Zhuojin Pipe Fitting Co., LtdAxial Compressive Load Bearing Capacity and Load Composition of Circular Concrete-Filled Steel Tube Columns
Literature Overview
This paper by Chen Likang and Huang Yuan from Hunan University, published in Progress in Steel Building Structures (2024, Vol. 26, No. 6, pp. 94–104), addresses a fundamental question in composite column engineering: how do the steel tube and the infill concrete each contribute to the ultimate axial load capacity of a circular concrete-filled steel tube (CCFST) column under interaction conditions? The research was supported by the Hunan Provincial Natural Science Foundation (Grant No. 2020JJ2003). The authors developed a three-dimensional finite element model using ABAQUS with a VUMAT subroutine to implement a novel concrete material constitutive law that captures the interaction between the steel tube and the core concrete. The model was validated against experimental data in terms of load-displacement curves, individual load-deformation curves for the steel tube and concrete, and lateral deformation curves.
Core Technical Points
The Interaction Mechanism
The central insight of this work is that the steel tube and the core concrete in a CCFST column do not act independently; rather, they interact through mutual confinement. Under axial compression, the concrete tends to expand laterally due to Poisson's effect, and the steel tube restrains this expansion, generating a confining pressure that enhances the compressive strength of the concrete. Conversely, the concrete provides lateral support to the steel tube, delaying local buckling and improving the effective buckling resistance of the thin-walled tube. This bidirectional interaction is the very reason why CCFST columns achieve load capacities significantly exceeding the simple sum of the individual steel and concrete capacities calculated as separate materials.
Finite Element Modeling Approach
The authors employed a VUMAT (Viscoelastic User Material) subroutine within the ABAQUS explicit dynamic framework to implement a new concrete constitutive model. This approach allows the model to capture the complex stress-strain behavior of confined concrete, including the transition from unconfined to confined states as the axial load increases. The use of an explicit dynamic solver, while typically associated with rate-dependent phenomena, was chosen here likely for its robustness in handling contact problems and material instabilities inherent in the interaction between steel and concrete. The model accounts for:
- Material nonlinearity of both steel and concrete
- Geometric nonlinearity (large deformations)
- Contact interaction between the steel tube inner surface and the concrete outer surface
- Progressive degradation of the concrete under triaxial compression
Key Parametric Variables
The parametric study examined four critical variables:
| Parameter | Symbol | Typical Range | Effect on Capacity |
|---|---|---|---|
| Concrete compressive strength | f_c | 20–80 MPa | Increases capacity linearly at low values, diminishing returns at high values |
| Steel tube yield strength | f_y | 235–460 MPa | Increases capacity proportionally |
| Diameter-to-thickness ratio | D/t | 10–100 | Higher ratios reduce capacity due to increased susceptibility to local buckling |
| Confinement ratio | ξ | 0.1–2.0 | Higher ratios significantly enhance capacity through increased confining pressure |
Proposed Mathematical Model
The authors derived a mathematical model for the peak axial load capacity of CCFST columns that introduces two novel coefficients:
- Steel tube circumferential stress proportion coefficient — this quantifies the actual circumferential stress state in the steel tube relative to the yield strength, accounting for the non-uniform stress distribution caused by the interaction with the core concrete.
- Concrete load-bearing contribution coefficient — this quantifies the fraction of the total capacity attributable to the concrete core, adjusted for the confinement effect.
These coefficients enable engineers to decompose the total capacity into its steel and concrete components, which is of considerable practical value for design verification, damage assessment, and retrofitting decisions.
Comparison with Existing Standards and Research
The proposed calculation model was compared against:
- Current Chinese design codes (GB 50936-2014, Technical Standard for Concrete-Filled Steel Tubular Structures)
- Existing research models in the international literature
The comparison revealed that the proposed formula achieves good computational accuracy while offering the unique advantage of decomposing the capacity into steel and concrete contributions. This decomposition capability is not readily available in most existing design codes, which typically provide only the total capacity without distinguishing the individual material contributions.
Engineering Practice Implications
From a metallurgical and welding engineering perspective, this research has several important implications:
- Material selection for the steel tube: The parametric study confirms that higher steel grades (higher f_y) directly improve column capacity, but the benefit is moderated by the confinement ratio. This means that for heavily confined columns (high ξ), the marginal benefit of using ultra-high-strength steel is reduced because the concrete contribution dominates.
- Welding quality and seam integrity: The load-bearing contribution of the steel tube depends on its circumferential stress state. In welded tubes (ERW, HFW, LSAW, SSAW), the weld seam represents a potential weakness where the circumferential stress may be unevenly distributed. Any welding defects — such as incomplete fusion, undercut, or lack of penetration — would disproportionately reduce the steel tube's contribution to the total capacity.
- Tube wall thickness and D/t ratio: The study confirms that higher D/t ratios reduce capacity. For welded tubes, achieving a low D/t ratio while maintaining weld quality is challenging. HFW and ERW processes can produce thinner walls with consistent weld quality, making them preferable for high-strength CCFST applications.
- Residual stress considerations: The interaction between the steel tube and the concrete is influenced by the residual stresses in the steel tube, which are directly related to the manufacturing process. For example, the high-frequency induction heating in HFW tubes produces a localized residual stress pattern that may affect the tube's initial buckling behavior under combined axial and hoop stresses.
Study Insights and Reflections
This paper makes a meaningful contribution to the theoretical understanding of CCFST columns by providing a mechanism-based decomposition of the load-bearing capacity. The introduction of the steel tube circumferential stress proportion coefficient and the concrete load-bearing contribution coefficient is particularly valuable because it bridges the gap between the simplified design formulas in current codes and the complex interaction mechanics revealed by finite element analysis. For practicing engineers involved in the fabrication and welding of steel tubes for composite columns, this work reinforces the importance of maintaining high welding quality and selecting appropriate tube geometries (D/t ratios) to ensure that the steel tube delivers its full design contribution to the composite column capacity. The validation of the finite element model against experimental data provides confidence that the proposed mathematical model is physically meaningful and not merely a curve-fitting exercise.