Civil Engineering Reference
In-Depth Information
benefi ts are greater deck effi ciency and lower weight, leading to economies in the
quantity of prestress, and savings in the foundations.
11.5.2 Voids
Voids may be circular, quasi-circular such as octagonal, or rectangular. Rectangular
voids are assimilated to multi-cell boxes and are covered in
11.7
. The diameter of
circular voids should be a maximum of 240 mm less than the deck thickness, and
preferably 300 mm, and not greater than 0.7 of the deck depth. The ribs between
voids should generally be 250-300 mm wide. The voids are usually stopped short of
the lines of piers, abutments and construction joints to create incorporated crossbeams,
Figure 11.14. Intermediate crossbeams are not required in such decks. Skews may be
easily accommodated.
Various methods have been used to create voids. The commonest is to use
expanded polystyrene, which has the advantage that it is light and easy to cut. In
theory, polystyrene voids can be made of any shape, either by building up rectangular
sections, or by shaping standard sections. In practice, the labour involved in building
up or cutting sections is not economical, and cylindrical voids are usually used. These
cylinders may be cut away locally to widen ribs, or to accommodate prestress anchors,
drainage gullies etc.
The voids must be held down to resist buoyancy, and this is usually achieved
by metal straps, wide enough to avoid cutting into the polystyrene under the very
considerable up-thrust. These straps may be anchored to the passive reinforcement
cage. However, as in most bridges this cage is not heavy enough to resist the fl otation
forces and would be lifted with the voids, the cage itself should be anchored through
the soffi t formwork. The penetrations of the formwork may be masked by siting them
in formwork features such as grooves.
Another solution that has been adopted is to anchor the voids to the reinforcement,
and then to pour a fi rst layer of concrete enclosing the bottom layer of steel, providing
the weight to resist fl otation. Before this lower layer has set, but once it has ceased to
be fl uid, casting is continued. Although effective, this technique requires very good
control of the casting process to avoid the twin dangers of creating a horizontal cold
joint, or of re-mobilising the up-thrust of the concrete.
Various other types of void have been adopted, such as thin spirally wound steel
tubes. These may well need internal stiffening to resist the unbalanced concrete forces
as one rib is cast before the other and need to be held down as for the polystyrene
void formers. They are much less adaptable to the needs of buried prestress anchors,
gullies etc.
Concrete drainage pipes or precast man-hole rings provide cheap and serviceable
voids where adaptability is not required. Their high self weight makes it possible
to hold them down to the reinforcing cage without the need for further anchorage
through the bottom shutter.
11.5.3 Prestress arrangements
Each rib generally accommodates either one or two prestress tendons. The latter is the
preferred arrangement as at the end of the deck one tendon may be swept up to carry
