Civil Engineering Reference
In-Depth Information
15
The construction of girder bridges
15.1 General
There is a close link between the designer's choice of bridge deck type and the most
appropriate method of construction. In general, solid and voided slabs and twin rib
decks are appropriate for the cast-in-situ construction of complete spans on falsework,
while box sections are most easily cast in-situ in short sections, or precast. One
reasonable rule of thumb is that if a deck is to be cast in-situ on falsework it should be
possible to pour a complete span in one continuous operation.
There is also a close link between the method of construction of a concrete deck
and the detailed design of the prestress. For instance, if a deck is designed to be built in
balanced cantilever, a change in the method of construction would require a redesign
of the deck. For this reason, the descriptions of the prestress layouts and of the various
methods of building decks are combined in this chapter.
The two principal components in the cost of building a bridge deck (apart from the
material content of the deck) are labour and plant. They are closely interdependent as
the cost of labour may be reduced by investing in plant to automate the construction
process. The balance between the investment in plant and the reliance on labour
depends principally on the scale of the project. The infl uence of scale on the choice of
the type of bridge deck and the method of construction is covered in Chapter 16.
15.2 Cast-in-situ span-by-span construction of continuous beams
15.2.1 General
Span-by-span construction is applicable to decks with spans that lie generally between
20 m and 45 m. Due to the diffi culties of casting boxes in-situ, decks are most commonly
solid or voided slab or twin rib, and are usually continuous.
However, some contractors have special skills in casting boxes in situ. The sharply
curved 50 m spans of a series of viaducts on A8 motorway in southern France, were
built at a rate of a span every three weeks on an articulated gantry, and were steam
cured in-situ, Figure 15.1. German contractors have great experience in building box
section viaducts span-by-span using self-launching gantries. For instance, the bridge
carrying the A14 autobahn over the Ahr Valley, built in 1975, used a 2,000 t gantry to
cast in-situ spans up to 106 m long, at a rate of a span in 14 days [1].
