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Steel Designers' Manual - 6th Edition (2003)
128 Bridges
in special locations. For example, an arch is a logical solution for a medium span
across a steep-sided ravine.
Through trusses are suitable for medium spans where the available construction
depth is limited. The vast majority of short- and medium-span highway bridges are
formed with composite construction because the highway profile can be arranged
to suit the depth available.
For short and medium spans the most important factor which influences the type
is the available construction depth. This particularly affects railway bridges because
it is rarely feasible to modify existing track levels. Where depth is limited then types
such as arch, truss or half-through plate girder offer an alternative solution. Types
of steel bridge are shown in Fig. 4.2 with their normal economic span range and the
world’s longest. Each is briefly described below.
4.3.1 Suspension bridges
Suspension bridges (Fig. 4.3) are used for the longest spans across river estuaries
where intermediate piers are not feasible. The cables form catenaries supporting
both sides of the deck and are tied to the ground usually by gravity foundations
sometimes combined with rock anchors. Thus ground conditions with firm strata at
or close to the surface of the ground are essential. Towers are usually twin steel or
concrete box members which are braced together above the roadway level. They
are designed so as to be freestanding under wind loading during construction until
the cables are installed. Cables are either a compacted bundle of parallel high tensile
steel strands (commonly 5 mm diameter) installed progressively by ‘spinning’ or
may be formed from a group of wire ropes. Deck hangers are wire ropes (or round
steel rods for light loading as for a footbridge) clamped to the cable and connected
to the deck at a spacing equal to the length of each deck unit erected, typically
18 m. The construction process for suspension bridges is more time consuming than
for other types because the deck cannot be installed until the towers, anchorages,
cable and hangers are constructed.
Depending upon ground conditions, the cables can be catenaries supporting side
spans. Cables may alternatively be straight from tower top to the ground anchorages
and merely support a main span, side spans being non-existent or formed as
short-span viaducts. Decks are either trusses with a steel orthotropic plate floor
spanning between or an aerofoil box girder. Footways are often cantilevered outside
the two sets of cables.
Aerodynamic behaviour must be considered in design because of the tendency
for the deck and cables to oscillate in flexure and torsion under ‘vortex shedding’
and other wind effects. This is due to the flexible nature and light weight of suspension
bridges illustrated by the collapse of the USA Tacoma Narrows Bridge in
1940, which had a very flexible narrow deck consisting of twin plate girders forming
a torsionally weak deck of ‘bluff’ shape prone to wind vortex shedding. Aerodynamic
considerations usually justify wind tunnel testing of models. The advantage
of an aerofoil box girder such as used on the Severn and Humber bridges is that