Incremental Launching of Steel Bridge Girders #1: Introduction

The Incremental Launching Methodology (ILM) is a construction technique primarily used for erecting bridge superstructures. It's particularly beneficial in locations with access challenges due to terrain or water presence. The process involves assembling girder segments within a launching bed at one end of the bridge, then incrementally moving the assembled unit forward, adding new segments at the back and pushing until the girder reaches its final position. This method offers several benefits, including reduced construction time, improved safety due to less work at height, and less environmental impact, making it especially advantageous for constructing bridges over challenging terrains or water bodies, busy roads, and areas with difficult access where traditional methods might be impractical or too expensive.

Steel girders being launched across Athabasca River, Ft. McMurray, AB

Incremental Launching Process

The incremental launching process for steel girders involves several stages:

• Preparation of the Launching Bed: The initial stage involves setting up a launching bed behind one of the bridge abutments. This bed serves as the assembly area for the steel girder segments and a runway from which the launching occurs.

Launch bed assembly of girder segments

• Assembly: Steel girder segments are assembled in the launching bay to form multi-girder units connected with cross frames and diaphragms ranging from two to as high as ten girder lines. These girder segments are typically between 15 to 50 meters in length. They are unloaded from delivery trucks and assembled using cranes and conventional steel erection methods, usually aligned with their transverse positioning on the bridge.

Driving system using strand jacks

• Launching Process: Once the requisite length of an assembled girder unit is completed, it is driven forward over temporary supports using a driving system powered by equipment such as hydraulic jacks, rams, cranes, or winches. The assembled girder unit is launched incrementally, typically moving 15 to 25 meters at a time. This process is repeated, adding new segments at the back end and driving the entire structure forward until it reaches the opposite abutment.

• Final Alignment and Jack Down: After the entire bridge has been launched, it is aligned, jacked down, and connected to the permanent bearings. Any necessary adjustments are made to ensure the bridge is in its correct final position before the permanent bearings are grouted.

Advantages of Incremental Launching for Steel Girders

• Cost-Effectiveness: ILM is cost-effective, especially for long bridges or those in difficult-to-access locations. It eliminates the need for extensive shoring, access roads and working platforms along the span, reducing material costs and labor.

• Safety: The method enhances safety by minimizing the need for workers to perform tasks at height or over water. Most construction activities occur on land, in a controlled environment, reducing the risks associated with steel erection.

• Minimal Environmental Impact: ILM is less disruptive to the environment compared to traditional methods. It minimizes disruption to the ecosystem below, such as rivers, valleys, or roadways.

• Efficiency: The incremental launching process can be faster than traditional methods, particularly for long spans. Its repetitive nature allows for a more streamlined and efficient construction timeline.

Challenges and Considerations

Despite its advantages, ILM presents several challenges:

• Initial Setup Costs: The setup for ILM, including the construction of the launching bed and acquisition of required equipment, can be costly and may have a long lead time. However, these initial costs are often offset by the savings during the construction process.

• Sophisticated Engineering: The method demands sophisticated engineering to ensure the deflections, stresses, and reactions during each step of the launch are accurately calculated. This ensures the strength and stability of the steel assembly at all stages. Meticulous planning is required to ensure that the launched steel assembly aligns correctly. It also needs constant monitoring to conform to the limitations outlined in the erection plans, which include wind restrictions. Misalignments can accumulate and magnify, leading to jamming. This can be challenging to correct and may result in costly delays.

• Technical Expertise: Successful implementation of ILM requires specialized technical expertise and experience in both engineering and implementation. Engineers and construction crews must be well-versed in the method to manage the complexities involved.

Conclusion

The Incremental Launching Method (ILM) for steel girders is an advanced bridge construction technique. It offers many advantages, especially in environmentally sensitive sites with difficult access. ILM involves assembling steel girder segments on land and then launching them into place. This improves safety, reduces environmental impact, and can be more cost-effective and efficient than traditional methods. In Canada, it's typically used to erect bridge structural steel in environments where conventional crane erection isn't feasible or becomes cost prohibitive.