Suspension Bridges - Structural Steel.

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 5mm 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  18m.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.

 Suspension bridges. (a) Three-span suspended; (b) straight back stays, viaduct side spans; (c) straight back stays, no side spans; (d) truss deck; (e) aerofoil box deck; (f) inclined hangers
Fig. 4.3 Suspension bridges. (a) Three-span suspended; (b) straight back stays, viaduct side
spans; (c) straight back stays, no side spans; (d) truss deck; (e) aerofoil box deck; (f)
inclined hangers

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  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.

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