Optimization Calculation Analysis of Concrete-Filled Steel Tube Arch Bridge Hoisting Process

Literature Overview

This paper by Zhang Jianmin, Zheng Jialian, and Xiao Rucheng from Tongji University and the Guangxi Zhuang Autonomous Region Department of Transportation, published in the China Journal of Highway and Transport in 2005, presents an optimization-based approach to the hoisting process analysis of a concrete-filled steel tube (CFST) arch bridge. Funded by the China Postdoctoral Science Foundation (Grant 2003034279), the study uses the Wu Xie Yangtze River Bridge, with a main span of 460 meters, as a case study. The research applies first-order optimization methods to determine the optimal construction state of the arch bridge, considering cable sag effects and structural geometric nonlinearity.

Core Technical Findings

Optimization Methodology

The study employs a first-order optimization calculation method to determine the reasonable construction state of the arch bridge. The objective function is defined as the final arch rib line shape after construction completion, and the design variables are the pre-bending angles of individual arch rib segments during construction. By establishing a direct relationship between measurable construction-stage structural state variables and the final-state objective function, the optimization process determines the cable tension forces and arch rib hoisting heights at each construction stage.

The key technical approach involves:

• Objective function: Final arch rib line shape matching the design profile
• Design variables: Pre-bending angles of arch rib segments
• Constraints: Cable tension limits, structural stress limits, geometric compatibility
• Optimization method: First-order optimization with consideration of cable sag and geometric nonlinearity

Case Study: Wu Xie Yangtze River Bridge

The Wu Xie Yangtze River Bridge is a through-type CFST arch bridge with a main span of 460 meters, representing one of the longest CFST arch bridges in the world at the time of construction. The bridge features a combination of steel arch ribs filled with concrete, tied by stay cables during construction, and converted to a self-supporting arch structure upon completion.

Key Results

The optimization calculation results demonstrate that introducing optimization theory into the construction process analysis of arch bridges is both feasible and reasonable. The method successfully determines the cable tension forces and arch rib hoisting heights at each construction stage, ensuring that the final arch rib line shape matches the design profile. The approach accounts for the complex interaction between cable sag effects and structural geometric nonlinearity, which are critical factors in the construction of long-span arch bridges.

The first-order optimization method provides a computationally efficient solution compared to higher-order methods, making it practical for real-time construction monitoring and adjustment. The method can be integrated with construction monitoring systems to provide feedback-based control of the hoisting process.

Welding and Fabrication Quality Considerations

The construction of CFST arch bridges involves extensive welding of steel arch rib segments, and the quality of these welds is critical to the structural integrity of the completed bridge. The following technical points are essential:

• Segment joint welding: The longitudinal butt welds between arch rib segments must be full-penetration welds per GB/T 985.1, with weld thickness equal to the thinner of the two joined plates. For thick-walled arch ribs (typically 20-40 mm), multi-pass welding with preheating is required to prevent cold cracking.
• Preheating requirements: For steel grades above Q345, preheating to 100-150°C is recommended before welding, with interpass temperature controlled to prevent excessive cooling rates. For Q390 and Q420 grades, preheating to 150-200°C may be necessary, particularly in cold weather conditions.
• Welding sequence for arch ribs: The welding sequence for arch rib segments should be planned to minimize residual stress and distortion. A symmetric welding pattern, proceeding from the center of the joint outward, is recommended. For large-diameter tubes, the circumferential weld should be completed before the longitudinal weld.
• Non-destructive testing: All longitudinal butt welds must undergo 100% RT (radiographic testing) per GB/T 3323 or UT (ultrasonic testing) per GB/T 11345, with acceptance criteria per GB/T 3323.1 or ISO 17636-1. For critical welds, both RT and UT may be required.
• Weld repair: Any weld defects identified during NDT must be repaired per a documented repair procedure. The repair procedure should specify grinding depth limits, preheating requirements, and re-testing criteria. Excessive repair may compromise the weld integrity and should be avoided through proper initial welding quality control.
• Heat input control: The heat input per pass should be controlled to prevent excessive HAZ softening and to minimize residual stress. For Q345 steel, the heat input should not exceed 25 kJ/mm, and for Q390 steel, the limit is 20 kJ/mm. Multi-pass welding with lower heat input per pass is preferred for thick sections.

Standards and Code Relevance

The design and construction of CFST arch bridges are governed by multiple standards, including:

The optimization methodology presented in this paper complements the design codes by providing a practical approach to construction sequence planning. The method ensures that the structural behavior during construction is compatible with the final design state, which is essential for maintaining the integrity of the steel arch rib welds throughout the construction process.

Engineering Practice Implications

For engineers involved in the design and construction of CFST arch bridges, the following recommendations emerge:

1. Construction optimization: The first-order optimization method should be applied to determine the optimal construction sequence, cable tension forces, and segment pre-bending angles. This approach minimizes construction-stage stresses and ensures the final arch shape matches the design profile.
2. Weld quality control: The welding of arch rib segments should follow a documented welding procedure specification (WPS) with qualified welding procedures (PQR). All welders should be certified per GB/T 9445 or ISO 9606, and weld quality should be verified through NDT.
3. Construction monitoring: Real-time monitoring of cable tension forces, arch rib displacements, and structural stresses should be integrated with the optimization model to provide feedback-based control of the construction process.
4. Material selection: The steel grade for arch ribs should be selected based on the structural demands and environmental conditions. Q345 is commonly used, but Q390 or Q420 may be required for larger spans or more severe loading conditions.
5. Concrete filling: The concrete filling of the steel arch ribs should be performed after the steel structure has been erected and tensioned to the design state. The concrete mix design should ensure adequate workability for filling the confined space within the arch rib, and the concrete should be properly cured to achieve the design strength.

Study Insights and Reflections

This research demonstrates the effectiveness of optimization-based methods in the construction planning of complex steel structures. The first-order optimization approach provides a computationally efficient solution that can be integrated with real-time construction monitoring, enabling adaptive control of the hoisting process.

From a structural engineering perspective, the successful application of optimization theory to CFST arch bridge construction highlights the importance of considering the construction sequence as an integral part of the structural design process. The construction-stage behavior of the structure must be compatible with the final design state, and the optimization method provides a systematic approach to achieving this compatibility.

The research also underscores the importance of welding quality in the construction of steel arch bridges. The integrity of the longitudinal butt welds between arch rib segments is critical to the structural performance of the completed bridge, and any compromise in weld quality could lead to catastrophic failure. The optimization method should be complemented by rigorous welding quality control procedures to ensure the structural integrity of the completed bridge.

Future research should explore the integration of advanced monitoring technologies, such as fiber optic sensors and wireless sensor networks, with optimization-based construction control methods. Additionally, the development of real-time optimization algorithms that can adapt to changing construction conditions (e.g., wind, temperature, equipment limitations) would further enhance the practical applicability of the methodology.