Experimental study of substandard RC frames enhanced with tube-in-tube buckling-restrained braces

Abstract

This thesis offers an advanced and comprehensive evaluation of the seismic retrofitting of substandard reinforced concrete (RC) frames using an innovative tube-in-tube buckling-restrained brace (TnT BRB) system. The research addresses a pervasive global problem: the critical seismic vulnerability of existing RC building stock constructed before the enforcement of modern design standards, particularly in regions such as Türkiye. These buildings are typified by insufficient ductility, poor reinforcement detailing, and limited lateral stiffness, leading to their disproportionate collapse and loss during strong earthquakes. The study begins by framing seismic retrofitting as a sustainable alternative to demolition, noting its benefits in reducing material consumption, construction waste, and embodied carbon. The research reviews the current spectrum of retrofitting options, noting that while column jacketing, shear walls, and fiber-reinforced polymers (FRPs) offer some improvements, they suffer from practical limitations including weight, construction complexity, and floor space loss. Buckling-restrained braces (BRBs) have emerged as an optimal solution due to their stable, symmetrical hysteretic behavior and consistent energy dissipation. Yet, traditional BRBs, especially mortar-filled types, present challenges in terms of mass, constructability, and post-earthquake inspection. Recent engineering advances have produced all-steel, tube-in-tube variants, which promise greater ductility, ease of replacement, and fabrication efficiency, but whose performance in deficient RC frames has not been robustly verified prior to this work. This survey sets the stage for the present work, which positions the TnT BRB as a solution engineered to overcome the disadvantages of mass, buckling risk, and replacement difficulty that beset earlier systems. The TnT BRB system is engineered with an internal load-bearing tube and an outer buckling-restrainer tube, separated by a carefully calibrated gap. The configuration includes a ring-shaped stopper and robust end plate and gusset connections, all optimized to enable rapid replacement and ensure the brace's performance in both tension and compression. Laboratory cyclic tests of the TnT BRB itself demonstrated fully symmetrical and ductile behavior, with maximum ductility reaching 10 and cumulative ductility of about 250, while keeping the compression overstrength close to 10%. This indicates minimal unbalanced force transfer to the host frame and highlights the potential for stable energy dissipation without sacrificing stiffness or strength. The study's primary significance lies in its empirical assessment of the TnT BRB for retrofitting substandard RC frames. A sophisticated experimental program was implemented using the 3 m × 3 m two-degree-of-freedom shake table at Allianz Teknik in Istanbul. Systematic shake-table experiments on two one-third-scale, one-story, one-bay frame specimens: a "bare reference frame" and an identical "frame retrofitted with a TnT BRB"were conducted. These frames are deliberately constructed using low-strength concrete of 9 MPa and minimal reinforcement to authentically represent the deficiencies found in the Turkish building stock from the 1980s. The test design employed strict similitude principles, including the use of 6 tons of additional roof mass to correctly simulate inertial effects and gravitational loading as observed in real structures. Fourteen steel blocks ,each 430 kg, are welded together and anchored to the slab to replicate prototype mass and inertia, ensuring the dynamic similitude required for valid seismic scaling. The TnT BRB itself is built as a dual-segment outer steel tube encasing a 42 × 1.5 mm inner tube, separated by a small annular gap and restrained by a mid-span stopper, with end connections detailed for field realism and minimized end rotation. Instrumentation is extensive and precise. High-frequency linear variable differential transformers (LVDTs) and accelerometers, together with twelve strategically placed strain gauges, allow for real-time monitoring of displacement, acceleration, and strain histories at critical locations, particularly the beam–column joints and brace elements. The test protocol begins with system identification using white noise and free vibration, then subjects both frames to a scaled version of the 2023 Kahramanmaraş earthquake record, selected for its contemporary relevance and spectral compatibility with the latest Turkish code requirements. In the first phase, both frames are loaded to 0.35g; subsequently, the TnT BRB-retrofitted frame is tested under the full 1g record to explore its ultimate capacity. In the control frame, the first sign of damage appeared at only 35% of the scaled ground motion, with progressive shear cracking at the beam–column joints, severe lateral displacement (maximum story drift of 5.5%), and rapid degradation of both strength and stiffness. The Park–Ang damage index for the bare frame reached 0.89, firmly in the near-collapse range. These outcomes align closely with the widespread field damage observed during the 2023 Kahramanmaraş earthquake, illustrated by the photographic evidence included in the study. The field data underscore that brittle joint failures, inadequate reinforcement anchorage, the use of substandard concrete, and poor construction practice were the predominant triggers of catastrophic loss. By contrast, the TnT BRB-retrofitted frame exhibited dramatic improvements in seismic performance. Under the same shaking, no significant damage was observed, with lateral displacements held below 2 mm and the structure remaining essentially elastic. When subjected to the full, unscaled ground motion (PGA ≈ 1g), the retrofitted frame showed peak lateral displacement of only 19.86 mm (story drift ≈ 0.02 rad), and the Park–Ang damage index rises modestly to 0.34, well below life safety limits, still without substantial concrete or reinforcement damage. The brace itself displayed minimal ductility demand (axial deformation under 5 mm and ductility below 3), remaining far from its ultimate capacity. The measured base shear increased significantly, reflecting enhanced lateral resistance, while the reduction in story drift and residual displacement was particularly notable. Detailed post-test inspection confirmed that the beam–column joints and critical gusset connections of the retrofitted frame remained virtually undamaged, even after the most severe loading. Analytical modeling using SeismoStruct V2025 provides a rigorous parallel to the laboratory programme. Fiber-based finite element models of all specimens are calibrated using measured material properties and validated against experimental data for natural periods, displacement histories, and top displacement, achieving discrepancies generally under 5%. A third (purely numerical) model employing a conventional X-type BRB enables direct comparison of energy dissipation and deformation patterns between retrofitting strategies. The models confirm that while the X-type BRB imparts greater initial stiffness, it produces less energy dissipation and is prone to earlier pinching and strength degradation. The TnT BRB, by contrast, sustains larger deformation cycles, maintains force symmetry, and provides superior ductility and resilience. The study goes beyond laboratory and analytical validations to integrate broad field reconnaissance from the 2023 Kahramanmaraş earthquake, presenting collapse and heavy damage rates by province, construction type, and period, and using extensive visual documentation to correlate the laboratory findings with actual building failures. The analysis reveals that improvements in Turkish seismic codes and construction practice since 2000 have reduced, but not eliminated, collapse rates, with older RC frames and those lacking joint confinement remaining highly vulnerable. The study concludes by synthesizing experimental and field evidence to recommend the widespread adoption of TnT BRB retrofitting for deficient RC frames. The results show that the TnT BRB can transform a frame from collapse-prone to resilient, with order-of-magnitude reductions in drift and damage even under very severe ground motions. The findings advocate for explicit inclusion of advanced, lightweight, and replaceable bracing systems in seismic retrofit codes, along with rigorous attention to joint detailing, construction quality, and field inspection.