Evaluation of SSI Effects for Mitigating the Seismic Risk of Historic Buildings

The documented damage due to earthquakes has shown that a seismic structural design is insufficient to have good safety conditions if the soil filter effect and the Soil-Structure Interaction (SSI) are not considered. An important modification of the seismic action evaluated according to the technical regulations can occur because of site effects. The dynamic response at the foundation level often differs remarkably from the free-field one because of the kinematic and inertial interaction. Mitigating the seismic risk represents a huge environmental challenge in medium-high seismic hazard areas. The paper proposes a manageable procedure for estimating at a large-scale the SSI effects on the seismic actions impacting ancient masonry churches present in some municipalities in the Etna volcano area (Catania, Italy). The investigated churches represent a valuable cultural heritage in Italy, the preservation of which is a serious environmental and social challenge. By identifying the churches at the highest seismic risk, it was possible to define the priority in implementing seismic improvement interventions, thus ensuring the conservation of the historical heritage and the safety of local communities. Local Site Response (LSR) analyses were performed in free-field conditions; then the Author analyzed the fundamental periods and the pertinent spectral accelerations of the investigated churches, considering both the flexible-base building configuration and the fixed-base one: S_a(T_SSI) and S_a(T_fixed). Their ratios were assumed as indexes of SSI effects. The procedure discussed here can easily be extended to other villages or cities.