Civil Engineering Reference
In-Depth Information
thickness increases at a rate of approximately 0.004 m per metre of span. One would
expect a variation either side of this mean line of about 10 per cent.
Bridges built by the method of incremental launching tend to require some 10-
15 per cent more concrete than those built by other methods, principally because they
have thicker and deeper webs, although if built using intermediate props to cut down
the launching span, this is less marked.
Twin rib type bridges are generally up to 20 per cent thicker than the mean, and
follow a steeper increase in thickness with increasing span, due to the preponderance
of their webs in their cross-section area. They follow a different logic, with a thickness
of 480 mm at 20 m, increasing at a rate of approximately 0.006 m/m.
Figure 8.8 includes only bridges designed by Benaim to British codes of practice,
or to very closely related codes such as those used in Hong Kong and Malaysia. This
historical record may need adjusting slightly where new regulations increase the cover
required for reinforcement to values greater than those described in
9.1.
It should not be forgotten that certain types of deck are generally associated with
specifi c designs of pier, and these may affect the overall economy of the project. Tee
beam decks (Chapter 10) which are very economical require substantial crossheads on
their columns, Figure 10.17, while twin rib decks (Chapter 12) which are heavier than
the norm, may be carried by very economical simple columns, Figure 12.1.
8.4.3 Reinforcement
The quantity of reinforcement in a fully prestressed concrete deck is not closely related
to the span. The bending reinforcement of the top slab, which accounts typically for
some half of the total, depends on its transverse span between webs and on the length
of the side cantilevers and is hence independent of the span of the deck, while the
bottom slab of a box girder is generally reinforced at nominal rates over most of its
length. Only the reinforcement of the webs, which increase in depth with span, is
clearly span related. Consequently at the preliminary design stage, the quantity of
reinforcement is better estimated as a rate per m
3
of concrete.
The rate of reinforcement achieved in a design is an accurate measure of the skill
and care with which the deck has been designed. Inexperienced designers will over-
reinforce a deck, often to the point of rendering it uncompetitive, and diffi cult to
build. Designers too concerned about their legal risks will over-reinforce in the hope
of eliminating the possibility of concrete cracking. Such designers often waste much
steel by over-providing the nominal reinforcement in concrete members. For instance,
reinforcing a 200 mm slab with four layers of T10 bars at 150 mm spacing gives rise
to a reinforcement rate of approximately 80 kg/m
3
, while adopting 16 mm bars at the
same spacing, which is not uncommon, gives a rate of over 200 kg/m
3
. The 10 mm
bars provide more than the minimum reinforcement required by the UK code for
slabs up to 350 mm thick, and in the right circumstances, are quite appropriate to a
bridge deck.
Even when a deck is designed expertly, minimising the reinforcement is time con-
suming, and may be diffi cult to achieve within limited design fees or to a tight deadline.
For instance, at a particular design section of a web, the live load cases that give the
maximum shear and the maximum transverse bending are usually not coincident, and
consequently some of the shear reinforcement can double as bending steel. It is clearly
