Refinement of unified design formula for CFST stub columns under axial compression

Driven by modern design requirements, concrete-filled steel tubular (CFST) columns with novel profiles are increasingly applied. It is of crucial importance to establish a reliable and convenient method for accurately evaluating the axial resistance of CFST columns with different profile types. A mechanism-based unified design method based on Chinese code GB 50936 was proposed by the authors in 2023. The method addressed the limitations of regression-based methods in previous studies and overcame the inconsistency in current codes. This study extended the previous research by proposing a general formulation, with which the specific unified design formulas based on current international codes can be established. The specific formulas based on GB 50936, EC4, and ACI 318 were presented in this study. Additionally, a simplified form for the shape-affected reduction factor in calculating the equivalent confining stress for concrete was proposed. The specific formulas were validated against 258 data from 21 independent studies, with the cubic strength of concrete ranging from 15.1 MPa to 115.0 MPa and the yield strength of steel tube between 194.0 MPa and 555.0 MPa. The unified design formula based on EC4 and GB 50936 accurately predicted the experimental axial resistance. In comparison with the current EC4 code method, the proposed method showed a substantial increase regarding accuracy and decrease in deviations.