Zhuojin Pipe Fitting Co., LtdAxial Compression Bearing Capacity of Circular Solid Double-Layer Steel Tube Concrete Columns
Literature Overview
This paper by Zhang Zhaoqiang, Zhao Junhai, and Yao Yong, published in the Journal of Southwest University of Science and Technology (2008, Vol. 23, No. 1, pp. 8-13), develops a theoretical model for the ultimate axial compression bearing capacity of circular solid double-layer steel tube concrete columns using the unified strength theory. The model accounts for the dual confinement effect of both inner and outer steel tubes on the concrete, as well as the reduction in longitudinal stress in the steel tubes due to hoop tension. The study analyzes the influence of the intermediate principal stress parameter b on ultimate bearing capacity and validates the theoretical results against experimental data from the literature.
Core Technical Content
The double-layer steel tube concrete column (also known as sandwich or double-skin steel tube concrete column) consists of an inner steel tube, an outer steel tube, and concrete filling the annular space between them. This configuration provides enhanced confinement compared to single-layer steel tube concrete columns, as both tubes exert lateral confining pressure on the concrete. The inner tube confines the concrete from the inside, while the outer tube confines from the outside, creating a dual-confinement mechanism.
Unified Strength Theory Framework
The unified strength theory, proposed by Yu Maochun, provides a comprehensive framework for describing the behavior of materials under complex stress states. The theory is governed by a single parameter b (0 ≤ b ≤ 1) that interpolates between different classical yield criteria:
| Parameter b Value | Corresponding Yield Criterion |
|---|---|
| b = 0 | Tresca criterion (maximum shear stress) |
| b = 1 | Von Mises criterion (octahedral shear stress) |
| 0 < b < 1 | Intermediate criteria considering intermediate principal stress |
The theoretical model developed in this paper derives the ultimate bearing capacity formula by:
- Establishing equilibrium equations for the concrete core under dual confinement.
- Applying the unified strength criterion to determine the failure condition of the confined concrete.
- Accounting for the Poisson effect in steel tubes—hoop tension reduces the effective longitudinal stress contribution.
- Integrating the stress contributions of inner tube, outer tube, and confined concrete to obtain total bearing capacity.
Key Design Parameters
| Parameter | Symbol | Typical Range | Effect on Capacity |
|---|---|---|---|
| Inner tube diameter | D1 | 100-400 mm | Larger D1 increases concrete volume |
| Outer tube diameter | D2 | 200-800 mm | Larger D2 increases confinement |
| Inner tube wall thickness | t1 | 4-16 mm | Thicker t1 increases confinement |
| Outer tube wall thickness | t2 | 6-25 mm | Thicker t2 increases confinement |
| Concrete strength | fck | 30-80 MPa | Higher strength increases capacity |
| Steel yield strength | fy | 235-460 MPa | Higher fy increases confinement |
| Unified strength parameter | b | 0-1 | Higher b generally increases capacity |
Technical Insights from a Steel Pipe Engineering Perspective
From a steel pipe fabrication standpoint, double-layer steel tube concrete columns present unique manufacturing challenges. The inner and outer tubes must be precisely aligned and spaced to maintain a uniform concrete annular gap. This requires:
- Dimensional accuracy: Both tubes must meet tight tolerance requirements on outer diameter and wall thickness. For seamless pipes, GB/T 8162 specifies tolerance classes; for welded pipes, GB/T 9711 (for API 5L grade) or GB/T 3091 (for GB grade) apply.
- Surface quality: The inner surface of the outer tube and the outer surface of the inner tube must be clean and free of defects to ensure proper bond with concrete. Surface roughness and any mill scale or coating must be evaluated for interface performance.
- Welding of tube ends: If the tubes are fabricated from multiple segments, the end welds must be high-quality to maintain structural integrity. GTAW (TIG) or SAW (submerged arc welding) processes are typically specified, with full-penetration butt joints and 100% UT inspection per GB/T 3323 or ASME Section V.
Confinement Mechanism Analysis
The dual-confinement mechanism can be analyzed as follows:
- The outer tube provides external confinement, preventing outward bulging of the concrete.
- The inner tube provides internal confinement, preventing inward buckling of the concrete core and creating a triaxial stress state.
- The interaction between the two tubes through the concrete creates a complex stress field that is more efficient than single-tube confinement.
The reduction in longitudinal steel stress due to hoop tension is a critical consideration. As the concrete expands laterally under compression, it exerts radial pressure on both tubes. This radial pressure induces hoop tension in the tubes, which—by the unified strength theory—reduces the effective longitudinal yield strength. This effect becomes more pronounced in tubes with thinner walls or lower yield strength.
Engineering Practice Implications
For steel pipe suppliers, double-layer steel tube concrete columns represent a niche but high-value market segment. The requirements for dimensional precision, material consistency, and surface quality exceed those of conventional structural steel pipes. Key quality control measures include:
- Hydrostatic testing per GB/T 241 to verify wall integrity.
- Dimensional inspection at multiple stations along the pipe length.
- Material certification with full chemical and mechanical test reports.
- Surface inspection (MT or PT) for critical applications.
The unified strength theory approach provides a more accurate prediction of bearing capacity compared to simplified models that ignore intermediate principal stress effects. For engineering design, this means that the actual capacity of double-layer steel tube concrete columns may be significantly higher than predictions based on von Mises or Tresca criteria alone, depending on the value of parameter b appropriate for the steel grade used.
Study Insights and Implications
The application of unified strength theory to double-layer steel tube concrete columns demonstrates the power of advanced constitutive modeling in structural engineering. For steel pipe engineers, the key takeaway is that the design of multi-layer composite steel structures requires careful consideration of the interaction between layers and the complex stress states that develop under loading. The theoretical framework developed here can be extended to other multi-layer composite configurations and provides a basis for optimizing the geometric parameters of double-layer systems for maximum efficiency.