## 1 Introduction  
In recent years, the need to enhance both seismic performance and energy efficiency in existing buildings has gained increasing recognition, which has been driven by the impact of recent seismic events, climate change and the vulnerabilities of older structures [1]. Reinforced concrete (RC) buildings, particularly those constructed in the post-war period without seismic considerations, constitute a significant portion of the Italian building stock. These buildings share common characteristics that make them highly vulnerable to seismic activity [2, 3]. Typically, RC buildings in Italy are composed of frames oriented in only one direction, with hollow brick infill walls lacking thermal insulation. This configuration is particularly susceptible to seismic forces and energy inefficiencies [4, 5]. Given the prevalence of these structures in urban environments, the need for effective seismic retrofitting solutions is urgent. Traditional methods, such as internal strengthening or the confinement of beam-column joints [6, 7, 8], though effective, can be invasive, time-consuming and disruptive to building occupants. A promising alternative is the use of external retrofit solutions made of various materials, such as timber, steel, concrete or aluminum alloys. For instance, external CLT timber panels were shown to increase the load bearing and displacement capacity and improve the global seismic response of RC buildings [9], while steel buckling restrained braces (BRBs) have been used to increase the strength and capacity for sustaining lateral deformations [10]. Steel exoskeletons offer an innovative solution for seismic retrofitting by providing additional lateral strength and stiffness to existing buildings while allowing for intervening only from the outside of the building, thus minimizing disruptions to the interior activities of occupants. Studies demonstrated the potential of steel exoskeletons to improve the seismic performance of RC buildings while also meeting structural, thermal and architectural requirements [11]. RC structures retrofitted with steel exoskeletons, at times in combination with local interventions, showed significant improvements in the global structural behavior, while also reducing the number of RC elements affected by brittle failure [12; 13]. Cold-formed lightweight steel systems offer an effective structural solution, as they combine the advantages of lightweight construction with strong performance during seismic events. Some studies focused on the design of this type of system by conducting both experimental tests and numerical analyses to assess the effect of earthquakes on lightweight structures [14; 15; 16; 17; 18; 19]. Furthermore, integrated solutions that combine structural and energy retrofitting are nowadays very competitive in the building sector and, therefore, represent an important issue in the research field. Traditional interventions, such as RC shear walls combined with insulation panels, allow for improving the thermal and seismic behaviors of existing RC buildings while also having a low environmental impact [20]. Innovative solutions, such as the so-called seismic energy coats, integrate various technologies for seismic retrofit and energy requalification using insulating panels, while also granting short execution times and low invasiveness [21; 22; 23; 24]. This research focused on a multi-story residential building located in northern Italy and was constructed in the late 1970s. It is an RC structure with the typical configuration with frames only in one direction. It was selected as a case study to evaluate the effectiveness of the Resisto 5.9 Tube system developed by the company Progetto Sisma Srl (Fiorano Modenese, MO, Italy). This recent solution for integrated retrofitting combines seismic reinforcement with energy efficiency improvements. Resisto 5.9 Tube features a cold-formed steel exoskeleton for seismic resistance combined with external thermal insulation panels [25].