# Operational Modal Analysis and Safety Assessment of a Historical Masonry Bell Tower
## 5 Conclusions  
In this paper, the safety vulnerability assessment of a historical masonry bell tower is described. In a first phase, a detailed survey was carried out by means of internal and external laser-scanning, thermography and visual inspection. Subsequently, materials were characterized through non- and semi-destructive testing, including sclerometer and penetrometer tests, single and double flat-jacks and tests on stone cores. The outcome of this phase was the picture of a structure made of low-quality materials and characterized by extensive cracking particularly vertically on the external walls. To characterize the global dynamic behavior, AVTs were performed. The results, post-processed using Frequency Domain Decomposition enhanced by the examination of the Modal Complexity Factor, indicated fundamental frequencies ranging in the constant branch of the design spectrum, and, more interestingly, the presence of similar modes associated with well separated frequencies. This was explained as a result of the physical separation of the structure due to the presence of the vertical cracks, thus indicating a significant effect of the damage state on the dynamic behavior. Motivated by these results, the final vulnerability assessment was carried out by means of EL1 and EL2 methodologies. Both approaches provided safety factors less than unity, thus indicating the need for seismic retrofitting. In particular, in the case of the EL1 approach the bottom section failure was estimated at a PGA less than that corresponding to the design spectrum at the Ultimate Limit State, as well as considering, in the authors' view, the rather optimistic behavior factor q = 2.8. In the case of EL2, a mechanism triggered by the observed vertical cracks provided activation ground acceleration equal to less than 60% of the design PGA at the ULS. This paper shows the information that can be obtained from documental research survey and experimental activities with the aim of vulnerability assessment of historical masonry towers. It also points out some limitations of current practice that need to be investigated in the future, such as for instance a more realistic behavior factor estimation. Further research will focus on effective and compatible retrofitting actions for the specific bell tower; more generally, embedding pre-existing damage in a detailed modeling framework will also be investigated, as this represents an important topic with critical relevance in the application of detailed modeling to historical heritage. Conceptualization, C.C., M.Z., A.L. and G.D.M. ; methodology, C.C., M.Z. and A.L. ; software, C.C. and A.L. ; validation, C.C., A.L. and M.Z. ; investigation, C.C., A.L. and M.Z. ; resources, G.D.M. ; writing--original draft preparation, C.C. and M.Z. ; writing--review and editing, C.C., M.Z., A.L. and G.D.M. ; visualization, C.C. and M.Z. ; supervision, G.D.M. ; project administration, S.F. and G.D.M. ; funding acquisition, G.D.M. All authors have read and agreed to the published version of the manuscript. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Mechanism**} & **Se(T1)\textbackslash{}**(Z)\textbackslash{}**/q** & **a\textbackslash{}^{*}\)** & **f\textbackslash{}**a,ULS** \\  & **(m/s\({}^{2}\))** & **(m/s\({}^{2}\))** & **(-)** \\ \hline
1 & 1.94 & 1.15 & 0.59 \\
2 & 1.39 & 1.96 & 1.41 \\
3 & 2.37 & 11.92 & 5.02 \\
4 & 1.94 & 4.00 & 2.06 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Structural check of collapse mechanisms relevant to EL2 procedure. **Funding:** This research was funded by European Union-Next Generation EU, Mission 4 Component 1 through the project PRIN 2022 PNRR \"SAFE_MOTION-Structural Assessment and development of innovative safeguard measures for historical masonry towers\" (CUP B53D23027300001). M.Z. is funded by MUR (Ministry of University and Research) through PON FSE 2014-2020 program (contract: 49-I-32603-3). **Data Availability Statement:** The raw data supporting the conclusions of this article will be made available by the authors on request.