7.5 SEISMIC DESIGN ISSUES
 
The information in this section summarizes the characteristics of retail 
facilities, notes their relationship to achieving good seismic performance, and 
suggests seismic risk management solutions that should be considered. 

Seismic Hazard and Site Issues
 
Unusual site conditions, such as a near-source location, poor soil 
characteristics, or other seismic hazards, may lead to lower performance than 
expected by the code design.  If any of these other suspected conditions are 
geologic hazards, a geotechnical engineering consultant should conduct a 
site-specific study. If defects are encountered, an alternative site should be 
considered (if possible) or appropriate soil stabilization, foundation and 
structural design approaches should be employed to reduce consequences of 
ground motion beyond code design values, or costly damage caused by 
geologic or other seismic hazards (see Chapter 3 for additional information).  If 
possible, avoid sites that lack redundant access and are vulnerable to bridge 
or highway closure. 

Structural System Issues 

Retail facilities are usually one or two stories; mall structures and "big boxes" 
are usually light steel frames or mixed steel frame/wood/con-crete/concrete 
masonry structures.  Reinforced concrete block masonry perimeter walls often 
provide lateral resistance; for these systems, connections of roof diaphragms 
to walls are critical. The large building size and long-span light-frame load 
bearing structures of many of these facilities often lead to large drifts (or 
sway) 
during earthquake shaking.  When designed to code minimums these drifts 
may be excessive and cause nonstructural damage, particularly to ceilings and 
partitions. 

Retail buildings are intrinsically simple in their architectural/structural 
configuration, and basically are large open box-like structures with few interior 
walls and partitions. This enables their structural design to be simple and their 
seismic design can be carried out using the basic equivalent lateral force 
analysis procedures with a good probability of meeting code performance 
expectations as far as life safety is concerned. The desire for low cost, 
however, coupled with a tendency to meet only the minimum code 
requirements, sometimes results in inadequately engineered and poorly 
constructed structures. The protection of non-structural components, systems 
and contents requires structural design to a higher performance level.  
Configuration irregularities are sometimes introduced for image reasons and 
the structural design may become more complex and expensive. 

Nonstructural System Issues 

The extensive use of light-steel-frame structures for retail facilities, together 
with the tendency for them to be designed to minimum codes and standards, 
has resulted in structures that are subject to considerable drift and motion.  
The result has been a high level of nonstructural damage, particularly to 
ceilings and lighting.  This kind of damage is costly and its repair is 
disruptive. 

In most "big box" stores the building structure forms only a weatherproof cover 
and is lightly loaded. Often there is no suspended ceiling and light fixtures are 
hung directly from the building's structure. The merchandise is stacked on 
metal storage racks, which provide vertical and lateral support.  These racks 
are supplied and installed by specialist vendors.  The correct sizing and 
bracing of these racks is critical because the merchandise is often heavy and 
located at a high elevation. Even if the racks remain, material may be displaced 
and fall on the aisles, which are often crowded. 

More upscale department stores have complete suspended ceilings and often 
have elaborate settings for the display of merchandise.  These can be 
hazardous to staff and shoppers. 

Excessive drift and motion (building sway) may also lead to damage to roof-top 
equipment and localized damage to water systems and fire suppression 
piping and sprinklers. 

The responsibilities within the design team for nonstructural component 
support and bracing design should be explicit and clear. The checklist for 
responsibility of nonstructural design in Chapter 12 (see Figure 12-5) provides 
a guide to establishing responsibilities for the design, installation, review and 
observation of all nonstructural components and systems.