The bridge launching method is also known as the incremental launching method. It is used for bridge decks that are greater than 250 m in length. In the bridge launching method of construction, the bridge deck is made in sections. This is done by pushing the structure of the bridge outwards using abutments towards the pier.
The incremental bridge launching method needs a considerable amount of analysis and design expertise. It also requires specialized bridge launching machine. This method is most suited for constructing bridges that have a constant radius of curvature. It is a very fast method of construction.
Bridge launching for construction
In the beginning of the construction, a highly mechanized prefabrication deck mould is set up behind one abutment. By doing this, there is no need for temporary support. After this, a rigid framework is attached. This enables the production of cast-in-situ segments. The in-situ deck segments can range anywhere from 5m to 30m in length.
After each segment is complete, it is placed on sliding bearings. Then, it is pushed through into the span. Also, a steel nose is placed in front of the first span formwork. This gives way for all necessary deck cantilever length while the span reaches the first pier.
The average rate of construction using the bridge launching method is 30m a week. The bridge launching method of construction becomes worthy of consideration for project sites that face some challenges. The challenges are:
1. Steep slows or deep valleys. This makes the delivery of materials difficult
2.Deep water crossings
3.Limited or prevention of access due to environmental restrictions
4.Limited access to area beneath the bridge construction. This could be because of heavy traffic on roadways or railways
To simplify fabrication and construction, the bridge that is supposed to be constructed using incremental bridge launching, is designed along a tangent alignment in both horizontal and vertical planes. The bridge sites that fit these ideal conditions are rare. This is especially the case when this scenario is combined with the close proximity of any of the site restrictions listed above. This makes it a more challenging process. It is still possible to construct a bridge along a curved alignment though, in either or both the planes.
It is a necessity for the construction surfaces to remain perfectly aligned with the superstructure, especially during the launch of operations. This is done to eliminate the relocation as well as the adjustment of lateral bearings.
Things to consider when using the bridge launching method of construction
1.There should be proper communication channels set between the contractor, monitoring consultant as well as the structural designer to monitor the to-be launched bridges.
2.There should also be a comprehensive monitoring program implemented. This can be used to alert either the contractor, consultant or designer in case something goes wrong.
3.There should be a pre-launch and post-launch survey of the structure.
4. There should be proper methods of monitoring the plumbness of the piers during the bridge launching and after
There are a lot of other things to be monitored during the bridge launching and after. Some of these are as follows:
1.The structural response of the piers
2.Touchdown forces during the bridge launching as well as the passage of the main structure over the pier
3.Girder flexural behaviour during the bridge launching
4.Load transfer mechanism that comes between girders
5. The horizontal force that is necessary for launching different stages of construction