DIAGNOSIS OF AN UNUSUAL STRUCTURAL INSTABILITY: THE CASE STUDY OF THE CATHEDRAL OF SAN LORENZO IN VITERBO

The cathedral of San Lorenzo is a Romanesque church sited in Viterbo (Lazio, Italy), founded in the XII century on a site characterized by a complex archaeological stratification. The building is a fine example of religious monumental architecture, built upon well-laid masonry walls, made of squared blocks of piperine stone and very thin layers of mortar. Its basilica-shaped plant is divided into three aisles by two rows of marble columns connected by arches. The structure, that has undergone many transformations during the centuries, was severely damaged during the II world war and a restoration executed in 1947 has brought it back to its romanesque appearance. Currently, a peculiar crack pattern can be observed in the structures, which is mainly characterized by recurring sub-vertical fissures appearing on the keystones of the arches that run along the central nave. The purpose of this paper is to illustrate the diagnostic path which enabled to rule out the most common damage mechanisms, as a main cause of the damages, and led to the identification of the most probable cause for the instabilities. A critical reading of the geometry of the cracks, supported by several static verifications, resulted in the exclusion of mechanisms associated with the arches thrust actions or with the compressive stresses in the walls. The hypothesis of soil settlement is rejected by the mechanical characteristics of the foundation soil, provided from a previous study. An original diagnostic hypothesis of tensile stresses induced by thermal variations is formulated and a fully coupled temperature-displacement analysis is carried out, with the FE software Abaqus, to investigate the effects of the thermal gradient on structures behavior.