LRFD of shallow foundations at the service limit state

Abstract

The design of shallow foundations typically involves two-steps: (1) the calculation of ultimate bearing capacity based on general bearing capacity theories and (2) the calculation of maximum contact pressure to produce an allowable total settlement. The allowable bearing capacity for the shallow foundation system is the lesser of the ultimate bearing capacity, divided by a factor of safety, and the computed maximum allowable contact pressure. Currently, these calculations utilise nominal values of soil strength and stiffness parameters and do not explicitly account for their uncertainty. This paper develops a method for the calibration of resistance factors for load and resistance factor design (LRFD) of shallow foundations at the service limit state. The bearing soil strength and stiffness parameters are assumed to be random variables and the standard elastic settlement equations are combined with the Monte Carlo simulation technique to develop a series of probabilistic pressure–settlement curves. Based on an allowable magnitude of total settlement, the pressure–settlement curves are analysed to compute the pertinent statistics for an allowable bearing resistance that are then utilised to develop the resistance factors. The computed resistance factors are observed to be highly variable and are dependent on design uncertainty and shallow foundation size.