The use of fluid viscous dampers (FVDs) together with isolators, frequent in near-fault buildings, is effective in reducing displacements of the isolation layer. Such a hybrid system is also beneficial in the case of inter-storey isolation with the aim of limiting P-Δ effects. However, previous research on base isolation shows that this additional damping may also be detrimental, as inter-storey drifts and floor accelerations may increase. This paper analyses the effectiveness of FVDs for enhanced seismic performance of systems with inter-storey isolation. A seven-floor building, with natural and lead rubber bearings between the second and third levels, was used as a case study, and a multi-objective optimal design was performed to identify the best damper parameters. In particular, time-history analyses with various natural records were carried out and two competing objectives were examined: minimisation of the deflection of the isolation layer and minimisation of the total drift of the superstructure. The results show not only the effectiveness of optimal FVDs but also the fact that their optimal linearity degree depends to a great extent on the non-linear seismic response of the structure, i.e., on the type of earthquake. The simplest design approach, consisting of applying an optimization algorithm for each design accelerogram, did not seem, in this case, to be sufficient to identify the best overall design solution. The design consequences of these findings are discussed.

Introduction

Inter-storey seismic isolation has attracted increasing interest in recent years, particularly in densely populated areas, as an alternative mitigation strategy to base isolation for both new and existing buildings. As the name suggests, the isolation system is incorporated between storeys rather than at the base of the structure, in view of architectural concerns, feasibility of construction, and performance benefits. Although base isolation for multi-storey buildings is a wellknown technique applied worldwide, it may sometimes clash with substantial economic and technical problems, which may limit its application. In particular, installing base isolation is straightforward for new buildings, but becomes complicated and expensive for existing ones, since excavation and temporary support works are required. Instead, the installation of inter-storey isolation is relatively simple and generally less expensive and disruption-free. It also allows extra floors to be constructed on an existing building (if its vertical capacity allows this) without increasing the total base shear demand, and thus represents an innovative and realistic retrofitting approach [1,2]. Firstly, base isolation is not as effective for medium/high-rise buildings as inter-storey isolation, because of the flexibility and bending-type behaviour of the latter [3]. Secondly, storey isolation can greatly increase design flexibility in high-rise and multipurpose buildings, by separating them into two independent structural parts which can be designed with different shapes, materials and functions, thus allowing them to become unique architectural features [4]. Examples of this application to irregular high-rise buildings are the Iidabashi First Building [4] and the Shiodome Sumitomo Building [5] in Japan, two multipurpose buildings having substructure and superstructure with different structural shape. In China, this technique was used to isolate 50 buildings (seven- or nine-storey RC frames) in Beijing, built on top of a two-storey platform covering a very large (∼۳ km2 ) railway area [6]. Built relatively recently, in the National Taiwan University campus, the Civil Engineering Research Building is a nine-storey pre-cast RC structure with an inter-storey isolation system installed between the second and third floors, which also includes viscous dampers [7]. Lastly, moving the isolation layer to the upper storeys reduces the need for a seismic gap, which is necessary to accommodate the expected displacement of isolators, but also expensive and sometimes impractical in densely-built urban areas.