Steel bridge beams play a crucial role in supporting heavy structures, and their design often involves the use of cambering or sweeping. Cambering refers to curving a beam about its strong axis, enhancing its load-bearing capacity and aesthetic appeal. In contrast, sweeping involves curving a member about its weaker axis. The choice between cold and heat methods for cambering steel bridge beams is a critical consideration in the fabrication process.
Cold Cambering:
Cold cambering is the predominant method employed in fabrication shops for inducing camber in steel members. This cost-effective and straightforward approach utilizes mechanical means to achieve the desired curvature. In this process, an I-beam is placed in a cambering machine equipped with two restrained pivots near its ends and one or two hydraulic jacks in the middle. These jacks apply force to induce camber into the beam.
The hydraulic jacks are advanced to stress the beam past its yield point, resulting in a permanent deflection. Once the jacks are retracted, the residual deformation remains, creating a uniform camber. The process may involve moving the beam back and forth to activate jacking forces at different locations, ensuring uniformity.
While cold cambering is efficient, precautions must be taken to prevent over-stressing the beam and exceeding its yield strength, which could lead to issues such as buckling or excessive thinning of the convex flange. In case of over-cambering, mild heat between 200°F and 550°F can be applied to correct the condition.
However, there are limitations to cold cambering, and adherence to standards like AASHTO/AWS D1.5 is crucial when proposing this bending method.
Heat Cambering:
Heat cambering is an alternative method that is more detailed, time-consuming, and labor-intensive compared to cold cambering. This approach requires the use of torches to apply heat to the flange and web of a beam, following specific triangular heating patterns. Unlike cold cambering, cutting torches cannot be used in heat cambering.
The process involves heating the steel to a temperature between 1,000°F and 1,150°F as rapidly as possible without overheating. Triangular heating patterns are strategically spaced along the length of the member, and the heating progresses slowly to achieve the desired camber.
After heating is complete, the steel naturally cools until it reaches 600°F, and quenching with oil, water, or cold air is prohibited. Further cooling with dry compressed air or fans moving ambient temperature air is permitted after the steel has naturally cooled to 600°F or lower.
Benefits of Cold Bending:
Cold bending offers several advantages over heat bending. It is less time-consuming, labor-intensive, and provides greater accuracy. Operators can achieve the desired camber more efficiently, and the process can be completed in a matter of minutes, resulting in significant labor and material savings.
For instance, using a cambering machine like the Bay-Lynx model allows an operator to process a bundle of W24 beams X 30 feet long with a required camber of 50mm in just four hours. In contrast, the heat method may take up to 24 hours plus consumables.
Conclusion
In conclusion, the choice between cold and heat cambering methods for steel bridge beams depends on factors such as efficiency, cost-effectiveness, and adherence to industry standards. While cold cambering is widely preferred for its simplicity and savings, heat cambering may be considered for specific applications that demand detailed precision.
Works Cited: Cambering: Cold Versus Heat for Steel Bridge Beams
