Civil Engineering Reference
In-Depth Information
Figure 10.6
Kwai Chung Viaduct (Photo: Phototèque VINCI et fi liales)
A reinforced concrete top slab, typically 200 mm thick, is allowed to span
transversally as far as possible and is then supported by the least number of beams.
This transverse span depends on the loading code, but is typically between 3.5 m
and 4 m. It is sometimes economical to increase slightly the thickness of this slab
to accommodate a larger transverse span and reduce by one the number of beams.
The side cantilevers are usually short; the cantilever bending moments must remain
compatible with the thickness of the slab as no moment turns down into the webs.
As the heels of the beams are not linked, the webs take no transverse bending
moment. Consequently, they may be made as thin as possible, subject only to the need
to provide corrosion protection to the reinforcement and to compact the concrete
in the heel (
9.5.2
). Since the prestress carries a proportion of the shear force, the
webs do not usually need thickening, except locally at the ends of the beams to carry
the concentrated force of the post-tensioning anchors at their ends. This thickening,
to a value dictated by the anchors, is usually extended forwards by approximately
a beam depth. The heel is shaped to facilitate the fl ow of concrete from the web
and is dimensioned to avoid over-stressing of the concrete at the fi rst tensioning
of the tendons. Its size depends essentially on the span/depth ratio of the deck, all
other factors being equal. Modern design practice for highway bridges is to provide
diaphragms at the beam-ends only.
Usually the top slab is given a slight haunch, generally 50 mm, with a 100 mm fi llet
at the junction with the web. As well as reducing shear stresses in the slab and improving
the arching action (
9.3.5
), this geometry aids striking of the shutter (
10.3.6
).
