Question

(4 points) Results from the bearing capacity equation indicate that the ultimate bearing capacity of a 4-ft by 4-ft shallow foundation is 8500 psf. For design you typically use factor of safety of 2.5. Results from settlement analysis indicate that a bearing stress of 4200 psf will result in settlement of 1 inch. If 1 inch is the allowable settlement of the proposed structure, what is the maximum allowable column load that should be placed on the foundation? Why?

Answer 1

there are two criteria for foundation design

i) shearing criteria --> ultimate capaity = 8500 psf ==> allowable bearing capacity = 8500/FOS = 8500/2.5 = 3400 psf

ii) settlement criteria --> allowable settlement = 1 inch , bearingstress for this = 4200 psf (note this is as per allowable settlement hence no need for further FOS)

allowable bearing stress = minimum of above two criteria

allowable column load = minimum(3400,4200) * footing area

=3400 * (4*4) = 54400 pound

thank you.