Bearing strength surfaces implied in conventional bearing capacity calculations

Abstract

The classic bearing capacity equation for a shallow foundation determines the vertical load which will initiate bearing failure. The factor of safety is this failure load divided by the applied vertical load. However, a shallow foundation might also need to sustain moment and shear; in some situations the vertical load does not change significantly and the action taking the foundation towards bearing failure is moment, in which case the vertical load becomes a stabilising influence. Bearing strength calculation under general actions is thus more complex than under vertical load only. In these situations the standard estimation of the available reserve of bearing strength can be quite misleading and fails to indicate how rapidly bearing strength diminishes with increasing moment. A good visual understanding comes by way of the bearing strength surface (the locus of all possible combinations of vertical load, horizontal shear and moment that will induce bearing failure). The bearing strength of the foundation is then not a single number but the combination of the vertical load, horizontal shear force and moment where the action path (locus of the combinations of vertical load, horizontal shear and moment applied to the foundation) intersects the bearing strength surface.