A simplified procedure for the seismic retrofit of bridges by seismic isolation: Part 2 - predimensioning of the isolation system

This study presents a simplified procedure for the seismic retrofit of bridges by means of isolation system (IS), applicable to bridges with an isostatic or continuous deck layout, supported by conventional bearings that can be replaced by seismic isolators. The procedure consists of two steps: (1) the assessment of suitability of the bridges for seismic isolation; (2) the preliminary design of the isolation system. The first part of the procedure is presented in a companion paper. This contribution presents the second step of the procedure. A nonlinear static analysis of the existing bridge is performed, and its capacity curve is determined. This curve is then transformed into that of the equivalent SDOF system. Combining the information of the nonlinear static analysis and the bridge characteristics in the ADRS plane, the minimum characteristics of the isolation system to achieve a preset performance point are derived. Two scenarios are considered: in the first one it is sufficient to shift the period of the deck; in the second one, in addition to period shifting, it is necessary to introduce damping to control the displacement of the deck. Once the minimum characteristics of the isolators have been defined, the type and model can be identified through a search into databases of commercial devices. The application of the procedure to a case-study bridge and the validation of the method are finally shown.