Civil Engineering Reference
In-Depth Information
shutters for the side cantilevers and for the outside face of the ribs were suspended
directly from the truss, while the shutter for the slab between ribs and for the inside
face of the ribs was carried by the steel girder that was suspended from the truss
during concreting, Figure 12.10. The reinforcing cage for each rib, complete with
prestressing tendons, was prefabricated on the bridge deck behind the construction
head, delivered to the rig by a purpose-designed trolley and lowered into the steel
shutters. The reinforcement of the slab was fi xed in-situ. The pier diaphragms were
omitted from the fi rst stage of construction and built subsequently, which allowed the
launching of the internal formwork. The lower girder was launched fi rst, and then
acted as a bridge to carry the main falsework, Figure 15.6. The rig built the bridge
at an average rate of a span every two weeks. As the bridge consisted of only 16
spans, there was no pressure to increase the rate of construction. However, this type
of mechanised falsework has the potential to build much more quickly.
It is undoubtedly possible to construct twin rib decks in-situ at a rate of two spans
per week, rivalling precast segmental for the longest viaducts. In order to achieve
such a rhythm, the reinforcing cage for an entire span complete with post-tensioning
tendons, weighing some 50 tons for a typical 12 m × 35 m span, would be prepared
behind one abutment, transported over the completed deck, and lowered into the
shutter. The casting and compaction of the 250 m
3
of concrete would take about
7 hours using two pumps, starting at the forward end. This would allow time for
the prestressing ducts to be coupled to the previous span, and the reinforcement at
the construction joint to be arranged. It would also give a few additional hours of
maturity to the concrete beneath the prestress anchors. Thus in one twelve-hour
shift the reinforcing cage could be placed and the concrete cast. The next evening,
when the concrete beneath the anchors had attained a strength of 25 MPa, half the
Figure 15.6
Viaduc d'Incarville: falsework truss being launched (Photo: Robert Benaim)
