Zhuojin Pipe Fitting Co., LtdAxial Compression Performance of Multi-Chamber Square CFST Short Columns
Literature Overview
Li Bin, Luo Kaini, and Gao Chunyan from Inner Mongolia University of Science and Technology and Nanjing University of Aeronautics and Astronautics published this study in the Concrete journal in 2019 (Issue 4, pages 37-40). Funded by the National Natural Science Foundation of China (Grant 51068021), the research investigates the axial compression behavior of multi-chamber square concrete-filled steel tube (CFST) short columns, with chamber quantity and inner-to-outer wall thickness ratio as the primary experimental parameters.
Core Technical Content and Experimental Results
The study tests nine multi-chamber square CFST short column specimens, systematically varying the number of internal chambers and the ratio of internal partition wall thickness to outer tube wall thickness. The fundamental innovation is the introduction of internal chamber plates within the square steel tube, which fundamentally alter the buckling behavior and confinement mechanism of the composite section.
The experimental results reveal a critical finding: conventional square CFST specimens exhibit significant outward bulging before reaching peak load, and after peak load, the bulging rapidly increases leading to weld fracture and specimen failure. This means that the effective confinement provided by the steel tube is not fully utilized in conventional designs.
Comparative Performance of Single-Chamber and Multi-Chamber Specimens
| Performance Indicator | Conventional Square CFST | Multi-Chamber Square CFST |
|---|---|---|
| Pre-peak bulging behavior | Significant outward bulging | Minimal bulging |
| Post-peak failure mode | Rapid weld fracture | Gradual, ductile failure |
| Confinement utilization | Incomplete | Fully utilized |
| Ductility after peak load | Poor | Excellent |
| Material utilization | Limited | Comprehensive |
| Deformation pattern | Large global buckling | Localized small half-wave buckling |
The internal chamber plates transform the large global buckling of the outer tube wall into localized small half-wave buckling patterns. This means that the tube wall does not undergo catastrophic outward deformation, and the specimen does not fail rapidly after reaching peak load. The post-peak ductility is excellent, and the material is fully utilized throughout the loading process.
Optimal Wall Thickness Ratio Determination
| Wall Thickness Ratio | Confinement Effect | Structural Behavior |
|---|---|---|
| 1.0 | Baseline reference | Moderate confinement |
| 1.5 | Optimal performance | Best balance of confinement and tension tying |
| 2.5 | Diminishing returns | Excessive material use without proportional benefit |
The study determines that a wall thickness ratio of 1.5 provides the optimal balance between the external steel tube's confinement of the concrete core and the internal chamber plates' tension-tying action on the outer tube wall. Ratios of 1.0 and 2.5 both perform inferior to the optimal ratio of 1.5.
Steel Pipe Manufacturing and Welding Implications
The multi-chamber CFST design introduces significant fabrication challenges for steel pipe manufacturers. The internal chamber plates must be precisely positioned and welded to the outer tube wall, creating multiple internal weld joints that must be inspected for quality. The welding of internal chamber plates presents unique challenges:
- Access for welding equipment and inspection is limited by the internal geometry of the tube
- Weld spatter and slag removal must be performed in confined spaces
- Heat input control is critical to prevent distortion of the thin internal chamber plates
- Radiographic inspection of internal welds requires specialized techniques or alternative NDE methods
The weld joints between the internal chamber plates and the outer tube wall are critical load transfer points. Under axial compression, these welds must resist the tension-tying action that transfers the confining force from the concrete core through the chamber plates to the outer tube. Any lack of fusion, porosity, or undercut at these welds can lead to premature weld failure and catastrophic loss of confinement effectiveness.
From a steel pipe quality control perspective, the dimensional accuracy of the square outer tube becomes even more critical in multi-chamber designs. Any deviation from squareness or any variation in wall thickness around the perimeter will create non-uniform concrete confinement and may lead to localized buckling at weaker sections. The tolerance requirements for squareness and wall thickness uniformity should be tighter than for conventional single-chamber CFST tubes.
Key Reflections and Study Insights
The multi-chamber concept represents a significant advancement in CFST design methodology, effectively solving the long-standing problem of incomplete confinement utilization in conventional square CFST columns. However, the realization of these theoretical benefits depends entirely on the fabrication quality of the internal chamber plate welds and the dimensional accuracy of the outer tube.
The determination of the optimal wall thickness ratio of 1.5 provides a clear design guideline for steel pipe manufacturers. This ratio dictates the relative thickness of the internal chamber plates compared to the outer tube wall, which has direct implications for material selection, welding procedure qualification, and quality control requirements. Manufacturers should develop specialized welding procedures for internal chamber plate attachment that account for the confined geometry and the critical nature of these weld joints in the overall structural performance.
Summary
This study demonstrates that multi-chamber square CFST short columns achieve significantly improved axial compression performance compared to conventional designs, with the optimal internal-to-outer wall thickness ratio determined as 1.5. The transformation of global buckling into localized small half-wave buckling through internal chamber plates results in excellent post-peak ductility and full material utilization. Steel pipe manufacturers must recognize the elevated quality requirements for multi-chamber CFST fabrication, particularly regarding internal weld quality, dimensional accuracy, and welding procedure qualification, as these factors are directly responsible for realizing the enhanced structural performance demonstrated in this research.