Seismic Fragility of Multispan Simply Supported Bridge with Drop Spans and Steel Bearings

Abstract

Multispan simply supported bridges with drop spans and steel bearings were commonly constructed in northern India during the 1970s, and little knowledge about their seismic behavior is available in the literature. Because these bridges are part of the existing transportation network, it is important to assess their seismic fragility and suggest possible retrofitting solutions, if required. A four-span model of such a bridge type was analytically investigated for its seismic response. The critical components of such bridges are rocker and roller-cum-rocker steel bearings, and their lateral force–deformation responses were obtained from nonlinear finite-element analyses. Incremental dynamic analyses were performed for the development of fragility curves for failure of the bridge along two horizontal directions. The damage criteria included either failure of bearings due to exceedance of respective displacement limits or dislodgement of the bridge deck due to unseating. It was observed that seismic demands for drop spans were small, and failure of main-span steel bearings was the primary cause for the bridge failure. Retrofitting of the bridge by replacing steel bearings with unanchored elastomeric pads is shown to significantly reduce the fragility of the retrofitted bridge.