Civil Engineering Reference
In-Depth Information
Figure 8.6
Single-cell box girder subjected to an eccentric point load
The capacity of the deck to re-centre loads is improved if the beams have signifi cant
torsional stiffness. The beams resist being twisted by local loading, and their relative
defl ections generate bending moments and vertical shears in the slab, transferring load
to the adjacent beam. This type of deck, known as a ribbed slab, may be designed
without any diaphragms, except at the abutments where support for the end of the
deck slab is essential. When the slab between the ribs is subjected to a concentrated
load, it becomes effectively built in, rather than supported on knife edges, as is the
case for torsionless beams. It can thus span further, allowing the number of ribs to be
reduced. Most typical carriageways can be carried by two such torsionally stiff ribs,
giving rise to the name 'twin rib decks'. This type of deck is described in Chapter 12,
and typical examples are shown in Figures 12.1 and 12.12. A very useful concept for
assessing the ability of a deck to re-centre loads is the transverse infl uence line for load
carried by each beam. For these twin rib type decks, this is shown in Figure 12.6.
The other extreme from the torsionless deck is a deck with a single central torsionally
stiff member such as a single-cell box girder, Figure 8.6. The concentrated live load
is only carried once, but at the expense of torsionally induced shear stresses in the
webs and slabs, which may require thicker members and more reinforcement. This
latter concept uses less material for longitudinal bending. This does not necessarily
mean that it is more economical as economy depends also on the quantity of materials
required for torsion and transverse bending and on the unit costs of materials, which
is itself dependent on the methods of construction.
8.4 Material quantities and costs
8.4.1 General
There are three principal components to the cost of a bridge deck:
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the quantity of materials
the unit cost of the materials
the cost of the erection method.
These three components are all within the control of the designer, although
consultation with the contractor, when this is contractually possible, should be
encouraged. Very approximately, the materials and the erection method each account
for half the cost of the deck.
