Contents lists available at ScienceDirect Soil Dynamics and Earthquake Engineering journal homepage: www.elsevier.com/locate/soildyn State of art in the worldwide evolution of base isolation design Antonello De Luca, Laura Giovanna Guidi ∗ University of Naples, Federico II, Department of Structures for Engineering and Architecture, Di.St. Tecchio Square, Fuorigrotta, Naples, Italy ARTICLEINFO Keywords: Base isolation system Design evolution Seismic isolators Sliding devices Design optimization Tributary area ABSTRACT In this paper, the evolution in design and implementation of base isolation systems (BIS) is traced by a worldwide reconnaissance review of case studies in the last 30 years. Changes occurred from the early pio- neering examples, affected by a fixed-base design approach, to more conscious applications are underlined. The evolution of design goes from original solutions, with low design vibration period (T) and small design dis- placements (δ), to more recent ones, targeting at ever increasing values of these design parameters. The growing awareness, in the scientific community, of high strong motion spectral values (in terms of displacement, velocity and acceleration), that have been recorded in real earthquakes, has determined the need to accomplish greater displacements in design, pushing the structure far away from acceleration plateau range (T > 4 s). The paper makes a proposal of classification of BIS design in three successive generations, each one marked by the occurrence of significant event, characterized by large spectral values of strong motion: first initial generation (1984–1994), second generation (1995–2004), after Kobe earthquake, third generation (2005–2018), after Chi Chi Taiwan and other recent earthquakes. Back analysis of remarkable case studies shows that the use of larger devices has been adopted to accom- modate, at the same time, greater displacements and device stability. In order to guarantee long design periods (very often larger than 4.0 s) with larger devices, a reduction in the number of bearing points is very often accomplished in design either by the use of large transferring system or by the adoption of larger structural grid (conforming to great tributary areas). In this way, a new positive aspect results in BIS design: the structural solution for the superstructure, freed from earthquake forces, ensures more freedom in organizing spaces thanks to the larger spans deriving from the increase of structural grids. The beneficial effects of the strategy of reducing the total number of BIS devices is analytically demonstrated in the paper. 1. Introduction: beginning and evolution of base isolation design Seismic isolation has become an effective design strategy to mitigate seismic hazard, as outlined in the 1990's by Kelly [1], De Luca and Serino [2], Giangreco and De Luca [3]. Through more than thirty years of practice, clear changes have occurred since the earliest applications as Foothill Communities Law and Justice Centre in San Bernardino (1984), described in detail by Tarics et al., in 1984 [4]. Considering the positive feedback in mitigating earthquake effects during strong events, such as Northridge ’94 and Kobe ’95, BIS appears the most effective solution to decouple the upper structure from the ground, as underlined by Nakashima et al. [5] and by Bertero and Bozorgnia [6]. In the evolution of this technology, a turning point is marked by Miyazaki [7]: he has defined the “next generation” of seismic isolation as the one that has to counteract unexpected spectral acceleration and displacement values, resulting from earthquakes such as Chi Chi Taiwan (1999). Later on, more destructive earthquakes have occurred such as Tohoku (2011) or Christchurch (2011, δ max > 90 cm) earthquakes (Carr [8], Wo- therspoon al. [9], Kam and Pampanin [10], Uma et al. [11]). They confirmed high and unexpected spectral values of displacements, ve- locities and accelerations. Changes occurred in the history of seismically isolated building in Japan have been indicated in chapter 3.1 of the Commentary of the fourth edition of Design Recommendations for Seismically Isolated Buildings by Architectural Institute of Japan (2016) [12]. It classifies four periods, tracing the main changes in seismically isolated building de- sign and technology. Here we propose a different classification of BIS design in three successive generations, all referring to an approximately 10 years long period. The passage from a generation to another is marked by earthquakes, read as turning points in the knowledge of earthquake engineering. This proposal of classification, presented in Table 1, gives a modern key to interpret the evolution of BIS. Looking at https://doi.org/10.1016/j.soildyn.2019.105722 Received 24 September 2018; Received in revised form 6 June 2019; Accepted 11 June 2019 ∗ Corresponding author. Department of Structures for Engineering and Architecture (Di. St.), University of Naples “Federico II”, Tecchio Square, Fuorigrotta, Naples, Italy. E-mail addresses: adeluca@unina.it (A. De Luca), lauragiovanna.guidi@unina.it (L.G. Guidi). Soil Dynamics and Earthquake Engineering 125 (2019) 105722 0267-7261/ © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/). T