Zhuojin Pipe Fitting Co., LtdNonlinear Finite Element Analysis of Composite CFST Axially Loaded Short Columns with Square Outer and Circular Inner Tubes
Overview and Research Background
This study by Wang Weihua, Yao Guohuang, and Wang Lingling from Huaqiao University and Tsinghua University presents a nonlinear finite element analysis of a novel composite CFST cross-section consisting of a square outer steel tube enclosing a circular inner steel tube, with concrete filling both the outer annular region and the inner tube. Funded by the China Postdoctoral Science Foundation (20110490417) and Huaqiao University Research Fund (11BS417), this research was published in the Journal of Beijing University of Technology in 2013. The work addresses the growing demand for high-capacity, ductile compression members in tall building cores and bridge piers.
Core Technical Content
The composite section design creates a dual-confinement mechanism where both the square outer tube and the circular inner tube provide lateral restraint to the concrete core. The authors established a nonlinear finite element model using ABAQUS that accurately captures the material nonlinearity of concrete (using the Concrete Damage Plasticity model), steel tube plasticity (using von Mises yield criterion with strain hardening), and the interface behavior between steel and concrete.
Key findings from the parametric analysis:
| Parameter | Range Studied | Effect on Ultimate Capacity | Effect on Residual Capacity |
|---|---|---|---|
| Inner tube diameter | 100-400 mm | First increases, then decreases | First increases, then decreases |
| Outer tube wall thickness | 4-12 mm | Monotonically increases | Monotonically increases |
| Inner tube wall thickness | 4-12 mm | Increases then plateaus | Increases then plateaus |
| Concrete strength (f_c) | 30-60 MPa | Increases linearly | Increases linearly |
| Steel yield strength (f_y) | 235-460 MPa | Increases linearly | Moderate increase |
The non-monotonic behavior of ultimate and residual capacity with respect to inner tube diameter is a particularly important finding. When the inner tube diameter is too small, the total steel area and concrete confinement are insufficient. When it is too large, the annular concrete region between the inner and outer tubes becomes too thin to develop full confinement pressure, and the inner tube itself may experience local buckling before the full composite capacity is reached.
Manufacturing and Welding Considerations
From a steel pipe fabrication standpoint, this composite section presents unique challenges:
- Inner tube fabrication: The circular inner tube must be manufactured to tight tolerances (typically ±1 mm on diameter) to ensure uniform concrete placement in the annular region. Seamless tubes or ERW tubes with strict dimensional control are preferred.
- Interface preparation: The inner tube surface must be prepared for concrete bonding, typically through mechanical roughening or the use of chemical bonding agents. Any coating or mill scale on the inner surface that is not removed will reduce the steel-concrete bond strength.
- Assembly welding: The connection between the inner and outer tubes (typically through longitudinal stiffeners or ring welds) must be designed to accommodate differential thermal expansion during concrete curing and service temperature variations.
- Concrete placement: The annular region between square outer and circular inner tubes creates geometric challenges for concrete placement. Vibration and compaction must be carefully controlled to avoid voids in the corners of the square section where concrete tends to segregate.
Analytical Framework and Validation
The finite element model was validated against experimental test results, showing good agreement in both ultimate load and load-deformation curves. The authors employed a fiber-section approach for parametric studies, which significantly reduced computational cost while maintaining accuracy. The modeling approach includes:
- Steel tubes: Shell elements with elastic-plastic material law and strain hardening
- Concrete: Solid elements with Concrete Damage Plasticity model
- Interface: Tie constraints or cohesive elements to model steel-concrete bond
- Boundary conditions: Pinned-pinned with eccentricity to simulate real loading
Engineering Implications and Design Recommendations
The research demonstrates that the composite square-circular CFST section offers superior ductility and residual capacity compared to conventional single-tube CFST columns. The dual-confinement mechanism ensures that even after local buckling of one tube, the other continues to confine the concrete core, providing a secondary load path.
For practical design, the optimal inner tube diameter ratio (d_inner/d_outer) appears to be in the range of 0.5-0.7, balancing the benefits of increased confinement against the risks of local buckling and concrete placement difficulties. The residual capacity ratio (post-buckling capacity/ultimate capacity) can reach 0.6-0.8 for well-designed composite sections, making them suitable for seismic applications where post-peak ductility is critical.
The parametric study reveals that wall thickness has a more pronounced effect on residual capacity than on ultimate capacity, suggesting that designers should prioritize adequate wall thickness over maximizing steel area for ductility-critical applications. This finding has direct implications for steel pipe procurement specifications and fabrication tolerances in composite CFST structural systems.