Steel bracing configurations for seismic retrofitting of a reinforced concrete frame

Steel bracings are among the most common and potentially effective technical solutions for enhancing the seismic safety of existing reinforced concrete structures. Nevertheless, several issues dealing with both analysis and design of steel bracings for retrofitting reinforced conrete frames remain unresolved. This paper presents a study inspired by a practical retrofitting project of a reinforced concrete frame with concentric X-bracings. Specifically, it proposes a discussion about the consequences and implications of assuming three different configurations for the steel diagonals, chosen from among the architecturally compatible ones. A detailed description of the as-built reinforced concrete frame is first presented, along with key information about the procedure followed to design the bracing diagonals. Then, after having outlined the numerical models employed to simulate the mechanical response of both concrete and steel members, the results of a series of non-linear time-history analyses are reported and discussed with the aim of highlighting the key advantages and disadvantages of adopting the three alternative bracing configurations. In this regard, particularly critical aspects, such as the actions transferred to foundations, the stresses achieved in beam-to-columns joints and the distribution of forces throughout the existing reinforced concrete members, are considered to compare the performance of the three alternative bracing patterns.