Zhuojin Pipe Fitting Co., LtdFinite Element Modeling of Concrete-Filled Steel Tubes Under Combined Compression-Bending-Torsion
Literature Overview
Wang Yuhang, Nie Jianguo, and Fan Jiansheng published this finite element study in the Journal of Harbin Institute of Technology in 2013 (Volume 45, Issue 8, pages 94–98). The research develops a refined "shell-solid" finite element model for CFST columns subjected to combined axial compression, bending, and torsion — a loading condition that arises in practical structures such as bridge piers, crane columns, and offshore platforms.
Finite Element Model Architecture
The model is constructed within the ABAQUS framework and employs a dual-domain discretization strategy:
| Component | Element Type | Material Model | Key Parameters |
|---|---|---|---|
| Steel tube | Shell elements (S4R) | Idealized elasto-plastic (bilinear) | Yield strength, tangent modulus |
| Concrete core | Solid elements (C3D8R) | Concrete damage plasticity (CDP) | Compressive strength, tensile strength, fracture energy |
| Interface | Tie constraint | Penalty-based contact | Friction coefficient |
Material Constitutive Models
Steel tube: The bilinear elasto-plastic model captures the yield plateau and strain hardening behavior. The tangent modulus is typically taken as 0.01–0.02 times the elastic modulus, consistent with experimental stress-strain curves for structural steels (Q235, Q345, Q390).
Concrete core: The Concrete Damage Plasticity (CDP) model in ABAQUS is selected because it allows independent calibration of compressive and tensile damage evolution. Key calibration parameters include:
- Dilatation angle: typically 20°–30° for normal concrete
- Flow potential eccentricity: 0.1 (default)
- Ratio of initial equibiaxial to uniaxial compressive yield stress: 1.167
- Fracture energy: derived from tensile strength and elastic modulus
Loading Protocol
The complex loading mode is implemented through a combination of:
- Axial displacement control at the column top
- Lateral displacement control to induce bending moment
- Rotational boundary conditions to apply torsional moment
This multi-axis displacement control approach is more robust than force control for capturing post-peak behavior, as it prevents numerical divergence when the structural response becomes unstable.
Validation and Results
The finite element model was validated against experimental data from published literature. The comparison showed good agreement in both the peak load and the post-peak softening branch. The model successfully reproduces:
- The torsional moment-rotation angle hysteresis loop
- The distribution of von Mises stress across the steel tube wall
- The localized crushing patterns of the concrete core
- The interaction between bending-induced tension and torsional shear
Engineering Relevance
Welding Process Implications
The combined loading state studied here has direct implications for the welding design of CFST column connections:
- Welding residual stress interaction — In practice, the residual stresses from tube-to-tube welding (typically flash butt welding or submerged arc welding for large-diameter tubes) interact with the operational stresses from combined loading. The finite element results can be used to superimpose welding residual stress fields (obtained from welding simulation) onto the operational stress field to assess fatigue life.
- Heat-affected zone (HAZ) degradation — Under combined compression-bending-torsion, the HAZ of circumferential welds experiences complex multiaxial stress states. The torsional component introduces shear stresses that are particularly detrimental to the toughness of the HAZ microstructure. Post-weld heat treatment (PWHT) or controlled cooling rates are essential for large-diameter tubes.
Quality Control Recommendations
| Test Method | Purpose | Acceptance Criteria |
|---|---|---|
| Hydrostatic pressure test | Verify concrete fill integrity | No pressure drop over 15 min at 1.5× design pressure |
| UT (TOFD/PAUT) | Detect internal defects in welds | ISO 17640 compliance |
| MT/PT | Surface crack detection at tube joints | ASME V Section acceptance |
| Charpy V-notch | HAZ toughness assessment | ≥ 27 J at service temperature |
Analytical Insights
The shell-solid approach offers a significant advantage over pure solid-element models: it reduces computational cost by approximately 60–70% while maintaining accuracy for thin-walled tube behavior. However, this approach requires careful attention to the shell-solid interface definition. The tie constraint used here assumes perfect bond, which is a reasonable approximation for well-filled CFST members but may be conservative for members with incomplete concrete placement.
The ability to extract point-by-point stress and strain data from the model is particularly valuable for fatigue assessment. In combined loading scenarios, the stress state at any point on the tube wall is triaxial, and conventional uniaxial fatigue curves are not directly applicable. The multiaxial stress history obtained from the FE model can be processed using fatigue criteria such as the Sines or Crossland criteria to estimate fatigue life.