PSI - Issue 78

Giulia Giuliani et al. / Procedia Structural Integrity 78 (2026) 952–959

953

Keywords: Seismic Isolation, Near-Fault Effects, Vertical Ground Motion, High Damping Rubber Bearings, Structural Seismic Design.

1. Introduction Base-isolated buildings can be idealized as dynamically decoupled systems in which isolation bearings effectively separate the seismic response of the superstructure from that of the substructure and foundation. When the isolation ratio, defined as the ratio between the fundamental period of the isolation system and that of the fixed-base superstructure, is sufficiently high, the superstructure behaves quasi-rigidly, and the system's dynamic response is predominantly governed by the elongated fundamental mode introduced by the isolation layer. Under these simplifying assumptions, the design process becomes relatively straightforward, contributing to the widespread adoption of base isolation as a seismic protection strategy in earthquake-prone regions. However, the design of base-isolated structures near active faults requires special consideration due to the complex nature of near-fault ground motions, a topic extensively addressed in the literature and codified in standards such as ASCE 7 (2022). Decades of recorded strong-motion data have deepened our understanding of near-fault effects, which manifest in both horizontal and vertical ground motion components. The most prominent horizontal effect is fault normal forward directivity, typically observed when rupture propagates toward a site at a velocity close to the shear wave speed, resulting in a large-amplitude, long-period velocity pulse (bilateral pulse). Additionally, fault-parallel fling-step effects can induce permanent displacements and are particularly pronounced on the hanging wall of reverse or oblique faults, often accompanied by significant vertical velocity pulses (Petricca et al., 2021). For base-isolated systems, these long-period, pulse-like horizontal ground motions can induce large displacements, particularly as isolation systems are increasingly designed with longer periods due to technological advancements in bearing performance. This trend heightens the risk of resonance with fault-normal velocity pulses (Erdik et al., 2023). In Europe, Eurocode 8 – Part 1 (EN 1998-1, 2005) requires that both horizontal and vertical seismic components be considered in the design of base-isolated buildings. For importance class IV structures located within 15 km of a potentially active fault capable of producing Mw ≥ 6.5 events, site -specific spectra incorporating near-fault effects must be considered. However, Eurocode 8 provides limited guidance on how to develop these spectra, and few studies have systematically examined the influence of vertical or combined vertical-horizontal ground motion on base-isolated structures. To address these gaps, this study investigates the influence of near-fault ground motion on the seismic design of base-isolated structures using advanced three-dimensional ground motion models (GMMs). The analysis employs the ITA18 model for Italy (Lanzano et al., 2019), which has been empirically adjusted for near-fault effects by Sgobba et al. (2021) using the global NESS2.0 database. Vertical excitation is modelled through the vertical-to-horizontal (V/H) spectral ratio proposed by Ramadan et al. (2021). The case study focuses on base-isolated reinforced concrete (RC) buildings equipped with High Damping Rubber Bearings (HDRBs), and where applicable, supplemented with Flat Sliders Bearings (FSBs). Four near-fault seismic scenarios are considered, representing high seismicity conditions typical of Central Italy. These scenarios assume a magnitude Mw = 7.4 event at distances of 0, 5, 15, and 30 km from the fault. Two RC building configurations, a 3-story and a 9-story structure, are analysed. Optimal base isolation designs are developed for each configuration and seismic scenario. Nonlinear time-history analyses are performed to assess the seismic performance of the structures, using ground motion records selected to match both the horizontal spectral demands at the isolation period and the vertical spectral content associated with the vertical vibration modes. The findings yield important insights into the displacement demands and vertical load effects on base-isolated systems in near-fault conditions, offering recommendations for enhancing seismic design practices within the framework of European codes. 2. Near source Ground Motion Characterization and Accelerogram Selection To assess the influence of near-fault earthquakes on the design and performance of base-isolated structures, a representative site located within the high-seismicity Apennine chain of Central Italy is considered. This region was notably impacted by the 2016 – 2017 seismic sequence, which generated ground motions with significant vertical components (Ramadan et al., 2021). The horizontal seismic demand is characterized using the empirical Ground