Among the various seismic retrofitting techniques, steel exoskeletons are distinguished by their non-invasive nature. However, only a few consolidated methodologies have been proposed for their design. The approach of several standard codes is based on the classification of elements according to their relative stiffness. In this way, a ratio between the stiffness of the exoskeletons and that of the building is taken as the main design parameter. In this study, a performancebased design approach was employed, with the inter-story drift of the building as the performance target. A sensitivity analysis was conducted to assess the impact of different inter-story drift thresholds on the structural behavior of the buildingexoskeleton system. For each threshold, an optimization process was conducted to identify the optimal number of exoskeletons, their placement around the building, and the dimensions of their elements. Finally, the stiffness ratios were determined for each optimal configuration and were compared to the threshold provided by the regulations. This comparison yielded interesting insights into the differences in the approaches.

The growing need for structural, energetic and architectonic refurbishment of the existing structures led to the emergence of alternative retrofitting techniques. Among them, steel exoskeletons stand out for their non-invasive and time-efficient nature, especially in cases where the relocation of the building’s activities is not allowed. In this research, the optimal exoskeleton configuration, in terms of number of exoskeletons and their position around the building, along with the sizing of their constituent elements, has been obtained through an optimization process. The optimization tool is based on a Genetic Algorithm with the aim of weight minimization, including as constraints a maximum allowable inter-storey drift for preserving the elastic behavior of the existing building, and the structural safety of the exoskeleton members. Two exoskeleton typologies were selected as case studies, employed for the retrofitting of an existing RC moment-resisting frame building. Furthermore, Life Cycle Assessment analyses were conducted on the optimal configurations considering two materials: steels with high and low percentages of recycled materials. The results yielded interesting insights into the structural performance of both exoskeleton typologies, as well as into the environmental characteristics of both studied materials.