Application of Steel Tube Concrete Structures in Urban Bridges

Literature Overview

This paper by Guo Jinqiong and colleagues, published in the Journal of Fuzhou University (Natural Science Edition, Volume 24, Issue 4, 1996, pp. 22-26), presents the design and application of steel tube concrete (SRC) structures in several urban bridges. The study focuses on SRC through-arch bridges, discussing aspects of structural design, aesthetic considerations, and construction methodology. The bridges featured elegant forms, lightweight structures, and simplified construction processes.

Core Technical Points

The research represents an early application of SRC technology in bridge engineering, demonstrating the following key advantages:

The through-arch configuration leverages the high compressive strength of SRC columns and ribs, while the steel tube provides formwork, lateral support, and post-construction durability.

Technical Interpretation from a Steel Pipe and Welding Perspective

Steel Tube Selection and Fabrication

For SRC bridge applications, the steel tubes serve as both structural members and permanent formwork. The selection criteria include:

• Material grade: Q235B or Q345B steel per GB/T 3091 or GB/T 8163
• Diameter and wall thickness: Typically 300-600 mm diameter with 8-16 mm wall thickness for bridge ribs
• Welding method: Longitudinal submerged arc welding (LSAW) or HFW for large diameters; seamless for smaller diameters
• Surface treatment: Shot blasting to Sa 2.5 or Sa 3 grade before concrete placement

Welding Quality Requirements

The welding of steel tube segments is critical for structural integrity:

The welding procedure specification (WPS) should be qualified per GB/T 9942 or ISO 15614. Pre-heat and interpass temperature control are essential for Q345B steel with wall thickness > 20 mm to prevent hydrogen-induced cracking.

Concrete Placement in Bridge Ribs

The placement of concrete inside curved steel tube ribs presents unique challenges:

• Climbing placement: Concrete must be placed from the springing line upward, with controlled climbing rates to prevent segregation.
• Vibration: Internal vibration is required, with care to avoid tube deformation.
• Joint management: Construction joints should be located at points of minimum shear force, typically at quarter-span points.
• Curing: Proper curing is essential, especially for exposed steel tube surfaces that may be subjected to environmental exposure.

Standards and Design Codes

The design of SRC bridge members should follow the composite action principles, where the steel tube and concrete core act together under loading. The bond between steel and concrete is assumed to be perfect, with no slip at the interface. This assumption is valid for normal-strength concrete but may require verification for ultra-high strength concrete.

Engineering Practice Implications

The application of SRC in urban bridges offers several practical advantages:

• Aesthetic integration: The exposed steel tube surface can be painted or coated to match urban design requirements. The smooth, curved form of the arch is visually appealing.
• Construction speed: Pre-fabricated steel tube segments can be erected quickly, with concrete placement following immediately. This reduces construction time compared to reinforced concrete arch bridges.
• Durability: The steel tube protects the concrete from environmental degradation, extending the service life of the bridge.
• Maintenance: The steel tube surface can be inspected and maintained independently of the concrete core.

However, several challenges must be addressed:

• Corrosion protection: The steel tube must be protected against corrosion, especially at weld joints and cut edges. Hot-dip galvanizing, paint systems, or cathodic protection may be required.
• Fire protection: Steel tubes lose strength at elevated temperatures. Fire-resistant coating or concrete encasement may be required for fire safety compliance.
• Inspection access: Access to the concrete core for inspection is limited. Non-destructive testing methods (UT, MT, PT) should be used for steel tube inspection.

Key Questions and Reflections

The paper does not address the long-term performance of SRC bridges under traffic loading and environmental exposure. Questions remain regarding:

• The durability of the steel-concrete interface under cyclic loading
• The effectiveness of corrosion protection systems over the design service life
• The inspection and maintenance requirements for SRC bridge members

From a welding perspective, the cyclic loading in bridge applications may lead to fatigue cracking at weld joints. Fatigue assessment per Eurocode 3 or GB/T 4656 should be performed for critical weld details.

Study Insights and Conclusions

This research demonstrates the viability of SRC technology for urban bridge applications, offering a combination of structural efficiency, aesthetic appeal, and construction simplicity. The through-arch configuration is particularly well-suited for medium-span urban bridges where visual elegance and structural performance are both important. From a steel pipe and welding quality perspective, the key requirements are proper steel tube selection, qualified welding procedures, thorough non-destructive testing, and effective corrosion protection. Engineers should ensure that the design, fabrication, and construction processes meet the relevant standards and specifications, with particular attention to weld quality and concrete placement control. The long-term success of SRC bridges depends on proper maintenance and inspection practices throughout the service life.