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EQUATION:
M -u = A -sfdy(d - - a/2)
(152)
where,
M -u =
ultimate moment capacity in rebound, in-lb
A -s =
total area of rebound tension reinforcement, in2
fdy =
dynamic design strength of reinforcement, psi
d - =
distance from extreme compression fiber to the centroid of
the rebound reinforcement, in
a = depth of the equivalent retangular stress block, in
(b) It is important to take into account the compression in the
concrete due to prestressing and reduce the strength available for rebound.
For a conservative design, it may be assumed that the compression in the
concrete due to prestressing is the maximum permitted by the ACI code, i.e.,
0.45 f'c.  Thus the concrete strength available for rebound is:
EQUATION:
0.85 f'dc - 0.45 f'c = 0.85f'dc - 0.45f'dc/DIF =
049f'dc
(153)
A more detailed analysis may be performed to determine the actual concrete
compression due to prestress.  In either case the maximum amount of rebound
EQUATION:
A -s < /= [(0.85f'dc - f)K1/fdy][(87000-nf)bd -/(87000 -
nf + fdy)]
(154)
where f is the compression in the concrete due to prestressing and all the
other terms have been defined previously.
If concrete compression is assumed to be 0.45f'c, Equation (154) becomes:
(0.49f'dcK1/fdy)(87,000 - .36 nf'dc)bd -
EQUATION:A -s< /= (155)
(87,000 - 0.36nf'dc + fdy)
g.  Connections.  One of the fundamental differences between a
cast-in-place concrete structure and one consisting of precast elements is
the nature of connections between members.  For precast concrete
structures, as in the case of steel structures, connections can be detailed
to transmit gravity loads only, gravity and lateral loads, or moments in
should be designed so that blast loads are transmitted to supporting members
through simple beam action.  Moment-resisting connections for blast
resistant structures would have to be quite heavy and expensive because of
the relatively large rotations, and hence induced stresses, permitted in
blast design.
2.08-225