How is bearing pressure/soil pressure calculated for isolated spread footings?


QUESTION:  How is bearing pressure/soil pressure calculated for isolated spread footings?

ANSWER:  For isolated spread footings, forces and moments obtained from each load combination, at the top of the footing, are applied at the centroid of the footing. The footing is treated as a member with infinite stiffness. The bearing pressure at each corner is calculated, as follows:

where:
P                 =   Axial force acting on the centroid of the footing
Mxx, Mzz    =   Moment acting on centroid of footing about global X and Z axes
Ixx, Izz        =   Moment of inertia with respect to global X and Z axes
x, z              =   Distance from the neutral axis of footing, along X, Z axes
The self-weight of the footing and surcharge are added to the bearing pressure under service and factored load combinations, in addition to the pressure due to column forces and moments.
This same procedure is used to calculate the bearing pressure for combined spread and strap footings. For multi-column piers, the result of forces and moments, with respect to the centroid of the footing, is computed and the above equation is used to obtain the bearing pressure.