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Elliptical concrete-filled stainless steel tubular (CFSST) columns are high-performance composite columns that have the enormous potentiality to be used in composite construction in the near future. However, to date, limited studies on the performance analysis of such columns under axial compression have been performed taking into account the accurate material laws of stainless steel and the confined concrete. This paper presents a mathematical model to investigate the axial performance of elliptical CFSST short columns. Accurate material laws of stainless steel and the confined concrete are incorporated in the numerical modeling to capture their true performance. Independent test results are utilized to validate the accuracy of the mathematical predictions. A parameter study is performed to study the important column parameters on their axial performance. The investigations of the accuracy of the existing design models for CFST columns in designing axially loaded elliptical CFSST columns are performed with the test and numerical predictions.

The accurate prediction of the concrete confinement effects in concrete-filled double-skin steel tubular (CFDST) columns composed of a circular section is vital in stimulating their true structural performance. However, existing concrete confinement models for such columns were developed based on limited studies and may not accurately predict their axial responses under axial loading. In this paper, a numerical model is developed to study the nonlinear performance of axially-loaded short CFDST columns incorporating an accurate concrete confinement model. Herein, a novel expression to calculate the lateral confining pressure on the confined concrete of the CFDST column is proposed considering the influences of a wide range of important column parameters. Furthermore, a reduction factor to accurately predict the post-peak behavior of confined concrete is suggested based on the previous test results. The accuracy of the numerical model incorporating the proposed concrete confinement model is verified against the experimental tests. The accuracy of the existing concrete confinement models is also evaluated and compared with the one proposed in this study. It is demonstrated that the proposed confinement model can yield the axial performance of CFDST columns more accurately. A parameter study is then performed to analyze the impacts of important column parameters on the axial performance of such a column. The applicability of the existing design standards in designing CFDST columns is verified. Lastly, a simple and accurate design equation to quantify the ultimate axial load is proposed.

Concrete-filled double steel tubular (CFDST) columns have been proposed in recent years. This paper presents the performance analysis and a new bearing capacity calculation method of circular CFDST stub columns under axial compression. In order to establish accurate finite element (FE) model, various constitutive models for both the concrete infill and steel tubes are analyzed and compared with experimental results in FE analysis, and the suitable constitutive models for sandwich concrete, core concrete and steel tube are proposed in the FE model developed. Based on the validated numerical model, the variation of confining stress between steel tube and concrete is analyzed in detail. In addition, parametric studies are conducted to investigate the effects of concrete strength, external tube thickness, inner tube thickness, diameter ratio of inner tube to external tube and yield strength of steel tube on the axial compressive performance of circular CFDST stub columns. A new simplified formula based on the confining pressure is proposed to evaluate the ultimate bearing capacity of the circular CFDST stub columns. Based on the comparisons with experimental results and numerical simulation results, it is shown that the proposed formulae for the circular CFDST stub columns appear to be satisfactory.