Seismic upgrading of existing RC structures through steel exoskeletons: comparison between bare and infilled frames

Several existing reinforced concrete (RC) structures are unable to fulfil the current structural safety standards with respect to seismic actions, owing to their inadequate structural detailing. However, recent Government policies tend to favour the reuse of older buildings, thus making the seismic retrofit of RC structures an increasingly common request to structural engineers. In this context, structure-level techniques, such as exterior steel bracing systems (“exoskeletons”), can help enhance the safety of existing structures without causing occupancy disruption. The development of an accurate numerical model, reproducing the nonlinear response of brace elements, existing RC members and infill panels, is fundamental to evaluate properly the capacity of the retrofitted structure, especially when damage to non-structural components needs to be prevented. Hence, the present paper outlines the performance assessment of an archetype existing RC structure (ideally located in a high seismic risk zone) equipped with steel exoskeletons. In particular, two possible retrofit designs are compared, considering two different performance levels either to ensure the structure serviceability or to prevent its collapse. The paper focuses on discussing the role and effects of masonry infill panels on the effectiveness of the steel bracing system. To this end, nonlinear static analyses were carried out, emphasizing the most relevant performance indicators characterizing both the bare frame structure and the infilled frame structure.