PSI - Issue 78

Giulia Giuliani et al. / Procedia Structural Integrity 78 (2026) 929–935

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3. Near-Source Input And Effect On Structures 3.1. Seismic Input Modelling

The approach adopted in this study to derive horizontal and vertical response spectra for near-fault sites involves selecting a representative seismic scenario for the site and applying suitable GMMs that account for near-fault effects. For the horizontal component, the ITA18 model (Lanzano et al. (2019)), a regional GMM specifically developed for Italy, is used. This model has been empirically adjusted for near-field conditions by Sgobba et al. (2021) using data from the global NESS2.0 database. To generate vertical spectra, the vertical-to-horizontal (V/H) spectral acceleration ratio proposed by Ramadan et al. (2021) is applied, incorporating an adjustment term specifically designed to improve accuracy in near-source scenarios. This enhanced model, referred to as ITA18-V(VH)-NESS, is combined with ITA18 to produce vertical response spectra consistent with the horizontal predictions. A peculiarity of the analysed GMMs is that they provide reliable predictions for periods up to 10 seconds which is of outmost importance if one wants to analyse structural behaviour of base-isolated structures. The analysis focuses on the CLO site, a representative location within the highly seismic Apennine chain of central Italy, where the maximum expected earthquake magnitude is Mw 7.4. Additional site parameters include a shear wave velocity (VS,30) of 300 m/s for the upper 30 meters of soil, and four Joyner-Boore distances (0, 5, 15, and 30 km) are considered to evaluate the variation in spectral response with distance from the fault. Spectral acceleration is expressed in terms of RotD50, which represents the median response across all horizontal orientations, offering a robust estimate of bidirectional ground motion (Boore (2010)). The resulting response spectra for Mw 7.4 are presented in Fig. 3 for horizontal in terms of pseudo-acceleration and displacement. The figure also includes zoomed-in views for the 2s – 3s range, which will later serve as key reference points for analysing the response of base-isolated structures. Moreover, in Fig. 3b, the response spectra in terms of displacement computed with ITA18 GMM, i.e., without considering near source effects, are also reported with dashed lines. It can be noted that near fault effects become less important as the distance from the fault increases, becoming negligible at a R JB distance of 30km. Fig. 4 show the vertical response spectra in terms of pseudo-acceleration and the V/H spectral acceleration ratio. The figure also includes zoomed-in views for the 0s – 0.15s range. Regarding the vertical component, it is important to emphasize that at distances of 0 km and 5 km, both within the 10 km threshold for near-source effects defined by Eurocode 8, the maximum pseudo-acceleration in the critical short-period range significantly exceeds 1g. Notably, for very short periods, the vertical component even surpasses the horizontal pseudo-acceleration, as shown in Fig. 4b, where the V/H spectral acceleration ratio exceeds 1. This indicates that for structures with vertical vibration periods within this range, vertical ground motion dominates and may temporarily nullify gravity loads.