The constant-ductility response spectra for vertical seismic isolated SDOF systems with asymmetric bilinear hysteresis model

The vertical components of ground motions are often exceptionally intense in near-fault regions, potentially causing significant seismic damage to structures, so controlling vertical dynamic responses through seismic isolation technology is a critical yet challenging issue that demands immediate attention. This study proposes a novel rubber shear device to achieve low vertical stiffness and substantial energy dissipation capacity. The rationality of its asymmetric bilinear hysteresis model is analyzed under the combined effect of gravity and vertical ground motion. Then, 100 near-fault vertical ground motions were selected as earthquake inputs, and the constant-ductility response spectra suitable for the asymmetric model were computed. The effects of natural period (T), ductility level (mu), and pre-yield stiffness ratio (alpha) on the spectra were thoroughly investigated. Appropriately increasing structural natural periods and incorporating energy dissipation through vertical isolation significantly reduces acceleration responses, with reductions of up to 70 % compared to elastic systems. Furthermore, the vertical dynamic displacement and residual displacement of vertically isolated structures remain within acceptable limits. For example, when T = 0.1 s, alpha= 0.1, and mu >= 15, the average maximum dynamic displacement corresponding to a peak ground motion acceleration of 1 m/s2 is only 1.7 mm. The inelastic response spectra provide theoretical validation for extending seismic isolation techniques to control vertical seismic responses, laying a crucial foundation for the performance-based design of vertical seismic isolation systems in structures.