Zhuojin Pipe Fitting Co., LtdAxial Compression Bearing Capacity of Circular Solid Double-Skin 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 in 2008 (Vol. 23, No. 1, pp. 8-13), presents a theoretical analysis of the axial compression ultimate bearing capacity of circular solid double-skin steel tube concrete (DSTC) columns using the unified strength theory. The research was supported by the Ministry of Education Doctoral Point Fund (Grant No. 20040710001) and the Shaanxi Provincial Natural Science Foundation (Grant No. 2005E204). The double-skin configuration consists of an outer steel tube, an inner steel tube, and a concrete core confined between them, creating a dual-confinement system.
Theoretical Framework
The unified strength theory, developed by Yu Maochun, provides a unified framework for describing the behavior of materials under complex stress states. Unlike traditional yield criteria (Tresca, von Mises), the unified strength theory can represent various yield surfaces through a single parameter b, where b = 0 corresponds to the Tresca criterion and b = 1 corresponds to the von Mises criterion. For concrete, the value of b typically falls between 0.5 and 0.8, depending on the concrete strength and aggregate characteristics.
The theoretical model considers two key mechanical effects:
- Dual confinement effect: Both the inner and outer steel tubes exert confining pressure on the concrete core. The outer tube confines the outer portion of the concrete, while the inner tube confines the inner portion. This creates a non-uniform confining pressure distribution across the concrete cross-section.
- Steel tube longitudinal stress reduction: As the steel tubes undergo hoop tension due to concrete radial expansion, the longitudinal stress in the steel tubes is reduced according to the yield criterion. This reduction in longitudinal stress must be accounted for in the bearing capacity calculation.
Derivation of Bearing Capacity Formula
The ultimate bearing capacity formula derived in this paper can be expressed conceptually as:
| Component | Contribution to Bearing Capacity | Key Variables |
|---|---|---|
| Outer steel tube | Axial compression minus hoop tension reduction | f_y1, A_s1, confining pressure p1 |
| Inner steel tube | Axial compression minus hoop tension reduction | f_y2, A_s2, confining pressure p2 |
| Concrete core | Enhanced by dual confinement | f_c, A_c, p1, p2, b parameter |
| Interface friction | Shear transfer between components | Interface friction coefficient |
The unified strength theory is applied to determine the stress state of the concrete under triaxial compression. The intermediate principal stress effect is explicitly considered, which is particularly important for the concrete confined between two steel tubes where the stress state is not purely axisymmetric.
Key Findings
The study reveals that the ultimate bearing capacity increases with the parameter b value, which is consistent with the physical interpretation that higher b values correspond to materials with greater resistance to shear failure. The theoretical predictions show good agreement with experimental results from the literature, validating the applicability of the unified strength theory to DSTC column analysis.
| Comparison Metric | Theoretical Prediction | Experimental Results | Deviation |
|---|---|---|---|
| Ultimate bearing capacity | Calculated from unified strength theory | Measured from tests | Generally within 10% |
| Failure mode | Predicted based on stress distribution | Observed in tests | Qualitatively consistent |
| Stress distribution | Non-uniform due to dual confinement | Measured by strain gauges | Reasonable agreement |
Technical Analysis and Engineering Practice
From a steel pipe manufacturing and structural engineering perspective, the double-skin steel tube concrete column represents an advanced structural system with several important implications:
- Steel tube selection: Both the inner and outer steel tubes should be selected from appropriate grades (typically Q235, Q345, or higher per GB/T 1591) to ensure adequate strength and ductility. The outer tube is typically larger in diameter and may be a welded pipe (HFW or LSAW per GB/T 9711), while the inner tube is often a seamless tube (per GB/T 8162 or 8163) for better dimensional accuracy.
- Welding considerations: If the double-skin tubes are connected by transverse welds or stiffeners, the welding quality is critical. The weld metal must match the base metal strength, and post-weld heat treatment may be required for high-strength steels to prevent hydrogen-induced cracking.
- Concrete placement: The concrete must be placed and compacted between the two tubes, which requires special construction techniques such as tremie concrete placement or specialized pumping methods. The concrete mix should have adequate workability to fill the annular space between tubes.
- D/t ratio control: Both tubes must satisfy local buckling limits. The outer tube, being larger in diameter, requires greater wall thickness to maintain the same D/t ratio as the inner tube.
Defect Analysis and Countermeasures
| Potential Defect | Cause | Countermeasure |
|---|---|---|
| Insufficient concrete fill between tubes | Poor workability, inadequate pumping pressure | Use self-compacting concrete, optimize pump pressure |
| Local buckling of inner tube | Excessive hoop stress from concrete confinement | Increase inner tube wall thickness, reduce concrete strength |
| Weld cracking at tube connections | High residual stress, hydrogen embrittlement | Preheat, control heat input, post-weld heat treatment |
| Uneven concrete confinement | Non-uniform concrete placement | Use vibration, ensure proper pump flow rate |
| Corrosion of inner tube | Carbonation penetration through concrete | Apply internal coating, use corrosion-resistant steel grade |
Study Insights and Conclusions
The application of the unified strength theory to double-skin steel tube concrete columns provides a rigorous theoretical foundation for predicting bearing capacity under complex stress states. The key insight is that the dual confinement from both steel tubes creates a non-uniform stress state in the concrete that cannot be adequately captured by simplified analytical models assuming uniform confinement. The parameter b in the unified strength theory effectively captures the intermediate principal stress effect, which is particularly significant for the concrete core in a double-skin configuration. For engineering practice, this theoretical framework enables more accurate design of DSTC columns, potentially leading to material savings through optimized steel tube dimensions. However, the practical implementation requires careful attention to construction quality, particularly in ensuring complete concrete fill between the two tubes and maintaining proper weld quality at all steel tube connections. The unified strength theory approach is particularly valuable for high-rise and heavy-load applications where DSTC columns may be employed to achieve high bearing capacity with compact cross-sections.