DESIGN GUIDANCE 6

In this chapter, design guidance is provided for limiting or mitigating the effects of 
terrorist attacks. Guidance is provided for each of the following aspects of design: site 
location and layout, architectural, struc
tural, building envelope, and mechanical 
and electrical systems. Sections 6.1 through 
6.5 pertain to attack using explosive 
weapons. Protective measures against 
chemical, biological, and radiological 
attacks are discussed in Section 
6.6. In each section, design guidance is 
discussed and recommendations are given 
for enhancing life safety. 
6.1 SITE LOCATION AND LAYOUT 
Because air-blast pressures decrease rapidly with distance, one of the most effective 
means of protecting assets is to increase the distance between a potential bomb and 
the assets to be protected. The best way to do this is to provide a continuous line of 
security along the perimeter of the facility to protect it from unscreened vehicles and 
to keep all vehicles as far away from critical assets as possible. 
This section discusses the perimeter and the approach 
to the building. For discussion about building shape 
and placement on the site, see Section 6.2, Architectural. 
6.1.1  Perimeter Line 
The perimeter line of protection is the outermost line 
that can be protected by facility security measures. The 
perimeter needs to be designed to prevent carriers of 
large-scale weapons from gaining access to the site. In 
design, it is assumed that all large-scale explosive 
weapons (i.e., car bombs or truck bombs) are outside 
this line of defense. This line is defended by both 
physical and operational security methods. 
It is recommended that the perimeter line be located as 
far as is practical from the building exterior. Many 
times, vulnerable buildings are located in urban areas 
where site conditions are tight. In this case, the options 
are obviously limited. Often, the perimeter line can be 
pushed out to the edge of the sidewalk by means of 
bollards, planters, and other obstacles. To push this line 
even further outward, restricting or eliminating parking 
along the curb can be arranged with the local authori-
ties, but this can be a difficult and time consuming 
effort. In some cases, eliminating loading zones and 
street/lane closings are an option.


6.1.2  Controlled Access Zones 
Access control refers to points of controlled access to the facility through the 
perimeter line. The controlled access check or inspection points for vehicles require 
architectural features or barriers to maintain the defensible perimeter. Architects and 
engineers can accommodate these security functions by providing adequate design 
for these areas, which makes it difficult for a vehicle to crash onto the site. 
Deterrence and delay are major attributes of the perimeter security design that should 
be consistent with the landscaping objectives, such as emphasizing the open nature 
characterizing high-population buildings. Since it is impossible to thwart all possible 
threats, the objective is to make it difficult to successfully execute the easiest attack 
scenarios such as a car bomb detonated along the curb, or a vehicle jumping the curb 
and ramming into the building prior to detonation. 
If space is available between the perimeter line and the building exterior, much can 
be done to delay an intruder. Examples include terraced landscaping, fountains, 
statues, staircases, circular driveways, planters, trees, high-strength cables hidden in 
bushes and any number of other obstacles that make it difficult to rapidly reach the 
building. Though individually these features may not be able to stop a vehicle, in 
combination, they form a daunting obstacle course. Other ideas for implementing 
secure landscaping features may be found in texts on Crime Prevention Through 
Environmental Design (CPTED). These concepts are useful for slowing down traffic, 
improving surveillance, and site circulation. 
On the sides of the building that are close to the curb, where landscaping solutions 
are limited, anti-ram barriers capable of stopping a vehicle on impact are 
recommended for high-risk buildings. Barrier design methods are discussed in more 
detail below. 
The location of access points should be oblique to oncoming streets so that it is 
difficult for a vehicle to gain enough velocity to break through these access locations. 
If the site provides straight-on access to the building, some mitigation options 
include concrete medians in the street to slow vehicles or, for high-risk buildings, 
use of anti-ram barriers along the curb capable of withstanding the impact of high-
velocity vehicles. 
Place parking as far as practical from the building. Off-site parking is recommended 
for high-risk facilities vulnerable to terrorist attack. If on-site surface parking or 
underground parking is provided, take precautions such as limiting access to these 
areas only to the building occu-pants and/or having all vehicles inspected in areas 
close-in to the building. If an underground area is used for a high-risk building, the 
garage should be placed adjacent to the building under a plaza area rather than 
directly underneath the building. To the extent practical, limit the size of vehicle that 
is able to enter the garage by imposing physical barriers on vehicle height.


6.1.3  Physical Protective Barriers 
There are two basic categories of perimeter anti-ram barriers; passive (or fixed) and 
active (or operable). Each is described below. 
6.1.3.1 Passive Barriers 
Passive barriers are those that are fixed in place and do not allow for vehicle entry. 
These are to be used away from vehicle access points. The majority of these are 
constructed in place. 
For lower-risk buildings without straight-on vehicular access, it may be appropriate to 
install surface-mounted systems such as planters, or to use landscaping features to 
deter an intrusion threat. An example of a simple but effective landscaping solution 
is to install a deep permanent planter around the building with a wall that is as high 
as a car or truck bumper. 
Individual planters mounted on the sidewalk resist impact through inertia and 
friction between the planter and the pavement. It can be expected that the planter 
will move as a result of the impact. For a successful design, the maximum 
displacement of the planter should be less than the setback distance to the building. 
The structure supporting the weight of the planter must be considered prior to 
installation. 
To further reduce displacement, the planter may be placed several inches below 
the pavement surface. A roughened, grouted interface surface will also improve 
performance. 
The traditional anti-ram solution entails the use of bollards (see Figure 6-1). Bollards 
are concrete-filled steel pipes that are placed every few feet along the curb of a 
sidewalk to prevent vehicle intrusion. In order for them to resist the impact of a 
vehicle, the bollard needs to be fully embedded into a concrete strip foundation that 
is several feet deep. The height of the bollard above ground should be higher than 
the bumper of the vehicle. The spacing of the bollards is based on several factors 
including ADA (American Disabilities Act) requirements, the minimum width of a 
vehicle, and the number of bollards required to resist the impact. As a rule of thumb, 
the center-to-center spacing should be between three and five feet to be effective. The 
height of the bollard is to be at least as high as the bumper of the design threat vehi-
cle, which is taken typically between two and three feet. 
An alternative to a bollard is a plinth wall, which is a continuous knee wall 
constructed of reinforced concrete with a buried foundation (see Figure 6-2). The 
wall may be fashioned into a bench, a base for a fence, or the wall of a planter. To be 
effective, the height needs to be at least as high as the vehicle bumper. 
For effectiveness, the barriers need to be placed as close to the curb as possible. 
However, the property line of buildings often does not extend to the curb. Therefore, 
to place barriers with foundations near the curb, a permit is required by the local 
authorities, which can be a difficult time-consuming effort to obtain. To avoid this, 
building owners are often inclined to place bollards along the property line, which 
significantly reduces the effectiveness of the barrier system. 
The foundation of the bollard and plinth wall system can present challenges. There 
are sometimes vaults or basements below the pavement that extend to the property 
line, which often require special foundation details. Unless the foundation wall can 
sustain the reaction forces, significant damage may occur. 
Below-ground utilities that are frequently close to the pavement surface present 
additional problems. Their location may not be known with certainty, and this often 
leads to difficulties during construction. This also can be a strong deterrent to 
selecting barriers with foundations as a solution. However, for high-risk facilities, it is 
recommended that these issues be resolved during the design phase so that a reliable 
anti-ram barrier solution can be installed. For lower-risk buildings without straight-
on vehicular access, it may be more appropriate to install sur-face-mounted systems 
such as planters or to use landscaping features to deter an intrusion threat. An 
example of a simple but effective landscaping solution is to install a deep permanent 
planter around the building with a wall that is at least as high as a car or truck 
bumper.


6.1.3.2   Active Systems 
At vehicular access points, active or operational anti-ram systems are required. There 
are off-the-shelf products available that have been rated to resist various levels of car 
and truck impacts. Solutions include: 
_ crash beams; 
. _ crash gates; 
. _ surface-mounted plate systems; 
. _ retractable bollards; and 
. _ rotating-wedge systems. The first three systems listed above generally have 
lower impact ratings than the last two listed. Check with the manufacturer to ensure 
that the 

system has been tested to meet the impact requirements for your project. 
It is important that the installation of hydraulically operated systems be performed by 
a qualified contractor to ensure a reliable system that will work properly in all 
weather conditions.



6.1.4 Effectiveness of Anti-Ram Barriers 
The effectiveness of an anti-ram barrier is based on the amount of energy it can 
absorb versus the amount of kinetic energy imparted by vehicle impact. The angle of 
approach reduces this energy in non-head-on situations, and the energy absorbed by 
the crushing of the vehicle also reduces the energy imparted to the barriers. The 
kinetic energy imparted to the wall is one-half the product of the vehicle mass and its 
impact velocity squared. Because the velocity term is squared, a change in velocity 
affects the energy level more than a change in vehicle weight. For this reason, it is 
important to review lines of approach to define areas where a vehicle has a long, 
straight road to pick up speed before impact. 
The vehicle weight used for the design of barriers typically ranges from 4000 lb for 
cars up to 15,000 lb for trucks. Impact velocities typically range from 30 mph for 
oblique impact areas (i.e., where the oncoming street is parallel to the curb) up to 50 
mph where there is straight-on access (i.e., where the oncoming street is 
perpendicular to the curb). 
The kinetic energy of the vehicle at impact is absorbed by the barrier system. For 
fixed systems (like a concrete bollard), the energy is absorbed through the 
deformational strain energy absorbed by the barrier, soil, and the vehicle. For 
movable systems (like a surface-mounted planter) energy is absorbed through shear 
friction against the pavement and vehicle deformation. 
Barrier effectiveness is ranked in terms of the amount of displacement of the system 
due to impact. Standard ratings defined by the federal government define the 
distance the vehicle travels before it is brought to rest. The most effective systems 
stop the vehicles within three feet, moderately effective barriers stop the vehicle 
within 20 feet, and the least effective systems require up to 50 feet.


6.1.5 	Checklist Š Site and Layout Design Guidance 
  Provide a continuous line of defense around the site as far from the 
building as practical.   Place vehicular access points away from 
oncoming streets.   Limit the number of vehicular entrances 
through the secured perimeter line 
  Use a series of landscape features to create an obstacle course
between the building and the perimeter. This approach is most 
effective if used in areas where there is ample setback.

  Design planters for the design-level impact to displace the planter a distance less 
than the setback. 
  Use anti-ram barriers along curbs, particularly on sides of the building that have a 
small setback and in areas where high-velocity impact is possible. 
  Use operable anti-ram barriers at vehicular access points. Select barriers rated to 
provide the desired level of protection against the design impact.


6.1.6 	Further Reading 
National Capital Planning Commission, 2001. Designing for Security in the NationÕs Capital, 
National Capital Planning Commission, Washington, D.C. 
http://www.ncpc.gov/whats_new/ITFreport.pdf 
U.S. Air Force, 1997, Installation Force Protection Guide, Air Force Center for 
Environmental Excellence, Brooks Air Force Base, Texas 
http://www.afcee.brooks.af.mil/dc/dcd/arch/force.pdf 
Crowe, T. D., 2000, Crime Prevention Through Environmental Design: Applications Of 
Architectural Design And Space Management Concepts (2nd Ed.), Butterworth-
Heinemann, Stoneham, Massachusetts. ISBN: 075067198X 
http://www.amazon.com/exec/obidos/ASIN/ 075067198X/103-7416668-
4182264#product-details 
Newman, O., 1996, Creating Defensible Space, U.S. Department of Housing and Urban 
Development, Washington, D.C. http:// 
www.huduser.org/publications/pdf/def.pdf



6.2 ARCHITECTURAL 
There is much that can be done architecturally to mitigate the effects of a terrorist 
bombing on a facility. These measures often cost nothing or very little if 
implemented early in the design process. It is recommended that protective design 
and security consultants are used as early as possible in the design process. They 
should be involved in the site selection and their input should be sought during 
programming and schematic design. 
6.2.1 Building Exterior 
This section discusses the building shape, placement, and exterior ornamentation. 
For a discussion of exterior cladding, see Section 6.4, Building Envelope. 
At the building exterior, the focus shifts from deterring and delaying the attack to 
mitigating the effects of an explosion. The exterior envelope of the building is most 
vulnerable to an exterior explosive threat because it is the part of the building closest 
to the weapon, and it is typically built using brittle materials. It also is a critical line of 
defense for protecting the occupants of the building. 
The placement of the building on the site can have a major impact on its 
vulnerability. Ideally, the building is placed as far from the property lines as possible. 
This applies not only to the sides that are adjacent to streets, but the sides that are 
adjacent to adjoining properties, since it is not certain who will occupy the 
neighboring properties during the life of the building. A common, but unfortunate 
practice is to create a large plaza area in front of the building, but to leave little 
setback on the sides and rear of the building. Though this practice may increase the 
monumental character of the building, it also increases the vulnerability of the other 
three sides. 
The shape of the building can have a contributing effect on the overall damage to the 
structure (see Figure 6-3). Re-entrant corners and overhangs are likely to trap the 
shock wave and amplify the effect of the air blast. Note that large or gradual re-
entrant corners have less effect than small or sharp re-entrant corners and 
overhangs. The reflected pressure on the surface of a circular building is less intense 
than on a flat building. When curved surfaces are used, convex shapes are preferred 
over concave shapes. Terraces that are treated as roof systems subject to downward 
loads require careful framing and detailing to limit internal damage to supporting 
beams. 
Generally, simple geometries and minimal ornamentation (which may become flying 
debris during an explosion) are recommended unless advanced structural analysis 
techniques are used. If ornamentation is used, it is preferable to use lightweight 
materials such as timber or plastic, which are less likely than brick, stone, or metal to 
become lethal projectiles in the event of an explosion. 
Soil can be highly effective in reducing the impact of a major explosive attack. 
Bermed walls and buried roof tops have been found to be highly effective for military 
applications and can be effectively extended to conventional construction. This type 
of solution can also be effective in improving the energy efficiency of the building. 
Note that if this approach is taken, no parking can be permitted over the building. 
Interior courtyards or atriums are other concepts for bringing light and a natural 
setting to the building without adding vulnerable openings to the exterior.


6.2.2 Building Interior 
In terms of functional layout, unsecured areas such as the lobby, loading dock, mail 
room, garage, and retail areas need to be separated from the secured areas of the 
building . Ideally, these unsecured areas are placed exterior to the main building or 
along the edges of the building. For example, a separate lobby pavilion or loading 
dock area outside of the main footprint of the building (see Figure 6-4) provides 
enhanced protection against damage and potential building collapse in the event of 
an explosion at these locations. Similarly, placing parking areas outside the main 
footprint of the building can be highly effective in reducing the vulnerability to 
catastrophic collapse. If it is not possible to place vulnerable areas outside the main 
building footprint, they should be placed along the building exterior, and the 
building layout should be used to create internal "hard lines" or buffer zones. 
Secondary stairwells, elevator shafts, corridors, and storage areas should be located 
between public and secured areas. 
When determining whether secured and unsecured areas are adjacent to one 
another, consider the layout on each floor and the relationship between floors. 
Secured occupied or critical areas should not be placed above or below unsecured 
areas. 
Adequate queuing areas should be provided in front of lobby inspection stations so 
that visitors are not forced to stand outside during bad weather conditions or in a 
congested line inside a small lobby while waiting to enter the secured areas. 
Occupied areas or emergency functions should not be placed immediately adjacent to 
the lobby, but should be separated by a buffer area such as a storage area or corridor. 
The interior wall area and exposed structural columns in unsecured lobby areas 
should be minimized. 
Vehicular queuing and inspection stations need to be accounted for in design of the 
loading docks and vehicle access points. These should be located outside the 
building along the curb or further away. A parking lane may be used for this purpose. 
Emergency functions and elevator shafts are to be placed away from internal parking 
areas, loading docks and other high-risk areas. In the 1993 World Trade Center 
bombing incident, elevator shafts became chimneys, transmitting smoke and heat 
from the explosion in the basement to all levels of the building. This hindered 
evacuation and resulted in smoke inhalation injuries. 
False ceilings, light fixtures, Venetian blinds, ductwork, air conditioners, and other 
nonstructural components may become flying debris in the event of an explosion. 
Wherever possible it is recommended that the design be simplified to limit these 
hazards. Placing heavy equipment such as air conditioners near the floor rather than 
the ceiling is one idea for limiting this hazard. Using fabric curtains or plastic 
vertical blinds rather than metal Venetian blinds, and using exposed ductwork as an 
architectural device are other ideas. Mechanically attaching light fixtures to the slab 
above as is done in high seismic areas is recommended. 
Finally, the placement of furniture can have an effect on injury levels. Desks, 
conference tables, and other similar furniture should be placed as far from exterior 
windows facing streets as practical. Desks with computer monitors should be oriented 
away from the window to prevent injury due to the impact of the monitor.


6.2.3 Checklist Š Architectural 
  Use simple geometries without sharp re-entrant corners.   Use lightweight 
nonstructural elements to reduce flying debris haz
ards.   Place the building on the site as far from the perimeter as practical.   Place 
unsecured areas exterior to the main structure or along the 
exterior of the building.
  Separate unsecured and secured areas horizontally and vertically 

using buffer zones and/or hardening of walls and floors.   Provide sufficient 
queuing areas at lobby and delivery entrances.   Limit nonstructural elements such 
as false ceilings and metal blinds 
on the interior. 
  Mechanically fasten light fixtures to the floor system above.
  Place desks and conference tables as far from exterior windows as 

practical.   Orient desks with computer monitors to 
face away from windows so the chair back faces the 
window, not the monitor.


6.2.4 Further Reading 
The American Institute of Architects, 2001, Building Security through Design: A Primer for 
Architects, Design Professionals, and their Clients, The American Institute of Architects, 
Washington, D.C. http:// www.aia.org/security 
GSA, 1999, Balancing Security and Openness: A Thematic Summary of a Symposium on Security 
and the Design of Public Buildings, General Services Administration, Washington, D.C. 
http://hydra.gsa.gov/pbs/pc/ gd_files/SecurityOpenness.pdf 
Hart, S., 2002, In the Aftermath of September 11, the Urban Landscape Appears 
Vulnerable and Random: Architects and Consultants Focus on Risk Assessment 
and Security Through Design, Architectural Record, March 2002, pages 135-160, New 
York, New York. http:// archrecord.construction.com/CONTEDUC/ARTICLES/ 
03_02_1.asp 
Nadel, B. A., 1998, Designing for Security, Architectural Record, March 
1998, pages 145-148, 196-197, New York, New York. http:// 
www.archrecord.com/CONTEDUC/ARTICLES/3_98_1.asp 
Council on Tall Buildings an Urban Habitat, 2002, Building Safety 
Enhancement Guidebook, Lehigh University, Bethlehem, Pennsylvania. 
http://www.ctbuh.org/



6.3 STRUCTURAL 
Given the evolving nature of the terrorist threat, it is impossible to predict what 
threats may be of concern during the lifetime of the building; it is therefore prudent 
to provide protection against progressive collapse initiated by a localized structural 
failure caused by an undefined threat. Because of the catastrophic consequences of 
progressive collapse, incorporating these measures into the overall building design 
should be given the highest priority when considering structural design approaches 
for mitigating the effects of attacks. 
Explicit design of secondary structural components to mitigate the direct effects of 
air-blast enhances life safety by providing protection against localized failure, flying 
debris, and air blast entering the building. It may also facilitate evacuation and 
rescue by limiting the overall damage level and improving access by emergency 
personnel. 
Specific issues related to structural protection measures are discussed separately in 
the sections below. 
6.3.1  Progressive Collapse 
ASCE-7 defines three ways to approach the structural design of buildings to mitigate 
damage due to progressive collapse. Each is described below with an emphasis on 
how the method is applied in the situation where explosive loads are the initiating 
cause of collapse. 
1.	Indirect Method: Consider incorporating general structural integrity measures 
throughout the process of structural system selection, layout of walls and 
columns, member proportioning, and detailing of connections to enhance 
overall structural robustness. In lieu of calculations demonstrating the effects of 
explosions on buildings, one may use an implicit design approach that 
incorporates measures to increase the overall robustness of the structure. These 
measures are discussed in the sub-sections below on structural systems, structural 
layout, and structural elements. This minimum standard is likely to be the 
primary method used for design of the type of buildings that are the focus of this 
primer. 
1. 2.	Alternate-Load-Path Method: Localize response by designing the structure 
to carry loads by means of an alternate load path in the event of the loss of a primary 
load-bearing component. The alternate-load-path method has been selected by 
agencies including the General Services Administration (GSA) as the preferred 
approach for preventing progressive collapse. This method provides a formal check 
of the capability of the structure to resist collapse following the removal of specific 
elements, such as a building column at the building perimeter. The method does not 
require characterization of the explosive threat. The structural engineer can usually 
perform the necessary analyses, with or without guidance from a protective design 
consultant. However, the analysis is likely to benefit from advice of the protective 
design consultant regarding element loss scenarios that should be considered in 
design. 
2. 3.	Specific Local-Resistance Method: Explicitly design critical vertical load-
bearing building components to resist the design-level explosive forces. Explosive 
loads for a defined threat may be explicitly considered in design by using nonlinear 
dynamic analysis methods. These are discussed below in the subsection on direct 
design methods with additional information in the subsection on structural 
elements. Blast-mitigating structural design or hardening generally focuses on the 
structural members on the lower floor levels that are closest to defined stationary 
exterior vehicle weapon threats. 

Useful references are provided at the end of this section that directly relate to 
progressive collapse prevention.


6.3.2 Building Structural Systems 
In the selection of the structural system, consider both the direct effects of air-blast 
and the potential for progressive collapse in the event that a critical structural 
component fails. 
The characteristics of air-blast loading have been previously discussed. To resist the 
direct effects of air-blast, the structural characteristics listed below are desirable. 
. _	Mass. Lightweight construction is unsuitable for providing air-blast 
resistance. For example, a building with steel deck (without concrete fill) roof 
construction will have little air-blast resistance. 
. _	Shear Capacity. Primary members and/or their connections should 
ensure that flexural capacity is achieved prior to shear failure. Avoiding brittle shear 
failure significantly increases the structureÕs ability to absorb energy. 

_	Capacity for Reversing Loads. Primary members and their connections should 
resist upward pressure. Certain systems such as prestressed concrete may have 
little resistance to upward forces. Seated connection systems for steel and precast 
concrete may also have little resistance to uplift. The use of headed studs is 
recommended for affixing concrete fill over steel deck to beams for uplift 
resistance. 
To reduce the risk of progressive collapse in the event of the loss of structural 
elements, the structural traits listed below should be incorporated. 
. _	Redundancy. The incorporation of redundant load paths in the ver-
tical-load-carrying system helps to ensure that alternate load paths are available in the 
event of failure of structural elements. 
. _	Ties. An integrated system of ties in perpendicular directions along the 
principal lines of structural framing can serve to redistribute loads during 
catastrophic events. 
. _	Ductility. In a catastrophic event, members and their connections may 
have to maintain their strength while undergoing large deformations. 

Historically, the preferred material for explosion-mitigating construction is cast-in-
place reinforced concrete. This is the material used for military bunkers, and the 
military has performed extensive research and testing of its performance. Reinforced 
concrete has a number of attributes that make it the construction material of choice. 
It has significant mass, which improves response to explosions, because the mass is 
often mobilized only after the pressure wave is significantly diminished, reducing 
deformations. Members can be readily proportioned and reinforced for ductile 
behavior. The construction is unparalleled in its ability to achieve continuity between 
the members. Finally, concrete columns are less susceptible to global buckling in the 
event of the loss of a floor system. 
Current testing programs are investigating the effectiveness of various conventional 
building systems; however, in general the level of protection that may be a achieved 
using these materials is lower than what is achieved using well-designed, cast-in-
place, reinforced concrete. The performance of a conventional steel frame with 
concrete fill over metal deck depends on the connection details. Pre-tensioned or 
post-ten-sioned construction provides little capacity for abnormal loading patterns 
and load reversals. The resistance of load-bearing wall structures varies to a great 
extent. More information about the response of these systems is described in the 
subsection on structural elements and in the section on exterior cladding in Section 
6.4, Exterior Envelope.


6.3.3 Structural Layout 
To enhance the overall robustness of the structure, the measures listed below are 
recommended. 
. _	In frame structures, column spacing should be limited. Large column 
spacing decreases likelihood that the structure will be able to redistribute load in 
event of column failure. 
. _	The exterior bay is the most vulnerable to damage, particularly for 
buildings that are close to public streets. It is also less capable of redistributing loads 
in the event of member loss, since two-way load distribution is not possible. It is 
desirable to have a shallow bay adjacent to the building exterior to limit the extent of 
damage. 
. _	Use of transfer girders is strongly discouraged. Loss of a transfer girder 
or one of its supports can destabilize a significant area of the building. Transfer 
girders are often found at the building exterior to accommodate loading docks or 
generous entries, increasing their vulnerability to air-blast effects. It is highly 
desirable to add redundant transfer systems where transfer girders are required. 
. _	In bearing-wall systems that rely primarily on interior cross-walls, 
interior longitudinal walls should be periodically spaced to enhance stability and to 
control the lateral progression of damage. 
. _	In bearing-wall systems that rely on exterior walls, perpendicular walls 
or substantial pilasters should be provided at a regular spacing to control the amount 
of wall that is likely to be affected. 


6.3.4 Direct Design Methods 
The direct design approach (Figure 6-5) to be used for the structural protective 
measures is to first design the building for conventional loads, then evaluate the 
structureÕs response to explosive loads and augment the design, if needed. Finally, 
the designer must make sure that all conventional load requirements are still met. 
This approach ensures that the design meets all the requirements for gravity and 
natural hazards in addition to air-blast effects. Take note that measures taken to 
mitigate explosive loads may reduce the structureÕs performance under other types of 
loads, and therefore an iterative approach may be needed. As an example, increased 
mass generally increases the design forces for seismic loads, whereas increased mass 
generally improves performance under explosive loads. Careful consideration 
between the protective design consultant and the structural engineer is needed to 
provide an optimized design. 
Nonlinear dynamic analysis techniques are similar to those currently used in 
advanced seismic analysis. Analytical models range from handbook methods to 
equivalent single-degree-of-freedom (SDOF) models to finite element (FE) 
representation. For SDOF and FE methods, numerical computation requires 
adequate resolution in space and time to account for the high-intensity, short-
duration loading and nonlinear response (Figure 6-6). Difficulties involve the 
selection of the model and appropriate failure modes, and finally, the interpretation 
of the results for structural design details. Whenever possible, results are checked 
against data from tests and experiments for similar structures and loadings. 
Charts are available that provide damage estimates for various types of construction, 
as a function of peak pressure and peak impulse, based on analysis or empirical data. 
Military design handbooks typically provide this type of design information. 
Components such as beams, slabs, or walls can often be modeled by a SDOF system 
and the governing equation of motion solved by using numerical methods. There are 
also charts available in text books and military handbooks for linearly decaying loads, 
which provide the peak response and circumvent the need to solve differential 
equations. These charts require only knowledge of the fundamental period of the ele-
ment, its ultimate resistance force, the peak pressure applied to the element, and the 
equivalent linear decay time to evaluate the peak displacement response of the 
system. The design of the anchorage and supporting structural system can be 
evaluated by using the ultimate flexural capacity obtained from the dynamic analysis. 
For SDOF systems, material behavior can be modeled using idealized elastic, 
perfectly-plastic stress-deformation functions, based on actual structural support 
conditions and strain-rate-enhanced material properties. The model properties 
selected provide the same peak displace-ment and fundamental period as the actual 
structural system in flexure. Furthermore, the mass and the resistance functions are 
multiplied by mass and load factors, which estimate the actual portion of the mass or 
load participating in the deflection of the member along its span. 
For more complex elements, the engineer must resort to finite-element numerical 
time integration techniques and/or explosive testing. The time and cost of the 
analysis cannot be ignored when choosing design procedures. Because the design 
process is a sequence of iterations, the cost of analysis must be justified in terms of 
benefits to the project and increased confidence in the reliability of the results. In 
some cases, an SDOF approach will be used for the preliminary design, and a more 
sophisticated approach using finite elements, and/or explosive testing may be used 
for the final verification of the design. 
A dynamic nonlinear approach is more likely than a static approach to provide a 
section that meets the design constraints of the project. Elastic static calculations are 
likely to give overly conservative design solutions if the peak pressure is considered 
without the effect of load duration. By using dynamic calculations instead of static, we 
are able to account for the very short duration of the loading. Because the peak 
pressure levels are so high, it is important to account for the short duration to 
properly model the structural response. In addition, the inertial effect included in 
dynamic computations greatly improves response. This is because by the time the 
mass is mobilized, the loading is greatly diminished, enhancing response. 
Furthermore, by accepting that damage occurs it is possible to account for the energy 
absorbed by ductile systems through plastic deformation. Finally, because the loading 
is so rapid, it is possible to enhance the material strength to account for strain-rate 
effects. 
In dynamic nonlinear analysis, response is evaluated by comparing the ductility (i.e., 
the peak displacement divided by the elastic limit displacement) and/or support 
rotation (the angle between the support and the point of peak deflection) to 
empirically established maximum values that have been established by the military 
through explosive testing. Note that these values are typically based on limited testing 
and are not well defined within the industry at this time. Maximum permissible 
values vary, depending on the material and the acceptable damage level. 
Levels of damage computed by means of analysis may be described by the terms 
minor, moderate, or major, depending on the peak ductility, support rotation and 
collateral effects. A brief description of each damage level is given below. 
Minor: Nonstructural failure of building elements such as windows, 
doors, cladding, and false ceilings. Injuries may be expected, and 
fatalities are possible but unlikely. 
Moderate: Structural damage is confined to a localized area and is usually 
repairable. Structural failure is limited to secondary structural members such as 
beams, slabs, and non-load-bearing walls. However, if the building has been 
designed for loss of primary members, localized loss of columns may be 
accommodated. Injuries and possible fatalities are expected. 
Major: Loss of primary structural components such as columns or transfer girders 
precipitates loss of additional adjacent members that are adjacent to or above the 
lost member. In this case, extensive fatalities are expected. Building is usually not 
repairable. 
Generally, moderate damage at the design threat level is a reasonable design goal 
for new construction.


6.3.5 Structural Elements 
Because direct explosion effects decay rapidly with distance, the local response of 
structural components is the dominant concern. General principles governing the 
design of critical components are discussed below. 
6.3.5.1   Exterior Frame 
There are two primary considerations for the exterior frame. The first is to design the 
exterior columns to resist the direct effects of the specified threats. The second is to 
ensure that the exterior frame has sufficient structural integrity to accept localized 
failure without initiating progressive collapse. The former is discussed in this 
section, the latter in the sub-section on structural integrity. Exterior cladding and 
glazing issues are discussed in Section 6.4, Building Envelope. 
Because columns do not have much surface area, air-blast loads on columns tend to 
be mitigated by "clear-time effects". This refers to the pressure wave washing around 
these slender tall members, and consequently the entire duration of the pressure 
wave does not act upon them. On the other hand, the critical threat is directly across 
from them, so they are loaded with the peak reflected pressure, which is typically 
several times larger than the incident or overpressure wave that is propagating 
through the air. 
For columns subjected to a vehicle weapon threat on an adjacent street, buckling and 
shear are the primary effects to be considered in analysis. If a very large weapon is 
detonated close to a column, shattering of the concrete due to multiple tensile 
reflections within the concrete section can destroy its integrity. 
Buckling is a concern if lateral support is lost due to the failure of a supporting floor 
system. This is particularly important for buildings that are close to public streets. In 
this case, exterior columns should be capable of spanning two or more stories 
without buckling. Slender steel columns are at substantially greater risk than are 
concrete columns. 
Confinement of concrete using columns with closely spaced closed ties or spiral 
reinforcing will improve shear capacity, improve the performance of lap splices in 
the event of loss of concrete cover, and greatly enhance column ductility. The 
potential benefit from providing closely spaced closed ties in exterior concrete 
columns is very high relative to the cost of the added reinforcement. 
For steel columns, splices should be placed as far above grade level as practical. It is 
recommended that splices at exterior columns that are not specifically designed to 
resist air-blast loads employ complete-pene-tration welded flanges. Welding details, 
materials, and procedures should be selected to ensure toughness. 
For a package weapon, column breach is a major consideration. Some suggestions 
for mitigating this concern are listed below. 
. _	Do not use exposed columns that are fully or partially accessible 
from the building exterior. Arcade columns should be avoided.

. _	Use an architectural covering that is at least six inches from the 
structural member. This will make it considerably more difficult to place a weapon 
directly against the structure. Because explosive pressures decay so rapidly, every inch 
of distance will help to protect the column. 

Load- bearing reinforced concrete wall construction can provide a considerable level 
of protection if adequate reinforcement is provided to achieve ductile behavior. This 
may be an appropriate solution for the parts of the building that are closest to the 
secured perimeter line (within twenty feet). Masonry is a much more brittle material 
that is capable of generating highly hazardous flying debris in the event of an 
explosion. Its use is generally discouraged for new construction. 
Spandrel beams of limited depth generally do well when subjected to air blast. In 
general, edge beams are very strongly encouraged at the perimeter of concrete slab 
construction to afford frame action for redistribution of vertical loads and to 
enhance the shear connection of floors to columns.


6.3.5.2   Roof System 
The primary loading on the roof is the downward air-blast pressure. The exterior bay 
roof system on the side(s) facing an exterior threat is the most critical. The air-blast 
pressure on the interior bays is less intense, so the roof there may require less 
hardening. Secondary loads include upward pressure due to the air blast penetrating 
through openings and upward suction during the negative loading phase. The 
upward pressure may have an increased duration due to multiple reflections of the 
internal air-blast wave. It is conservative to consider the downward and upward loads 
separately. 
The preferred system is cast-in-place reinforced concrete with beams in two 
directions. If this system is used, beams should have continuous top and bottom 
reinforcement with tension lap splices. Stirrups to develop the bending capacity of 
the beams closely spaced along the entire span are recommended. 
Somewhat lower levels of protection are afforded by conventional steel beam 
construction with a steel deck and concrete fill slab. The performance of this system 
can be enhanced by use of normal-weight concrete fill instead of lightweight fill, 
increasing the gauge of welded wire fabric reinforcement, and making the 
connection between the slab and beams with shear connector studs. Since it is 
anticipated that the slab capacity will exceed that of the supporting beams, beam end 
connections should be capable of developing the ultimate flexural capacity of the 
beams to avoid brittle failure. Beam-to-column connections should be capable of 
resisting upward as well as downward forces. 
Precast and pre-/post-tensioned systems are generally considered less desirable, 
unless members and connections are capable of resisting upward forces generated 
by rebound from the direct pressure and/or the suction from the negative pressure 
phase of the air blast. 
Concrete flat slab/plate systems are also less desirable because of the potential of 
shear failure at the columns. When flat slab/plate systems are used, they should 
include features to enhance their punching shear resistance. Continuous bottom 
reinforcement should be provided through columns in two directions to retain the 
slab in the event that punching shear failure occurs. Edge beams should be provided 
at the building exterior. 
Lightweight systems, such as untopped steel deck or wood frame construction, are 
considered to afford minimal resistance to air-blast. These systems are prone to 
failure due to their low capacity for downward and uplift pressures.


6.3.5.3   Floor System 
The floor system design should consider three possible scenarios: air-blast loading, 
redistributing load in the event of loss of a column or wall support below, and the 
ability to arrest debris falling from the floor or roof above. 
For structures in which the interior is secured against bombs of moderate size by 
package inspection, the primary concern is the exterior bay framing. For buildings 
that are separated from a public street only by a sidewalk, the uplift pressures from a 
vehicle weapon may be significant enough to cause possible failure of the exterior bay 
floors for several levels above ground. Special concern exists in the case of vertical 
irregularities in the architectural system, either where the exterior wall is set back 
from the floor above or where the structure steps back to form terraces. The 
recommendations of Section 6.3.5.2 for roof systems apply to these areas. 
Structural hardening of floor systems above unsecured areas of the building such as 
lobbies, loading docks, garages, mailrooms, and retail spaces should be considered. 
In general, critical or heavily occupied areas should not be placed underneath 
unsecured areas, since it is virtually impossible to prevent against localized breach in 
conventional construction for package weapons placed on the floor. 
Precast panels are problematic because of their tendency to fail at the connections. 
Pre-/post-tensioned systems tend to fail in a brittle manner if stressed much beyond 
their elastic limit. These systems are also not able to accept upward loads without 
additional reinforcement. If pre-/post-tensioned systems are used, continuous mild 
steel needs to be added to the top and the bottom faces to provide the ductility needed 
to resist explosion loads. 
Flat slab/plate systems are also less desirable because of limited two way action and 
the potential for shear failure at the columns. When flat slab/plate systems are 
employed, they should include features to enhance their punching shear resistance, 
and continuous bottom rein-forcement should be provided across columns to resist 
progressive collapse. Edge beams should be provided at the building exterior.


6.3.5.4 Interior Columns 
Interior columns in unsecured areas are subject to many of the same issues as exterior 
columns. If possible, columns should not be accessible within these areas. If they are 
accessible, then obscure their location or impose a standoff to the structural 
component through the use of cladding. Methods of hardening columns (already 
discussed under Section 6.3.5.1, Exterior Frame) include using closely spaced ties, 
spiral reinforcement, and architectural covering at least six inches from the struc-
tural elements. Composite steel and concrete sections or steel plating of concrete 
columns can provide higher levels of protection. Columns in unsecured areas should 
be designed to span two or three stories without buckling in the event that the floor 
below and possibly above the detonation area have failed, as previously discussed.


6.3.5.5   Interior Walls 
Interior walls surrounding unsecured spaces are designed to contain the explosive 
effects within the unsecured areas. Ideally, unsecured areas are located adjacent to the 
building exterior so that the explosive pressure may be vented outward as well. 
Fully grouted CMU (concrete masonry unit) block walls that are well reinforced 
vertically and horizontally and adequately supported laterally are a common solution. 
Anchorage at the top and bottom of walls should be capable of developing the full 
flexural capacity of the wall. Lateral support at the top of the walls may be achieved 
using steel angles anchored into the floor system above. Care should be taken to 
terminate bars at the top of the wall with hooks or heads and to ensure that the upper 
course of block is filled solid with grout. The base of the wall may be anchored by 
reinforcing bar dowels. 
Interior walls can also be effective in resisting progressive collapse if they are 
designed properly with sufficient load-bearing capacity and are tied into the floor 
systems below and above. 
This design for hardened interior wall construction is also recommended for 
primary egress routes to protect against explosions, fire, and other hazards 
trapping occupants.



6.3.6 Checklist Š Structural 
  Incorporate measures to prevent progressive collapse. 
  Design floor systems for uplift in unsecured areas and in 
exterior 
bays that may pose a hazard to occupants.
  Limit column spacing.
  Avoid transfer girders.
  Use two-way floor and roof systems.

  Use fully grouted, heavily reinforced CMU block walls that are properly anchored 
in order to separate unsecured areas from critical functions and occupied secured 
areas. 
  Use dynamic nonlinear analysis methods for design of critical structural 
components.


6.3.7 Further Reading 
Listed below are publicly available references on structural hardening. Other 
references are given in the chapter on Weapons Effects. Existing design criteria are 
given in the chapter on Design Approach. 
Mays, G.C. and Smith, P.D. (editors), 1995, Blast Effects on Buildings: Design of Buildings 
to Optimize Resistance to Blast Loading., American Society of Civil Engineers, Reston, 
Virginia. 
Conrath, E., et al., 1999, Structural Design for Physical Security: State of the Practice. 
Structural Engineering Institute of American Society of Civil Engineers, Reston, 
Virginia. 
Ettouney, M., Smilowitz, R., and Rittenhouse, T., 1996, Blast Resistant Design of 
Commercial Buildings. Practice Periodical on Structural Design and Construction, Vol. 1, 
No. 1, American Society of Civil Engineers. 
http://ojps.aip.org/dbt/dbt.jsp?KEY=PPSCFX&Vol-ume=1&Issue=1 A preprint of 
the final article is available at http:// www.wai.com/AppliedScience/Blast/blast-
struct-design.html 
American Society of Civil Engineers,1997, Design of Blast Resistant Buildings in 
Petrochemical Facilities. American Society of Civil Engineers, Reston, Virginia. 
American Society of Civil Engineers, 2002, Vulnerability and Protection of Infrastructure 
Systems: The State of the Art. An ASCE Journals Special Publication compiling 
articles from 2002 and earlier available online 
https://ascestore.aip.org/OA_HTML/aipCCtpSctD-spRte.jsp?section=10123 
References on Progressive Collapse: 
American Society of Civil Engineers, 2002, Minimum Design Loads for Buildings and Other 
Structures, ASCE-7, Reston Virginia, ISBN: 0-7844-0624-3, 
http://www.asce.org/publications/ 
dsp_pubdetails.cfm?puburl=http://www.pubs.asce.org/ ASCE7.html?9991330 
General Services Administration, 2000, Progressive Collapse Analysis and Design Guidelines 
for New Federal Office Buildings and Major Modernization Projects, U.S. General Services 
Administration, Washington, D.C. 
http://www.oca.gsa.gov/about_progressive_collapse/progcol-lapse.php 
Burnett, E. F. P., 1975, The Avoidance of Progressive Collapse, Regulatory Approaches to the 
Problem, National Bureau of Standards, Washington 
D.C. 
Conrath, E., 2000, Interim Antiterrorism/Force Protection Construction Standards Š Progressive 
Collapse Guidance, Contact US Army Corps of Engineers Protective Design Center, 
ATTN: CENWO-ED-ST, 215 N. 17th Street, Omaha, Nebraska, 68102-4978, phone: 
(402) 221-4918.



6.4 BUILDING ENVELOPE 
Exterior wall/cladding and window systems and other openings are discussed in this 
section. For a discussion of the roof and exterior frame, see Section 6.3.5, Structural 
Elements. 
6.4.1 Exterior Wall/Cladding Design 
The exterior walls provide the first line of defense against the intrusion of the air-
blast pressure and hazardous debris into the building. They are subject to direct 
reflected pressures from an explosive threat located directly across from the wall 
along the secured perimeter line. If the building is more than four stories high, it 
may be advantageous to consider the reduction in pressure with height due to the 
increased distance and angle of incidence. The objective of design at a minimum is 
to ensure that these members fail in a ductile mode such as flexure rather than a 
brittle mode such as shear. The walls also need to be able to resist the loads 
transmitted by the windows and doors. It is not uncommon, for instance, for bullet-
resistant windows to have a higher ultimate capacity than the walls to which they are 
attached. Beyond ensuring a ductile failure mode, the exterior wall may be designed 
to resist the actual or reduced pressure levels of the defined threat. Note that special 
reinforcing and anchors should be provided around blast-resistant window and door 
frames. 
Poured-in-place, reinforced concrete will provide the highest level of protection, but 
solutions like pre-cast concrete, CMU block, and metal stud systems may also be 
used to achieve lower levels of protection. 
For pre-cast panels, consider a minimum thickness of five inches exclusive of reveals, 
with two-way, closely spaced reinforcing bars to increase ductility and reduce the 
chance of flying concrete fragments. The objective is to reduce the loads transmitted 
into the connections, which need to be designed to resist the ultimate flexural 
resistance of the panels. Also, connections into the structure should provide as 
straight a line of load transmittal as practical. 
For CMU block walls, use eight-inch block walls, fully grouted with vertically centered 
heavy reinforcing bars and horizontal reinforcement placed at each layer. 
Connections into the structure should be designed to resist the ultimate lateral 
capacity of the wall. For infill walls, avoid transferring loads into the columns if they 
are primary load-carrying elements. The connection details may be very difficult to 
construct. It will be difficult to have all the blocks fit over the bars near the top, and it 
will be difficult to provide the required lateral restraint at the top connection. A 
preferred system is to have a continuous exterior CMU wall that laterally bears against 
the floor system. For increased protection, consider using 12-inch blocks with two 
layers of vertical reinforcement. 
For metal stud systems, use metal studs back-to-back and mechanically attached, to 
minimize lateral torsional effects. To catch exterior cladding fragments, attach a wire 
mesh or steel sheet to the exterior side of the metal stud system. The supports of the 
wall should be designed to resist the ultimate lateral out-of-plane bending capacity 
load of the system. 
Brick veneers and other nonstructural elements attached to the building exterior 
are to be avoided or have strengthened connections to limit flying debris and to 
improve emergency egress by ensuring that exits remain passable.


6.4.2 Window Design 
Windows, once the sole responsibility of the architect, become a structural issue 
when explosive effects are taken into consideration. In designing windows to 
mitigate the effects of explosions they should first be designed to resist conventional 
loads and then be checked for explosive load effects and balanced design. 
Balanced or capacity design philosophy means that the glass is designed to be no 
stronger than the weakest part of the overall window system, failing at pressure levels 
that do not exceed those of the frame, anchorage, and supporting wall system. If the 
glass is stronger than the supporting members, then the window is likely to fail with 
the whole panel entering into the building as a single unit, possibly with the frame, 
anchorage, and the wall attached. This failure mode is considered more hazardous 
than if the glass fragments enter the building, provided that the fragments are 
designed to minimize injuries. By using a damage-lim-iting approach, the damage 
sequence and extent of damage can be controlled. 
Windows are typically the most vulnerable portion of any building. Though it may be 
impractical to design all the windows to resist a large-scale explosive attack, it is 
desirable to limit the amount of hazardous glass breakage to reduce the injuries. 
Typical annealed glass windows break at low pressure and impulse levels and the 
shards created by broken windows are responsible for many of the injuries incurred 
during a large-scale explosive attack. 
Designing windows to provide protection against the effects of explosions can be 
effective in reducing the glass laceration injuries in areas that are not directly across 
from the weapon. For a large-scale vehicle weapon, this pressure range is expected on 
the sides of surrounding buildings not facing the explosion or for smaller explosions 
in which pressures drop more rapidly with distance. Generally, it is not known on 
which side of the building the attack will occur, so all sides need to be protected. 
Window protection should be evaluated on a case-by-case basis by a qualified 
protective design consultant to develop a solution that meets established objectives. 
Several recommended solutions for the design of the window systems to reduce 
injuries to building occupants are provided in Figure 6-7. 
Several approaches that can be taken to limit glass laceration injuries. One way is to 
reduce the number and size of windows. If blast-resistant walls are used, then fewer 
and/or smaller windows will allow less air blast to enter the building, thus reducing 
the interior damage and injuries. Specific examples of how to incorporate these ideas 
into the design of a new building include (1) limiting the number of windows on the 
lower floors where the pressures would be higher during an external explosion; (2) 
using an internal atrium design with windows facing inward, not outward; (3) using 
clerestory windows, which are close to the ceiling, above the heads of the occupants; 
and (4) angling the windows away from the curb to reduce the pressure levels. 
Glass curtain-wall, butt glazed, and Pilkington type systems have been found to 
perform surprisingly well in recent explosive tests with low explosive loads. In 
particular, glass curtain wall systems have been shown to accept larger deformations 
without the glass breaking hazardously, compared to rigidly supported punched 
window systems. Some design modifications to the connections, details, and member 
sizes may be required to optimize the performance. 
6.4.2.1   Glass Design 
Glass is often the weakest part of a building, breaking at low pressures compared to 
other components such as the floors, walls, or columns. Past incidents have shown 
that glass breakage and associated injuries may extend many thousands of feet in 
large external explosions. High-velocity glass fragments have been shown to be a 
major contributor to injuries in such incidents. For incidents within downtown city 
areas, falling glass poses a major hazard to passersby and prolongs post-incident 
rescue and clean-up efforts by leaving tons of glass debris on the street. At this time, 
the issue of exterior debris is largely ignored by existing criteria. 
As part of the damage-limiting approach, glass failure is not quantified in terms of 
whether breakage occurs or not, but rather by the hazard it causes to the occupants. 
Two failure modes that reduce the hazard posed by window glass are 
_ glass that breaks but is retained by the frame and _ glass fragments exit the frame and fall within three to ten feet of 
the 
window. The glass performance conditions are defined based on empirical data 
from explosive tests performed in a cubical space with a 10- foot dimension (Table 6-
1). The performance condition ranges from 1, which corresponds to not breaking, to 
5, which corresponds to hazardous flying debris at a distance of 10 feet from the 
window (see Figure 6-8). Generally a performance condition 3 or 4 is considered 
acceptable for buildings that are not at high risk of attack. At this level, the window 
breaks and fragments fly into the building but land harmlessly within 10 feet of the 
window or impact a witness panel 10 feet away, no more than 2 feet above the floor 
level. The design goal is to achieve a performance level less than 4 for 90 percent of 
the windows. 
The preferred solution for new construction is to use laminated annealed (i.e., float) 
glass with structural sealant around the inside perimeter. For insulated units, only 
the inner pane needs to be laminated. The lamination holds the shards of glass 
together in explosive events, reducing its ability to cause laceration injuries. The 
structural sealant helps to hold the pane in the frame for higher loads. Annealed 
glass is used because it has a breaking strength that is about one-half that of heat-
strengthened glass and about one-fourth as strong as tempered glass. Using annealed 
glass becomes particularly important for buildings with lightweight exterior walls 
using for instance, metal studs, dry wall, and brick fa¨ade. Use the thinnest overall 
glass thickness that is acceptable based on conventional load requirements. Also, it is 
important to use an interlayer thickness that is 60 mil thick rather than 30 mil thick, 
as is used in conventional applications. This layup has been shown to perform well in 
low-pressure regions (i.e., under about 5 psi). If a 60 mil polyvinyl butaryl (PVB) layer 
is used, the tension membrane forces into the framing members need to be 
considered in design. 
To make sure that the components supporting the glass are stronger than the glass 
itself, specify a window breakage strength that is high compared to what is used in 
conventional design. The breakage strength in window design may be specified as a 
function of the number of windows expected to break at that load. For instance, in 
conventional design, it is typical to use a breakage pressure corresponding to eight 
breaks out of 1000. When a lot of glass breakage is expected, such as for an explosive 
incident, use a pressure corresponding to 750 breaks out of 1000 to increase 
confidence that the frame does not fail, too. Glass breakage strength values may be 
obtained from window manufacturers.


6.4.2.2   Mullion Design 
The frame members connecting adjoining windows are referred to as mullions. 
These members may be designed in two ways. Using a static approach, the breaking 
strength of the window glass is applied to the mullion; alternatively, a dynamic load 
can be applied using the peak pressure and impulse values. The static approach may 
lead to a design that is not practical, because the mullion can become very deep and 
heavy, driving up the weight and cost of the window system. It may also not be 
consistent with the overall architectural objectives for the project. 
As with frames, it is good engineering practice to limit the number of interlocking 
parts used for the mullion.


6.4.2.3 Frame and Anchorage Design 
Window frames need to retain the glass so that the entire pane does not become a 
single large unit of flying debris. It also needs to be designed to resist the breaking 
stress of the window glass. 
To retain the glass in the frame, a minimum of a 1/4-inch bead of structural sealant 
(e.g., silicone) should be used around the inner perimeter of the window. The 
allowable tensile strength should be at least 20 psi. Also, the window bite (i.e., the 
depth of window captured by the frame) needs to be at least 1/2 inch. The structural 
sealant recommendations should be determined on a case-by-case basis. In some 
applications, the structural sealant may govern the overall design of the window 
system. 
Frame and anchorage design is performed by applying the breaking strength of the 
window to the frame and the fasteners. In most conventionally designed buildings, 
the frames will be aluminum. In some applications, steel frames are used. Also, in 
lobby areas where large panes of glass are used, a larger bite with more structural 
sealant may be needed. 
Inoperable windows are generally recommended for air-blast mitigating designs. 
However, some operable window designs are conceptually viable. For instance, 
designs in which the window rotates about a horizontal hinge at the head or sill and 
opens in the outward direction may perform adequately. In these designs, the window 
will slam shut in an explosion event. If this type of design is used, the governing 
design parameter may be the capacity of the hinges and/or hardware.


6.4.2.4 Wall Design 
The supporting wall response should be checked using approaches similar to those 
for frames and mullions. It does not make sense, and is potentially highly hazardous, 
to have a wall system that is weaker than windows. Remember that the maximum 
strength of any wall system needs to be at least equal to the window strength. If the 
walls are unable to accept the loads transmitted by the mullions, the mullions may 
need to be anchored into the structural slabs or spandrel beams. Anchoring into 
columns is generally discouraged, because it increases the tributary area of lateral 
load that is transferred into the columns and may cause instability. 
The balanced-design approach is particularly challenging in the design of ballistic-
resistant and forced-entry-resistant windows, which consist of one or more inches of 
glass and polycarbonate. These windows can easily become stronger than the 
supporting wall. In these cases, the windows may need to be designed for the design 
threat air-blast pressure levels under the implicit assumption that balanced-design 
conditions will not be met for larger loads.


6.4.2.5   Multi-hazard Considerations 
Under normal operating conditions, windows perform several functions listed below. 
. _ They permit light to enter building. 
. _ They save energy by reducing thermal transmission. 
. _ They make the building quieter by reducing acoustic transmission. 

Explosions are one of a number of abnormal loading conditions that the building 
may undergo. Some of the others are 
. _ fire, 
. _ earthquake, 
. _ hurricane, 
. _ gun fire, and 
. _ forced entry. When developing a protection strategy for windows to mitigate 
the 

effects of a particular explosion threat scenario, it is important to consider how this 
protection may interfere with some of these other func-tions or other explosion 
threat scenarios. Some questions that may be worthwhile to consider are listed 
below. 
. _	If an internal explosion occurs, will the upgraded windows increase 
smoke inhalation injuries by preventing the smoke to vent through windows that 
would normally break in an explosion event? 
. _	If a fire occurs, will it be more difficult to break the protected windows 
to vent the building and gain access to the injured? 
. _	Will a window upgrade that is intended to protect the occupants 
worsen the hazard to passersby?




6.4.3 Other Openings 
Doors, louvers, and other openings in the exterior envelope should be designed so 
that the anchorage into the supporting structure has a lateral capacity greater than 
that of the element. 
There are two general recommendations for doors. 
. _	Doors should open outward so that they bear against the jamb during 
the positive-pressure phase of the air-blast loading. 
. _	Door jambs can be filled with concrete to improve their resistance. 

For louvers that provide air to sensitive equipment, some recommendations are given 
below. 
. _	Provide a baffle in front of the louver so that the air blast does not have 
direct line-of-sight access through the louver. 
. _	Provide a grid of steel bars properly anchored into the structure 
behind the louver to catch any debris generated by the louver or 
other flying fragments.



6.4.4 Checklist Š Building Envelope 
Cladding 
  Use the thinnest panel thickness that is acceptable for 
conventional loads.   Design cladding supports and the 
supporting structure to resist the ultimate lateral resistance 
of the panel. 
  Design cladding connections to have as direct a load transmission path into the 
main structure as practical. A good transmission path minimizes shear and 
torsional response. 
  Avoid framing cladding into columns and other primary vertical 
load-carrying members. Instead frame into floor diaphragms.


Windows 
  Use the thinnest glass section that is acceptable for conventional 
loads.

  Design window systems so that the frame anchorage and the sup-
porting wall are capable of resisting the breaking pressure of the
window glass.

  Use laminated annealed glass (for insulated panels, only the interior 
pane needs to be laminated).   Design window frames with a minimum 
of a 1/2-inch bite.   Use a minimum of a 1/4-inch silicone sealant 
around the inside glass perimeter, with a minimum tensile strength of 
20 psi.



6.4.5 Further Reading 
Norville, H.S., Harville, N., Conrath, E.J., Shariat, S., and Mallone, S., 1999, Glass-
Related Injuries in Oklahoma City Bombing, Journal of Performance of Constructed 
Facilities,Vol. 13, No. 2. http:// www.pubs.asce.org/WWWdisplay.cgi?9902006 
Emek, M. and Tennant, D., 1998, "Energy Absorbing Blast Mitigating Window 
Systems", Glass Magazine, McLean, Virginia. 
Smilowitz, R. and Tennant, D., 1998, "Curtainwall Systems and Blast Loading," 
Glass Magazine, McLean, Virginia.



6.5 MECHANICAL AND ELECTRICAL SYSTEMS 
In the event of an explosion directed at a high-occupancy building, the primary 
objective is to protect people by preventing building collapse. Secondary goals are to 
limit injuries due to flying building debris and the direct effects of air blast entering 
the building (i.e., impact due to being thrown or lung collapse). Beyond these life-
safety concerns, the objective is to facilitate building evacuation and rescue efforts 
through effective building design. This last objective is the focus of this section. 
Issues related specifically to chemical, biological, and radiological threats are 
discussed under a separate section with that heading. 
The key concepts for providing secure and effective mechanical and electrical 
systems in buildings is the same as for the other building systems: separation, 
hardening, and redundancy. Keeping critical mechanical and electrical functions as 
far from high-threat areas as possible (e.g., lobbies, loading docks, mail rooms, 
garages, and retail spaces) increases their ability to survive an event. Separation is 
perhaps the most cost-effective option. Additionally, physical hardening or protec-
tion of these systems (including the conduits, pipes, and ducts associated with life-
safety systems) provides increased likelihood that they will be able to survive the 
direct effects of the event if they are close enough to be affected. Finally, by providing 
redundant emergency systems that are adequately separated, there is a greater 
likelihood that emergency systems will remain operational post-event to assist 
rescuers in the evacuation of the building. 
Architecturally, enhancements to mechanical and electrical systems will require 
additional space to accommodate additional equipment. Fortunately, there are many 
incremental improvements that can be made that require only a small change to the 
design. Additional space can be provided for future enhancements as funds or the 
risk justify implementation. 
Structurally, the walls and floor systems adjacent to the areas where critical 
equipment are located need to be protected by means of hardening. Other areas 
where hardening is recommended include primary egress routes, feeders for 
emergency power distribution, sprinkler systems mains and risers, fire alarm system 
trunk wiring, and ducts used for smoke-control systems. 
From an operational security standpoint, it is important to restrict and control access 
to air-intake louvers, mechanical and electrical rooms, telecommunications spaces 
and rooftops by means of such measures as visitor screening, limited elevator stops, 
closed-circuit television (CCTV), detection, and card access-control systems. 
Specific recommendations are given below for (1) emergency egress routes, (2) the 
emergency power system, (3) fuel storage, (4) transformers, (5) ventilation systems, 
(6) the fire control center, (7) emergency elevators, (8) the smoke and fire detection 
and alarm system, (9) the sprinkler/standpipe system, (10) smoke control system, and 
(11) the communication system. Air intakes are covered in Section 6.6. 
6.5.1 Emergency Egress Routes 
To facilitate evacuation consider these measures. 
. _	Provide positive pressurization of stairwells and vestibules. 
. _	Provide battery packs for lighting fixtures and exit signs. 
. _	Harden walls using reinforced CMU block properly anchored at 
supports.


. _	Use non-slip phosphorescent treads. 
. _	Do not cluster egress routes in single shaft. Separate them as far as 
possible. 
. _	Use double doors for mass evacuation. 
. _	Do not use glass along primary egress routes or stairwells. 


6.5.2 Emergency Power System 
An emergency generator provides an alternate source of power should utility power 
become unavailable to critical life-safety systems such as alarm systems, egress 
lighting fixtures, exit signs, emergency communications systems, smoke-control 
equipment, and fire pumps. 
Emergency generators typically require large louvers to allow for ventilation of the 
generator while running. Care should be taken to locate the generator so that these 
louvers are not vulnerable to attack. A remote radiator system could be used to 
reduce the louver size. 
Redundant emergency generator systems remotely located from each other enable 
the supply of emergency power from either of two locations. Consider locating 
emergency power-distribution feeders in hardened enclosures, or encased in 
concrete, and configured in redundant routing paths to enhance reliability. 
Emergency distribution panels and automatic transfer switches should be located in 
rooms separate from the normal power system (hardened rooms, where possible). 
Emergency lighting fixtures and exit signs along the egress path could be provided 
with integral battery packs, which locates the power source directly at the load, to 
provide lighting instantly in the event of a utility power outage.


6.5.3 Fuel storage 
A non-explosive fuel source, such as diesel fuel, is acceptable for standby use for 
emergency generators and diesel fire pumps. Fuel tanks should be located away from 
building access points, in fire-rated, hardened enclosures. Fuel piping within the 
building should be located in hardened enclosures, and redundant piping systems 
could be provided to enhance the reliability of the fuel distribution system. Fuel 
filling stations should be located away from public access points and monitored by 
the CCTV system.


6.5.4 Transformers 
Main power transformer(s) should be located interior to the building if possible, 
away from locations accessible to the public. For larger buildings, multiple 
transformers, located remotely from each other, could enhance reliability should one 
transformer be damaged by an explosion.


6.5.5 Ventilation Systems 
Air-intake locations should be located as high up in the building as is practical to 
limit access to the general public. Systems that serve public access areas such as mail 
receiving rooms, loading docks, lobbies, freight elevators/lobbies should be isolated 
and provided with dedicated air handling systems capable of 100 percent exhaust 
mode. Tie air intake locations and fan rooms into the security surveillance and alarm 
system. 
Building HVAC systems are typically controlled by a building automation system, 
which allows for quick response to shut down or selectively control air conditioning 
systems. This system is coordinated with the smoke-control and fire-alarm systems. 
See Section 6.6 on chemical, biological, and radiological protection for more 
information.


6.5.6 Fire Control Center 
A Fire Control Center should be provided to monitor alarms and life-safety 
components, operate smoke-control systems, communicate with occupants, and 
control the fire-fighting/evacuation process. Consider providing redundant Fire 
Control Centers remotely located from each other to allow system operation and 
control from alternate locations. The Fire Control Center should be located near the 
point of firefighter access to the building. If the control center is adjacent to lobby, 
separate it from the lobby using a corridor or other buffer area. Provide hardened 
construction for the Fire Control Center.


6.5.7 Emergency Elevators 
Elevators are not used as a means of egress from a building in the event of a life-safety 
emergency event, as conventional elevators are not suitably protected from the 
penetration of smoke into the elevator shaft. An unwitting passenger could be 
endangered if an elevator door opens onto a smoke filled lobby. Firefighters may 
elect to manually use an elevator for firefighting or rescue operation. 
A dedicated elevator, within its own hardened, smoke-proof enclosure, could enhance 
the firefighting and rescue operation after a blast/fire event. The dedicated elevator 
should be supplied from the emergency generator, fed by conduit/wire that is 
protected in hardened enclosures. This shaft/lobby assembly should be sealed and 
positively pressurized to prevent the penetration of smoke into the protected area.


6.5.8 	Smoke and Fire Detection and Alarm System 
A combination of early-warning smoke detectors, sprinkler-flow switches, manual 
pull stations, and audible and visual alarms provide quick response and notification 
of an event. The activation of any device will automatically start the sequence of 
operation of smoke control, egress, and communication systems to allow occupants 
to quickly go to a safe area. System designs should include redundancy such as 
looped infrastructure wiring and distributed intelligence such that the severing of 
the loop will not disable the system. 
Install a fire-alarm system consisting of distributed intelligent fire alarm panels 
connected in a peer-to-peer network, such that each panel can function 
independently and process alarms and initiate sequences within its respective zone.


6.5.9  	Sprinkler/Standpipe System 
Sprinklers will automatically suppress fire in the area upon sensing heat. Sprinkler 
activation will activate the fire alarm system. Standpipes have water available locally in 
large quantities for use by professional fire fighters. Multiple sprinkler and standpipe 
risers limit the possibility of an event severing all water supply available to fight a fire. 
Redundant water services would increase the reliability of the source for sprinkler 
protection and fire suppression. Appropriate valving should be provided where 
services are combined. 
Redundant fire pumps could be provided in remote locations. These pumps could 
rely on different sources, for example one electric pump supplied from the utility 
and/or emergency generator and a second diesel fuel source fire pump. 
Diverse and separate routing of standpipe and sprinkler risers within hardened 
areas will enhance the systemÕs reliability (i.e., reinforced masonry walls at stair 
shafts containing standpipes).


6.5.10  Smoke-Control Systems 
Appropriate smoke-control systems maintain smoke-free paths of egress for building 
occupants through a series of fans, ductwork, and fire-smoke dampers. Stair 
pressurization systems maintain a clear path of egress for occupants to safe areas or to 
evacuate the building. Smoke-control fans should be located higher in a building 
rather than at lower floors to limit exposure/access to external vents. Vestibules at 
stairways with separate pressurization provide an additional layer of smoke control.


6.5.11 Communication System 
A voice communication system facilitates the orderly control of occupants and 
evacuation of the danger area or the entire building. The system is typically zoned by 
floor, by stairwell, and by elevator bank for selective communication to building 
occupants. 
Emergency communication can be enhanced by providing 
. _	extra emergency phones separate from the telephone system, con-
nected directly to a constantly supervised central station; 
. _	in-building repeater system for police, fire, and EMS (Emergency 
Medical Services) radios; and

. _	redundant or wireless firemanÕs communications in building. 


6.5.12 	Checklist Š Mechanical and Electrical Systems 
  Place all emergency functions away from high-risk areas in protected 
locations with restricted access.   Provide redundant and separated 
emergency functions.   Harden and/or provide physical buffer zones 
for the enclosures around emergency equipment, controls, and wiring. 
  For egress routes, provide battery packs for exit signs, use non-slip 
phosphorescent treads, and double doors for mass evacuation.   Avoid 
using glass along primary egress routes or stairwells.   Place emergency 
functions away from structurally vulnerable areas such as transfer 
girders.   Place a transformer interior to building, if possible.   Provide 
access to the fire control center from the building exterior.


6.5.13 Further Reading 
Building Owners and Managers Association International, 1996, How to Design and 
Manage Your Preventive Maintenance Program, Building Owners and Managers 
Association International, Washington, D.C. 
http://www.boma.org/pubs/bomapmp.htm 
Kurtz,N.D., Hlushko, A., and Nall, D., 2002, Engineering Systems and an 
Incremental Response to Terrorist Threat, Building Standards Magazine, July-
August 2002, Whittier, California, pages 24-27. 
Craighead, G., 2002, High-Rise Security and Fire Life Safety, 2nd Edition, Academic Press, 
ISBN 0750674555 http://www.amazon.com/exec/ obidos/tg/detail/-
/0750674555/qid=/br=1-/ref=br_lf_b_//t/103-7416668-
4182264?v=glance&s=books&n=173507#product-details 
Council on Tall Buildings and Urban Habitat, 2001, Task Force on Tall Buildings: "The 
Future," Council on Tall Buildings and Urban Habitat, Lehigh University, 
Bethlehem, Pennsylvania. 
http://www.lehigh.edu/ctbuh/htmlfiles/hot_links/report.pdf



6.6 	CHEMICAL, BIOLOGICAL, AND RADIOLOGICAL 
PROTECTION 
This section discusses three types of air-borne hazards. 
1. 1.	A large exterior release originating some distance away from the 
building (includes delivery by aircraft). 
2. 2.	A small localized exterior release at an air intake or other opening in 
the exterior envelope of the building. 
3. 3.	A small interior release in a publicly accessible area, a major egress 
route, or other vulnerable area (e.g., lobby, mail room, delivery receiving). 

Like explosive threats, chemical, biological and radiological (CBR) threats may be 
delivered externally or internally to the building. External ground-based threats may 
be released at a standoff distance from the building or may be delivered directly 
through an air intake or other opening. Interior threats may be delivered to 
accessible areas such as the lobby, mailroom, or loading dock, or they may be 
released into a secured area such as a primary egress route. This discussion is limited 
to air-borne hazards. 
There may not be an official or obvious warning prior to a CBR event. While you 
should always follow any official warnings, the best defense is to be alert to signs of a 
release occurring near you. The air may be con-taminated if you see a suspicious 
cloud or smoke near ground level, hear an air blast, smell strange odors, see birds or 
other small animals dying, or hear of more than one person complaining of eye, 
throat or skin irritation or convulsing. 
Chemicals will typically cause problems within in seconds or minutes after exposure, 
but they can sometimes have delayed effects that will not appear for hours or days. 
Symptoms may include blurred or dimmed vision; eye, throat, or skin irritation; 
difficulty breathing; excess saliva; or nausea. 
Biological and some radioactive contaminants typically will take days to weeks before 
symptoms appear, so listen for official information regarding symptoms. 
With radioactive "dirty" bombs, the initial risk is from the explosion. Local 
responders may advise you to either shelter-in-place or evacuate. After the initial 
debris falls to the ground, leaving the area and washing will minimize your risk from 
the radiation. 
Buildings provide a limited level of inherent protection against CBR threats. To some 
extent, the protection level is a function of how airtight the building is, but to a 
greater extent it is a function of the HVAC systemÕs design and operating parameters. 
The objectives of protective building design as they relate to the CBR threat are first 
to make it difficult for the terrorist to successfully execute a CBR attack and second, to 
minimize the impact (e.g., life, health, property damage, loss of commerce) of an 
attack if it does occur. 
In order to reduce the likelihood of an attack, use security and design features that 
limit the terroristÕs ability to approach the building and successfully release the 
CBR contaminant. Some examples are listed below. 
. _	Use security stand-off, accessibility, and screening procedures similar 
to those identified in the explosive threat mitigation sections of this document (see 
Chapter 5, Design Approach and Section 6.1, Site Location and Layout). 
. _	Recognize areas around HVAC equipment and other mechanical 
systems to be vulnerable areas requiring special security consider-
ations. 

. _	Locate outdoor air intakes high above ground level and at inaccessible 
locations. 

. _	Prevent unauthorized access to all mechanical areas and equip-
ment. 

. _	Avoid the use of ground-level mechanical rooms accessible from 
outside the building. Where such room placement is unavoidable, doors and air vents 
leading to these rooms should be treated as vulnerable locations and appropriately 
secured. 
. _	Treat operable, ground-level windows as a vulnerability and either avoid 
their use or provide appropriate security precautions to minimize the vulnerability. 
. _	Interior to the building, minimize public access to HVAC return-air 

systems. Further discussion of some of these prevention methods is provided 
below. 
6.6.1 Air intakes 
Air intakes may be made less accessible by placing them as high as possible on the 
building exterior, with louvers flush with the exterior (see figure 6-9). All 
opportunities to reach air-intakes through climbing should be eliminated. Ideally, 
there is a vertical smooth surface from the ground level to the intake louvers, without 
such features as high shrubbery, low roofs, canopies, or sunshades, as these features 
can enable climbing and concealment. To prevent opportunities for a weapon to be 
lobbed into the intake, the intake louver should be ideally flush with the wall. 
Otherwise, a surface sloped at least 45 degrees away from the building and further 
protected through the use of metal mesh (a.k.a. bird screen) should be used. Finally, 
CCTV surveillance and enhanced security is recommended at intakes. 
In addition to providing protection against an air-borne hazard delivered directly 
into the building, placing air-intakes high above ground provides protection 
against ground-based standoff threats because the concentration of the air-borne 
hazard diminishes somewhat with height. Because air-blast pressure decays with 
height, elevated air intakes also provide modest protection against explosion 
threats. Furthermore, many recognized sources of indoor air contaminants (e.g., 
vehicle exhaust, standing water, lawn chemicals, trash, and rodents) tend to be 
located near ground level. Thus, elevated air intakes are a recommended practice in 
general for providing healthy indoor air quality. 
In the event that a particular air intake does not service an occupied area, it may not 
be necessary to elevate it above ground level. However, if the unoccupied area is 
within an otherwise occupied building, the intake should either be elevated or 
significant precautions (tightly sealed construction between unoccupied/occupied 
areas, unoccupied area maintained at negative pressure relative to occupied area) 
should be put in place to ensure that contaminants are unable to penetrate into the 
occupied area of the building.


6.6.2 Mechanical Areas 
Another simple measure is to tightly restrict access to building mechanical areas 
(e.g., mechanical rooms, roofs, elevator equipment access). These areas provide 
access to equipment and systems (e.g., HVAC, elevator, building exhaust, and 
communication and control) that could be used or manipulated to assist in a CBR 
attack. Additional protection may be provided by including these areas in those 
monitored by electronic security and by eliminating elevator stops at the levels that 
house this equipment. For rooftop mechanical equipment, ways of restricting (or at 
least monitoring) access to the roof that do not violate fire codes should be pursued.


6.6.3 	Return-Air Systems 
Similar to the outdoor-air intake, HVAC return-air systems inside the building can be 
vulnerable to CBR attack. Buildings requiring public access have an increased 
vulnerability to such an attack. Design approaches that reduce this vulnerability 
include the use of ducted HVAC returns within public access areas and the careful 
placement of return-air louvers in secure locations not easily accessed by public occu-
pants. 
The second objective is to design to minimize the impact of an attack. For many 
buildings, especially those requiring public access, the ability to prevent a 
determined terrorist from initiating a CBR release will be a significant challenge. 
Compared to buildings in which campus security and internal access can be strictly 
controlled, public-access buildings may require a greater emphasis on mitigation. 
However, even private-access facilities can fall victim to an internal CBR release, 
whether through a security lapse or perhaps a delivered product (mail, package, 
equipment, or food). Examples of design methods to minimize the impact of a CBR 
attack are listed below. 
. _	Public access routes to the building should be designed to channel 
pedestrians through points of noticeable security presence. 
. _	The structural and HVAC design should isolate the most vulnerable 
public areas (entrance lobbies, mail rooms, load/delivery docks) both physically and 
in terms of potential contaminant migration. 
. _	The HVAC and auxiliary air systems should carefully use positive 
and negative pressure relationships to influence contaminant
migration routes.


Further discussion of some of these prevention methods is provided below.


6.6.4 	Lobbies, Loading Docks, and Mail Sorting Areas 
Vulnerable internal areas where airborne hazards may be brought into the building 
should be strategically located. These include lobbies, loading docks, and mail 
sorting areas. Where possible, place these functions outside of the footprint of the 
main building. When incorporated into the main building, these areas should be 
physically separated from other areas by floor-to-roof walls. Additionally, these areas 
should be maintained under negative pressure relative to the rest of the building, but 
at positive-to-neutral pressure relative to the outdoors. To assist in maintaining the 
desired pressure relationship, necessary openings (doors, windows, etc.) between 
secure and vulnerable areas should be equipped with sealing windows and doors, 
and wall openings due to ductwork, utilities, and other penetrations should be 
sealed. Where entries into vulnerable areas are frequent, the use of airlocks or vesti-
bules may be necessary to maintain the desired pressure differentials. 
Ductwork that travels through vulnerable areas should be sealed. Ideally, these areas 
should have separate air-handling units to isolate the hazard. Alternatively, the 
conditioned air supply to these areas may come from a central unit as long as 
exhaust/return air from these areas is not allowed to mix into other portions of the 
building. In addition, emergency exhaust fans that can be activated upon internal 
CBR release within the vulnerable area will help to purge the hazard from the 
building and minimize its migration into other areas. Care must be taken that the 
discharge point for the exhaust system is not co-located with expected egress routes. 
Consideration should also be given to filtering this exhaust with High Efficiency 
Particulate Air (HEPA) filtration. For entrance lobbies that contain a security 
screening location, it is recommended that an airlock or vestibule be provided 
between the secured and unsecured areas.


6.6.5 Zoning of HVAC Systems 
Large buildings usually have multiple HVAC (heating, ventilation, air-conditioning) 
zones, each zone with its own air-handling unit and duct system. In practice, these 
zones are not completely separated if they are on the same floor. Air circulates among 
zones through plenum returns, hallways, atria, and doorways that are normally left 
open. Depending upon the HVAC design and operation, airflow between zones on 
different floors can also occur through the intentional use of shared air-
return/supply systems and through air migrations via stairs and elevator shafts. 
Isolating the separate HVAC zones minimizes the potential spread of an airborne 
hazard within a building, reducing the number of people potentially exposed if there 
is an internal release. Zone separation also provides limited benefit against an 
external release, as it increases internal resistance to air movement produced by wind 
forces and chimney effect, thus reducing the rate of infiltration. In essence, isolating 
zones divides the building into separate environments, limiting the effects of a single 
release to an isolated portion of the building. Isolation of zones requires full-height 
walls between each zone and the adjacent zones and hallway doors. 
Another recommendation is to isolate the return system (i.e., no shared returns). 
Strategically locate return air grills in easily observable locations and preferably in 
areas with reduced public access. 
Both centralized and decentralized shutdown capabilities are advantageous. To 
quickly shut down all HVAC systems at once in the event of an external threat, a 
single-switch control is recommended for all air-exchange fans (includes bathroom, 
kitchen, and other exhaust sources). In the event of a localized internal release, 
redundant decentralized shutdown capability is also recommended. Controls should 
be placed in a location easily accessed by the facility manager, security, or 
emergency response personnel. Duplicative and separated control systems will add 
an increased degree of protection. Further protection may be achieved by placing 
low-leakage automatic dampers on air intakes and exhaust fans that do not already 
have back-draft dampers.


6.6.6  Positive Pressurization 
Traditional good engineering practice for HVAC design strives to achieve a slight 
overpressure of 5-12 Pa (.02-inch-.05-inch w.g.) within the building environment, 
relative to the outdoors. This design practice is intended to reduce uncontrolled 
infiltration into the building. When combined with effective filtration, this practice 
will also provide enhanced protection against external releases of CBR aerosols. 
Using off-the-shelf technology (e.g., HEPA), manually triggered augmentation 
systems can be put into place to over-pressure critical zones to intentionally impact 
routes of contaminant migration and/or to provide safe havens for sheltering-in-
place. For egress routes, positive-pres-surization is also recommended, unless of 
course, the CBR source is placed within the egress route. Design parameters for such 
systems will depend upon many factors specific to the building and critical zone in 
question. Care must be taken that efforts to obtain a desired pressure relationship 
within one zone, will not put occupants in another zone at increased risk. Lastly, the 
supply air used to pressurize the critical space must be appropriately filtered (see 
filtration discussion below) or originate from a non-contaminated source in order to 
be beneficial.


6.6.7 Airtightness 
To limit the infiltration of contaminants from outside the building into the building 
envelope, building construction should be made as airtight as possible. Tight 
construction practices (weatherization techniques, tightly sealing windows, doors, 
wall construction, continuous vapor barriers, sealing interface between wall and 
window/door frames) will also help to maintain the desired pressure relationships 
between HVAC zones. To ensure that the construction of the building has been per-
formed correctly, building commissioning is recommended throughout the 
construction process and prior to taking ownership to observe construction practices 
and to identify potential airflow trouble spots (cracks, seams, joints, and pores in the 
building envelope and along the lines separating unsecured from secured space) 
before they are covered with finish materials.


6.6.8 Filtration Systems 
To offer effective protection, filtration systems should be specific to the particular 
contaminantÕs physical state and size. Chemical vapor/gas filtration (a.k.a. air 
cleaning) is currently a very expensive task (high initial and recurring costs) with a 
limited number of design professionals experienced in its implementation. Specific 
expertise should be sought if chemical filtration is desired. Possible application of 
the air cleaning approach to collective protection zones (with emergency activation) 
can assist in significantly reducing the cost though the protection is limited to the 
reduced size of the zone. 
Most "traditional" HVAC filtration systems focus on aerosol type contaminants. The 
CBR threats in this category include radioactive "dirty bombs", bio-aerosols, and some 
chemical threats. Riot-control agents and low-volatility nerve agents, for example, are 
generally distributed in aerosol form; however, a vapor component of these chemical 
agents could pass through a filtration system. HEPA filtration is currently considered 
adequate by most professionals to achieve sufficient protection from CBR particulates 
and aerosols. However, HEPA filtration systems generally have a higher acquisition 
cost than traditional HVAC filters and they cause larger pressure drops within the 
HVAC system, resulting in increased energy requirements to maintain the same 
design airflow rate. Due to recent improvements in filter media development, signifi-
cant improvements in aerosol filtration can be achieved at relatively minimal 
increases in initial and operating costs. Also important is that incremental increases 
in filtration efficiency will generally provide incremental increases in protection 
from the aerosol contaminant. 
In 1999, the American Society of Heating, Refrigeration, and Air Conditioning 
Engineers (ASHRAE) released Standard 52.2-1999. This standard provides a system for 
rating filters that quantifies filtration efficiency in different particle size ranges to 
provide a composite efficiency value named the Minimum Efficiency Reporting Value 
(MERV). MERV ratings range between 1 and 20 with a higher MERV indicating a 
more efficient filter. Using the MERV rating table, a desired filter efficiency may be 
selected according to the size of the contaminant under consideration. For example, 
a filter with a MERV of 13 or more will provide a 90% or greater reduction of most 
CBR aerosols (generally considered to be at least 1-3 um in size or larger) within the 
filtered airstream with much lower acquisition and maintenance costs than HEPA 
filtration. 
Efficiency of filtration systems is not the only concern. Air can become filtered only 
if it actually passes through the filter. Thus, filter-rack design, gasketing, and good 
quality filter sources should all play a role in minimizing bypass around the filter. 
The use of return-air filtration systems and the strategic location of supply and 
return systems should also be carefully employed to maximize effective ventilation 
and filtration rates.


6.6.9 Detection Systems 
Beyond the measures discussed above, there is the option of using detection systems 
as part of the protective design package. In general, affordable, timely, and practical 
detection systems specific to all CBR agents are not yet available. However, for aerosol 
contaminants, nonspecific detection equipment can be employed to activate 
response actions should a sudden spike in aerosol concentration of a specific size 
range be detected. If the spike were detected in an outdoor intake for example, this 
could trigger possible response options such as damper closure, system shutdown, 
bypass to alternate air intake, or rerouting the air through a special bank of filters. 
Such protective actions could occur until an investigation was performed by trained 
personnel (i.e., check with adjacent alarms, and review security tape covering outdoor 
air intake). Unless foul play was discovered, the entire process could be completed 
within 10 minutes or less and without alarming occupants. The initial cost of such a 
system is relatively modest (depending upon the number of detectors and response 
options incorporated into the design), but the maintenance requirements are 
relatively high. Similar monitoring systems could be employed to trigger appropriate 
responses in high-threat areas such as mailrooms, shipping/receiving areas, or 
entrance lobbies. The approach could also be expanded to incorporate some of the 
newer chemical detection technologies, though the low threshold requirements may 
generate a substantial number of false positives. As technology progresses, detector 
availability and specificity should continue to expand into the general marketplace. 
It is recognized that at this time, detection systems are not appropriate for many 
buildings. Consider using higher-efficiency filtration systems initially and design 
HVAC systems so that detection systems can be easily integrated into the HVAC 
control package at a later date.


6.6.10 	Emergency Response Using Fire/HVAC Control Center 
Certain operations that are managed at the Fire Control Center can play a 
protective role in the response to a CBR incident. Examples of such operations and 
how they could be used are given below. 
. _	Purge fans. These can be used to purge an interior CBR release or to 
reduce indoor contaminant concentrations following building exposure to an 
external CBR source. (Note: In practice, some jurisdictions may recommend purging 
for chemical and radiological contaminants but not for biological contaminants, 
which may be communicable and/or medically treatable.) 
. _	Communication Systems. Building communication systems that 
allow specific instructions to be addressed to occupants in specific 
zones of the building can play a significant role in directing occu-
pant response to either an internal or external release. 

. _	Pressurization Fans. These provide two functions. First, the ability to 
override and deactivate specific positive-pressure zones may be beneficial in the event 
that a known CBR source is placed into such an area. Second, areas designated for 
positive pressurization (generally for smoke protection) may also become beneficial 
havens for protection from internal and external CBR releases, if they are supplied by 
appropriately filtered air. 
. _	HVAC Controls. The ability to simultaneous and individually 
manipulate operation of all HVAC and exhaust equipment from a single location may 
be very useful during a CBR event. Individuals empowered to operate such controls 
must be trained in their use. 

The provision of simple floor-by-floor schematics showing equipment locations 
and the locations of supply and return louvers will aid the utility of this control 
option. 
_	Elevator Controls. Depending upon their design and operation, the ability to 
recall elevators to the ground floor may assist in reducing contaminant migration 
during a CBR event.


6.6.11 	Evolving Technologies 
Many of the challenges relating to CBR terrorism prevention will be facilitated with 
the introduction of new technologies developed to address this emerging threat. As 
vendors and products come to market, it is important that the designer evaluate 
performance claims with a close level of scrutiny. Vendors should be willing to 
guarantee performance specs in writing, provide proof of testing (and show certified 
results) by an independent, reputable lab, and the testing conditions (e.g., flow rate, 
residence time, incoming concentrations) should be consistent with what would be 
experienced within the ownerÕs building. For CBR developments, proof of federal 
government testing and acceptance may be available.


6.6.12 	Checklist Š Chemical, Biological & Radiological Protective 
Measures 
  Place air intakes servicing occupied areas as high as practically possible 
(minimum 12 feet above ground). GSA may require locating at fourth floor or 
above when applicable. 
  Restrict access to critical equipment.
  Isolate separate HVAC zones and return air systems.
  Isolate HVAC supply and return systems in unsecured areas.
  Physically isolate unsecured areas from secured areas.
  Use positive pressurization of primary egress routes, safe havens, 

and/or other critical areas.   Commission building throughout construction and 
prior to taking 
ownership.   Provide redundant, easily accessible shutdown capabilities.   For 
higher levels of protection, consider using contaminant-specific 
filtration and detection systems.   Incorporate fast-acting, low-leaking dampers.   
Filter both return air and outdoor air for publicly accessible build
ings. 
  Select filter efficiencies based upon contaminant size. Use reputable filter media 
installed into tight-fitting, gasketed, and secure filter racks. 
  For higher threat areas (mail room, receiving, reception/screening lobby): 
. _	Preferably locate these areas outside the main building footprint. 
. _	Provide separate HVAC, with isolated returns capable of 100% exhaust. 
. _	Operate these areas at negative pressure relative to secure portion of 
the building. 
. _	Use air-tight construction, vestibules, and air locks if there is high 
traffic flow. 

_	Consider installation of an emergency exhaust fan to be activated 
upon suspected internal CBR release.   Lock, secure, access-log, and 
control mechanical rooms.   In public access areas, use air diffusers 
and return air grills that are secure or under security observation.   
Zone the building communication system so that it is capable of 
delivering explicit instructions, and has back-up power. 
  Create safe zones using enhanced filtration, tight construction, 
emergency power, dedicated communication systems, and appro-
priate supplies (food, water, first aide, and personal-protective 
equipment).


6.6.13 Further Reading 
Miller, J., 2002, Defensive Filtration, ASHRAE Journal, Vol. 44, No. 12, pp. 18-23, 
December 2002. 
American Society of Heating, Refrigerating, and Air Conditioning Engineers, 2002, 
Risk Management Guidance for Health and Safety under Extraordinary Incidents. ASHRAE 
Winter Meeting Report, January 12, 2002. http://xp20.ashrae.org/ABOUT/ 
Task_Force_RPT_12Jan02.pdf 
Centers for Disease Control and Prevention / National Institute for Occupational 
Safety and Health, 2002, Guidance for Protecting Building Environments from Airborne 
Chemical, Biological, or Radiological Attacks. Publication No. 2002-139, Cincinnati, 
Ohio. www.cdc.gov/ niosh/bldvent/2002-139.html 
Central Intelligence Agency, 1998. Chemical, Biological, Radiological Incident Handbook. 
Central Intelligence Agency, Washington, D.C. 
http://www.cia.gov/cia/publications/cbr_handbook/cbrbook.htm 
American Society of Heating, Refrigerating and Air-Conditioning Engineers, 2003, 
Report of Presidential Ad Hoc Committee for Building Health and Safety under Extraordinary 
Incidents on Risk Management Guidance for Health, Safety and Environmental Security under 
Extraordinary Incidents, Washington, D.C. http://xp20.ashrae.org/about/extraordi-
nary.pdf 
Centers for Disease Control and Prevention / National Institute for Occupational Safety 
and Health, 2003. Guidance for Filtration and Air-Cleaning Systems to Protect Building 
Environments from Airborne Chemical, Biological, and Radiological Attacks, Publication No. 
2003-136, Cincinnati, Ohio. http://www.cdc.gov/niosh/docs/2003-136/2003-136.html