UC Berkeley

The improved seismic performance and cost-effectiveness of two innovative performance-enhancement technologies in typical reinforced concrete bridge construction in California were assessed in an analytical and experimental study. The technologies considered were lead rubber bearing isolators located underneath the superstructure and fiber-reinforced concrete for the construction of bridge piers.

A typical five-span, single column-bent reinforced concrete overpass bridge was redesigned using the two strategies and modeled in OpenSees finite element program. Two alternative designs of the isolated bridge were considered; one with columns designed to remain elastic and the other such that minor yielding occurs in the columns (maximum displacement ductility demand of 2). The analytical model of the fiber-reinforced concrete bridge columns was calibrated using the results from two bidirectional cyclic tests on approximately ¼-scale circular cantilever column specimens constructed using concrete with a 1.5% volume fraction of high-strength hooked steel fibers, relaxed transverse reinforcement, and two different longitudinal reinforcement details for the plastic hinge zone.

Pushover and nonlinear time history analyses using 140 ground motions were carried out for the different bridge systems. The PEER performance-based earthquake engineering methodology was used to compute the post-earthquake repair cost and repair time of the bridges. Fragility curves displaying the probability of exceeding a specific repair cost and repair time thresholds were developed. The total cost of the bridges included the cost of new construction and post-earthquake repair cost required for a 75 year design life of the structures. The intensity-dependent repair time model for the different bridges was computed in terms of crew working days representing repair efforts. A financial analysis was performed that accounted for a wide range of discount rates and confidence intervals in the estimation of the mean annual post-earthquake repair cost.

Despite slightly higher initial construction costs, considerable economic benefits and structural improvements were obtained from the use of the two performance-enhancement techniques considered, in comparison to the fixed-base conventionally reinforced concrete bridge, especially seismic isolation. The isolation of the bridge superstructure resulted in a significant reduction in both column and abutment displacement and force demands. The repair time of the isolated bridges was also significantly reduced, leading to continuous operation of the highway systems and reduced indirect economic losses. The experimental and analytical results also demonstrated that the use of fiber-reinforced concrete to build bridge columns leads to improved damage-tolerance, shear strength, and energy dissipation under cyclic loading compared to conventional reinforced concrete columns. These improvements result in better seismic performance and lower total 75-year cost of the fiber-reinforced column bridges.