Calibration of Resistance Factors for Drilled Shafts for the New FHWA Design Method
The Louisiana Transportation Research Center has released a report that presents the reliability-based analyses for the calibration of the resistance factor for load and resistance factor design of axially loaded drilled shafts. Here is the abstract:
The Load and Resistance Factor Design (LRFD) calibration of deep foundation in Louisiana was first completed for driven piles (LTRC Final Report 449) in May 2009 and then for drilled shafts using 1999 FHWA design method (Oâ€™Neill and Reese method) (LTRC Final Report 470) in September 2010. As a continuing effort to implement the LRFD design methodology for deep foundations in Louisiana, this report will present the reliability-based analyses for the calibration of the resistance factor for LRFD design of axially loaded drilled shafts using Brown et al. method (2010 FHWA design method). Twenty-six drilled shaft tests collected from previous research (LTRC Final Report 449) and eight new drilled shaft tests were selected for statistical reliability analysis; the predictions of total, side, and tip resistance versus settlement behavior of drilled shafts were established from soil borings using both 1999 FHWA design method (Brown et al. method) and 2010 FHWA design method (Oâ€™Neill and Reese method). The measured drilled shaft axial nominal resistance was determined from either the Osterberg cell (O-cell) test or the conventional top-down static load test. For the 30 drilled shafts that were tested using O-cells, the tip and side resistances were deduced separately from test results. Statistical analyses were performed to compare the predicted total, tip, and side drilled shaft nominal axial resistance with the corresponding measured nominal resistance. Results of this showed that the 2010 FHWA design method overestimates the total drilled shaft resistance by an average of two percent, while the 1999 FHWA design method underestimates the total drilled shaft resistance by an average of 21 percent. The Monte Carlo simulation method was selected to perform the LRFD calibration of resistance factors of drilled shaft under strength I limit state. The total resistance factors obtained at different reliability index (Beta) were determined and compared with those available in literature. Results of reliability analysis, corresponding to a target reliability index (Beta) of 3.0, reveals resistance factors for side (Phi-side), tip (Phi-tip), and total resistance factor (Phi-total) are 0.26, 0.53, and 0.48, respectively for the 2010 FHWA design method and 0.39, 0.52, and 0.60, respectively for the 1999 FHWA design method. The side and total resistance factors calibrated using the 2010 FHWA design method are less than those calibrated using the 1999 FHWA design method.