Resistance factor calculations for LRFD of driven piles based on setup effects

Setup effects, one of a large number of uncertainties in geotechnical engineering, can strongly enhance the bearing capacity (ultimate resistance) of driven piles after initial installation, especially for ultimate shaft resistance. Based on load and resistance factor design (LRFD) principle and the reliability theory, this paper incorporates the setup effects to propose a methodology for separately calculating the resistance factors for ultimate base and shaft resistance for reliability-based design (RBD) of driven piles. The proposed approach can clearly explain the different sources of uncertainties of the ultimate resistance, including those from ultimate base and shaft resistances. Meanwhile, the discussions on the uncertainties of resistance and load and the probability characteristics of setup effects are presented, and the effects of four relevant parameters (setup effect factor, ratio of dead to live loads ρ = LD/LL, target reliability index, and ratio of ultimate base resistance to load ψ = Rult,b/(LD + LL)) on resistance factors are analyzed. Parametric study indicates that, setup effects and target reliability index significantly influence on the optimal ultimate base and shaft resistance factors for RBD of driven piles, and ρ = LD/LL has the limited effect on the optimal ultimate base and shaft resistance factors. Varying ψ = Rult,b/(LD + LL) obviously causes the change of the optimal ultimate base resistance factor although it slightly influences the optimal ultimate shaft resistance factor. Therefore, the appropriate choice of target reliability index, and the accurate evaluations of setup effects and ψ = Rult,b/(LD + LL) have great roles on resistance factor calculations for LRFD for driven piles.