Issues on the modal characterization of large monumental structures with complex dynamic interactions

Vibration-based monitoring techniques are currently used for damage assessment especially in monumental structures. In fact, dynamic tests allow investigation of structural behaviour of the building system through minimally invasive actions, which is important when preservation of the integrity of historical buildings is required. The Sanctuary of Vicoforte (Cuneo, Italy) has great architectural significance and contains the largest masonry oval dome in the world (axes 37.15 m × 24.80 m). The first dynamic campaign on this monumental building was performed in 2008. During this campaign, ambient vibrations could be measured in a limited portion of the oval dome, hence a first dynamic characterization was accomplished thanks to a recursive model-driven procedure. Some of the identified values were considered uncertain because the setups that were not able to fully capture the global behaviour. In 2015, a permanent dynamic monitoring system was installed on the dome-drum area of the building through Optimal Sensor Placement (OSP) procedures. New ambient response data were analysed in order to better define the dynamic properties of the structure as a whole, considering possible dynamic interactions. The most recent identification sessions, while retrieving and confirming the main results obtained in 2008, also supply elements that could not be appreciated in the first dynamic campaign. In any event, the Basilica's dynamic characterization requires very complex data processing and modelling operations due to the presence of dynamic interaction both with the soil and with other elements of the structure, and because of uncertainties about mass and material properties. This paper summarises the process that has finally led to the dynamic characterization of this stunning architectural structure.