From Digital Survey to Stability Assessment in San Tomè, Almenno

The Rotunda of San Tomè in Almenno San Bartolomeo (Bergamo, Italy) is an example of Lombard Romanesque architecture. Built during 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. Inside, a double-level row of eight columns surrounds the central space, concluding with 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, with overlay of materials, making the interpretation of the palimpsest a challenge. On the other hand, this left room for new research to clarify the evolution of architecture. The most recent intervention involved inserting metal hoops to contain cracks in the vaults andwalls of the church. To combine advancements in historical and archaeological knowledge with safety monitoring, an integrated survey campaign was conducted. 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 F. de Dartein, an engineer and expert on medieval architecture, did not even report it. It was likely reconstructed during the 19th century renovation by E. Fornoni. This project may have involved opening the summit oculus, which appears eccentric with respect to the dome axis in the current survey. The surveys also allowed the detection of the slight deformation of the dome,whose deviations from a supposed perfectly hemispherical geometry can be taken as indicators of the displacements suffered by the walls. These findings demonstrate how the information provided by an accurate survey can be used to understand the structural behavior of a historical building and its deformations over time, becoming essential for a coherent and effective consolidation project.