Seismic Response of Existing Masonry‐Infilled RC Frame Structures Retrofitted with Steel Exoskeletons

Considering the needs for reducing carbon emissions and except for extreme cases of very degraded existing structures, research on technologies allowing continued use or upgrading of older buildings has become a priority topic. Generally, research on upgrading of reinforced concrete frame structures certainly has a prominent role, because of the large number of buildings of this type. Particularly, seismic upgrading is a common request to structural engineers in earthquake‐prone regions, where structure‐level techniques, such as exterior steel bracing systems (“exoskeletons”), can help enhance safety of existing structures without disrupting occupancy and, possibly, allowing integration with other interventions intended at improving the energy efficiency. Usually, seismic analysis neglects the contribution of masonry infills, although their lateral stiffness and resistance could be detrimental to the efficiency of the new bracing system and there could be interactions in the inelastic range impairing the original bare frame design. To investigate this issue, a numerical analysis was carried out considering an archetype existing frame structure located in a high seismic hazard zone, which was meant to be seismically upgraded by the addition of newly designed steel exoskeletons. More specifically, two possible retrofit designs were compared, considering two different performance levels either to ensure the structure serviceability or to prevent its collapse. Based on results from both static and dynamic non‐linear analyses, this paper discusses the role and effects of masonry infill panels on the effectiveness of the steel bracing system, emphasising the most relevant performance indicators characterizing both the bare frame structure and the infilled frame structure.