The launch nose is a temporary steel structure attached to the front of the bridge girders during the launching process. Its primary function is to reduce the bending moments and shear forces experienced by the bridge girders as they transition over the supports. Essentially, the launch nose helps manage deflections and maintain the structural integrity of the girders during the critical phases of the launch.
Key Functions of the Launch Nose
Reduction of Bending Moments: As the steel girders are launched across a span, they remain unsupported for a considerable length before reaching the next pier. The launch nose helps reduce the cantilever moment by providing a lighter leading edge, decreasing the bending stresses in the main girder during the launch.
Smooth Transition Across Supports: The launch nose facilitates a smoother transition from the cantilever condition to an end support. It allows the leading edge of the girders to gradually engage with the pier roller supports, minimizing dynamic effects and reducing the risk of damage during this process.
Control of Deflection: By extending the length of the structure without adding significant weight, the launch nose helps control deflection. This is crucial in preventing excessive sagging, which could compromise the structural integrity of the steel girders or interfere with the launching components.
Design Considerations for a Launch Nose
Designing an effective launch nose requires careful consideration of several factors:
Length and Weight: The length of the launch nose ranges from 30% to 60% of the span length. This proportion ensures that the moments are adequately reduced while still allowing the nose to function effectively. The weight of the launch nose should be minimized to avoid adding unnecessary load to the girders during launch.
Structural Stiffness: While the launch nose must be lightweight, it also needs to be sufficiently stiff to maintain its shape and function under the loads encountered during launching. The design should balance these requirements, often using trussed or box-shaped sections to achieve the necessary stiffness without excessive weight.
Attachment Mechanism: The connection between the launch nose and the steel girders must be secure yet detachable once the launch is complete. Bolted connections are commonly used, allowing for easy removal and reuse of the launch nose in other projects if desired.
Aerodynamic Considerations: In some cases, especially for longer launches, aerodynamic forces can play a role. The design of the launch nose should minimize wind resistance and avoid flutter or other dynamic instabilities during the launch.
Case Study: Athabasca Launching Nose
In the incremental launching of the Athabasca Bridge, a specially designed launch nose was crucial for managing the structural demands. The launch nose had an inclined bottom flange attached to each leading girder segment, reducing bending moments and ensuring a smooth transition across supports. Cross and plan braces connected the nose segments, forming a unified structural system that controlled deflection during the launch.
Key Design Features
Inclined Bottom Flange: The inclined bottom flange geometry of the launch nose was essential for compensating for the downward deflection of the cantilever and ensuring touchdown on the pier roller supports as the girders approached each pier. When the nose reached a pier, the inclined flanges made contact with Hilman rollers positioned at the pier. These rollers acted as guides, helping correct the vertical alignment of the girders and ensuring a smooth, controlled engagement with the pier supports.
Transition Mechanism: A major challenge in the Athabasca project was designing a mechanism to transition the Hilman rollers from the inclined flange of the launch nose to the horizontal flange of the main girders. This was achieved with a rotating flange plate, known as a "rocker." The rocker, slightly longer than the Hilman roller, was attached to the launch nose via a steel pin, allowing it to rotate and align with the inclinations of the launching nose flange and the girder flange.
Mechanism of Operation
The operation of the rocker assembly during the launch can be broken down into four key steps:
Initial Contact (Step 1): As the launch nose approached the Hilman roller, the rocker assembly made initial contact. The path of least resistance guided the nose to align with the roller, initiating the transition process.
Pivot and Rotation (Step 2): As the distance between the pivot points of the rocker and the Hilman roller minimized, the rocker began to rotate. This rotation was triggered simply by the forward pushing force exerted on the girders, causing the launch nose to lift slightly.
Controlled Transition (Step 3): During this rotation, the Hilman roller remained stationary relative to the rocker due to the principle of minimum deformation energy. This ensured that the rotation was smooth and controlled, preventing any abrupt movements that could destabilize the girders.
Completion of Transition (Step 4): Once the rotation was complete, the Hilman roller resumed movement and cleared the rocker, allowing the girder to fully transition from the inclined launch nose to the horizontal girder flange.
Conclusion
The launch nose used in the Athabasca Bridge project showcases the crucial role these structures play in incremental launching. By incorporating an inclined bottom flange and a well-designed rocker mechanism, the launch nose effectively managed bending moments, controlled deflection, and ensured a smooth transition across supports. The innovative use of the rocker assembly, guided by the principle of minimum deformation energy, offered a simple yet effective solution to the challenges of transitioning between the inclined and horizontal flanges.
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