A vulnerability index evaluation for masonry bell towers in Venice

The skyline of Venice is still nowadays characterized by a high density of slender bell-towers, corresponding to a huge number of churches that marked the built environment of the ancient town, constituting both religious and administrative references. It is the character itself of such structures that enforces a particular attention to the stability and safety the time during, also if the astonishing event of the sudden collapse of St. Marco bell-tower in 1904 is by now nearly forgotten. More than 90 bell-towers has been classified on the basis of a research conducted by the “Cultural Heritage and Landscape” Office of Venice, collecting a lot of historical, geometrical and qualitative information about these buildings. The problem was then to find a reasonable method to compare each other these structures, in order to fix a vulnerability scale helpful also for establishing the priority choice of intervention on a territorial scale, in the spirit of the “Guide-Lines” for seismic risk evaluation and reduction on the cultural heritage”, according to Italian Building Code. With the help of a GIS based on the survey previously made, some global limit states have been defined on the basis of a limited number of geometrical or mechanical parameters (qualitatively obtained by visual inspection or building character survey), in order to highlight possible critical situations of the whole tower. The evaluation of different “vulnerability indexes”, homogeneous and comparable among them, allowed for each tower to outline the higher risk factor and to set up an overall rating of bell-tower structures examined, comparing in this way also objects that are heterogeneous buy shape, materials and building history.