Zhuojin Pipe Fitting Co., LtdAxial Compression Capacity Calculation of Composite CFST Using Double-Shear Unified Strength Theory
Literature Overview
This 2012 publication by Zhang Yufen, Zhao Junhai, and Liu Yiting appeared in the journal "Mechanics and Practice" (Volume 34, Issue 3, pages 36-42). The study was funded by the National Natural Science Foundation of China (Grant No. 51008027), Central University Research Business Fee (CHD2011ZD009), and the Tsinghua University Key Laboratory of Civil Engineering Safety and Durability Open Fund. The authors from Tsinghua University, Chang'an University, and the Naval Engineering Quality Supervision Station developed a theoretical framework for calculating the axial compression bearing capacity of composite concrete-filled steel tubes (double-tube CFST) using the double-shear unified strength theory.
Theoretical Framework and Methodology
The double-tube CFST system features both an inner and outer steel tube confining the concrete core, creating a dual-confinement effect that fundamentally alters the stress state of the concrete and steel components. The researchers analyzed the axial compression stress state of the composite section and applied the double-shear unified strength theory to determine the ultimate axial compression strength of both the concrete and steel tube components independently.
The unified strength theory is a more comprehensive approach than traditional Mohr-Coulomb or Drucker-Prager criteria because it accounts for the intermediate principal stress effect, which is particularly significant in confined concrete where the lateral stress state is complex and non-trivial. The double-shear criterion considers both shear failure on planes at different orientations, providing a more accurate representation of concrete failure under multiaxial stress conditions.
Key Derivations and Formulation Components
| Component | Theoretical Treatment | Key Parameters |
|---|---|---|
| Concrete core | Double-shear unified strength criterion | Intermediate principal stress coefficient, material tensile-to-compressive ratio |
| Steel tube (inner) | Yield strength with confinement effect | Inner tube diameter-to-thickness ratio |
| Steel tube (outer) | Yield strength with confinement effect | Outer tube diameter-to-thickness ratio |
| Confinement force | Tightening force between tube and concrete | Interaction coefficient |
| Strength reduction factor | Steel tube strength at ultimate state | Derived from equilibrium and compatibility |
The paper derives the steel tube strength reduction coefficient at the ultimate state and calculates the tightening force (confining force) between the steel tube and concrete. The calculated axial compression bearing capacity was compared with experimental data, demonstrating good agreement and validating the applicability of the double-shear unified strength theory for composite CFST calculations.
Parametric Analysis and Design Guidance
The study provides relationships between the inner circular steel tube's diameter-to-thickness ratio and diameter with the axial compression bearing capacity improvement coefficient. This information is directly applicable to the optimization of double-tube CFST design. The parametric analysis reveals that:
- Increasing the inner tube diameter generally improves the confinement efficiency and thus the overall bearing capacity.
- The diameter-to-thickness ratio of the inner tube affects local buckling behavior, which in turn influences the effectiveness of the confinement mechanism.
- The interaction between inner and outer tube confinement effects is not simply additive but exhibits synergistic behavior at certain parameter combinations.
Engineering Practice Application
From a manufacturing perspective, the double-tube CFST system requires careful consideration of fabrication tolerances. The concentricity between inner and outer tubes is critical, as eccentricity would create uneven concrete thickness and non-uniform confinement. The welding connections between the two tube systems must be designed to transfer the confining forces without premature local failure.
For steel pipe suppliers and fabricators, this research provides quantitative guidance on how tube geometry parameters affect structural performance. The diameter-to-thickness ratio relationships derived in this paper can inform specifications for tube procurement and fabrication, ensuring that the geometric proportions selected will deliver the expected confinement benefits.
Study Insights and Limitations
The strength of this work lies in its rigorous theoretical derivation combined with experimental validation. The use of the unified strength theory represents a methodological advancement over conventional design approaches that neglect the intermediate principal stress effect. However, the study focuses on axial compression, and the extension to combined loading conditions (axial compression with bending) would require additional investigation. The practical application of double-tube CFST is still limited, and further research on long-term behavior, including creep and fatigue, would be beneficial for widespread adoption. The theoretical framework established here provides a solid foundation for future code development and design standardization.