###### Abstract  
The process of seismic retrofitting for inadequate RC frames is vital for enhancing structural integrity in areas susceptible to earthquakes. This research investigates a novel tube-in-tube (TnT) buckling restrained brace (BRB) system aimed at improving the seismic performance of these substandard RC frames. By targeting significant weaknesses inherent in older RC constructions, the TnT BRB introduces a lightweight, all-steel configuration that eliminates the need for traditional mortar or concrete infill materials. Experimental shake table testing on two one-third scaled RC frame models was conducted to compare the seismic performance of an unretrofitted control frame and a frame retrofitted with the TnT BRB system. Results indicate significant enhancements in lateral strength, ductility, and energy dissipation capacity in the retrofitted frame, demonstrating stable and symmetrical hysteresis loops and reduced stiffness degradation compared to conventional X-breaced systems. Analytical modeling corroborated these experimental findings, confirming the TnT BRB's superior capability in absorbing seismic energy and preventing premature structural failures. This investigation emphasizes both the practical and financial benefits of integrating the TnT BRB into seismic retrofitting strategies while recommending further research to optimize the system, specifically addressing issues related to local denting, frictional wear, and alignment to bolster its effectiveness in practical applications. structural resilience; shake table testing; tube-in-tube buckling restrained braces 2020  
Predominant structural inadequacies that contribute to these damage manifestations encompass insufficient spacing of transverse reinforcement, substandard stirrup detailing characterized by 90-degree hooks, inadequate concrete cover leading to substantial spalling, improperly executed lap splices, and insufficient confinement in critical beam-column joint areas. These weaknesses markedly enhance the vulnerability to brittle shear failures and soft-story mechanisms during seismic occurrences. Typically, these buildings were designed primarily to support vertical loads, with little or no consideration for lateral forces. Consequently, they demonstrate a tendency toward soft-story failure mechanisms, rapid degradation of stiffness, and diminished ductility when subjected to cyclic loading, resulting in an increased risk of collapse [9]. The choices available for the demolition and reconstruction of these structures are frequently restricted by both practical and financial limitations. This situation underscores the critical necessity of seismic retrofitting as an essential measure to mitigate vulnerabilities associated with earthquakes. The primary aim of retrofitting is to enhance the lateral strength, ductility, and energy dissipation capacity of these buildings. By doing so, retrofitting helps equip them to endure seismic activities of predetermined intensity, ensuring they do not fail or collapse during such events. In the past five years, the engineering sector has focused on developing effective seismic retrofitting techniques for inadequate RC frames. Traditional methods such as column jacketing, shear walls, and the use of fiber-reinforced polymers (FRPs) aim to reduce vulnerabilities by enhancing member strength and deformation capacity [10]. However, these techniques have practical and financial limitations, including construction demands, increased structural weight, and disruption of usable floor space [11; 12]. Despite improvements in cross-sectional capacity, older RC structures still face brittle failure modes due to poor joint detailing or insufficient reinforcement. This underlines the need for supplementary damping systems to enhance energy dissipation and interstory drift distribution [13].