Experimental Investigation into the Failure Behaviour of Square Stainless-Steel CFDST Short Columns

This paper presents an experimental investigation into the axial compressive behaviour of square stainless steel concrete-filled double-skin tubular (SS-CFDST) short columns, focusing on the effects of member size, tube slenderness, and concrete strength. A total of 21 specimens with outer widths from 150 to 300 mm were tested under concentric axial compression, covering systematic variations in tube slenderness, hollow ratio, and concrete strength. The results show that member size strongly influences both strength and ductility: larger specimens exhibited lower normalised axial resistance and reduced deformation capacity than geometrically similar small-scale columns, despite meeting width-to-thickness limits. This is attributed to elastic local buckling of the outer stainless-steel tube, which limits confinement and promotes stability-controlled failure. Outer-tube slenderness was the most influential geometric parameter, while high-strength concrete increased axial resistance but reduced ductility due to limited dilation and brittle behaviour. Existing design provisions do not adequately capture the combined effects of confinement, local buckling, and size-dependent stability, leading to unconservative predictions for large members. The experimental data and mechanistic insights provide a benchmark for developing numerical models and improved design approaches that account for size effects and stability-controlled behaviour in square SS-CFDST columns.