Post-earthquake Traffic Capacity of Modern Bridges in California

Evaluation of the post-earthquake capacity of a bridge to carry self-weight and traffic loads is essential for a safe and timely re-opening of the bridge after an earthquake. Although modern highway bridges in California designed using the Caltrans Seismic Design Criteria are expected to maintain at minimum a gravity load-carrying capacity during both frequent and extreme seismic events, as of now, there are no validated, quantitative guidelines for estimating the remaining load carrying capacity of the bridges after an earthquake event. In this study, experimental and analytical methods were combined to evaluate the post-earthquake traffic load carrying capacity of a modern California highway overpass bridge. An experimental study on models of circular reinforced concrete bridge columns was performed first to investigate the relationship between earthquake-induced damage in bridge columns and the capacity of the columns to carry axial load in a damaged condition. The earthquake-like damage was induced in the column specimens in bi-lateral quasi-static lateral load tests. The damaged column specimens were then tested in compression to failure to evaluate their remaining axial load strength. It was found that well-confined modern bridge columns loose approximately 20% of their axial load capacity after sustaining displacement ductility demands of 4.5 in both principal directions of the bridge. Typical California highway overpass bridges are designed such that they are not expected to develop displacement ductility demands larger than 4.0 in design-basis earthquake events. These test results were used to calibrate a finite element model of a bridge column. This bridge column model was incorporated into a hybrid model of a typical California overpass bridge and tested using the hybrid simulation technique. This typical bridge is a straight 5-span overpass with single-column bents. During these hybrid simulations a heavy truck load was applied on the bridge immediately after the earthquake to study the behavior of the damaged bridge under such truck load. The hybrid bridge model safely carried the applied truck load after surviving an earthquake that induced displacement ductility demands of 4.7 and 6.7 in the longitudinal and transverse direction of the bridge, respectively. The finite element model of the typical California overpass bridge was validated using the data from hybrid simulations. The validated model of the typical bridge was used to evaluate its post-earthquake truck load capacity in an extensive parametric study that examined the effect of different ground motions and bridge modeling parameters such as the boundary conditions imposed by the bridge abutments, location of the truck on the bridge, and amount of bridge column residual drift. The principal outcomes of this study are the following findings. A typical modern California highway bridge overpass is safe for traffic use after an earthquake if none of its columns failed, i.e. none of the column main reinforcing bars fractured, and if its abutments are still capable for restraining torsion of the bridge deck about its longitudinal axis. If any of the columns failed, i.e. if broken column reinforcing bars were discovered in a post-earthquake inspection, the bridge should be closed for regular traffic. Emergency traffic with weight, lane and speed restrictions may be allowed on a bridge whose columns failed if the abutments can restrain torsion of the bridge deck. These findings pertain to the bridge configuration investigated in this study. Additional research on the post-earthquake traffic load capacity of different bridge configurations is strongly recommended