Civil Engineering Reference
In-Depth Information
Figure 18.4
Undertrussed deck
reactions to the deck through downstand posts, Figure 18.4. Just as extradosed decks
are essentially continuous as the additional strength is provided over the supports,
undertrussed decks are essentially statically determinate.
As the author has no experience of designing such bridges, no detailed description
will be given. However, it does appear that this is a rather under-used genre, and there
is certainly scope for exploiting it for economy and for visual effect.
18.4 Cable-stayed bridges
18.4.1 General considerations
When compared with a girder bridge of equivalent span, a cable-stayed bridge trades
off its cheaper deck against the additional cost of the expensive stays and towers. The
tonnage of stay required is directly proportional to the total weight carried, of which
a large proportion is the self weight of the deck. Thus in order to minimise the cost of
stays, some decades ago, the deck of a cable-stayed bridge spanning, say, 400 m would
have been a steel truss or girder, with a steel orthotropic deck.
More recently, the excessive fi rst cost of such steel decks led designers to adopt
composite construction for long-span cable-stayed bridges, with steel main girders
and a concrete top slab. However, this is not a very rational design concept for a
cable-stayed deck. The inclined stays impart large compressive forces to the deck. As
the concrete slab is initially in tension due to restrained heat of hydration shortening,
and is subject to creep when in compression, such decks are designed so that the steel
girders carry all the compression, and as is well known, steel is not economical in
compression.
The stays carry virtually all the overall shear force and bending moment, leaving
the deck to carry only local actions. Consequently only two longitudinal girders are
generally needed even for the widest of decks, and replacing the steel webs and bottom
fl anges of the girders by concrete need not represent a major increase in weight. The
longitudinal compression in the deck then becomes benefi cial, reducing the amount
of prestress or reinforcement that is necessary to carry the local live load bending
moments. By this reasoning, concrete decks are gradually being used for ever longer
spans, currently up to about 500 m.
The towers are also sensitive to the weight of the deck, but less than the stays,
as they have a very considerable self weight of their own and their design is heavily
infl uenced by environmental loads.
18.4.2 Span arrangements and tower heights
The consensus is that the length of the side span should be 43 per cent of the main
span while the height of the tower to the uppermost stay anchorage should be one-fi fth
