Zhuojin Pipe Fitting Co., LtdCyclic Axial Compression Performance of Irregular Multi-Chamber CFST Columns
Literature Overview
The study by Yin Fei, Xue Suduo, Cao Wanlin, Dong Hongying, and Wu Haipeng from Beijing University of Technology investigates the cyclic axial compression behavior of irregular multi-chamber concrete-filled steel tube (CFST) columns, using the Z15 Building in Beijing as a prototype structure. The research is funded by the National Natural Science Foundation of China (grants 51578020 and 51808014) and published in the Journal of Harbin Institute of Technology, Volume 51, Issue 12, 2019, pages 94-103. Three large-scale specimens were tested under cyclic axial loading to evaluate the influence of internal rebar cages and internal circular steel tubes on structural performance.
Core Technical Content
The Z15 Building features an irregular multi-chamber steel-concrete composite column system that deviates significantly from conventional circular or square CFST columns. The authors designed three full-scale specimens with different internal configurations to isolate the contribution of each structural element. The key experimental variables included the presence or absence of an internal rebar cage and an internal circular steel tube within the multi-chamber hollow section.
From a steel pipe manufacturing perspective, the internal circular steel tubes used in these columns would typically be seamless steel tubes or high-frequency welded (HFW) tubes conforming to standards such as GB/T 8163 or GB/T 9948. The multi-chamber outer shell represents a complex fabricated steel component, likely involving longitudinal submerged-arc welded (LSAW) plate construction or precision-rolled sections. The welding connections between the internal and external steel elements are critical to overall structural integrity, as they must transfer shear forces and prevent relative slip between the concrete core and the steel shell under cyclic loading.
Key Findings and Engineering Implications
The experimental results reveal several important conclusions with direct implications for structural steel design and fabrication:
- The internal rebar cage effectively improves initial stiffness and load-bearing capacity, but its contribution is primarily through reinforcing the concrete core rather than enhancing the steel tube interaction.
- The internal circular steel tube further improves initial stiffness and load capacity, while significantly enhancing ductility, cumulative energy dissipation, and post-yield performance.
- All international codes (ACI 318, Eurocode 4, GB 50017, AS 4100) produce calculated axial capacities significantly lower than experimental values because none account for the contribution of cross diaphragms or the different yielding mechanisms of internal and external steel plates.
This last finding is particularly significant for design engineers. The code-based capacity calculations systematically underestimate the actual performance of multi-chamber CFST columns, which means that existing design methodologies may be overly conservative for such complex geometries. This conservatism can lead to unnecessary material usage and increased project costs, or conversely, if engineers attempt to "correct" for this by reducing member sizes, they may introduce safety risks.
Welding and Fabrication Considerations
From a fabrication standpoint, the multi-chamber CFST column presents several welding challenges:
| Fabrication Aspect | Technical Requirement | Potential Defect | Countermeasure |
|---|---|---|---|
| Outer shell longitudinal weld | Full-penetration SAW, minimum 100% UT per GB/T 11345 | Lack of fusion, undercut | Preheat to 100-150°C for plates >25 mm; use back-gassing |
| Cross diaphragm attachment weld | Fillet weld with full throat, MT inspection | Cracking at weld toe | Control interpass temperature <250°C; post-weld bake if required |
| Internal tube to outer shell | Slot welds or continuous fillet welds | Incomplete penetration | Use GTAW for root pass; control weld sequence to minimize distortion |
| Rebar cage to steel shell | Welded or mechanical couplers | Heat-affected zone embrittlement | Prefer mechanical couplers near steel shell; if welding, use low-hydrogen electrodes |
The cyclic loading nature of the test imposes fatigue considerations on all welds. The weld toe regions, particularly at cross diaphragm connections, are susceptible to fatigue crack initiation under repeated loading. In seismic design applications, these details must be evaluated according to ductile fracture criteria as outlined in AISC 341 or the equivalent Chinese standard JGJ 166.
Study Insights
The research highlights a critical gap in current design codes regarding complex multi-chamber CFST systems. The authors recommend that future research develop a dedicated capacity calculation method that explicitly accounts for cross diaphragm contributions and the differential yielding of inner and outer steel plates. For practicing engineers, the key takeaway is that the inclusion of an internal circular steel tube is not merely an incremental improvement but a transformative enhancement to ductility and energy dissipation—properties that are essential in seismic zones. The fabrication of such columns demands rigorous weld quality control, with particular attention to the cyclic loading performance of all welded joints.