Evaluation of equipment fragility curve in isolation building considering the vertical seismic ground motion

Seismic isolation structures have been introduced in a wide variety of fields as devices to enhance safety during earthquakes. In the nuclear field, although many studies have been conducted for introducing seismically isolated nuclear reactors, this has not been realized yet. To ensure higher reliability and safety of seismic isolation systems, probabilistic risk assessment, which consists of seismic hazard analysis, seismic fragility analysis, and system analysis, plays an important role. This paper aims to investigate the effect of the ultimate characteristics of isolation devices and vertical ground motion on the damage risk of isolators and equipment on the building. As a result, in the case of the isolation layer, it was confirmed that the horizontal response dominates the risk since the effect of vertical motion tends to be small. On the other hand, when the natural period of equipment is approaching either a horizontal or vertical period of the isolation system, the risk of equipment is tended to be underestimated. Thus, the consideration of the effects of vertical motion in addition to horizontal motion and the ultimate characteristics of isolators should be required for the risk assessment of seismic isolation systems.