Influence of the connections between the existing building and the strengthening exoskeleton

Holistic retrofit solutions for RC buildings carried out from outside by means of structural exoskeletons have been widely investigated in the last years. The effectiveness of such a renovation approach emerges both in the construction time, when addressing the barriers to the renovation such as the inhabitant relocation and the building disruption, and when broadening the time frame of the analyses when shifting from the construction time to a life cycle perspective. The potential of the holistic approach becomes clear in reducing costs, impacts on the inhabitants and impacts on the environment over the building life cycle. Within such a new perspective, new technology options are needed, and their performance has to be investigated. This work addresses the influence on the global response of the connections between the existing building and the seismic retrofit solution carried out from outside. A parametric evaluation of the retrofitted structure is conducted by means of nonlinear time history analyses on a simplified 2 DOFs system considering the existing building, the additional external retrofit, and their mutual connection. The results show that the connections play a fundamental role in the calibration of both stiffness and strength of the retrofit exoskeleton and must be therefore considered in the design phase. Indeed, considering the sole stiffness of the retrofit system leads to a substantial underestimation of the required exoskeleton stiffness compared to the case where the stiffness of both the exoskeleton and the connections are considered. Finally, a case study resembling a RC building typical of the European post-WWII building stock is considered for the validation of the proposed design procedure by means of nonlinear time history analyses.