Dynamic monitoring of cultural heritage assets: the bell tower of San Francesco church in Pisa (Italy)

The preservation of cultural heritage assets from natural and man-made disasters is paramount for our community because of their architectural and social value. Cultural heritage assets are directly linked to the economy of a region through cultural tourism. They also have a symbolic value for the community: their damage and partial/total collapse may have a huge impact on social cohesion, sustainable development and psychological well-being. Among natural hazards, earthquakes are the most dangerous for ancient buildings, which are usually built following empirical knowledge and reflecting the tradition of a community. Moreover, modifications over time, local repairs or partial/total reconstructions can even worsen the seismic performance of cultural heritage assets. In this context, dynamic Structural Health Monitoring systems represent a powerful tool to control the health state of the structure, manage maintenance interventions and optimise retrofitting. This paper presents the preliminary results of a research work aimed at defining a probabilistic framework for monitoring systems for cultural heritage assets. In particular, in this paper the experimental campaign on the bell tower of the San Francesco church in Pisa is described. The San Francesco bell tower is a unique example of masonry tower, since it rests on the transept walls of the church (12 m above the ground level). For this reason, it represents an interesting case study to test limits and advantages of the dynamic monitoring of masonry towers. The experimental campaign is aimed at determining the modal characteristics of the structure, i.e., frequencies and mode shapes, by recording the effect of ambient vibrations on the tower. These observations are then elaborated through Operational Modal Analysis techniques to characterise the structural modal behaviour. A finite element model has been finally calibrated by comparing numerical eigenfrequencies and mode shapes with experimental ones. The calibrated model will be used in future steps of the project to understand the complex dynamic behaviour of this unique masonry tower and to assess its seismic fragility.