Civil Engineering Reference
In-Depth Information
14
Counter-cast technology for box
section decks
14.1 General
As has been stated elsewhere in this topic, building box girder bridges in long lengths
in-situ is a slow and diffi cult task, prone to defects and delays, tying up expensive
falsework for relatively long periods. These diffi culties have been the stimulus for the
adoption of segmental methods of construction, such as cast-in-situ balanced cantilever
and incremental launching. However, both these methods are only applicable to
bridges of suitable layout and geometry. Many bridges, particularly in towns, have
span layouts and geometry that are not regular, constrained by the urban fabric.
This diffi culty was tackled in bridges built in the 1960s and 1970s, including some
major urban viaducts, such as the A40 Westway viaduct in London, and the Mancunian
Way viaduct in Manchester, by precasting the deck in short transverse segments, and
assembling them on falsework. Narrow, unreinforced joints were cast in mortar or
small-aggregate concrete to complete the deck, which was subsequently prestressed.
Although this allowed complex box sections to be cast under factory conditions, this
method of construction combined some of the disadvantages of both precast and cast-
in-situ construction. Falsework was still required, in addition to the typical precasting
costs of the multiple handling of heavy segments, their transport and their careful
adjustment on the falsework. On curved alignments, the precast deck segments had to
be made trapezoidal in plan, as the joints were too narrow to accommodate the taper.
In addition, coupling tendon ducts across the joint and pouring the joint itself were
diffi cult and time consuming.
In 1962, the French contractor Campenon Bernard built the Choisy-le-Roi Bridge
across the river Seine, adopting a brilliant variation of this form of construction. The
bridge deck was cast in short transverse slices as before, but each segment used the
previous one as an end shutter. Consequently they were a perfect fi t, and could be
reassembled on site without the need for a cast-in-situ joint. This greatly speeded up
erection, and made it possible to adopt several methods of deck assembly, adapted to
a wide variety of alignments. But of course it removed any possibility of adjusting the
position of the segments; the precast alignment had to be perfect.
The precast segments may be fabricated in either the long line or the short line
method.
