TERRORIST THREATS 2

2.1 OVERVIEW OF POSSIBLE THREATS 
This primer addresses several types of terrorist threats, which are listed below. 
Explosive Threats: 
. _ Vehicle weapon 
. _ Hand-delivered weapon 

Airborne Chemical, Biological, and Radiological Threats: 
. _ Large-scale, external, air-borne release 
. _ External release targeting building 
. _ Internal release Although it is possible that the dominant threat mode may 
change in the future, bombings have historically been a favorite tactic of terrorists. 
Ingredients for homemade bombs are easily obtained on the open market, as are the 
techniques for making bombs. Bombings are easy and quick to execute. Finally, the 
dramatic component of explosions in 

terms of the sheer destruction they cause creates a media sensation that is highly 
effective in transmitting the terroristÕs message to the public.


2.2 EXPLOSIVE ATTACKS 
From the standpoint of structural design, the vehicle bomb is the most important 
consideration. Vehicle bombs are able to deliver a sufficiently large quantity of 
explosives to cause potentially devastating structural damage. Security design 
intended to limit or mitigate damage from a vehicle bomb assumes that the bomb is 
detonated at a so-called critical location(see Figure 2-1). The critical location is a 
function of the site, the building layout, and the security measures in place.  For a 
vehicle bomb, the critical location is taken to be at the closest point that a vehicle 
can approach, assuming that all security measures are in place. This may be a parking 
area directly beneath the occupied building, the loading dock, the curb directly 
outside the facility, or at a vehicle-access control gate where inspection takes place, 
depending on the level of protection incorporated into the design. 
Another explosive attack threat is the small bomb that is hand delivered. Small 
weapons can cause the greatest damage when brought into vulnerable, unsecured 
areas of the building interior, such as the build-
TERRORIST THREATS 
ing lobby, mail room, and retail spaces. Recent events 
around the world make it clear that there is an increased 
likelihood that bombs will be delivered by persons who 
are willing to sacrifice their own lives. Hand-carried 
explosives are typically on the order of five to ten pounds 
of TNT equivalent. However, larger charge weights, in 
the 50 to 100 pounds TNT equivalent range, can be 
readily carried in rolling cases. Mail bombs are typically 
less than ten pounds of TNT equivalent. 
In general, the largest credible explosive size is a 
function of the security measures in place. Each line of 
security may be thought of as a sieve, reducing the size of 
the weapon that may gain access. Therefore the largest 
weapons are considered in totally unsecured public 
space (e.g., in a vehicle on the nearest public street), and 
the smallest weapons are considered in the most secured 
areas of the building (e.g., in a briefcase smuggled past 
the screening station). 
TERRORIST THREATS 
Two parameters define the design threat: the weapon size, measured in equivalent 
pounds of TNT, and the standoff. The standoff is the distance measured from the 
center of gravity of the charge to the component of interest. 
The design weapon size is usually selected by the owner in collaboration with security 
and protective design consultants (i.e., engineers who specialize in the design of 
structures to mitigate the effects of explosions). Although there are few unclassified 
sources giving the sizes of weapons that have been used in previous attacks 
throughout the world, security consultants have valuable information that may be 
used to evaluate the range of charge weights that might be reasonably considered for 
the intended occupancy. Security consultants draw upon the experience of other 
countries such as Great Britain and Israel where terrorist attacks have been more 
prevalent, as well as data gathered by U.S. sources. 
To put the weapon size into perspective, it should be noted that thousands of 
deliberate explosions occur every year within the United States, but the vast majority 
of them have weapon yields less than five pounds. The number of large-scale vehicle 
weapon attacks that have used hundreds of pounds of TNT during the past twenty 
years is by comparison very small. 
The design vehicle weapon size will usually be much smaller than the largest credible 
threat. The design weapon size is typically measured in hundreds of pounds rather 
than thousands of pounds of TNT equivalent. The decision is usually based on a 
trade-off between the largest credible attack directed against the building and the 
design constraints of the project. Further, it is common for the design pressures and 
impulses to be less than the actual peak pressures and impulses acting on the 
building. This is the approach that the federal government has taken in their design 
criteria for federally owned domestic office buildings. There are several reasons for 
this choice. 
1. 1.	The likely target is often not the building under design, but a high-risk 
building that is nearby. Historically, more building damage has been due to 
collateral effects than direct attack. 
2. 2.	It is difficult to quantify the risk of man-made hazards. However, 
qualitatively it may be stated that the chance of a large-scale terrorist attack occurring 
is extremely low. A smaller explosive attack is far more likely. 
3. 3.	Providing a level of protection that is consistent with standards 
adopted for federal office buildings enhances opportunities for leas-

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ing to government agencies in addition to providing a clear state
ment regarding the buildingÕs safety to other potential tenants. 
4.	The added robustness inherent in designing for a vehicle bomb of moderate size 
will improve the performance of the building under all explosion scenarios.


2.3 FURTHER READING 
Technical Support Working Group, ___, Terrorist Bomb Threat Stand-Off Card with 
Explanation of Use, Technical Support Working Group, Washington, D.C. 
http://www.tswg.gov/tswg/prods_pubs/ newBTSCPress.htm 
U.S. Department of the Treasury / Bureau of Alcohol, Tobacco and Firearms, 1999, 
Vehicle Bomb Explosion Hazard And Evacuation Distance Tables, Department of the 
Treasury, Washington, D.C. (Request in writing, address information available at 
http://www.atf.treas.gov/ pub/fire-explo_pub/i54001.htm 
Federal Bureau of Investigation, 1999, Terrorism in the United States, U.S. Department of 
Justice, Federal Bureau of Investigation, Counterterrorism Division, Washington, 
DC. http://www.fbi.gov/publications/ terror/terror99.pdf 
The U.S. Department of State, 2002, Patterns of Global Terrorism 2001. The U.S. 
Department of State, Washington, DC. http:// 
www.state.gov/s/ct/rls/pgtrpt/2001/pdf/ 
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