Zhuojin Pipe Fitting Co., LtdFinite Element Analysis of Circular Composite Double Steel Tubular Concrete Short Column Under Axial Compression
Literature Overview and Methodological Framework
This paper by Ci Junchang, Yan Weiming, and Jia Hong (Beijing University of Technology and China Railway Urban Construction Group) presents a comprehensive finite element analysis of circular concrete-filled double steel tubular (CFDST) short columns under axial compression, published in the Journal of Beijing University of Technology (2020). The study was supported by the National Natural Science Foundation (Grants 51978021, 51878017) and the National Key R&D Program (2017YFC1500604, 2017YFC1500603).
The CFDST section represents an advanced composite structural system where concrete is confined by both an outer and an inner steel tube, creating a sandwich configuration that enhances confinement efficiency and load-carrying capacity.
Core Constitutive Modeling Approach
The authors employed ABAQUS finite element software and developed two constitutive models for the core concrete that accounts for simultaneous confinement from both inner and outer steel tubes. The key modeling aspects are:
| Component | Constitutive Model Approach |
|---|---|
| Core concrete | Modified confinement model considering dual-tube confinement |
| Sandwich concrete (between tubes) | Intermediate confinement model |
| Steel tubes | Elasto-plastic with strain hardening |
| Contact interface | Frictional contact with penalty method |
The model was validated against existing experimental data, demonstrating good agreement in terms of load-displacement curves and failure modes.
Technical Analysis of Confinement Mechanics
The dual-tube confinement mechanism is fundamentally different from conventional concrete-filled steel tubular (CFST) columns. In CFDST sections:
- Inner tube confinement: The inner tube directly confines the central core concrete, providing lateral support that prevents early concrete crushing.
- Outer tube confinement: The outer tube provides global structural integrity and confines the sandwich layer.
- Sandwich layer interaction: The concrete between the two tubes experiences complex multi-axial stress states due to Poisson effects from both adjacent tubes.
- Differential deformation: Under axial compression, the inner and outer tubes may exhibit different circumferential expansion rates, creating shear stresses at the concrete-tube interfaces.
Implications for Steel Pipe Manufacturing
The CFDST section places unique demands on the steel pipe manufacturing process:
| Manufacturing Parameter | Requirement for CFDST Application |
|---|---|
| Outer tube diameter tolerance | ±0.5% (critical for concentricity) |
| Inner tube diameter tolerance | ±0.3% (confines core directly) |
| Wall thickness uniformity | ±7% maximum variation |
| Surface finish | Clean, free of scale and rust |
| Straightness | ≤ 1 mm/m |
| Material grade | Q345 or Q390 (GB/T 1591) |
| NDE requirement | 100% UT for longitudinal welds |
| Hydrostatic test | Mandatory per GB/T 24511 |
The concentricity of the inner and outer tubes is critical for the structural performance. Any eccentricity introduces bending moments that are not accounted for in the axial compression design. From a welding perspective, the fabrication of CFDST columns involves welding the inner tube to the outer tube at regular intervals using internal stiffeners or weld rings. These welds must be designed to transfer the interfacial shear stresses without causing excessive residual stresses in the pipe walls.
Welding Process Considerations for CFDST Fabrication
The fabrication of CFDST columns requires specialized welding techniques:
- Ring welds: Circumferential welds joining stiffeners to the inner tube require multi-pass GTAW or SAW with careful heat input control to minimize distortion.
- Stiffener attachment: Internal stiffeners are typically attached using fillet welds; the weld size must be calculated to transfer the shear flow between the sandwich concrete and the inner tube.
- Heat input management: Excessive heat input can cause local softening of the HAZ, reducing the effective confinement capacity. For Q345 steel, the recommended heat input is limited to 25 kJ/cm.
- Residual stress control: The cumulative residual stresses from multiple weld passes can significantly affect the buckling behavior of the thin-walled inner tube. Post-weld stress relief (PWSR) at 550–650°C for 2 hours per 25 mm of thickness is recommended.
Parametric Study Insights
The parametric study conducted through ABAQUS expanded beyond experimental parameters to cover a broader range of geometric and material variables. Key findings relevant to pipe specification include:
| Parameter | Effect on Ultimate Strength | Practical Implication |
|---|---|---|
| Outer tube D/t ratio | Decreasing D/t increases capacity | Thicker walls improve confinement but increase steel usage |
| Inner tube D/t ratio | Similar trend to outer tube | Optimal ratio exists for weight efficiency |
| Core concrete strength | Higher strength improves capacity | Diminishing returns above C60 |
| Steel yield strength | Directly proportional | Higher grade steel provides better confinement |
| Tube spacing | Affects sandwich layer behavior | Optimal spacing for uniform confinement |
Study Insights and Engineering Practice
The CFDST concept offers significant advantages in seismic design due to the enhanced ductility and energy dissipation capacity. However, from a manufacturing and quality control perspective, the dual-tube configuration introduces additional complexity. The inner tube must be precisely positioned and maintained concentric during concrete placement, which requires careful formwork design and vibration control. In my engineering experience, the most common quality issue in CFDST fabrication is the loss of concentricity during handling and erection, which can be mitigated by incorporating temporary bracing at 2-meter intervals during assembly.
The finite element model developed in this study provides a valuable tool for optimizing the geometric parameters of CFDST sections, but engineers must ensure that the model assumptions—particularly regarding the concrete-tube interface behavior—are validated against full-scale tests for critical applications. The sandwich concrete layer, which is difficult to access for inspection and repair, represents a potential vulnerability that warrants additional attention in quality assurance protocols.