# Intermediate Isolation System with Nonlinear Lower Structure and Isolation System
## 7 Conclusions  
In this paper, time history analyses were carried out on 3D FE models of an existing masonry building, selected as a case study, and on some design configurations of the same building, consisting in vertical extensions through IIS. The aim was to assess the validity and accuracy of the prediction derived from response spectrum analyses on lumped-mass models for preliminary design purpose. Conventional extension has been also considered for comparison.  
Based on the results of the linear and nonlinear THA, the following conclusions can be derived:  
* The dynamic behaviour of the 3D FE model of the extended building can be reasonably estimated through the simplified 3 DOF IIS model;
* Based on the results of linear THA, the peak displacement demand in the lower structure is reduced for low-medium values of \(T_{ISO}\) (zone 2, mass damping effect);
* Considering the nonlinear behaviour either for the masonry structure only, or for the isolation system only, a reduced response in the existing building is still obtained for low-medium values of \(T_{ISO}\) (zone 2, mass damping effect);
* Considering the hysteretic behaviours of both the masonry structure and the isolation system, some reductions in the displacement response are obtained, but only for medium-large values of \(T_{ISO}\) (transition zone and beyond);
* When a building should be vertically expanded, the adoption of an isolation interface between the existing and the new structures usually enhances the seismic performance of the existing structural portion with respect to the case of a vertical extension conventionally realised (without isolation system).
* However, fully nonlinear analyses, accounting for the hysteretic behaviour of both masonry structure and isolation system, have highlighted that design solutions that maximize the mass damping effect can lead to an amplification, rather than a reduction in the seismic response.
* The results have also highlighted a remarkable influence of the modal damping values adopted in nonlinear THA on the structural response. When the lower structure exhibits significant inelastic deformations, the design of isolated vertical extension in the mass damping zone seems not advisable.  
* It is worth underlying that all the above results have been obtained with reference to a single case study. In order to confirm the seismic performance that can be achieved through IIS, other case studies should be considered for further analyses and deeper insights.  
Conceptualization, F.E., D.F. and E.M.; Methodology, F.E., M.A., D.F. and E.M.; Software, F.E., M.A. and D.F.; Validation, F.E., D.F. and E.M.; Formal analysis, F.E., M.A. and D.F.; Investigation, F.E., M.A., D.F. and E.M.; Resources, F.E. and E.M.; Data curation, F.E.; Writing--original draft, F.E., D.F. and E.M.; Writing--review & editing, F.E., D.F. and E.M.; Visualization, F.E. All authors have read and agreed to the published version of the manuscript.  
This research was partially supported by the projects DPC--ReLUIS 2022-2024, WP15--Contributi normativi relativi a Isolamento e Dissipazione: CUP G53C22000200001 \"Progetto ReLUIS 2022-2024.  
Not applicable.  
The authors declare no conflict of interest.