Seismic retrofit of fabricated steel exterior beam-to-column connections using knee braces

A novel fabricated steel structure exterior beam-to-column connection with knee braces is presented in this study, to improve the seismic performance and internal force distribution of semi-rigid connections. A design methodology for the new knee brace system was developed based on force equilibrium and displacement compatibility principles, recommending a range of shear coefficient values suitable for various joint internal force requirements. Moreover, three external beam-to-column connections were developed, produced, and examined under cyclic reversed loads. These include two knee-braced connection specimens with varying design parameters, as well as one comparison connection specimen. The experimental results indicate that the knee braces significantly enhance the stiffness and strength of the beam-column connections. Compared with the comparison specimen, the average peak bearing capacity of the specimens with square steel tube-knee brace (SST-KB) and angle steel-knee brace (AS-KB) increased by 1.39 times and 1.63 times, respectively, and the initial stiffness increased by 2.37 times and 2.18 times, respectively. Stress from the beam flanges is effectively redistributed to the outside of the knee braces, providing better protection for the joint area. Failure modes such as knee brace fracture, knee brace compressive buckling, and local buckling of the beam flanges are observed. The specimen with AS-KB demonstrates better seismic performance than the SST-KB, showing superior potential for stable energy dissipation. When loaded to 64 mm, the cumulative energy dissipation of the two specimens with SST-KB and AS-KB increased by 34.5 % and 81.9 %, respectively, compared to the comparison specimen. The AS-KB effectively controls the progression of component damage, slows the degradation of overall strength, and facilitates the functional recovery of the connection system. This experimental study provides valuable references for designing and applying fabricated semi-rigid steel frames.