## 3 Results
### Bell Tower Experimental Vibration Frequencies and Mode Shapes Detection  
The SSR graph of spectra related to recordings acquired along the same vertical shows four large amplitude peaks related to bell tower vibration frequencies (Figure 3b,c), in the frequency range 0.5-10 Hz. A first large amplitude peak is observable at 1.406 Hz in  
Figure 6: Damping plot obtained using the Random Decrement Method (RDM) and corresponding Fourier spectra (FFT) for the C8 sampling point (see Figure 4a for location). The damping was evaluated for the fundamental vibration frequency identified. The continuous black line corresponds to the mean of the random decrement, the dashed lines correspond to the standard deviation, and the solid red line corresponds to the fitted exponentially decreasing function. the north-south component for the spectra associated with vertical A and at 1.438 Hz for vertical B and C (synchronous signals). The second vibration frequency identified emerges at 3.531 Hz in all the spectra. Two further peaks are at frequencies of 5.531 Hz and 9.25 Hz on the SSR related to vertical A; these peaks are present with slight frequency variations in the SSR graphs of vertical B and C (Figures 3b,c and 4b) (Table 1). The SSR spectra of the east-west component show the first three frequency peaks at the same frequency in all the verticals investigated; the vibration frequencies detected are equal to 1.406 Hz, 3.5 Hz, and 5.531 Hz. Only the fourth peak, identified at 9.063 Hz, shows a small variation in the frequency value for vertical B (Figures 3b,c and 4d) (Table 1). The signal synchronization performed on the recordings for vertical C allowed the reconstruction of the mode shapes for the frequencies identified on both horizontal components (Figure 4c,e). The shape relative to the first mode shows an almost linear trend with increasing amplitude from the base to the top of the bell tower; this indicates behavior dominated by a fundamental flexural mode. The behavior is also similar in the EW direction, with increasing amplitudes up to the top. However, the stiffness may be slightly different, as the somewhat lower frequency suggests. The shape of the second mode, for the north-south component, is characterized by an increasing amplitude from the base to the sixth floor, while at the eighth floor, the amplitude does not increase. In contrast, the east-west component shows a linearly increasing trend from the base upward. The shape of the third mode presents a more complex pattern with two reversals of curvature for both components; again the slightly lower frequency for the east-west component suggests a slight difference in stiffness compared to the north-south component. In the fourth mode, the modal shape for the two components follows the same pattern, becoming even more complex with several inversions of curvature for points at different heights in the bell tower. The C8 acquisition performed on the highest floor of the bell tower was further processed using the RDM method to obtain a damping estimation that characterizes the structure (Figure 6). The RDM analysis was applied to the first vibration frequency, easily detectable and clearly distinct from the other modes. The damping percentage values obtained are low, less than 1%, for both horizontal components. These damping values highlight the predominant influence of the dissipation mechanisms intrinsic to the material and the linear elastic response of the structure, linked to the fact that the analysis is carried out based on small stress, such as environmental seismic noise.