Civil Engineering Reference
In-Depth Information
Figure 15.8
Route 3 Gantry (Photo: Benaim)
15.3.2 Statically determinate spans with internal tendons and glued joints
One of the most economical options for a long viaduct is to design it as a series of
statically determinate spans. For highway bridges, groups of up to 10 spans may be
linked together at the top slab to minimise the number of expansion joints. Although
such decks need up to 20 per cent more prestress than continuous decks, their
simplicity and speed of erection overcome this disadvantage. Once a span is complete
the construction of the following span may start immediately; creating continuity takes
up at least one additional day per span. The span length for such decks should in
general not exceed 45 m, with a span/depth ratio preferably of about 14-16 for UK
loading.
On the 10 km long West Rail Viaducts which were designed by Benaim and are
typical of the method, underslung girders, very similar to that shown in Figure 15.7,
carry on their top boom a slide track that may be polished stainless steel or rails. The
point of support of the deck segments may be some distance from the root of the
cantilever, particularly if the deck is curved, and the side cantilevers must be reinforced
for this sagging moment, Figure 15.9. The segments are placed on sledges that slide or
roll on the track, Figure 15.10. The sledges are each equipped with lifting jacks and
small lateral jacks that allow precise positioning of the segments once they are brought
into contact. The lifting jacks must have suffi cient travel so that they may provide a
true alignment to the deck despite the defl ection of the gantry.
All the segments for a span are loaded onto the gantry, so that it takes up its defl ection.
A gap of about 200-300 mm is left between the fi rst segment and the remainder for
the application of the epoxy. Once the resin has been applied, the two fi rst segments
are brought together and clamped using temporary prestress, the procedure being
