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 b. Pressure Design. Durations of blast loads acting on structures
designed for a low pressure range are extremely long in comparison to
impulse (short duration) loads. Here, the structure responds to the peak
pressure.
c. Pressure and Duration Design. Structures subjected to pressures in
the intermediate range are designed to respond to the combined effects of
both the pressure and impulse associated with the blast output.
6. PRIMARY AND SECONDARY FRAGMENTS.
The distinction between primary and
secondary fragments is as follows:
a. Primary Fragments. All weapons have some kind of cover or case if
for no other reason than to make shipping and handling of the explosive
charge safer and easier. A fragment from such a casing or cover is denoted
a "primary fragment." The size and velocities of primary fragments are
functions of the type of casing material and type and quantity of explosives
encased. A detail description of the fragmentation of cased explosives is
presented in section 2, paragraph 11 of this manual. Chapter 8 of NAVFAC
P-397 deals with the response or behavior of concrete to primary fragment
impact. This chapter is updated in the report by Healy, et al., titled
Primary Fragment Characteristics and Impact Effects on Protective Barriers.
b. Secondary Fragments. In the event of an explosion, both primary and
secondary fragments are ejected in all directions. Primary fragments have
been defined in the preceding paragraph; hence, secondary fragments are
usually objects in the path of the resulting blast wave that are accelerated
to velocities which can cause impact damage. Secondary fragments can vary
greatly in size, shape, initial velocity and range. A complete and detailed
analysis of secondary fragments is presented in section 6.2.2. of DOE/TIC
11268, A Manual for the Prediction of Blast and Fragment Loadings on
Structures.
7. STRUCTURAL ANALYSIS AND DESIGN. Design and analysis of a structure for
blast loads is a dynamics problem. A structural-dynamic problem differs
from its static-loading counterpart in two important respects. The first
difference to be noted is the time-varying nature of the dynamic problem.
Because the load and the response vary with time, it is evident that a
dynamic problem does not have a single solution, as a static problem does.
A succession of solutions corresponding to all times of interest in the
response history can be established. However, a more fundamental
distinction between static and dynamic problems is illustrated in Figure 1.
If a single degree-of-freedom system (such as a mass, M, connected to a
spring having a stiffness, K) is subjected to a static load, P, as shown in
Figure 1a, the deflected shape depends directly on the applied load and can
be calculated from P. On the other hand, if a load P(t) is applied
dynamically, as shown in Figure 1b, the resulting displacements of the
system are associated with accelerations which produce inertia forces
resisting the accelerations. In general, if the inertia forces represent a
significant portion of the total load equilibrated by the internal elastic
forces of a system, then the dynamic character of the problem must be
accounted for in its solutions. However, if the applied load, P(t), is
applied very slowly such that the inertia forces are negligible (x = 0),
then the applied load equals the resisting force in the spring and the load
is considered a static load, although the load and time may be varying.
Usually for blast design, the structure can be replaced
2.08-3
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