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(3) Ultimate Resisting Moment.
(a) To resist the buildup of soil pressures beneath the
structure, the ultimate unit resisting moment of the foundation extension
should be computed as:
EQUATION:
Mu = (Asfs)[d  (a/2)]/b
(158)
where,
Mu
=
ultimate unit resisting moment, inlb/in
As
=
area of tension reinforcement within the width b, in2
fs
=
static design stress for reinforcement, psi
d
=
distance from extreme compression fiber to tension
reinforcement, in
a = Asfs/0.85bf'c, the depth of equivalent rectangular
stress block, in
f'c = specified compressive strength of concrete, psi
(b) To develop the strength of a blast wall, the ultimate
unit resisting moment of the foundation should be calculated using Equation
(159):
EQUATION:
Mu = Asfdsdc/b
(159)
where,
fds = dynamic design strength for the reinforcement (see Section
56 of NAVFAC P397).
dc = distance between centroids of the compression and tension
reinforcement, in.
(4) Minimum Flexural Reinforcement. The recommended minimum
areas of flexural reinforcement are listed in Table 31. These insure the
proper structural behavior of the foundation and also prevent excessive
cracking and deformations under conventional loads.
TABLE 31
Minimum Area of Flexural Reinforcement
Reinforcement
Oneway element
Twoway element
Main
As = 0.0025 bd
As = 0.0025 bd
Other
As = 0.0010 bTc[*]
As = 0.0018 bd
[*]Tc = total thickness of foundation
2.08306


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