Bending Process for Large Steel Tube Arch Bridge Ribs

Literature Overview

The paper by He Shumin, published in Railway Construction in 2000, documents the bending process for steel tube arch ribs of the Guijiang No. 3 Bridge in Wuzhou City. Large steel tube arch bridges are increasingly used for medium and long-span crossings due to their aesthetic appeal, structural efficiency, and constructability. The arch rib bending process is a critical fabrication step that determines the geometric accuracy, residual stress distribution, and overall structural performance of the arch. This paper provides practical insights into jig fabrication, pre-bending preparation, and temperature control during the bending operation.

Core Technical Content

Jig Fabrication

The bending jig (or forming fixture) is the primary tool for shaping the steel tube into the required arch profile. For large-diameter steel tubes, the jig must be designed to:

• Accommodate the tube diameter: The jig must have sufficient clearance for the tube to pass through while maintaining close proximity for controlled deformation.
• Provide uniform pressure: The contact surfaces must distribute the bending force uniformly to avoid localized yielding or buckling.
• Maintain geometric accuracy: The jig profile must match the designed arch curve within tight tolerances, typically ±3 mm for large arch ribs.

The jig fabrication process involves:

1. Template preparation: A full-scale template of the arch curve is laid out on the fabrication floor, and the jig profile is marked based on this template.
2. Structural fabrication: The jig is typically fabricated from structural steel beams and plates, with the contact surfaces machined or ground to achieve the required surface finish.
3. Support system: The jig must be supported on a rigid foundation to prevent deflection under bending loads. For large arch ribs, the support system may include reinforced concrete foundations or heavy steel frames.
4. Adjustment mechanisms: Some jig designs include adjustable elements to accommodate minor variations in the arch profile or to allow for corrections during the bending process.

Pre-Bending Preparation

Before bending, the straight steel tubes must undergo several preparation steps:

The steel grade selection is critical for the bending process. Common grades for steel tube arch ribs include Q345q (bridge steel) and Q370q, which offer a good balance of strength and ductility. The yield strength, tensile strength, and elongation at fracture must be verified to ensure the material can withstand the plastic deformation induced during bending without cracking.

Temperature Control During Bending

Temperature control is a critical aspect of the bending process for large steel tube arch ribs. The paper emphasizes that the bending operation should be performed within a controlled temperature range to ensure:

• Uniform deformation: Temperature gradients across the tube cross-section can cause uneven plastic deformation, leading to ovality and residual stress.
• Material properties: The yield strength and ductility of steel are temperature-dependent. At lower temperatures, the material becomes more brittle and prone to cracking. At higher temperatures, the yield strength decreases, which may require higher bending forces to achieve the desired shape.
• Residual stress management: Controlled temperature helps manage the residual stress distribution, which is critical for the long-term structural performance of the arch rib.

The typical temperature control parameters are:

• Ambient temperature: 10–35°C during the bending operation.
• Tube surface temperature: Monitored to ensure it does not deviate from the ambient temperature by more than 10°C.
• Heating (if required): For very thick-walled tubes or high-strength steels, pre-heating may be required to improve ductility. The pre-heating temperature is typically 100–200°C, applied uniformly using gas burners or induction heating.

Bending Process Details

The bending process for large steel tube arch ribs typically involves the following steps:

1. Tube insertion: The straight tube is inserted into the bending jig, with the start point aligned to the jig reference mark.
2. Progressive bending: The tube is bent progressively along its length, with the bending force applied incrementally to avoid sudden plastic deformation.
3. Shape verification: At regular intervals (typically every 3–5 meters), the bent profile is measured and compared to the design curve. Any deviation beyond the tolerance is corrected by adjusting the bending force or re-bending the affected section.
4. Final verification: After the complete bending operation, the entire arch rib profile is surveyed using total station or laser scanning to verify the geometric accuracy.

The bending force required depends on the tube diameter, wall thickness, steel grade, and bend radius. For a typical Q345q steel tube with 2000 mm diameter and 20 mm wall thickness, the bending force can exceed 500 kN. The bending machine must be capable of applying this force with sufficient control to avoid over-bending or under-bending.

Residual Stress and Distortion Control

The bending process induces significant residual stresses in the arch rib. These stresses arise from the non-uniform plastic deformation across the tube cross-section and can affect the structural performance in several ways:

• Buckling resistance: Residual stresses reduce the effective buckling resistance of the arch rib, particularly under compressive loading.
• Fatigue life: Residual tensile stresses at the weld seams can accelerate fatigue crack initiation and propagation.
• Geometric stability: Residual stresses can cause the arch rib to distort over time, particularly under thermal cycling or sustained loading.

To manage residual stresses, the following measures are typically employed:

1. Post-bending stress relief: Thermal stress relief (heating to 550–650°C and holding for 2–4 hours) or mechanical stress relief (shot peening or vibration stress relief) may be applied after bending.
2. Controlled bending rate: Bending at a controlled rate allows for more uniform plastic deformation and reduces the magnitude of residual stresses.
3. Symmetric bending: Where possible, the bending process should be designed to produce symmetric deformation, which minimizes the net residual stress.

Engineering Practice Considerations

The paper documents the practical challenges encountered during the fabrication of the Guijiang No. 3 Bridge arch ribs. Key lessons learned include:

• Jig accuracy is paramount: Even minor errors in the jig profile are magnified in the final arch rib geometry. The jig must be fabricated and verified with the same rigor as the arch rib itself.
• Temperature monitoring is essential: In outdoor fabrication environments, temperature can vary significantly between day and night, or between sun-exposed and shaded areas. Continuous temperature monitoring and adjustment of the bending parameters are necessary.
• Sequential bending reduces distortion: Bending the arch rib in multiple sections, rather than in one continuous operation, reduces the overall distortion and allows for intermediate corrections.
• Quality control at every step: Non-destructive testing (UT, MT) should be performed before and after bending to verify that no new defects have been introduced during the deformation process.

Study Insights and Implications

This paper provides valuable practical guidance for engineers involved in the fabrication of large steel tube arch ribs. While the technology has advanced since 2000, with the introduction of CNC-controlled bending machines and advanced simulation tools, the fundamental principles remain the same. The emphasis on jig accuracy, temperature control, and progressive bending is still relevant and should be incorporated into modern fabrication procedures.

For contemporary projects, the integration of finite element analysis (FEA) for bending process simulation can complement the empirical approach described in this paper. FEA can predict the residual stress distribution, distortion, and required bending force before the actual fabrication begins, allowing for optimization of the process parameters. However, the practical experience documented in this paper remains an essential reference for troubleshooting and process refinement on the fabrication floor.