Influence of soil-structure interaction on the site-specific seismic demand to masonry towers

Joint analysis of hazard and fragility is becoming a widespread and unavoidable tool to estimate losses caused by earthquakes. The application of such analysis to buildings on soft soil is improved when the dynamic soil - foundation - structure interaction is considered, since both the reference hazard on site and the structural performance are modified by soil compliance. This study investigates and compares the site-specific seismic demand required to squat or slender masonry towers settled on stiff or soft soil. To this aim, 96 non-linear dynamic analyses were performed on 3D models, including soil, foundation and structure and excited by fourteen unscaled records of real earthquakes. Distributions of the equivalent bending rotation of structure and of peak and residual rotations of foundation were calibrated on numerical results and convoluted with the hazard curves of three Italian cities, to compute the mean annual rate of exceeding different values of structural and foundation demand. The resulting curves highlight that soil deformability enhances the bending demand and produces a not negligible permanent tilt in towers.