The seismic vulnerability of towers' masonry spires: learning from history

The May 2012 Emilia earthquake has once again underlined the high vulnerability of the spires that often complete masonry towers and belfries. These daring structures, which are typical of the Lombard architecture, but later spread throughout Italy, can have the shape of a circular or polygonal cone and are usually made of clay brick masonry, sometimes topped by a stone decorative element. Indeed, this building feature was once even more widespread, as many cases were found of towers whose spire was damaged by an earthquake and that was then restored or rebuilt with a different – more stable – shape. Starting from the observation of numerous real cases, both in recent and historical earthquakes, some considerations are made on the type of collapse and on the shape and position of the damaged area. The recurrent types of seismic damage, consisting in the overturning or shear sliding of the top of the cone, are then analyzed with finite element models. One interesting observation that was made during the on site inspections refers to a very particular strengthening solution found in the bell tower of the Parma Cathedral, adopted as a paradigmatic example: a large timber log was hung to the top of the spire, anchoring the angel-shaped weather vane. Its weight contributes to the stability of the top part of the spire increasing both the shear strength and the overturning stability. Also this technical solution has been modelled in order to verify its contribution to the stability of the structure and to propose a possible up-to-date version for the reconstruction or strengthening of the recently damaged spires.