Civil Engineering Reference
In-Depth Information
without pier diaphragms the ribs will rotate until the moments at the rib centre line
have been brought into equilibrium. This rotation will apply bending moments to the
abutment diaphragms, and, amplifi ed by creep, will affect the design of the bearings,
and in extreme cases may affect the serviceability of the deck. For a deck with pier
diaphragms, any imbalance of the transverse moments will cumulate as torque in the
ribs at each pier, Figure 12.11.
The issues that control the design of the intermediate slab are closely related to the
behaviour of these decks described in
12.2
. The slab is subjected to two superimposed
modes of bending; 'fi eld' bending due to the relative defl ections and rotations of the
ribs, and 'local' bending under the wheel loads themselves. The logic controlling the
thickness of the side cantilever and of the intermediate slab is described in
9.2
and
9.3.6
respectively. Typical forms of some twin rib decks are shown in Figure 12.12.
12.5 Ribbed slabs and skew bridges
Ribbed slabs are particularly suited to skew decks. A conventional box section deck
with side cantilevers carried on two bearings on each skew pier is heavily stressed in
torsion as the twin supports impose a bending axis that is not perpendicular to the
bridge centre line, Figure 12.13 (a).
However, each rib of an equivalent twin rib deck may bend about a perpendicular
axis and there will be no signifi cant torsion due to the skew geometry. In the heavily
skewed deck shown in Figure 12.13 (b), the piers beneath one rib are located
approximately opposite the mid-spans of the other. Live loads will defl ect the ribs
causing transverse bending and shear in the slab, transferring load to the pier section of
the other rib. Thus a skew arrangement of piers may even reduce the live load bending
moments in a twin rib deck, leading to savings in prestress.
This skew arrangement of piers also highlights an essential difference between
prestressed and reinforced decks. In a reinforced deck, the ribs defl ects downwards
under both permanent loads and live loads, causing such heavy bending of the slab that
the arrangement would probably not be feasible. On the other hand, in the prestressed
deck described above, the ribs defl ect slightly upwards under the combined effects of
prestress and dead load, and downwards under live loads, leading to relatively light
transverse bending of the slab.
It is frequently the case that the cancelling of self-weight defl ections by prestress
makes feasible structural arrangements that would not be viable in reinforced concrete.
The engineer should always consider the defl ection of his structures.
12.6 Heat of hydration effects on twin rib decks
These effects were highlighted by the severe cracking during construction of a deck
designed by the author. The deck consisted of two wide, solid prestressed concrete ribs
1.5 m deep, connected by a thin reinforced concrete top slab, with side cantilevers.
The design of the prestressing was carried out in the normal way by analysing the
deck at the tenth points of each span. In common with most ribbed slab bridge decks,
the bottom fi bre was very lightly compressed under self weight plus prestress. In fact,
at the fi rst tenth point of the main span adjacent to the intermediate piers, the stress
