Effect of Vertical Ground Motions on the Structural Response of Highway Bridges

The 1971 San Fernando earthquake in California caused highway bridges to collapse because of excessive longitudinal movement at expansion joints and supports. Since then, the California Department of Transportation (Caltrans) has installed longitudinal cable restrainers to prevent such collapse. In the study presented in this report, two existing design methods, the Caltrans and AASHTO methods, were evaluated for simply-supported bridges. In addition, three new design methods for restrainers were proposed and evaluated. The adequacy of the design methods was determined using nonlinear response history analyses of a large number of two- and five-span bridges. The models represented highway bridges that would be retrofitted with restrainers. All of the procedures considered in this study were found to be effective in preventing spans from unseating in most bridges even under strong earthquakes. In the most critical cases with narrow supports and skew, however, it was found that the Caltrans and a modified version of the Caltrans methods (a new method developed in this study) were the procedures that prevented unseating.