Behaviour and design of square concrete-filled double skin tubular columns with inner circular tubes

• Behaviour of concrete-filled double skin tubular (CFDST) columns has been examined.
• Validated 3D FE models for CFDST columns under compression have been developed.
• Based on FE and test results, the provisions of EC4 and the AISC were assessed.
• Existing design rules were found to provide many predictions on the unsafe-side.
• Modified proposals were suggested to provide safe-side predictions for CFDST columns.

Concrete-filled double skin tubular (CFDST) columns consist of two concentric steel tubes, the void between which is filled with concrete. For the current studied configuration, a square tube is used for the outer columns, which enables more straightforward fabrication and installation of beam-to-column joints compared with the case of circular outer columns. To date there have been no significant applications of square CFDST columns with inner circular tubes worldwide, partly due to the lack of design provisions. This paper addresses the axial compressive performance of these columns. Nonlinear finite element (FE) analyses are employed to conduct parametric studies, having first validated the models against available experimental data. The depth-to-thickness (D/t) ratios of the outer tubes were varied such that all cross-section classes were considered. Based on the generated numerical results, a new design model is proposed to provide reliable predictions for the ultimate axial strength of short CFDST columns. The work is then extended to investigate the behaviour of slender CFDST columns. Comparisons of the generated FE ultimate loads for the slender columns are made with strength predictions according to the provisions of Eurocode 4 (EC4) and the AISC 360 Specification for CFST columns, but with allowance made for the inner tube. It is found that the AISC provisions generally give good predictions of the strengths of the CFDST columns, while EC4 gives rather conservative predictions for the columns with slender and very-slender cross-sections.