Modal testing of a reinforced concrete Gerber-type bridge

This study focuses on the modal identification of the ”Ponte delle Grazie”, located in Faenza, northern Italy. The structure crosses the Lamone River and consists of a reinforced concrete deck with a Gerber-type configuration. The bridge, constructed in 1950, was found to be in a critical state of deterioration during inspections conducted in 2017, with significant damage observed in the rocker bearings, Gerber saddles, and lateral girders. These findings led to urgent repair interventions. A follow-up evaluation, carried out in line with Italian bridge guidelines, confirmed major structural issues. As a result, additional repair works were undertaken in 2024. Given these conditions, the bridge was deemed an ideal candidate for continuous monitoring via a permanent system, which was installed that same year as part of the DIGI-BRIDGE research project. This paper presents the results of dynamic tests carried out prior to the installation of the permanent monitoring system, aimed at assessing the dynamic behavior of the bridge. The tests were carried out using different acceleration measurement systems: one system consists of MEMS accelerometers, which was also used for the subsequent permanent monitoring of the bridge; a second system uses piezoelectric accelerometers and is intended to provide a more accurate characterization of the modal properties; the third is an innovative system based on fiber optic sensors. In the paper, results obtained from the different monitoring systems are discussed and compared.