# Operational Modal Analysis and Safety Assessment of a Historical Masonry Bell Tower
## 1 Introduction  
Italy, renowned for its rich cultural heritage, is home to an extraordinary number of historical bell towers, built over the past centuries and predominantly constructed from masonry, and often so important as to shape the country's identity and landscape. The historical importance of these bell towers cannot be overstated. Many of them date back to medieval times, representing the architectural style and construction techniques of their respective eras. They were often built to signify the presence of a church or a cathedral, serving both religious and communal purposes. Over the centuries, these bell towers have witnessed several historical events and have become integral parts of their communities. However, their historical and cultural significance is juxtaposed with their structural and seismic vulnerabilities, particularly during seismic events. Past seismic events provide valuable insights into the behavior of masonry bell towers under earthquake loading. Reconnaissance performed on masonry churches in the aftermath of the last Italian earthquakes (e.g., L'Aquila 2009, Emilia-Romagna 2012 and Central Italy 2016-2017) testified that bell towers may experience severe damage during seismic events [1,2]. Historical records and modern case studies of seismic damage to these structures reveal common patterns of failure and highlight the most vulnerable aspects of their design and construction. For this reason, similarly to churches [3], and as suggested in Ministerial Italian Guidelines for Cultural Heritage [4], the seismic assessment of masonry bell towers can be performed at different evaluation Levels (ELs), from simplified mechanical approaches (EL1) to more accurate and sophisticated analyses (EL2, EL3). In particular, according to EL1 [5; 6; 7], the tower is represented as a variable-cross-section vertical beam subjected to lateral forces simulating seismic forces. A simplified yet realistic representation of the plan view at each floor is required, along with the definition of a very limited number of homogeneous material properties (compression strength). EL2 [5; 7; 8; 9; 10] needs an accurate geometric representation of the bell tower and the vulnerability estimate is given by the results of analyses at collapse, which are performed for the most representative mechanisms. Finally, EL3 [5; 7; 11; 12; 13; 14; 15] is based on accurate numerical modeling of the structure, requiring detailed surveying, material characterization and geometrical representation. Although it is not included in the Italian Guidelines, the existence of a lower-detailed level EL0 should be also mentioned [16; 17; 18]. Such methodologies, in which the bell tower is identified by very few typological parameters, allow for defining a vulnerability ranking rather than an estimate of safety. It is clear that the evaluation level selected for a specific case primarily depends on two key factors: the available information regarding the structure and the objective of the analysis (with global interventions requiring an EL3 assessment). However, the reliability of these methodologies should also be linked to the actual behavior of the structure under investigation, including any degradation or cracking phenomena. The use of more detailed levels of assessment is not always the optimal solution, particularly when significant uncertainties are associated with mechanical properties and strength parameters in numerical models. In the present study, a case study of the bell tower of St. Lucia Church in Cellole (Caserta Province) is addressed. Based on wide preliminary investigation activity, including a geometrical survey, tests on materials and dynamical characterization, the main structural features of the tower were identified. Given the presence of a significant cracking pattern, the assessment of the tower was performed by adopting EL1 and EL2 methodologies for in-plane and out-of-plane collapse modes, respectively.