BUILDING DESIGN GUIDANCE 3

3-1

BUILDING DESIGN GUIDANCE

This chapter addresses explosive blast and CBR concerns 
from terrorist attacks, highlighting mitigation measures 
that may be applied to building elements, including architectural, 
structural, and building envelope systems. After the site 
design considerations to enhance protection presented in Chapter 
2 have been taken into account (recognizing that many may not 
be applicable to buildings in urban settings), additional building 
design measures, such as hardening, must be considered to protect 
building occupants. That is, when the desired level of protection 
cannot be achieved through site design, building envelope design 
measures must be considered. Catastrophic collapse of the 
building is a primary concern. Historically, the majority of fatalities 
that occur in terrorist attacks directed against buildings are due to 
building collapse. This was true for the Oklahoma City bombing in 
1995 when 87 percent of the building occupants who were killed 
were in the collapsed portion of the Murrah Federal Building; however, 
other threats such as CBR agents should also be considered. 

When considering mitigation measures for explosive blast threats, 
the primary strategy is to keep explosive devices as far away from 
the building as possible (maximize stand-off distance). This is 
usually the easiest and least costly way to achieve a desired level of 
protection. In cases where sufficient stand-off distance is not available 
to protect the building, hardening of the buildings structural 
systems may be required, as well as design to prevent progressive 
collapse. In addition, designers should try to minimize hazardous 
flying debris during an explosive event because a high number 
of injuries can result from flying glass fragments and debris from 
walls, ceilings, and non-structural features. The hardening of the 
building envelope should be balanced so that the columns, walls, 
and windows have approximately equal response for damage and 
injury/casualty for the design basis threat weapon at the available 
stand-off distance. Window design is the element that is usually 
the most diverse in conventional construction. Good blast engineering 
is a multi-disciplinary effort that requires the concerted 
efforts of the architect, structural engineer, mechanical engineer, 
and the other design team members in order to achieve a balanced 
building envelope.

When considering mitigation measures for CBR hazards, the HVAC 
systems are of particular concern. A building can provide protection 
against CBR agents released outdoors if the flow of fresh air is 
filtered or interrupted; however, HVAC systems can also become an 
entry point and distribution system for hazardous contaminants. If 
installed, HVAC air filtration and air-cleaning systems can reduce 
the effects of a CBR agent by removing the contaminants from the 
air within a building. There are a variety of ways to protect building 
occupants from airborne hazards. These protective measures can be 
as simple as defining a protective action plan or as complex as strict 
design measures practical only for new construction.

Building design should be optimized to facilitate emergency 
evacuation, rescue, and recovery efforts through effective placement, 
structural design, and redundancy of emergency exits and 
critical mechanical/electrical systems. Through effective structural 
design, the overall damage levels may be reduced to make it easier 
for people to get out safely and allow emergency responders to 
enter safely. The designer must also balance measures to protect 
people with the requirements of the ADAAG, UFAS, NFPC, and 
all applicable local building codes. 

 The primary focus of this chapter is the protection of buildings 
where the occupants are the primary asset. In this case, the objective 
of the designer is to save lives by mitigating building damages 
and reducing the chances of catastrophic collapse of the building, 
at least until the building is fully evacuated. The measures described 
in this chapter are designed to minimize the loss of life 
through deterrence and detection, as well as strengthening of the 
building against a variety of terrorist tactics. The design team must 
determine which measures are appropriate and cost-effective for 
incorporation into the building design. The measures presented 
here are not all-inclusive, and additional technical information for 
implementation can be found in the referenced documents.


3.1 ARCHITECTURAL

A lot 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. Architectural considerations 
include building configuration, space 
design, and building detailing. It 
is recommended that architects be 
brought into the design process as 
early as site selection, to optimize 
the protection provided. FEMA 430 
contains an expanded discussion of 
incorporating security components in 
architectural design.

3.1.1 Building 
Configuration

The vertical or horizontal profile of 
a building has implications for its 
protection. As with the discussion of 
clustered versus dispersed buildings, 
designers should balance a number 
of relevant considerations to the extent 
that site, economic, and other 
factors allow. Some of the relevant 
considerations include the following:

Low, Large-footprint Buildings:

. Distribute people, assets, and operations across a wider area, 
to limit damage 

. Use vegetation, terrain, and other screening elements to 
protect from hostile surveillance 

. Maximize the benefits of green roof technologies, which can help 
reduce a buildings heat signature and lower its visual profile

Leadership in Energy and Environmental Design (LEED)

As with CPTED, some of the Leadership in Energy and Environmental 
Design (LEED) concepts complement security concerns and others conflict 
with physical security principles. The LEED Green Building Rating System 
represents the U.S. Green Building Councils (USGBCs) effort to provide 
a national standard for what constitutes a green building. Through 
its use as a design guideline and third-party certification tool, it aims to 
improve occupant well-being, environmental performance, and economic 
returns of buildings using established and innovative practices, standards, 
and technologies.1 LEED is a voluntary building assessment tool that is 
most applicable to commercial, institutional, and high-rise residential 
construction. Owners, architects, and engineers must work together to strike 
a balance between building design objectives.

LEED looks at six basic categories: Sustainable Sites, Water Efficiency, 
Energy and Atmosphere, Materials and Resources, Indoor Environmental 
Quality, and Innovation and Design Process. Within each category, points 
are awarded for achieving specific goals. A total of 69 points is possible. A 
score of 26-32 points achieves basic certification; 33-38 achieves Silver; 39-
51 achieves Gold; and 52-69 points achieve Platinum certification. The LEED 
rating is awarded after the project has been documented by the USGBC.

Another goal in the LEED effort is to encourage more sustainable construction 
practices. LEED encourages manufacturers to provide materials that:

. contain high recycled content and sustainable use raw materials

. are manufactured close to the construction site

. have low volatile organic compound emissions

. are designed to minimize energy consumption and packaging

1 U.S. Green Building Council, LEED Green Building Rating System for New 
Construction & Major 
Renovations Version 2.1, November 2002. http://www.usgbc.org

. Require the use of additional measures (at additional cost) 
to prevent introduction of CBR agents due to easier access to 
HVAC intakes by intruders

Tall, Small-footprint Buildings:

. Suffer damage to a greater percentage of their faades, 
structures, and interiors at best, and catastrophic damage or 
collapse at worst, should a large blast occur near the building if 
not constructed with progressive collapse prevention in mind

. Elevate occupied areas above vegetation, terrain, and other 
screening elements, making it potentially more difficult to 
protect interior spaces from outside surveillance

. Minimize the amount of impervious surface, contributing to 
a reduction in stormwater runoff, which reduces the need 
for culverts, drainage pipes, manholes, and other covert site 
access and weapon concealment opportunities

. Provide greater opportunity to elevate HVAC intakes to 
prevent the introduction of CBR agents

The shape of the building may also contribute to the overall 
damage to the structure. For example, U or L shaped buildings 
tend to trap shock waves, which may exacerbate the effect of 
explosive blasts. For this reason, it is recommended that re-entrant 
corners be avoided. In general, convex rather than concave shapes 
are preferred for the exterior of the building. For example, circular 
buildings act to reduce the air-blast pressures because the 
angle of incidence of the shock wave increases more rapidly than 
in a rectangular building. The following design considerations are 
recommended:

. Reduce a buildings vulnerability to attack by using earth-
sheltered design 

. Orient buildings horizontally rather than vertically to reduce 
the buildings profile and exposure

. Place the ground floor elevation of a building at 4 feet above 
grade to prevent vehicle ramming

. Avoid eaves and overhangs, because they can be points of high 
local pressure and suction during blasts; when these elements 
are used, they should be designed to withstand blast effects

. Orient glazing perpendicular to the primary facade to reduce 
exposure to blast and projectiles (see Figure 3-1)

. Avoid exposed structural elements (e.g., columns) on the 
exterior of the facility

. Provide pitched roofs to allow deflection of launched 
explosives

. Avoid re-entrant corners on the building exterior where blast 
pressures may build up (see Figure 3-2)

 

SOURCE: U.S. AIR FORCE, INSTALLATION FORCE PROTECTION GUIDE


Figure 3-1 Glazed areas perpendicularly oriented away from streets





3.1.2Space Design

Unsecured areas should be physically separated from the main 
building to the extent possible. For example, a separate lobby 
pavilion or loading dock outside the main footprint provides 
enhanced protection against damages and potential building collapse 
in the event of an explosion. Similarly, placing parking areas 
outside the main footprint of the building can be highly effective 
in reducing the vulnerability to catastrophic collapse.

The protection of the building interior can be divided into two categories: 
functional layout and structural layout. In terms of functional 
layout, public areas such as the lobby, loading dock, mail room, garage, 
and retail areas need to be separated from the more secured 
areas of the facility. This can be done by creating internal hard 
lines or buffer zones, using secondary stairwells, elevator shafts, corridors, 
and storage areas between public and secured areas.

In lobby areas, the architect would be wise to consider the 
queuing requirements in front of the 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.


SOURCE: U.S. AIR FORCE, INSTALLATION FORCE 
PROTECTION GUIDE

Figure 3-2 Re-entrant corners in a floor plan


Emergency functions (e.g., sprinkler systems and generators, 
which are critical for mitigating the effects of an explosion) and 
elevator shafts should be placed away from internal parking areas 
and loading docks. 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 caused smoke inhalation 
injuries. When it is not possible to separate mechanical areas and 
parking, the walls need to be designed to resist explosive forces. 
The following design measures should be considered:

. Do not collocate high-risk facilities with lower risk tenants. For 
example, a post office or supply center/room should not be 
located in the same building as a childcare facility.

. Locate key assets as far into the interior of a building as possible.

. Place areas of high visitor activity away from key assets.

. Locate critical assets in spaces that are occupied 24 hours 
per day.

. Locate assets in areas where they are visible to more than one 
person.

. Eliminate hiding places within the 
building.

. Use interior barriers to differentiate 
levels of security within a building.

. Stagger doors located across from 
one another in interior hallways to 
limit the effects of a blast through a 
structure (see Figure 3-3).

. Provide foyers with reinforced 
concrete walls, and offset interior 
and exterior doors.

. Consider methods to facilitate the 
venting of explosive forces and 
gases from the interior spaces to the 

SOURCE: U.S. AIR FORCE, INSTALLATION FORCE PROTECTION GUIDE


Figure 3-3 Offset doors through foyer



outside of the structure. Examples of such methods include the 
use of blow-out panels and window system designs that provide 
protection from blast pressure applied to the outside, but that 
readily fail and vent if exposed to blast pressure on the inside.

. Physically isolate lobbies, mailrooms (includes various mail 
processing areas), loading docks, and other entry and storage 
areas from the rest of the building. These are areas where bulk 
quantities of CBR agents are likely to enter a building. Building 
doors, including vestibule and loading dock doors, should 
remain closed when not in use.

. Design buildings so that lobbies, mailrooms, and loading docks 
do not share a return-air system or return pathway (e.g., ceiling 
plenum) with other areas of the building. Some of these 
measures are more feasible for new construction or buildings 
undergoing major renovation. 

3.1.3 Other Design Considerations

When designing high-risk buildings, engineers and architects 
should consider the following:

. Safe havens. The innermost layer of protection within a 
physical security system is the safe haven. Safe havens are 
not intended to withstand a disciplined, paramilitary attack 
featuring explosives and heavy weapons. The safe haven 
should be designed such that the time attackers need to 
penetrate the protected area is greater than the time that first 
responders need to reach the protected area. For additional 
information on safe havens, see FEMA 428, Primer to Design 
Safe School Projects in Case of Terrorist Attacks.

. Office locations. Offices considered to be high risk (more 
likely to be targeted by terrorists) should be placed or glazed 
so that the occupants cannot be seen from an uncontrolled 
public area such as a street. Whenever possible, these spaces 
should face courtyards, internal sites, or controlled areas. 
If this is not possible, suitable obscuring glazing or window 
treatment should be provided, including ballistic-resistant 
glass, blast curtains, or other interior protection systems.

. Mixed occupancies. High-risk tenants should not be housed 
with low-risk tenants. Terrorists may identify some targets 
based on their symbology, visibility, ideology, political views, 
potential for publicity, or simply the consequences of their loss. 

. Public toilets and service areas. Public toilets, service spaces, 
or access to vertical circulation systems should not be located 
in any non-secure areas, including the queuing area before 
visitor screening at the public entrance.

. Retail uses in the lobby. Retail and other mixed uses, which have 
been encouraged in public buildings by the Public Buildings 
Cooperative Use Act of 1976, create spaces that are open and 
inviting. Although important to the public nature of the buildings, 
the presence of retail and other mixed uses may present a 
risk to buildings and their occupants and should be carefully considered 
on a project-specific basis during project design. In areas 
exposed to potential terrorist attacks, retail and mixed uses may 
be accommodated through such means as separating entryways, 
controlling access, and hardening shared partitions, as well as 
with special security operational countermeasures.

. Stairwells. Stairwells required for emergency egress should be 
located as remotely as possible from areas where blast events 
might occur and, wherever possible, should not discharge into 
lobbies, parking, or loading areas. 

. Mailroom. The mailroom should be located away from facility 
main entrances areas containing critical services, utilities, 
distribution systems, and important assets. In addition, the 
mailroom should be located at the perimeter of the building 
with an outside wall or window designed for pressure relief. It 
should have adequate space for explosive disposal containers. 
An area near the loading dock is a preferred mailroom 
location. Where these rooms are located in occupied areas 
or adjacent to critical utilities, walls, ceilings, and floors, they 
should be blast- and fragment-resistant. Significant structural 
damage to the walls, ceilings, and floors of the mailroom 
is acceptable; however, the areas adjacent to the mailroom 
should not experience severe damage or collapse.





. Non-structural elements. False ceilings, light fixtures, venetian 
blinds, ductwork, air conditioners, and other equipment 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; 
using curtains rather than venetian blinds, and using exposed 
ductwork as an architectural device are others.

3.2 BUILDING STRUCTURAL AND NON-
STRUCTURAL SYSTEMS

3.2.1 Building Design to Achieve a Desired 
Protection Level

The assessment process described in Chapter 1 determines the 
level of protection sought for the building structure and defines 
the threat/hazard specific to the facility. Explosive blast threats 
usually govern building structural design for high-risk buildings. 
A structural engineer should determine the building design features 
needed to achieve the desired level of protection against the 
design blast threat, considering collapse of the building, as well as 
incipient injuries and fatalities. In addition, Chapter 4 discusses 
other structural systems related to explosive blast.

3.2.2 Progressive Collapse

Progressive collapse is a situation where local failure of a primary 
structural component leads to the collapse of adjoining members, 
which, in turn, leads to additional collapse. Hence, the total 
damage is disproportionate to the original cause. Progressive collapse 
is a chain reaction of structural failures that follows from 
damage to a relatively small portion of a structure. Information on 
progressive collapse can also be found in FEMA 427, Primer for Design 
of Commercial Buildings to Mitigate Terrorist Attacks.

All buildings should be designed with the intent of reducing the 
potential for progressive collapse as a result of an abnormal loading 
event, regardless of the required level of protection. The following 
structural characteristics (from GSA Progressive Collapse Analysis and 
Design Guidelines for New Federal Office Buildings and Major Modernization 
Projects, November 2000) should be considered in the initial 
phases of structural design. Incorporation of these features will 
provide a more robust structure and decrease the potential for progressive 
collapse. Designers should consider the following:

. Redundancy. The use of redundant lateral and vertical force 
resisting systems is highly encouraged when considering 
progressive collapse. Redundancy tends to promote a more 
robust structure and helps to ensure that alternate load paths 
are available in the case of a structural element(s) failure. 
Additionally, redundancy provides multiple locations for 
yielding to occur, which increases the probability that damage 
will be constrained.

. The use of ductile structural elements and detailing. It is 
critical that both the primary and secondary structural 
elements be capable of deforming well beyond the elastic 
limit, without experiencing structural collapse. The use 
of ductile construction materials (i.e., steel, cast-in-place 
reinforced concrete, etc.) for both the structural elements 
and connection detailing is encouraged. The capability of 
achieving a ductile response is imperative when considering 
an extreme redistribution of loading such as that encountered 
in structural element(s) failure.

. Capacity for resisting load reversals. Both the primary and 
secondary structural elements should be designed to resist 
load reversals in case of a structural element(s) failure.

. Capacity for resisting shear failure. Primary structural 
elements maintain sufficient strength and ductility under an 
abnormal loading event to preclude a shear failure. If the 
shear capacity is reached before flexural capacity, the sudden, 
non-ductile failure of the element could potentially lead to a 
progressive collapse of the structure.

Both the GSA and DoD take a threat-independent approach to 
progressive collapse. The goal of a threat-independent approach 
is not to prevent collapse from a specific design threat, but to 
control and stop the continuing spread of damage after localized 
damage or localized collapse has occurred.

The GSA and DoD require that the structural response of a 
building be analyzed in a test that removes a key structural element 
(e.g., vertical load carrying column, section of bearing wall, 
beam, etc.) to simulate local damage from an explosion. If effective 
alternative load paths are available for redistributing the 
loads, originally supported by the removed structural element, the 
building has a low potential for progressive collapse. The details 
of the GSA and DoD guidelines and criteria can be found in GSA 
Progressive Collapse Analysis and Design Guidelines for New Federal 
Office Buildings and Major Modernization Projects (November 2000) 
and DoD Unified Facilities Criteria (UFC) 4-010-01 (31 July 2002). 
Although these criteria provide specific guidance on which structural 
elements must be analyzed for removal from the structural 
design configuration, they do not provide specific guidance for 
choosing an engineering structural response model for verifying 
the effectiveness of alternate load paths. 

Several other design codes and guidelines in use throughout the 
world (notably in the United Kingdom and Sweden) require some 
form of analysis or measures to reduce the potential for progressive 
collapse. However, there is no specific engineering design 
method prescribed for the structural design process to prevent 
progressive collapse. Unless a building is being designed to meet 
the GSA or DoD criteria, it is up to the owner and the design team 
to decide how much progressive collapse analysis and mitigation 
to incorporate into their design. 

To address blast resistance (Chapter 4 contains a detailed discussion 
of explosive blast theory) and to minimize the possibility of 
progressive collapse, the priority of upgrades should be based on 
the relative importance of a structural or non-structural element, 
in the order below:

. Primary structural elements. These are the essential parts 
of the buildings resistance to catastrophic blast loads and 
progressive collapse (e.g., columns, girders, roof beams, and 
the main lateral resistance system).

. Secondary structural elements. These include all other load 
bearing members (e.g., floor beams, slabs, etc.).

. Primary non-structural elements. These are the elements 
(including their attachments) that are essential for life safety 
systems or elements that can cause substantial injury if failure 
occurs (e.g., ceilings or heavy suspended mechanical units).

. Secondary non-structural elements. These include elements 
not covered in primary non-structural elements (e.g., 
partitions, furniture, and light fixtures).

Priority should be given to the critical elements that are essential 
to mitigating the extent of collapse. Designs for secondary structural 
elements should minimize injury and damage. Consideration 
should be given to reducing damage and injury from primary as well 
as secondary non-structural elements. For example, if an explosive 
event causes the local failure of one column, which results in major 
collapse within a structural bay, a design that mitigated progressive 
collapse would preclude the additional loss of primary structural 
members beyond this localized damage zone (i.e., the loss of additional 
columns, main girders, etc.). This would not necessarily 
preclude the additional loss of secondary structural or non-structural 
elements outside the initial zone of localized damage, provided 
the loss of such members is acceptable for that performance level 
and the loss does not precipitate the onset of progressive collapse.

3.2.3 Loads and Stresses

Structures should be designed to resist blast loads. The DoD designates 
the level of blast protection a building must meet based on 
how many occupants it contains and its function. The demands on 
the structure will be equal to the combined effects of dead, live, 
and blast loads. Blast loads or dynamic rebound may occur in directions 
opposed to typical gravity loads.

For purposes of designing against progressive collapse, loads 
should be defined as dead load plus a realistic estimate of actual 
live load. The value of the live load may be as low as 25 percent 
of the code-prescribed live load. The design should use ultimate 
strengths with dynamic enhancements based on strain rates. Allowable 
responses are generally post elastic.

3.2.4 Good Engineering Practice Guidelines

The following guidelines are commonly used to mitigate the effects 
of blast on structures and to mitigate the potential for progessive 
collapse. Details and more complete guidance are available in the 
references below. These guidelines are not meant to be complete, 
but are provided to assist the designer in the initial evaluation and 
selection of design approaches. For higher levels of protection from blast, 
cast-in-place reinforced concrete is normally the construction type of 
choice. Other types of construction such as properly designed and 
detailed steel structures are also allowed. Several material and construction 
types, although not disallowed by these criteria, may be 
undesirable and uneconomical for protection from blast. 

. Consider incorporating internal 
damping into the structural 
system to absorb the blast impact. 

. The use of symmetric 
reinforcement can increase the 
ultimate load capacity of the 
structure. 

. Consider wire mesh in plaster to 
reduce the incidence of flying 
fragments.

. Avoid the use of masonry when 
blast is a threat. Masonry walls 
break up readily and become 
secondary fragments during 
blasts. 

The following additional references are recommended:

. Biggs, John M. Introduction to Structural Dynamics. McGraw-Hill. 
(1964).

. The Institute of Structural Engineers. The Structural Engineers 
Response to Explosive Damage. SETO, Ltd., 11 Upper Belgrave Street, 
London SW1X8BH. (1995).

. Mays, G.S. and Smith, P.D. Blast Effects on Buildings: Design of 
Buildings to Optimize Resistance to Blast Loading. Thomas Telford 
Publications, 1 Heron Quay, London E14 4JD. (1995).

. National Research Council. Protecting Buildings from Bomb Damage. 
National Academy Press. (1995). 


. The use of multiple barrier materials and construction 
techniques can sometimes accomplish the same goal with less 
expense than a single material or technique.

. The designer should recognize that components might act in 
directions for which they were not designed. This is due to 
the engulfment of structural members by blast, the negative 
phase, the upward loading of elements, and dynamic rebound 
of members. Making steel reinforcement (positive and negative 
faces) symmetric in all floor slabs, roof slabs, walls, beams, 
and girders will address this issue. Symmetric reinforcement 
also increases the ultimate load capacity of the members.

. Lap splices should fully develop the capacity of the 
reinforcement.

. Lap splices and other discontinuities should be staggered.

. Deflections around certain members, such as windows, should 
be controlled to prevent premature failure. Additional reinforcement 
is generally required.

. In general, column spacing should be minimized so that reasonably 
sized members can be designed to resist the design 
loads and increase the redundancy of the system. A practical 
upper level for column spacing is 30 feet for the levels of blast 
loads described herein.

. In general, floor to floor heights should be minimized. Unless 
there is an overriding architectural requirement, a practical 
limit is generally less than or equal to 16 feet.

. It is recommended that the designer use fully grouted and 
reinforced concrete masonry unit (CMU) construction when 
CMU is selected.

. It is essential that the designer actively coordinate structural 
requirements for blast with other disciplines, including 
architectural and mechanical.


. The use of one-way wall elements spanning from floor-to-
floor is generally a preferred method to minimize blast loads 
imparted to columns.

. In many cases, the ductile detailing requirements for seismic 
design and the alternate load paths provided by progressive 
collapse design assist in the protection from blast. The 
designer must bear in mind, however, that the design 
approaches are, at times, in conflict. These conflicts must be 
worked out on a case by case basis.

. It is recommended that architectural or structural features 
be used that deny contact with exposed primary vertical load 
members. A minimum stand-off of at least 6 inches from these 
members is required.

3.2.5 Building Materials

All building materials and types acceptable under model 
building codes are allowed; however, special consideration 
should be given to materials that have inherent flexibility and 
that are better able to respond to load reversals (i.e., cast in 
place reinforced concrete and steel construction). Careful detailing 
is required for material such as pre-stressed concrete, 
pre-cast concrete, and masonry (brick and concrete masonry 
unit) to adequately respond to the design loads. The construction 
type selected must meet all performance criteria of the 
specified level of protection.

3.2.6 Methods and References

All building components requiring blast resistance should be designed 
using established methods and approaches for determining 
dynamic loads, structural detailing, and dynamic structural response. 
Design and analysis approaches should be consistent with those in 
the technical manuals or the GSA Security Design Criteria and the 
American Society of Civil Engineers Minimum Design Loads for Buildings 
and Other Structures, ASCE 7. Alternative analysis and mitigation 
methods are permitted, provided that the performance level is attained. 
A peer group should evaluate new and untested methods.



3.3 BUILDING ENVELOPE

3.3.1 Building Exterior

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 the most vulnerable to an exterior 
explosive threat because it is the part of the building closest 
to the weapon. 

It also is a critical line of defense for protecting the occupants 
of the building from CBR threats. Significant quantities of air 
can enter a building by means of infiltration through unintentional 
leakage paths in the building envelope. Such leakage 
is of more concern during an exterior CBR release, such as a 
large-scale attack, than for a directed terrorist act. The reduction 
of air leakage is a matter of tight building construction in 
combination with building pressurization. Although building 
pressurization may be a valuable CBR-protection strategy in 
any building, it is much more likely to be effective in a tight 
building. However, to be effective, filtration of building supply 
air must be appropriate for the CBR agent introduced. Although 
increasing the air tightness of an existing building can 
be more challenging than during new construction, it should 
still be seriously considered.

The design philosophy to be used here is that simpler is better. 
Generally, simple geometries, with minimal ornamentation 
(which may become flying debris during an explosion) are recommended. 
If ornamentation is used, it is recommended that it 
consist of lightweight material such as timber or plastic, which is 
less likely to become a projectile in the event of an explosion than, 
for example, brick, stone, or metal.

Soil can be highly effective in reducing the impact of a major explosive 
attack. Bermed walls and buried rooftops 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 should 
be permitted on top of the building. 

3.3.2 Exterior Wall Design

The exterior walls provide the first line of defense to prevent air-
blast pressures and hazardous debris from entering the building. 
They would be 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. At a 
minimum, the objective of design is to ensure that these members 
fail in a flexible mode 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 bullet-resistant windows to 
have a higher ultimate capacity than the walls to which they are attached. 
Beyond ensuring a flexible failure mode, the exterior wall 
may be designed to resist the actual or reduced pressure levels of 
the defined threat. 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, reinforced 
CMU block, and metal studs may also be used to achieve lower 
levels of protection. 

For pre-cast panels, consider a minimum thickness of 5 inches with 
two-way reinforcing bars spaced not greater than the thickness of 
the panel. Connections into the structure should provide a straight 
line of load transmittal, using as few connecting pieces as possible.

For CMU block walls, use 8-inch block walls, fully grouted with 
vertical centered reinforcing bars placed in each cell and horizontal 
reinforcement at each layer. Connections into the structure 
should be able to resist the ultimate lateral capacity of the wall. 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 reinforcement.


For metal stud systems, use metal studs back to back and mechanically 
attached, to minimize lateral torsion effects. To catch exterior 
cladding fragments, attach a wire mesh to the exterior side of the 
metal stud system. The supports of the wall are to be designed to 
resist the ultimate lateral capacity load of the system.

Exterior Walls. For the design of exterior walls, consider the 
following:

. Exterior walls should resist the actual pressures and impulses 
acting on the exterior wall surfaces from the threats defined 
for the facility.

. Exterior walls should be capable of withstanding the dynamic 
reactions from the windows.

. Shear walls that are essential to the lateral and vertical load 
bearing system, and that also function as exterior walls, should 
be considered primary structures. Design exterior shear walls to 
resist the actual blast loads predicted from the threats specified.

. Special consideration should be given to construction types 
that reduce the potential for collapse where exterior walls are 
not designed for the full design loads.

. By U.S. military standards (per Army Technical Manual 5-853), 
a medium protection level for walls would be the equivalent 
of 4-inch concrete with #5 reinforcing steel at 6-inch intervals 
each way or 8-inch CMU with #4 reinforcing steel at 8-inch 
intervals. TM 5-853 provides other alternatives for low, 
medium, and high protection.

Cladding and Finishes. Designers should consider the following:

. Substitute strengthened building elements and systems when 
stand-off distances cannot be accommodated.

. Use ductile materials capable of very large plastic 
deformations without complete failure.

. Provide blast-resistant walls when a high threat is present.

. Consider use of sacrificial exterior wall panels to absorb blast.

. Use earthtone-colored materials and finishes on exterior 
surfaces to diminish the prominence of a building.

. Consider reinforced concrete wall systems in lieu of masonry 
or curtain walls to minimize flying debris in a blast.

. Reinforced wall panels can protect columns and assist in 
preventing progressive collapse, because the wall will assist in 
carrying the load of a damaged column.

Exterior Column Design. Exterior columns should be designed 
to resist the peak reflected pressure of an explosive event, including 
the load transmitted by supported walls. Because these 
elements are slender, the air-blast tends to wash around them 
if they are free standing, reducing the duration of the loading. 
This is referred to as a clearing time effect. Although it is conservative 
to neglect these effects, it may be advantageous to take 
them into account in some situations, like close-in explosions. 
The design must provide for both the axial and flexible loads 
and stability. For concrete columns, spiral reinforcing or closed 
stirrups should be placed at a spacing of not less than half the 
minimum thickness of the member to ensure that adequate 
confinement and ductility are provided. For steel columns, the 
connections are the most vulnerable part of the design. If possible, 
place the base plate below ground level and protect with 
concrete and use multi-story segments for the lower floors.

3.3.3 Window Design

Window systems on the exterior faade of a building should be designed 
to mitigate the hazardous effects of flying glass during an 
explosion event. Designs should integrate the features of the glass, 
connection of the glass to the frame (bite), and anchoring of the 
frame to the building structure to achieve a balanced design. 
This means all the components should have compatible capacities 
and theoretically would all fail at the same pressure-pulse levels. 
In this way, the damage sequence and extent of damage are controlled. 
Table 3-1 presents six GSA glazing protection levels based 
on how far glass fragments would enter a space and potentially injure 
its occupants. Figure 3-4 depicts how far glass fragments could 
enter a structure for each GSA performance condition. 

The divide between performance conditions 3a and 3b can be 
equated to the threshold of injury. The divide between performance 
conditions 4 and 5 can be equated to the threshold of 
lethality. The GSA glazing performance conditions shown below 
correlate with the DoD levels of protection presented in Table 3-2.
Glass Design. Four types of glass are commonly used in window 
glazing systems: annealed glass, heat strengthened glass, fully 
thermally tempered glass, and polycarbonate. Other types of glass 
materials exist, but are not commonly used in typical commercial 
window systems. Of the four common types, annealed glass and 
fully thermally tempered glass are the type of windows for most office 
buildings. 

Table 3-1: Glazing Protection Levels Based on Fragment Impact Locations1

PerformanceCondition

ProtectionLevel

HazardLevel

Description of WindowGlazing Response

1

Safe

None

Glazing does not break. No visible damage to glazing or frame.

2*

Very High

None

Glazing cracks, but is retained by the frame. Dusting or very small 
fragments near sill or on floor acceptable. 

3a*

High

Very Low

Glazing cracks. Fragments enter space and land on floor no more than 3.3 
feet from the window.

3b*

High

Low

Glazing cracks. Fragments enter space and land on floor no more than 10 
feet from the window.

4*

Medium

Medium

Glazing cracks. Fragments enter space and land on floor and impact a 
vertical witness panel at a distance of no more than 10 feet from the 
window at a height no greater than 2 feet above the floor.

5*

Low

High

Glazing cracks and window system fails catastrophically. Fragments enter 
space, impacting a vertical witness panel at a distance of no more than 10 
feet from the window at a height greater than 2 feet above the floor.


* In conditions 2, 3a, 3b, 4 and 5, glazing fragments may be thrown to the 
outside of the protected space toward the detonation location.

1 From GSA PBS-PQ100.1, Facilities Standards for the Public Building Service, 
June 14, 1996


Annealed glass, also known as float, plate, or sheet glass, is the 
most common glass type used in commercial construction. Annealed 
glass is of relatively low strength and upon failure, fractures 
into razor sharp, dagger-shaped fragments (see Figure 3-5). 

Fully thermally tempered glass (TTG) is typically four to five times 
stronger than annealed glass with a design stress of 16,000 psi 
(112,000 kPa). The fracture characteristics of tempered glass are 
superior to those of annealed glass. Upon failure, it will eventu



Figure 3-4 Side view of a test structure illustrating performance 
conditions of Table 3-2

Table 3-2: 
Correlation of GSA Glazing Performance Conditions and DoD Levels of Protection 
for New Buildings

GSA Glazing Performance Condition

Corresponding DoD Level of Protection 
for New Buildings

1

High

2

Medium

3a

Low

3b/4

Very Low

5

Below antiterrorism (AT) Standards


ally fracture into small cube-shaped fragments. Breakage patterns 
of side and rear windows in American automobiles are a good 
example of the failure mode of thermally tempered glass. Current 
building codes generally require thermally tempered glass anywhere 
the public can physically touch the glass such as entrance doors and 
sidelights. Although thermally tempered glass exhibits a relatively 
safe failure mode for conventional usage, failure under blast loading 
still presents a significant health hazard. Results from blast tests 
reveal that, upon fracture, TTG fragments may be propelled into 
cohesive clumps that only fragment upon impact into smaller rock 
salt-type fragments. Even if the tempered glass breaks up initially into 
small fragments, the blast overpressure can propel the fragments at a 
high enough velocity to constitute a severe hazard.

Wire-reinforced glass is a common glazing material. It consists of 
annealed glass with an embedded layer of wire mesh. Its primary 
use is as a fire-resistant and forced entry barrier. Wire-reinforced 
glass has the fracture and low strength characteristics of annealed 


Figure 3-5 An unprotected window subject to a large explosion





glass and, although the wire binds some fragments, it still ejects a 
considerable amount of sharp glass and metal fragments. Wire-reinforced 
glass is not recommended for blast-resistant windows. 

Laminated glass is a pane with multiple glass layers and a pliable 
interlayer material (usually made from polyvinyl butyral (PVB)) 
between the glass layers. Combining interlayer bonding materials 
with layers of glass produces cross-sections that perform well 
against blast, ballistic, and forced entry attacks. The interlayer acts 
as the glue that bonds the multiple layers of glass into a single 
pane of a given thickness and provides a membrane response after 
the glass layers crack under loading. Laminated glass offers significant 
advantages over monolithic glass. It is stronger and, if failure 
occurs, the interlayer material may retain most of the glass fragments. 
Also, if a projectile passes through the glass, most spalling 
glass fragments will be retained. Increased safety for fragment 
retention can be obtained in the event of catastrophic failure from 
an explosive blast by placing a decorative crossbar or grillwork 
on the interior of the glazing. Note, crossbars must be mounted 
across the center of mass of each window pane to be effective. 

Another treatment used for mitigating the effects of an explosive 
attack is security window film. The polyester film used in commercial 
products is commonly referred to as fragment retention film 
(FRF), safety film, security film, protective film, or shatter-resistant 
film. These films are adhered to the interior surface of the window 
to provide fragment retention and reduce the overall velocity 
of the glass fragments at failure. Fragment retention film combines 
a strong pressure sensitive adhesive with a tough polyester 
layer. Four methods are used to attach security film to windows: a 
daylight attachment (applied only to the vision opening); edge-to-
edge attachment (applied out to the edge of the glass); wet-glazed 
attachment (daylight or edge-to-edge film with a silicone bead 
along edge of the frame); and a mechanical attachment. Because 
fragment retention film applies directly to the glass surface of a 
window pane, it is beneficial for retrofitting existing windows as 
well as on new windows. 


Fragment retention film behaves similarly to relatively thin laminated 
and polycarbonate glazing in terms of fragmentation. It is 
available in common thicknesses of 2, 4, 7, and 10 mils. Fragment 
retention film improves the performance of the glass under blast 
loading to varying degrees, depending on the thickness, quality, 
and type of film installation. The best performance is achieved 
when the film is installed into the bite of the glazing or is connected 
to the frame. Fragment retention film can also provide 
solar control benefits. As with laminates, increased safety can be 
obtained with window films by placing a decorative crossbar or 
grillwork on the interior of the glazing.

Thermoplastic polycarbonates are very strong and suitable for 
blast- and forced entry-resistant window design. Monolithic polycarbonate 
is available in thicknesses up to 1/2 inch, but can be fused 
together to obtain any thickness needed. However, polycarbonate 
is expensive and subject to environmental degradation (especially 
from exposure to aromatic hydrocarbons) and abrasion. Local 
building codes should be consulted when considering polycarbonates. 
There are several fire safety issues associated with its use 
(thermoplastic polycarbonate is rated as a class CC-1 material and 
will often test with a smoke density rating over 500). Additionally, 
because of its strength, local fire codes may require a percentage of 
polycarbonate glazing to pop out for emergency egress.

Frame and Anchorage Design. Window frames need to retain the 
glass so that the entire pane does not fall out and also should 
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 (i.e., silicone or polyvinyl butyral) 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. In some applications (e.g., the lobby area 
where large panes of glass are used), a larger bite with more 
structural sealant may be needed. 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; however, in some applications, 
steel frames are used. 

Mullion and Wall Design. The frame members connecting adjoining 
windows are referred to as mullions. These members may 
be designed using a static approach when the breaking strength 
of the window glass is applied to the mullion, or a dynamic load 
may be applied using the peak pressure and impulse values. 
Although the static approach may seem easier, it often yields 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. In addition, it may not be consistent with the overall 
architectural objectives of the project. A dynamic approach is 
likely to provide a section that meets the design constraints of the 
project. To accomplish this, a single-degree-of-freedom solution 
is often used. The governing equation of motion may be solved 
using numerical methods. There are also charts available for linearly 
decaying loads that circumvent the need to solve differential 
equations. These charts only require that the fundamental period 
of the mullion (including the tributary area of the window glass), 
the ultimate resistance force of the mullion, the peak pressure, 
and the equivalent linear decay time are known.

Peak lateral response of the mullion is to be limited to a 2-degree 
support rotation. Also, the displacement ductility is to be limited to a 
4-degree support rotation. As with frames, it is good engineering practice 
to limit the number of interlocking parts used for the mullion. 

A similar approach may be used for checking the response of 
the supporting wall response. It makes no sense to have blast 
mitigating windows if they are stronger than the wall that they 
are anchored into. The maximum strength of any window and 
anchorage system should be equal to the wall strength. This becomes 
particularly important in the design of ballistic-resistant 
and forced entry mitigating windows, which consist of one or 
more inches of glass and polycarbonate. These windows can easily 
become stronger than the supporting wall. In some applications, 
even the use of tempered glass can become problematic.


Design up to Specified Load. Window systems design (glazing, 
frames, anchorage to supporting walls, etc.) on the exterior 
facade should be balanced to mitigate the hazardous effects 
of flying debris in an explosive event. The walls, anchorage, 
and window framing should fully develop the capacity of the 
glazing material selected. The designer may use a combination 
of methods such as government produced and sponsored computer 
programs (e.g., Window Lite Analysis Code (WINLAC), 
Safety Viewport Analysis Code (SAFEVU), Blast-Resistant Window 
Program (BLASTOP), and Window Glazing Analysis Response 
and Design (WINGARD)) coupled with test data and recognized 
dynamic structural analysis techniques to show that the glazing 
either survives the specified threats or the post damage performance 
of the glazing protects the occupants in accordance with 
the conditions specified in Table 3-2. In general, laminated glass 
is the preferred glazing material for new construction. Tests 
have shown that laminated glass performs well under blast loads 
if mounted in properly designed window frames and it can be 
engineered to offer the highest levels of protection from glass 
fragments. 

Limitation of Glass Hazard Mitigation. Keep in mind that the 
pressures exerted on a building in a large explosion (e.g., a truck 
bomb) are often significantly greater than the pressures for which 
protected windows are designed. For these large events, the upgraded 
solutions may not be effective, except for windows on the sides of the 
building not facing the explosion or adjacent buildings. This is particularly 
true if structural damage occurs. Flying debris generated by 
structural damage typically causes more severe injuries than window 
damage alone; however, blast mitigating window designs are expected 
to be effective for a large number of threats where the pressures 
are low. Two such scenarios include a package bomb near the 
building and a truck bomb that goes off a block away.

Although these solutions do provide protection at modest pressure 
levels, they are not a magic shield. The threat of attack still 
exists and injuries may still occur if an attack occurs. These measures 
will be most effective if considered a last resort measure 
used in conjunction with a full range of physical and operational 
security measures at the facility.

General Guidelines for Windows and Glazing. General guidelines 
for windows and glazing include the following:

. Do not place windows adjacent to doors because, if the 
windows are broken, the doors can be unlocked.

. Minimize the number and size of windows in a facade. If 
possible, limit the amount of glazed area in building facades 
to 15 percent. The amount of blast entering a space is directly 
proportional to the amount of openings on the facade.

. Consider using burglary- and ballistic-resistant glazing in high-
risk buildings.

. Consider using laminated glass in place of conventional glass.

. Consider window safety laminate (such as mylar) or another 
fragment retention film over glazing (properly installed) to 
reduce fragmentation.

. Consider placing guards, such as grills, screens, or meshwork, 
across window openings to protect against covert entry. Affix 
protective window guards firmly to the structure.

. Consider installing blast curtains, blast shades, or spall shields 
to prevent glass fragments from flying into the occupied space.

. Consider curtains, blinds, and shades to limit entry of 
incendiary devices.

. Consider narrow recessed windows with sloped sills because they 
are less vulnerable than conventional windows (see Figure 3-6).

. Consider windows with key-operated locks because they provide 
a greater level of protection than windows with simple latches. 
Stationary, non-operating windows are preferred for security. 

. Position the operable section of a sliding window on the inside 
of the fixed section and secure it with a broomstick, metal 
rod, or similar device placed at the bottom of the track. 

. Provide horizontal windows 6 feet above the finished floor to 
limit entry.

. Harden the windows by using steel window frames securely 
fastened or cement grouted to the surrounding structure.

Additional Glazing Requirements. Additional glazing requirements 
include the following:

. Ballistic windows, if required, should meet the requirements 
of Underwriters Laboratory (UL) 752 Bullet-Resistant 
Glazing for a level appropriate for the project. Glassclad 
polycarbonate or laminated polycarbonate are two types of 
acceptable glazing material.

. Security glazing, if required, should meet the requirements of 
the American Society for Testing and Materials (ASTM) F1233 
or UL 972, Burglary-Resistant Glazing Material.

. Glazing should meet the minimum performance specified 
in Table 3-2; however, special consideration should be given 
to frames and anchorages for ballistic-resistant windows and 
security glazing because their inherent resistance to blast may 
impart large reaction.


Figure 3-6 Narrow and recessed windows with sloped sills

SOURCE: U.S. AIR FORCE, INSTALLATION FORCE PROTECTION GUIDE


. Resistance of window assemblies to forced entry (excluding 
glazing) should meet the requirements of ASTM F 588 for a 
grade appropriate for the project.

. Interior glazing should be minimized where a threat exists. 
The designer should avoid locating critical functions next to 
high-risk areas with glazing, such as lobbies, loading docks, etc.

Multi-hazard Considerations. Under normal operating conditions, 
windows function in a variety of ways, including:

. Allowing light into a building

. Conserving energy by reducing thermal transmission 

. Reducing noise through acoustic transmission 

Explosions are one of a number of abnormal loading conditions 
that the building may be designed to mitigate. Some of the others 
are fire, earthquake, hurricane, gunfire, and forced entry.

In 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 functions or other explosion threat scenarios. Some questions 
that may be worthwhile to consider are:

. If an internal explosion occurs, will the upgraded windows 
increase smoke inhalation injuries by preventing the smoke 
from venting through windows that would normally break in 
an explosion event?

. If a fire occurs, will it be more difficult to break protected 
windows in order to vent the building and gain access to the 
injured?

. Will a window upgrade that is intended to protect the 
occupants worsen the hazards to passersby?


3.3.4 Doors

A door system includes the door, frame, and anchorage to the 
building. As part of a balanced design approach, exterior doors 
in high-risk buildings should be designed to withstand the 
maximum dynamic pressure and duration of the load from the 
design threat explosive blast. Other general door considerations 
are as follows:

. Provide hollow steel doors or steel-clad doors with steel frames.

. Provide blast-resistant doors for high threats and high levels of 
protection.

. Limit normal entry/egress through one door, if possible.

. Keep exterior doors to a minimum while accommodating 
emergency egress. Doors are less attack-resistant than adjacent 
walls because of functional requirements, construction, and 
method of attachment. 

. Ensure that exterior doors open outward from inhabited 
areas. In addition to facilitating egress, the doors can be 
seated into the door frames so that they will not enter the 
buildings as hazardous debris in an explosion.

. Replace externally mounted locks and hasps with internally 
locking devices because the weakest part of a door system is 
the latching component. 

. Install doors, where practical, so that they present a blank, flush 
surface to the outside to reduce their vulnerability to attack.

. Locate hinges on the interior or provide concealed hinges to 
reduce their vulnerability to tampering.

. Install emergency exit doors so that they facilitate only exiting 
movement.

. Equip any outward-opening double door with protective 
hinges and key-operated mortise-type locks.

. Provide solid doors or walls as a backup for glass doors in foyers.

. Strengthen and harden the upright surfaces of door jambs.



3.3.5 Roof System Design

For an explosive threat, the primary loading on the roof is downward 
over-pressure. Secondary loads include upward pressure due to the 
blast penetrating through openings and upward suction during the 
negative loading phase. The upward pressures may have an increased 
duration due to multiple reflections of the air-blast internally. It is 
conservative to consider the downward and upward loads separately. 

The preferred system is to use poured-in-place reinforced concrete 
with beams in two directions. If this system is used, beams should 
have stirrups along the entire span spaced not greater than one half 
the beam depths. A second system uses steel frames with a concrete 
and metal deck slab to achieve higher levels of protection. For this 
system to work well, a two-way system of reinforcing bars spaced not 
more than the total thickness of the slab should be provided. Also, 
puddle welds or other robust systems are needed along the perimeter 
to resist the shear forces on the slab as it deflects downward. 

Less desirable systems include metal plate systems without concrete, 
and precast and pre/post tensioned systems. The metal plate system 
is prone to fail due to its light weight and direct pressure, or by the 
negative phase pressure that creates a suction effect on the roof. 
Precast panels are problematic because of the tendency to fail at 
the connections. Pre/post tensioned systems tend to fail in a brittle 
manner if stressed beyond their elastic limit and they also are 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 flexibility needed to 
resist explosion loads.

Flat slab/plate systems are also less desirable because of limited 
two-way action and the potential of shear failure at the columns. 

Miscellaneous Roof System. Designers should consider the following:

. Designing buildings with a sacrificial sloping roof that is above 
a protected ceiling (see Figure 3-7).

. Controlling access to roofs to minimize the possibility of 
aggressors placing explosives or chemical, biological, or 
radiological agents there or otherwise threatening building 
occupants or critical infrastructure. For new buildings, 
eliminate all external roof access by providing access from 
internal stairways or ladders, such as in mechanical rooms. For 
existing buildings, eliminate external access where possible or 
make roof access ladders removable, retractable, or lockable.

. Protecting roof openings to a facility from covert entry by 
installing screens or grates or attaching Intrusion Detection 
System sensors.

3.4 MECHANICAL SYSTEMS

Mechanical system design standards address limiting damage to 
critical infrastructure and protecting building occupants against 
CBR threats. The primary goal of a mechanical system after a 
terrorist attack should be to continue to operate key life safety 
systems. This can be accomplished by locating components in 
less vulnerable areas, limiting access to mechanical systems, and 
providing a reasonable amount of redundancy. Other aspects of 
mechanical systems are discussed in Section 2.10.


Figure 3-7 Sacrificial roof


During an interior bombing event, smoke removal and control 
are of paramount importance. The designer should consider the 
fact that, if window glazing is hardened, a blast may not blow out 
windows, and smoke may be trapped in the building. In the event 
of a blast, the available smoke removal system may be essential 
to smoke removal, particularly in large, open spaces. This equipment 
should be located away from high-risk areas (e.g., garages 
and loading docks). The system controls and power wiring to the 
equipment should be protected, and the system should be connected 
to emergency power to provide smoke removal. Smoke 
removal equipment should be provided with standalone local 
control panels that can continue to individually function in the 
event the control wiring is severed from the main control system.

Designers should consider the following:

. Do not mount plumbing, electrical fixtures, or utility lines 
on the inside of exterior walls, but, when this is unavoidable, 
mount fixtures on a separate wall at least 6 inches from the 
exterior wall face.

. Avoid placing plumbing on the roof slab.

. Avoid suspending plumbing fixtures and piping from the ceiling.

. Reduce the number of utility openings, manholes, tunnels, air 
conditioning ducts, filters, and access panels into the structure.

. Locate utility systems away from likely areas of potential 
attack, such as loading docks, lobbies, and parking areas.

. Protect building operational control areas and utility feeds to 
lessen the negative effects of a blast.

. Design operational redundancies to survive all kinds of attack.

. Use lockable systems for utility openings and manholes where 
appropriate. Infrequently used utility covers/manholes can be 
tack-welded as an inexpensive alternative to locking tamper-
resistant covers.


Key HVAC System Considerations. The following HVAC design 
measures (from Centers for Disease Control and Prevention, National 
Institute for Occupational Safety and Health, Guidance for 
Protecting Building Environments from Airborne Chemical, Biological, 
or Radiological Attacks, May 2002) should be considered to mitigate 
the risk of CBR threats for high security buildings. HVAC protective 
actions are discussed in Chapter 5. 

. Elevating the fresh-air intakes to reduce the potential for 
hazardous materials entering a building from a ground-level 
outdoor release is most easily applied in new construction 
(see Figures 3-8 and 3-9). This has two main benefits. 
The first benefit is that it provides passive security against 
malicious acts, which makes it more difficult for a container of 
hazardous material to be inserted directly into the buildings 
HVAC system and to be conveyed to various parts of the 
building. The second benefit is that it makes it less likely that 
high concentrations of hazardous material will occur at the 
intakes if there is a ground-level release near the building. 

. Locating ground-level intakes near streets or parking areas 
can cause exhaust fumes to be drawn indoors under certain 
conditions of wind and stability (see Figure 3-10). In elevating 
the intakes, the dilution increases with the distance from 
the source. In stable conditions, contaminants released near 
the ground will likely remain close to the ground unless the 
airflow over the building lifts it upward. Contaminants that 
are heavier than air will also tend to remain close to the 
ground under calm conditions.

. Placing intakes at the highest practical level on the building is 
beneficial. For protection against malicious acts, the intakes 
should also be covered by screens so that objects cannot be 
tossed into the intakes or into air wells from the ground (see 
Figure 3-10). Such screens should be sloped to allow thrown 
objects to roll or slide off the screen, away from the intake. 
Many existing buildings have air intakes that are located at or 
below ground level. For those that have wall-mounted or below-
grade intakes close to the building, the intakes can be elevated 

Figure 3-8 Example of protecting outdoor air intakes

SOURCE: CDC/NIOSH, PUBLICATION NO. 2002-139, GUIDANCE FOR PROTECTING BUILDING 
ENVIRONMENTS FROM AIRBORNE 
CHEMICAL, BIOLOGICAL, OR RADIOLOGICAL ATTACKS, MAY 2002


Figure 3-9 
Example of elevated air 
intake

SOURCE: CDC/NIOSH, PUBLICATION NO. 2002-139, GUIDANCE FOR 
PROTECTING BUILDING ENVIRONMENTS FROM AIRBORNE CHEMICAL, 
BIOLOGICAL, OR RADIOLOGICAL ATTACKS, MAY 2002


by constructing a plenum or external shaft over the intake 
(see Figure 3-11). An extension height of 12 feet will place the 
intake out of reach of individuals without some assistance.

. Effectively elevating intakes has practical limits. A plume or cloud 
of hazardous materials can reach the intakes, particularly if the 
source is large and distant. For low-rise buildings (i.e., those 
having a width more than twice the height), a plume originating 
at ground level near the building will travel over the building 
rather than around it; thus, the wind will convey contaminants to 
the top of the building, with some dilution occurring.

. For existing buildings with air intakes below grade, at ground 
level, or wall-mounted outside secure areas, some protection 
can be gained with physical security measures (e.g., placing 
fencing, surveillance cameras, and motion detectors around 
the intakes to facilitate monitoring by security personnel). 
These measures can help prevent malicious acts, but are less 
effective than elevating the intakes, because ground level 
releases under certain conditions can enter the intakes from 
points outside the area fenced or under surveillance.


Figure 3-10 Another example of protecting outdoor air intakes

SOURCE: CDC/NIOSH, PUBLICATION NO. 2002-139, GUIDANCE FOR PROTECTING BUILDING 
ENVIRONMENTS FROM AIRBORNE 
CHEMICAL, BIOLOGICAL, OR RADIOLOGICAL ATTACKS, MAY 2002


. Physical security for mechanical rooms to prevent the direct 
introduction of hazardous materials into the system of ducts 
that distributes air to the building should be maintained. This 
includes locking and controlling the access to all mechanical 
rooms containing HVAC equipment.


Figure 3-11 
Example of enclosing an 
existing vulnerable air 
intake

Ground-level Outdoor Air Intake

Raised-level Outdoor Air Intake

SOURCE: CDC/NIOSH, PUBLICATION NO. 2002-139, GUIDANCE FOR PROTECTING 
BUILDING ENVIRONMENTS FROM AIRBORNE CHEMICAL, BIOLOGICAL, OR RADIOLOGICAL 
ATTACKS, MAY 2002

. Public access to building roofs should be prevented. Access to the 
roof may allow entry to the building and access to air intakes and 
HVAC equipment (e.g., self-contained HVAC units, laboratory or 
bathroom exhausts) located on the roof. From a physical security 
perspective, roofs are like other entrances to the building and 
should be secured appropriately. Roofs with HVAC equipment 
should be treated like mechanical areas. Fencing or other 
barriers should restrict access from adjacent roofs.

. Access to building operation systems by outsiders should be 
restricted. A building staff member should escort maintenance 
workers throughout their service visit and should visually 
inspect their work before final acceptance of the service. 
Alternatively, building managers can ensure the reliability of 
pre-screened service personnel from a trusted contractor.

. Access to information on building operations (including 
mechanical, electrical, vertical transport, fire and life safety, 
security system plans and schematics, and emergency 
operations procedures) should be strictly controlled. Such 
information should be released to authorized personnel 
through the development of an access list and controlled copy 
numbering.

. To prevent widespread dispersion of a contaminant released 
within lobbies, mailrooms, and loading docks, their HVAC 
systems should be isolated and the areas maintained at a 
negative pressure relative to the rest of the building, but at 
positive pressure relative to the outdoors. Physical isolation 
of these areas (well-sealed floor to roof-deck walls, sealed 
wall penetrations) is critical to maintaining the pressure 
differential. It requires special attention to ensure airtight 
boundaries between these areas and adjacent spaces. In some 
building designs (those having lobbies with elevator access, 
for example), establishing a negative pressure differential 
presents a challenge. A qualified mechanical engineer can 
assist in determining if the recommended isolation is feasible 
for a given building.

. Large buildings usually have multiple HVAC zones, with each 
zone served by its own air handling unit and duct system. In 
practice, these zones are not completely separated if they are 
on the same floor. Air flows between zones through hallways, 
atria, and doorways that are normally left open. Isolating the 
separate HVAC zones minimizes the potential spread of an 
airborne hazard within a building, and reduces the number 
of people potentially exposed if an internal release occurs. 
Zone separation provides a limited benefit against an external 
release. It increases internal resistance to air movement that 
is produced by wind forces and chimney effect, therefore 
reducing the rate of infiltration. In essence, isolating zones 
divide the building into separate environments, limiting the 
effects of single release to an isolated portion of the building. 
Isolation of zones requires full-height walls between each zone 
and its adjacent zone and hallway.

. Consider shelter-in-place rooms or areas where people can 
congregate in the event of an outdoor release. The goal is to 
create areas where outdoor air infiltration is very low. Usually 
such rooms will be in the inner part of the building in an area 
with no exterior windows. The rooms should have doors that 
are effective at preventing airflow and should contain staging 
supplies such as duct tape and plastic to help further seal the 
areas from the hallways. Typically, restrooms are a bad choice, 
because they have exhaust ducts that lead directly to the 
outside. Opening and closing a conventional hinged door can 
pump large amounts of air into the room. If practical, replace 
the door with a code compliant sliding door to reduce this 
effect. Shelter-in-place is discussed in detail in Section 5.2. 

. Many central HVAC systems have energy management and 
control systems that can regulate airflow and pressures within 
a building on an emergency response basis. Some fire alarm 
systems provide useful capabilities during CBR events. In some 
cases, the best response option (given sufficient warning) 
might be to shut off the buildings HVAC and exhaust 
system(s), thus avoiding the introduction of a CBR agent 
from outside. In other cases, interior pressure and airflow 
control may prevent the spread of a CBR agent released in 
the building and/or ensure the safety of egress pathways. The 
decision to install emergency HVAC control options should 
be made in consultation with a qualified mechanical engineer 
who understands the ramifications of various HVAC operating 
modes on building operation and safety systems. 

. HVAC control may not be appropriate in all emergency 
situations. Protection from CBR attacks depends upon 
the design and operation of the HVAC system and the 
nature of the CBR agent release. Lobbies, loading docks, 
and mailrooms might be provided with manually operated 
exhaust systems, activated by trained personnel to remove 
contaminants in the event of a known release, exhausting air 
to an appropriate area. Manipulation of the HVAC system 
could minimize the spread of an agent. If an HVAC control 
plan is pursued, building personnel should be trained to 
recognize a terrorist attack and know when to initiate the 
control measures. For example, emergency egress stairwells 
should remain pressurized (unless they are known to contain 
the CBR source). Other areas, such as laboratories, clean 
rooms, or pressure isolation rooms in hospitals, may need to 
remain ventilated. All procedures and training associated with 
the control of the HVAC system should be addressed in the 
buildings Emergency Response Plan (ERP).

. Ducted returns offer limited access points to introduce a 
CBR agent. The return vents can be placed in conspicuous 
locations, reducing the risk of an agent being secretly 
introduced into the return system. Non-ducted return air 
systems commonly use hallways or spaces above suspended 
ceilings as a return-air path or plenum. CBR agents 
introduced at any location above the suspended ceiling in a 
ceiling plenum return system will probably migrate back to 
the HVAC unit and, without highly efficient filtration for the 
particular agent, redistribute it to occupied areas. Buildings 
should be designed to minimize interaction between air-
handling zones. This can be partially accomplished by limiting 
shared returns. Where ducted returns are not feasible or 
warranted, hold-down clips may be used for the accessible 
areas of suspended ceilings that serve as the return plenum. 
This issue is closely related to the isolation of lobbies and 
mailrooms, because shared returns are a common way for 
contaminants from these areas to disperse into the rest of the 
building. These modifications may be more feasible for new 
building construction or those undergoing major renovation.

. A rapid response, such as shutting down an HVAC system, may 
involve closing various dampers, especially those controlling the 
flow of outdoor air (in the event of an exterior CBR release). 
When the HVAC system is turned off, the building pressure 
compared to outdoors may still be negative, drawing outdoor 
air into the building via many leakage pathways, including 
the HVAC system. Consideration should be given to installing 
low leakage dampers to minimize this flow pathway. Damper 
leakage ratings are available as part of the manufacturers 
specifications and range from ultra-low to normal categories. 
Assuming that there is some warning prior to a direct CBR 
release, the speed with which these dampers respond to a 
close instruction can also be important. From a protective 
standpoint, dampers that respond quickly are preferred over 
dampers that might take 30 seconds or more to respond.

Emergency Plans, Training, and Procedures for HVAC Systems. 
All buildings should have current emergency plans to address 
fire, weather, and other types of emergencies. In light of past 
U.S. experiences with anthrax and similar threats, these plans 
should be updated to consider CBR attack scenarios and the associated 
procedures. Emergency plans should have procedures 
for communicating instructions to building occupants, identifying 
suitable shelter-in-place areas (if they exist), identifying 
appropriate use and selection of personal protective equipment 
(i.e., clothing, gloves, respirators), and directing emergency 
evacuations. Individuals developing emergency plans and procedures 
should recognize that there are fundamental differences 
between chemical, biological, and radiological agents. In general, 
chemical agents will show a rapid onset of symptoms, while the 
response to biological and radiological agents will be delayed. 
Issues such as designated areas and procedures for chemical 
storage, HVAC control or shutdown, and communications with 
building occupants and emergency responders, should all be addressed. 
The plans should be as comprehensive as possible, but, 
as described earlier, protected by limited and controlled access. 
When appropriately developed, these plans, policies, and procedures 
can have a major impact upon occupant survivability in 
the event of a CBR release. Staff training, particularly for those 
with specific responsibilities during an event, is essential and 
should cover both internal and external events. Holding regularly 
scheduled practice drills, similar to the common fire drill, allows 
for plan testing, as well as occupant and key staff rehearsal of the 
plan, and increases the likelihood for success in an actual event. 
For protection systems in which HVAC control is done via the 
energy management and control system, emergency procedures 
should be exercised periodically to ascertain that the various control 
options work (and continue to work) as planned.

Periodic training of HVAC maintenance staff in system operations 
and maintenance should be conducted. This training should include 
the procedures to be followed in the event of a suspected 
CBR agent release. Training should also cover health and safety 
aspects for maintenance personnel, as well as the potential health 
consequences to occupants of poorly performing systems. Development 
of current, accurate HVAC diagrams and HVAC system 
labeling protocols should be addressed. These documents can be 
of great value in the event of a CBR release.

Procedures and preventive maintenance schedules should be 
implemented for cleaning and maintaining ventilation system 
components. Replacement filters, parts, etc., should be obtained 
from known manufacturers and examined prior to installation. It 
is important that ventilation systems be maintained and cleaned 
according to the manufacturers specifications. To do this requires 
information on HVAC system performance, flow rates, damper 
modulation and closure, sensor calibration, filter pressure loss, 
filter leakage, and filter change-out recommendations. These 
steps are critical to ensure that protection and mitigation systems, 
such as particulate filtration, operate as intended.

3.5 ELECTRICAL SYSTEMS

The major security functions of the electrical system are to maintain 
power to essential building services, especially those required 
for life safety and evacuation; provide lighting and surveillance to 
deter criminal activities; and provide emergency communications. 
Thus, the operability of electrical systems is an important element 
for deferring terrorist attacks and can become a critical component 
for life safety systems after an attack. Designers should consider the 
following recommendations for buildings requiring high security:

. Emergency and normal electric panels, conduits, and 
switchgear should be installed separately, at different 
locations, and as far apart as possible. Electric distribution 
should be run from separate locations.

. Emergency generators should be located away from loading 
docks, entrances, and parking. More secure locations include 
the roof, protected grade level, and protected interior areas. 

. Fuel tanks should be mounted near the generator, given the 
same protection as the emergency generator, and sized to 
store an appropriate amount of fuel. A battery and/or UPS 
could serve a smaller building or leased facility.

. Conduits and lines should be installed outside to allow a 
trailer-mounted generator to connect to the buildings 
electrical system. If tertiary power is required, other methods 
include generators and feeders from alternative substations.

. Site lighting should be coordinated with the CCTV system.

. Emergency lighting should be provided in restrooms.

. Building access points should be illuminated to aid in threat 
detection. 

. Self-contained battery lighting should be provided in stairwells 
and for exit signs.

. Suspending electrical conduits from the ceiling should be avoided.

. Adequate lighting of perimeters and parking areas should be 
provided to aid in visual surveillance and to support the use of 
physical security systems.

3.6 FIRE PROTECTION SYSTEMS

The fire protection system inside the building should maintain life 
safety protection after an incident and allow for safe evacuation of 
the building when appropriate. Although fire protection systems 
are designed to perform well during fires, they are not traditionally 
designed to survive bomb blasts. To enhance the performance 
of fire protection systems, especially in the case of an explosive 
blast, the designer should consider the following:

. The fire protection water system should be protected from 
single-point failure in case of a blast event. The incoming 
line should be encased, buried, or located 50 feet away from 
high-risk areas. The interior mains should be looped and 
sectionalized.

. To increase the reliability of the fire protection system in 
strategic locations, a dual pump arrangement should be 
considered, with one electric pump and one diesel pump. 
The pumps should be located away from each other.

. All security locking arrangements on doors used for egress 
must comply with requirements of the National Fire 
Protection Association (NFPA) 101, Life Safety Code.

3.7COMMUNICATIONS SYSTEMS

For buildings requiring greater protection, the designer should 
consider the following:

. Redundant communications. The facility could have a 
second telephone service to maintain communications in 
case of an incident. A base radio communication system with 
antenna should be installed in the stairwell, and portable sets 
distributed on floors. This is the preferred alternative.

. Radio telemetry. Distributed antennas could be located 
throughout the facility if required for emergency 
communications through wireless transmission of data.

. Alarm and information systems. Alarm and information 
systems should not be collected and mounted in a single 
conduit, or even collocated. Circuits to various parts of the 
building should be installed in at least two directions and/
or risers. Low voltage signal and control copper conductors 
should not share conduits with high voltage power conductors. 
Fiber-optic conductors are generally preferred over copper.

. Empty conduits. Empty conduits and power outlets can be 
provided for future installation of security control equipment.

. Mass notification. All inhabited buildings should have a timely 
means to notify occupants of threats and give instructions 
as to responses. Building communications systems should 
provide real-time notification of occupants and passersby 
in the immediate vicinity of the building during emergency 
situations. The information relayed should be specific enough 
to determine the appropriate response actions.

3.8 ELECTRONIC SECURITY SYSTEMS

Electronic security, including surveillance, intrusion detection, 
and screening, is a key element of facility protection. Many aspects 
of electronic security and the posting of security personnel have 
been adequately dealt with in other criteria and guideline documents. 
These criteria primarily address access control design, 
including stair and lobby design, because access control must 
be considered when design concepts for a building are first conceived. 
Although fewer options are available for modernization 
projects, some designs can be altered to consider future access control 
objectives.

The purpose of electronic security is to improve the reliability and 
effectiveness of life safety systems, security systems, and building 
functions. When possible, accommodations should be made for 
future developments in security systems.

This chapter is not a design guide for Electronic Security Systems 
(ESS). The following criteria are only intended to stress those 
concepts and practices that warrant special attention to enhance 
public safety. Consult design guides pertinent to the specific 
project for detailed information about electronic security. A description 
of Electronic Security Systems is provided in Appendix D.

For control centers and building management systems, designers 
should consider the following:

. The Operational Control Center (OCC), Fire Command 
Center (FCC), and Security Control Center (SCC) may be 
collocated. If collocated, the chain of command should be 
carefully pre-planned to ensure the most qualified leadership 
is in control for specific types of events. Secure information 
links should be provided between the OCC, FCC, and SCC.

. A Backup Control Center (BCC) should be provided in a 
different location, such as a manager's or engineer's office. If 
feasible, an off-site location should be considered.

. A fully redundant BCC should be installed (this is an 
alternative to the above).

. Basic intrusion detection devices should be provided: 
magnetic reed switches for interior doors and openings, 
glass break sensors for windows up to scalable heights, and 
balanced magnetic contact switch sets for all exterior doors, 
including overhead/roll-up doors. Roof intrusion detection 
should be reviewed.

. Monitoring should be done at an off-site facility.

. An on-site monitoring center should be used during normal 
business hours and be operational 24 hours.

. A color CCTV surveillance system with recording capability 
should be provided to view and record activity at the perimeter 
of the building, particularly at primary entrances and exits. A 
mix of monochrome cameras should be considered for areas 
that lack adequate illumination for color cameras.

The following considerations apply when lighting systems are intended 
to support CCTV assessment or surveillance: field of view 
of the camera; lighting intensity levels; maximum light-to-dark 
ratio; scene reflectance; daylight-to-darkness transitions; camera 
mounting systems relative to lighting; spectral response of the 
camera; cold-start time; and restrike time.

3.9 ENTRY-CONTROL STATIONS

Figure 3-12 Physical security devices


Entry-control stations should be provided at main perimeter entrances 
where security personnel are present (see Figure 3-12). In 
addition, entry-control stations should be located close to the perimeter 
entrance to permit people inside the station to maintain 
constant surveillance over the entrance and its approaches. Additional 
considerations at entry-control stations include:

. A holding area for unauthorized vehicles or those needing 
further inspection should be established. A turnaround area 
should be provided so that traffic is not impeded.

. Control measures such as displaying a decal on the window or 
having a specially marked vehicle should be established.

. Entry-control stations that are manned 24 hours each day 
should have interior and exterior lighting, interior heating 
and cooling (where appropriate), and a sufficient glassed 
area to afford adequate observation for people inside. Where 
appropriate, entry-control stations should be designed for 
optimum personnel ID and movement control. Each station 
should include a telephone, a radio, and badge racks (if 
required).

. Signs should be erected to assist in controlling authorized 
entry, to deter unauthorized entry, and to preclude accidental 
entry. Signs should be plainly displayed and legible from 
any approach to the perimeter from a reasonable distance. 
The size and coloring of a sign, its letters, and the interval of 
posting must be appropriate to each situation.

. Entry-control stations should be hardened against attacks 
according to the type of threat. The methods of hardening 
may include:

 Reinforced concrete or masonry

 Steel plating

 Bullet-resistant glass

 Commercially fabricated, bullet-resistant building 
components or assemblies


3.10PHYSICAL SECURITY SYSTEMS

Physical security concerns the physical measures designed to 
safeguard people; prevent unauthorized access to equipment, 
installations, material, and documents; and safeguard against 
terrorist attacks. As such, all security operations face new and 
complex physical security challenges across the full spectrum of 
operations. Challenges relative to physical security include the 
control of populations, information dominance, multi-national 
and interagency connectivity, antiterrorism, and the use of physical 
security assets as a versatile force multiplier.

The rapid evolution of physical security equipment technology 
leads to physical security challenges, which are exponentially multiplied 
by the introduction of the information age (see Appendix 
D). Physical security challenges must be understood, and measures 
must be taken to minimize them to enhance the protection 
of people within a facility. 

3.11 SUMMARY OF BUILDING ENVELOPE 
MITIGATION MEASURES

A general spectrum of building envelope mitigation measures 
ranging from the least protection, cost, and effort going to the 
greatest protection, cost, and effort is provided below. Detailed 
discussions of individual measures can be found earlier in the 
chapter. Please note this is a nominal ranking of mitigation 
measures. In practice, the effectiveness and cost of individual mitigation 
measures may deviate from this example based on specific 
applications. 


 Ensure that exterior doors into inhabited areas open outward. Ensure emergency 
exit doors only 
facilitate exiting.

 Secure roof access hatches from the interior. Prevent public access to 
building roofs.

 Restrict access to building operation systems.

 Conduct periodic training of HVAC operations and maintenance staff.

 Evaluate HVAC control options. 

 Install empty conduits for future security control equipment during initial 
construction or major 
renovation.

 Do not mount plumbing, electrical fixtures, or utility lines on the inside of 
exterior walls.

 Minimize interior glazing near high-risk areas.

 Establish emergency plans, policies, and procedures.

 Establish written plans for evacuation and sheltering in place.

 Illuminate building access points.

 Restrict access to building information.

 Secure HVAC intakes and mechanical rooms.

 Limit the number of doors used for normal entry/egress.

 Lock all utility access openings.

 Provide emergency power for emergency lighting in restrooms, egress routes, 
and any meeting room 
without windows.

 Install an internal public address system.

 Stagger interior doors and offset interior and exterior doors.

 Eliminate hiding places.

 Install a second and separate telephone service.

 Install radio telemetry distributed antennas throughout the facility.

 Use a badge identification system for building access.

 Install a CCTV surveillance system.

 Install an electronic security alarm system.

 Install rapid response and isolation features into HVAC systems.

 Use interior barriers to differentiate levels of security.

 Locate utility systems away from likely areas of potential attack.

 Install call buttons at key public contact areas.

 Install emergency and normal electric equipment at different locations.

 Avoid exposed structural elements.

 Reinforce foyer walls.

 Use architectural features to deny contact with exposed primary vertical load 
members.

 Isolate lobbies, mailrooms, loading docks, and storage areas.

 Locate stairwells remotely. Do not discharge stairs into lobbies, parking, or 
loading areas.

 Elevate HVAC fresh-air intakes.

 Create shelter-in-place rooms or areas.

 Separate HVAC zones. Eliminate leaks and increase building air tightness.

 Install blast-resistant doors or steel doors with steel frames. 

 Physically separate unsecured areas from the main building.

 Install HVAC exhausting and purging systems.

 Connect interior non-load bearing walls to structure with non-rigid 
connections.

 Use structural design techniques to resist progressive collapse.

 Treat exterior shear walls as primary structures.

 Orient glazing perpendicular to the primary faade facing uncontrolled vehicle 
approaches.

 Use reinforced concrete wall systems in lieu of masonry or curtain walls.

 Ensure active fire system is protected from single-point failure in case of a 
blast event.

 Install a Backup Control Center (BCC).

 Avoid eaves and overhangs or harden to withstand blast effects.

 Establish ground floor elevation 4 feet above grade.

 Avoid re-entrant corners on the building exterior.

 [end of Chapter 3]