Bending Process for Steel Pipe Arch Ribs in Large Arch Bridges

Literature Overview

This paper by He Shumin, published in Railway Engineering (Vol. 40, No. 6, 2000), describes the bending process for steel pipe arch ribs in the Wuzhou Guijiang Third Bridge. The author, representing the Second Engineering Division of China Railway 18th Engineering Bureau, presents a practical engineering approach to cold bending large-diameter steel pipes into the required arch geometry. The classification code U445.47 places this work within bridge engineering, specifically arch bridge design and construction. The paper covers three key aspects: jig fabrication, straight pipe pre-bending preparation, and temperature control during the bending process.

Engineering Background and Requirements

Large steel tube arch bridges have become increasingly popular for medium and long span applications due to their aesthetic appeal, structural efficiency, and construction flexibility. The arch rib, formed from large-diameter steel pipes, must be bent to a precise geometric profile that matches the design arch curve. The bending process must produce a smooth, continuous curve without introducing unacceptable deformations, residual stresses, or material degradation.

For the Wuzhou Guijiang Third Bridge, the arch ribs are constructed from steel pipes with large diameters and relatively thin walls. The bending requirements include achieving a specific radius of curvature, maintaining dimensional accuracy within tight tolerances, and ensuring that the material properties are not adversely affected by the bending process. The cold bending approach was selected over hot bending to avoid the complexities and costs associated with heating large-diameter pipes, while still achieving the required curvature.

Jig Fabrication

The jig (mandrel or bending form) is the critical tooling that defines the final geometry of the bent arch rib. The jig must be fabricated to precise dimensional tolerances, as any error in the jig geometry will be transferred directly to the bent pipe. The paper describes the fabrication of a jig that matches the design arch curve, typically constructed from steel plates and structural members welded into a rigid frame.

The jig design must account for several factors. First, the elastic springback of the steel pipe after bending must be compensated for by oversizing the jig curvature. The springback angle depends on the material properties (yield strength, elastic modulus), the pipe geometry (diameter, wall thickness), and the bending radius. For structural steel pipes, the springback factor is typically in the range of 1.05 to 1.15, meaning the jig must be made with a radius 5% to 15% smaller than the desired final radius.

The jig fabrication process involves precise layout of the arch curve on the jig plates, followed by welding of the structural members to create the required shape. After fabrication, the jig must be inspected for dimensional accuracy using surveying equipment, and any deviations must be corrected before use. The jig is typically assembled on-site, with modular segments bolted or welded together to form the complete bending form.

Straight Pipe Pre-Bending Preparation

Before the bending process begins, the straight steel pipes must undergo several preparation steps. First, the pipes are inspected for dimensional accuracy, including outer diameter, wall thickness, and straightness. Pipes with out-of-tolerance dimensions are rejected or corrected before bending. Second, the pipes are marked with reference points at regular intervals along their length, which will be used to verify the bending accuracy during and after the process.

The pipe ends are prepared to ensure smooth engagement with the jig and bending equipment. Any burrs, scale, or surface contaminants are removed, and the pipe ends may be beveled to facilitate insertion into the bending mechanism. The paper emphasizes the importance of ensuring that the pipe material is in a suitable condition for cold bending. Pipes that have been previously heat-affected from welding or that have undergone cold work in other operations may have altered mechanical properties that affect the bending behavior.

The pre-bending preparation also includes the application of lubricants or protective coatings to the pipe surface to reduce friction during the bending process and prevent surface damage. The selection of lubricant depends on the steel grade and the bending equipment used. For structural steel pipes, common lubricants include graphite-based compounds, synthetic oils, or specialized bending compounds.

Temperature Control During Bending

The temperature control aspect of the bending process is a critical technical consideration. During cold bending, plastic deformation generates heat due to the work hardening of the steel. This localized heating can cause temperature gradients within the pipe wall, leading to non-uniform deformation and potentially inducing residual stresses.

The paper describes temperature monitoring and control measures implemented during the bending of the arch ribs. The bending rate must be controlled to prevent excessive temperature rise. If the bending is performed too rapidly, the heat generated by plastic deformation cannot dissipate sufficiently, leading to localized temperature increases that may approach or exceed the critical temperature for tempering of the steel. For structural steels such as Q235 or Q345, temperatures above approximately 300°C can cause significant changes in mechanical properties, including reduction in yield strength and potential for strain aging.

The temperature control strategy involves monitoring the pipe temperature at multiple locations along the bending zone using contact thermometers or infrared pyrometers. When the temperature exceeds a predetermined threshold, the bending operation is paused to allow heat dissipation. The bending rate is adjusted based on the pipe diameter, wall thickness, and steel grade, with larger and thicker pipes requiring slower bending rates to manage heat generation.

Quality Control and Dimensional Verification

After bending, each arch rib segment must be inspected for dimensional accuracy, surface condition, and structural integrity. The dimensional verification includes checking the radius of curvature at multiple points along the arch, the out-of-roundness of the pipe cross-section, and the overall length. The acceptable tolerances for these parameters are typically specified in the bridge design drawings and applicable standards.

Non-destructive testing may be required to detect any defects introduced during the bending process. Magnetic particle testing or ultrasonic testing can detect surface and subsurface cracks that may have initiated at the inner curvature of the bend due to tensile stresses. Visual inspection is performed to identify surface deformations, wrinkles, or other visual anomalies.

Key Reflections and Study Insights

This paper provides valuable practical insights into the cold bending of large-diameter steel pipes for arch bridge construction. The emphasis on jig fabrication accuracy, pipe preparation, and temperature control reflects a systematic approach to process quality management that is applicable to similar forming operations in pipe fabrication.

From a metallurgical perspective, the cold bending process induces plastic deformation that work-hardens the steel, particularly at the inner curvature where tensile strains are highest. This work hardening increases the yield strength locally but reduces the ductility, creating a potential concern for fatigue performance and seismic resilience. The temperature control measures described in the paper help mitigate this concern by preventing excessive heat generation that could cause additional metallurgical changes.

The jig fabrication process described in this paper is analogous to the forming dies used in pipe fitting fabrication. The principles of dimensional accuracy, material selection, and structural rigidity are common to both applications. Engineers involved in pipe fitting manufacturing can draw useful parallels from this bridge engineering application, particularly regarding the importance of compensating for elastic springback and ensuring precise dimensional control of forming tools.

The temperature control aspect of this paper is particularly relevant for engineers who may be involved in hot forming operations as well. The understanding of heat generation during plastic deformation, the temperature thresholds for material property changes, and the strategies for managing thermal effects are all transferable to other forming processes such as hot bending, welding, and heat treatment.

In conclusion, this paper offers a practical engineering case study that demonstrates the application of fundamental forming principles to a specific bridge construction challenge. The systematic approach to jig design, pipe preparation, and temperature control provides a valuable reference for engineers working on similar steel pipe forming operations. The emphasis on quality control and dimensional verification underscores the importance of process discipline in achieving the required geometric accuracy for structural applications.