Shear performance degradation of square CFST through-diaphragm joints with debonding defects under cyclic loading: Experimental and numerical studies

Interface debonding defects may occur in the joint core region of concrete-filled steel tube (CFST) structures due to construction imperfections and environmental effects, potentially damaging the structure's mechanical performance. This study investigated the shear performance of cruciform square CFST through-diaphragm joints with four-sided debonding defects in the joint core region. Quasi-static tests were conducted on four specimens, including two with preset defect rates of 7.77% and 15.11%. The degradation in failure mode, shear capacity, stiffness, and energy dissipation capacity was analyzed. The results indicated that the debonding defects changed the yielding mode of the steel tube from diagonal bulging to indentation-dominated behavior. By disrupting the concrete diagonal compression strut load-transfer mechanism and weakening the internal restraint of the steel tube, the debonding defects reduced the peak strength by 33.2%–38.1%. In addition, the initial stiffness and energy dissipation capacity of the joints were reduced by up to 21.5% and 28.8%, respectively. A finite element (FE) model was developed and validated using ABAQUS, and 24 models were established for parametric analysis. A defect degradation model considering the relative debonding height and relative debonding gap was developed and incorporated into a defect-adjusted strength formulation. The model was validated against 20 additional FE samples, yielding a mean prediction-to-FE ratio of 0.960 and a standard deviation of 0.043, and showed good agreement with the test results. Within the investigated range, defect-control indicators were provided as preliminary reference values for controlling the predicted defect-induced strength reduction to approximately 5% and 15%, respectively.