Seismic analysis and retrofitting of the highway bridge of the rivers Tanaro and Bormida

Seismic retrofitting of existing bridges, aimed at enhancing their safety level under earthquake conditions according to current design standards, is often a challenging task. When designing the strategy for seismic retrofitting, in fact, the practitioner have often to face remarkable limitations to the possible interventions, mostly due to the geometry of the original structures and to construction site accessibility issues. A meaningful example of this task is represented by the concrete bridge over the Tanaro and Bormida rivers of the A21 Italian highway between the cities of Turin, Alessandria and Piacenza, managed by Satap S.p.A in Turin. The overall bridge is characterized, in fact, by three different structural types consisting of: (a) a box girder deck, (b) a deck supported by regular r.c. beams and (c) a deck supported by p.c. beams. The analysis of the seismic vulnerability of the bridge in its ‘as-built’ configuration has been carried out according to the current Italian code DM 14/01/2008. The resulting seismic shear forces on the bridge piers and foundation piles are larger than their shear capacity, showing the need for seismic retrofitting. Instead of using conventional strengthening techniques which increase the capacity of the structure to meet the expected demand, in this case, the strategy of seismic demand reduction was adopted, which consists in minimizing the demand caused by the given earthquake to such an extent that it becomes less than the capacity of the existing structure. This strategy was pursued by adopting a seismic isolation system made of friction pendulum bearings, that are particularly suitable as energy dissipating elements and allow the reduction of seismic forces and, at the same time, of the displacements (this leading to lower dimensions of the expansion joints). The present paper shows and discusses the results of the vulnerability analysis and the design of the retrofitting intervention