System Identification and Long-Term Modal Tracking of an Earthquake-Damaged Masonry Bell Tower

The vibration-based assessment of historical masonry towers increasingly relies on operational modal analysis (OMA) and long-term tracking tools capable of distinguishing genuine structural changes from environmental and operational variability. In this work, a 14th-century masonry bell tower in Fossano (Italy) serves as a full-scale benchmark for system identification and modal tracking methodologies. The key characteristic of the case study lies in its current structural condition: the structure was severely cracked by an 1887 earthquake and, later in recent times (2012), subjected to provisional hooping. Following insights from a previous (2011) ambient vibration test (AVT) campaign using a dense sensor layout, a reduced permanent setup was established in 2024 for continuous, persistent monitoring. Subspace-based identification allowed the extraction of stable modal parameters despite sparse sensing, with notable economic and practical improvements. The comparison between the recent identifications and 2011 results shows a statistically significant increase in natural frequencies (up to ~10%), attributable to the 2012 tie-rod confinement system rather than environmental effects. Modal tracking over several months in the last year confirmed narrow-band frequency fluctuations, mostly dominated by temperature variability, while preserving clear detectability of stiffness-related shifts. Damping ratios remained too dispersive to serve as reliable indicators, reinforcing the importance of robust natural frequency-based tracking strategies. These recent results highlight how AVT-informed modal tracking can support seismic assessment and upgrade design in heritage towers, enabling three important outcomes: (i) quantification of positive effects from already-deployed structural intervention, (ii) early detection of anomalies, and (iii) evidence-based comparison of different strengthening strategies.