Required Embedment Length of Column Reinforcement Extended into Type II Shafts

California Department of Transportation (Caltrans) seismic design specifications on the embedment length of column reinforcement terminating in a Type II (enlarged) shaft are very conservative, especially for large-diameter columns, complicating the construction work and increasing construction costs. This report presents an experimental and analytical investigation to determine the minimum embedment length required and the transverse reinforcement required in the bar anchorage regions of these shafts. Experiments were carried out to investigate the bond strength and cyclic bond deterioration of large-diameter bars and to evaluate the adequacy of the development length requirements in the AASHTO LRFD Bridge Design Specifications. Results have been used to develop, calibrate, and validate a semi-empirical bond-slip model for bars embedded in well-confined concrete. The model can successfully reproduce bond deterioration caused by cyclic bar-slip reversals and the tensile yielding of the bar. While the development length tests have indicated that the AASHTO requirements are adequate to develop the expected yield and tensile strengths of a large-diameter bar, further numerical studies using finite element models and Monte Carlo simulations have indicated that they do not have sufficient reliability to develop the full tensile capacity of a bar when uncertainties in material properties and construction quality are considered. New design recommendations on the minimum embedment length have been proposed. Recommendations on the transverse reinforcement required in the bar anchorage region of a shaft are also provided.

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