On site investigation and crack monitoring of an ancient bell-tower

The bell tower of Arcisate (Italy), a stonework masonry structure built toward the end of the 4th century, shows heavy signs of structural damage, that are signified by several cracks, sometimes passing through the masonry thickness. As it is well known, ancient towers are often characterized by widespread crack patterns, whose primary cause is not always clear, but can be generally related to the original function, the construction technique, the load history, and soil settlements. Nevertheless these damages can sometimes bring to collapse. In the case of the Arcisate tower, that looks like a rather simple structure at first sight (square plan), some geometrical variations, i.e. internal and external tapers that are quite difficult to be quantified, are of fundamental importance for the sound evaluation of the out-of-plumb and the crack pattern which in turn affect the structural behaviour. In order to identify the causes of the observed damage, and to assure the safety of the structure, a research project was undertaken, ranging from the historic documentation, to on-site testing and monitoring; all the information gathered are instrumental for the proper conceptual design of the intervention. The analysis of the results obtained from the survey and monitoring operations allowed to adequately locate new onsite tests, and consequently better identify the damage causes, that are probably: some differential settlements, high compressive stresses due to the dead load, fatigue cycles due to temperature variation.