Design Criteria

Custom Search

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