Seismic Vulnerability Assessment and Retrofitting Design of a Multispan Highway Bridge: Case Study

Seismic vulnerability is a critical issue in Italy, because the bulk of existing structures were designed before enforcement of the new national building code in 2008 and the ensuing seismic risk map, which assigns a nonnegligible peak ground acceleration (PGA) to previously nonseismic areas. Thus, seismic retrofitting becomes of primary importance even though often challenging, because remarkable limitations mostly due to structure geometry, site features, and economical issues should be tackled. All of the aforementioned aspects were a primary concern for the Tanaro and Bormida Bridges (Alessandria, Italy), discussed herein, of the A21 Highway. The main criticalities came from (1) the presence of three different superstructure deck types (simply supported RC and prestressed concrete beam decks, and a continuous prestressed concrete box girder deck), (2) geometrical and feasibility restraints (limited deck-to-deck gaps and bearing heights), and (3) instream foundations. Whereas vulnerability assessment showed the need for retrofitting, seismic isolation of the bridge with friction pendulum bearings (FPBs) was studied. The preliminary design was performed through the simplified linear single-degree-of-freedom (SDOF) model, and the final solution was checked by means of linear modal analyses via finite-element simulations. The use of a SDOF model proved to be very useful in designing the isolation system thanks to its flexibility and possibility of easily managing the main parameters. The final designed isolation system was shown to be very effective owing to the significant reduction (even higher than 80% in the present work) of the base shear force and the limitation of the seismic displacements allowed by high energy dissipation capability.