Practical formula for predicting axial strength of circular-CFST columns considering size effect

A practical and accurate unified formula for predicting the axial compressive strength of short-long circular CFST columns is proposed based on the regression analysis of 368 available experimental data. The formula is applicable to large, thin-walled, and high-strength circular CFST columns, which was established through two processes: (1) Based on 273 experimental data of short specimens, the axial strength of short columns was determined numerically and normalized with respect to the yielding load of the steel tube, thus showing a superior exponential correlation with the modified confinement index ξ; and (2) The numerical model was improved by considering the effect of length-to-diameter (L/D) ratio for long columns. According to the parametric analysis, the size effect is found to have a certain influence on the axial strength of large CSFT short columns and the primary affecting factors to the size effect are the compressive strength of in-filled concrete (f_c) and the outer diameter to thickness (D/t) ratio of the steel tubes. Therefore, the size effect is better to be considered in a strength model, as proposed. The strength reduction caused by the size effect is characterized by ξ which involves the parameters of f_c, D, and t. In comparison with the measured strengths from the test specimens, the proposed prediction model provides a more accurate and stable estimate on column design strength than the existing design codes.