Calibration of load and resistance factor design - Resistance factors for drilled shaft design
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The AASHTO load and resistance factor design specification was approved for use in 1994 with phi factors determined from fitting allowable stress design (ASD). Unfortunately, the latter did not provide resistance factors for shafts founded in sand, gravel, and rock or identify the influence of construction (dry, wet, and cased). Using a database of 273 shafts, of which 185 failed (settlement equal to 5% of the diameter) and have sufficient soil information, the resistance factors were determined for a number of different AASHTO design methods based on probability of failure. Effects of skin friction, combined skin and end bearing, and method of construction were investigated for different soils/rock. It was found that all the methods had resistance factors between 0.3 and 0.6, which correspond to a factor of safety of 2.5 to 4.5 in ASD. Current FHWA design methods gave the highest resistance factors in soils with the cased method of construction. In the case of rock, the dry hole method of construction and the intermediate geomaterials design method gave the highest resistance factors.
