Load-Level Isolation System for industrial racks: Evaluations on a case study structure

Recent Italian earthquakes have dramatically demonstrated the high seismic vulnerability of industrial racks, raising awareness of the need to increase their seismic resilience in industry, academia, and society in general. Originally, these structures were conceived and designed to meet load-bearing requirements for static loads only. Although recent racking regulations contain clear requirements against seismic actions, most of the racks still in use today do not comply with them. To reduce their vulnerability, but also to increase the resilience of the newly designed racks, specific base isolation systems have recently been studied as an alternative to conventional strengthening solutions (e.g. thicker profiles, bracing systems). In this context, the Load-Level Isolation System (LLIS), i.e. the isolation system directly applied to some load levels of the rack, represents a new and interesting seismic mitigation technique, which deserves to be further explored. When properly designed, the LLIS allows the load units to act as tuned mass dampers, thus controlling the dynamic response of the structure. This paper aims to evaluate the effectiveness of the LLIS by analyzing a 5-span, 7-level case study rack, with the RBRL isolation system applied to the top two levels. The structure is analyzed through 3D finite element modeling and time-history analysis, using a set of bidirectional natural events. The effects of the LLIS are evaluated in terms of displacement and acceleration profiles, as well as axial and shear forces at the base of some uprights. These results are compared with those obtained by the same rack without LLIS and by the latter strengthened by means of a longitudinal bracing system, thus demonstrating the potential of the LLIS. Specifically, it is sufficient to isolate a few load levels to obtain significant reductions in the displacement profiles and in the maximum forces at the base of the structure.