Zhuojin Pipe Fitting Co., LtdStability Analysis of Large-Span Steel Tube Concrete Arch Bridge During Slewing Construction
Literature Overview
The paper by Xiao Guanghong and Zhang Qiuling, published in the Journal of Chongqing Jiaotong University (Natural Science) (2007, Vol. 26, No. 5, pp. 40-43), presents a stability analysis of a large-span steel tube concrete (STC) arch bridge during the slewing construction phase, using the Yonghe Bridge in Guangxi as a case study. The authors employ MSC.Nastran finite element analysis software to conduct both linear and nonlinear stability analyses and demonstrate that nonlinear analysis is essential for meaningful stability evaluation of such structures. This work is significant because it addresses a critical safety concern in the construction of large-span arch bridges, where the structural behavior during construction differs substantially from the final service condition.
Technical Background
Slewing construction is a method used for large-span arch bridges where the arch ribs are erected in segments on one side of the span and then rotated (slewed) into their final position. During the slewing process, the arch ribs are subjected to complex loading conditions that differ significantly from the final in-service state. The stability of the arch ribs during slewing is a critical design and construction consideration, as loss of stability can lead to catastrophic structural failure.
The Yonghe Bridge, located in Guangxi Province, China, is a large-span STC arch bridge that employs the slewing construction method. The bridge combines the high strength and stiffness of steel tubes with the compressive strength of concrete, creating a composite structural system that offers excellent load-bearing capacity and durability. The steel tubes provide confinement to the concrete, enhancing the compressive strength and ductility of the infill concrete, while the concrete provides fire protection and corrosion resistance to the steel tubes.
Methodology and Analysis Approach
The authors established a finite element model of the Yonghe Bridge using MSC.Nastran, a widely used structural analysis software in the aerospace and civil engineering industries. The model was used to perform two types of stability analysis:
- Linear stability analysis: This approach assumes that the structure remains within the linear elastic regime and that the equilibrium path is straight. It provides the theoretical critical buckling load based on the initial stiffness matrix of the structure.
- Nonlinear stability analysis: This approach accounts for geometric nonlinearity (large displacements and rotations), material nonlinearity (if applicable), and the change in stiffness as the structure deforms. It traces the equilibrium path and identifies the limit point or bifurcation point where stability is lost.
The comparison between linear and nonlinear results reveals significant differences in the predicted critical loads, underscoring the importance of nonlinear analysis for accurate stability assessment.
Key Findings
The analysis results demonstrate that the linear stability analysis significantly overestimates the critical buckling load compared to the nonlinear analysis. This overestimation is attributed to several factors that are not captured by linear analysis:
| Analysis Type | Critical Load Prediction | Accuracy |
|---|---|---|
| Linear stability analysis | Higher critical load | Overestimates stability |
| Nonlinear stability analysis | Lower critical load | More realistic prediction |
The nonlinear analysis reveals that the arch ribs undergo significant geometric nonlinearity during the slewing process, with large displacements and rotations that substantially reduce the structural stiffness. The initial imperfections, including fabrication tolerances and erection deviations, are also captured in the nonlinear analysis, providing a more realistic representation of the actual structural behavior.
Engineering Practice Implications
For engineers involved in the design and construction of large-span STC arch bridges, this study provides several important takeaways:
- Nonlinear analysis is essential: Linear stability analysis should not be used as the sole basis for stability evaluation during construction. Nonlinear analysis that accounts for geometric nonlinearity and initial imperfections is necessary for realistic predictions.
- Construction phase design: The structural design must consider the most unfavorable loading conditions that occur during the construction sequence, not just the final service conditions. The slewing phase may represent the critical condition for stability.
- Monitoring and control: During actual slewing operations, real-time monitoring of structural responses such as displacements, stresses, and strains is essential to ensure that the structure remains within acceptable limits.
The study also highlights the importance of accurate modeling of the structural system, including the boundary conditions, support stiffness, and the interaction between the steel tubes and the concrete infill. In STC arch bridges, the composite action between the steel and concrete is a key factor in the structural performance, and the degree of composite action may vary during the construction sequence as the concrete cures and shrinks.
Study Insights and Reflections
While this paper provides valuable insights into the stability behavior of STC arch bridges during slewing construction, several aspects warrant further discussion. The paper does not provide detailed information on the specific nonlinear analysis parameters used, such as the convergence criteria, the number of load steps, and the treatment of initial imperfections. These parameters can significantly influence the accuracy of the nonlinear analysis results.
Additionally, the study focuses on the Yonghe Bridge as a specific case, and the results may not be directly applicable to other bridge configurations. The stability behavior of STC arch bridges is sensitive to several design parameters, including the span-to-rise ratio, the steel tube dimensions, the concrete grade, and the boundary conditions at the arch springs. A parametric study would provide more generalizable insights.
From a broader perspective, this work underscores the importance of construction phase analysis in the design of complex structures. In modern bridge engineering, the construction sequence is often a critical design factor, and the structural behavior during construction can govern the design in cases where the construction loads are more severe than the service loads. Engineers must ensure that the structural system has adequate stability margins throughout the entire construction sequence, not just in the final service condition.