Zhuojin Pipe Fitting Co., LtdStrength Performance of T-Shaped CFST Columns Under Biaxial Bending
Literature Overview
This paper by Lei Min, Shen Zuyuan, Li Yuanqi, and Luo Jinhui, published in Journal of Tongji University (Natural Science) in 2016 (Vol. 44, No. 3, pp. 348-354), investigates the strength behavior of T-shaped concrete-filled steel tube (CFST) short columns under uniaxial and biaxial eccentric compression. The authors employ a fiber model program using the equivalent uniaxial constitutive relationship for the core concrete of T-shaped CFST columns to analyze the influence of various parameters on section strength and propose simplified calculation methods for design.
Core Technical Framework
T-shaped CFST columns combine two rectangular or square steel tubes connected in a T-configuration, offering improved bending stiffness about one axis while maintaining compact cross-sectional dimensions. The study focuses on section strength under combined axial force and bending moments, which is critical for column design under realistic loading conditions.
The fiber model approach divides the cross-section into numerous small elements (fibers), each assigned appropriate material constitutive laws. For the steel tubes, a bilinear or multilinear elastic-plastic model is used, while for the core concrete, the equivalent uniaxial constitutive relationship accounts for the confining effect of the steel tube walls.
Key Parameters Analyzed
| Parameter | Symbol | Influence on Interaction Curve |
|---|---|---|
| Steel yield strength | f_y | Moderate influence on curve shape |
| Concrete compressive strength | f_c | Moderate influence on curve shape |
| Tube wall slenderness ratio | t/D | Significant influence on curve shape |
| Section limb slenderness ratio | b/t | Significant influence on biaxial interaction curve |
| Loading angle | θ | Significant influence on normalized N-M curve shape |
| Axial compression ratio | n_0 | Significant influence on biaxial interaction curve shape |
Main Findings
The parametric study reveals several important trends:
- Concrete working capacity coefficient and loading angle have a significant influence on the shape of the normalized axial force-moment interaction curves. The concrete working capacity coefficient reflects the efficiency with which the concrete contributes to section strength, and the loading angle determines the orientation of the bending moment relative to the T-section's principal axes.
- Section limb slenderness ratio and axial compression ratio significantly affect the shape of the normalized biaxial moment interaction curves. Higher limb slenderness ratios reduce the effective width of the concrete core and decrease the confinement effectiveness, while higher axial compression ratios shift the interaction curves toward the axial compression axis.
- Simplified calculation methods are proposed based on full plastic section assumptions and regression analysis of extensive numerical results. These simplified methods cover pure bending, uniaxial eccentric compression, and biaxial eccentric compression cases, and their results agree well with the fiber model analysis.
Engineering Practice Implications
For structural engineers designing T-shaped CFST columns, this study provides the following practical guidance:
- Design orientation: The loading angle relative to the T-section's principal axes significantly affects section strength. Engineers should orient the T-section such that the stem (the longer limb) aligns with the primary bending direction to maximize bending resistance.
- Slenderness control: The section limb slenderness ratio (b/t) should be controlled to prevent local buckling of the steel tube walls. For typical structural applications, the limb slenderness ratio should not exceed the limits specified in relevant codes such as GB 50936 or EN 1993-1-1.
- Biaxial bending consideration: In real structures, columns are rarely subjected to pure uniaxial bending. The biaxial interaction curves developed in this study enable more accurate assessment of column capacity under combined loading, which is essential for seismic design and wind loading scenarios.
- Welding quality: T-shaped CFST columns typically involve welding of two tube segments at the junction. The weld quality at the T-junction directly affects the structural integrity and load transfer capacity. Engineers should ensure that welds meet the requirements of relevant welding standards such as AWS D1.1 or ISO 3834.
Study Insights and Reflections
The fiber model approach used in this study is a powerful tool for analyzing complex CFST section behavior, and the proposed simplified methods make the results accessible for practical design. The identification of loading angle as a critical parameter is particularly valuable for engineers who must consider the orientation of T-sections in their structural designs. The simplified calculation methods, while based on full plastic assumptions, provide reasonable accuracy for preliminary design and code-level calculations. However, engineers should be aware that the simplified methods may underestimate the capacity of sections with very high concrete strength or very low steel yield strength, where the assumptions of proportional behavior may not hold. Future research should extend these methods to consider the effects of material nonlinearity, geometric imperfections, and residual stresses on the interaction curves.
The paper contributes significantly to the design methodology for T-shaped CFST columns and provides engineers with practical tools for evaluating section strength under complex loading conditions. The combination of rigorous numerical analysis and simplified design formulas represents a balanced approach that bridges the gap between research and practice.