The pushing system involves equipment and mechanisms that drive the girders along the launch path. Incremental launching relies on the controlled application of force to move the girders incrementally. The choice of pushing system depends on the project's specific needs, including the weight of the girders, site conditions, and the desired precision in movement.
Pushing Equipment
Hydraulic jacks are commonly used devices that apply controlled force to push girders forward in small increments. These jacks can be positioned at a fixed point, such as the abutment, or at a moving point like the tail end of the girders. Hydraulic jacks offer precise control over movement and can generate significant force. The precision comes from the ability to easily synchronize them for uniform pushing. However, they require sophisticated anchoring systems, are expensive, and generally need specialized expertise to operate.
Hydraulic rams are similar to jacks but are often used with a movable sled at the girder ends. The rams push the sled along launch rails providing a continuous surface to distribute the load. They are suitable for heavy launches that require high precision, consistent force, and controlled movement speed. Typically, they are anchored to sequential points on the launch rail and need constant retraction and reconnection to the next anchor point after each full stroke. The launch rail and its supports must safely dissipate the horizontal and vertical forces from the rams to the launch bed. This system requires careful alignment of launch rails and is typically slower and more expensive compared to other systems. They have been considered but never used in any incremental launch application that the author has been involved in.
Winch systems use cables to pull the girders forward. The winch is typically anchored at one end, and the cable is attached to the tail end of the girder through a distribution beam or a sled that supports the girder. Winch systems are simple and cost-effective for lighter loads and can be used in areas with limited access for large machinery. However, they have limited capacity for very heavy loads. They are difficult to anchor at abutments, requiring a Deadman buried sufficiently in the ground instead, and need precise cable management to avoid tangling or slippage.
In some cases, cranes can be used as the driving mechanism. One option is to push girders forward by applying force directly to the tail end of the girders through a distribution beam or a sled attached to the girders. This method is more common in lighter launches where the push force can be adequately transferred through friction from the crawler crane tracks to the ground. Another option is using a crane for vertical lifting of a line around a pulley system to apply a horizontal pull on the girders. This applies a force on the pulley with a significant vertical component that must be anchored to the ground, usually with a Deadman, unless the pulley can be anchored to a permanent substructure like the abutment cap.
Crane-pushed systems are flexible and can adapt to various site conditions without requiring a permanent pushing setup. However, they offer less precise control over movement and are limited by the crane's capacity and reach.
Launch Rails
In scenarios where a movable sled is used at the ends of the girders, launch rails are essential components of the pushing system. The sled, which supports the girder ends, moves along these rails, reducing friction and providing stability during the launch. A common solution is using a channel section oriented sideways, with the bottom cavity filled with timber and resting on periodic concrete pads on compacted ground. These channels provide a continuous surface for the sled to slide on, ensuring even load distribution across the launch bed.
Anchoring of Pushing Forces
Proper anchoring is essential for stabilizing the pushing system and ensuring the effective transfer of forces to the ground. The specific anchoring requirements depend on the type of pushing equipment used:
Hydraulic Jacks: Typically anchored to the abutment, hydraulic jacks transfer longitudinal forces to the ground through the abutment's foundation, which must be designed to handle these loads effectively.
Hydraulic Rams: Forces from hydraulic rams are transferred to the launch rails, which are anchored to the launch bed through intermediate footings that distribute the load and ensure stability during the launch.
Winch Systems: Winches can be anchored either directly to the abutment or independently to the ground using a Deadman. This anchoring approach ensures that the pulling forces are resisted and safely transferred into the ground.
Crane Systems:
Crane-Pushed: The pushing force is transferred through the crane’s friction with the ground, requiring no additional anchoring.
Crane Lifting with Pulley: Vertical forces from the pulley system need to be anchored either to a Deadman in the ground or to the abutment cap beam, ensuring safe force transfer into the ground.
Comparison of Pushing Equipment
When choosing the appropriate pushing equipment for incremental launching, engineers must consider the specific needs of the project, including the weight of the girders, the distance to be covered, and the site conditions. Below is a summary comparison of the equipment discussed:
Equipment | Advantages | Disadvantages |
Hydraulic Jacks | High precision, significant force, easy synchronization | Expensive, requires operational expertise |
Hydraulic Rams | Suitable for heavy launches, consistent force, effective load distribution | Requires sophisticated rail system for anchorage, force distribution and precise alignment, slower overall operation |
Winch Systems | Cost-effective, suitable for lighter loads | Limited capacity, cable management and anchorage challenges |
Crane-Pushed Systems | Flexible, adaptable to site conditions | Suitable for lighter launches, less precise, crane capacity limits |
Conclusion
The pushing system is a critical component in incremental launching, providing the necessary force to move the structure incrementally across the span. From hydraulic jacks and rams to winch systems and crane-assisted methods, each option offers distinct advantages and challenges, making the choice of equipment highly dependent on the specific project requirements. Launch rails are required for movable end supports for sled-mounted girders. They provide a stable, continuous surface, ensuring even load distribution and alignment during the launch process. Carefully considering the specific project conditions, selecting a suitable pushing system is important for the successful and cost-effective execution of an incremental launch.
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