PSI - Issue 78

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

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during the 2016 Central Italy seismic sequence (Savoia et al. (2017)), and for precast industrial buildings during the 2012 Emilia earthquakes (Di Sarno et al. (2010)). RC structures behave differently, as their columns can sustain some tensile forces. However, even in these cases, large vertical ground motions can induce excessive tensile or compressive stresses, potentially leading to severe structural damage (see Di Sarno et al.. (2012)). An even more critical issue arises in base-isolated buildings located near active faults, especially if near-source effects are not explicitly considered during design, as highlighted in recent studies (Erdik (2023)). This vulnerability is particularly concerning given the growing adoption of base-isolated systems in high-seismicity areas, due to the superior protection they offer. The concern stems from the fact that while isolation bearings are designed with low lateral stiffness (to achieve a long isolation period), they typically possess high vertical stiffness. As a result, their vertical vibration periods are similar to those of fixed-base structures, placing them in the range of high vertical pseudo-acceleration. In base-isolated buildings equipped with concave curved surface bearings (CCSBs), uplift is a major concern. Uplift is prohibited by codes because it can amplify axial compressive forces through dynamic effects and reduce global friction, thereby increasing horizontal displacements. For high-damping rubber bearings (HDRBs), which can tolerate moderate tensile forces, the risks involve exceeding code-imposed tensile limits and experiencing significant axial compression. These issues, when combined with high horizontal displacement demands, may result in bearing buckling. Considering the displacement response spectra depicted in Fig 3b for an equivalent damping ratio of 5%, it can be noted that at an isolation period of 3 seconds, the displacement demand at 0 km is 0.5 m, and 0.45 m at 5 km. When applying a damping reduction factor of 0.7, corresponding to an equivalent damping of 15% typical for HDRBs, these displacements reduce to 0.35 m and 0.25 m, respectively, which are still considerable. Such large displacements necessitate the use of large-diameter isolators to avoid buckling, along with additional low-friction flat sliders to preserve a sufficiently long isolation period for horizontal seismic mitigation. This issue becomes even more critical in multi-story buildings, where rocking-induced axial force fluctuations on isolators can further compromise performance. Lastly, it is worth noting that at distances under 5 km, horizontal pseudo-accelerations at T is = 3s reach 0.225g and 0.375g. After applying the 0.7 reduction factor, these become 0.16g and 0.26g, respectively, values that remain significant for superstructure design. 4. Conclusions The analysis conducted in this study confirms that near-fault effects have a significant influence on both horizontal and vertical seismic demands. In particular, while the Mw 6.5 Italian earthquake showed moderate horizontal displacement demands, it exhibited particularly intense vertical pseudo-accelerations, in some instances exceeding 1g. Most structures, including both masonry and reinforced concrete (RC), tend to have low vertical vibration periods, making them especially vulnerable to vertical accelerations in near-fault conditions. This vulnerability is further heightened in systems that cannot resist tensile forces, such as masonry buildings and precast RC structures lacking mechanical connections between beams and columns, due to the potential loss of frictional resistance under vertical seismic loading. Base-isolated buildings, which are typically designed to mitigate horizontal seismic forces, may also be at considerable risk when subjected to near-fault vertical ground motions. Concave Curved Surface Bearings (CCSBs), for instance, are susceptible to uplift, which can lead to amplified axial compressive forces and increased horizontal displacements. Similarly, High-Damping Rubber Bearings (HDRBs), though capable of withstanding moderate tensile forces, may experience excessive tension or even buckling when exposed to the combined effects of vertical and horizontal demands. Although current seismic design codes, including Eurocode 8, acknowledge the importance of near-source effects, they lack specific guidelines for deriving near-fault response spectra. There is a clear need to incorporate advanced ground motion models (GMMs) that account for near-fault effects on both horizontal and vertical components, as well as to establish appropriate procedures for selecting accelerogram sets that simultaneously match both target spectra. Furthermore, base-isolated structures constructed in near-fault regions require dedicated design provisions. To this end, further research is necessary to refine and enhance code guidance, ensuring the reliable performance of base isolated systems under the unique challenges posed by near-fault seismic ground motions.