The Garisenda Tower in Bologna: Effects of degradation of selenite basement on its static behaviour

The Garisenda tower in Bologna, a 48 m tall structure with a square base of 7.45 meters per side, is characterized by an overall out of plumb of 3.32m in the South-East direction. Its construction dates back to the XI century and, due to its impressive leaning, in 1350–1353 the original height of 60m was reduced to the 48m of the present day (Cavani 1903; Giordano 2000). The tower can be seen as partitioned in a lower portion, with walls composed by two external leaves of selenite stones filled with rubble conglomerate, and an upper portion where the external leaves are made of masonry bricks. Recent investigations have proved that selenite blocks of the basement have been altered as a result of (a) exposition to high temperatures during important fires, that took place at the end of XIV and XVII centuries, and possibly because of the presence of forges (that were demolished at the end of the XIX centuries) and (b) high level of humidity in the inner lower part of the tower. This process has produced a gradual local disintegration of the selenite stones, leading in some case to a reduction of the original 50 to 60 cm thickness by an amount of about 20 cm. The contribution submitted to this conference is aimed at clarifying this important aspect, linked to the ageing and damage of structural stones and the related consequences in terms of stress distribution and concentrations that could induce fracture propagation and sudden collapse of the tower basement.