APPENDIX D (Continued)
Controlling Event. This refers to an explosive threat source, expressed as net
equivalent TNT charge that produces the most damage to the target. The controlling event may
not be the maximum event. It is a function of explosive size, standoff distance, and location that
produces the most damage to the structure.
Design Events. This is a combination of explosive threats, expressed as net
equivalent TNT charge at a separation distance, that comprise a range of possible attack
scenarios. This includes a variety of events of different sizes and locations.
Example. The variation of location is significant in evaluating the range of damage
possible. For example, consider a maximum credible event as a vehicle carrying 4,000 pounds
(1,818 kg) of explosives at a standoff distance of 500 feet (152 m) to the perimeter boundary.
The sets of design events that need to be considered are the maximum event, as well as other
scenarios. One such scenario might be a car carrying 500 pounds (227 kg) of explosives that
could maneuver within 100 feet (30 m) of the structure (assuming large vehicles are restricted
from approaching the structure). Another scenario could be a motorcycle or bicycle with 50
pounds of explosives (23 kg) concealed in the frame that could maneuver within 50 feet (15 m),
or be manually carried over or through the perimeter barrier and placed in or near the structure.
The controlling event is the scenario that produces the most damage to the structure.
It may or may not be the maximum event. In this example, the 500-pound (227-kg) explosive at
100-foot (30 m) standoff is the controlling event, because it would create the highest blast
pressures; the 50-pound (23kg) event would create the most localized damage.
Another major concern is the placement of the explosive. All locations must be
considered to determine the position that would produce the greatest damage to the structure.
Blast Effects. The material developed for this section is based on vehicle bomb
threats that can range from 50-pound (23-kg) to 40,000-pound (18,181-kg) bombs. These charge
weights are considered the net equivalent weights of an uncased spherical TNT charge, the
standard explosive used for assessing blast effects. The specific threat to structure should be
based on available local intelligence or mandated requirements.
The type of explosive and its shape are known to affect blast yield. However, these
factors tend to be relatively minor until a scaled distance of 10 is reached, because blast damage
is related to the cube root of the charge weight. Designers typically assume the worst case of a
hemispherically shaped explosive. The confinement around the explosive is also a factor in
defining explosive yield. Limited testing on the effects of a vehicle on confinement of a small
explosive charge shows that pressures are about 10 percent lower than free-air detonations.