Civil Engineering Reference
In-Depth Information
Figure 16.1
Typical alternatives for a variable-depth wide deck subject to UK loading
16.2.2 Precast
As the size of the bridge provides inadequate repetition for precast segmental
construction, and the spans are too long for precast beams, precast construction is not
appropriate for this bridge.
16.4 The bridge is 500 m long
16.4.1 Cast-in-situ
With a twin deck arrangement, there is 1,000 m of deck to build. If the spans can be
less than 40 m, the twin rib, twin deck arrangement is likely to be the most economical.
However, with at least 25 spans to build, it is worth investing in plant to speed up the
method of construction and save on labour costs. The birdcage scaffold should be
replaced with a semi-mechanised falsework rig that can build a span every 2 weeks
(
15.2
and Figure 12.2).
The complete deck erection, allowing for a learning curve and the time to relocate
the falsework for the second deck, would be about 65 weeks, or 2.6 weeks per span on
average. If the rate of substructure construction may be designed to match the rate of
deck erection, the substructure only needs a short start over the deck. The falsework
and deck formwork are relatively quick to design and fabricate, so a relatively short
mobilisation period helps compensate for the slow construction.
A disadvantage of the twin deck arrangement is the need for four columns per pier,
or two columns with crossheads, or pier diaphragms in the deck. This may be overcome
by adopting a single deck in a super twin rib layout, Figure 12.17. This type of deck,
