Robustness of constant load and resistance factor design factors for drilled shafts in multiple strata

Abstract

This study addresses the issue of adopting constant resistance factors for the LRFD approach to the axial compression of drilled shafts in multiple strata of clay, sand, gravel, and rock. The focus is on how to achieve a uniform reliability level for a wide range of stratum scenarios with constant or nonconstant LRFD factors. The main finding is that a uniform reliability level cannot be achieved if the LRFD factors are constant and are independent of the thicknesses or relative contributions of the strata. This conclusion is confirmed in three separate ways from examination exercises: (1) by using LRFD factors recommended in a recent Federal Highway Administration manual, (2) by using tailormade LRFD factors derived from optimization, and (3) by using LRFD factors calibrated from the first-order reliability method. A possible strategy of calibrating nonconstant LRFD factors without resulting in a proliferation of factors is proposed in this study. The use of this approach is demonstrated by a design example. © 2013 American Society of Civil Engineers.