Steel exoskeletons for seismic upgrading of RC frame buildings: Analysis of various design criteria

The vast majority of reinforced concrete (RC) structures built in Eu rope after World War II are not compliant with contemporary structural safety standards due to their lack of ductility and poor structural details. Thus, nowa days Government policies promote the reuse of existing buildings, to mitigte the environmental consequences of demolition and reconstruction of such struc tures. As a result, seismic upgrading of RC structures has become a frequent demand for civil engineers. To tackle this challenge, a combination of member level and structure-level techniques can be employed, such as the introduction of external steel bracing systems called "exoskeletons." These exoskeletons can enhance structural capacity for structures with low lateral stiffness and many non-conforming members. However, the effectiveness of steel exoskeletons is influenced by the specific detailing of connections against brace buckling and potential brittle failures in the upgraded RC frame. Therefore, it is crucial to de velop an accurate numerical model to capture the nonlinear response of brace elements and the existing RC structure, especially in infrequent or highly de manding earthquakes. Despite the lack of design criterion for this structural ty pology in the Scientific Literature, this paper presents a comprehensive analysis and modelling of an existing RC frame structure upgraded with exoskeletons, highlighting the influence of design choices and nonlinear modelling on the fi nal seismic performance of the upgraded structural system.