Zhuojin Pipe Fitting Co., LtdComparative Analysis of Axial Compression Bearing Capacity Calculation Methods for Double-Skin Steel Pipe Concrete Columns
Literature Overview
This paper by Chen Jianwei, Su Youpo, and Li Xin, published in the Natural Science journal of Fuzhou University in 2013, presents a comparative study of three calculation methods for the axial compression bearing capacity of double-skin steel pipe concrete (DSC) short columns. The research is supported by the National Natural Science Foundation of China and addresses a growing area of structural engineering where conventional single-skin SRC columns are insufficient for extreme load conditions. From a steel pipe fabrication and welding standpoint, this study is relevant because double-skin configurations introduce additional welding complexity, require precise concentricity between inner and outer tubes, and demand rigorous quality control of both tube geometries.
Core Technical Content
A double-skin steel pipe concrete column consists of an inner steel tube, an outer steel tube, and concrete infilled between the two tubes and within the inner tube. This configuration provides enhanced load-bearing capacity and ductility compared to single-skin SRC columns, making it suitable for applications such as nuclear containment structures, blast-resistant buildings, and heavy industrial columns. The paper evaluates three calculation methods: the unified theory method, the independent tube method, and the interaction method, and proposes an improved unified theory approach that better matches experimental results.
Calculation Methods Compared
The following table summarizes the three calculation methods and their key assumptions:
| Method | Core Assumption | Treatment of Concrete-Confinement | Applicable Range | Accuracy vs. Test Data |
|---|---|---|---|---|
| Unified Theory (original) | Composite section acts as one unit | Both tubes contribute to confinement | All DSC columns | Overestimates by 10-20% |
| Independent Tube Method | Inner and outer tubes act independently | Only inner tube confines concrete | Moderate confinement ratio | Underestimates by 5-15% |
| Interaction Method | Tubes interact through concrete | Partial confinement from both tubes | All DSC columns | Reasonable but complex |
| Improved Unified Theory | Modified confinement contribution | Weighted contribution from both tubes | All DSC columns | Within ±5% of test data |
The paper's key finding is that the improved unified theory method, which modifies the original unified theory by accounting for the differential confinement contributions of the inner and outer tubes, provides the best agreement with experimental data. The original unified theory overestimates capacity because it assumes both tubes contribute equally to concrete confinement, whereas in reality the inner tube provides more effective confinement due to its direct contact with the concrete.
Welding and Fabrication Implications for Double-Skin Tubes
The double-skin configuration introduces significant fabrication challenges that are not addressed in the structural analysis but are critical for ensuring that the calculated bearing capacity is achieved in practice:
- Concentricity control: The inner and outer tubes must be concentric within ±2 mm tolerance to ensure uniform concrete thickness and uniform confinement pressure. Any eccentricity creates stress concentrations that can initiate premature failure.
- Inner tube welding: The inner tube is typically welded to the outer tube at discrete connection points using fillet welds or bolted connections. These connections must transfer the radial confinement forces from the concrete to the outer tube.
- Concrete pouring: The annular space between the tubes must be filled with concrete without voids, which requires careful pouring technique and vibration control. The inner tube must be held in position during pouring to prevent displacement.
- NDT coverage: Both inner and outer tube welds require 100% radiographic or ultrasonic inspection because hidden defects in the annular region are difficult to detect and repair after concrete placement.
Material Properties and Standards
The paper references experimental data from DSC column tests, which provides valuable information about the material properties required for reliable performance. The following table summarizes typical material specifications for double-skin SRC columns:
| Component | Typical Specification | Standard Reference |
|---|---|---|
| Outer steel tube | Q345, Q390, or Q460 | GB/T 1591 |
| Inner steel tube | Q345 or Q390 | GB/T 1591 |
| Concrete | C40 to C60 | GB/T 50081 |
| Outer tube diameter | 300–600 mm | Design-specific |
| Inner tube diameter | 150–300 mm | Design-specific |
| Outer wall thickness | 10–20 mm | Design-specific |
| Inner wall thickness | 8–15 mm | Design-specific |
| Concrete cover (annular) | 20–40 mm | Design-specific |
The paper's improved unified theory method provides a practical calculation approach for engineers designing double-skin SRC columns, and the method's accuracy within ±5% of test data makes it suitable for design applications where safety factors of 1.5 to 2.0 are applied.
Engineering Practice Insights
The comparative analysis of calculation methods has direct implications for the design and fabrication of double-skin SRC columns. Engineers should adopt the improved unified theory method for design calculations because it provides the best balance of accuracy and simplicity. The method also identifies the parameters that most influence bearing capacity, allowing engineers to optimize the steel tube geometry and material selection for cost-effective designs.
From a fabrication standpoint, the paper's findings reinforce that the inner tube's confinement contribution is more significant than the outer tube's, which means that the inner tube's fabrication quality and welding integrity are more critical to achieving the calculated bearing capacity. This insight should guide the allocation of quality control resources during fabrication, with greater emphasis on inner tube dimensional accuracy and weld quality. The paper also serves as a reminder that analytical methods, while valuable for design, must be validated against experimental data, and that the gap between predicted and actual capacity can be significant if the wrong calculation method is used.