Development Review of CFST Rigid Skeleton Arch Bridges in China

Literature Overview

This comprehensive review paper by Zhao Renda and Zhang Zhengyang from Southwest Jiaotong University, published in Bridge Construction (2016, Vol. 46, No. 6, pp. 45-50), provides a systematic summary of the development status, design theory, construction methods, and span progression of composite steel tube concrete (CFST) rigid skeleton arch bridges in China. The paper synthesizes findings from a large number of such bridges to identify current research trends and future directions.

Structural Characteristics and System Conversion

CFST rigid skeleton arch bridges represent a hybrid structural system that combines the advantages of steel arch construction speed with the ultimate strength and stiffness of composite steel-concrete action. The term "rigid skeleton" refers to the steel CFST arch ribs that serve as permanent structural members throughout construction and service, as opposed to temporary steel skeletons that are removed after concrete placement.

The increased number of structural system conversions introduces complexity in both design and construction. Each conversion stage represents a potential source of error accumulation, and the residual stresses from each stage affect the final stress state. This is particularly relevant from a welding and fabrication standpoint, as each erection stage involves field welding of CFST arch rib segments.

Design Calculation Methods

The paper highlights the stress superposition method as the approach that best reflects the true stress state during construction stages. This method accounts for the time-dependent effects of concrete shrinkage and creep that develop after each construction stage. The alternative approaches—such as the full composite section method or the elastic stage method—may not accurately capture the progressive load transfer from steel skeleton to composite section.

From a metallurgical perspective, the design must consider the different thermal expansion coefficients of steel and concrete, which induce additional stresses at the steel-concrete interface. The bond strength between the steel tube and concrete core is critical for composite action, and this bond can be affected by the steel tube surface condition, concrete mix design, and curing conditions.

Construction Methods

Two primary construction methods are identified for CFST rigid skeleton arch bridges:

Both methods require precise control of steel tube fabrication tolerances and welding quality. The spinning method involves lifting and rotating massive CFST arch segments, placing significant demands on temporary connection details and welding sequences. The cable-suspended method requires careful sequencing of cable tensioning to control arch rib deflection and prevent local buckling of the steel tubes.

Span Enhancement Strategies

Three approaches to increase span capacity are identified:

1. Strengthening the rigid skeleton through increased steel tube wall thickness or higher-strength steel grades
2. Reducing the main arch self-weight through lightweight concrete or hollow steel tubes
3. Adopting combined construction methods that leverage the advantages of multiple techniques

The selection of steel grade is particularly significant for span extension. Higher-strength steels such as Q420 or Q460 (equivalent to ASTM A572 Gr.60 or higher) can reduce steel tonnage while maintaining structural capacity. However, increased steel strength introduces challenges in welding—higher preheat temperatures, stricter hydrogen control, and more rigorous non-destructive testing become necessary.

Quality Control Considerations

The construction of CFST arch bridges involves numerous field-welded joints that are critical to structural integrity. Key quality control measures include:

The steel tubes used for arch ribs are typically large-diameter welded steel pipes (LSAW or HFW), and their fabrication quality directly affects the long-term performance of the arch bridge. Residual stresses from pipe fabrication, if not properly relieved, can interact with service loads to accelerate fatigue crack initiation.

Research Gaps and Future Directions

The paper identifies three areas requiring further research: refined bridge model testing, simulation program development, and durability design. From a materials and welding perspective, the durability concern is particularly acute—CFST arch bridges are often exposed to corrosive environments (marine atmosphere, deicing salts), and any corrosion of the steel tube interior can compromise the composite action. The internal corrosion of CFST members is difficult to inspect and repair, making prevention through proper coating and corrosion protection essential.

Study Implications and Outlook

This review provides a valuable roadmap for the continued development of CFST rigid skeleton arch bridges in China. The emphasis on construction-stage behavior and structural system conversion highlights the importance of construction engineering expertise alongside structural design. For welding engineers, the review underscores the critical role of field welding quality in ensuring the long-term performance of these complex structures. The future of CFST arch bridges lies in the integration of advanced high-strength steels, improved welding procedures, and sophisticated construction monitoring systems that can detect and correct deviations during each construction stage.