Stability Analysis of Large-Span Steel Tube Arch Bridge During Rotation Construction

Literature Overview

The paper by Xiao Guanghong and Zhang Qiuling from Chongqing Jiaotong University, published in the Journal of Chongqing Jiaotong University (Natural Science Edition) in 2007 (Vol. 26, No. 5, pp. 40–43), presents a stability analysis of a large-span steel tube concrete arch bridge during its rotation construction phase. Using the Yonghe Bridge in Guangxi as a case study, the authors established a computational model and employed the large-scale spatial analysis software MSC.Nastran to compare linear and nonlinear stability analyses. The central conclusion is that nonlinear analysis is essential for accurately evaluating the stability of large-span steel tube concrete arch bridges during construction, as linear analysis alone may produce misleading results.

Core Technical Findings

The study addresses a critical engineering challenge: the temporary structural state of a steel tube arch during rotation construction is fundamentally different from the final service state. During rotation, the arch is subjected to asymmetric loading, temporary boundary conditions, and potentially unfavorable load paths that can trigger instability modes not present in the final structure.

Linear vs. Nonlinear Stability Analysis

The key finding is that linear stability analysis, which assumes small displacements and elastic material behavior, significantly overestimates the critical buckling load for the arch during rotation construction. This is because the rotation process introduces large geometric deformations that fundamentally alter the stiffness matrix of the structure. The nonlinear analysis, which accounts for geometric nonlinearity (large displacement effects) and potentially material nonlinearity (plasticity in the steel tube), provides a more realistic and conservative estimate of the stability margin.

Technical Analysis of Rotation Construction Stability

Structural Behavior During Rotation

During rotation construction, the steel tube arch is typically erected on one side of the span and then rotated about a pivot point to its final position. This process involves several critical stability considerations:

1. Geometric nonlinearity: As the arch rotates, the deformation of the structure becomes large relative to its dimensions, invalidating the small-displacement assumptions of linear analysis. The stiffness matrix must be updated at each load increment to account for the changed geometry.
2. Boundary condition changes: The support conditions at the pivot and the temporary supports change throughout the rotation process. The transition from one support configuration to another can introduce instability windows.
3. Dynamic effects: Although the study focuses on static stability, the rotation process involves dynamic loading that may trigger transient instability. The quasi-static analysis used in the study is conservative but may not capture dynamic amplification effects.
4. Load eccentricity: The weight of the arch, combined with construction loads (equipment, workers, temporary bracing), creates eccentric loading that can induce torsional instability in the steel tube arch.

Role of MSC.Nastran in the Analysis

MSC.Nastran is a widely used finite element analysis software that supports both linear and nonlinear structural analysis. For this application, the software was configured to perform:

• Linear eigenvalue buckling analysis: To determine the theoretical critical buckling loads under idealized conditions.
• Nonlinear static analysis with geometric nonlinearity: To simulate the actual rotation process and identify the true stability limit.
• Post-buckling analysis: To evaluate the post-buckling behavior and the available stability margin beyond the critical load.

The comparison between linear and nonlinear results reveals that the nonlinear critical load is significantly lower than the linear prediction, highlighting the importance of nonlinear analysis for construction phase stability assessment.

Engineering Practice Implications

For engineers involved in the design and construction of large-span steel tube arch bridges, this study has several important implications:

1. Mandatory nonlinear analysis: Construction phase stability analysis must include nonlinear methods. Linear analysis alone is insufficient and may lead to unsafe designs.
2. Construction monitoring: Real-time monitoring of deflections, strains, and stresses during rotation construction is essential to detect any deviation from the predicted behavior.
3. Temporary bracing design: The temporary bracing and support systems must be designed based on nonlinear analysis results, with adequate safety margins.
4. Construction sequencing: The rotation speed, pause points, and load application sequence must be carefully planned to avoid instability during critical rotation angles.
5. Factor of safety: The factor of safety against buckling during construction should be determined from nonlinear analysis, typically with a minimum value of 1.5 to 2.0 depending on the project risk assessment.

Key Questions and Reflections

A question that arises is whether the study considered the effect of temperature gradients on the steel tube during rotation construction. Solar radiation can create significant temperature differences between the sunlit and shaded surfaces of the steel tube, inducing thermal stresses that may reduce the stability margin. Additionally, the study does not address the effect of wind loading during rotation, which can be a significant lateral load for tall arch structures.

Another consideration is the material behavior of the steel tube. If the steel tube has already been filled with concrete before rotation, the composite action between the steel and concrete must be modeled accurately. If the rotation is performed before concrete infilling, the stability analysis must consider the behavior of the hollow steel tube alone, which has different buckling characteristics.

Study Insights and Implications

This research reinforces a fundamental principle in structural engineering: construction phase stability must be analyzed separately from service phase stability, and nonlinear methods are indispensable for accurate assessment. The Yonghe Bridge case study provides a practical demonstration of how linear analysis can be dangerously optimistic for large-span arch bridges during rotation construction. For future projects, engineers should adopt a rigorous nonlinear analysis workflow that includes geometric nonlinearity, material nonlinearity, and boundary condition changes throughout the construction sequence. The findings also underscore the importance of construction monitoring and real-time feedback to ensure that the actual behavior matches the predicted behavior throughout the rotation process.