# Operational Modal Analysis and Safety Assessment of a Historical Masonry Bell Tower
## 4 Seismic Assessment  
### Methodology  
The observations collected during the survey and testing phases have direct consequences for the most suitable analysis approaches to use in the safety assessment. Firstly, the very low compressive strength detected through experimental testing directly leads to EL1 approach, where compression strength is the only significant material property. In addition, the significant cracking pattern observed in the tower makes it paramount to investigate the influence of local mechanisms and out-of-plane collapse modes on the safety of the bell tower, i.e., to use EL2 methodologies. Conversely, the use of sophisticated modeling approaches such as Finite Element Modeling (EL3) does not appear fully justified for two main reasons. On one hand, previous research [24] has clearly shown that the use of detailed material models that include softening and damage evolution may provide unreliable results if not coupled with extensive material characterization estimating less understood parameters, e.g., tensile strength, fracture energy, dilatancy parameters, etc., which is very difficult to perform in situ. On the other hand, the presence of pre-existing damage in the structure poses extraordinary challenges, especially in the context of macroscale modeling of masonry structures, and no established methods are available in the literature yet to determine realistically the effect of this damage on the final capacity. For all these reasons, lower detail levels (i.e., EL1 and EL2) were considered more suitable in this work to estimate the seismic vulnerability of the case under study.  
In particular, according to EL1 approach, the tower is considered as a cantilever, variable-cross-section member subjected to its own weight, as well as a system of horizontal forces generated by the earthquake, assumed proportional to mass and height (linear acceleration profile). In addition to this code-compliant force system, a second one with accelerations proportional to the experimental mode shapes has been considered in this work. The collapse is due to combined compression and bending in a generic section, which provoke masonry crushing in the compressed area, as a result of partialization caused by the lack of tensile strength.  
In the EL2 methodology, conversely, the seismic safety of the structure is evaluated by determining the ground acceleration that transforms the entire structure, or its significant individual parts (i.e., macro-elements), into a mechanism. This assessment can be conducted through linear kinematic analysis. The application of this method requires the analysis of the local mechanisms considered significant for the structure. These can be hypothesized based on the knowledge of the seismic behavior of and damage to similar structures, as observed in previous earthquakes or identified by the presence of specific crack patterns, even if non-seismic in nature. Additionally, in studying the local mechanisms, other aspects must also be considered: the quality of the connection between the masonry walls, the texture and arrangement of the masonry, the presence and efficiency of tie rods, the interactions with other structural elements or adjacent buildings, and phases of transformation that may be identifiable through construction details (e.g., heterogeneity of the masonry, juxtaposition of different types of walls, etc.).