Effect of RC slab on the seismic behavior of CFDST column to steel beam joint with top flange welded and bottom flange bolted connections

Concrete-filled double-skin steel tubular (CFDST) structures demonstrate excellent seismic performance, showing great potential for application in high-rise composite frames. However, the seismic behavior of joints is significantly influenced by the presence of RC slabs and the type of column-to-beam connection. In this study, four CFDST column to steel beam top flange welded and bottom flange bolted joints with RC slabs were fabricated and tested. The results indicate that all joints experienced beam-end failures, including tensile fractures or compressive buckling near the welded interface between the top flange and the upper ring plate, bolt slippage at the lower flange connection, and crushing of the RC slab. Increasing the RC slab thickness improved the moment capacity and stiffness, while greater slab width led to reduced ductility and energy dissipation. The core concrete and inner circular steel tube acted as additional seismic defense layers, providing multiple levels of protection for structural integrity. Additionally, the use of multiple bolts at the lower flange connection helped suppress brittle fracture. Finite element models (FEM) were established and validated against experimental results, showing strong agreement in failure modes, moment capacity, and stiffness. The validated models were used to investigate the effects of slab thickness, slab width, reinforcement strength, concrete strength, and shear stud spacing on joint behavior. Additionally, theoretical models for sagging and hogging moment conditions were proposed and shown to be consistent with both experimental and numerical results, offering design-oriented insights for CFDST joint applications.