How do you calculate the strap footing bearing pressures?
In the design of a strap footing the system is assumed infinitely rigid and acts as a unit, so that the longitudinal bending moments are resisted entirely by the strap beam. The calculation of the bearing pressures implies the calculation of the upward reaction forces at the center of both footings by taking moments about the other footing, by simple Statics, as shown below. Even when there’s an eccentricity between the applied loads and the reactions, the footings cannot rotate due to the restraining action of the strap beam between the two columns.
Note that by doing this the system actually works as a unit and some loads may be transferred from one footing to the other, for example the exterior reaction might not necessarily be equal to the exterior loads (P1 ≠ R1 and P2 ≠ R2), where P1 and P2 represent the applied service loads. As a result, since the reactions are applied at the center of the footings, the bearing pressures are uniformly distributed.
Once the service reactions are calculated, the two footings may be sized so that the resulting bearing pressures are under the allowable limits established in the soils report. ASDIP FOUNDATION accurately calculates the soil bearing pressures under the exterior and interior footings for the applied service loads, as shown in the picture below.