Zhuojin Pipe Fitting Co., LtdBearing Capacity of Circular Hollow Sandwich Steel Tube Concrete Columns Using Unified Strength Theory
Literature Overview
The paper by Zhao Junhai, Guo Hongxiang, and Wei Xueying, published in the Journal of Architecture and Civil Engineering in 2005 (Volume 22, Issue 1, pages 50-54), addresses the bearing capacity analysis of circular hollow sandwich steel tube concrete columns. This research is funded by the Shaanxi Provincial Natural Science Foundation and the Ministry of Education Doctoral Program, underscoring the academic significance of this novel structural system. The authors employ the unified strength theory to develop a theoretical formula for the ultimate bearing capacity and validate it against existing experimental data from the literature.
Structural Configuration and Design Rationale
The circular hollow sandwich steel tube concrete column consists of an outer circular steel tube and an inner circular steel tube with a concrete layer sandwiched between them. This configuration differs from the conventional steel tube concrete (CFT) column by introducing an internal hollow core, which reduces the overall weight of the column while maintaining a high load-bearing capacity. The hollow interior also provides convenient access for utilities, inspection, and maintenance, making this system particularly attractive for multi-story buildings and bridge piers.
| Configuration Type | Description | Weight Reduction vs Solid CFT | Key Advantage |
|---|---|---|---|
| Solid CFT column | Single steel tube with solid concrete fill | Baseline | Highest compressive strength |
| Circular hollow sandwich CFT | Outer tube + concrete ring + inner tube + hollow core | 15-25% | Weight reduction with maintained capacity |
| Double-tube CFT (solid core) | Outer tube + concrete + inner tube + concrete fill | 5-10% | Maximum confinement efficiency |
Unified Strength Theory Application
The unified strength theory, originally developed by Yu Wei-yin, provides a generalized framework that encompasses various yield and failure criteria as special cases. By introducing the material-dependent parameter b, which ranges from 0 to 1, the theory can transition smoothly between the Tresca criterion (b = 0) and the Mohr-Coulomb criterion (b = 1). This flexibility is particularly valuable for analyzing the biaxial and triaxial stress states that exist within the sandwich concrete layer.
The authors analyze two critical factors influencing the bearing capacity: the tension-to-compression strength ratio of the steel tube material, and the intermediate principal stress effect on the confined concrete. The tension-to-compression strength ratio (R) is a material property that varies with steel grade and heat treatment condition. For common structural steels such as Q235 and Q345, this ratio is approximately 0.95 to 1.0, but can deviate significantly for high-strength steels or steels subjected to elevated temperatures.
The intermediate principal stress effect is particularly important in the sandwich configuration because the concrete layer between the two steel tubes experiences a complex stress state that is neither purely uniaxial nor purely biaxial. The unified strength theory captures this effect through the parameter b, which is calibrated based on experimental data for confined concrete under various stress paths.
Derivation of Ultimate Bearing Capacity Formula
The theoretical formula for the ultimate bearing capacity of the circular hollow sandwich CFT column is derived by integrating the stress distribution across the cross-section, considering the confinement effect on the concrete ring and the membrane stress in both steel tubes. The formula takes into account the geometric parameters (outer diameter D, outer wall thickness t1, inner diameter d, inner wall thickness t2), material properties (steel yield strength fy, concrete cylinder strength fc, tension-to-compression ratio R), and the intermediate principal stress parameter b.
The derived formula is validated against experimental results reported in the literature, showing good agreement with average deviations within acceptable engineering tolerances. The validation confirms that the unified strength theory provides a more accurate prediction than the conventional uniaxial confinement model, particularly for columns with high strength-to-thickness ratios.
Comparison with Conventional CFT Columns and Engineering Value
The authors conclude that the circular hollow sandwich CFT column possesses the same advantages as the conventional solid CFT column, including high compressive capacity, good ductility, and excellent resistance to local buckling. The key additional benefit is the significant weight reduction achieved through the hollow interior, which translates to reduced foundation loads and improved seismic performance due to lower inertial forces.
| Comparison Metric | Solid CFT Column | Hollow Sandwich CFT Column | Relative Difference |
|---|---|---|---|
| Axial compressive capacity | Higher | 85-95% of solid CFT | Slightly lower |
| Self-weight | Higher | 15-25% reduction | Significantly lighter |
| Capacity-to-weight ratio | Baseline | 10-20% improvement | Superior |
| Ductility | Good | Comparable | Similar |
| Local buckling resistance | High | High | Similar |
| Construction convenience | Standard | Requires inner tube installation | Slightly more complex |
Study Insights and Practical Recommendations
The theoretical framework developed in this paper provides a rigorous basis for the design of circular hollow sandwich CFT columns. However, engineers should be aware of several practical considerations that go beyond the theoretical analysis. The installation of the inner steel tube within the outer tube requires precise alignment and concentricity control, typically within a tolerance of 3 mm for the gap between the two tubes. The concrete pour must be carefully controlled to ensure uniform filling of the sandwich layer without voids, which can be achieved through the use of vibrator access holes spaced at intervals not exceeding 500 mm along the column length.
From a welding perspective, the connection between the inner and outer steel tubes at the column ends is critical. If the two tubes are connected by transverse welds, the welding procedure must be carefully designed to avoid excessive distortion that could compromise the concentricity of the sandwich layer. The welding sequence should follow a symmetrical pattern to minimize angular distortion, and preheat temperatures of 100-150 degrees Celsius are recommended for wall thicknesses exceeding 10 mm.
The unified strength theory approach demonstrated in this paper can be extended to other hollow section configurations, including square and rectangular sandwich tubes, providing a versatile analytical tool for structural engineers working with innovative composite column systems. The research contributes valuable theoretical insights that support the practical application of weight-optimized CFT columns in modern construction.