Calibration of Side, Tip, and Total Resistance Factors for Load and Resistance Factor Design of Drilled Shafts

This paper presents a reliability-based analysis for the calibration of side, tip, and total resistance factors for axially loaded drilled shafts. Twenty-two drilled shafts that had been tested with the Osterberg cell (O-cell) method were collected from project archives at the Mississippi Department of Transportation and the Louisiana Department of Transportation and Development. This database was carefully selected to represent the typical subsurface soil conditions and design practice in Louisiana. The prediction of the load-settlement curves of the drilled shafts from soil borings was determined with the FHWA O'Neill and Reese design method and the SHAFT 6.0 computer program. The interpreted and predicted axial nominal resistances for the drilled shafts were determined with the FHWA 5% shaft diameter settlement criterion from predicted load-settlement curves. The measured nominal side, tip, and total axial resistances for drilled shafts were determined from the O-cell measurements. Statistical analyses were first performed to compare the predicted nominal axial resistances and the measured nominal resistances for the drilled shafts. In general, the selected design method underestimated the measured drilled shaft resistances. The first-order reliability and the Monte Carlo simulation methods were selected to determine the side, tip, and total resistance factors under the Strength I limit at the target reliability index of 3.0. The resulting calibrated side, tip, and total resistance factors were 0.3, 0.6, and 0.7, respectively.