UC San Diego

Enlarged (Type II) pile shaft foundations are used frequently in reinforced concrete bridges because of the convenience in construction and efficiency in post- earthquake inspection and repair. According to the specifications of the California Department of Transportation (Caltrans), the diameter of a Type II shaft should be at least 610 mm (2 ft) larger than that of the column. Hence, the column reinforcement extended into the pile shaft can be perceived as forming a non-contact splice with the pile shaft reinforcement. Because of the lack of data, the seismic design specifications of Caltrans on the embedment length of column reinforcement in Type II shafts are very conservative for large-diameter columns, which could complicate the construction work and entail high construction costs. This dissertation presents an experimental and analytical investigation to characterize the bond between concrete and reinforcing steel when a reinforced concrete member is subjected to severe cyclic loading, and determine the minimum embedment length required for column longitudinal reinforcement extended into a Type II shaft. Experiments were carried out to investigate the bond strength and cyclic bond deterioration of large-diameter bars (No. 11, 14, and 18) commonly used in large-diameter bridge columns and piles. The experimental results have been used to develop, calibrate, and validate a phenomenological bond-slip model for bars embedded in well-confined concrete. The model successfully reproduces bond deterioration caused by cyclic bar-slip reversals and tensile yielding of the bar, and has been implemented in an interface element in a finite element program. A physics-based dilatant interface model formulated with a multi-surface plasticity concept has also been developed and implemented in the finite element program to simulate bond-slip under a broad range of confinement situations. With the phenomenological bond- slip model, nonlinear finite element analysis has been conducted to extrapolate results of development length tests conducted on large-diameter bars, and assess the reliability of the development lengths required in the AASHTO LRFD Bridge Design Specifications. Finally, two large-scale tests on column-pile shaft assemblies were conducted. The tests were combined with finite element analysis to evaluate the conservatism of the current Caltrans specifications, and provide new design recommendations that can significantly reduce the embedment length required for column reinforcement, while ensuring an appropriate performance of the column-pile shaft connections under severe seismic loads