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