Zhuojin Pipe Fitting Co., LtdAxial Compression Performance Analysis of Composite Rectangular versus Ordinary Rectangular Steel Tube Concrete Columns
Literature Overview
This paper, published in Journal of Shihezi University (Natural Science Edition) (Vol. 34, Issue 4, 2016, pp. 506-511) by Liu Li, He Mingsheng, and Tang Jianping, investigates and compares the axial compression performance of ordinary rectangular steel tube concrete (SRC) columns and composite (复式) rectangular steel tube concrete columns. The research was supported by the National Natural Science Foundation of China (Project No. 51368052). The study focuses on columns with a large aspect ratio of 1.7, which is a practical concern in many building applications where rectangular column sections are preferred for architectural and space utilization reasons.
Background and Motivation
Steel tube concrete columns combine the compressive strength of concrete with the confinement effect of steel tubes, providing excellent load-bearing capacity and ductility. Circular steel tubes are the most common and well-studied configuration, but rectangular tubes are increasingly used in building construction due to their compatibility with architectural layouts and efficient use of corner spaces.
The challenge with rectangular steel tube concrete columns is the stress concentration at the corners, which leads to premature failure. The authors propose a composite configuration—a rectangular steel tube with an internal circular steel tube (核心圆钢管)—to improve the stress distribution and overall performance. This composite design introduces an additional steel ring that redistributes the internal forces and delays the onset of local buckling.
Experimental Program and Methodology
Specimen Design
The study involved two types of specimens:
- Ordinary rectangular SRC columns: A single rectangular steel tube filled with concrete.
- Composite rectangular SRC columns: A rectangular steel tube containing an internal circular steel tube, with concrete filling the space between the inner and outer tubes.
Both types were designed with the same overall cross-sectional dimensions and the same aspect ratio of 1.7 (long side to short side). The steel tubes were made of common structural steel grades, and the concrete was of a standard high-strength grade suitable for column applications.
Loading and Measurement
The specimens were subjected to monotonic axial compression loading. The load-displacement behavior was recorded, and the failure modes were observed and documented. Strain gauges were likely placed at critical locations to monitor the strain distribution, particularly at the corners of the rectangular section and along the long and short sides.
Key Findings and Technical Analysis
Failure Mode Comparison
The most significant finding is the difference in failure modes between the two column types:
| Feature | Ordinary Rectangular SRC | Composite Rectangular SRC |
|---|---|---|
| Initial failure location | Corners of rectangular section | Short side near column end |
| Primary failure mechanism | Corner cracking followed by long-side bulging | Short-side bulging near ends |
| Failure progression | Localized at corners, then spreads | More distributed, with internal tube providing redistribution |
| Post-peak behavior | Relatively rapid capacity loss | More gradual capacity degradation |
The ordinary rectangular SRC column fails first at the corners due to stress concentration, where the high compressive stress in the concrete combined with the lack of confinement from the steel tube (which is far from the corner in the diagonal direction) leads to premature cracking. After corner failure, the load redistributes to the long sides, which then experience outward bulging as the concrete loses its ability to resist lateral pressure.
The composite column, by contrast, has the internal circular steel tube that provides additional confinement in the central region. This redistributes the internal forces away from the corners and toward the short sides near the column ends, where the failure ultimately occurs. The presence of the internal tube also provides a more uniform confinement pressure across the cross-section, delaying the onset of local buckling.
Load-Bearing Capacity and Ductility
| Parameter | Ordinary Rectangular SRC | Composite Rectangular SRC | Improvement |
|---|---|---|---|
| Ultimate load capacity | Baseline | +12.5% | Significant |
| Ductility | Baseline | +4.9% | Moderate |
| Post-peak capacity retention | Lower | Higher | Improved |
The 12.5% improvement in load-bearing capacity is substantial and practically significant. This improvement is attributed to the additional steel in the internal circular tube and the improved confinement effect. The 4.9% improvement in ductility, while smaller in percentage terms, is important for seismic applications where energy dissipation capacity is critical.
Engineering Practice Implications
Design Considerations
The composite rectangular SRC column concept offers several advantages for practical design:
- Higher load capacity: The 12.5% improvement allows for either larger loads to be carried or smaller cross-sections to be used for the same load, resulting in material savings.
- Improved ductility: The enhanced ductility is particularly valuable for seismic design, where columns must undergo significant inelastic deformation without catastrophic failure.
- Predictable failure mode: The shift from corner failure to short-side failure near the ends provides a more predictable and potentially more damage-tolerant failure mechanism.
Cost-Benefit Analysis
The composite configuration requires additional steel for the internal circular tube, which increases the material cost. However, the improved performance may allow for:
- Reduced concrete volume due to higher confinement efficiency.
- Reduced overall column size for the same load capacity.
- Improved seismic performance that may reduce the need for additional structural measures.
Quality Control Considerations
The composite configuration introduces additional construction challenges:
- The internal circular tube must be precisely positioned within the outer rectangular tube.
- The concrete must be properly placed and compacted in the annular space between the two tubes.
- The connection between the internal and external tubes must be designed to ensure composite action.
Study Insights and Reflections
This research demonstrates that the addition of an internal steel tube to a rectangular SRC column can significantly improve its structural performance. The key insight is that the failure mode shift from corner-dominated to side-dominated is not merely a change in failure location but represents a fundamental improvement in the stress distribution and load path. The internal tube acts as a confinement ring that redistributes the internal forces, creating a more efficient structural system.
The research also highlights an important principle in structural engineering: that the configuration of a structural member can be as important as the material properties. The composite design leverages the geometric arrangement to improve performance without requiring higher-grade materials, which is a cost-effective approach to structural improvement.
For engineers designing SRC columns, this research suggests that the composite configuration should be considered for applications where high load capacity, ductility, and predictable failure behavior are required, particularly in seismic regions. The additional complexity of the composite design is justified by the significant performance improvements demonstrated in this study.