Retrofit of Column-Bent Cap Connections of Alaska Bridges for Seismic Loadings: Damage Evaluation

Typical bridges in the state of Alaska are characterized by columns with excessive reinforcing ratios encased in a steel shell that is partially embedded in the bent cap; analysis of existing bridges indicate that the bent caps are inadequately designed for flexure, shear, and joint shear for the feasible moments that would be imposed upon them during a seismic event. This research proposes a method to retrofit such bridges to perform in a ductile manner during a seismic event. Three 80% scale column-bent cap t-connections were built to model such typical as-built bridges in Alaska. They were retrofitted to improve their response to a lateral loading and then tested under a reversed cyclic load. From a moment curvature analysis of a prototype column and bent cap, it was decided to reduce the longitudinal reinforcement ratio of the column at the joint to increase its ductility at lower lateral load levels and subsequently to reduce the moment demand on the adjacent bent caps. As such, the moment ratio of the bent cap over the column was increased thus forcing the plastic hinge to form at the top of the column, as this is the current seismic design practice. Using existing strut and tie models for joint design, additional reinforcement was provided to prevent joint shear failure. The retrofit was applied by enlarging the bent cap in the joint region to allow for proper placement of the additional reinforcement. The specimens were then loaded in a reversed cyclic loading and results from this research program are presented and discussed in this paper