GNGTS 2021 - Atti del 39° Convegno Nazionale

333 GNGTS 2021 S essione 2.2 AN EMPIRICAL MODEL FOR THE VERTICAL-TO-HORIZONTAL SPECTRAL RATIOS FOR ITALIAN EARTHQUAKES INCLUDING NEAR-SOURCE EFFECTS G. Lanzano, F. Pacor, F. Ramadan, Fadel, C. Smerzini Introduction Vertical ground motion may play a crucial role for unreinforced masonry constructions, prominent in countries such as Italy, whose dynamic behavior is essentially governed by friction. As shown in [Liberatore et al. 2019], for these constructions, cycles of increments of axial loads can induce repetitive reductions of friction force, leading, therefore, to more extensive failures in small-cohesion structures. This finding is also supported by Di Michele et al. (2020) who concluded that fluctuations in the axial load strongly affect the flexural and shear capacity of masonry walls and a higher incidence of such effects is found for near-source records of large magnitude events. In general, two main approaches can be used to develop vertical design seismic spectra in the framework of a Probabilistic Seismic Hazard Assessment (PSHA): (1) perform hazard integrations using Ground Motion Models (GMM) specifically developed for the vertical response spectral ordinates [Chiou and Youngs 2013;Bozorgnia and Campbel 2016; Gülerce et al. 2017; Çağnanet al. 2017]; (2) use a GMM for the Vertical-to-Horizontal (vertical-to- horizontal (VH) response spectral acceleration ratios to scale the horizontal Uniform Hazard Spectrum (UHS). The first approach has the main limitation of posing a potential inconsistency between the disaggregation scenarios controlling the horizontal and vertical hazard, leading to serious obstacles in the selection of three-component ground motion time histories for dynamic structural analyses. For these reasons, the most commonly used approach is to generate the vertical spectrum by making use of empirical models for VH ratios [Gülerce and Abrahamson 2011; Bommer et al. 2011; Akkar et al. 2014; Bozorgnia and Campbell 2016; Poggi et al. 2019]. This approach, although simplified, avoids the issues of performing vector-valued PSHA [Bazzurro and Cornell 2002] including both horizontal and vertical components and the full treatment of their correlation. The main aim of this study is to develop an empirical GMM for VH ratios of Spectral Acceleration (SA), Peak Ground Acceleration (PGA) and Peak Ground Velocity (PGV) for Italian crustal earthquakes. The regression is calibrated using the same dataset adopted by Lanzano et al. (2018) to develop the horizontal GMM, thus ensuring consistency between the two models, both in terms of validity range and functional form. To account for the effect of near-source conditions on the VH ground motion, a near-source correction term is proposed, following the Referenced Empirical Approach [Atkinson 2008; 2010]. In this approach, for a given GMM, a corrective term is computed on the basis of the residuals between the prediction and additional data, suitable to model specific effects on the ground motion. In our case, the corrective term for the reference ITA18 GMM is calibrated based on the analysis of residuals with respect to a worldwide dataset of near-source recordings, namely, NESS1.0 [Pacor et al. 2018]. Figure 1 shows the conceptual framework at the basis of the methodology proposed to develop the Italian VH GMM accounting for near-source effects.