Partial factors calibration based on reliability analyses for square footings on granular soils

Abstract

Partial safety factors for square footings for highway bridges resting on granular soils have been determined based on reliability analyses. Some example cases are chosen based on a database which includes detailed information of 1869 actually constructed highway bridge pier shallow foundations in one fiscal year in Japan. The designs obtained, using calculations based on the bearing capacity equations by Meyerhof and by Brinch Hansen as modified by Vesic, are compared. The uncertainties involved in the bearing capacity equations are investigated through a comprehensive literature review. The seismic forces determined from the peaks over threshold analysis and fitted to a general Pareto distribution have been considered. The first-order reliability method (FORM) and Monte Carlo simulations (MCS) are employed to determine and compare the 100-year failure probabilities of the shallow foundations designed. It is found that FORM gives a considerably lower failure probability than MCS. Finally, partial factors obtained using calculations based on the two bearing capacity equations are carefully reviewed. It is found that the design value method used to determine partial factors by FORM does not appropriately give either the partial factors or the failure probability for the case of shallow foundation design where the performance function is highly non-linear and some of the basic variables follow distributions that are far from the normal distribution.