Aspects of seismic assessment and retrofit of existing highway bridges: Column retrofit design and footing stiffness

This research shows the results of two studies; the first was conducted on retrofitted one-way hinged base columns and fixed base columns subjected to constant axial compression and cyclic lateral load in the strong and weak direction. The second study was conducted on three footings of an existing bridge subjected to dynamic forces through an eccentric mass shaker, which was attached to the footings. The objectives of this research were to upgrade the performance of RC tapered bridge columns, which have inadequate dowel anchorage in both columns and footings, and to develop a simple procedure for the computation of stress for spread footings supporting bridges.; In the first part of this research, four 0.4-scale models were tested. The first two specimens had one-way hinge details at the base. In one specimen, the base plate in the retrofit was designed to remain elastic, while in the other specimen, the base plate was designed to yield. The other two specimens had fixed base details; one was tested in the strong direction, while the other was tested in the weak direction. A steel jacket was placed on each of the retrofitted columns to improve the shear strength and the concrete confinement. To ensure sufficient moment transfer, these steel jackets were connected to the footings by a base plate and bolts, which were anchored to the footing. Results from testing the retrofitted specimens showed that connecting the steel jacket to the base plate provided an effective retrofitting method and enabled the columns to reach their design strength under lateral cyclic loading. The retrofit also enhanced the shear strength and ductility capacity of the columns. In addition to the experimental study, design recommendations and design procedures were presented.; The second part of this study presents the results of an experimental and analytical study conducted on three footings of an existing bridge. The bridge is located on Interstate 80 south east of Verdi, Nevada. Six tests were conducted by shaking each footing in the longitudinal and the short directions of the footing. Each footing was subjected to sinusoidal vibration force, which was applied through an eccentric mass shaker. Twenty-three accelerometers were used to measure the response of the soil, bridge footings, and the bent cap of the bridge during shaking. A three-dimensional finite element model of the last two spans of the bridge was developed using computer program COSMOS/M. Then, a linear response history analysis was performed to determine the steady state acceleration response of the three footings. During the analysis, the soil-footing interaction was modeled using a spring-dashpot model. Several refinements were applied to the response history analysis until the predicted steady state peak acceleration at the footing level matched the measured steady state peak acceleration on the corresponding footing during the dynamic field test. A design guideline and recommendations, which provide the basis for the computation of footing stiffness using spring models, were developed based on the experimental and the analytical results.