Longitudinal seismic response of concrete substructure-to-steel superstructure integral bridge connections

A research program consisting of both experimental testing and force transfer modeling was conducted at the University of California in San Diego (UCSD) to characterize the behavior of a bridge design detail that integrates a steel superstructure with a concrete substructure using a concrete bent cap. When an integral bridge is subjected to a longitudinal seismic event, a moment is imposed on the girders that must be transferred to the column. This moment is transferred through torsional action in the bent cap. Because the bent cap is a designated capacity protected component, it must remain essentially elastic during the force transfer. Therefore, the research objective was to establish a behavior profile of the bent cap connection to define limit states. Clear definition of the entire behavior was needed to ensure that the proposed design recommendations placed the bent cap performance in the elastic range for a given seismic event.; The effect of two design parameters on the bent cap torsional behavior and moment capacity was investigated in a series of four component tests, with the most promising detail to be validated in a final system test. The two parameters investigated were (1) bent cap reinforcement (post-tensioned versus conventionally reinforced), and (2) girder web configuration inside the bent cap (with or without bearing stiffeners). The specimens with post-tensioned bent caps exhibited less crack dilation during the initial loading stages than the specimens with conventionally reinforced bent caps. The specimens with girder web bearing stiffeners reached maximum capacities approximately 25% higher than the specimens without girder web stiffeners.; The component test results lead to the recommended design for the system test of stiffened steel girders integrated with a post-tensioned bent cap. The system test demonstrated that the bent cap could be designed to perform essentially elastically while the column developed an inelastic mechanism.; Based on the experimental findings and force transfer models, design guidelines for an integral bridge were developed. The procedure for determining the bent cap torsional strength for a given earthquake, as well as recommended limits, is outlined in a design example.