## 4 Conclusions  
The research focused on applying a cold-formed exoskeleton, known as Resisto 5.9 Tube, to a reinforced concrete (RC) structure for seismic-energy retrofitting. Resisto 5.9 is made of steel-breaced frames connected to the existing structure with chemical anchors and integrated with insulation panels for the improvement of the thermal performance. The investigation was addressed to the assessment of the structural behavior of a building before and after the installation of the exoskeleton. The aim of the study was to show the effectiveness of the proposed design procedure to obtain a solution that was able to improve the global seismic behavior of a case study representing a common typology of RC buildings erected in Italy in the 1970s. The system was initially designed using an iterative procedure that determined the required thickness of the seismic-resistant bracings based on the specific needs of the structure. The design was set on a target displacement that was identified based on the capacity curve of the existing structure before the first instance of damage (d\({}^{*}\)\({}_{\mathrm{dam}}\) = 0.55 cm). Then, an iterative procedure allowed for determining the required stiffness for the Resisto 5.9 Tube that aimed to obtain a similar ductility of the structure before and after the application of the exoskeleton (\(\mu\) = 2.16). Due to large openings on the ground floor that limited the application of the exoskeleton, the design specified a bracing thickness of 2 mm for the first level and 1 mm for the upper levels. The effectiveness of the design was evaluated through nonlinear static analysis of the structure both before and after the system application. The results demonstrated that the exoskeleton significantly enhanced the seismic performance of the existing RC buildings. From the comparison of the capacity curves of the structure before and after the retrofit, it was observed that the ultimate and yielding displacements increased in both the longitudinal and transverse directions. The shear strength increased by 18.5% and 27.5% in the longitudinal and transverse directions, respectively, while the initial stiffness increased only in the longitudinal direction with an 8.3% increase. The seismic safety index improved from 0.26 to 0.42, which surpassed the minimum required increase of 0.1, as stipulated by the Italian Technical Standard for seismic upgrading. Additionally, the application of the Resisto 5.9 Tube resulted in a reduction of seismic risk, which advanced the seismic risk class from F to E. In conclusion, the Resisto 5.9 allowed for increasing the strength and the overall base shear of the existing building, and thus, improved the global seismic behavior of the structure consistently with other investigations on external exoskeletons found in literature. The ongoing research aims to further assess the system's efficacy, including its performance in the presence of infill walls, which were only represented as a uniform load on the perimeter beams in this study. Despite the challenges posed by large openings, the positive impact of the Resisto 5.9 Tube on the case study was confirmed, which demonstrated its effectiveness in improving seismic resilience even when considering complex geometrical configurations. Additional investigations will be performed to take into consideration not only the structural behavior but also the improvement of the energy performances of the building due to the presence of the integrated retrofit system. Figure 12: Seismic risk class of the RC building: before (**a**) and after (**b**) the retrofit intervention. **Author Contributions:** Conceptualization, A.F. ; methodology, A.F. ; software, E.M.; validation, A.F. ; formal analysis, E.M.; investigation, E.M. and A.F. ; resources, A.F. ; data curation, E.M.; writing--original draft preparation, E.M.; writing--review and editing, A.F. ; visualization, A.F. ; supervision, A.F. ; project administration, A.F. ; funding acquisition, A.F.