APPENDIX D (Continued)
When an explosive detonates, it undergoes a rapid chemical reaction that propagates
through the explosive material and converts the explosive into a very hot, dense, high-pressure
gas. This energy is released in several forms, including sound, heat, light, and shock wave. The
blast effects of an explosion are in the form of a shock wave that expands outward from the
explosive surface at very high velocities. As this wave expands outwardly, it decays in strength
(amplitude), but increases in duration.
Components of an Explosion. When a bomb explodes, the most important
mechanism for damaging a structure is the shock wave. Ground shock, cratering, fragmentation,
and fire are also factors that should be considered, but these usually have a minor effect when
compared to the shock wave.
Shock Wave. The shock wave damages a target by the action of high pressures
loaded on the target, usually many times the ordinary loads for which the structure was designed.
There are two aspects of the pressure wave that produce damage: the peak amplitude
of the pressure and the duration of the pressure (i.e., how long the pressure acts on the structure).
The integral part of the pressure-duration shock pulse is termed the "impulse" and is represented
graphically as the area under the curve in a plot of the pressure-time pulse (Figure D-1). The
impulse is equal to the amount of momentum imparted to the structure.
Peak pressure also controls the response for rigid structures, such as a boxlike,
reinforced-concrete building with a relatively short natural period. However, if the building is
flexible, such as a steel frame with a long natural period compared to the duration of the shock
wave, then the damage will be caused by the impulse imparted on the structure. Multiple
reflections caused by pressure waves bouncing off surrounding buildings can vary greatly, and
the wave shape will change depending on the site.