this increased "effective charge weight" in a particular design situation
may only be made for those cases which are sufficiently simple and well
tried to justify this action. Such modifications must be approved by the
cognizant military construction agency.
BLAST PRESSURE OUTPUT.
a. Blast Phenomena. The blast effects of an explosion are in the form
of a shock wave composed of a high-pressure shock front which expands
outward from the center of the detonation, with intensity of the pressure
decaying with distance and as a function of time. The magnitude and other
characteristics of the blast loads associated with the explosion are a
function of the following:
Location of the explosive relative to the structure.
Amplification of the pressure by its interaction with the
ground, barrier, etc.
Besides NAVFAC P-397, there are several good texts which deal with
the basic physics of air blast and the prediction of blast wave properties
for (HE) explosives. One of these is Explosions in Air by Baker.
b. TNT Equivalents. The major quantity of blast effects data presented
in NAVFAC P-397 and other similar manuals pertains to the blast pressure
output of TNT explosions.. These data can be extended to include other
potentially mass-detonating materials whose shapes differ from those
considered in these manuals, by relating the explosive energy of the
effective charge weight of these materials to that of an equivalent weight
of TNT. For blast-resistant design in general, the TNT equivalent should
be based upon a pressure and impulse relationship depending upon the
anticipated pressure-design range. Comparison of the heats of detonation
of other explosives can help in determining their TNT equivalences. TNT
equivalences for different explosives arc presented in Table 1.
c. Cased Explosives. Some of the energy released when a cased charge
detonates is lost through strain energy to break up the casing and through
kinetic energy to accelerate the fragments of the casing. No longer are
the blast parameters simply a function of scaled standoff distance
(R/W1/3), but they become a function of other variables such as casing
weight, Wc, charge weight, W, etc. As stated in Class Notes by Keenan,
effective charge weight, Weff, for computing blast pressures from a cased
charge is less than the total charge weight, W, and the difference
increases with the ratio of metal case weight to the charge weight, Wc/W.
Weff is given by Equation (12a).
Weff = F x W
where F is given by:
F = 0.6 + 0.4/(1 + 2Wc/W)