Steel Tube Concrete Structures in Urban Bridge Applications

Literature Overview

This paper by Guo Jinqiong and colleagues from Fuzhou University presents a review of steel tube concrete (CFST) applications in urban bridge design, with particular emphasis on middle-through arch bridges. Published in the Journal of Fuzhou University (Natural Science Edition) in 1996 (Vol. 24, No. 4), the study reflects the early-stage adoption of CFST technology in Chinese urban bridge engineering. Supported by the Fujian Provincial Natural Science Foundation, the research documents practical design experiences and highlights the advantages of CFST arch bridges in urban environments.

Core Technical Content and Design Philosophy

The paper introduces several urban bridges designed in recent years that employ CFST middle-through arch configurations. The design philosophy emphasizes three integrated considerations:

1. Aesthetic form: The arch geometry provides elegant structural lines that complement urban landscapes
2. Structural efficiency: CFST members achieve high strength-to-weight ratios, enabling slender, visually appealing members
3. Constructability: Prefabricated steel tubes can be erected rapidly, minimizing traffic disruption in urban settings

The middle-through arch configuration places the arch rib between the deck levels, allowing the arch to carry vertical loads through compression while the deck truss system transfers loads to the arch through hangers. This arrangement provides clear separation between vehicular traffic and structural elements, improving both aesthetics and maintenance accessibility.

Technical Parameters and Structural Characteristics

The structural advantages of CFST arch ribs in bridge applications include:

• High compressive capacity: The composite action between steel tube and concrete core provides bearing capacities exceeding the sum of individual components
• Excellent post-peak ductility: The steel tube confines the concrete, preventing brittle failure and providing energy dissipation capacity during seismic events
• Reduced member size: Compared to reinforced concrete arch ribs, CFST arches can be significantly smaller in cross-section for equivalent load capacity
• Corrosion protection: The steel tube encases the concrete, providing physical protection against environmental exposure
• Construction speed: Prefabricated CFST segments can be erected rapidly, reducing construction time and traffic disruption

Steel Pipe Manufacturing and Welding Considerations

For bridge-scale CFST applications, the steel pipe requirements are considerably more demanding than for building applications:

Pipe manufacturing requirements:

• Seamless pipes or HFW welded pipes conforming to GB/T 8162 or API 5L standards
• Diameter tolerances within ±0.5% of nominal
• Wall thickness uniformity within ±10% of specified minimum
• Surface quality meeting SA 2.5 (white metal) minimum standard for inner surface
• Hydrostatic testing at 1.5 times the design pressure

Welding considerations for arch rib segments:

• Field butt-weld connections between prefabricated segments require qualified WPS procedures
• Longitudinal submerged-arc welding (LSAW) may be used for larger diameter tubes
• Weld HAZ properties must be verified through hardness testing (typically within ±1 HRC of base metal)
• Post-weld heat treatment may be required for thick-walled tubes (>25 mm) to relieve residual stresses
• Non-destructive testing (RT or UT) coverage should meet relevant code requirements (typically 100% for primary structural welds)

Connection design:

• Arch rib-to-support connections (often internal diaphragm or end plate type) are critical load transfer elements
• Bolted connections at arch crown and haunch regions require careful fit-up and torque control
• The composite action between steel tube and concrete at connection regions must be verified

Engineering Practice Experiences

The documented bridge projects demonstrate several practical lessons:

1. Concrete placement: Pumping C40-C60 concrete into large-diameter tubes requires careful management of pump pressure, flow rate, and placement sequence to avoid air entrapment and segregation. The use of superplasticizers and appropriate aggregate grading is essential.
2. Segmentation and joint design: For long arch ribs, prefabrication in manageable segments (typically 6-12 m lengths) with field welding is the practical approach. Joint locations should be placed at regions of low bending moment to minimize structural impact.
3. Quality verification: After concrete placement, the integrity of the concrete fill must be verified. Methods include impact-echo testing through the steel tube wall, ultrasonic pulse velocity measurements, or internal inspection using borescopes.
4. Long-term performance: The composite action between steel and concrete develops over time as the concrete cures and shrinks. Initial design assumptions regarding interface friction and bond may need adjustment based on long-term monitoring data.

Study Insights and Development Implications

This early-stage documentation of CFST bridge applications in China provides valuable historical context for the technology's evolution. The emphasis on combining structural efficiency with aesthetic form reflects the unique demands of urban infrastructure, where bridges must serve both functional and visual purposes. The middle-through arch configuration with CFST ribs represents a rational structural choice that balances engineering performance with urban design requirements.

From a steel pipe engineering perspective, the bridge application scale demands higher quality standards than building applications. The larger diameters, thicker walls, and more demanding load conditions require rigorous manufacturing controls, comprehensive welding qualification, and thorough non-destructive testing. The lessons learned from these early projects have informed subsequent standard development, including the Chinese code JGJ/T 222 for steel tube concrete structures, which incorporates bridge-specific provisions.

The continued development of CFST bridge technology has led to increasingly sophisticated applications, including longer spans, higher strength materials, and more refined connection details. The foundational work documented in this paper established the practical feasibility of the technology in China's urban bridge sector and contributed to its subsequent widespread adoption.