RETROFITTING BY SEISMIC ISOLATION OF EXISTING BRIDGES: FAST PROCEDURE FOR THE PRELIMINARY DESIGN

In recent years increasing attention has been devoted to existing bridges, facing issues varying from maintenance to seismic rehabilitation. Isolation has proved to be a viable option for reducing the seismic vulnerability of these structures, as witnessed by the large number of bridges retrofitted using this technology. Replacing conventional with seismic isolation bearings (SIBs) proves to be an effective solution to reduce seismic forces, avoiding increasing the resistance of the substructures designed according to old codes. The study aims at proposing a fast procedure for the preliminary design of the seismic retrofit of existing bridges by seismic isolation. The procedure, which is applicable to simply supported and continuous deck bridges, consists of two steps. In the first one, the effectiveness of seismic isolation for the considered structure is checked by assessing the ability of the bridge substructures to resist the inertial forces associated to their own masses, under the assumption that the deck is fully isolated, and its acceleration is reduced to a small fraction of the acceleration that would be induced in the as-built structure. In the event of positive outcome of this first check, the method moves to the evaluation of the characteristics of the SIBs. The preliminary design is performed in the Acceleration-Displacement Response Spectrum (ADRS) space; the procedure, aiming at keeping the response of the substructures within the elastic range, determines the maximum displacement of the deck as a function of the seismic demand, considering both the deflection of the piers and the displacement of existing joints, and calculates the required stiffness and damping of SIBs. The proposed procedure is eventually applied to a case study bridge and nonlinear dynamic analyses are performed to validate the method.