Civil Engineering Reference
In-Depth Information
The substructure will be more expensive due to the increased dead weight of the
deck. Also, if the pier consists of one central column a crosshead will be necessary to
carry the two boxes. If the pier consists of one column beneath each box, the cost of
the foundation to carrying a live load that consists of a single heavy vehicle (such as
the British HB Vehicle) is increased as it is carried twice.
If the deck is made of precast segments a 15 m wide segment, 3.5 m long is likely
to weigh of the order of 80 tons, while the narrower segments will weigh about 30
tons, and may more easily be transported by road. The casting cells for the smaller
boxes would be smaller and cheaper, the lifting equipment lighter, and the precasting
run doubled.
The practical maximum width of deck that may be carried by these two small boxes
without thickening the top slab is defi ned by 2.8 m long side cantilevers and a span
between boxes of 6 m, giving an overall width of 20.1 m, Figure 13.10 (c). The webs
would need to be increased in width to about 450 mm at the piers. A single box
could still be used for this width, with typically a box that is 10 m wide at the top
Figure 13.10
Strategies for wide decks
