Comparison of seismic performance of three restrainers for multiple-span bridges using fragility analysis

Steel restrainer cables for multiple-frame bridges in California in the United States showed the effectiveness to prevent the unseating at the internal hinges during the past several earthquakes. After that, the steel restrainer cables are being tried to apply for multiple-span-simply-supported (MSSS) bridges in the central and the southeastern regions in the United States. In addition, shape memory alloy (SMA) bars in tension are being studied for the same application. In multiple-frame bridges, the developed seismic forces are transferred to piers through the restrainers; however, in MSSS bridges, the seismic forces are transferred to abutments by the restrainers. Therefore, the abutment’s behavior should be investigated as well. This study assesses the seismic performance of the three types of restrainers—steel restrainer cables, SMA bars in tension, and SMA bars in bending for MSSS bridges—due to moderate to strong ground motions using a deterministic seismic analysis. Also, a fragility analysis is conduced to assess the seismic resistance for the overall bridge system. For these analyses, the bending test of an SMA bar is conducted and its analytical model is determined. Then, nonlinear time history analyses are conducted to assess the seismic responses of the as-built and the retrofitted bridges. The deterministic analysis illustrates how the restrainers influence the components of the bridge. However, it could not explain the effects on the whole bridge system. The fragility analysis shows the effects of the three restrainers on the overall bridge system. The fragility analysis indicates that the SMA bars in bending are the most effective ones.