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:
= [phi][3.5 - (2.5Mcr)/Vud] x [1.9(f'c)1/2
[3.5 - (2.5Mcr)/Vud] < /= 6(f'c)1/2
capacity reduction factor = 0.85 for all sections
nominal permissible shear stress carried by concrete, psi
applied design load moment at the critical section, in-lb
total applied design shear force at critical section, lb
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
vc = [phi][1.9(f'c)1/2 + 2,500pw]
< 2.28 [phi] (f'c)1/2
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.