Design of Steel Tube Concrete Arch Bridge at Dongdaqiao in Chaoyang

Overview and Context

The paper by Zhang Lianzhen, Huang Qiao, and Yang Dawei from the Harbin Institute of Technology presents the design of the Dongdaqiao main bridge in Chaoyang City, which employs a through-type steel tube concrete tied-arch bridge configuration with spans of 30 m + 120 m + 30 m. Published in the "Journal of Highway and Transportation Research" in 2005, this study provides detailed discussions on overall design, structural characteristics, internal force calculations, and structural design considerations. As a materials and welding specialist, I appreciate the importance of understanding the structural behavior of steel tube concrete (CFST) members, as the material properties and connection details directly influence the long-term performance of such bridges.

Structural Configuration and Design Philosophy

The bridge adopts a mid-deck steel tube concrete tied-arch configuration, which offers several advantages over conventional bridge types. The steel tubes provide confinement to the concrete core, enhancing the compressive strength and ductility of the composite section. The tied-arch arrangement transfers the horizontal thrust from the arch ribs to the deck system through tie rods, eliminating the need for massive abutments to resist horizontal forces.

Key Design Parameters

The design philosophy emphasizes the unified theory of steel tube concrete, where the mechanical parameters (stiffness and strength) account for the confining pressure developed between the steel tube and the concrete core. This confining pressure, often called the "tightening force" or "confinement pressure," develops as the concrete expands laterally under compressive loading and is restrained by the steel tube. The unified theory provides a more accurate representation of the composite behavior compared to simplified approaches that treat the steel and concrete as independent components.

Dynamic Characteristics and Vibration Control

One of the critical aspects highlighted in this paper is the vibration performance of the bridge. Steel tube concrete arch bridges, particularly those with long spans, are susceptible to vehicle-induced vibrations that can affect serviceability and comfort. The design team addressed this concern through several measures:

1. Increasing the stiffness of the deck system to raise the natural frequencies away from typical vehicle excitation frequencies
2. Enhancing the stiffness of transverse bracing between arch ribs to improve torsional rigidity and reduce differential vibration between ribs
3. Incorporating damping mechanisms in the deck-arch connection details
4. Performing detailed dynamic analysis to identify critical vibration modes and frequencies

Internal Force Analysis and Stability

The internal force analysis considers both static and dynamic loading conditions. The arch ribs experience predominantly compressive forces with bending moments arising from eccentric loading and thermal effects. The steel tube concrete composite section provides high load-bearing capacity with relatively slender dimensions, which is advantageous for aesthetic and functional considerations.

The stability analysis addresses:

• Overall stability of the arch structure under asymmetric loading
• Local buckling of the steel tube under compressive stress
• Buckling of slender compression members in the deck truss system
• Lateral-torsional buckling of the arch ribs

The steel tube confinement significantly improves the post-buckling behavior of the concrete core, providing a degree of reserve capacity that enhances structural safety.

Engineering Practice Considerations

From a welding and fabrication perspective, the steel tube concrete arch bridge presents several challenges:

• Steel tube fabrication: The steel tubes must be fabricated with high geometric accuracy to ensure proper fit-up and uniform concrete placement. Longitudinal weld seams, if present, must be thoroughly inspected using non-destructive testing methods such as ultrasonic testing (UT) or radiographic testing (RT).
• Connection details: The connections between steel tubes and other structural elements (tie rods, bracing, deck girders) require careful welding design to accommodate the combined stresses from axial loads, bending moments, and shear forces.
• Concrete placement: The concrete must be placed carefully to ensure full compaction within the steel tube, avoiding voids that could lead to local buckling or reduced load transfer efficiency. Vibratory placement is essential, and the concrete mix design must ensure adequate workability for placement within confined spaces.
• Quality control: Regular inspection of weld quality, concrete strength, and structural geometry is essential throughout the construction process.

Material Selection and Welding Considerations

The selection of steel grade for the arch tubes should consider:

• Adequate yield strength to support the design loads
• Sufficient ductility for seismic performance and post-buckling behavior
• Weldability for fabrication and field erection
• Corrosion resistance appropriate for the environmental conditions
• Compatibility with the concrete mix in terms of thermal expansion and bond strength

For welding, the steel tube connections should be designed to minimize residual stresses and distortion. Preheating may be required for thicker sections or higher-strength steels to prevent cold cracking. Post-weld heat treatment or stress relief may be necessary for critical connections to reduce residual stresses that could affect long-term fatigue performance.

Study Insights and Reflections

This paper provides valuable insights into the design and analysis of steel tube concrete arch bridges. The unified theory approach to mechanical parameter selection is particularly important, as it provides a more realistic representation of the composite behavior. The emphasis on dynamic characteristics is commendable, as vibration performance directly affects serviceability and user comfort.

For engineers involved in the design and construction of CFST structures, the key lessons include:

• The importance of considering the interaction between steel and concrete in both strength and stiffness calculations
• The need for detailed dynamic analysis to identify and mitigate vibration issues
• The critical role of connection design in overall structural performance
• The necessity of rigorous quality control during fabrication and construction

The paper also highlights the importance of stability analysis in the design of slender arch structures. The steel tube confinement provides beneficial post-buckling behavior, but the design must still ensure adequate safety margins against various buckling modes.

In summary, this literature serves as an excellent reference for engineers involved in the design of steel tube concrete arch bridges. The comprehensive treatment of structural analysis, dynamic characteristics, and stability considerations provides a solid foundation for the design of similar structures. The emphasis on the unified theory of steel tube concrete and the practical measures taken to improve dynamic performance are particularly noteworthy and should be considered in future design projects.