Ductility Analysis of Type II Pile Shafts

Large diameter Cast-In-Drilled-Hole (CIDH) pile shafts have been widely used to support bridge columns due to the significant cost savings. Two types of pile shafts are classified in the California Department of Transportation (Caltrans) Seismic Design Criteria. Type I shafts may have a plastic hinge formed below ground in the pile shaft. Type II shafts are usually enlarged and the plastic hinge forms at the shaft/column interface. Based on the ductility response and capacity design concept, it is very important to have a plastic hinge formed in the column, not in the foundation system in order to avoid the difficulty of post-earthquake inspection and the high cost associated with repair of the damaged foundation system. Therefore, Type II pile shafts should be a preferred alternative. Since Type II shafts are much stronger than columns, the assumption of a “fixed” base at the interface between column and shafts is widely adopted in engineering practice. In this paper, a procedure that incorporates surrounding soil properties into the ductility analysis of Type II pile shafts is developed. The interactions between soil and shafts are represented by the Beam-on-Winkler-Foundation model. The Capacity Spectrum Method is introduced to calculate the ductility of the column-shaft system. The influences of soil-pile interaction are evaluated through the examples.