Zhuojin Pipe Fitting Co., LtdEccentric Compression Behavior of Square Steel Tube Concrete Columns with Built-in High-Strength Angle Steel
Research Background and Motivation
This 2021 paper published in the Journal of Architecture and Civil Engineering investigates the eccentric compression performance of square steel tube concrete (STC) columns incorporating built-in high-strength angle steel members. The research was conducted at the State Key Laboratory of Subtropical Building Science, South China University of Technology, and was supported by the National Key R&D Program and the National Natural Science Foundation of China. The study addresses a practical engineering challenge: how to optimize the use of steel in composite columns to achieve desired structural performance while maintaining material efficiency. This topic is directly relevant to steel pipe and structural steel engineering because it examines the interaction between steel tube components and supplementary steel reinforcement under complex loading conditions.
Experimental Program
The study involved the fabrication and testing of 10 specimens: 8 columns with built-in high-strength angle steel and 2 traditional square STC columns without built-in angle steel. The specimens were subjected to eccentric compression loading to evaluate their load-carrying capacity, deformation behavior, and ductility characteristics.
Test Parameters
| Parameter | Range | Number of Levels |
|---|---|---|
| Steel ratio (angle steel mass / total steel mass) | 15% to 35% | 3 levels |
| Net gap between angle steel and tube inner wall | 20 mm to 60 mm | 3 levels |
| Eccentricity ratio (e/h) | 0.1 to 0.3 | 3 levels |
Specimen Configuration
The square steel tubes used in the specimens were fabricated from structural steel conforming to Chinese standards. The high-strength angle steel was positioned symmetrically within the steel tube, with a controlled net gap between the angle steel legs and the tube inner walls. The concrete was cast within the tube after the angle steel was installed, creating a composite cross-section. The steel ratio, defined as the mass of angle steel divided by the total steel mass (tube plus angle steel), was the primary variable controlling the distribution of steel between the tube and the internal reinforcement.
Experimental Results and Key Findings
Load-Carrying Capacity
The experimental results demonstrated that, when the total steel mass is kept approximately constant, the eccentric compression load-carrying capacity of columns with built-in high-strength angle steel can be made comparable to that of traditional STC columns without built-in angle steel. This finding is significant because it suggests that the distribution of steel between the tube and internal reinforcement can be optimized without sacrificing structural capacity.
However, the study also revealed that the net gap between the angle steel and the tube inner wall has a more pronounced effect on load-carrying capacity than the steel ratio itself. This is attributed to the influence of the gap on the confinement effectiveness and the bond development between the concrete and the angle steel. A smaller gap promotes better composite action but may complicate concrete placement and compaction.
Stiffness and Ductility
When the total steel mass was held constant, the equivalent stiffness and ductility of the built-in angle steel specimens showed limited variation compared to the traditional STC specimens. This finding indicates that the redistribution of steel between the tube and internal reinforcement does not significantly alter the overall deformation characteristics of the column. The ductility, characterized by the displacement at peak load relative to the yield displacement, remained within an acceptable range for all tested configurations.
Steel Utilization
A particularly important finding was that, at the peak eccentric compression load, the high-strength angle steel in the compression zone (and/or the tension zone) had fully utilized its strength capacity. This confirms that the built-in angle steel is not merely a passive reinforcement but actively participates in load carrying, reaching its full material potential before the column fails. This efficient utilization of high-strength steel is a key advantage of the proposed column configuration.
Practical Calculation Method
The authors proposed a practical calculation method for the eccentric compression load-carrying capacity of the built-in angle steel STC columns. The method accounts for the composite action between the steel tube, the concrete core, and the built-in angle steel, incorporating the confinement effect of the tube on the concrete and the contribution of the angle steel to both compression and tension resistance.
| Component | Contribution to Capacity | Calculation Approach |
|---|---|---|
| Steel tube | Confinement of concrete, direct load sharing | Modified concrete stress-strain model with confinement |
| Concrete core | Primary compressive load carrier | Confined concrete model with enhanced strength and ductility |
| Built-in angle steel | Additional compression/tension capacity | Elastic-plastic material model |
| Composite interaction | Enhanced overall performance | Equilibrium and compatibility conditions |
The proposed method was validated against the experimental results and showed good accuracy, with predictions generally within acceptable engineering tolerances of the measured values.
Engineering Practice Implications
From a steel pipe and structural steel engineering perspective, this study offers several practical insights:
- Material optimization: The study demonstrates that high-strength steel can be efficiently utilized in composite column design, allowing for material savings without compromising structural performance. This is particularly relevant in applications where steel cost is a significant design driver.
- Fabrication considerations: The installation of built-in angle steel within a square steel tube requires careful fabrication and assembly. The controlled net gap between the angle steel and the tube wall must be maintained during fabrication to ensure proper concrete placement and composite action. Welding connections between the angle steel and the tube, if used, must be designed and executed to avoid compromising the tube wall integrity.
- Quality control: The eccentric compression test results highlight the importance of ensuring proper concrete placement and compaction within the tube, particularly in the regions adjacent to the built-in angle steel. Incomplete concrete filling or voids near the angle steel would significantly reduce the confinement effectiveness and composite action.
Study Reflection
This research represents a thoughtful approach to optimizing composite column design through the strategic use of high-strength steel. The finding that the net gap between the angle steel and the tube wall is more influential than the steel ratio itself is a practical insight that can guide fabrication and assembly decisions. The demonstrated full utilization of high-strength steel at peak load validates the engineering concept and provides confidence in the proposed calculation method. For steel pipe manufacturers and structural engineers, this study highlights the potential for innovative composite designs that leverage the strengths of different steel products within a unified structural system.