Seismic analysis and retrofit of mid-America bridges

The lack of seismic design practices in the Central and Southeastern United States (Mid-America), coupled with the potential for strong ground motion has led to a significant hazard to the bridge infrastructure in the Mid-America region. This hazard is amplified when one considers the fact that the bridge infrastructure in Mid-America is a key component in the transportation network of the United States.; A comprehensive analytical study is conducted to assess the seismic vulnerability of typical bridges in Mid-America. Detailed inventory studies are performed to obtained a profile of the characteristics of typical bridges in Mid-America. Next, nonlinear analytical models of six typical bridge types are developed using DRAIN-2DX nonlinear analysis program. Using the analytical models and synthetic ground motion for several typical cities in Mid-America, the seismic response of the bridges are assessed. The results show that several classes of bridges are vulnerable to significant damage at relatively low levels of ground motion. Several retrofit measures, focusing on the bridge superstructures are evaluated. The analytical studies show that the most effective retrofit measure for increasing the seismic resistance of most of the typical bridges is using lead-rubber bearings to isolate and provide damping to the superstructure. Finally, fragility curves for the typical bridges are developed for both as-built and retrofitted conditions. These curves, which account for the uncertainty in both ground motion and material properties of the bridge, confirm the results of the deterministic analyses. These fragility curves will be a key tool that can be used for loss estimation, and network vulnerability studies. In particular, the fragility curves developed in this study can form the basis for decision making for the bridge community in Mid-America.