The influence of the inner steel tube on the compression behaviour of the concrete filled double skin steel tube (CFDST) columns

Concrete Filled Double Skin Steel Tube (CFDST) columns which comprise two concentric steel tubes and shell concrete between them is a favourable member of composite columns family. The CFDST columns have the potential to offer equal or superior characteristics compared to the classical Concrete Filled Steel Tube (CFST) counterparts. Although CFDST columns have been the focus of experimental and numerical studies, there is limited knowledge on the contribution of the inner steel tube to the compression performance. Since the inner steel tube is the key member which motivates the deployment of the CFDST columns in the field, it is imperative to characterize its contribution. This paper conveys the findings of an experimental study which was conducted to explore the effect of the inner steel tube on the mechanical response of CFDST columns under axial compression. Two different D/t ratios and concrete classes (normal strength and high strength) were utilized and in total sixteen specimens were tested to failure. To be able to analysis the impact of the parameters, failure modes, compression loading versus end shortening curves and the concrete-steel contribution ratios were detailed. The results show that the CFDST columns could attain greater performance over CFST columns when a proper configuration of the inner steel tube is used. Furthermore, the inner tube works consistently with the shell concrete and plays an important role in mitigating failure mode of CFDST even with high strength concrete (HSC). Finally, the prediction formulations were employed and examined against the test results to specify the convenient method for design processes.