Substructure Mass Participation Effect on the Performance-Based Seismic Design Method for Isolated Bridges

Abstract

Many of the procedures used in bridge design are force-based and may be considered reasonable design approaches that will lead to safe structures. But they do not directly address performance criteria at the initial stage of the design. On the other hand, the previously presented displacement-based design procedures of isolated bridges only considered the displacement of the deck, which means the pier’s seismic performance and the substructure mass effects are ignored. This paper presents a new performance-based design method for isolated RC bridges considering the contribution of substructure mass. This method allows the designer that selects the performance level of the superstructure and substructure at the beginning of the design procedure. In this respect, the two degrees of freedom analytical model of the isolated bridge containing substructure mass is presented to quantify its effect on the performance of the seismically isolated bridge. The proposed design method is applied to two continuous-span bridges with regular and irregular substructures, and different target performance levels, including elastic and yielded substructures. The results of the proposed design method have been compared with nonlinear time history analyses, AASHTO, and National Cooperative Highway Research Program simplified methods. The results showed that the seismic performances are close to the results of the nonlinear time history analyses. However, the other design approaches, which ignore the substructure mass, underestimate the responses of the substructure.