Feasibility Study of 700 m Span CFST Arch Bridge Design and Construction

Literature Overview

This paper by Zheng Jialian, Wang Jianjun, Mou Tingmin, Feng Zhi, Han Yu, and Qin Dayan, published in China Engineering Science (2014, Vol. 16, No. 8, pp. 33-37), presents a feasibility study for the design and construction of a 700 m span concrete-filled steel tube (CFST) arch bridge. The study builds upon the design and construction technologies developed for 500 m span CFST arch bridges and examines the key technical challenges associated with scaling up to the 700 m span class. The research was conducted by the Guangxi Zhuang Autonomous Region Department of Transportation, the Guangxi Zhuang Autonomous Region Highway and Bridge Engineering Co., Ltd., and the Sichuan Provincial Department of Transportation Highway Planning, Survey, and Design Institute.

Background and Motivation

China has made remarkable progress in CFST arch bridge construction, with the world's longest CFST arch bridges all located in China. The 500 m span class has been successfully achieved with multiple bridges in service. The 700 m span class represents the next frontier, requiring significant advances in design, fabrication, and construction technologies. The feasibility study aims to demonstrate that the 700 m span class is technically achievable and economically competitive.

Key Technical Challenges and Solutions

The paper identifies several key technical challenges for 700 m span CFST arch bridges and proposes solutions for each:

Arch Rib Fabrication

The arch ribs of a 700 m span CFST arch bridge are significantly larger than those of 500 m span bridges. The arch rib chord tubes may have diameters of 1.5 m or more and wall thicknesses of 40-60 mm. The fabrication of such large tubes requires:

• Plate rolling: The steel plates must be rolled into curved shapes with precise dimensional control. The rolling process must be carefully controlled to avoid excessive thinning or strain hardening of the plates.
• Longitudinal welding: The longitudinal welds of the arch rib tubes are critical structural elements. For tubes of this size, the welding process must be carefully designed to minimize residual stresses and distortion. Multi-pass welding with appropriate preheating and interpass temperature control is essential.
• Circumferential welding: The circumferential welds joining the arch rib segments must also be executed with high quality. The welding position (typically horizontal or overhead) and access constraints must be considered in the welding procedure design.

Arch Rib Installation

The installation of the arch ribs is a critical construction phase that requires careful planning and execution. The paper discusses the use of cantilever construction or cable-stayed construction methods for the arch rib installation. The key considerations include:

• Temporary works: The design of temporary supports, cables, and anchorage systems.
• Construction sequence: The optimization of the construction sequence to minimize temporary loads and ensure structural stability during each construction stage.
• Erection tolerances: The control of geometric tolerances during erection to ensure that the final arch rib shape matches the design profile.

Concrete Pouring into Arch Rib Tubes

The pouring of concrete into the arch rib tubes is a critical process that must be carefully controlled to ensure proper filling and compaction. For 700 m span bridges, the length of the arch rib tubes is significantly greater than for smaller spans, presenting challenges for:

• Concrete flowability: The concrete mix must have adequate flowability to fill the entire length of the tube without segregation or bleeding.
• Air removal: The removal of trapped air during the pouring process is critical to ensure full concrete filling. The use of vibrators and air release devices is essential.
• Concrete strength development: The strength development of the concrete during the curing period must be monitored to ensure that the arch rib can be loaded at the appropriate time.

Technical Parameters and Design Considerations

The selection of steel grade and concrete strength grade is critical for the structural performance of the arch bridge. The steel grade should be selected to provide adequate strength and toughness for the arch rib tubes, while the concrete strength grade should be selected to provide adequate compressive strength and durability for the concrete core.

Welding Quality and Inspection Requirements

For a 700 m span CFST arch bridge, the welding quality requirements are extremely demanding. The following welding-related considerations are critical:

• Welding procedure qualification: All welding procedures must be qualified in accordance with relevant standards (such as GB/T 9948 or AWS D1.1) before production welding begins.
• Welder qualification: All welders must be qualified for the specific welding process, position, and material combination.
• Preheat and interpass temperature control: For thick-walled sections, preheating and interpass temperature control are essential to prevent hydrogen-induced cracking and ensure adequate weld toughness.
• Post-weld heat treatment: PWHT may be required for thick-walled sections to relieve residual stresses and prevent delayed cracking.
• NDT requirements: All critical welds must be subject to 100% NDT, including ultrasonic testing (UT) for volumetric defects and magnetic particle testing (MT) for surface-breaking defects.

The welding of the arch rib tubes for a 700 m span bridge is a significant undertaking that requires careful planning, execution, and quality control. The welding engineer must work closely with the design engineer and the construction team to ensure that the welding requirements are met throughout the fabrication and construction process.

Economic and Technical Competitiveness

The paper concludes that the construction of 700 m span CFST arch bridges does not present any technical barriers and is a strong competitor among bridge types for this span class. The economic competitiveness of CFST arch bridges is attributed to:

• The efficient use of materials, with the steel tube providing both structural strength and confinement for the concrete core.
• The relatively simple construction methods, which do not require extensive temporary works.
• The long service life and low maintenance requirements of CFST structures.

However, the paper also acknowledges that the economic competitiveness must be evaluated on a case-by-case basis, considering factors such as site conditions, material costs, and construction access.

Study Insights and Reflections

This feasibility study provides a valuable roadmap for the design and construction of 700 m span CFST arch bridges. The paper demonstrates that the key technical challenges can be overcome with appropriate design, fabrication, and construction technologies. The conclusion that there are no technical barriers to the construction of 700 m span CFST arch bridges is encouraging and should inspire further research and development in this area.

From a welding perspective, the fabrication of the arch rib tubes for 700 m span bridges represents a significant challenge that requires advanced welding technologies and rigorous quality control. The welding engineer must be prepared to develop and qualify welding procedures for thick-walled sections, implement advanced NDT techniques, and manage the welding process to minimize residual stresses and distortion.

In summary, this paper is a forward-looking feasibility study that demonstrates the technical viability of 700 m span CFST arch bridges. The paper provides valuable guidance for the design and construction of future CFST arch bridges in this span class and highlights the importance of welding quality and construction technology in achieving the desired structural performance.