Keywords: anti-seismic systems, seismic isolation, energy dissipation, seismic input, new constructions, retrofit, seismic codes.

Seismic (or base) isolation is a design technique that reduces the force demand on structures by isolating them from the damaging effect of the ground motion. It functions primarily by lengthening the period of the structure. This approach contrasts with conventional design schemes that rely on inelastic action of various structural elements to dissipate earthquake energy. The isolation alternative reduces the force demand on the structure and thereby limits inelastic deformation; it provides a level of performance well beyond the normal code requirements with potential for substantial life-cycle cost reduction. In contrast to conventional technology, seismic isolation offers the possibility of protecting the contents and secondary features (such as cladding and windows) of the building because seismic forces transmitted to the structure are reduced; the safety of occupants and passers-by is thus also enhanced. Isolation is achieved with specially designed bearings placed between the building and its foundations that provide both flexibility and energy absorption capability while supporting the weight of the structure.

When seismic isolation cannot be used (i.e. when the soil is too soft, or the superstructure is too flexible and, especially, in the absence of a sufficiently large structural gap around the latter, compatible with the superstructure displacement), other techniques to enhance the seismic performance of structures are available, in particular energy dissipation. Dampers are inserted in the superstructure in suitable positions as to enable them to dissipate a large amount of the seismic energy which penetrates into the structure. Several kinds of such devices have been developed and used (elastic-plastic, viscous, visco-elastic and friction dampers, so called buckling restrained braces, etc.).

This paper, based on the authors' most recent experience and on the data provided by other members of the Anti-Seismic Systems International Society (ASSISi), gives an overview of the worldwide applications and of the behaviour of seismically isolated buildings during recent earthquakes, including some preliminary data on the effects of the 2011 Christchurch and Tohoku events. Particular attention is paid to the isolated buildings in Italy, in the context of the effects of recent quakes. Based on the italian experience, some important conditions for the correct use of anti-seismic systems are presented and discussed.

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