Influence of the ground on the structural identification of a bell-tower by ambient vibration testing

The article revolves around the structural identification technique through dynamic in situ tests trying to unravel the contribution of Soil Structure Interaction (SSI). The issue is addressed by considering the bell tower of Santa Sofia in Benevento (Italy) as a case study. Together with the homonymous church, the monastery and the fountain, the bell tower is part of a monumental complex included in the UNESCO World Heritage List since 2011. As the tower is completely separated from the other buildings of the complex, from a structural viewpoint it is quite a simple structure but likely to be affected by SSI. The latter phenomenon in some way could dictate the tower static and dynamic response, thus affecting the overall structural dynamic identification process. Invasive investigations are not usually allowed on cultural heritage due to preservation requirements. Thus, the paper proposes a simple but effective procedure for obtaining the data set required for both structure and soil characterization through noninvasive in-situ tests. The dynamic response of the subsoil at the tower site was firstly identified by reproducing the experimental amplification function provided by geophysical tests. Later, the results of an ambient vibration test performed on the bell tower have been interpreted and used to calibrate two refined 3D finite element models of the tower, one having the structure with a restrained base and another with a base compliant with the reality found. In the latter case, a suitable portion of the soil below the tower was included in the analysis domain. The contribution of the ground has been ascertained to move towards a reliable dynamic identification of the built cultural heritage.