(2) Roof and Side flail. In calculating the pressures on the roof
and side walls of a structure, a step-by-step analysis of the wave
propagation should be made. However, because of the complexity of such an
analysis, an approximate method of calculating the pressure-time history is
outlined in Chapter 4, Section V of NAVFAC P-397.
(3) Rear Wall. In most design cases, the primary reason for
determining the blast loads acting on the rear wall is to determine the
overall drag effects on the building. The recommendations outlined in
Chapter 4 of NAVFAC P-397 should be followed for local design of the rear
wall. However, for analyses of frames and similar structures, it is
recommended that the positive phase blast loads acting on the rear wall of a
structure be taken equal to 60 percent of the incident overpressure. This
is true for incident pressures of approximately 10 psi or less. For higher
pressures, the procedures of NAVFAC P-397 should be followed for both the
design and analysis of rear walls.
9. PRESSURE INCREASE WITHIN A STRUCTURE. Leakage of pressures through any
openings in a structure occurs when such a structure is engulfed by a blast
wave. The interior of the structure experiences an increase in its ambient
pressure in a time that is a function of the structure's volume, area of
openings, and applied exterior pressure and duration. A method of
determining the average pressure inside the structure is outlined in Section
VI, Chapter 4 of NAVFAC P-397 and also in the report Accidental Explosions
and Effects of Blast Leakage into Structures by Kaplan and Price.
a. Blast Characteristics. The blast characteristics of a multiple
explosion can be very different from that measured for a single charge or
one of separated charges. The pressure-time relationships will depend upon
the interaction of the individual waves themselves. A minimum amount of
theoretical and experimental data is available to characterize blast waves
from multiple detonations. A good source of information on this subject is
the text by Zaker, Blast Pressures from Sequential Explosions.
b. Blast Loading on Side Walls of a Cubicle Due to Simultaneous
Explosions. A procedure for determining the blast loading on side walls of
a cubicle due to simultaneous explosions is outlined in Keenan's Class
Notes. It should be emphasized that the impulses calculated by this
procedure will be conservative. Other sources of information on this matter
are; Blast Effects of Simultaneous Multiple Charge Detonations by Horkanson,
High Explosive Multi-Burst Air Blast Phenomena (Simultaneous and
Non-Simultaneous Detonations) by Reisler, et al., and The Air Blast from
Simultaneously Detonated Explosive Spheres by Armendt, et al.
a. Initial Fragment Velocities. The most common method for calculating
the initial velocity of fragments resulting from the detonation of cased
explosives is the Gurney method described in the reports The Initial
Velocities of Fragments from Bombs, Shells and Grenades and The Mass
Distribution of Fragments from Bombs. Shells and Grenades. The report by
Healey, et al., Primary Fragment Characteristics and Impact Effects on
Protective Barriers, presents an in-depth discussion of primary fragment
characteristics and part of this report is reproduced here.