A direct displacement-based method for the seismic design of bridges on bi-linear isolation devices

This paper presents an extension of the displacement-based design procedure for bridges supported on hysteretic isolation bearings. An equivalent damping ratio, derived from the particular characteristics of bridges supported on lead rubber bearings (LRBs), is proposed. The trend of the equivalent damping ratio that is obtained is similar to that of the hysteretic energy bearing dissipation, over the whole range of inelastic displacement demands that is expected for this type of device. The proposed method emphasizes material strain control by means of the lateral displacement of isolator bearings and the lateral displacement of the pier top. The response is obtained directly from the elastic displacement response spectrum and is applicable to regular bridges with rigid superstructures that can be idealized as a single degree of freedom (SDOF) system. The proposed methodology improves the displacement prediction capability of the linear equivalent model when it is applied to bridges supported on LRB isolators, and low data scatter is obtained, especially for displacements. Pier displacements are slightly underestimated and base shears are overestimated, compared to inelastic time-history results.