Flexural behaviour and design of circular concrete-filled tube beams: Effective stress method

Abstract

The current AISC Specification lacks clear guidance on the flexural design of circular concrete-filled tube (CCFT) beams, while updates to material strength restrictions and the P-M interaction design curve apply exclusively to rectangular shapes. In response, this paper builds on a series of the author’s prior research to focus on the unique characteristics of CCFT beams, covering a wide range of material strength and geometric properties. A mechanical-based model, coupled with 169 experimental test outcomes, is utilized to develop an effective plastic stress distribution model for predicting the flexural capacity of conventional and high-strength CCFT beams with high precision. The proposed model accounts for the interaction between steel and concrete, including concrete confinement, steel local buckling, and the bi-axial state of stresses on steel. The results demonstrate that steel local buckling in circular filled shapes is unlikely, while the author's expression, as established in a prior study, for concrete confinement under pure compression applies to beams. Statistical validation indicates that the proposed effective stress model achieves the highest predictive efficiency compared to current design codes, with a coefficient of variation (CoV) of 12.6 % and a coefficient of determination (R²) of 0.99. This study offers an easy-to-use design expression for structural engineers by eliminating the current need for section classifications and represents a crucial step for researchers investigating the axial-flexure interaction behavior of conventional and high-strength CCFT members.