Zhuojin Pipe Fitting Co., LtdExperimental Study on Compression-Bending Performance of Giant CFST Bifurcated Column Joints
Literature Overview
This study, published in the Journal of Beijing University of Technology (2018, Vol. 44, No. 1, pp. 88-96), presents experimental research on the compression-bending performance of giant steel tube concrete (CFST) bifurcated column joints. The research is conducted against the engineering background of the Beijing "China Zun" tower under construction, which represents one of the most challenging structural engineering projects in China. The authors are Yang Guang, Cao Wanlin, Dong Hongying, Yang Weibiao, and Tian Shichuan, affiliated with Beijing University of Technology, Heilongjiang Bayi Agricultural University, and Beijing Institute of Architectural Design and Research. The work is funded by the National Natural Science Foundation of China (Project No. 51578020).
Core Technical Content
The primary research objective is to understand how construction measures influence the bearing capacity of giant CFST bifurcated column joints. Two specimens with different structural configurations were subjected to eccentric compression under cyclic loading. The study examines failure characteristics, bearing capacity, lateral flexural stiffness, lateral deformation, and strain distribution patterns.
Key Experimental Findings
The experimental results reveal several critical observations:
- Failure mode is predominantly bending failure, with the failure location correlated to the stiffness distribution of the bifurcated joint.
- Lateral deformation concentrates primarily in the column branch with larger lateral displacement.
- Before steel tube yielding, the irregular cross-section of the column branch conforms to the plane section assumption; after steel yielding, it approximately satisfies the plane section assumption.
- When lateral construction measure contributions are ignored and only the steel tube confinement effect is considered for bearing capacity calculation, significant discrepancies arise between calculated and measured values.
Technical Parameters and Observations
| Parameter | Observation |
|---|---|
| Failure mode | Bending failure dominant |
| Failure location | Related to joint stiffness distribution |
| Lateral deformation | Concentrated in larger displacement branch |
| Plane section assumption | Valid before yielding, approximately valid after yielding |
| Bearing capacity calculation | Steel tube confinement alone insufficient |
| Combined confinement effect | Steel tube + diaphragm produces stronger constraint |
Engineering Practice Implications
The study carries significant implications for the design of mega-structure joints in super-tall buildings. The China Zun tower, at 528 meters, demands joints that can withstand extreme combined loading conditions. The finding that the combined confinement effect of steel tubes and diaphragms is substantially stronger than steel tube confinement alone suggests that designers must account for both contributions in capacity calculations.
From a welding and fabrication perspective, the bifurcated joint geometry presents considerable challenges. The irregular cross-section requires precise pipe forming, cutting, and welding operations. The weld quality at the bifurcation zone directly influences the stiffness distribution and failure location. Engineers should pay special attention to:
- Weld penetration and fusion quality at the branch-to-column junction
- Residual stress distribution from welding, which may interact with the applied eccentric compression
- Heat-affected zone (HAZ) properties in high-strength steel tubes used in such mega-structures
- Post-weld heat treatment to relieve residual stresses that could affect cyclic loading performance
Key Questions and Reflections
The study raises an important question about the adequacy of current design codes for mega-scale CFST joints. Traditional design methods often simplify the confinement effect to steel tube contribution alone, which this research demonstrates leads to significant underestimation of bearing capacity. The combined effect of steel tube and diaphragm confinement is nonlinear and geometry-dependent, making it difficult to capture in simplified analytical models.
The cyclic loading protocol used in this study is particularly relevant for seismic design considerations. The observation that plane section assumptions hold reasonably well even after steel yielding provides confidence in using standard plastic hinge models for seismic analysis of such joints, provided the HAZ and weld quality are properly controlled during fabrication.
The engineering practice connection is direct: for projects like China Zun, detailed finite element analysis incorporating realistic weld properties and combined confinement effects should supplement code-based calculations. The test results serve as validation benchmarks for such advanced analytical models.
Study Insights
This research represents an important bridge between experimental structural engineering and practical mega-structure design. The emphasis on the combined confinement effect of steel tubes and diaphragms challenges conventional design approaches and calls for more comprehensive analytical models. For steel pipe manufacturers and fabricators, the study underscores the critical importance of joint geometry precision, weld quality, and material uniformity in determining the ultimate performance of these complex structural elements. The work demonstrates that for mega-scale CFST structures, the interaction between steel pipe confinement, concrete behavior, and construction measures cannot be treated independently in design calculations.