# Retrofit of Existing Reinforced Concrete (RC) Buildings: Steel vs. RC Exoskeletons
## 1 Introduction  
Continuous experimentation and research in the field of architecture and engineering have led to important results in terms of requirements and technical skills. Today, many of the buildings constructed since the Second World War are in a critical situation, as they are not sustainable [1; 2; 3]; they do not meet various contemporary standards, both in terms of living comfort and energy; and above all, from a structural point of view, these buildings often do not meet modern safety requirements with respect to vertical load [4], and are not designed to accommodate seismic action. Most of these buildings have reached the end of their useful life (50 years) according to Italian standard requirements [5]. It is thus evident that a dramatic situation emerges, in which entire housing sectors are highly obsolete and vulnerable, and very often located within highly degraded urban contexts. There is a clear need for action to address the performance and structural deficiencies of post-World War II buildings and to reduce the current unsustainable energy waste through requalification operations [6]. The retrofit of such buildings is a solution that aims to adapt past constructions to contemporary levels and standards, while at the same time ensuring that these interventions produce important results in the structural, energy and architectural fields, without neglecting the urban context in which they are located [7]. With the spread of the theme of sustainable redevelopment combined with that of energy saving [8; 9; 10], research has been directed towards high-performance and effective solutions. These no longer involve the classic demolition and reconstruction of obsoleteand vulnerable buildings, but promote their valorisation through reversible and more economically advantageous interventions. This produces a series of advantages in economic terms, but also from the point of view of finding useful raw materials for new construction and the disposal of rubble resulting from demolition [11; 12; 13]. Today, there are a myriad of interventions available for the retrofit of such buildings [14; 15; 16; 17]. However, the number and type of these possible interventions are drastically reduced when the most important requirement is to avoid interruption to the operation of buildings under intervention. Reference is thus made to an innovative solution which first appeared on Russian soil in the late 1800s and early 1900s, when [PERSON] developed a self-supporting structural system [18]. This type of intervention, well known as an \"exoskeleton\", consists of an additive strategy in which the existing building is wrapped in a steel or reinforced concrete (RC) framework that is rigidly connected to the primary structure, thus effectively modifying the dynamic response of the system and remedying the vulnerabilities typical of this type of building [19; 20; 21; 22]. The present work is part of an extended research project aimed at developing a holistic retrofit intervention strategy for existing RC buildings, so that--through a multidisciplinary approach--the problem can be addressed in unified way to achieve adequate levels of structural safety, liveability and functionality. In more detail, this research project aims to study, in addition to the effects of exoskeleton systems, the effects of their connection systems, as well as the possible improvements (from both energetic and seismic points of view) to the infill walls. To improve the energy aspects of these types of buildings, and at the same time the seismic-related aspects, the use of natural materials such as cork and basalt fibre is envisaged. There are, in fact, plans to apply natural thermo-plaster with fibre composite matrices along the perimeter walls of these buildings to achieve a beneficial dual effect for energy and seismic issues. From the energy point of view, such an intervention would make it possible to reduce consumption and even achieve performance levels close to nZEB (nearly zero-energy building).