Dynamic investigations before and after the strengthening of a masonry arch bridge

The bridge crossing the Gesso River is a multi-span masonry arch bridge built in the 19th century in Cuneo, Piedmont, Italy. Due to extended local degradation and damage, the bridge recently underwent a significant strengthening intervention. Ambient vibration tests (AVTs) were performed both before and after the strengthening to assess the effectiveness of the repairs. The paper presents the results of the dynamic investigations, identifying the modal characteristics of the masonry bridge through different techniques. The pre-intervention analysis revealed clear anomalies, including a sort of "frequency splitting" phenomenon and irregularities in the mode shapes, that were localized in the regions of maximum masonry decay. After the strengthening works, the identified modal parameters showed an increase of natural frequencies, along with the resolution of previously identified mode shape irregularities, indicating a clear improvement of the bridge structural condition. As a final remark, the presented results highlight the value of operational modal analysis (OMA) as a non-destructive tool for validating the effectiveness of rehabilitation measures.