Urban scale risk assessment including SSI and site amplification

Large-scale risk analysis is typically performed considering existing fragility curves, calculated in most cases without adequately accounting for local site amplification (SA) and soil-structure interaction (SSI) effects.
Nevertheless, foundation flexibility and local site effects may lead to a substantial margin of fragility or loss estimates, especially for structures resting on soft soil. Including these effects on the city-scale vulnerability analysis is challenging due to the entailed complexity of defining the whole interacting urban system. We propose a novel framework for the fragility assessment of structures considering the influence of SSI and local site amplification effects, suitable for large-scale applications. The applicability of the proposed approach is based on globally available data regarding the soil, the foundation, and the building portfolio. Site amplification is considered directly in the resulting fragility curves using site response analyses. An improved taxonomy is adopted to make the approach implementable in the OpenQuake software, introducing V_S,30 as a proxy for the site and SSI effects. Finally, following the performance-based earthquake engineering framework, the outcomes of the methodological framework are adopted to estimate the nominal probability of failure for selected building classes belonging to the majority of structural types of the city of Thessaloniki, Northern Greece. The main findings demonstrate that the conventional way of calculating fragility curves may lead to an incorrect seismic risk evaluation, especially in soft soil formations.