Combined Vertical and Horizontal Components of Near‐Source Earthquakes and Impact on Base‐Isolated Structures

Over recent decades, the collection of seismic data has improved the understanding of near-fault ground motion effects, which involve both horizontal and vertical components. Among the most significant effects are fault-normal directivity, which concentrates seismic energy into an intense, long-period pulse, and fault-parallel fling step, which causes permanent ground displacement. In dip-slip faulting scenarios (such as reverse and normal faults), significant vertical acceleration also occurs, as highlighted by recent studies, which have shown that vertical acceleration can exceed the horizontal component at short spectral periods. This research proposes a systematic approach to evaluate the combined effects of the vertical and horizontal components of near-fault ground motion – including pulse-like effects – for different near-source scenarios. The approach is applied to base-isolated structures equipped with high-damping rubber bearings (HDRBs), either alone or in combination with flat slider bearings (FSBs). The results, consistent with previous experimental and numerical studies on similar isolation systems, indicate that the vertical component does not influence the horizontal response of the hybrid isolation system or the superstructure, but it can cause uplift of FSBs, cavitation of HDRBs, and very large vertical accelerations in the superstructure. Furthermore, for scenarios similar to the one considered, they provide insight into the fault distances at which these phenomena may pose significant challenges for base-isolated buildings.