Steel Vertical Extension of Existing Buildings with Isolation System: Diagram-Based Design Approach and Validation Through a Case Study

This paper deals with vertical extensions, realized in structural steelwork and isolated at their base, to meet the fast growth of urban centres located in medium or high seismicity zones. Beyond coping the growing real estate demand, the adoption of a steel structure allows to quickly erect the new addition on the existing building without interrupting the activities hosted therein. At the same time, the inelastic engagement of the existing structure can be reduced during the occurrence of earthquakes. In fact, a mass damper effect can be exerted by the isolated vertical extension on the existing structure thanks to their relative motion. The design effectiveness of this solution for existing buildings with inelastic behaviour has been already investigated by the authors in previous works. A design framework and a five-step chart-based design procedure has been proposed by using IIS design spectra to predict the nonlinear response of the existing building. Then, the simplified procedure has been validated by comparing the predicted inelastic response to numerical results obtained from nonlinear time history analyses of several two-degree-of-freedom models. In this paper, the outcomes of the simplified design procedure are validated by the results of NTHAs carried out on more refined 3D FE models of different design solutions. For this aim, an aggregate of masonry building is selected as case study and erected through several configurations of isolated vertical addition characterized by the same steel structure and different isolation periods. Some design implications are finally discussed.