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 c. Design Criteria. The design criteria listed below are also
presented in Stea's report for the two types of foundation extensions
utilized in protective structures.
(1) Thick Foundation Extension, i.e., *ln/d < 5, where *ln is
the clear span of member to the face of the support and d is distance from
extreme compression fiber to the centroid of tension reinforcement
(a) Allowable shear stress carried by the concrete is:
EQUATIONS:
vc
= [phi][3.5 - (2.5Mcr)/Vud] x [1.9(f'c)1/2
(156a)
+ 2,500pw(Vud/Mcr)]
and:
[3.5 - (2.5Mcr)/Vud] < /= 6(f'c)1/2
(156b)
where,
[phi]
=
capacity reduction factor = 0.85 for all sections
vc
=
nominal permissible shear stress carried by concrete, psi
Mcr
=
applied design load moment at the critical section, in-lb
Vu
=
total applied design shear force at critical section, lb
d
=
distance from extreme compression fiber to centroid of
tension reinforcement, in
f'c = specified compressive strength of concrete, psi
pw = As/bd, the ratio of area of flexural reinforcement to
area of concrete within depth, d, and width, b.
b = width of compression face, in
(b) The critical section for shear is taken as 15 percent of
the clear span (0.15 *ln) measured from the face of the support.
(2) Thin Foundation Extension, i.e., *ln/d > 5.
(a) Allowable shear stress carried by concrete is expressed
as:
EQUATION:
vc = [phi][1.9(f'c)1/2 + 2,500pw]
< 2.28 [phi] (f'c)1/2
(157)
Equation (157) is identical to Equation 5-10 of NAVFAC P-397. In fact,
the provisions of Section 5-3 of NAVFAC P-397 are utilized to determine
the permissible shear stress.
(b) The critical section for shear is taken at a distance, d,
from the face of the support.
2.08-305
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