Ultimate Flexural Strength of Circular Concrete-Filled Steel Tubular Beam-Columns Considering Stress Gradient and Scale Effect

According to the “AIJ Recommendations for Design and Construction of Concrete Filled Steel Tubular Structures”, the ultimate flexural strength of concrete-filled steel tubular (CFST) beam-columns is calculated based on full-plastic strength. However, this approach may overestimate the actual ultimate flexural strength of circular CFST beam-columns, particularly when the diameter-to-thickness ratio is large and/or high-strength materials are used. To address these limitations, a strength evaluation formula that considers the stress gradient and the extreme compression fiber strain of concrete has been proposed for cases beyond the application range of the AIJ Recommendations. Nevertheless, when high-strength materials are employed, the influence of the scale effect of concrete must also be considered, in addition to the stress gradient, to ensure accurate estimation of ultimate strength. In this study, a simplified evaluation method is proposed for calculating the ultimate flexural strength of circular CFST beam-columns, incorporating the stress gradient, extreme compression fiber strain of concrete, and scale effect of concrete. Additionally, the study investigates the appropriate strain at the extreme compression fiber of concrete necessary for accurate strength prediction. The proposed method can accurately evaluate the ultimate flexural strength, regardless of the steel yield strength or the diameter-to-thickness ratio.