Zhuojin Pipe Fitting Co., LtdSpatial Stability Analysis of Large-Span Concrete-Filled Steel Tube Arch Bridges During Concrete Pouring Stage
Literature Overview
This paper by Xu Kaiming and Zhang Mingzhong, published in Road Machinery and Construction Mechanization (2008, Vol. 25, Issue 12), addresses a critical construction-phase stability problem encountered in the erection of large-span concrete-filled steel tube (CFST) arch bridges. Using the Jingyang River Bridge in Hubei Province as the engineering case, the authors established a three-dimensional finite element model in ANSYS to evaluate both linear and nonlinear buckling stability during the in-situ concrete pouring process. The work was supported by the Ministry of Education Doctoral Program Research Fund (20050247029).
Core Technical Content
The fundamental engineering challenge addressed here is that during the concrete pouring stage, the steel arch tube transitions from being a slender, empty steel tube—governed primarily by elastic buckling—to a composite CFST member with dramatically improved stiffness and load-bearing capacity. This transition is not instantaneous; at any given moment during pouring, the arch consists of partially filled and partially empty segments, creating a highly non-uniform structural system whose stability margin varies along the span and through time.
The authors employed a two-pronged analytical approach:
- Linear eigenvalue buckling analysis to identify critical buckling modes and theoretical buckling loads for each construction condition.
- Nonlinear geometric and material stability analysis to capture post-buckling behavior, accounting for initial imperfections, material nonlinearity, and progressive concrete hardening.
The construction sequence was discretized into multiple pouring stages, each representing a distinct structural condition. For each condition, the stability coefficient (safety factor against buckling) was computed, and the corresponding instability mode shapes were extracted.
Key Technical Findings and Interpretation
The most significant finding is that the stability margin is lowest at intermediate pouring stages—not at the beginning (empty tube) nor at the end (fully filled CFST). This counterintuitive result arises because:
- At early stages, the empty steel tube, while slender, has uniform properties and predictable buckling behavior.
- At intermediate stages, the asymmetry of the partially filled arch creates eccentric loading, secondary bending moments, and differential stiffness that reduce the effective buckling resistance.
- The added self-weight of partially poured concrete increases the axial compression without providing proportional stiffness improvement.
| Analysis Aspect | Linear Buckling | Nonlinear Stability |
|---|---|---|
| Geometry | Pristine, no imperfections | Includes initial geometric imperfections |
| Material | Elastic (linear) | Elastic-plastic (bilinear or multilinear) |
| Load path | Eigenvalue problem | Incremental loading with equilibrium iterations |
| Safety factor | Theoretical upper bound | Realistic, lower values |
| Applicable condition | Preliminary screening | Final design verification |
The nonlinear analysis revealed that the actual stability coefficients were significantly lower than the linear eigenvalue results—typically 30% to 50% reduction—confirming that linear analysis alone is insufficient for construction-phase safety assessment of CFST arch bridges.
Engineering Practice Implications
From a steel pipe fabrication and welding quality perspective, this analysis underscores several critical requirements:
- Steel tube dimensional accuracy: Ovality, out-of-straightness, and wall thickness uniformity directly affect the initial imperfection amplitude used in nonlinear buckling analysis. For large-diameter tubes (typically 1500–2000 mm for large-span arch bridges), controlling ovality within ±0.5% of nominal diameter is essential.
- Weld quality: The longitudinal weld seams in LSAW or UOE fabricated tubes must achieve full penetration with no lack of fusion or porosity, as localized weakness at welds reduces the effective section modulus and can trigger premature local buckling.
- Residual stress management: Residual stresses from welding and cold bending (for UOE tubes) interact with the compressive stresses during construction, accelerating instability. Post-weld stress relief or controlled forming sequences are recommended.
Study Insights and Independent Reflection
Having worked extensively with large-diameter fabricated steel tubes for bridge applications, I find the emphasis on the intermediate pouring stage particularly instructive. In practice, construction teams often focus monitoring efforts on the initial erection and the final loading, but the transition phase—where stability is most critical—is frequently under-monitored. I would recommend integrating real-time strain and deflection monitoring at the critical sections identified by the finite element model, with automated alarm thresholds set at 70% of the predicted nonlinear buckling load.
Furthermore, the paper's approach can be extended by incorporating temperature effects during concrete pouring (hydration heat) and wind loading during construction, which are not considered here but are significant in outdoor construction environments. The coupling of thermal expansion of the partially filled tube with structural instability represents a potential failure mode that warrants further investigation.