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 b. Configuration of Charge and Interpretation of Different Parameters.
Figure 18 shows the configuration of the charge and the interpretation of
the different parameters required for the data presented in this section.
Figures 19 through 24 present the blast pressure data as fixed functions of
[lambda]y, with values of [lambda]y varying between 0.25 and 40.
Because of the scatter in the experimental pressure data used to construct
Figures 19 through 24 and the uncertainties involved in making the necessary
extrapolations, the curves in this section should be considered accurate
only to within +/-30 percent. The following parameters are used in the
aforementioned figures:
[lambda]d = d/W1/3, scaled charge depth
[lambda]x =
R/W1/3, scaled horizontal distance
[lambda]y =
y/W1/3, scaled vertical distance
8.
EXTERNAL BLAST LOADS ON STRUCTURES
a. Forces Acting on Structures. The blast wave generated by an
explosion in air is characterized by its transmission velocity, U; by a peak
incident pressure, Pso; by a positive phase duration, to; and by a
specific impulse, is. For each pressure range, there is a particle or
wind velocity associated with the blast wave that causes a dynamic pressure
on objects in the path of the wave. The values of the peak dynamic
pressure, qo, vs. peak incident pressure are shown in Figure 4-66 of
NAVFAC P-397.
b. Calculating Durations of Positive and Negative Pressure Phases. For
design purposes, it is necessary to establish the variation or decay of both
the incident and dynamic pressures with time, since the effects on a
structure subjected to a blast loading depend upon the intensity-time
history of the loading as well as on the peak intensity. Equations are
provided in Chapter 4, Section V of NAVFAC P-397 to be used in calculating
the durations of the positive and negative pressure phases. However, the
rise time for the negative pressure should be taken as one-third of its
duration, not one-eighth as recommended in NAVFAC P-397.
c. Aboveground Rectangular Structures. The interaction of an incident
blast wave with an object is a complicated process and to reduce the complex
problem of blast to reasonable terms, it will be assumed here that:
1.
The structure is generally rectangular in shape;
2.
The incident pressure is in the order of 200 psi or less; and
3. The object being loaded is in the region of the Mach reflection
(NAVFAC P-397).
(1) Front Wall. For a rectangular aboveground structure at low
pressure ranges, the variation of pressure with time on the side facing the
detonation is illustrated in Figure 4-63 of NAVFAC P-397. When the incident
wave interacts with the structure's front wall, the pressure is immediately
raised from zero to the reflected pressure, Pr, which is a function of the
incident pressure and the angle of incidence between the shock front and the
structure face. The time required to relieve the reflected pressures and
the pressure acting on the front wall after this time can be calculated
using Equations 4-5 and 4-6 of NAVFAC P-397, respectively.
2.08-35
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