Bearing Characteristics of Large-Diameter Variable-Section Steel Tube Composite Piles Based on Fiber Optic Monitoring

Literature Overview

The paper by Cui Yunliang, Wang Xin, Zhou Lianying, Wang Haifeng, and Zhou Feng, published in the Journal of Central South University (Volume 51, Issue 6, 2020, pages 1627–1636), presents a comprehensive study on the bearing characteristics of super-long large-diameter variable-section steel tube composite piles using distributed fiber optic monitoring technology. The research focuses on Pile No. 45 of the Yushan Bridge, where static load tests were conducted using the self-weight of the pier and box girder as the load, with distributed fiber optic sensors providing full-length monitoring of pile behavior.

Core Technical Approach

The study employs a combination of field monitoring and numerical simulation to characterize the behavior of variable-section steel tube composite piles. The distributed fiber optic monitoring system provides continuous strain measurements along the entire pile length, offering a level of detail that conventional discrete sensors cannot achieve. The ABAQUS finite element model was calibrated against the measured data to validate the modeling parameters.

Key findings on bearing characteristics:

The parametric study revealed that increasing steel tube thickness and reducing shear ring spacing both enhance the lateral stiffness of the composite pile.

Steel Tube Design and Fabrication Considerations

From a steel pipe manufacturing perspective, this research provides valuable insights into the design of large-diameter steel tubes for composite pile applications:

1. Variable-section transition: The sudden change in axial force at the variable-section transition creates stress concentrations that require careful design of the transition detail. The steel tube diameter change should be gradual or reinforced with transition rings to avoid stress concentrations.
2. Wall thickness optimization: Thicker steel tubes enhance lateral stiffness but may inhibit side friction resistance development. An optimal wall thickness must balance these competing effects.
3. Shear ring spacing: The spacing of shear rings (or stiffening rings) significantly affects the composite action between the steel tube and the surrounding soil-concrete composite. Closer spacing improves load transfer but increases fabrication complexity and cost.
4. Material selection: For marine environments, the steel tube must resist corrosion from seawater. Options include:

• Weathering steel (e.g., Q355NH per GB/T 4171)
• Duplex stainless steel for critical applications
• Corrosion-resistant coatings with cathodic protection

Welding Quality Requirements for Composite Pile Connections

The fabrication of large-diameter steel tube composite piles involves critical welding operations:

The welding of shear rings to the steel tube inner or outer surface requires careful attention to weld geometry and residual stress. Excessive welding heat input can cause local distortion of the steel tube, affecting the composite action with the surrounding concrete and soil. The welding sequence should be planned to minimize cumulative distortion, and post-weld stress relief may be required for critical connections.

Fiber Optic Monitoring and Quality Assurance

The successful application of distributed fiber optic monitoring for pile behavior assessment has implications for steel pipe quality control:

1. The monitoring data can be used to verify that the steel tube is performing as designed
2. Any anomalous strain patterns may indicate fabrication defects such as incomplete welds or material discontinuities
3. The long-term monitoring data can be used to assess the corrosion rate of the steel tube in the marine environment
4. The monitoring system can detect early signs of structural distress, enabling preventive maintenance

Study Insights and Implications

This research demonstrates that variable-section steel tube composite piles are a viable solution for super-long pile applications in marine environments, and that distributed fiber optic monitoring provides an effective means of characterizing their behavior. For steel pipe manufacturers, the key design parameters are steel tube wall thickness, shear ring spacing, and the geometry of the variable-section transition. The finding that the upper half of the pile experiences inhibited side friction resistance due to the steel tube presence suggests that the steel tube design should be optimized to minimize this effect, possibly through surface roughening or the use of friction-enhancing coatings. The significant contribution of pile tip resistance to the overall bearing capacity highlights the importance of pile driving quality and the integrity of the steel tube at the pile toe. Engineers should ensure that the steel tube maintains its structural integrity throughout the driving process, as any damage at the pile toe can significantly reduce the bearing capacity.