Incremental launching - General
In situations where the deck is straight (or with constant curvature) and there is a requirement to avoid providing false work in the spanned area, incremental launching may be appropriate. Segments are built at the end of the previous segment and pushed in place. The process is continued until the entire bridge is constructed.
Normally a steel beam or truss element is connected to the leading edge of the bridge to reduce the cantilever moment in the main deck. This steel element is referred to as the nosing. Sliding bearings are installed over the intermediate supports to facilitate movement of the deck.
The critical design loading experienced by the deck is normally that of the launch. This normally leads to span lengths in the range of 40 to 60m if no temporary work is used to enhance the deck strength during launch. However, these figures are only a guide. For example, the concrete deck of the Hungerford Millennium Bridge in London, with spans of about 45 m, was supported by a steel truss along its whole length during incremental launching. This was necessary as the concrete deck was not deep enough to provide the stiffness and strength required during construction.
See also Concrete Society Current Practice Sheet 151, Incrementally launched concrete bridges, by Simon Bourne (Benaim), CONCRETE, July 2007, pp. 7–8. [Note that copies of Current Practice Sheets can be downloaded form the Members´ Area of the Concrete Society web site.]