Fragility curves of URM buildings in aggregate considering the interaction with soil and among nearby footings

Abstract The paper examines unreinforced masonry (URM) buildings, which are common in small historic centres around the world. These buildings are often constructed in aggregate, a configuration that not only results in significant structural interaction but also interaction with and through the foundation soil. The seismic performance of an aggregate can be influenced by foundation-soil-foundation interaction (FSFI), in addition to standard soil-foundation-structure interaction (SFSI) and site effects (SE). While reliable and time-efficient approaches are available in the literature to address all these issues for standalone buildings, buildings in aggregate are frequently modelled as isolated and fixed at their base, particularly when developing fragility curves. This paper investigates the effects of SFSI and FSFI on the period and damping ratio estimates of typical URM buildings. Specifically, it examines the impact of SE, SFSI, and FSFI on the fragility curves of two aggregated URM buildings. These latter are representative of Visso, a town heavily affected by the 2016–2017 Central Italy earthquake, known for site amplification phenomena due to soft soils. Fragility curves were developed through nonlinear dynamic analyses of equivalent 3D frame models of the two archetypes, analysed under both fixed and compliant base conditions. In the latter scenario, the structural model is equipped with springs at its base, with stiffness first calibrated to account for SFSI effects and subsequently adjusted to include the additional FSFI contribution. The results indicate a higher fragility in the fixed-base model. Specifically, the ratio of the median values of the fragility curves for the compliant base model to the fixed-base model ranges from 20 to 60%. Finally, the results from the cross-interacting models exhibited slightly higher values than those considering only SFSI, suggesting a moderate impact from the additional contribution of footing-footing interaction, at least for the case studies examined.