Civil Engineering Reference
In-Depth Information
spacing greater than about 2.5 m, it is most likely that the top slab will have to be
thickened at the root of the cantilever, increasing the self weight of the deck.
Figure 10.10 (c) shows the third arrangement, generally favoured by the author.
Here the top slab is precast over its full depth, but over a reduced width. The remainder
of the top slab is then cast in-situ. The width of the precast slab may be chosen to suit
the circumstances; the wider the slab the less concrete there is to cast in a second
phase, but the heavier the beams. Typically, the top slab of the beam is 1.2 m to 1.7 m
wide.
Once the beams have been launched into place, the top fl ange gives a good working
surface. The shutter for the cast-in-situ slab may be simply suspended from the precast
stub cantilevers. The cast-in-situ or prefabricated parapet may be attached directly to
the precast beam, or alternatively the width of the deck may be increased by casting on
a short in-situ side cantilever.
The top slab cast with the beams is usually inclined to the crossfall of the road.
However, if the precast slab is narrow, it may be cast horizontal, with the cast-in-
situ slab more steeply inclined, Figure 10.10 (d). The road surfacing will then be of
variable thickness.
The construction joint between the precast and the in-situ slab needs to be designed in
shear friction, requiring projecting reinforcement. Although a vertical joint is perfectly
safe if correctly designed, an additional factor of safety may easily be introduced by
giving a small inclination to the joint surface, which in any event should be roughened.
A variety of reinforcement arrangements for different widths of cast-in-situ slab are
shown in Figure 10.11. If the projecting reinforcement of adjacent beams overlaps,
more care is required in placing the beams. The top slab may also be transversally
prestressed, in which case all projecting steel may be omitted, subject of course to
thoughtful design.
If the deck is not to be waterproofed, consideration must be given to the risk of
water seepage through the slab construction joints.
10.3.4 Prestress layout
The reasoning governing the layout of the prestressing cables in precast Tee beams has
been described in Chapter 5.
10.3.5 Reinforcement layout
As there is no transverse bending moment in the webs, and much of the shear is carried by
the prestressing tendons, typical highway bridge decks are generally lightly reinforced,
with typically 16 mm shear links near the beam-ends, and 12 mm links elsewhere, with
10 mm distribution steel. Railway bridges with their heavy live loads could well require
larger diameter reinforcement. Heavier steel will be required immediately behind the
prestress anchors, and in the zone of dispersion of the prestress. Also, as there are no
bending stresses in the deck due to differential settlement and temperature gradients,
and no signifi cant heat of hydration effects, there is no need to provide longitudinal
passive reinforcement for a fully prestressed design, other than corner bars for the
links, and a light surface grillage on the external surfaces of the heel, using 10 mm or
12 mm bars. The top slab will of course be more heavily reinforced to carry the traffi c
loading.
