A review of the design equations for the axial capacities of circular concrete-filled double-skinned tubular columns

This paper reviews the accuracy of design equations of concrete-filled steel tube (CFST) columns given by GB-50,936, AIJ-2008, AS/NZS 2327, and CSA S16-2019 by expanding the design equations of these codes and incorporating the role of the inner tube. An overall analysis compares the accuracy of different codes in predicting the axial capacity of concrete-filled double-skinned tubular column (CFDST) members. Further, the effect of critical parameters on the axial behaviour of CFDST columns, including geometric and material variables, is systematically analyzed for their different ranges. A detailed evaluation is carried out for each code, and the effect of these parameters on axial strength is compared among the codes, assessing the applicability in different ranges of these parameters. The results showed that CSA S16-2019 produced the most conservative predictions (94.6%) while also giving the highest experimental-to-predicted strength ratio (1.33). The findings observed that CSA S16-2019 yielded maximum conservative results (94.6%), but the average of experimental results to code prediction was the highest, i.e., 1.33. GB-50,936, AIJ-2008, and AS/NZS 2327 provided high percentages of unconservative predictions of the axial capacities of CFDST columns. Finally, the CSA S16-2019 was used to develop a new equation by considering the constraint effect of the inner tube on the axial capacity of CFDST columns. The proposed equation produced accurate and conservative predictions for the axial bearing capacities of CFDST columns with an experimental to predicted strength ratio of 1.19, with a coefficient of variance of 0.15. The innovation of this study lies in extending the applicability of international CFST design codes to CFDST columns, systematically evaluating the role of inner tube parameters, including grade matching, assessing yielding and buckling thickness criteria, and proposing a CSA-based modification that explicitly incorporates the inner tube confinement effect. This approach improves the accuracy and reliability of axial capacity predictions while ensuring consistency with international design practice.