Column Reinforcement Extended into Enlarged (Type II) Shafts

Professor P. Benson Shing

In spite of recent changes, the seismic design specifications of the California Department of Transportation (Caltrans) on the development of longitudinal column reinforcement in enlarged (Type II) CIDH shafts are still very conservative. For large-diameter bridge columns, #14 and 18 bars are common. With the older specifications that were recently replaced, these large-diameter bars would call for very long development lengths, which could significantly drive up the construction costs. Little research data are available on the cyclic bond-slip behavior of large-diameter bars to assess the development and splice length requirements in current design codes.

In a project sponsored by Caltrans, Professor Shing is leading an experimental and analytical study to characterize the bond-slip behavior of large-diameter bars, assess the level of conservatism in current Caltrans specifications, and develop new design recommendations that can further minimize the required development length and improve the performance of these shafts under extreme seismic load conditions. The study includes basic bond-slip experiments and bar pullout experiments as well as detailed nonlinear finite element modeling of the bond-slip behavior in column-shaft assemblies. Finally, four large-scale column-shaft assembles are being tested in the Charles Lee Powell Structural System Laboratory at UC San Diego to validate the design recommendations and finite element models developed in this study. This research can lead to a significant reduction of the depth of the column rebar cage and construction joint in a shaft, which will result in significant cost savings in construction and safer work conditions.