AISC suggests the following method to determine the required thickness of bearing plate, using a maximum bending stress of .75 σy psi (AISC Sec 1.5.1.4.8):
1. Determine the required minimum base plate area, A = P/p. The column load (P) is applied uniformly to the base plate within a rectangular area (shaded ). The dimensions of this area relative to the column section's dimensions are .95 d and .80 b.
The masonry foundation is assumed to have a uniform bearing pressure (p) against the full area (A = BX D) of the base plate. See Table 1 for allowable values of p.
2. Determine plate dimensions B and D so that dimensions m and n are approximately equal. As a guide, start with the square root of required plate area (A). Table 2 lists standard sizes of rolled plate used for bearing plates.
3. Determine overhanging dimensions m and n, the projection of the plate beyond the assumed (shaded ) rectangle against which the load (P) is applied.
m = 1/2 ( D - 0.95 d )
n = 1/2 ( B - .80 b )
4. Use the larger value of m or n to solve for required plate thickness (t) by one of the following formulas:
t = m sqrt( 3p/ r )
t = n sqrt( 3p/ r )
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