooms and corridors, which 
led to the collapse of suspended acoustical ceilings and light fixtures. Figure 
4-147 illustrates the extent of the interior damage.

[Begin figure]
Figure 4-147 is a photo showing glass shards and other debris in the lobby area 
housing retail shops and offices. 
[end figure]

Damage also occurred on other facades, including collapse of a glass curtain 
wall and a portion of a brick veneer/unreinforced CMU wall. (Figure 4-148). A 
large unreinforced brick chimney collapsed on a roof, and a large rooftop air 
handling unit (20 x 7 x 9 feet) was shifted several feet. A substantial amount 
of water entered the building at various places where the envelope was breached. 

[Begin figure]
Figure 4-148 is a photo of a collapsed brick veneer/CMU wall at a mechanical 
room. The cast-in-place concrete wall (arrow) remained, but the brick veneer was 
blown away.
[end figure]

Much of the aggregate ballast (11/2-inch nominal diameter) was blown from the 
roof, and broke numerous vehicle windows in the parking lot.

There were 54 patients in the facility when the tornado struck around 9:15 p.m. 
Staff was not aware of the approaching tornado until just before it struck. Most 
of the patients and newborn infants were moved into hallways as the tornado 
struck the building. Some remained in their beds during the few seconds of the 
tornado impact. People were injured, but none seriously. Because of the 
extensive damage, it was necessary to completely evacuate the hospital after the 
storm. Evacuation was completed by around 2:00 a.m. (about 5 hours after the 
tornado).

Within a couple of days, an urgent care facility was temporarily set up in a 
tent. Temporary modular buildings were also brought in to provide space for an 
expanded array of healthcare services for the community. After approximately 3 
months of study, it was decided to demolish the entire hospital complex and 
build a new facility. An interim facility was expected to be completed by the 
fall of 2007. 

[Begin text box]
In addition to the impacts on delivery of healthcare to the community, the 
damage had potential impacts on the economy. The hospital had approximately 700 
employees and was one of the largest employers in the county. Had there been 
significant interruptions in meeting payroll, or had it been decided to close 
the facility and rely on a hospital that was approximately 40 miles away, the 
loss of jobs would likely have been difficult on the community.
[End text box]

As with the previous case study, the damage investigation of this facility 
validates several of the recommendations provided in Section 4.5. In particular, 
this case study validates the recommendations pertaining to use of roof decks, 
exterior walls, and doors, as recommended in sections 4.3.2.1, 4.3.3.2, and 
4.3.3.6, and the use of laminated glass window assemblies that are designed to 
resist the test Missile E. In those instances where there is little or no 
warning of an impending tornado strike, maintaining building envelope integrity 
is crucial to providing protection to patients and staff, and in minimizing 
disruption of services.




4.6 CHECKLIST FOR BUILDING VULNERABILITY OF HOSPITALS EXPOSED TO HIGH WINDS 

The Building Vulnerability Assessment Checklist (Table 4-3) is a tool that can 
help in assessing the vulnerability of various building components during the 
preliminary design of a new building, or the rehabilitation of an existing 
building. In addition to examining design issues that affect vulnerability to 
high winds, the checklist also examines the potential adverse effects on the 
functionality of the critical and emergency systems upon which most critical 
facilities depend. The checklist is organized into separate sections, so that 
each section can be assigned to a subject expert for greater accuracy of the 
examination. The results should be integrated into a master vulnerability 
assessment to guide the design process and the choice of appropriate mitigation 
measures.

[Begin table]
Table 4-3: Checklist for Building Vulnerability of Hospitals Exposed to High 
Winds

Vulnerability Section: General

Vulnerability: What is the age of the facility, and what building code and 
edition was used for the design of the building?

Guidance: Substantial wind load improvements were made to the model building 
codes in the 1980s. Many buildings constructed prior to these improvements have 
structural vulnerabilities. Since the 1990s, several additional changes have 
been made, the majority of which pertain to the building envelope. 
Older buildings, not designed and constructed in accordance with the practices 
developed since the early 1990s, are generally more susceptible to damage than 
newer buildings.

Vulnerability: Is the hospital older than 5 years, or is it located in a zone 
with basic wind speed greater than 90 mph?

Guidance: In either case, perform a vulnerability assessment with life-safety 
issues as the first priority, and property damage and interruption of service as 
the second priority.

Vulnerability Section: Site

Vulnerability: What is the design wind speed at the site? Are there topographic 
features that will result in wind speed-up?

Guidance: ASCE 7

Vulnerability: What is the wind exposure on site?

Guidance: Avoid selecting sites in Exposure D, and avoid escarpments and hills

Vulnerability: Are there trees or towers on site?

Guidance: Avoid trees and towers near the facility. If the site is in a 
hurricane-prone region, avoid trees and towers near primary access roads.
Road access Provide two separate means of access.

Vulnerability: Is the site in a hurricane-prone region?

Guidance: ASCE 7. If yes, follow hurricane-resistant design guidance.

Vulnerability: If in a hurricane-prone region, are there aggregate surfaced 
roofs within 1,500 feet of the facility?

Guidance: Remove aggregate from existing roofs. If the buildings with aggregate 
are owned by other parties, attempt to negotiate the removal of the aggregate 
(e.g., consider offering to pay the reroofing costs).

Vulnerability Section: Architectural 

Vulnerability: Will the facility be used as a shelter?

Guidance: If yes, refer to FEMA 361.

Vulnerability: Are there interior non-load-bearing walls?

Guidance: Design for wind load.

Vulnerability: Are there multiple buildings on site in a hurricane-prone region?

Guidance: Provide enclosed walkways between buildings that will be occupied 
during a hurricane.

Vulnerability: Are multiple elevators needed for the building?

Guidance: Place elevators in separate locations served by separate penthouses.

Vulnerability Section: Structural Systems - Section 4.3.2

Vulnerability: Is a pre-engineered building being considered?

Guidance: If yes, ensure the structure is not vulnerable to progressive 
collapse. If a pre-engineered building exists, evaluate to determine if it is 
vulnerable to progressive collapse.

Vulnerability: Is precast concrete being considered?

Guidance: If yes, design the connections to resist wind loads. If precast 
concrete elements exist, verify that the connections are adequate to resist the 
wind loads.

Vulnerability: Are exterior load-bearing walls being considered?

Guidance: If yes, design as MWFRS and C&C. 

Vulnerability: Is an FM Global-rated roof assembly specified?

Guidance: If yes, comply with FM Global deck criteria.

Vulnerability: Is there a covered walkway or canopy?

Guidance: If yes, use “free roof” pressure coefficients from ASCE 7.Canopy decks 
and canopy framing members on older buildings often have inadequate wind 
resistance. Wind-borne debris from canopies can damage adjacent buildings and 
cause injury. 

Vulnerability: Is the site in a hurricane-prone region?

Guidance: A reinforced cast-in-place concrete structural system, and reinforced 
concrete or fully grouted and reinforced CMU walls, are recommended.

Vulnerability: Is the site in a tornado-prone region?

Guidance: If yes, provide occupant protection. See FEMA 361.

Vulnerability: Do portions of the existing facility have long-span roof 
structures (e.g., a gymnasium)?

Guidance: Evaluate structural strength, since older long-span structures often 
have limited uplift resistance.

Vulnerability: Is there adequate uplift resistance of the existing roof deck and 
deck support structure?

Guidance: The 1979 (and earlier) SBC and UBC, and 1984 (and earlier) BOCA/NBC, 
did not prescribe increased wind loads at roof perimeters and corners. Decks 
(except cast-in-place concrete) and deck support structures designed in 
accordance with these older codes are quite vulnerable. The strengthening of the 
deck attachment and deck support structure is recommended for older buildings.

Vulnerability: Are there existing roof overhangs that cantilever more than 2 
feet?

Guidance: Overhangs on older buildings often have inadequate uplift resistance.

Vulnerability Section: Building Envelope - Section 4.3.3

Vulnerability: Exterior doors, walls, roof systems, windows, and skylights.

Guidance: Select materials and systems, and detail to resist wind and wind-
driven rain.

Vulnerability: Are soffits considered for the building?

Guidance: Design to resist wind and wind-driven water infiltration. If there are 
existing soffits, evaluate their wind and wind-driven rain resistance. If the 
soffit is the only element preventing wind-driven rain from being blown into an 
attic space, consider strengthening the soffit.

Vulnerability: Are there elevator penthouses on the roof?

Guidance: Design to prevent water infiltration at walls, roof, and mechanical 
penetrations.

Vulnerability: Is a low-slope roof considered on a site in a hurricane-prone 
region?

Guidance: A minimum 3-foot parapet is recommended on low-slope roofs .

Vulnerability: Is an EOC, healthcare facility, shelter, or other particularly 
important hospital in a hurricane-prone region?

Guidance: If yes, a very robust building envelope, resistant to missile impact, 
is recommended.

Vulnerability: Is the site in a tornado-prone region?

Guidance: To minimize generation of wind-borne missiles, avoid the use of brick 
veneer, aggregate roof surfacing, roof pavers, slate, and tile.

Vulnerability: Are there existing sectional or rolling doors?

Guidance: Older doors often lack sufficient wind resistance. 

Vulnerability: Does the existing building have large windows or curtain walls?

Guidance: If an older building, evaluate their wind resistance.

Vulnerability: Does the existing building have exterior glazing (windows, glazed 
doors, or skylights)?

Guidance: If the building is in a hurricane-prone region, replace with impact-
resistant glazing, or protect with shutters.

Vulnerability: Does the existing building have operable windows?

Guidance: If an older building, evaluate its wind-driven rain resistance.

Vulnerability: Are there existing exterior non-load-bearing masonry walls?

Guidance: If the building is in a hurricane- or tornado-prone region, strengthen 
or replace.

Vulnerability: Are there existing brick veneer, EIFS, or stucco exterior 
coverings?

Guidance: If the building is in a hurricane-prone region, evaluate attachments. 
To evaluate wind resistance of EIFS, see ASTM E 2359 (2006).

Vulnerability: Are existing exterior walls resistant to wind-borne debris?

Guidance: If the building is in a hurricane-prone region, consider enhancing 
debris resistance, particularly if dealing with an important hospital.	

Vulnerability: Are there existing ballasted single-ply roof membranes?

Guidance: Determine if they are in compliance with ANSI/SPRI RP-4. If non-
compliant, take corrective action.

Vulnerability: Does the existing roof have aggregate surfacing, lightweight 
pavers, or cementitious-coated insulation boards?

Guidance: If the building is in a hurricane- prone region, replace the roof 
covering to avoid blow-off.

Vulnerability: Does the existing roof have edge flashing or coping?

Guidance: Evaluate the adequacy of the attachment. 

Vulnerability: Does the existing roof system incorporate a secondary membrane?

Guidance: If not, and if the building is in a hurricane-prone region, reroof and 
incorporate a secondary membrane into the new system. 

Vulnerability: Does the existing building have a brittle roof covering, such as 
slate or tile?

Guidance: If the building is in a hurricane-prone region, consider replacing 
with a non-brittle covering, particularly if it is an important hospital.

Vulnerability Section: Exterior-Mounted Mechanical Equipment

Vulnerability: Is there mechanical equipment mounted outside at grade or on the 
roof?

Guidance: Anchor the equipment to resist wind loads. If there is existing 
equipment, evaluate the adequacy of the attachment, including attachment of 
cowlings and access panels.

Vulnerability: Are there penetrations through the roof?

Guidance: Design intakes and exhausts to avoid water leakage.

Vulnerability: Is the site in a hurricane-prone region?

Guidance: If yes, place the equipment in a penthouse, rather than exposed on the 
roof.

Vulnerability Section: Exterior-Mounted Electrical and Communications Equipment 

Vulnerability: Are there antennae (communication masts) or satellite dishes?

Guidance: If there are existing antennae or satellite dishes and the building is 
located in a hurricane-prone region, evaluate wind resistance. For antennae 
evaluation, see Chapter 15 of ANSI/TIA-222-G-2005.

Vulnerability: Does the building have a lightning protection system?

Guidance: See Sections 3.3.4.4 and 3.4.4.2 for lightning protection system 
attachment. For existing lightning protection systems, evaluate wind resistance 
(Section 3.6.3.1)

Vulnerability Section: Municipal Utilities

Vulnerability: Is the site in a hurricane-prone region?

Guidance: See Section 4.3.5.1 for emergency and standby power recommendations.

Vulnerability: Is the emergency generator(s) housed in a wind- and debris-
resistant enclosure?

Guidance: If not, build an enclosure to provide debris protection in a 
hurricane-prone region.

Vulnerability: Is the emergency generator’s wall louver protected from wind-
borne debris?

Guidance: If the building is in a hurricane-prone region, install louver debris 
impact protection.

Vulnerability: Is the site in a hurricane-prone region?

Guidance: If yes, an independent water supply and alternative means of sewer 
service are recommended, independent of municipal services. 
[end table]




4.7 REFERENCES AND SOURCES OF ADDITIONAL INFORMATION 

Note: FEMA publications may be obtained at no cost by calling (800) 480-2520, 
faxing a request to (301) 497-6378, or downloading from the library/publications 
section online at http://www.fema.gov.

American Association of State Highway and Transportation Officials, Standard 
Specifications for Structural Supports for Highway Signs, Luminaires and Traffic 
Signals, LTS-4-M and LTS-4-12, January 2001 and October 2003.

American Concrete Institute, ASCE, The Masonry Society, Building Code 
Requirements for Masonry Structures, ACI 530/ASCE 5/TMS 402, 2005.

American Institute of Architects, Buildings at Risk: Wind Design Basics for 
Practicing Architects, 1997. 

American National Standards Institute/SPRI RP-4, Wind Design Standard for 
Ballasted Single-Ply Roofing Systems, 2002.

American National Standards Institute/SPRI ES-1, Wind Design Standard for Edge 
Systems Used in Low Slope Roofing Systems, 2003.

American National Standards Institute/Telecommunications Industry Association, 
Structural Standards for Antenna Supporting Structures and Antennas, ANSI/TIA-
222-G, August 2005.

American Red Cross, Standards for Hurricane Evacuation Shelter Selection, 
Publication 4496, July 1992, rev. January 2002.

American Society of Civil Engineers, Structural Engineering Institute, Minimum 
Design Loads for Buildings and Other Structures, ASCE/SEI 7-05, Reston, VA, 
2005.

American Society for Testing and Materials, Standard Test Method for Field 
Determination of Water Penetration of Installed Exterior Windows, Skylights, 
Doors, and Curtain Walls by Uniform or Cyclic Static Air Pressure Difference, 
ASTM E1105, 2000.

American Society for Testing and Materials, Standard Test Method for Structural 
Performance of Sheet Metal Roof and Siding Systems by Uniform Static Air 
Pressure Difference, ASTM E1592, 2000.

American Society for Testing and Materials, Standard Test Method for Structural 
Performance of Exterior Windows, Curtain Walls, and Doors by Cyclic Static Air 
Pressure Differential, ASTM E1233, December 2000.

American Society for Testing and Materials, Standard Practice for Installation 
of Exterior Windows, Doors and Skylights, ASTM E2112, 2001.

American Society for Testing and Materials, Standard Test Method for Performance 
of Exterior Windows, Curtain Walls, Doors, and Impact Protective Systems 
Impacted by Missile(s) and Exposed to Cyclic Pressure Differentials, ASTM E1886, 
2005.

American Society for Testing and Materials, Standard Specification for 
Performance of Exterior Windows, Curtain Walls, Doors and Impact Protective 
Systems Impacted by Windborne Debris in Hurricanes, ASTM E1996, 2005.

American Society for Testing and Materials, Standard Specification for Zinc 
Coating (Hot-Dip) on Iron and Steel Hardware, ASTM A153, April 2005.

American Society for Testing and Materials, Standard Specification for Steel, 
Sheet, Cold-Rolled, Carbon, Structural, High-Strength Low-Alloy, High-Strength 
Low-Alloy with Improved Formability, Solution Hardened, and Bake Hardenable, 
ASTM A1008/A1008M, 2006.

American Society for Testing and Materials, Test Method for Field Pull Testing 
of an In-Place Exterior Insulation and Finish System Clad Wall Assembly, ASTM E 
2359, 2006.

American Society for Testing and Materials, Standard Test Method for Structural 
Performance of Exterior Windows, Doors, Skylights, and Curtain Walls by Cyclic 
Air Pressure Differential, ASTM E1233, 2006.

American Society of Heating, Refrigeration and Air-Conditioning Engineers, Inc. 
(ASHRAE), Ventilation for Acceptable Indoor Air Quality, Standard 62.1, 2004.

American Society of Heating, Refrigeration and Air-Conditioning Engineers, Inc. 
(ASHRAE), Standard 62.1 User’s Manual, 2006.

Brick Industry Association, Anchored Brick Veneer-Wood Frame Construction, 
Technical Notes 28, Revised August 2002.

Brick Industry Association, Wall Ties for Brick Masonry, Technical Notes 44B, 
Revised May 2003.

Brick Industry Association, Brick Veneer/Steel Stud Walls, Technical Notes 28B, 
December 2005.

Door & Access Systems Manufacturers Association, Connecting Garage Door Jambs to 
Building Framing, Technical Data Sheet #161, December 2003.

FM Global, Loss Prevention Data for Roofing Contractors, Norwood, MA, dates 
vary.

Factory Mutual Research, Approval Guide, Norwood, MA, updated quarterly.
FEMA, Building Performance: Hurricane Andrew in Florida, FEMA FIA-22, 
Washington, DC, December 1992.

FEMA, Building Performance: Hurricane Iniki in Hawaii, FEMA FIA-23, Washington, 
DC, January 1993.

FEMA, Corrosion Protection for Metal Connectors in Coastal Areas, FEMA Technical 
Bulletin 8-96, Washington, DC, August 1996. 

FEMA, Typhoon Paka: Observations and Recommendations on Building Performance and 
Electrical Power Distribution System, FEMA-1193-DR-GU, Washington, DC, March 
1998.

FEMA, Hurricane Georges in Puerto Rico, FEMA 339, Washington, DC, March 1999.

FEMA, Oklahoma and Kansas Midwest Tornadoes of May 3, 1999, FEMA 342, 
Washington, DC, October 1999.

FEMA, Coastal Construction Manual, Third Edition, FEMA 55, Washington, DC, 2000.
FEMA, Design and Construction Guidance for Community Shelters, FEMA 361, 
Washington, DC, July 2000.

FEMA, Installing Seismic Restraints for Mechanical Equipment, FEMA 412, 
Washington, DC, December 2002.

FEMA, Tornado Protection, Selecting Safe Areas in Buildings, FEMA 431, 
Washington, DC, October 2003.

FEMA, Installing Seismic Restraints for Electrical Equipment, FEMA 413, 
Washington, DC, January 2004. 

FEMA, Installing Seismic Restraints for Duct and Pipe, FEMA 414, Washington, DC, 
January 2004.

FEMA, Taking Shelter From the Storm: Building a Safe Room Inside Your House, 
FEMA 320, Washington, DC, March 2004.

FEMA, MAT Report: Hurricane Charley in Florida, FEMA 488, Washington, DC, April 
2005.

FEMA, MAT Report: Hurricane Ivan in Alabama and Florida, FEMA 489, Washington, 
DC, August 2005.

FEMA, Home Builder’s Guide to Coastal Construction Technical Fact Sheet Series, 
FEMA 499, August 2005.

FEMA, Attachment of Brick Veneer in High-Wind Regions—Hurricane Katrina Recovery 
Advisory, Washington, DC, December 2005. 

FEMA, MAT Report: Hurricane Katrina in the Gulf Coast, FEMA 549, Washington, DC, 
July 2006. 

FEMA, Attachment of Rooftop Equipment in High-Wind Regions—Hurricane Katrina 
Recovery Advisory, Washington, DC, May 2006, rev. July 2006.

FEMA, Rooftop Attachment of Lighting Protection Systems in High-Wind Regions—
Hurricane Katrina Recovery Advisory, Washington, DC, May 2006, rev. July 2006. 

Institute of Electrical and Electronics Engineers, Inc., National Electrical 
Safety Code, ANSI/IEE/C2, 2006.

International Code Council, 2006 International Building Code, ICC IBC-2006, 
March 2006.

McDonald, J.R. and Lea, P.A, A Study of Building Damage Caused by Wind Forces, 
Institute for Disaster Research, Texas Tech University, 1978. 





Appendix A - ACRONYMS

ABFE - Advisory Base Flood Elevation
ACI - American Concrete Institute
ADA - Americans with Disabilities Act 
ANSI - American National Standards Institute
ASCE - American Society of Civil Engineers 
ASHRAE - American Society of Heating, Refrigeration and Air-Conditioning 
Engineers, Inc.
ASTM - American Society for Testing and Materials

BCFH - Boulder Community Foothills Hospital
BFE - Base Flood Elevation
BIA - Brick Industry Association 
BOCA - Building Officials and Code Administrators International, Inc.
BURs - Built-Up Roofs 

C&C - Components and Cladding 
CMU - Concrete Masonry Unit

DFE - Design Flood Elevation 

EERI - Earthquake Engineering Research Institute 
EIFS - Exterior Insulation Finish Systems 
ELF - Equivalent Lateral Force
EMS - Emergency Medical Service
EOC - Emergency Operation Center
ER - Emergency Room

FEMA - Federal Emergency Management Agency
FIRMs - Flood Insurance Rate Maps 
FISs - Flood Insurance Studies 
FMG - Factory Mutual Global
FMR - Factory Mutual Research 

GAO - General Accounting Office 

HSSA - Hospital Seismic Safety Act (California)
HVAC - Heating, Ventilating, and Air-Conditioning

IBC - International Building Code 
ICBO - International Conference of Building Officials
ICC - International Code Council 
ICU - Intensive Care Unit
IEE - Institute of Electrical and Electronics Engineers, Inc.

KCH - Kona Community Hospital

LACDHS - Los Angeles County, Department of Health Services
LPS - Lightning Protection System

M/E/P - Mechanical/Electrical/Plumbing
MEPS - Molded Expanded Polystyrene 
MMI - Modified Mercalli Intensity
MOB - Medical Office Building 
MWFRS - Main Wind-Force Resisting System 

NBC - National Building Code 
NEHRP - National Earthquake Hazard Reduction Program
NFIP - National Flood Insurance Program 
NFPA - National Fire Protection Association
NIBS - National Institute of Building Sciences
NOAA - National Oceanic and Atmospheric Administration
NSC - Nonstructural Seismic Coordinator

OR - operating room 
OSB - Oriented Strand Board
OSHPD - Office of Statewide Health Planning and Development (California)

PCI - Precast/Prestressed Concrete Institute 
PGA - Peak Ground Acceleration
PMRs - Protected Membrane Roof

REACH - Rapid Evaluation and Assessment Checklist

SBC - Standard Building Code 
SBCCI - Southern Building Code Congress International, Inc. 
SDC - Seismic Design Category
SPF - Sprayed Polyurethane Foam

TIA - Telecommunications Industry Association 

USACE - U.S. Army Corp of Engineers
UBC - Uniform Building Code
URM - Unreinforced Masonry
USGS - U. S. Geological Survey 

VA - U.S. Department of Veterans Affairs 
VSI - Vinyl Siding Institute






Appendix B - GLOSSARY OF TERMS

100-year flood. See “base flood.”

A
Astragal. The center member of a double door, which is attached to the fixed or 
inactive door panel.

B

Base flood. The flood having a 1 percent chance of being equaled or exceeded in 
any given year, commonly referred to as the “100-year flood.” The base flood is 
the national standard used by the NFIP and all Federal agencies for the purposes 
of requiring the purchase of flood insurance and regulating new development. 

Base flood elevation (BFE). The height of the base (1 percent or 100-year) flood 
in relation to a specified datum, usually the National Geodetic Vertical Datum 
of 1929, or the North American Vertical Datum of 1988. 

Base isolation. Also called seismic isolation.  A design concept that reduces 
the earthquake motions in the building superstructure by isolating the building 
from ground motions.

Basic wind speed. A 3-second gust speed at 33 feet above the ground in Exposure 
C. (Exposure C is flat open terrain with scattered obstructions having heights 
generally less than 30 feet.) Note: Since 1995, ASCE 7 has used a 3-second peak 
gust measuring time. A 3-second peak gust is the maximum instantaneous speed 
with a duration of approximately 3 seconds. A 3-second peak gust speed could be 
associated with a given windstorm (e.g., a particular storm could have a 40-mph 
peak gust speed), or a 3-second peak gust speed could be associated with a 
design-level event (e.g., the basic wind speed prescribed in ASCE 7).

Building configuration. Size, shape, and proportions of the building; size, 
shape, and location of structural elements; and the type, size, and location of 
nonstructural elements.

Building, enclosed. A building that does not comply with the requirements for 
open or partially enclosed buildings.

Building, open. A building having each wall at least 80 percent open. This 
condition is expressed by an equation in ASCE 7.

Building, partially enclosed. A building that complies with both of the 
following conditions:
1. The total area of openings in a wall that receives positive external pressure 
exceeds the sum of the areas of openings in the balance of the building envelope 
(walls and roof) by more than 10 percent.
2. The total area of openings in a wall that receives positive external pressure 
exceeds 4 square feet, or 1 percent of the area of that wall, whichever is 
smaller, and the percentage of openings in the balance of the building envelope 
does not exceed 20 percent.

These conditions are expressed by equations in ASCE 7.

Building, period. The rate at which a building will vibrate as a result of 
ground motion.

Building, regularly shaped. A building having no unusual geometrical 
irregularity in spatial form.

Building, simple diaphragm. An enclosed or partially enclosed building in which 
wind loads are transmitted through floor and roof diaphragms to the vertical 
main wind-force resisting system.

C

Coastal Flooding. The accumulation of water experienced along the Atlantic, 
Gulf, and Pacific coasts, and the Great Lakes due to storm surges, extratropical 
systems, tsunamis, and sometimes wind-driven waves. 

Components and cladding (C&C). Elements of the building envelope that do not 
qualify as part of the main wind-force resisting system.

Coping. The cover piece on top of a wall exposed to the weather, usually made of 
metal, masonry, or stone, and sloped to carry off water.

D

Damping. The rate at which natural vibration decays as a result of the 
absorption of energy. In buildings it is an inherent nature to resonate 
inefficiently to vibration depending on structural connections, kinds of 
materials, and nonstructural elements used. “Damping” design measures can reduce 
the magnitude of seismic forces. 

Design flood. The greater of the following two flood events:  (1) the base 
flood, affecting those areas identified as special flood hazard areas on a 
community’s Flood Insurance Rate Map (FIRM); or (2) the flood corresponding to 
the area designated as a flood hazard area on a community’s flood hazard map or 
otherwise legally designated.

Design flood elevation (DFE). The elevation of the design flood, including wave 
height, relative to the datum specified on a community’s flood hazard map.

Downburst. Also known as a microburst. A powerful downdraft associated with a 
thunderstorm. 

Down-slope wind. A wind blowing down the slope of mountains (frequently occurs 
in Alaska and Colorado).

Drift. The term used in seismic design to describe the horizontal deflection of 
structural members in response to seismic forces.

Dry floodproofing. An adjustment, modification, or addition of a feature, or any 
combination thereof, that eliminates or reduces the potential for flood damage 
by sealing walls and closing openings to keep water from entering a building. 

Ductility. The characteristic of certain materials—steel in particular—to fail 
only after considerable distortion or deformation has occurred.

E

Escarpment. Also known as a scarp. With respect to topographic effects, a cliff 
or steep slope generally separating two levels or gently sloping areas. 

Equivalent Lateral Force (ELF) Procedure. A procedure used in building design 
that allows adjustments of the design force for varying site seismicities, 
alternative soil types, different structural and nonstructural systems and 
materials, different building heights, and occupancies of varying importance. 

Epicenter. The epicenter is the place on the surface of the earth under which an 
earthquake rupture originates, often given in degrees of latitude and longitude.

Exposure. The characteristics of the ground roughness and surface irregularities 
in the vicinity of a building. ASCE 7 defines three exposure categories—
Exposures B, C, and D. 

Extratropical storm. A cyclonic storm that forms outside of the tropical zone. 
Extratropical storms may be large, often 1,500 miles (2,400 kilometers) in 
diameter, and usually contain a cold front that extends toward the equator for 
hundreds of miles.

F

Fault. A fault is a fracture along which the blocks of earth’s crust on either 
side have moved relative to one another parallel to the fracture. 

Federal Emergency Management Agency (FEMA). The Federal Emergency Management 
Agency is the Federal agency which administers the National Flood Insurance 
Program (NFIP).

Flashing. Any piece of material, usually metal or plastic, installed to prevent 
water from penetrating a structure.

Flood Insurance Rate Map (FIRM). The official map of a community on which FEMA 
has delineated both the special hazard areas, and the risk premium zones 
applicable to the community.

Flood Insurance Study (FIS). An engineering study performed by FEMA to identify 
flood hazard areas, flood insurance risk zones, and other flood data in a 
community; used in the development of the FIRM.

Floodplain. Any land area, including the watercourse, that is susceptible to 
partial or complete inundation by water, from any source.

Floodplain management regulations. Zoning ordinances, subdivision regulations, 
building codes, or special-purpose ordinances that set flood-resistant standards 
for new construction, land use, and development.

Flood profile. A graph of computed flood elevations at points located along a 
riverine waterway. A flood profile typically is available for a waterway that 
has Base Flood Elevations (BFEs) shown on the Flood Insurance Rate Map (FIRM). 
Flood profiles are usually found in the Flood Insurance Study (FIS) report.

Floodway. The channel and that portion of the floodplain that is to be reserved 
to convey the base flood, without cumulatively increasing the water surface 
elevation more than a designated height.

Floodway fringe. The area of the floodplain outside of the floodway, where 
floodwaters may be shallower and slower. 

Freeboard. A factor of safety, usually expressed in feet above a flood level, 
for purposes of flood­plain management. Freeboard also compensates for the many 
unknown factors that could contribute to flood heights greater than the height 
calculated for a selected size flood and floodway conditions, such as wave 
action, constricting bridge openings, and the hydrological effect of 
urbanization of the watershed. A freeboard of from 1 to 3 feet is often applied 
to critical facilities.

G

Glazing. Glass or a transparent or translucent plastic sheet used in windows, 
doors, and skylights.

Glazing, impact-resistant. Glazing that has been shown, by an approved test 
method, to withstand the impact of wind-borne missiles likely to be generated in 
wind-borne debris regions during design winds.

Ground motion. The movement of the earth’s surface from earthquakes or 
explosions. Ground motion is produced by waves that are generated by sudden slip 
on a fault or sudden pressure at the explosive source, and travel through the 
earth and along its surface.

H

Hurricane-prone regions. Areas vulnerable to hurricanes; in the United States 
and its territories de­fined as:
1. The U.S. Atlantic Ocean and Gulf of Mexico coasts, where the basic wind speed 
is greater than 90 miles per hour.
2. Hawaii, Puerto Rico, Guam, U.S. Virgin Islands, and American Samoa.

Human intervention. The presence and active involvement of people necessary to 
enact or implement floodproofing measures prior to the onset of flooding.

Hydrodynamic load. Loads imposed by water flowing against and around an object 
or structure, including the impacts of debris and waves.

Hydrostatic load. Load (pressure) imposed on an object or structure by a 
standing mass of water; the deeper the water, the greater the load or pressure 
against the object or structure.

I

Impact-resistant covering. A covering designed to protect glazing, which has 
been shown by an approved test method to withstand the impact of wind-borne 
missiles likely to be generated in wind-borne debris regions during design 
winds.

Importance factor, I. A factor that accounts for the degree of hazard to human 
life and damage to property. Importance factors are given in ASCE 7.

L

Landslides. The slipping of soil and rock on sloping ground triggered by 
earthquake ground motion.

Liquefaction. The temporary change of loose granular soils and sand in the 
presence of water from a solid to a liquid state when subjected to ground 
shaking. 

Lowest floor. The lowest floor of the lowest enclosed area (including basement). 
An unfinished or flood-resistant enclosure, usable solely for parking of 
vehicles, building access, or storage, in an area other than a basement area, is 
not considered a building’s lowest floor, provided that the enclosure is 
compliant with flood-resistant requirements.

M

Magnitude. The magnitude is a number that characterizes the relative size of an 
earthquake. Magnitude is based on measurement of the maximum motion recorded by 
a seismograph. Best known scales are “Richter magnitude,” and “moment 
magnitude.” The moment magni­tude (Mw) scale, based on the concept of seismic 
moment, is uniformly applicable to all sizes of earthquakes.  

Main wind-force resisting system. An assemblage of structural elements assigned 
to provide sup­port and stability for the overall structure. The system 
generally receives wind loading from more than one surface.

Mean roof height, (h). The average of the roof eave height and the height to the 
highest point on the roof surface, except that, for roof angles of less than or 
equal to 10 degrees, the mean roof height shall be the roof eave height.

Missiles. Debris that could become propelled into the wind stream.  

Modified Mercalli Intensity (MMI) scale. Intensity scale used to measure the 
level of earthquake damage.  

Moment Frame. Frames in which structural members and joints resist lateral 
forces by bending. There are “ordinary,” “intermediate,” and “special” moment 
frames. The latter provide the most resistance.

N

National Flood Insurance Program (NFIP). A Federal program to identify flood-
prone areas nation­wide, and make flood insurance available for properties in 
communities that participate in the program. 

NEHRP. The Federal National Earthquake Hazard Reduction Program, enacted in 
1977, to reduce potential losses from earthquakes by funding research in 
earthquake prediction and hazards and to guide the implementation of earthquake 
loss-reduction programs. 

Nor’easter. Nor’easters are non-tropical storms that typically occur in the 
eastern United States, any time between October and April, when moisture and 
cold air are plentiful. They are known for dumping heavy amounts of rain and 
snow, producing hurricane-force winds, and creating high surfs that cause severe 
beach erosion and coastal flooding. A nor’easter is named for the winds that 
blow in from the northeast and drive the storm along the east coast and the Gulf 
Stream, a band of warm water that lies off the Atlantic Coast. 

O

Openings. Apertures or holes in the building envelope that allow air to flow 
through the building envelope. A door that is intended to be in the closed 
position during a windstorm would not be considered an opening. Glazed openings 
are also not typically considered openings. However, if the building is located 
in a wind-borne debris region and the glazing is not impact-resistant or 
protected with an impact-resistant covering, the glazing is considered an 
opening.

P

Passive Energy Dissipation. The reduction of earthquake forces in a building by 
the introduction of devices designed to dissipate the earthquake energy in a 
controlled manner using friction, hydraulics, or deformation of material 
specially placed for this purpose.

Peak Ground Acceleration. The largest acceleration that occurs during 
earthquake-induced ground motion.

R

Racking. Lateral deflection of a structure resulting from external forces, such 
as wind or lateral ground movement in an earthquake.

Resonance. The increase in vibrations and accelerations to a vibrating building 
from the transmission of ground motion.

Response spectrum. A characterization of ground motion (representing the suite 
of spectral or­dinates) measuring the extent of shaking different structures 
will experience based on their natural period of vibration.  

Richter magnitude scale. Developed in 1935 by Charles F. Richter of the 
California Institute of Technology. The magnitude of an earthquake is determined 
from the logarithm of the amplitude of waves recorded by seismographs. 
Adjustments are included for the variation in the distance between the various 
seismographs and the epicenter of the earthquakes. On the Richter Scale, 
magnitude is expressed in whole numbers and decimal fractions. For example, a 
magnitude 5.3 might be computed for a moderate earthquake, and a strong 
earthquake might be rated as magnitude 6.3. Because of the logarithmic basis of 
the scale, each whole number increase in magnitude represents a tenfold increase 
in measured amplitude; as an estimate of energy, each whole number step in the 
magnitude scale corresponds to the release of about 31 times more energy than 
the amount associated with the preceding whole number value. 

Ridge. With respect to topographic effects, an elongated crest of a hill 
characterized by strong relief in two directions.

Riverine Flooding. The accumulation of runoff from rainfall or snowmelt, such 
that the volume of flow exceeds the capacity of waterway channels and spreads 
out over the adjacent land.

S

Seiche. A wave that oscillates in lakes, bays, or gulfs from a few minutes to a 
few hours as a result of seismic or atmospheric disturbances.
Seismic ground motion. The movement of ground surfaces in every direction 
simultaneously, back and forth, side to side, and up and down during an 
earthquake.

Seismograph. Also known as seismometer, is an instrument used to detect and 
record earth­quakes. Generally, it consists of a mass attached to a fixed base. 
During an earthquake, the base moves and the mass does not. The motion of the 
base with respect to the mass is commonly transformed into an electrical 
voltage. The electrical voltage is recorded on paper, magnetic tape, or another 
recording medium. This record is proportional to the motion of the seismometer 
mass relative to the earth, but it can be mathematically converted to a record 
of the absolute motion of the ground. 

Shear. A force that causes parts of a material to slide past one another in 
opposite directions

Shear wall. Solid wall that resists shear forces, used in buildings constructed 
in earthquake and high-wind zones.

Sheetflow. Rainfall runoff that flows over relatively flat land without 
concentrating into streams or channels.

Spectral Acceleration. The acceleration to be experienced by structures of 
different periods.

Stiffness. Rigidity, or resistance to deflection or drift. A measure of 
deflection or of staying in alignment within a certain stress.

Stillwater elevation. The elevation that the surface of coastal flood waters 
would assume in the absence of waves, referenced to a datum.

Straight-line wind. A wind blowing in a straight line with wind speeds ranging 
from very low to very high (the most common wind occurring throughout United 
States and its territories). 

Substantial damage. Damage of any origin sustained by a structure, whereby the 
cost of restoring the structure to its predamage condition equals or exceeds 50 
percent of the market value of the structure before the damage occurred (or 
smaller percentage if established by the authority having jurisdiction). 
Structures that are determined to be substantially damaged are considered to be 
substantial improvements, regardless of the actual repair work performed. 

Substantial improvement. Any reconstruction, rehabilitation, addition, or other 
improvement of a structure, the cost of which equals or exceeds 50 percent of 
the market value of the structure (or smaller percentage if established by the 
authority having jurisdiction) before the start of the improvement. 

T

Torsion. Twisting around an axis. The center of the mass does not coincide with 
the center of re­sultant force of the resisting building elements causing 
rotation or twisting action in plans and stress concentrations. Symmetry in 
general reduces torsion. 

Tsunami. An unusually large sea wave produced by submarine earth movement or a 
volcanic eruption.


Wet floodproofing. Permanent or contingent measures and construction techniques, 
applied to a structure or its contents, that prevent or provide resistance to 
damage from flooding while allowing floodwaters to enter the structure. 
Generally, this includes properly anchoring the structure, using flood-resistant 
materials below the BFE, protection of mechanical and utility equipment, and the 
use of openings or breakaway walls. 

Wind-borne debris regions. Areas within hurricane-prone regions located:
1. Within 1 mile of the coastal mean high water line, where the basic wind speed 
is equal to or greater than 110 mph, and in Hawaii.
2. In areas where the basic wind speed is equal to or greater than 120 mph.