Interpretation Criteria to Evaluate Resistance Factors for Axial Load Capacity of Drilled Shafts

Abstract

Reliability-based analyses are presented for calibration of the resistance factor for axially loaded drilled shafts with eight interpretation criteria. A database of 26 drilled shafts representing the typical design practice in Louisiana was created for statistical reliability analyses. Predictions of load–settlement curves of drilled shafts from soil borings were determined by using the FHWA design method (O'Neill and Reese method) via the SHAFT computer program. Interpreted predicted and measured drilled shaft axial nominal resistances were determined through use of eight criteria. Measured drilled shaft axial nominal resistances were determined from either Osterberg cell tests or conventional top-down static load tests. Statistical analyses were performed to compare the predicted total drilled shaft nominal axial resistance with the measured nominal resistance interpreted by using eight criteria. Results show that the selected design method underestimates the measured drilled shaft resistance for all eight criteria. This underestimation increases as the mean settlement for each criterion increases. The Monte Carlo simulation method was used to perform the load and resistance factor design (LRFD) calibration under the Strength I limit state. The total resistance factors for the interpretation criteria were determined. Results show that interpretation criteria have a great effect on calibrated resistance factors. The lowest resistance factor is 0.15 for L1 criteria, and the highest resistance factor is 0.41 for Davisson criteria. The resistance factor by FHWA criteria (5%B) is compared with those in the literature. LRFD calibration showed that the FHWA design method has a resistance factor of 0.40 at the target reliability index of 3.0.