In situ static and dynamic investigations on the “Torre Grossa” masonry tower

Many ancient masonry towers represent one of the most relevant characteristics of both the Italian and the whole European territory. To preserve their cultural and economic value, they often call for a deeper investigation to assess their state of conservation. This paper offers a contribution to the issue of monitoring and analysis of monumental masonry tower, by discussing an experimental investigation survey made on an illustrative masonry tower: the Italian Medieval “Torre Grossa” (Big Tower) of San Gimignano in Toscana (Italy). The tower is part of the UNESCO cultural heritage after 1990 together with the city centre; it dates back to the thirteenth century and its height is about 55 m (one of the tallest masonry tower in Italy) with a square cross-section whose side is about 9.5 m. During the experimental campaign, both static and dynamic tests were performed. Static tests (flat-jack and laboratory test on cored specimens) were used to estimate the mechanical properties and the ultimate strength of masonry. Dynamic tests led to the measurements of the natural frequencies and corresponding modal shapes of the whole monument. By using the finite element technique, a 3D model of the tower was built (macro-modelling) and it was calibrated on the basis of the in situ investigation survey; the degree of restrain offered by the neighbouring buildings was estimated by tuning the numerical results with those obtained from the dynamic tests. The paper, by discussing the design and results of the experimental campaign, aims at offering a contribution to both the investigation and the structural behaviour of historic masonry towers providing useful hints to deepen the knowledge on their structural behaviour.