Zhuojin Pipe Fitting Co., LtdAxial Compressive Capacity of Inner-Circular Outer-Square Composite Steel Tube Concrete Short Columns
Literature Overview
The study by Zhang Yang, Qian Jiaru, Ji Xiaodong, and Cao Wanlin (2011), published in World Information on Earthquake Engineering (Volume 27, Issue 3, pages 40-46), investigates the axial compressive behavior of a novel composite column configuration: an inner circular steel tube surrounded by an outer square steel tube, with concrete filling the annular space between them. The research was supported by the Beijing Science and Technology Plan Major Project (D09050600370000), the Beijing Education Commission Key Project (KZ200910005008), and a Tsinghua University Independent Research Project (20101081766). Ten specimens were tested under concentric axial compression at the Ministry of Education Key Laboratory of Structural Safety and Durability, Department of Civil Engineering, Tsinghua University.
Core Technical Content
The experimental program examined the load-bearing capacity, failure modes, and post-peak ductility of inner-circular outer-square composite SRC short columns. The key findings challenge conventional assumptions about confinement mechanisms in composite columns.
Key Experimental Results
| Observation | Detail |
|---|---|
| Peak load state | Square tube longitudinally yielded, transversely not yet yielded |
| Failure mode | Square tube outward bulging, longitudinal local tearing |
| Concrete condition at failure | Loosened and locally crushed between square and circular tubes |
| Post-peak ductility | Most specimens maintained ≥70% of peak load at average compressive strain of 0.11 |
| Stiffness prediction accuracy | Calculated compressive stiffness averaged 83.6% of measured values |
Confinement Assumption Analysis
A significant finding is that the assumption providing only axial load capacity from the steel tubes without considering lateral confinement produced the best agreement with test results. This suggests that the interaction between the inner circular tube and the outer square tube does not create an effective confinement mechanism for the intervening concrete.
Technical Interpretation and Engineering Practice Relevance
Steel Pipe Manufacturing Implications
The composite configuration described requires two distinct steel tube products: a circular seamless or welded tube (inner) and a square hollow section (outer). The manufacturing quality of both components is critical.
- Circular inner tube: Typically seamless (conforming to GB/T 8162 or ASTM A53) or ERW welded (conforming to GB/T 3091 or ASTM A53), with wall thickness tolerance requirements of ±10% per EN 10210.
- Square outer tube: Cold-formed square hollow section (conforming to GB/T 6728 or EN 10219), where the corner radius and wall thickness uniformity significantly affect buckling behavior.
The welding quality of the square tube — particularly at the longitudinal and transverse seams — becomes critical because the failure mode involves longitudinal local tearing of the square tube. Any pre-existing weld defects, lack of fusion, or undercuts could initiate this tearing mode prematurely.
Design and Analysis Recommendations
- The assumption that steel tubes provide only axial capacity (no confinement contribution) should be adopted in design calculations for this specific configuration to avoid overestimating capacity.
- The 83.6% stiffness prediction accuracy indicates that conventional stiffness models slightly overestimate the actual compressive stiffness, which is conservative for serviceability but should be noted for dynamic analysis.
- The post-peak ductility (maintaining 70% of peak load at 0.11 strain) is favorable for seismic applications, but the failure mode (longitudinal tearing) is brittle in nature and should be addressed through detailing.
Study Insights and Reflections
This study provides valuable experimental data on a hybrid composite configuration that has received limited attention in the literature. The finding that the conventional confinement assumption overestimates capacity is particularly important for engineers who might otherwise apply standard SRC confinement models to this configuration without justification. The failure mode observed — longitudinal tearing of the square tube — suggests that the square tube's transverse restraint capacity is insufficient to prevent local buckling of the longitudinal faces. For pipe manufacturers, this underscores the importance of ensuring uniform wall thickness and high-quality welds in square hollow sections intended for structural composite applications. The research also opens questions about whether alternative composite geometries might better exploit the confinement potential of multiple steel tubes.