Automated post-earthquake damage detection in a monumental bell tower by continuous dynamic monitoring

In this paper, one year of vibration monitoring data of the masonry bell tower of the Basilica of San Pietro in Perugia, Italy, are presented, alongside with a numerical evaluation of the effects of earthquakeinduced damages on the natural frequencies of the structure. The analysis ends with a practical demonstration of the effectiveness of a vibration-based SHM technique in automatically detecting such earthquake-induced damages and in tracking damage severity. The tower consists of three different architectural parts: the shaft and the belfry with dodecagonal and hexagonal cross sections, respectively, and the cusp on top. The peculiar shape of the tower and the various structural and architectural interventions made throughout the centuries, result in complex dynamic characteristics, which have been identified by means of specific ambient vibration tests. The paper also presents a tuned FEM model of the structure that is used for performing the damage sensitivity analysis, whereby a parametric reduction of the elastic moduli of the weakest structural parts is considered. Overall, the presented analysis demonstrates that the monitoring system allows to automatically detect typical damage conditions in the tower caused by low return period earthquakes.