Zhuojin Pipe Fitting Co., LtdAxial Compression Capacity of Thin-Walled Square Concrete-Filled Steel Tube Columns
Literature Overview
This experimental and analytical study by Hao Wenxiu, Cui Boyun, and Xu Xiao from Hebei Agricultural University, published in Industrial Construction (2023, Vol. 53, No. 7, pp. 109-115), investigates the axial compression behavior of thin-walled square concrete-filled steel tube (CFT) columns. Ten specimens were tested under axial compression, varying the slenderness ratio, wall thickness, and concrete strength. A finite element model was developed and validated, and simplified calculation formulas were proposed in reference to GB 50936-2014. The research was funded by the Hebei Province Key R&D Program (Grant 21327209D).
Experimental Setup and Results
The ten thin-walled square CFT column specimens were designed to vary three key parameters: slenderness ratio, wall thickness, and concrete strength. The loading process was divided into three stages: elastic, elastic-plastic, and descending. The failure mode was consistent across all specimens: crushing of the core concrete accompanied by circumferential buckling bands on the exterior of the steel tube.
| Parameter | Variation Range | Effect on Capacity |
|---|---|---|
| Slenderness ratio | Multiple values | Higher slenderness reduces capacity |
| Wall thickness | Multiple values | Thicker walls increase capacity |
| Concrete strength | C20 to C50 | Higher strength improves composite efficiency |
The study found that for members with a steel ratio between 0.02 and 0.06, using concrete of grade C20 to C50 significantly improves the composite efficiency of the steel-concrete structure. This is a practically important finding for material selection in thin-walled CFT applications.
Finite Element Analysis
A finite element model was developed using standard software and validated against the experimental results. The model demonstrated good accuracy and reliability in predicting the load-displacement behavior and ultimate capacity of thin-walled square CFT columns.
The finite element analysis revealed that the stability coefficient for different parameter variations can be expressed by a single equation using the regular slenderness ratio as the independent variable. This simplification is valuable for practical design purposes.
Simplified Design Formula
Based on the experimental and analytical results, simplified calculation formulas for the axial compression capacity of thin-walled square CFT columns were proposed, referencing the provisions of GB 50936-2014. The calculated values showed good agreement with the test results.
The key design insight is that for thin-walled CFT columns, the concrete strength has a pronounced effect on composite efficiency, particularly when the steel ratio is in the range of 0.02 to 0.06. This suggests that for thin-walled applications, the economic optimization of concrete grade is more impactful than for conventional thick-walled CFT members.
Fabrication and Welding Implications
For thin-walled square CFT columns, the fabrication and welding considerations are distinct from those of conventional thick-walled CFT members:
- Wall thickness sensitivity: Thin walls are more susceptible to distortion during welding. Pre-heating and interpass temperature control are critical to minimize angular distortion and out-of-plane deformation.
- Welding process selection: For thin-walled sections, GTAW (TIG) or FCAW (flux-cored arc welding) may be preferred over SMAW to achieve better control of heat input and weld geometry.
- Post-weld straightening: Any angular distortion at butt welds in thin-walled columns will reduce the effective section properties and compromise the axial compression capacity. Mechanical or thermal straightening should be performed and verified by dimensional inspection.
- Surface preparation for concrete filling: The internal surface of thin-walled tubes should be clean and free of mill scale or welding slag to ensure proper bond with the concrete fill.
Key Reflections
This study provides valuable quantitative data on the axial compression behavior of thin-walled square CFT columns, a configuration that is increasingly used in modern construction for its material efficiency and architectural flexibility. The finding that concrete strength significantly affects composite efficiency in the thin-walled regime has direct implications for material specification and cost optimization. For welding engineers, the emphasis on dimensional accuracy and distortion control in thin-walled fabrication is reinforced: even small weld-induced distortions can significantly affect the structural performance of thin-walled CFT members.