Finite element modelling of concrete-filled double-skin short compression members with CHS outer and SHS inner tubes

Composite steel-concrete construction utilising the benefits of both materials has extensively been used in structural engineering with the ability to be used in onshore and offshore applications. One of these elements is the concrete-filled double-skin tubes (CFDST) which is a concept that has been developed in recent years. In this column, the steel tubes can support considerable amounts of loads while providing permanent formwork to the concrete. Likewise, the steel tubes add confinement to the concrete whereas the concrete infill delays the local buckling of the steel tubes. This paper provides the behaviour of CFDST short columns under concentric compressive loads. The specimens studied consist of an outer skin, which is a circular hollow section (CHS), and an inner skin, which is a square hollow section (SHS), with the annulus filled with concrete while the inner tube is completely empty. A finite element (FE) analysis is generated in order to analyse the performance of such columns. Therefore, to assure the accuracy of the modelling of these specimens, FE models with concentric axial loads are developed and compared against results from past experiments. In view of this, different stress-strain curves for structural steel and concrete infill are identified, and those that provided the best curve fittings were selected for the parametric study. Accordingly, the best combination of the constitutive models of both the steel (suiting the cold-formed tubes) and the concrete (filled in double-skin tubes) is found based on previous research and considered herein. The study aims at examining the effect of various diameter-to-thickness (Do/to) ratios, width-to-thickness (Bi/ti) ratios and material properties such as nominal compressive strength and nominal yield strength on the fundamental behaviour of CFDST. In the parametric study, high nominal compressive strengths are tested, and the steel tubes are cold-formed from different design yield strengths. Overall, this paper provides important conclusions for current circular CFDST short columns with an inner SHS and an outer CHS.