State of art in the worldwide evolution of base isolation design

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 pioneering 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 displacements (delta), 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 accommodate, 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.