Effect of local thickness enhancement on seismic performance of special-shaped steel beam-to-square CFST column joints with inclined internal diaphragms

Special-shaped steel beam-to-square concrete-filled steel tubular (CFST) column joints with internal diaphragms are increasingly used in complex steel structures, but their local failure control and seismic design remain insufficiently understood. This study investigated the effect of local thickness enhancement on the seismic performance of such joints with inclined internal diaphragms (IIDs), including unequal-depth beam (UDB) joints and staggered beam (SB) joints. Four full-scale specimens were tested under cyclic loading, including two benchmark specimens and two strengthened specimens with increased column wall and diaphragm thicknesses. The benchmark specimens exhibited premature diaphragm fracture and column face tearing, whereas the strengthened specimens showed a clear transition to beam-end flange fracture and local buckling at larger drift ratios. The strengthened joints also developed fuller hysteretic loops, higher flexural resistance, slower strength and stiffness deterioration, improved ductility, and much greater energy dissipation. Panel-zone shear deformation and strain responses further confirmed that local thickness enhancement effectively suppressed early damage concentration in the joint core and promoted a beam-end-dominated inelastic mechanism. Finite element (FE) models were then developed and validated against the test results. Based on the validated models, a parametric study was conducted to quantify the effects of diaphragm thickness, diaphragm inclination angle, diaphragm opening diameter, and column wall thickness. Finally, a simplified method for calculating the beam-end yield moment was proposed and validated, with all predictions falling within ±20% of the corresponding test or FE results and most within ±10%.