Beam–column joints play a critical role in the seismic response of reinforced concrete moment-resisting frames (RC-MRFs), particularly in older structures with non-seismic detailing. observations consistently identify corner beam–column joints as the most vulnerable regions, where bidirectional loading demands and torsional effects in plan-irregular buildings often initiate structural failure. In this study, a bidirectional retrofit approach based on the joint-enlargement concept and implemented using a three-dimensional post-tensioned steel scheme is pro posed for non-seismically detailed corner joints in existing RC structures. The effectiveness of the proposed retrofit scheme is experimentally evaluated under unidirectional and non-simultaneous bidirectional cyclic loading protocols. An intensive loading protocol was employed to capture bidirectional interaction effects on the structural response in the principal loading direction, accounting for the damage history induced by transverse cyclic loading. The test specimens were designed to reflect common field conditions observed deficiencies in older non-seismically detailed reinforced concrete buildings, including the absence of transverse reinforcement within the joint core and inadequate bond capacity of beam longitudinal reinforcement. Experimental results demonstrated that the proposed retrofitting scheme provided reliable seismic performance even under severe conditions, including bidirectional loading, absence of column axial loads, and pre-retrofit seismic damage. The retrofit effectively prevented brittle shear and bond-slip failures and shifted plastic hinge formation from the joint to the beam, with the joint remaining essentially elastic. The retrofitted specimens exhibited significant improvements in seismic performance, including a threefold increase in cumulative energy dissipation, a twofold increase in ductility and equivalent hysteretic damping, a 62% increase in initial stiffness, a 45% reduction in damage index, and a 33.5% increase in lateral load capacity, with negligible in-cycle stiffness degradation. Bidirectional loading caused severe degradation in the control specimens, including reductions of 56% in initial stiffness, 40% in effective stiffness, 35% in energy dissipation, 30% in peak load capacity, and 20% in ductility. By contrast, these adverse effects were effectively eliminated in the retrofitted specimens.

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