Seismic retrofitting of substandard RC frames using tube-in-tube buckling-restrained braces: shake table evaluation and field insights

This study presents a comprehensive evaluation of an all-steel Tube-in-Tube Buckling-Restrained Brace (TnT-BRB) system designed for the seismic retrofitting of substandard reinforced concrete (RC) frames. A significant portion of existing RC buildings, particularly those constructed prior to the adoption of modern seismic design codes, suffer from inadequate ductility, poor joint detailing, and insufficient lateral load resistance. To address these vulnerabilities, this research integrates large-scale cyclic testing of TnT-BRB components with shake table experiments on one-third scale, single-story RC frame specimens. The experimental program involved two geometrically identical frames: one unretrofitted and one equipped with the TnT BRB system. Subjected to progressively scaled horizontal ground motions, the retrofitted frame exhibited pronounced improvements in lateral stiffness, deformation capacity, and energy dissipation. By contrast, the bare frame sustained brittle joint failures, diagonal shear cracking, and considerable residual drift at moderate excitation levels. When subjected to ground motions scaled to replicate the 2023 Kahramanmara & scedil; earthquake, the retrofitted specimen remained structurally intact and stable, with the brace yielding in a controlled manner as intended. These outcomes were validated through detailed strain, acceleration, and displacement histories, as well as through post-test inspections. Field observations from the 2023 Kahramanmara & scedil; earthquake further reinforce the system's practical feasibility. Collectively, the findings demonstrate the TnT BRB system's capacity to transform vulnerable RC frames into ductile, seismically resilient structures. The study supports the integration of such systems into performance-based retrofitting frameworks, offering a scalable and repairable solution for improving the seismic safety of existing RC buildings in high-risk regions.