To improve the seismic performance of semi-rigid steel frame beam–column joints connected by T-stubs, reinforced T-stubs formed via wedge-shaped and thickening modifications are proposed. Taking the middle column joints in steel frames as the research objects, three types of beam–column joints are designed by adopting ordinary, wedge-shaped, and thickened wedge-shaped T-stubs. To conduct a comparative analysis of the seismic performance of the test specimens, this study imposes low-cycle cyclic loads on the column ends of each specimen along their major-axis and minor-axis in-planes. This loading protocol is adopted to simulate the dynamic responses of the specimens under bidirectional seismic action. Comparing the macroscopic failure phenomena of the specimens, the influence of reinforced T-stubs on the plastic development mode of the joints is analyzed. Based on seismic indicators such as hysteresis characteristics, skeleton curves, stiffness degradation, and energy dissipation capacity, the energy dissipation capacity of the specimens along the major-axis is greater than that along the minor-axis, but their deformation capacity is slightly reduced. The bearing capacity, energy dissipation, and rotational stiffness could be improved by reinforced T-stubs, but the deformation capacity is reduced to varying degrees. The stiffness degradation rate of the specimen adopting wedge-shaped T-stubs shows a more obvious accelerating trend. Through the comparative analysis of the three specimens based on the energy damage index, the results indicate that wedge-shaped T-stubs significantly increase the damage degree of the specimens, but thickened wedge-shaped T-stubs have a relatively small impact on the evolution of joint damage.

In this paper, the quasi-static reciprocating loading test was carried out for the T-stub steel middle column joints with blind-bolt connections. As the thickness of the T-stub steel increased, the damage failure modes were different. The load–displacement curve, cumulative energy dissipation, equivalent viscous damping coefficient, damage index and other characteristics of the joints were analysed. When T-stub steel was used as a connector, its thickness affected the damage development mode. As the thickness of T-stub steel increased, the bearing capacity and plastic displacement were improved, the energy dissipation capacity was significantly increased, and the rotational stiffness retention ability of the joint was improved, but the damage index value was not significantly increased. Finally, the damage index and the rotational stiffness degradation coefficient under each loading level were numerically fitted, and the polynomial and exponential rotational stiffness models were established, taking into account the damage index. These two models took into account the reduction in rotational stiffness caused by damage under each level of load, and the rotational stiffness under each loading level could be obtained from the damage index and the initial rotational stiffness.

Four full-scale exterior beam-to-column connections comprising H-section beams and square hollow-section (SHS) columns were tested under low-cycle reversed loading to investigate two engineering-oriented T-stub section configurations and the effects of the presence or absence of triangular stiffeners. Failure modes, moment–rotation response, stiffness degradation, energy dissipation, and cumulative damage were evaluated, together with nonlinear finite element simulations and a modified Park–Ang damage assessment. All specimens progressed from bolt-hole slip through plastic deformation to localized fracture. In the unstiffened connections, damage concentrated near the T-stub flange-to-web junction; stiffeners redistributed critical demand toward the stiffener welds, adjacent T-stub webs, and SHS column walls. The maximum differences in initial rotational stiffness relative to J1A were 15.69% and 15.07% in the positive and negative loading directions, indicating that the elastic-stage response reflected the combined deformability of the T-stub, blind-bolt assembly, and column wall. The maximum increases in yield moment, peak-resistance moment, and ductility coefficient were 20.59%, 43.71%, and 45.91%, respectively. Complete-history energy dissipation varied non-monotonically across the tested configurations. The finite element model reproduced the global and local responses, while the damage-index results showed overall correspondence with the observed failure progression. The findings emphasize stiffness compatibility and rational distribution of plastic demand rather than maximum local stiffness.

To investigate the effects of end-plate thickness on the seismic performance and damage evolution of K-shaped eccentrically braced frames with end-plate connections (EPEBFs), two scaled specimens with different end-plate thicknesses were tested under low-cycle reversed loading. The influence of end-plate thickness on mechanical behavior and seismic response was evaluated from failure modes, hysteretic response, skeleton curves, ductility, energy dissipation, and stiffness degradation, together with damage assessment using different models. Refined finite element models were then established for parametric analysis. The results show that EPEBFs exhibit a well-defined plastic development path. Damage in the reference specimen was mainly concentrated in the link and its end-plate connection region, thereby protecting the frame columns, beams, and braces. For the two thicknesses tested, the specimen with thicker end plates showed higher lateral resistance, greater ultimate deformation capacity, and better late-stage resistance retention, but lower displacement ductility, indicating enhanced absolute deformation capacity but reduced post-yield deformation reserve. Thicker end plates also altered stress transfer and plastic development in the connection regions, causing damage to extend toward the frame-beam ends. Among the damage models considered, the elastic–plastic energy dissipation ratio model better captured the accumulation of energy dissipation and plastic deformation. The finite element results agreed well with the tests. Within the investigated range, increasing end-plate thickness mainly improved ultimate deformation capacity and late-stage resistance retention, without a clear monotonic effect on lateral resistance. The findings provide a basis for prefabricated design, damage control, and post-earthquake replacement and repair of EPEBFs.