Development and validation of external steel joints retrofitted by knee braces

External beam-column joints are generally recognized as critical components in ductile steel frame structures. The incorporation of minimally invasive and easily replaceable knee braces offers an effective retrofitting solution. This study investigates the feasibility of enhancing the seismic behavior of external beam-column joints in steel structures through the utilization of knee braces. The mechanical behavior of knee-braced retrofitted joints was analyzed to derive expressions for internal forces in the beam, column, and joint connection components. By utilizing principles of force balance and deformation compatibility between the beam, column, and joint components, a shear coefficient expression was formulated to quantify the knee brace’s impact on the internal forces within the beam and column. Furthermore, methods were developed to calculate the bending and shear capacities of the joint, as well as the axial capacity of the knee brace. The failure modes and sequences of the joint were predicted using a strength hierarchy evaluation method, and the validity of this approach was confirmed through experimental data from both retrofitted and non-retrofitted joints. Finally, the impact of critical design parameters, such as horizontal projection length, installation angle, and section type of the knee brace, on the strength hierarchy and failure modes of external steel frame joints was examined, providing valuable insights for the design of knee-braced retrofitted joints.