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 (3) The load-mass factors, used to convert the mass of the actual
system to the equivalent mass, are given in Table 10. For prestressed
elements the load-mass factor in the elastic range is used. An average of
the elastic and plastic range load-mass factors is used in the design of
non-prestressed elements.
(4) The equivalent single degree-of-freedom system is defined in
terms of its ultimate resistance, ru, equivalent elastic deflection XE,
and natural period of vibration, TN. The dynamic load is defined by its
peak pressure, B, and duration, T. For a triangular pulse load, Figure 6-7
of NAVFAC may be used to determine the response of an element in terms of
its maximum deflection, Xm, and the time to reach maximum deflection,
tm.
(5) Recommended maximum deflection criteria for precast elements is
as follows:
(a) For prestressed flexural members:
[theta]max < /= 2deg. or [mu]max < /= 1, whichever
governs
(b) For non-prestressed flexural members:
[theta]max < /= 2deg. or [mu]max < /= 3, whichever
governs
(c) For compression members:
[mu]max < /= 1.3
where [theta]max = maximum support ratio and [mu]max = maximum ductility
ratio.
f. Rebound. Precast elements will vibrate under dynamic loads, causing
negative deflections after the maximum deflection has been reached. The
negative forces associated with these negative deflections may be predicted
using Figure 6-8 of NAVFAC P-397.
(1) Non-prestressed elements. The design of non-prestressed precast
elements for the effects of rebound is the same as for cast-in-place
members. See paragraph 3.g. and 4.e. of Section 3 for a discussion of
rebound effects in beams and columns respectively.
(2) Prestressed elements. In prestressed elements, non-prestressed
reinforcement must be added to what is the compression zone during the
loading phase to carry the tensile forces of the rebound phase. The rebound
resistance will be determined from Figure 6-8 of NAVFAC P-397, but in no
case will it be less than one-half of the resistance available to resist the
blast load.
(a) The moment capacity of a precast element in rebound is as
follows:
2.08-224
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