Impact of Restoration on the Dynamic Behavior of a Historic Bell Tower

The protection of architectural heritage and monuments is a critical challenge, particularly in regions exposed to seismic hazards. This paper presents the experimental assessment of the bell tower of the Basilica of Santa Maria degli Angeli in Assisi (Italy), which was damaged during the 2016–2017 seismic sequence and subsequently subjected to restoration interventions. Ambient Vibration Tests (AVTs) were performed on the substructure of the Basilica’s bell tower, both before and after the restoration campaign. Operational Modal Analysis (OMA) was applied to identify the main dynamic properties, including natural frequencies and mode shapes. The comparison of pre- and post-intervention results provides a direct and quantitative evaluation of the changes in behavior. The results show a consistent increase in natural frequencies, indicating a recovery of structural stiffness, while high Modal Assurance Criterion (MAC) values confirm the preservation of mode shapes. These findings demonstrate that the restoration intervention effectively improved the structural condition of the bell tower without altering its global dynamic response.