In the incremental launching of steel bridge girders, vertical sliding supports and lateral guides are critical components that ensure controlled, safe movement of the girders. These elements provide the necessary load-bearing capacity, minimize friction, and maintain precise alignment during the launch process. Misalignment or excessive friction can lead to significant challenges, including excessive restraining stresses and potential jamming of the structure—both of which can cause delays. Therefore, careful selection and design of these systems are essential to the success of any incremental launching operation.
Vertical Sliding Supports
Vertical sliding supports are vital for carrying the vertical loads of the girders while facilitating smooth longitudinal movement. Common solutions include roller supports and temporary sliding bearings, selected based on project-specific and site conditions.
Roller Supports
Roller supports are widely used due to their ability to handle significant loads while minimizing friction. For bridge launching, steel rollers are typically arranged at piers and temporary supports.
Single Rollers: Ideal for lighter girders or shorter spans, these provide a straightforward solution when space and load capacity aren't major concerns. Their use depends on the girder's ability to withstand the roller support reaction throughout its launch length. Pedestrian bridges might employ single steel rollers supporting up to 50 tons each, while steel bridges can use rollers handling up to 250 tons. Typically, single rollers are mounted on custom-designed rocker assemblies. A pin allows free rotation, with the bottom part connecting to the pier or abutment and the roller sitting atop the assembly, above the pin hinge.
Roller Nests: For heavier girders or truss bottom chords, roller nests—consisting of multiple rollers grouped in a frame—are used to distribute the load more effectively. In cases of multi-span bridges with girder spans of 100 meters, roller reactions can reach 300–500 tons. Roller nests distribute these loads and reduce concentrated stresses on the girder flange. This configuration was successfully employed during the launch of the Deh Cho Bridge truss, where pins at two levels allowed the multiple rollers to conform to the geometry and longitudinal slope of the truss chord or girder soffit.
Temporary Sliding Bearings
Temporary sliding bearings, often incorporating PTFE (commonly known as Teflon) surfaces to reduce friction, are frequently used as alternatives to rollers. PTFE sliding bearings offer greater load-carrying capacity than basic Teflon surfaces, especially in high-load applications. These bearings are ideal for space-constrained situations, providing compact solutions without compromising load distribution or friction management. They're particularly suitable for lighter applications, such as crane-assisted launches of single spans, and can be custom-designed by an erection engineer. For heavier applications, it's advisable to engage a bearing manufacturer to ensure the design meets the required performance specifications for the launch.
Positioning
The Vertical Sliding Supports can be positioned directly above the permanent bearings using a straddle frame, where the vertical reactions can be transferred through the permanent bearings, subject to designer approval. The frame, meanwhile, carries the horizontal loads, and any rotations are articulated at the roller support pins. Alternatively, the supports can be positioned longitudinally offset from the permanent bearings if sufficient room exists on the abutment or pier cap shelves. Either approach will require a final lowering operation to transfer the superstructure from the temporary supports to the permanent bearings.
For example, during the Sombrio Bridge launch, Hilman-manufactured rollers with a working load capacity of 250 US tons were used. The proprietary roller units consisted of a loop of smooth steel cylinders connected by a chain, moving around a central load-bearing steel plate with very low friction. These rollers were mounted on custom-designed rockers, which, in turn, were positioned on frames straddling the permanent bearings at the pier and North Abutment locations. This arrangement allowed early installation of permanent bearings without obstructions and ensured precise load transfer during the launch. See photo below.
Lateral Guides
Lateral guides are employed to maintain the alignment of the girders during launching and to restrain against lateral forces such as wind. Any unintended lateral movement could result in misalignment that may be difficult to correct or could induce additional forces, complicating the launch.
Adjustable Guide Rollers
Adjustable guide rollers are positioned at key points along the launch path—typically at piers and abutments at the exterior girder lines. They are also strategically placed at intermediate supports within the launch bed, with their positioning determined by the overall wind resistance requirements during the launch. These rollers can be adjusted to fine-tune the alignment of the girders, which is especially important for curved bridges. The adjustability of these rollers allows for real-time corrections during the launch to ensure that the girders follow the intended path.
For instance, during the launch of the Sombrio Bridge, lateral guides were installed at the north abutment and pier to control the alignment of the girders and provide wind restraint when the launch was paused. The guides bore upon the edge of the girder flange through a layer of PTFE, minimizing friction while allowing smooth lateral corrections. These guides were equipped with hydraulic jacks, which could push the girders back into position when necessary. In case of strong winds, additional tie-down cables were available on-site to enhance transverse resistance.
Fixed Guide Rails
Fixed guide rails offer a more rigid system that enforces precise control of the girders’ lateral movement. These rails are particularly useful in environments where lateral deviations must be tightly controlled, such as when launching girders over narrow rivers or through urban areas. In launch beds, rails are often used to provide continuous support for the moving girder’s tail end. These rails conform the girders to their intended path while also serving as a barrier against any lateral shifts.
Conclusion
Vertical sliding supports and lateral guides are critical to the success of the incremental launching method for steel bridge girders. The careful selection of these components based on load-bearing capacity, friction management, and alignment control is essential for ensuring smooth and safe movement during the launch. Regular monitoring of these systems throughout the launch process is crucial to detect any misalignments or issues early. By utilizing advanced products from suppliers such as Hilman, Enerpac, and VSL, engineers can optimize the launching process, mitigate risks, and increase the overall efficiency of bridge construction projects.
Comentários