Seismic Assessment of the Portogruaro Bell Tower Using a Three-Dimensional Soil–Structure Interaction Approach

The Portogruaro Bell Tower (Italy) represents a challenging case study for seismic assessment owing to its pronounced inclination, slender geometry, and foundation on deformable alluvial deposits. While fixed-base models are commonly adopted in engineering practice, they may neglect the influence of soil–structure interaction (SSI) on the dynamic response and seismic performance of historic masonry towers. This study presents a three-dimensional numerical investigation aimed at quantifying the role of SSI in the seismic assessment of the tower through a computationally efficient modeling approach. A coupled three-dimensional finite element model of the bell tower and the surrounding soil domain was developed to explicitly simulate the soil–structure system. Both the tower and the supporting ground were discretized using solid finite elements, allowing the interaction between the structure and the deformable foundation to be directly reproduced. A comprehensive sensitivity analysis was first performed by varying the linear and nonlinear mechanical properties of the soil to investigate their influence on the global structural response, including natural vibration characteristics, displacement demand, and stress distribution. The outcomes of the sensitivity analyses were then used to identify representative foundation conditions for subsequent seismic simulations. The seismic performance of the tower was evaluated under multiple earthquake scenarios representative of the local seismic hazard through nonlinear numerical analyses. The results demonstrate that the mechanical properties of the supporting soil significantly influence the structural response, affecting both the dynamic characteristics and the seismic demand imposed on the tower. The study highlights the importance of accounting for soil–structure interaction in the seismic assessment of inclined masonry towers and shows that simplified fixed-base assumptions may lead to inaccurate estimations of structural safety. The proposed methodology provides an effective numerical framework for investigating the influence of foundation conditions on the seismic behavior of historic masonry towers while maintaining a computational effort suitable for engineering applications.