GNGTS 2019 - Atti del 38° Convegno Nazionale

678 GNGTS 2019 S essione 3.2 All acquisitions have been carried out in a short period of time (about 3 hours), during which the weather and atmospheric conditions did not change significantly. Fig. 2 shows the spectral ratios graphs of vertical 1; the tower’s resonance vibration frequencies correspond to the spectral peaks observable in the HSSR spectra. Fig. 2 - Vertical 1 HSSR analysis. a) North/South direction; b) East/West direction Validation of experimental results with Midas Fea®. In order to validate the results obtained by the experimental analysis, a 3-dimensional finite element modelling of the bell tower was analyzed (Fig. 3a) (Betti and Vignoli, 2008; Betti and Vignoli, 2011) using the software Midas Fea® (Castellaro et al. , 2017; Castellaro et al. , 2019; D’Amico et al. 2017; Lunedei et al. , 2014). The tower was modelled as a linear elastic isolated structure, with a Young’s modulus of 2800 N/mm 2 , a Poisson ratio of 0.3 and a specific weight of 22 kN/m 3 . The parameters chosen are in agreement with the result of numerous researches carried out on structures with similar characteristics and by consulting the Italian technical regulation. The fundamental frequency obtained by the modal analysis is equal to 3.36 Hz (Fig. 3b, c), which is slightly different from the value 3.97 Hz, obtained experimentally. This difference could be attributed to the estimate of the mechanical parameters and to having considered the tower as isolated respect to the church. The slight difference between experimental and numerical frequencies suggests therefore the presence of a weak connection between the church and the tower. It is important to highlight that the results are compared only in terms of the resonance frequencies and that the full dynamic characterization is only possible after an experimental reconstruction of the modal shapes. Discussions and conclusions. The geophysical method used in this study has several advantages. Compared to other procedures, the acquisition phase is extremely easy and not invasive. Moreover artificial energy sources are not necessary. The above-mentioned considerations and the obtained results show how this fast technique is particularly suitable