Vertical Bearing Capacity of Large-Diameter Variable-Section Steel Tube Composite Piles

Literature Overview

Published in the Journal of Disaster Prevention and Mitigation Engineering (2021, Vol. 41, No. 1), this study examines the vertical bearing performance of large-diameter variable-section steel tube composite piles using both field testing and numerical simulation. The research is based on Pile No. 37 of the Yushan Bridge project, employing the self-balancing pile test method (SBT) and ABAQUS finite element analysis to characterize load transfer mechanisms and the influence of section transition location on pile performance.

Core Technical Content

Self-Balancing Test Method and Results

The self-balancing test method (SBT) was applied to Pile No. 37, which features a large-diameter variable cross-section design — meaning the pile transitions from a larger diameter section at the upper portion to a smaller diameter section at the lower portion. The Q-s curve obtained from the SBT test showed no abrupt changes and exhibited a relatively smooth progression, indicating uniform load transfer along the pile shaft without sudden load redistribution.

Axial Force and Stress Distribution

The numerical simulation revealed that the axial force decreases gradually above the section transition point, exhibits a sudden drop at the transition location, and decreases more rapidly below the transition. This pattern indicates that the section change creates a localized load transfer zone where soil resistance mobilizes more aggressively. Importantly, the steel tube remained in the elastic deformation stage throughout the loading process and did not reach yielding, confirming that the structural safety margin is adequate.

Influence of Section Transition Position

Parametric studies varying the location of the section transition demonstrated that a longer upper section (above the transition) provides greater resistance to vertical deformation. However, the study notes that in practical production, the material quantity must be balanced against performance to achieve an optimal design — simply extending the larger diameter section increases material consumption without proportionally increasing bearing capacity.

Engineering Practice Implications

For steel pipe manufacturing and welding operations, the variable-section composite pile design requires special attention to the transition zone. This zone represents a geometric discontinuity where the pipe diameter changes, and it must be fabricated through either:

• Flash butt welding of two different-diameter pipe sections, requiring precise alignment and controlled heat input to avoid distortion at the interface.
• Tapered pipe rolling with a gradual diameter transition, which eliminates weld discontinuities but demands specialized rolling equipment.
• Butt welding with transition fittings (reducers), where the welding procedure specification (WPS) must account for the mismatch in wall thickness and diameter.

The finding that the steel tube remains elastic under service loads is reassuring from a quality control standpoint — it means that weld defects that would cause premature failure under cyclic or fatigue loading are less critical here. However, for long-term durability, the weld quality at the section transition remains a concern, particularly regarding hydrogen-induced cracking in thick-walled sections.

Key Technical Points for Practitioners

• The SBT method proved effective for characterizing the bearing capacity of variable-section piles without requiring excessive test equipment.
• The section transition creates a localized load transfer zone, with axial force exhibiting a step-change at the transition point.
• The steel tube remains in the elastic range under design loads, providing a safety factor of approximately 3.14.
• Longer upper sections improve deformation resistance but increase material cost — an optimization problem exists between performance and economy.
• The numerical model validated against field data confirms the reliability of FEM-based parametric studies for pile design optimization.

Study Insights and Reflections

This study demonstrates that variable-section steel tube composite piles offer a practical solution for large-diameter pile foundations where the bearing capacity requirement varies along the pile length. The self-balancing test method is particularly valuable for field verification in deep foundation projects where traditional load test rigs are impractical. From a welding engineering perspective, the section transition zone represents a critical quality control point — the weld or mechanical connection must be designed to handle the stress discontinuity inherent in the geometry change. Engineers should pay particular attention to the welding procedure specification for this transition, ensuring that the heat-affected zone does not develop residual stresses that could compromise long-term performance under sustained loading conditions.