Zhuojin Pipe Fitting Co., LtdVertical Bearing Performance of Large-Diameter Variable-Section Steel Pipe Composite Piles
Literature Overview
This study by Wei Gang, Wang Xin, Cui Yunliang, Zhou Lianying, Wang Haifeng, and Zhou Feng from Zhejiang University and Zhejiang Communications Construction Group investigates the vertical bearing capacity of large-diameter variable-section steel pipe composite piles, using the Yushan Bridge as a case study. Published in the Journal of Disaster Prevention and Mitigation Engineering, Volume 41, Issue 1, 2021, pages 46-54, the research combines self-balancing pile load testing with ABAQUS numerical simulation.
Technical Background and Methodology
The steel pipe composite pile (also known as a steel pipe reinforced concrete pile or steel tube reinforced pile) represents a hybrid foundation system where a steel pipe is embedded within or forms part of the pile shaft. The variable-section design introduces a transition in diameter at a specific depth, which is intended to optimize the load distribution between skin friction and end-bearing resistance.
The Yushan Bridge pile No. 37 served as the test specimen. The self-balancing test method (also called the O-cell method or balanced load test) was employed, which involves installing an O-cell device at a predetermined depth within the pile to measure upward and downward resistance independently. This method is particularly advantageous for large-diameter piles where conventional loading platforms would be impractical or excessively costly.
From a pipe manufacturing perspective, the large-diameter steel pipes used in such composite piles are typically spiral submerged-arc welded (SSAW) or longitudinal submerged-arc welded (LSAW) pipes conforming to SY/T 5037 or GB/T 9711. The variable section transition may involve a manufactured transition fitting or a field-welded enlargement, which presents significant welding challenges.
Key Results
The self-balancing test produced a Q-s curve without abrupt changes, indicating stable load transfer throughout the test. The calculated single pile bearing capacity reached 120,056 kN, which is 3.14 times the maximum design vertical load at the pile head (38,212 kN), indicating adequate safety margin.
The numerical simulation results aligned well with the measured data, validating the modeling approach and parameter selection. The analysis revealed that:
| Parameter | Above Variable Section | At Variable Section | Below Variable Section |
|---|---|---|---|
| Axial force distribution | Gradual decrease | Sudden change (discontinuity) | Rapid decrease |
| Steel pipe stress state | Elastic deformation | Stress concentration | Elastic deformation |
| Load transfer mechanism | Skin friction dominant | Transition zone | End-bearing contribution |
The steel pipe remained in the elastic deformation stage throughout the loading process and did not yield, which is a critical finding for design verification. The end-bearing resistance was confirmed to exist at the pile toe, contributing to the overall capacity.
Engineering Practice Implications
The finding that longer pile segments above the variable section improve vertical deformation resistance introduces an important design trade-off. While increasing the length above the transition point enhances stiffness, it also increases material consumption and fabrication complexity. In practice, engineers must optimize this parameter considering:
- Geotechnical conditions at the transition depth, which determine the skin friction capacity available above the variable section.
- The cost-benefit analysis of additional steel pipe length versus the required settlement control.
- Construction logistics, particularly the handling and driving of extra-long pile sections.
For the welding of the variable-section transition, the following quality considerations apply:
- The transition weld must achieve full penetration with a smooth geometric profile to minimize stress concentration.
- Welding sequence must be carefully planned to control angular distortion, which is particularly critical for large-diameter pipes where even small angular deviations can affect driving performance.
- Post-weld heat treatment (PWHT) may be required for pipe thicknesses exceeding 32 mm to relieve residual stresses and prevent delayed hydrogen cracking, especially when using high-strength pipe grades such as X65 or X70 per API 5L.
- Non-destructive testing should include both ultrasonic testing (UT) for volumetric defects and magnetic particle testing (MT) for surface-breaking defects at the transition weld.
Study Insights
This research demonstrates that the variable-section design is an effective approach for optimizing the vertical bearing performance of large-diameter composite piles. The numerical model validation provides confidence in using FEA for parametric studies of variable-section location. For steel pipe manufacturers and fabricators, the key takeaway is that the transition fitting or field-welded enlargement must be designed with careful attention to weld geometry, residual stress management, and dimensional tolerance control. The 3.14 safety factor achieved in this case study suggests that the design methodology is robust, but engineers should not extrapolate this margin to other geotechnical conditions without site-specific analysis. The study reinforces the importance of integrating numerical modeling with physical testing for the design of complex foundation systems involving large-diameter steel pipes.