###### Abstract
Among the array of retrofitting methodologies available, buckling restrained braces (BRBs) are distinguished from other supplemental damping devices by their consistent hysteretic behavior and established effectiveness in energy dissipation. Initially engineered for use in steel framing, BRBs comprise a steel core that is intentionally designed to yield under axial tension and compression forces. This core is encased within a restraining mechanism, typically a tube filled with either steel or mortar, which serves to inhibit global buckling [14]. The design of the steel core allows it to alternate between tension and  
Figure 1: Typical damage to substandard RC frames observed after the 6 February 2023 Kahramanmaras earthquakes (all photographs by the authors, 2023). compression without experiencing premature buckling, resulting in nearly symmetrical load-displacement loops. This characteristic enables BRBs to provide enhanced energy absorption capabilities compared to conventional concentric braces [15]. The efficacy of BRBs in new steel construction has led to increased research interest in their adaptation for use in substandard RC frames [16]. By acting as a specialized damping component, a properly engineered BRB can alleviate inelastic demands placed on existing RC elements, thereby mitigating shear and flexural stresses in critical structural components such as columns, beams, and joints [17]. In light of the observable advantages that conventional BRBs offer, it is essential to recognize the practical challenges that both researchers and engineers encounter when attempting to integrate these systems into older RC frames. Primarily, the effectiveness of BRBs hinges on the presence of reliable anchorages and well-designed gusset plates, which are crucial for effectively transmitting substantial axial forces to beam-column junctions. However, numerous older frames exhibit deficiencies in transverse reinforcement, rendering them unsuitable for such force transfer [18]. Additionally, the inclusion of mortar or concrete infill within the restraining tube can create complexities in the fabrication process, lead to an increase in the overall weight of the system, and introduce difficulties related to future inspection and maintenance activities [19]. In light of recent developments, an innovative tube-in-tube (TnT) BRB concept has gained prominence, presenting itself as a potentially lighter and more constructible alternative to conventional infill-based designs [20; 21]. This approach diverges from traditional methods that utilize mortar or concrete for buckling restraint, instead employing two concentric steel tubes. The inner tube serves as the primary yielding core, while the outer tube offers continuous lateral support. A significant key benefit linked to this configuration can be identified as its stable hysteretic performance. Recent experimental research reveals that TnT braces can attain ductility levels that are on par with, or potentially exceed, those found in conventional BRBs [22]. The tube-in-tube design still represents a relatively innovative approach for enhancing the performance of substandard RC frames, evidenced by the limited number of extensive experimental studies documented in existing literature. Important unresolved issues pertain to the local denting phenomenon observed at the mid-span of the core tube, potential friction or unintended bearing occurrences between the interconnected tubes, and the dependability of connection systems within RC elements that are susceptible to cracking or cover spalling. [PERSON] and [PERSON] [23] emphasized that even minor misalignments in tube geometry or significant ovalization resulting from compression can adversely affect the hysteretic behavior of a brace. Therefore, TnT BRBs offer advantages over conventional concrete-encased BRBs in terms of cost-efficiency due to their less labor-intensive fabrication process and relatively small structural weight.