Seismic behavior of a masonry bell-tower with verticality defect

The seismic behaviour of slender structures, such as masonry towers, is dominated by bending which may lead to overturning. Since the geometrical survey of towers is hard to compute due to the prevalent longitudinal development of the structure, the shape inputted in the FEM-habitat (Finite Element Method) is commonly regularised (e.g. in cross-section, openings, global shape, etc.). This assumption may make the computation significantly more time-saving together with the increase of the model robustness. At the same time, it may compromise the accuracy of the theoretical prediction. The present study is aimed to report and discuss a seismic vulnerability analysis of a Heritage masonry bell-tower, dated back to the 14th century, placed in the south-east of Italy. The geometry of the structure was assessed by using a drone flying around the building and taking a series of photo afterwards computer-elaborated. The orientation of the photos, associated with the relative position of the drone in the 3D-space, allowed to assess a cloud of points belonging to the bell-tower, leading to an accurate geometrical survey. This process evidenced a verticality defect of the structure. After different investigations aimed to assess the mechanical properties of the masonry and the structural details, a FEM-analysis was achieved in order to compute the capacity under horizontal forces. Linear (modal) and non-linear (push-over) analyses were performed. Moreover, a nonlinear kinematic analysis was able to individuate the weakest rigid-body mechanism. The main results are presented and discussed in the paper, by evidencing that the lack of verticality produces an unsatisfactory seismic vulnerability index. In fact, the first failure mode consisted in the diagonal cracking at the middle height of the tower and the consequent overturning of the top-body, under the seismic acceleration at ultimate limit state (according to the Italian Technical Code).