Stability and Ultimate Bearing Capacity of Large-Span CFST Arch Bridges

Literature Overview

This paper by Yan Quansheng from South China University of Technology and Xu Shengqiao from the Railway Professional Design Institute was published in 2003 in the journal Railway Standard Design. Using the Maocaojie Grand Bridge in Nanxian, Hunan Province as a case study, the authors analyzed the elastic stability and ultimate bearing capacity of large-span steel tube concrete (CFST) arch bridges during both construction and operational stages. The work is notable for its consideration of staged construction effects, progressive section formation, and the ultimate capacity of individual structural members.

Structural Characteristics of CFST Arch Bridges

Large-span CFST arch bridges combine the advantages of steel tube construction and concrete infill, providing high structural efficiency, good ductility, and excellent resistance to local buckling. The steel tube provides formwork during construction, confinement to the concrete core, and structural continuity, while the concrete core provides compressive strength, thermal mass, and corrosion protection.

Typical Design Parameters

Elastic Stability Analysis

The elastic stability analysis considered the staged construction process, recognizing that the structural stiffness changes progressively as each section of the arch is erected and concreted. During construction, temporary supports and partial structural stiffness significantly influence the stability behavior, and the elastic critical load may be lower than that of the completed structure.

Construction Stage Effects

The analysis revealed that the elastic critical load during the construction stage, particularly when temporary supports are being removed, can be significantly lower than the operational stage critical load. This finding has direct implications for construction sequencing and temporary support design, requiring engineers to ensure adequate stability margins at all stages of construction.

Ultimate Bearing Capacity Analysis

The ultimate bearing capacity analysis incorporated nonlinear material behavior, geometric nonlinearity, and the progressive formation of cross-sections during construction. The analysis accounted for the interaction between the steel tube and concrete core, including the confinement effect of the steel tube on the concrete and the shear transfer at the steel-concrete interface.

Key Findings on Ultimate Capacity

1. The ultimate bearing capacity of the arch ribs is governed by the combined action of steel tube yielding and concrete crushing, with the steel tube providing confinement that enhances the concrete's compressive capacity beyond its unconfined strength.
2. The staged construction process affects the residual stress distribution within the arch ribs, which can reduce the ultimate capacity by 5-15% compared to a monolithic construction assumption.
3. The slenderness ratio of individual arch rib segments is a critical factor in determining the buckling mode and ultimate capacity, particularly for the haunch regions where bending moments are largest.

Engineering Practice Integration

The case study of the Maocaojie Grand Bridge provides practical guidance for the design of similar large-span CFST arch bridges. The following engineering recommendations emerge from the study:

• Construction sequencing should be carefully planned to maintain adequate stability margins at all stages, with particular attention to the temporary support removal phase.
• The progressive section formation effect should be incorporated into stability analyses, as the simplified assumption of a fully formed cross-section can be unconservative for construction stage design.
• Welding quality of the steel tube segments is critical to achieving the assumed composite action between the steel tube and concrete core, and rigorous welding procedure qualification and inspection are essential.
• The arch rib cross-section should be designed to accommodate the combined effects of elastic stability and ultimate capacity, with the governing case often being the construction stage rather than the operational stage.

Study Insights and Outlook

This research represents an early but important contribution to the structural analysis of large-span CFST arch bridges, addressing the complex interaction between construction staging and structural stability. The methodology adopted, which combines elastic stability analysis with nonlinear ultimate capacity analysis across construction stages, provides a comprehensive framework for evaluating the safety of CFST arch bridges.

For engineers involved in the fabrication and welding of CFST arch bridge components, the study highlights the importance of ensuring uniform material properties and weld quality throughout the steel tube segments, as variations in material behavior can significantly affect the stability and ultimate capacity of the arch structure. The progressive section formation effect also has implications for welding schedule planning, as the residual stress state from sequential welding of individual segments can influence the overall structural response.

The study's findings remain relevant to contemporary practice, particularly as large-span CFST arch bridges continue to be constructed for transportation infrastructure worldwide. Future research should extend the analysis to include fatigue performance under traffic loading, seismic response of the composite arch structure, and long-term degradation effects such as corrosion of the steel tube and carbonation of the concrete core.