Civil Engineering Reference
In-Depth Information
One most often sees inverted T and M beams placed at 1 m centres, giving a
continuous soffi t. Permanent falsework is then placed and a reinforced concrete slab,
generally 160 mm thick cast on top, Figure 10.1 (a). This form of construction is not
economical unless it is essential to minimise the depth of the deck. It has the further
disadvantage that it is not possible to inspect the internal beams during the life of the
bridge.
In all cases when it is not necessary to minimise depth, deeper beams, more widely
spaced, would result in signifi cant savings. Typically a spacing of 2 m is possible, halving
the number of beams, Figure 10.1 (b). Of course each beam will be more expensive,
containing more concrete, prestress and reinforcement, but these are marginal costs;
so much of their cost, including storage, transport, delivery and placing, is almost
independent of their material content. The slab will have to be slightly thicker, say
180-200 mm, but slab concrete is cheap.
The most economical way of supporting the beams is to rest each of them
on a laminated rubber bearing carried by a crosshead located beneath the deck,
Figure 10.2 (a). This is not very good looking, and a variation is to design a crosshead
of inverted T cross section, that is mainly within the thickness of the deck, with a
bearing ledge protruding below, Figure 10.2 (b). The bearing ledge may be incorporated
into the depth of the deck by using halving joints at the beam ends. However this
signifi cantly increases the cost of the beams, which derive their economy from their
standardisation. The crossheads form a very important component of the cost of these
Figure 10.1
Standard precast beams
