## 2 RC-Framed Skin for Retrofit Intervention on Existing Buildings  
In this section, the main structural characteristics of the proposed RC-framed skin technology are briefly presented, together with a description of the in situ installation phases, which allow avoiding the interruption of use of the building and significantly reducing the invasiveness of the intervention.  
Figure 1 shows the detail of the proposed retrofitting technology, which consists of a RC frame rigidly connected to the external facade of the existing building by means of anchor rods placed at every floor level. The anchor rods must be designed to guarantee the respect of the hierarchy of strength principle, i.e., the connections must be sized to withstand with adequate over-strength the shear actions, so behaving elastically for the target seismic action.  
The RC-framed skin is casted on site by using prefabricated expanded polystyrene (EPS) modules as formwork, which ensures also the thermal enhancement of the building.  
The proposed system is characterized by a modular geometry with variable dimensions for both beams and columns, variable RC reinforcement, and variable column interspace. All these parameters can be designed to achieve a specific target of seismic safety level. Actually, the geometric parameters of EPS modules, strictly related to both the cross sections and interspace of frame elements, can be optimized depending on the characteristics of the existing building and of the selected seismic action.  
In detail, columns can be designed with a square cross-section in the range 150 to 300 mm, while the transversal beams with a rectangular cross-section with base equal to the side of the column plus 50 mm and height variable in the range 300 to 500 mm. Typical column interspace is 1200 mm, but some adjustments are usually required to fit the facade opening geometry. The beams are placed in contact and connected with the existing structure in order to maximize the effectiveness of the connection system.  
Concerning the transversal reinforcement layout of the frame elements, continuous spirals are designed for the columns and rectangular stirrups for the beams. It is worth noting that, in order to ensure adequate confinement of the beam-column joint, the continuous spiral reinforcement of the column is provided across the joint itself.  
The geometry of the system, and particularly of the EPS modules, is specifically shaped to realize an additional thick plaster on the external surface of the building. The external finishing plaster is reinforced with a steel mesh pre-assembled on the EPS modules and connected to the reinforcement of RC-framed skin by means of anchor rods.  
The proposed retrofitting technology aims to limit invasiveness and interruption of use of the building; therefore, one of the main advantages of this technology is the possibility of working from the outside without internal intervention, by applying the RC-framed skin to the external facades.  
The construction of the retrofitting system is of the platform type (i.e., floor by floor starting from the bottom) and consists of three main phases: (a) preparation of the facade and construction of the foundation beam; (b) installation of the prefabricated EPS modules and casting of the reinforcement frame; (c) realization of the external plaster and finishing of the facade, see [13] for the details.  
It is worth noting that the existing foundation system may be inadequate for the new actions transmitted by the RC-framed skin, then, a simple connection to the existing foundation beams can be insufficient. In those cases, a retrofit of the foundation system may be needed. Nevertheless, the proposed technology provides for widespread interventions along the entire perimeter of the building, so generally leading to lower actions to the foundation if compared to the case of more localized retrofitted intervention, such as the addition of new shear walls.