Comparison of Resistance Factors Between the 1999 and the 2010 FHWA Design Methods for LRFD Design of Drilled Shafts

This paper presents the reliability-based analyses for the calibration of the resistance factors for Load and Resistance Factor Design (LRFD) design of axially loaded drilled shafts using both the 2010 Federal Highway Administration (FHWA) and the 1999 FHWA design methods. Thirty-four drilled shaft test cases were collected for this study. The shaft nominal axial resistance was first estimated using both the 2010 FHWA and the 1999 FHWA design methods. Statistical analyses were then performed to compare the predicted and the measured drilled shaft nominal axial resistances. Results of the analyses showed that the 2010 FHWA design method overestimates the measured drilled shaft resistance, while the 1999 FHWA design method underestimates the measured drilled shaft resistance. The LRFD calibration of resistance factors of drilled shaft under strength I limit state was performed. The resistance factors were determined at different reliability indices (β). Results of reliability analysis at a target reliability index (βT) of 3.0 showed that the resistance factors calibrated using the 2010 FHWA design method are less than those calibrated using the 1999 FHWA design method (0.48 vs. 0.60). However, the 2010 FHWA design method gave a slightly higher efficiency factor than the 1999 FHWA design method did (0.48 vs. 0.47).