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This paper reports a study on the Intermediate Isolation System (IIS) applied to existing buildings. This kind of application is particularly suitable when a vertical addition is planned for buildings in seismic zones; in such a case, an isolation system can be placed at the base of the extension to prevent the increase, or, better, to reduce the seismic demand on the existing structure. In previous works, parametric response spectrum analyses have been carried out on lumped mass models by varying the period of the isolation system. As a result, a sort of IIS design spectrum has been derived and used for selecting design solutions for the vertical extension that minimize the overall seismic response. In this paper, the above design indications are assessed in the light of nonlinear time history analyses, accounting for the hysteretic response of the existing structure and the nonlinear behaviour of the isolation system. The IIS configurations are analysed and the results are discussed and compared in terms of peak response. In light of the obtained analysis results, the effectiveness and robustness of IIS applications for vertical extensions are discussed, and design implications are suggested.

Mass damping is a well known principle for the reduction of structural vibrations and applied in tall building design in a variety of configurations. With mass usually small (around 1% of building mass), the properly "tuned" mass damper (TMD) shows great effectiveness in reducing wind vibrations, but minor advantages under earthquake excitations. The above limitation can be surpassed by utilizing relatively large mass TMD. For this purpose, two different solutions are here proposed. In both cases, the idea is to separate the building into two or more parts, thus allowing for a relative motion between them, and activating the mass damping mechanism. In the first solution, the building is subdivided along elevation into an upper and a lower structure, separated by means of an intermediate isolation system (IIS). In the second solution, by revisiting the classical mega-frame typology, the exterior full-height structure provides the global strength and stiffness, and secondary structures, extending between two transfer levels, are physically detached from the main structure at each floor and isolated at transfer level. Simplified lumped-mass models are developed for illustrating the dynamic behaviour of the two solutions and carrying out parametric analyses. Procedures for deriving optimum values of design parameters are also proposed and compared to the parametric study.