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  • Writer's pictureRaj Singh

Incremental Launching of Steel Bridge Girders #2: Planning

Planning the launch of bridge steel girders using the Incremental Launching Method (ILM) involves several crucial steps. This process ensures that the construction is efficient, safe, and cost-effective. Key considerations include the number of lines to be launched, the choice between a launch nose and temporary bents, the determination of nose length, and the selection of a pushing system.


Deciding on the Number of Lines

The first step is to decide on the number of girder lines to be launched simultaneously. Ideally, bridges designed for incremental launching should have an even number of girder lines. This flexibility allows for either simultaneous launching of all lines or two identical launches.

Gilbert River Bridge - 2 x 4 girder line launches
Gilbert River Bridge - 2 x 4 girder line launches

For instance, the Gilbert River Bridge used two identical launches of four lines each due to the availability of a pre-existing launch nose suitable to connect with four lines. In contrast, the Athabasca River Bridge launched all ten lines simultaneously to meet schedule constraints, requiring a custom launch nose, thereby doubling up on the temporary works but cutting the launch duration approximately in half.


Athabasca River Bridge - 1 x 10 girder line launch
Athabasca River Bridge - 1 x 10 girder line launch

When configuring an odd number of girder lines, the process can become less efficient from a launching perspective. This is because the temporary works and equipment are controlled by the larger line's launch, making them somewhat redundant for the smaller launch. However, opting for an odd number of girder lines might still be appropriate when optimizing the permanent steel structure. In some cases, the design and load distribution of the final bridge may benefit from having an odd number of girders. This could lead to more efficient use of materials and better structural performance in the long term.


Designer Obligations

The designer of the bridge must ensure the girders can be incrementally launched if site conditions make conventional crane erection very challenging or unfeasible. However, it is not the designer's obligation to optimize the launch design. They must ensure the permanent steel has adequate strength and stability for launching. While doing so, they can make reasonable assumptions for temporary works such as a launch nose and/or intermediate temporary shoring towers to avoid adding additional permanent steel for temporary erection stage demands. Balancing the initial challenges of launching efficiency with the advantages in the permanent structure is something the designer of the bridge could consider in their conceptual design.


They should clearly document the assumptions made regarding the temporary works on the design plans, such as the launch nose length and weight, and the locations of any temporary bents. They must also detail the girders to accommodate a launch procedure, such as keeping the soffit of the bottom flange flush (transitioning the flange thicknesses in the web) and splitting the bottom splice plate to allow rollers to pass through.


Launch Nose vs. Temporary Bents

The erection engineer, along with the erector, can choose between using a launch nose or temporary bents based on preferences, available equipment, cost, and duration considerations. The length of the launch nose is determined by the strength of the girder cantilever. The greater the cantilever capacity, the shorter the nose, and vice versa. Preliminary strength analysis is essential to assess the girder cantilever's strength and deflection capabilities to provide an adequate nose geometry.


Some projects may reduce or eliminate the nose length by using intermediate temporary towers if there are cost advantages. This approach places temporary supports at strategic intervals along the bridge span. Using these temporary towers can minimize or negate the need for a launch nose, which is designed to reduce cantilever demands on the girder as it extends across the span. This can lead to significant cost savings since constructing and installing a long launch nose can be expensive and time-consuming. Therefore, while using a launch nose is common in incremental launching, intermediate temporary towers could provide a cost-effective alternative for projects where relatively short and off-the-shelf shoring towers can be used.


Launch Nose Optimization

Determining the nose length involves analyzing the girder cantilever strength and deflections for various nose lengths. The starting point is usually the design engineer's assumption about nose length and weight. In instances where a custom nose needs to be fabricated, the erection engineer optimizes its size by minimizing the nose weight and length. It's important to note that the touchdown point of the nose can be several meters behind the nose tip, resulting in a longer cantilever than the span being launched across. The taper in the launch nose counters the downward deflection of the cantilever to ensure the nose lands above the temporary supports. If a pre-existing nose is available, it must be verified to ensure it meets both the strength requirements of the resulting cantilever and the geometric requirement of the nose landing above the temporary support upon touchdown.


Custom launch nose designed and fabricated for the Athabasca River Bridge launch
Custom launch nose for the Athabasca River Bridge

For example, a 57m long pre-existing launch nose was used for the Gilbert River Bridge, while the Athabasca River Bridge used a 22m long custom nose. The Gilbert River nose was not optimized for this particular launch, as a generic nose from the erector's inventory was adapted to fit several launch situations. Even though it wasn't optimized, reusing an existing nose for multiple bridges saves cost and time. On the other hand, for Athabasca, there was no previous nose available, so a custom nose was designed and fabricated. While it was optimized for this specific launch, it may not be suitable for reuse.


In certain instances, the addition of a minimal amount of permanent steel may negate the necessity for a launch nose. For example, the Sombrio Bridge, featuring spans of 40m and 82m, had girders capable of cantilevering most of the longer span when launched from the abutment of the shorter span. Introducing a small quantity of permanent steel would enable them to nearly reach the distant abutment without a nose. Subsequently, a crane situated behind the abutment would lift the tip onto the permanent bearings. Opting for thicker permanent steel plates and the use of a crane, the erector decided against the expense of designing and fabricating a nose.


Selecting a Pushing System

The pushing system is a crucial component of the Incremental Launching Method (ILM), as it applies the necessary force to move the assembled girder segments forward along the launching bed. Selecting the appropriate system depends on several factors, including the weight of the girder segments, the length of the launch, and the specific site conditions. Below are some common options for pushing systems and their considerations:


  • Hydraulic Jacks: Hydraulic jacks are frequently used due to their precision and control. They can generate substantial pushing force and are adaptable to different site conditions. For example, the Athabasca River Bridge utilized 50-ton hollow core jacks at each girder line. These jacks pulled on strands anchored at the abutment, effectively moving the girder segments forward. The hydraulic jacks' reaction force must be securely dissipated into the ground to ensure stability.

Athabasca River Bridge - Pushing system using hydraulic hollow core jacks
Athabasca River Bridge - Pushing system using hydraulic hollow core jacks
  • Hydraulic Rams: Similar to hydraulic jacks, hydraulic rams are capable of exerting high levels of force. They are typically used in conjunction with a series of pushing and pulling cycles to incrementally move the girder segments. Hydraulic rams are advantageous for their strength and ability to handle heavy loads but require careful alignment and support to function effectively.


  • Winches: Winches provide a cost-effective solution for pushing girder segments, especially in smaller projects. They use cables or ropes wound around a drum to pull the segments forward. While winches are simpler and less expensive than hydraulic systems, they might not provide the same level of control and precision, making them less suitable for larger or more complex launches.


  • Excavators: In some cases, excavators can be adapted to push girder segments. This method is typically used for shorter spans or where the weight of the segments is within the excavator's pushing capacity. Excavators provide flexibility and mobility but might lack the precision required for longer or more complex launches.


Excavator pushing the launch
Excavator pushing the launch

Planning the Launch Bed

The launch bed must be planned based on available space and the preferred length of girder segments for preassembly. A longer launch bed reduces launch time as a longer length of segments can be assembled and pushed, allowing for fewer overall launches and thus expediting the construction process. This means that more significant portions of the bridge can be incrementally moved into place in a single operation, improving efficiency and potentially reducing labor costs over time. However, this benefit comes at the expense of increased ground preparation costs.

Intermediate supports in the launch bed
Temporary intermediate supports in the launch bed

The bed must withstand the loads from the intermediate supports supporting the segments, usually compacted to a proctor density of 98%. Preparing a longer launch bed requires more extensive site work, including grading, compacting, and possibly reinforcing the ground to ensure it can support the weight and movement of the assembled girder segments. These additional preparation steps can significantly add to the initial project costs, necessitating a careful cost-benefit analysis to determine the optimal length of the launch bed for the specific project requirements.


Vertical Support System

The vertical supports on which the girder assembly is launched are generally provided using rollers. These rollers play a crucial role in facilitating the smooth and controlled movement of the girder segments during the launching process. The sizing and placement of these supports are critical during the planning stage to ensure smooth and efficient launching.


Typical roller assembly support
Typical roller assembly support

Roller assemblies are used to distribute the load and prevent excessive stress on any single point. The material of the rollers is also a consideration, as it must be strong enough to withstand the weight and friction of the moving segments while maintaining durability over multiple uses. Typically, rollers are placed at regular intervals along the length of the launching bed and on top of each abutment and pier/bent . Proper alignment of the rollers is crucial for the safe and efficient movement of the girders. Misaligned rollers can cause uneven loading, leading to potential structural damage or even failure during the launch. Stability is also a key concern; the rollers must be anchored securely to withstand the horizontal and vertical forces exerted during the incremental pushing of the girder segments.


There are different types of rollers that can be used depending on the specific requirements of the project. Fixed rollers are stationary and provide a stable base for the girder segments to move over, making them ideal for straightforward launching paths. Swivel rollers can rotate to accommodate changes in direction, which is useful for more complex launching paths that require flexibility. Hydraulic rollers offer the capability to be adjusted in height and angle, providing adaptability to varying terrain and structural requirements. Common suppliers of these types of rollers include Hillman, known for their high-quality mechanical motion control solutions, and Enerpac, which offers a wide range of hydraulic tools and equipment.


Conclusion

Effective planning is crucial for the successful implementation of the Incremental Launching Method. Properly deciding on the number of lines, choosing between a launch nose and temporary bents, determining nose length, and selecting a pushing system ensures a safe, efficient, and cost-effective bridge construction process.


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