b. Interior Blast Loadings. Methods of calculating the average blast
impulse on the walls and roofs of fully vented cubicles of various sizes and
configurations are presented in NAVFAC P-397. The general procedure in
predicting the impulse involves selecting the particular configuration of
the cubicle from the 180 different cases listed in NAVFAC P-397 (such as the
number of reflecting surfaces, charge position in cubicles, volume of
cubicle and standoff distance). Usually, interpolation is required for one
or more of the parameters which define a given situation. However, a
computer program has been developed which executes the interpolation
procedure using numerical tables equivalent to Figures 4-17 through 4-62 in
NAVFAC P-397. Section 8 of this manual provides more information on the
availability of the computer program.
c. Exterior Blast Environment. Several tests were performed in cubicle
structures where the length (L) to height (H) ratio of the back walls was
approximately equal to 1.0, while the charge weight-to-volume ratio varied
between 0.2 and 2.0. The pressure-distance data is presented in Figure 4-63
of NAVFAC P-397. For large charge weights-to-structure ratios, the
pressure-distance relationships in all directions from the structures will
approach that for an unconfined surface explosion. Another investigation
was performed by Keenan and Tancreto on 3-wall cubicles with and without
roofs. The data obtained by the two investigations on the blast environment
outside such cubicles are presented in the following paragraphs.
(1) Three-Wall Cubicle Without Roof. For a charge located at the
geometric center of a 3-wall cubicle, the envelope curves for the peak
pressures behind each wall of the cubicle are plotted as a function of
scaled distances in Figure 13. The peak pressure, (Pso)max, is also
plotted as a function of W/V in Figure 14. Figure 11 should be used in
conjunction with Figure 14. In some cases (small values of R/W1/3 or
W/V), Pso from Figure 13 will exceed (Pso)max from Figure 14. In
these cases, (Pso)max is the maximum peak pressure outside the cubicle.
(2) Three-Wall Cubicle with Roof. The envelope of peak pressures
outside a 3-wall cubicle with a non-frangible roof is shown in Figure 15.
At any scaled distance, there is no clear influence of cubicle geometry or
W/V on the peak pressures out the open front; therefore, the curves apply to
all values of W/V. As stated in paragraph 6.c.(1), Pso should not exceed
(Pso)max obtained from Figure 14.
d. Design Loads. Some criteria are outlined in Figures 16 and 17 for
predicting the design loading in and around fully and partially vented
cubicles. Some engineering judgment is required in using these figures.
AIRBLAST FROM UNDERWATER EXPLOSIONS.
a. Predicting Parameters of a Shock Wave. Figures 4-5 and 4-12 of
NAVFAC P-397 provide a means of predicting the peak pressures, impulses,
velocities, and other parameters of a shock wave for a bare spherical TNT
explosive charge detonated in free air and on or very near ground surface.
These parameters change, however, if the explosion occurs under shallow
water. A portion of the work done by M.M. Swisdak is reproduced here. A
detailed description of the work is provided in the report titled Explosion
Effects and Properties: Part 1, Explosion Effects in Air, NSWC/WOL TR-116.