Page 247 - Forensic Structural Engineering Handbook

LOADS AND HAZARDS: THEIR NATURE, MAGNITUDE, AND CONSEQUENCES 7.31

Blast Environment
In all explosions, there are two primary sources of pressure forces that can affect buildings:
shock fronts and expansion of hot gases. The detonation of explosives creates a shock front
that radiates away from the center of the explosion at supersonic speeds. It is generated by
the initial detonation of the explosive. Following the shock front is a pressure wave that is
generated by the expanding, hot gases from the rapid consumption of the explosive fuel. It
is, in effect, a wind created by the movement of the air. The shock front “shell” usually is
quite thin: Pressures increase to a maximum as a step function, and they diminish rapidly.
The air movement that follows the shock front has associated with it the momentum of the
air mass: After the outward velocity peaks and diminishes, there is often a period of rebound
when air movement reverses direction, and negative pressures are induced on surfaces facing
the explosion site.
In general, the very short duration of very high blast loading (usually on the order of
several milliseconds) imparts an impact load on buildings and structural elements with rel-
atively low vibrational frequencies. To survive, structural elements must have sufficient
strength and energy-absorbing ability to arrest the resulting motion before being destroyed.
However, the impulse function is dependent on the magnitude of the blast, the efficiency
of the explosive, and the distance between the detonation point and the receiving structure.
With smaller charges, lower efficiency, and larger distance, the effective pressures dimin-
ish, but the durations lengthen. Progressively, the nature of the load changes from impulse
to dynamic to quasistatic in the limit, depending on element’s natural frequency.
Exterior Blasts. Exterior blasts can be classified into three groups: free air explosions, air
explosions, and ground explosions. Free air explosions are those that occur in the atmos-
phere, remote to objects that might affect the expansion of shock fronts and hot gases.
While such explosions might not be very interesting for the design of blast-resistant struc-
tures, their character is the basis for the description of blasts of other types.
Free Air Explosions. In a free air explosion, the shock front advances away from the
26
detonation point in a spherical pattern .
For most explosives, the static overpressure initially is many times the ambient air pressure.
1/3
Its magnitude is a function of the scaled distance, z = R/W , in which R is the radius from
the center of the detonation, and W is the TNT equivalent mass of the charge. As such, the
strength of the advancing shock front initially diminishes rapidly with distance.
At the same time, the blast creates the maximum dynamic overpressure at the leading
surface of the shock front, due to moving air. 26
In all cases, the duration of the overpressure is very short, and the pressure magnitude
and, therefore, the potential influence that an explosion might have on an object such as a
building diminish rapidly with distance.
Air Explosions. Air explosions are those that occur in the air, but closer to the ground
than to objects such as buildings that might be affected by the blast. Typically, detonations
are characterized as air explosions if the horizontal distance between ground zero (a point
on the ground vertically below the detonation point) and the object is at least 3 times the
height of blast above the ground. In blasts of this type, the strength of the advancing shock
front can be substantially higher than that for free air explosions. As the incident shock
front radiates in a spherical pattern away from the detonation point, it reaches and is
reflected off the ground surface before it reaches the object. The reflected front, which con-
tinues away from the point of reflection, can combine with the advancing incident front to
create a “mach front” of greater magnitude than the incident front. In addition, the combi-
nation of these fronts creates a front pattern that radiates away from ground zero with a
shape that approximates a vertical cylindrical. The pressure of this front often is taken as