Behaviour and design of axially-loaded square concrete-filled stainless steel double-skin tubular short columns with circular inner tubes

During the last 20 years, the construction of concrete-filled double-skin tubular (CFDST) members for usage in harsh corrosive environments has recently involved the use of stainless steel. Nevertheless, relatively little research has been done on how the square hollow section (SHS) outer and circular hollow section (CHS) inner CFDST (SC-CFDST) columns behave under concentric compressive loads. In this paper, research is being done on the behaviour and resistance of these SC-CFDST short columns with stainless steel outer tubes and carbon steel inner tube. A numerical model was firstly built using ABAQUS software, and its validity was confirmed through comparisons with available experimental test results in the literature. Through parametric analyses, the influence of key parameters on the behaviour of SC-CFDST short columns has been thoroughly examined. Then, a confinement stress analysis has been carried out to explore such mechanism, hence, the key parameters affecting the behaviour of these members have been thoroughly discussed. The results showed that the sandwiched concrete strength and outer tube slenderness ratio have the greatest influence on the ultimate resistance and ductility of these composite columns. The resistances of SC-CFDST short columns were then used to evaluate the applicability of the design standards provided by the North American (AISC 360 and ACI 318) and European (EC4) specifications and other design procedures from the literature. The results showed that EC4 and existing theoretical models, provide slightly unconservative predictions on average with unacceptable reliability, while both of the North American specifications yield conservative predictions with relatively high reliability indices. Accordingly, the design models of the North American specifications can be safely used to design the SC-CFDST short columns with inner circular carbon steel tubes and outer square stainless steel tubes. However, to provide a balance between cost and safety, a new modification has been made to the EC4 design formula to accurately predict the axial design strength of SC-CFDST short columns.