Steel Exoskeletons for Seismic Upgrading of RC Frame Buildings: Analysis of Various Design Criteria

The vast majority of reinforced concrete (RC) structures built in Europe after World War II do not meet the current structural safety standards due to their lack of ductility and poor structural detailing. Thus, nowadays Government policies promote the reuse of existing buildings, to mitigate the environmental consequences of demolition and reconstruction of such structures. As a result, seismic upgrading of RC structures has become a frequent task 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”, which can enhance structural capacity in structures characterized by 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 develop an accurate numerical model to capture the nonlinear response of brace elements and the existing RC structure, especially in infrequent or highly demanding earthquakes. With the aim to formulate consistent design criteria for this structural typology, 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 final seismic performance of the upgraded structural system.