Engineering and Construction

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.

Fig. 4.4 Cable-stayed bridges. (a) Harp; (b) fan; (c) single tower; (d) single plane; (e) single plane; (f) twin plane

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, 20m) 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 250m. 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.