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

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.