## 1 Introduction  
The study of masonry towers attracts numerous scholars. Many research on their seismic performance, which can be quite diverse depending on the dynamic features of the tower. Specifically, in some cases, masonry towers are the first structures to suffer damages or collapse, see Figures 0(a) and 1, while in some instances they are the sole to endure, see Figure 0(b).  
For these reasons, the assessment of the dynamic response of masonry towers is essential before any retrofitting intervention. However, practitioners are still not acquainted with dynamic identification, which can provide useful information, especially in the case of slender structures, like belfries.  
Besides, dozens of papers present the dynamic response of masonry towers to ambient excitation. Most of the researchers discuss the results of Operational Modal Analysis (OMA) by tuning finite element models to the observed dynamics [1; 2; 3; 4]. A few researchers show the outcomes of continuous dynamic monitoring and the impact of ambient parameters over the modal parameters [5]. Further, some papers present the coupling between the towers and the ringing bells and assess the effects of the bells on the seismic performance [6; 7].  
Despite the numerous researchers on masonry belfries, each belfry is a stand-alone case and, still, may add pieces of information on the understanding of the dynamics of slender masonry structures.  
A few papers present in-depth investigations on the dynamics of masonry towers via rigorous experimental, analytical and numerical approaches.  
Among them, [PERSON] et al. [7] endeavoured to determine an accurate analytical description of both the bells' motion and their actions using experimental measurements and a rough mechanical model. They [7] concluded that the dynamic effects of the bells over the considered belfry (Tower of Matilde) could be described analytically with surprising accuracy by merely representing the motion of every single bell as an equivalent pendulum.  
The authors of the current paper, clinging to this evidence, attempted to interpret the dynamics of the San Silvestro bell tower in L'Aquila, by modelling the more massive bell with an equivalent pendulum and the tower with a single degree-of-freedom (DOF) oscillator.  
The bells' swinging can seriously affect the seismic performance of masonry towers. Although a considerable distance between the resonance frequency of the tower and the frequency of oscillation of the bell can prevent unfortunate consequences, the seismic performance mostly depends on the frequency features of the earthquake.  
The paper presents the case study of the San Silvestro belfry, discussed via experimental tests and analytical modelling. The first two sections describe the case study and the outcomes of OMA. In the following part, a two-DOF dynamic systems tuned to the experimental eigenfrequencies give information on the expected coupling between the structure and the more massive bell under earthquake.  
Figure 1: Pictures of damaged masonry towers during seismic sequences, taken by Italian Fire Department: (**a**) an almost collapsed masonry tower struck by the 1997 earthquake in Umbria (Italy), (**b**) the masonry tower in Amatrice (Italy), which did not collapse despite the neighbouring masonry structures were all raised to the ground by the 2016 earthquake.  
Figure 2: (**a**) collapse mechanisms of the stem; (**b**) collapse mechanisms of the bell-cell.