GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 2.2 GNGTS 2023 Proposal for defining the vertical design spectrum in the Italian seismic norm C. Smerzini 1 , S. Sgobba 2 , F. Ramadan 2 , G. Lanzano 2 , R. Paolucci 1 , F. Pacor 2 1 Department of Civil and Environmental Engineering, Politecnico di Milano, Milano, Italy 2 INGV - Istituto Nazionale di Geofisica e Vulcanologia, Milano, Italy Introduction In general, there exist two main classes of approaches to develop vertical design seismic spectra in the framework of a Probabilistic Seismic Hazard Assessment (PSHA): (1) perform hazard integrations using GMMs specifically developed for the vertical response spectral ordinates (e.g. Bozorgnia et al. 2019; Gülerce and Abrahamson 2017; Ca ğ nan et al. 2017), separately from those for the horizontal components, to derive either a vertical Uniform Hazard Spectrum (UHS) or Conditional Mean Spectrum (CMS, Baker 2011); (2) use a GMM for the VH spectral ratios, as accomplished in this work (Ramadan et al. 2021), to scale suitably either the horizontal UHS or CMS. The main limitation of the first approach, although straightforward, is that disaggregation of hazard may lead to different earthquake scenarios controlling the horizontal and vertical spectra. Such inconsistency may pose obstacles to site-specific engineering studies, such as in the selection of hazard-consistent three-component ground motions to be used in dynamic time history analyses of structures. For these reasons, the most commonly used approach is to generate the vertical spectrum by making use of VH GMMs. Among this class of approaches, as discussed in Gülerce and Abrahamson (2011), there are three main options, depending, primarily, on the type of reference spectrum, either a UHS or a CMS, and, secondarily, for the CMS, whether the correlation between the residuals of the H and VH GMMs are explicitly accounted for. In this work, we propose an update of the vertical design spectrum for the Italian seismic norm following a simpler approach, suitable for application in a normative framework, similarly to the one adopted in NEHRP (2020). A corrective VH spectral ratio is proposed to scale the elastic horizontal design spectrum and, then, obtain the vertical elastic design spectrum, as follows: (1) where