Dynamic validation of a masonry arch bridge seismic retrofit

This paper presents the dynamic characterization of a retrofitted masonry arch bridge. The Gresal Bridge, located in a seismic area in the North-East of Italy, was rated highly vulnerable to seismic hazard, mostly due to the slenderness of its high piers. A retrofit intervention has been carried out, and a new r.c. slab has been built under the pavement, anchored to the piers with high strength vertical ties and restrained at the abutments, to create a new resistance arrangement withstanding inertial forces. The dynamic behaviour has been initially assessed with numerical models comparing the response of the bridge before and after the repair, and has subsequently been tested by the Output-Only technique to detect the variation of the modal response induced by the strengthening intervention. The dynamic tests have shown the retrofitted structure to be less rigid than expected and, after calibration, a good agreement was found between the numerical frequencies and the experimental records captured on the retrofitted bridge.