Concrete-filled double skin steel tubes with large aspect ratio under axial compression

This study investigates the axial compressive behavior of long circular CFDST columns through experimental testing and finite element (FE) analysis. Three prismatic and three tapered with a uniform length of 3000 mm and aspect ratios ranging from 10 to 12 were tested under concentric axial compression. The load-displacement responses exhibited three distinct stages: elastic, elastoplastic, and post-peak softening, with stiffness degradation and global stability highly sensitive to the aspect ratio. Specimens with lower aspect ratios failed predominantly by local buckling of the outer steel tube accompanied by concrete crushing, whereas those with higher aspect ratios failed by global flexural buckling. Tapered specimens exhibited more nonuniform stress distributions along the height, with local buckling preferentially initiating at the smaller end. A validated FE model was developed to conduct a parametric study on key design variables. Predictions from T/CCES 7–2020 are found to be generally conservative for the slender column regime, and a modified axial capacity formula is proposed, showing good agreement with experimental and FE results, with a mean predicted to measured ratio of 0.98 and a coefficient of variation of 0.041.