Civil Engineering Reference
In-Depth Information
Figure 9.44
Custom-designed gulley in thin cantilever slab
f) Flat gullies in side cantilever
Box section decks with wide side cantilevers regularly pose drainage problems, as the
gutters usually are situated towards the end of the cantilever where the slab is thin.
Troughs or hollow kerbs may solve the problem, if the gradients permit.
Alternatively, very fl at gullies may be custom designed, in glass reinforced plastic
(GRP) or, if enough are required, in cast iron. Such custom-made gullies, associated
with 100 mm drainage pipes, may be accommodated in slabs as thin as 300 mm,
Figure 9.44. It is important in the hydraulic design of such gullies that care is taken to
avoid dead areas where rubbish may accumulate. The gullies may need to be located
so that the transverse pipes do not clash with the prestressing ducts. These gullies may
discharge into a carrier pipe located within the box girder. Alternatively, if the areas
and gradients of the road permit, they may be located opposite each pier, and the
water piped directly to ground level. If the required gully spacing is less than the span,
hollow kerbs or troughs may be used, with discharge pipes at piers.
In countries where rainfall is heavier than in the UK, national regulations may impose
silt traps at each gully, as well as minimum pipe diameters, 150 mm for instance. When
it is impossible to hide the drainage it is up to the designer to fi nd a system that is
neat and does not look like an afterthought. For instance, by adopting hollow kerb
drainage if necessary, with discharge pipes placed opposite each pier, the silt trap and
the transverse discharge pipe may be housed in a downstand rib that becomes visually
associated with the pier.
g) East Moors Viaduct
An original solution was adopted for the East Moors Viaduct. The deck was 13 m wide
and 914 m long. The longitudinal alignment had a summit curve, and the S-shaped
horizontal alignment required a change in crossfall. Gullies in the kerb discharged into
carrier pipes situated beneath a precast footpath, Figure 9.2.
9.8 Waterproofi ng
Any bridge deck that will be treated with de-icing salts must have a waterproof
membrane. Many proprietary products are available with standard details to solve the
most common problems.
Water that seeps through the black-top and onto the membrane will be put under
high pressure by the weight of heavy wheels above. It will either be forced upwards,
