The Inter-story Isolation System (IIS) is a technique that employs a seismic isolation layer between building stories, effectively dividing the structure into dynamically independent parts. Its main application concerns high-rise buildings but is also gaining popularity as an effective seismic retrofit technique. Indeed, by adding one or more isolated stories on top of the existing structure and employing a careful design of the isolation and dissipation system, it is possible to significantly reduce the seismic forces at the base of the building, enabling the superstructure to behave as a Tuned Mass Damper (TMD).

Among the main issues with this technique is the need to control the drift in the isolation system and the accelerations in the superstructure. In fact, excessive drift in the isolation layer can generate significant P-∆ effects in the substructure or, in some cases, cause hammering phenomena with adjacent buildings. Moreover, if the isolation system is designed as a TMD, the accelerations in the superstructure may be significantly higher compared to those of a classical base-isolated building. This can represent a problem in the presence of sensitive facilities. For these reasons, it is useful to address the problem with a multi-objective approach.

This study first presents a multi-objective optimization of the IIS parameters. Then, a design model based on the optimization results is calibrated. The model allows the calculation of optimal non-dimensional isolation parameters (representing stiffness and damping) based on the structural characteristics and a performance parameter, which allows to define the degree of optimization to be achieved among the various objective functions considered.

Finally, the paper presents the application of the design model and the effectiveness of the IIS technique for retrofit in a simple case study structure. The objective of the intervention is to raise the existing structure by one seismically isolated story in order to increase the serviceable space and improve the seismic performance of the existing building. The effectiveness of the IIS is assessed through time history analyses applied to the structural model. The results validate both the reliability of the proposed design model and the effectiveness of the technique.