Inter-storey seismic isolation is increasingly gaining attention. One of the main related issues is the need to limit the relative displacement between substructure and superstructure, while maintaining a good seismic performance of the superstructure. As shown in some studies, fluid viscous dampers (FVDs) mounted in isolation systems are effective in reducing isolator deflection but can be harmful by amplifying inter-storey drifts and floor accelerations. Additionally, the effectiveness of FVDs for inter-storey applications was investigated only recently, and specific approaches for their optimisation and performance evaluation are missing. Therefore, this paper proposes a method for the optimal multi-objective design of FVDs, based on the definition of appropriate surrogate response models, which allows for rationally comparing the FVD effects for a wide range of dampers and structures. In particular, the optimal FVD parameters are provided in a dimensionless form, so that they can be predicted by design equations of general validity within the range of the structures analysed. This method is applied to a stock of regular structures with various vibration periods of superstructure, isolation and substructure, examining a linear and a non-linear isolation system and a set of natural records, in order to comprehensively assess the effects of FVDs and their non-linearity on the seismic performance of these structures. Finally, prediction models of optimal FVD parameters are provided based on the results obtained and are applied to three case studies as an example.

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Recent Italian earthquakes have shown the high seismic vulnerability of pallet racking systems. In the down-aisle direction and in the absence of bracing systems, these structures are very flexible moment-resisting frames. Instead, in the cross-aisle direction they consist of slender trusses, stiffened by various bracing systems; the latter, although necessary for lateral stability, attract significant seismic accelerations, which can cause the stored goods to fall, posing a threat to human safety. To reduce this risk while increasing the rack structural performance, some mitigation systems were proposed, based on specific base-isolation or dissipation devices. In this paper, an innovative passive control system is investigated, i.e., the Load-Level Isolation System (LLIS), which consists of applying isolators to the load level to control the movement of pallets in the cross-aisle direction. The LLIS is based on the Tuned Mass Damper (TMD) strategy and exploits the high payload of these structures. Among the major uncertainties of this system are the amount of isolated mass and its position within the structure. Therefore, for a case study rack, the LLIS parameters (isolation stiffness and damping ratio) are optimized for various arrangements of this system, considering cases with one or two isolated levels. The applied optimization procedure is an extension of the classical TMD approaches. The effectiveness of the optimized LLISs is then investigated through bidirectional Time-History analyses on 3D Finite Element rack models. The results show that using the LLIS, even on a single load-level, can greatly reduce the upright stresses and the cross-aisle displacements and accelerations, and that the optimal position of the LLIS is in the upper part of the rack.