Base isolation in near-fault scenario: a design-oriented investigation

Base isolation is a widely recognized strategy for mitigating seismic demands in structures, effectively enhancing safety by reducing inertial forces and limiting structural damage. However, its use in near-fault sites, typically within 15 km of a fault, requires additional considerations due to unique ground motion characteristics not fully addressed by current seismic design codes. Near-fault motions are marked by forward directivity in the horizontal component and significant vertical accelerations, particularly near dip-slip faults. Directivity effects often generate large, long-period velocity pulses, which critically increase displacement demands on base-isolated systems, especially as isolation periods lengthen. Simultaneously, vertical motions can exhibit vertical-to-horizontal spectral acceleration ratios exceeding unity at short periods (0.05–0.10 s), with amplified effects in soft soils. While vertical excitations have been studied for vertical-sensitive structures, their implications for isolated systems remain underexplored. This study investigates the seismic response of base-isolated buildings with High Damping Rubber Bearings (HDRBs), both independently and in combination with low-friction Flat Slider Bearings (FSBs), under near-fault inputs. Ground Motion Models developed for near-fault scenarios were used to derive horizontal and vertical spectra within a Eurocode-based design framework. Results highlight the critical role of fault proximity: as distance decreases, displacement demands rise sharply, requiring larger isolator volumes and additional FSBs to meet performance targets. Vertical motions significantly affect isolator compressive and tensile behaviour, potentially causing uplift in sliders and overstressing HDRBs. These findings reinforce the need to explicitly account for vertical effects in the seismic design of isolated structures near active faults.