Additional structural systems coupled to existing buildings for seismic retrofit

Limiting the interference of the intervention on existing buildings can be an important issue in seismic retrofit. Indeed, local interventions on critical structural elements is often incompatible with the use of the building. In many cases, the interruption of activities, such as strategic or public interest ones, is not allowed. In other cases, there could be the need of preserving the architectural asset avoiding interference of the interventions with the aesthetics. Further limitations may arise from urban planning or administrative aspects not allowing modifications to the building envelope. In seismic retrofit, the use of additional structural systems coupled to existing buildings has therefore spread in recent years. These systems are typically seismic-resistant steel frames equipped with an independent foundation and capable of taking most of the seismic action upon themselves. The present paper deals with these systems and in particular with the use of steel exoskeletons and endoskeletons suitably coupled to the existing seismic-resistant elements. The objective of the technic is to transfer most of the seismic action to the additional structure avoiding to intervene directly on the existing structure. The latter will be required to bear only a reduced rate of the design seismic action limited to the pre-existing capacity level. In order to obtain this result, the additional systems must be designed as characterized by high stiffness or capable to subtract a large mass rates from the existing structure. The coupling system is therefore fundamental, it is calibrated according to: the structure on which to intervene (r/c, masonry); the type of interference to be resolved; the type of additional system used (endoskeleton, exoskeleton).