APPENDIX D (Continued)
Confined Detonation. In most instances, more damage is inflicted on a building by a
confined blast detonating inside a structure than from an external explosion. When an explosive
detonates in a confined space, the initial shock wave is amplified by the reflections of the blast
wave on the internal walls of the structure. Generally, a portion of the structure will blow out,
venting the blast. During this process, the full or partial confinement of the detonation produces
a buildup of high-temperature gases. These gases result in a relatively long duration of pressure,
termed gas pressure. The amplitude and duration of the gas pressure are functions of the charge
weight, venting area, and volume of the confined space.
The environment within a confined or partially confined space is much more severe
than that of an open-space detonation. A vehicle bomb allowed to penetrate into a building will
cause significantly more damage than if it is detonated outside the building. However, when a
bomb detonates within a structure, there is usually less damage to the surrounding buildings,
because most of the blast is absorbed by the target structure.
Blast Effects From Distant Explosions. The following is a description of how blast
loads on structures are determined. This is not a detailed procedure for use in engineering
calculations, but rather an overview of the subject. It is limited to rectangular, above-ground
structures that are distant from the explosion and subjected to a plane-wave shock front.
As discussed earlier, when a bomb explodes, a hemispherically expanding shock
wave is formed. The forces acting on a structure associated with a shock wave depend on the
peak incident pressure, the impulse of the incident pressure, and the dynamic wind pressures
acting on the structure. For each incident pressure level there is a blast wind. In the wake of this
blast wind, there is a secondary blast wind that is composed of air particles rushing in to fill the
vacuum left by the shock wave. This secondary blast wind is referred to as a dynamic drag
pressure and is responsible for drawing debris away from the building at considerable distances.
For any given incident, the forces imparted to a structure can be divided into four
a) The force resulting from the incident pressure;
b) The force associated with the dynamic wind pressures;
c) The force resulting from reflection of the incident pressure (termed reflected
pressure) striking the building;
d) Pressures associated with the negative phase of the shock wave.