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(3) The loadmass factors, used to convert the mass of the actual
system to the equivalent mass, are given in Table 10. For prestressed
elements the loadmass factor in the elastic range is used. An average of
the elastic and plastic range loadmass factors is used in the design of
nonprestressed elements.
(4) The equivalent single degreeoffreedom 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 67
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 nonprestressed 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 68 of NAVFAC P397.
(1) Nonprestressed elements. The design of nonprestressed precast
elements for the effects of rebound is the same as for castinplace
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, nonprestressed
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 68 of NAVFAC P397, but in no
case will it be less than onehalf of the resistance available to resist the
blast load.
(a) The moment capacity of a precast element in rebound is as
follows:
2.08224


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