Civil Engineering Reference
In-Depth Information
15.4.3 Construction of the deck
Typically, a hammerhead at least 6 m long is cast off falsework on each pier. A traveller
is then lifted onto the hammerhead and builds a fi rst deck segment. This traveller is
then launched forwards, clearing the hammerhead for the second traveller that will
build the balancing half span. The pair of travellers then proceeds to build the two
adjacent half spans in balanced pairs of segments, Figure 15.15.
As each pair of segments is completed, prestressing cables are threaded and stressed.
When the balanced cantilevers are complete, a mid-span stitch, usually 2-3 m long,
is cast, using one of the travellers as falsework. Continuity cables are then threaded
through the stitch and stressed.
The travellers used to build cantilever bridges are usually simple steel trusses that
suspend a platform on which is placed the shutter. They are counterbalanced so that
they are stable under their own self weight, and are tied down to the deck to resist
the weight of concrete. The main design criterion for the truss is to limit its defl ection
as the concrete is cast. The bottom slab which is cast fi rst may be cracked by this
defl ection at its junction with the previous segment as the main weight of concrete is
poured. It is common practice to leave a strip some 300 mm wide between the new
bottom slab and the previous segment, which is then cast once the webs and top slab
are complete.
Each deck segment is normally between 3.5 m and 5 m long. After a short learning
curve, it is traditional to cast a pair of segments each week. A reasonable approximation
to a construction programme is to assume that the hammerhead takes 6-8 weeks to
build, the fi rst pair of segments four weeks, the second pair three weeks, the third and
fourth pair two weeks, and one week per pair thereafter.
Figure 15.15
Travellers for River Dee Bridge, Newport, UK (Photo: Edmund Nuttall/Molyneux
Photography)
