Nonlinear Analysis of Large-Span Steel Tube Concrete Stiffening Skeleton Arch Bridges

Overview and Analytical Framework

This paper by Yang Bingcheng, Wu Gangrou, and Liu Jian from Chang'an University presents a nonlinear finite element analysis of a large-span steel tube concrete (STC) stiffening skeleton arch bridge, using a real bridge project as the case study. The researchers employed ANSYS to establish a full-bridge three-dimensional model and conducted both linear and nonlinear analyses throughout the entire construction process. The study investigates how section stiffness changes during construction affect the nonlinear behavior of the bridge and evaluates the rationality of the staged construction design.

The stiffening skeleton arch bridge configuration combines the advantages of steel tube concrete for the main arch ribs with a stiffening truss or girder for load distribution and stability. This hybrid system is particularly suitable for large spans where pure concrete arches would be too heavy and pure steel arches would be too flexible.

Key Technical Findings

The nonlinear analysis revealed that the construction process significantly influences the final stress state and deflection profile of the bridge. As the construction progresses through different stages (erection of steel skeleton, concrete infill, stiffening truss installation, and deck loading), the section stiffness changes substantially, which in turn affects the load distribution and internal forces. The nonlinear effects become more pronounced in later construction stages when the structure approaches its final configuration.

The study emphasizes that the rationality of the staged construction design (dividing the arch into segments and rings) directly affects the nonlinear response of the structure. Poorly planned construction sequences can lead to excessive temporary stresses, permanent deformations, or even structural instability during construction.

Implications for Steel Pipe Design and Fabrication

For the steel pipes used as the stiffening skeleton, the nonlinear analysis provides critical design information. The pipes must be designed to withstand not only the final service loads but also the temporary loads and stresses that occur during construction. This often means that the steel pipe cross-sections must be sized for construction-stage forces, which can be more demanding than service-stage forces.

The welding quality of steel pipe connections is paramount in stiffening skeleton bridges. Each connection must transfer the design forces with adequate safety margins, and the weld details must accommodate the thermal expansion and contraction that occurs during construction. Friction welding or flash butt welding may be preferred for field splices of large-diameter steel pipes, while submerged arc welding is typically used for factory-welded joints.

The steel grade selection should consider both the service requirements and the construction sequence. For example, if the steel skeleton must support significant temporary loads before concrete infill, a higher-strength steel grade may be required. However, higher-strength steels often have lower ductility, which must be balanced against the need for adequate energy dissipation capacity.

Engineering Practice Considerations

The study reinforces the importance of construction-stage analysis in the design of large-span STC arch bridges. Engineers should not rely solely on service-stage analysis but must also verify that the structure is safe and stable throughout all construction stages. This requires close coordination between the design engineer, construction engineer, and steel pipe fabricator.

The nonlinear analysis also highlights the importance of accurate material property characterization. The stress-strain behavior of the steel pipe material, including yield strength, strain hardening, and ultimate strength, must be accurately represented in the finite element model. Material test data from the actual production heat should be used rather than nominal specification values.

Study Insights and Implications

The most significant contribution of this paper is its demonstration that construction-stage nonlinear effects can dominate the design of large-span STC stiffening skeleton arch bridges. Engineers who neglect these effects risk designing structures that are inadequate during construction, even if they appear adequate in the final service configuration.

From a steel pipe fabrication perspective, this study emphasizes the need for precise dimensional control and consistent mechanical properties. Steel pipes with variations in wall thickness or yield strength along their length can create unexpected stress concentrations during construction stages. Quality control procedures should include dimensional inspection at multiple locations and mechanical testing at representative points along the pipe length.