###### Abstract
It is important to emphasize that TnT BRBs can be 5 to 7 times lighter than their concrete-encased counterparts with equivalent strength. This can be attributed to the absence of concrete encasing, which might substantially increase the overall mass of the structure and the inertia forces acting upon it consequently. Further, the symmetrical hysteretic behavior without significant compression overstrength (e.g., a compression adjustment factor of 1.10 on average) obtained from experimental studies could lead to up to a 35% reduction in the material cost owing to the small, unbalanced brace forces in the inelastic stage [24; 25; 26; 27]. This research aims to analyze an innovative TnT BRB system for inadequate RC frames, focusing on performance characteristics across various drift ratios, including load displacement profiles, energy dissipation efficiency, and failure modes. The study is grounded in the significant 2023 Kahramanmaras (Pazarck) seismic event with a magnitude of 7.7. A systematic assessment of structural performance was conducted through shake table experiments on two one-third scale, single-story, single-bay frame models. The first model served as a reference frame typical of substandard RC constructions, while the second integrated an advanced tube-in-tube system to enhance structural resilience. An extensive account of the experimental methodologies utilized is provided, followed by a detailed examination of the results that juxtaposes the performance indicators of both non-retrofitted and TnT braced reinforced concrete frames. Additionally, the seismic behavior of a traditional X-type brace was subjected to an analytical assessment relative to the new tube-in-tube design. This analysis indicated that the novel configuration facilitates superior energy dissipation and diminishes lateral stiffness, both of which are essential elements for enhancing the overall structural efficacy during seismic occurrences. Prospects and Challenges of All-Steel Tube-in-Tube Buckling Restrained Braces for Seismic-Resistant Structures  
Steel concentrically braced frames (CBFs) utilize brace yielding and buckling mechanisms to dissipate energy during seismic events; however, conventional braces are prone to early buckling and exhibit insufficient ductility, resulting in premature failure. To address these limitations, buckling controlled braces (BCBs) have been introduced. BCBs incorporate an external restraining mechanism designed to inhibit both global and local buckling of the brace, thus preserving stable hysteretic performance even under significant cyclic deformation. This innovation bears a resemblance to BRBs but typically employs an all-steel design, foregoing concrete infill, which enhances the practicality and feasibility of BCBs for both new constructions and retrofitting projects. Among the prominent configurations of BCBs are the TnT BRB, consisting of an internal steel tube brace surrounded by an outer hollow steel section with a minimal gap, and the channel-encased BCB, which encapsulates an existing brace, usually a hollow structural section (HSS) tube, with two steel channels that are stitched together. To enhance brevity, these systems will be designated as TnT BRBs. Initial findings indicate that these designs markedly increase the ductility of the braces, bolster their capacity for energy dissipation, and successfully delay the onset of fracture. A schematic representation of a tube-in-tube buckling restrained brace configuration is given in Figure 2.  
Note that the lateral support provided by the encasing is expected to prevent global buckling entirely. Although preventing the formation of local plastic deformations entirely is not possible, it is intended that the outer tube delay the local buckling until reaching an expected ductility level.