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Steel Designers' Manual - 6th Edition (2003)
it discourages vortex shedding and reduces the wind forces to be resisted by
the towers and substructures. The Severn (1966) and Bosporus Bridge (1970)
use inclined hangers that form a truss system which helps to reduce any
tendency to oscillate. Suspension bridges behave as non-linear structures under
asymmetric deck loading so that deflections may be significant. Behaviour under
such loading depends upon the combined gravity stiffness and the flexural rigidity
of the deck or stiffening girder. The type is less suitable for heavy loading such as
railway traffic, especially for short spans. Suspension bridges are sometimes suitable
for medium spans carrying pedestrian or light traffic.
4.3.2 Cable-stayed bridges
Cable-stayed bridges (Fig. 4.4) are of a suspension form using straight cables which
are directly connected to the deck.The structure is self-anchoring and therefore less
dependent upon good foundation conditions, but the deck must be designed for
the significant axial stress from the horizontal component of the cable forces. The
construction process is quicker than for a suspension bridge because the cables and
deck are erected at the same time and the amount of temporary works is reduced.
Either twin sets of cables are used or alternatively for dual carriageways a single
plane of cables and tower can be located in the central reserve space. Two basic
forms of cable configurations are used, either ‘fan’ or ‘harp’. A fan layout minimizes
bending effects in the structure due to its better triangulation but anchorages can
be less easy to incorporate into the towers. The harp form is often preferred where
there are more than, say, four cables. The number of cables depends on the span
and cable size, which is often selected such that each fabricated length of deck (say,
20 m) contains an anchorage at one end to suit a cantilever erection method. Bridges
either have two towers and are symmetrical in elevation or have a single tower as
suited to the site.
Floors are generally an orthotropic steel plate but composite slabs can be used
for spans up to about 250 m. A box girder is essential for bridges having a single
plane of cables to achieve torsional stability, but otherwise either box girders or twin
plate girders are suitable. Aerodynamic oscillation is a much less serious problem
than with suspension bridges but must be considered. Some bridges with plate
girders incorporate non-structural aerodynamic edge fairings.
It is essential to use cables of maximum strength and modulus at a high working
stress so that sag due to self-weight, which produces non-linear effects, is negligible.
Cables are normally of parallel wires or prestretched locked coil wire rope. During
erection the cable lengths are adjusted or prestressed so as to counteract the dead
load deflections of the deck arising from extension of the cables.
4.3.3 Arch bridges
Selection of type 131
Arch bridges (Fig. 4.5) are suitable in particular site conditions. An example is
a medium single span over a ravine where an arch with spandrel columns will