Circular concrete-filled double skin tubular short columns with external stainless steel tubes under axial compression

Lean duplex stainless steel (EN 1.4162) has recently gained significant attention for its higher structural performance, similar corrosion resistance and lower cost compared to the austenitic steel. This paper presents the nonlinear finite element (FE) analysis, behaviour and design of circular lean duplex stainless steel-concrete-carbon steel double skin tubular (CFDST) short columns under compression. The finite element (FE) model for CFDST short columns is developed and validated by comparisons with experimental results available in the literature. The FE model is used to investigate the fundamental behaviour of axially loaded CFDST short columns with various parameters. The results show that the ultimate axial strength of CFDST short columns increases significantly with increasing the concrete compressive strength or with decreasing the hollow ratio. However, increasing the ratio of inner-to-outer thickness or increasing the yield strength of the inner carbon steel tube leads to insignificant increase in the ultimate axial strength of the short column. The accuracy of the design models given in ACI code and by Han et al. (2011) and of the modified continuous strength method (CSM) for CFDST short columns is examined against the FE and experimental results. It is demonstrated that the ACI code and Han et al. model provide conservative strength predictions of CFDST short columns, with a relatively high margin of safety. The ultimate axial strengths of CFDST short columns predicted by the CSM are unconservative. A new design model is proposed and shown to be a reliable predictor of the ultimate axial strength of CFDST short columns under axial loading.