Fabrication of hollowed-out bridge frame bends

Hollow bridge frame bends can be fabricated using heat bending, cold bending, or specialized methods like rotary-draw and induction bending, with careful control to maintain structural integrity.

Methods for Bending Hollow Structural Sections (HSS)

1. Cold Bending Techniques

• Pyramid Rolling: The HSS member is passed through three rollers arranged in a pyramid shape. The distance between rollers is adjusted incrementally to achieve a constant radius bend. This method is suitable for large-radius curves and maintains the cross-sectional shape with minimal distortion .
• Gag Pressing / Point Bending: A hydraulic ram applies point loads at specific locations (typically at one-third points of the member) to create bends. This method is effective for cambering or large-radius curves and allows precise control over the bend .
• Synchronized Incremental Cold Bending: Multiple points along the member are loaded in a synchronized sequence, gradually forming the desired radius. This method minimizes distortion and is suitable for tighter bends than standard cold bending . 2. Heat Bending Techniques
• Hot Bending: Localized heating using a flame or furnace softens the steel, allowing it to be bent to the desired radius. This method is commonly used for repair or when tighter bends are required .
• Induction Bending: A short section of the HSS is heated using an electric coil, then drawn around a die and quenched. This allows precise bends with tighter radii and minimal flattening compared to conventional hot bending .
• Heat Straightening / Heat Curving: These methods are used to correct or form bends in steel bridge components. Controlled heating and cooling cycles induce plastic deformation while maintaining material properties . 3. Rotary-Draw Bending
• The HSS is clamped into a die and drawn around it to achieve the desired bend radius. This method is ideal for repeated production of identical bends and can handle small radii with minimal distortion .

Considerations for Hollow Bridge Frame Bends

• Material Properties: Bending can alter yield strength and residual stresses. Heat methods require careful temperature control to avoid weakening the steel .
• Cross-Section Integrity: Hollow sections are prone to flattening or ovalization. Methods like rotary-draw or induction bending help maintain the cross-section .
• Bend Radius: The minimum achievable radius depends on the HSS dimensions, wall thickness, and bending method. Cold bending is generally limited to larger radii, while heat-assisted methods allow tighter curves .
• Fabrication Planning: For bridge frames, bends should be coordinated with design specifications, including camber, alignment, and connection points, to ensure proper fit during assembly .

Practical Tips

• Use gradual, incremental bending to reduce stress concentrations.
• For tight bends, consider induction or rotary-draw bending to minimize distortion.
• Always verify bend geometry against design drawings before final assembly.
• Consider post-bend inspection for residual stresses, cracks, or deformation, especially in critical load-bearing members. By selecting the appropriate bending method and carefully controlling the process, hollowed-out bridge frame bends can be fabricated efficiently while maintaining structural performance and safety.