The lecture will show how shallow foundation behaviour under general loading (vertical load, horizontal shear, and moment) is more interesting than the standard bearing capacity equation suggests. In fact, manipulation of the equation reveals that beautiful bearing strength surfaces are implied. These surfaces are the locus of all possible combinations of vertical load, horizontal shear, and moment, which will induce bearing failure of a shallow foundation.
The background to this statement will be elaborated and the equivalence between conventional bearing strength calculations and those using the BSS will be demonstrated. The proposal will be made that calculations using the conventional equation are likely to be more attractive in a design office setting and, with the visual understanding provided by bearing strength surface thinking, better informed. In this way the “black-box” aspect of the conventional bearing strength equation is illuminated.
Applications to foundations for gravity retaining walls, both under static load and earthquake loading, will be considered. The discussion will be extended to modelling shallow foundations for multi-storey buildings under earthquake loading incorporating data obtained from field testing of near prototype scale rocking foundations and observations from the behaviour of large buildings in Christchurch on shallow foundations in gravels during the events of 2010 and 2011. Finally, the preparation of input data for structural analysis software modelling foundation-soil interaction with a bed of springs will be considered.
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