Fully coupled nonlinear analyses of one building on one soil profile showed that inelastic dynamic soil–structure interaction (DSSI) can reduce seismic demand and shift fragility curves toward lower vulnerability (Sáez et al., 2011), but the authors themselves left open how general and why this effect is. Fifteen years of subsequent case studies have produced typology-specific and soil-specific results — including published cases where soil–structure interaction increases collapse probability or drift demand — while two 2026 state-of-the-art reviews still conclude that SSI effects 'can either be beneficial or detrimental depending on the conditions'. This problem asks for a general, mechanistically attributed, and observationally validated account of how inelastic DSSI modifies the seismic fragility of buildings: how demand reductions and fragility shifts generalize across building types, foundation systems, soil stratifications, and ground-motion characteristics (including strong and near-fault pulse-like motions); how the net demand change decomposes into radiation damping versus modified hysteretic soil damping; and whether SSI-informed fragility curves can be validated against empirical damage data.

Background

Seismic vulnerability of buildings is commonly expressed through fragility curves: functions giving the probability that a structure exceeds a specified damage state (e.g., the four levels slight, moderate, extensive, complete used in loss-assessment frameworks such as HAZUS) as a function of a ground-motion intensity measure such as peak ground acceleration (PGA). The standard simplification is the fixed-base assumption, in which the structure is analyzed as if rigidly clamped to the ground and the foundation input motion equals the free-field motion. In reality the supporting soil is deformable: the foundation modifies the motion transmitted to the structure (kinematic interaction) and the soil–foundation flexibility lengthens the system's natural period while adding energy dissipation (inertial interaction) — together, soil–structure interaction (SSI). When the soil itself responds nonlinearly under strong shaking (shear-modulus degradation, hysteretic energy dissipation, yielding) and the soil–foundation–structure system is analyzed in the time domain with nonlinear finite elements, the phenomenon is called inelastic dynamic SSI (DSSI). Two dissipation channels accompany DSSI: radiation damping, the energy carried away from the foundation by waves propagating into the unbounded soil, and material (hysteretic) damping, the energy dissipated inside the soil by its nonlinear stress–strain response; classical studies of linear SSI, such as Ambrosini (2006), showed that the relative weight of these two damping channels matters for the system response, but treated linear soil behavior. Sáez, Lopez-Caballero and Modaressi-Farahmand-Razavi (2011) computed fragility curves from fully coupled nonlinear finite-element analyses including inelastic DSSI, with a sensitivity strategy to size the synthetic ground-motion database, and found a general reduction of seismic demand when DSSI is included, reflected in shifted (less vulnerable) fragility curves. They explicitly delimited their conclusions: one building geometry, one approximately calibrated nonlinear soil profile, no motions strong enough to populate the Extensive and Complete damage levels, no clean separation of the radiation-damping and modified-hysteretic-damping contributions, no recorded damage data to validate the curves, and a two-dimensional model that cannot capture torsional or localized three-dimensional effects. Later work has filled pieces of this agenda case by case — fragility modifiers for reinforced concrete (RC) dual buildings across soil profiles (Petridis and Pitilakis, 2020), non-ductile and high-rise RC frames (Rajeev and Tesfamariam, 2012; Karapetrou et al., 2015), steel moment-resisting frames (Hamidia et al., 2021), near-fault pulse-like motions (Birzhandi and Halabian, 2018; Mashhadi and Homaei, 2023; Debnath et al., 2026), torsionally coupled systems (Shirzadi et al., 2019), and urban-scale assessment including SSI (Amendola and Pitilakis, 2022, 2024) — but with typology-specific, sometimes opposite-signed findings, and recent reviews (Shayah, 2025; Singhai et al., 2026; Islam et al., 2026) still report that whether SSI is beneficial or detrimental remains condition-dependent and that damping modeling and its quantification are unresolved issues.

Problem Statement

Determine, in a general and mechanistically attributed way, how inelastic dynamic soil–structure interaction modifies the seismic fragility of buildings. This comprises three jointly needed components serving one objective — a validated general account replacing the case-by-case literature: (1) Generality: establish how the DSSI-induced seismic demand reductions and fragility shifts found for one building on one nonlinear soil profile generalize across building types, foundation systems, soil stratifications, and ground-motion characteristics, including whether DSSI remains beneficial for very large demands and for near-fault pulse-like motions that populate the Extensive and Complete damage levels. (2) Mechanisms: within the coupled nonlinear soil–foundation–structure system, isolate and quantify the relative contributions of radiation damping (energy radiated away by waves propagating into the unbounded soil) and of modified hysteretic soil damping (material dissipation changed by the nonlinear soil response beneath the foundation) to the net change in seismic demand. (3) Validation: test the SSI-informed fragility curves against observational evidence — recorded building-damage databases or instrumented case studies, matched to the characterized site, foundation, and building conditions of their records and identified explicitly by the answer — extending the modeling to three-dimensional effects such as torsion and localized member failure where needed. The three components are jointly needed to answer the question: the general condition-dependent result across structural system, foundation and soil characteristics, and ground-motion regime for when inelastic DSSI reduces, leaves unchanged, or increases fragility; the quantified damping-mechanism attribution that explains those conditions; and the observational validation of the resulting fragility curves.

References

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