Il rilievo per la diagnosi dei dissesti in San Tomè di Almenno

The San Tomè Rotunda in Almenno San Bartolomeo is an emblematic example of the originality of Lombard Romanesque architecture. Built in the mid-12th century, it appears as a cylindrical structure with an apse that extends in the northeast direction surmounted by two smaller cylindrical volumes. The first contains the dome, and the second houses the lantern. Inside, a double-level row of eight columns surrounds the central volume, concluded by a hemispherical dome topped with an oculus. Starting in the 19th century, the growing interest in the unique configuration of the temple led to a series of restoration efforts. These efforts resulted in a well-preserved building but also in the overlay of materials, making the interpretation of the palimpsest a challenge. On the other side, this left room for new researches to clarify the evolution of the architecture. The most recent intervention, which occurred a decade ago, involved inserting metal hoops to contain cracks in the vaults and walls of the church. To combine advancements in historical and archaeological knowledge with safety monitoring, the University of Bergamo conducted a survey campaign. The results shed light on several issues, including the lantern. Drawings from the 18th and 19th centuries depict the lantern as not being connected to the hall, and de Dartein did not even report it. It was likely reconstructed during Fornoni's renovations, which may have involved opening the summit oculus. The summit oculus appears eccentric with respect to the dome axis, supporting the hypothesis concerning its opening during the restorations of the early twentieth century. The surveys also allowed the detection of the slight deformation of the dome, whose deviations from hemispherical geometry can be taken as indicators of the displacements suffered by the walls. These findings demonstrate how the information provided by a survey can be used for understanding the structural behaviour of a historical building during time and can be crucial for a coherent and effective consolidation project.