The towers of San Gimignano. Architectural diagnostic for knowledge and conservation

As part of the RISEM project (Seismic Risk in Monumental Buildings), funded by the Regione Toscana, a study is being done on the vulnerability of the San Gimignano towers. It addresses the problem in several ways, from geological and geotechnical aspects, to architectural and structural modelling. The subject of the direct investigation and the survey of the towers presented rather complex operational problems, due to the fragmentation of ownership which usually characterises them and the difficulties of connecting the different parts of each tower. It is probably due to these problems that, despite the interest aroused by these structures, there is no existing reliable survey of the actual consistency of the towers. Two complementary survey methods were designed. The interior rooms had to be surveyed using traditional manual techniques, while the exterior shape using advanced tools such as a laser scanner. By this process complete longitudinal and transversal sections and plans (sometimes even ten different levels) were obtained and drawn for almost all the towers. The survey results allowed to identify the original structure of the towers and their transformation over the years. This transformation was sometimes really incisive beyond the outward appearance of these massive structures, which can be appreciated by strolling through the streets of San Gimignano. It was amazing to discover that, in almost all cases, the interior room is very limited in size in comparison to the size of the building structure. In the case of the Salvucci towers, the northern tower, 37.25m high, has an interior room 1.3m in width and 2m in lenght, while the perimeter walls are 2.05m thick. In this type of situation there were no windows, while the beams of the interior wooden floors were either supported on stone brackets or inserted into holes in the wall. Equally significant is the transformation of these structures when adapted for housing, which took place when they were incorporated into the flanking buildings. New doors were opened, the inner room was nearly always increased reducing the thickness of the walls, sometimes more than halved. The transformation of the Ardinghelli towers is a significant case. In the northern tower the upper inner walls were completely dismantled for their entire height and the material was reutilised to raise the height of the tower, which left a limited wall thickness. Today we can appreciate their volume emerging from the roof of the surrounding buildings, but in reality the towers have almost completely lost the effective structural integrity which characterised their original consistency.