Civil Engineering Reference
In-Depth Information
Figure 12.3
Viaduc d'Incarville, Autoroute de Normandie: self-launching rig (Photo Robert
Benaim)
Twin rib bridges are essentially a cast-in-situ solution for spans between 20 m and
45 m, and are very well adapted to the span-by-span construction of long continuous
viaducts.
12.2 Behaviour of twin rib decks
12.2.1 Length of load and end fi xity of the slab
A particular characteristic of twin rib decks without diaphragms is that the end fi xity of
the slab varies with the loaded length. When a short load (measured along the span) is
applied to the centre of the slab, the twisting of the ribs is inhibited by the slab either
side of the loaded patch and consequently the slab is substantially fi xed ended, with the
support moments carried in torsion in the ribs. As the load becomes longer, the ribs are
freer to twist and the end restraint of the slab reduces until, when the full length of the
deck is loaded, it behaves as simply supported on the rib axes, Figure 12.4. As vehicle
loading is made up of both long and short loads, this behaviour must be understood,
and refl ected in the appropriate calculation model. For instance, under the effect of
lane loading applied to complete spans the deck slab is weakly restrained while under
localised wheel or truck loading it is virtually fi xed ended. Both these loading cases
may co-exist.
