Since ‘80s the use of external additive structures, commonly called exoskeletons, is considered one of the possible alternatives for seismic retrofit of existing r.c. structures with low dissipative capacity. The first Japanese and American codes dealing with structural rehabilitation issues, as well as many applications on the use of steel devices at the international level, are testimony of this trend, especially in high seismic hazard areas. Nowadays, the use of this intervention strategy has become of great actuality, not only because it can be implemented in a safe way without interrupting the building use, but also because it can be effectively adopted, in cases of restructuring operations with lateral addition, for the integrated (formal, energetic and functional) retrofit of the entire construction. In the present work, after a thorough state-of-the art of the main researches and applications on steel exoskeletons, their typological classification into families and the definition of the key project parameters, indispensable to both properly conceive and design such systems, have been performed.
