ESTABLISHING STANDOFF DISTANCE
General. This section provides guidance and data on establishing standoff distances
for a facility susceptible to a terrorist vehicle bomb attack. It is based on the design and testing
of structures to resist the effects of explosions. Standoff distances can be established, based on
land value and availability or on the building construction information contained in this section.
There may be tradeoffs between these factors, based on the cost of land versus the cost of
building hardening. Hardening costs for the structure will increase with decreasing standoff
distance. Also, the cost of the perimeter vehicle barrier system must be included in the cost
analysis, because the barrier system cost will increase as the perimeter is expanded to provide
greater standoff distances.
Building Performance Levels. The initial phase in designing structures to resist the
effects of vehicle bombs is to develop the design criteria that describes what will happen when a
bomb detonates near the structure. To develop design criteria for the performance of a structure
under some threat, it is important to define:
a) Performance goals that establish how the structure should perform under the
expected blast load.
b) Expected range of threat that will establish the blast loading conditions on the
c) Amount of damage allowed that will ensure that performance goals are met.
The performance goals establish a common basis of communication between the user
and design engineer. The various levels of performance that allow an increasing level of
building functionality after an attack are defined below.
For all performance levels, the application of FRF to existing glazing, or replacement
of existing glazing systems with glazing cross-sections that will withstand the applied loads, is
recommended to reduce injury levels to personnel occupying these structures.
Possible Collapse. This level refers to buildings designed for no protection against an
explosive threat. Standard construction will be completely destroyed at this performance level.
Non-Repairable. This level refers to buildings designed and sited for minimal
protection against an explosive threat. The building will be heavily damaged, but will not
undergo "progressive collapse." If a building collapses under structural loading, it dramatically
increases the possibility that lives will be lost. This level of design allows for full disruption of
the building and a high probability that the building will not be repairable. All standard window