Hazard-dependent soil factors for site-specific elastic acceleration response spectra of Italian and European seismic building codes – an update from recorded accelerograms

Most seismic building codes worldwide allow the definition of the seismic action (horizontal component of ground motion) using a simplified approach based on modifying the ordinates of an elastic acceleration or displacement response spectrum expected on outcropping bedrock through appropriate soil factors. The procedure is only suitable for geotechnically stable soil sites (i.e. non-liquefiable). The details of the method may differ from code to code but, all share the same idea of classifying soil deposits into a restricted number of categories based on the geotechnical characteristics of the soil deposit and the weighted average of the shear wave velocity of the top 30 m (i.e., V_S30) or a surrogate parameter such as the site’s fundamental period. Finally, specific soil factors are associated with each soil category to scale the ordinates of the elastic response spectrum defined on outcropping bedrock and flat topographic surface. Being a method specifically developed for design, the search for a balance between simplicity and accuracy increases the uncertainty of the results. Recent studies based on analyzing recorded strong motion data and numerical simulations have raised doubts about the reliability of this approach, given the tendency of the current soil factors to either underestimate or overestimate the horizontal acceleration at the free surface of the soil deposit. If, onto one hand, the underestimation of the seismic action is related to the level of safety, on the other hand, the overestimation of the seismic action may lead to overdesign with an increase in construction costs. In 2018 the authors of this paper have published an article on this journal on assessing the reliability of current Eurocode 8 and the Italian building code (NTC18) soil factors using the results of a large number of numerical simulations. In this paper the same authors update their 2018 study by including strong motion data from real recordings. Updated hazard-dependent soil factors for Eurocode 8 and the Italian building code (NTC18) are defined by complementing numerical and real ground motion data. The role of epistemic uncertainty in specifying soil amplification factors is highlighted also through a comparison with soil factors calculated from other international building codes (e.g. 2021 IBC and ASCE 7–16) and recent publications.