Page 248 - Forensic Structural Engineering Handbook

7.32 CAUSES OF FAILURES

uniform with height. As with free air explosions, the magnitude of the shock diminishes
rapidly with distance.
Surface Explosions. Surface explosions probably are of greatest relevance for most
designs and investigations, because the most frequent explosions affecting buildings occur
near the ground. Explosives that are detonated on the ground create hemispherical shock
fronts that radiate upward and outward from the detonation point. This form is the result of
the instantaneous reflection of the initial blast from the ground. Theoretically, the reflec-
tive surface at the location of the detonation can cause the magnitude of the pressure to dou-
ble. In fact, the ground is not a perfect reflector (some energy is absorbed by the creation
of a crater and seismic waves). Therefore, the shock is seldom amplified by more than
approximately 80 percent.
Interior Blasts. Interior blasts also are of three types: fully confined explosions, partially
vented explosions, and fully vented explosions.
Fully Confined Explosions. Fully confined explosions occur in enclosed spaces. To
survive, the structure that surrounds such explosions must be able to contain and absorb all
the energy released by the explosion, including the shock front, expanding gases, and heat.
Partially Vented Explosions. Partially vented explosions occur in confined spaces that
have perhaps 5 to 10 percent of their exterior surfaces open to the atmosphere. In explo-
sions of this type, very complex patterns of reflections of the shock from the surfaces of the
volume amplify the effective impulse exerted on the structure, and pressures can build sub-
stantially as the expanding gas is released relatively slowly to the atmosphere through con-
stricted openings.
Fully Vented Explosions. Fully vented explosions are characterized by many reflec-
tive surfaces near to the detonation point, but with much of the perimeter surface open to
the atmosphere. An explosive detonated in a building with one wall totally open to the
atmosphere is an example of a fully vented explosion. In these explosions, there will be sig-
nificant amplification of shock through many reflections from surfaces, but gas pressure
buildup usually is not more significant than for exterior explosions.
In buildings subjected to forces due to explosions, most exterior blasts are surface
explosions and most interior blasts are partially vented explosions.

Determination of Blast Loads
To determine the magnitude of blast effects, it is important to know the size of the explo-
sive charge, the nature of the explosive, and the location of the explosion relative to the
receiving object. With the size and nature of the explosive, the blast characteristics can be
determined and an equivalent TNT charge weight can be estimated. This equivalent TNT
charge weight is essential for the use of most blast effects formulas.
Alternatively, the size and efficiency of the charge can be estimated from the physical
evidence at the site. Distances to which debris is thrown can indicate the magnitude of a
blast. The depth of the blast crater can be correlated with the position of the explosive
device and the amount of explosive. 27
Once the magnitude of the explosion has been estimated, the nature of the influence on
a structure can be assessed. First, the pressures and loading durations must be estimated.
When the distance between the detonation point and the nearest face of a building
exceeds approximately twice the larger dimension of that face of the building, the incident
pressures reaching the near surface of the building are relatively uniform. However, when
the incident shock reaches the rigid wall surface, the moving air is further compressed as it
is reflected from the surface of the building. This causes an amplification of the pressure
on the near face of the building. Surfaces that are close to large charges can experience