Impact of the soil-structure interaction on the seismic vulnerability assessment of irregular masonry infilled RC buildings on soft soils

This study investigates the seismic vulnerability of two existing high-rise reinforced concrete frame (RCF) buildings with plan and vertical irregularities and masonry infills, considering the role of soil–structure interaction (SSI). Both structures are located on soft alluvial deposits of the Guadalquivir River and rest on shallow foundations. To capture the influence of SSI on dynamic response, nonlinear three-dimensional finite element models were developed in OpenSees, incorporating direct soil modelling and advanced constitutive laws for soil behaviour. A total of 42 bidirectional ground motion records, comprising 21 motions representative of hard soils and 21 of soft soils (Eurocode 8 Types A and C), were employed for linear and nonlinear time-history analyses. Key seismic response parameters—including elastic and inelastic inter-storey drifts, global damage indices, and code-defined damage states (EC8, FEMA 356, Risk-UE, and ATC-40)—were evaluated under both fixed-base and SSI-integrated conditions. Results demonstrate that SSI contributes to increased drift demands, altered collapse mechanisms, and redistribution of damage across the structural system. Despite compliance with inter-storey drift limits in many cases, global damage indices reveal significant vulnerability, even at low drift levels. These outcomes highlight a misalignment between drift-based code thresholds and actual damage patterns when SSI is present. The findings underscore the necessity of incorporating SSI into performance-based seismic design approaches, particularly for mid- to high-rise masonry-infilled RCF buildings on soft soils, to ensure more accurate vulnerability assessments and resilient structural performance.