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

Literature Overview

The 2020 paper by Cui Yunliang and colleagues, published in the Journal of Central South University, investigates the bearing characteristics of ultra-long large-diameter variable cross-section steel tube composite piles using distributed fiber optic sensing technology and ABAQUS finite element simulation. The study focuses on Pile No. 45 of the Yushan Bridge, utilizing the self-weight of the pier and box girder as the loading mechanism for static load testing. This research is highly relevant to pipeline and steel tube engineering because it addresses the behavior of large-diameter steel tubes under complex loading conditions, including the effects of variable cross-section (step changes in diameter or wall thickness) on load distribution and bearing capacity.

Core Technical Findings

The study demonstrates several important findings regarding the behavior of variable cross-section steel tube composite piles:

The finding that the axial force exhibits a sudden change at the variable cross-section location is particularly significant from a welding perspective. This step change in axial force creates a stress concentration at the transition zone, which is typically welded. The weld at the variable cross-section transition is therefore a critical location that requires special attention in terms of welding procedure, NDT coverage, and residual stress management.

Welding Challenges at Variable Cross-Section Transitions

The variable cross-section in large-diameter steel tube composite piles typically involves a transition from a smaller diameter tube to a larger diameter tube, or from a thinner wall to a thicker wall. This transition is achieved through welding a transition sleeve or collar, creating a dissimilar joint in terms of geometry and potentially material. The welding challenges include:

• Geometric discontinuity: The change in diameter or wall thickness creates a geometric discontinuity that acts as a stress concentrator. The stress concentration factor at the weld toe can reach 2.0-3.0 under axial loading, significantly increasing the risk of fatigue crack initiation.
• Thermal mismatch: The different wall thicknesses on either side of the weld lead to asymmetric cooling rates, which can cause angular distortion and uneven residual stress distribution. For a transition from 20 mm to 30 mm wall thickness, the thinner section cools faster, creating a differential contraction that pulls the joint into an angular misalignment.
• Heat input control: The thicker section requires higher heat input to achieve full fusion, while the thinner section is susceptible to burn-through and excessive HAZ grain growth. A multi-pass welding procedure with carefully controlled heat input per pass is essential.
• Welding position: The transition weld is often in a vertical or overhead position, which increases the difficulty of maintaining consistent weld geometry and fusion quality.

Fiber Optic Monitoring and Weld Quality Verification

The use of distributed fiber optic sensing (DAS) for monitoring the pile provides a unique opportunity to verify welding quality in situ. The strain profile measured along the pile can reveal anomalies at weld locations:

1. Strain discontinuity: A sudden change in strain at a weld location may indicate lack of fusion or reduced effective cross-sectional area at the weld.
2. Strain concentration: Elevated strain at a weld location compared to the surrounding base metal indicates a stress concentration effect, which may be due to geometric discontinuity or weld defects.
3. Strain asymmetry: If the strain profile is asymmetric about the weld centerline, it may indicate angular distortion or uneven residual stress from the welding process.

For engineering practice, I recommend that fiber optic monitoring data be used as a supplementary quality control tool for weld verification, particularly for large-diameter steel tube piles where traditional NDT methods may have limited accessibility. The strain profile can be compared with finite element predictions to identify weld locations that require additional inspection or repair.

Standards and Quality Control Considerations

The bearing characteristics of steel tube composite piles are governed by the welding quality at critical locations, including the pile tip weld, the variable cross-section transition weld, and any splice welds along the pile length. The following standards and requirements are relevant:

For the variable cross-section transition welds, I recommend 100% UT inspection per GB/T 11345 with phased array technique, supplemented by 100% MT inspection of the weld toe region. The acceptance criteria should be stricter than standard requirements, with no lack of fusion, no undercut exceeding 0.5 mm, and no porosity exceeding 20% of the weld throat area.

Key Questions and Reflections

The paper raises an important question about the optimal location of the variable cross-section transition. The finding that the transition location affects pile bearing performance suggests that there is an optimal position that maximizes capacity while minimizing stress concentration. From a welding perspective, the optimal transition location should also minimize the difficulty of welding, favoring positions where the weld can be performed in a favorable position (flat or horizontal) rather than vertical or overhead.

Another consideration is the effect of the shear rings on the welding sequence and residual stress state. The shear rings, which enhance lateral stiffness, are typically welded to the inner surface of the steel tube. The welding of multiple shear rings creates a complex residual stress field that interacts with the axial stress from the pile loading. A systematic welding sequence, starting from the pile tip and progressing upward, should be adopted to minimize distortion and ensure that the residual stress state is compatible with the expected loading conditions.

Study Insights and Engineering Practice Integration

This research provides valuable insights into the behavior of large-diameter variable cross-section steel tube composite piles, with direct implications for welding design and quality control. The key takeaway for welding engineers is that the variable cross-section transition weld is a critical location that requires special attention in terms of welding procedure, NDT coverage, and residual stress management. The fiber optic monitoring technology demonstrated in the study offers a promising supplementary tool for verifying weld quality in situ. For future projects involving large-diameter steel tube composite piles, I recommend that the welding quality assurance plan include: (1) a qualified WPS specifically developed for the variable cross-section transition weld, with rigorous heat input control and preheating requirements; (2) 100% PAUT inspection of all transition welds with acceptance criteria stricter than standard requirements; (3) fiber optic monitoring data analysis as a supplementary weld verification tool; and (4) a systematic welding sequence plan based on FEA simulation to minimize distortion and residual stress at critical locations.