# Operational Modal Analysis and Safety Assessment of a Historical Masonry Bell Tower
## 2 The Case Study: The Bell Tower of the St. Lucia Church in Cellole (CE)
### Structural Features, Material Characteristics and State of Conservation  
To define the main structural features of the tower and to mechanically characterize the existing materials, a series of non-destructive and minimally destructive tests were conducted. The material used for the construction of the tower is gray tuff masonry, a soft volcanic stone typical of this geographical area. Its compressive strength is generally quite low compared to other stones and materials (e.g., clay bricks) used across the Italian peninsula for masonry structures. This condition was confirmed by sclerometer tests performed on the masonry units, which returned an average compressive strength of 0.7 MPa.  
The quality of mortar, particularly its compressive strength, was also assessed through non-invasive tests (i.e., penetrometer). The instrument used had a lower threshold of admissible strength range of 0.4 MPa. All tests returned strength values below this threshold, indicating very poor mortar quality.  
The bad quality of masonry was confirmed by compressive tests on tuff samples and double flat jack tests, which returned compressive strengths of 0.86 MPa and 0.65 MPa, respectively.  
Beyond the mechanical characteristics of the existing material, the tower is in a poor state of conservation, with visible signs of degradation and cracking. Specifically, the visual inspection revealed that many of the squared gray tuff blocks showed signs of detachment, especially around the openings. In the bell chamber, many blocks appeared displaced, disconnected and infested with weeds due to the disrepair affecting the structure. The natural deterioration of the masonry, plaster and roof, combined with lack of maintenance, has resulted in a structure with serious conservation and stability issues.  
Furthermore, significant vertical cracking was observed on the east and west facades, affecting the external walls of the second, third and fourth levels. The presence of cracks on the west facade was clearly confirmed through thermographic inspections conducted on-site from the ground level and with the support of a drone (Figure 4). On the other hand, the situation was even more severe on the opposite wall (east facade), where the cracks were visible to the naked eye and further confirmed by thermal imaging (Figure 5). The presence of these vertical cracks may be connected to the configuration of the last sector, whose construction clearly took place in a latter phase compared to the remaining bell tower. The abrupt change in the plan section may be responsible for the non-uniform distribution of vertical stresses on the masonry below. Furthermore, as shown in Figure 3, the tower is configured as the juxtaposition of two hollow sections, whose connection is far from perfect. This, together with the possible presence of settlements, out-of-plumb and past horizontal forces, may explain the detachment of the south facade from the north part.  
Figure 4: Cracking phenomenon observed on the west facade: inspection with thermal imager from ground (**a**) and with drone (**b**). Cracks are circled in white.