Experimental Study on the Seismic Performance of Exterior RC Beam-Column Joints with U-Shaped Steel-Strengthened Beams

U-shaped steel strengthening can substantially enhance the mechanical performance of reinforced concrete (RC) beams while having minimal impact on their cross-sectional dimensions and self-weight. However, the influence of strengthening RC beams with U-shaped steel casing on the overall seismic performance of frame joints remains insufficiently investigated through experimental studies. To address this gap, four exterior beam-column joint specimens were tested under quasi-static cyclic loading without column axial load. The specimens comprised one reference specimen and three specimens with U-shaped steel-strengthened RC beams, with the U-shaped steel plate thickness as the primary variable. The failure modes, hysteretic behavior, load-carrying capacity, deformation response, stiffness degradation and energy dissipation performance were evaluated. The results showed that all specimens exhibited plastic-hinge-dominated failures at the beam ends, while U-shaped steel strengthening provided only limited improvement in the overall lateral load-carrying capacity. Compared with the reference specimen, the strengthened specimens exhibited fuller hysteretic loops and improved energy dissipation performance at large drift ratios. At a drift ratio of 6%, the energy dissipated per hysteretic loop by specimens SJ1, SJ2, and SJ3 increased by 6.5%, 14.3%, and 8.1%, respectively, compared with the reference specimen NJ1. The corresponding equivalent viscous damping coefficients increased by 15.4%, 16.5%, and 8.9%, respectively. The average test-termination-based displacement ratios of the strengthened specimens exceeded 3.2, indicating considerable deformation capacity up to the prescribed termination drift ratio of 8%.