Seismic safety assessment of the tower of S. Maria Maggiore cathedral in Guardiagrele, Italy

This paper presents the results of seismic vulnerability assessment of an ancient tower erected in the XIV century and located in Guardiagrele, Chieti province (Italy). An extensive in situ experimental campaign was conducted to identify the material properties and the geometry of the tower as well as the stratigraphy of the supporting soil. The tower natural frequencies and mode shapes are extracted from ambient vibration data using state-of-the-art system identification techniques. The onsite investigation results were used to calibrate a 3D finite element model of the tower in which the foundation soil was explicitly modelled in order to study the effects of soil-foundation-structure interaction. Results from the modal analysis showed a good correlation between the elastic properties of the model and the ambient vibration data. Realistic linear and nonlinear constitutive models for cyclic loading were used for the structural and soil materials. The tower was modelled with damage-coupled plasticity both in tension and compression. The ground was modelled with linear base conditions to study the influence of the ground flexibility on the tower response. The seismic safety was evaluated with nonlinear time-history analyses (NLTHA). Results to date show the importance of the selection of the input ground motion for the NLTHA as well as the importance of the correct evaluation of the structure geometry and mechanical properties. In particular, the results indicate that a rigorous evaluation of the seismic vulnerability should consider the historical seismic activity related to the seismogenic zones as well as the frequency content of the input.