Numerical Investigation on the Axial Behavior and Confinement Effect of Concrete-Filled Steel Tube Stub Columns

Concrete-filled steel tubes (CFST) exhibit superior axial performance compared with hollow steel tubes due to the confinement interaction between steel and concrete. Understanding how geometric and material parameters influence this enhancement is essential for rational composite design. In this study, a three-dimensional finite element model is developed in ABAQUS to investigate the monotonic axial behavior of steel tube stub columns with and without concrete infill. The model incorporates geometric imperfections, nonlinear constitutive laws, and a contact-based steel–concrete interface, and is validated against published experimental results. A parametric study is then conducted by varying the diameter-to-thickness ratio, steel yield strength, and concrete infill condition. The axial load–displacement responses, stress evolution, and damage development are examined, and two quantitative indices are introduced to evaluate performance: the load enhancement factor associated with concrete confinement and the deformation capacity ratio. The results show that concrete infill significantly improves axial capacity and deformation stability, while the effectiveness of confinement decreases with increasing section slenderness. Higher steel strength increases peak load but alters the post-peak response depending on tube thickness. The findings provide numerical evidence for optimizing tube geometry and material combinations in CFST stub columns under axial compression.