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Suspension Bridges

The cable of a suspension bridge is in tension, enabling it to be much narrower and cheaper than an arch of the same span.

Simple, isn't it.  All you have to do is walk across.  But it's not very convenient, and utterly impossible for vehicular traffic.  You need a flat deck, as in the next picture.

With the deck high above the floppy cables, this looks unstable, and it is.  This construction can be used only for spans that are short enough for a stiff deck to transmit lateral forces to the anchorages.  Some early iron trusses were made like this, one advantage being the that the top chord of a truss is in compression, and has to be thick, and so it may as well carry the deck, especially for railways.  The Traversina footbridge in Switzerland was built like this, using wood among various other materials.

As we all know, the standard suspension bridge looks more like the picture below (showing one half only) with the attendant cost of the towers and anchorages.

Severn1ABCD.jpg (35544 bytes)Here is a picture of a suspension bridge.

The functions of the various parts are easy to understand.  

The towers hold the cable up.

The anchorages pull the cable outwards and downwards.  

The hangers connect the deck to the main cable.  

And the deck is there to carry the traffic.

The diagram below shows an anchorage that relies on its weight to hold the cable.  The moment of the weight about the toe must be greater than the moment of the pull of the cable about that same point.  The diagram includes the inequality.  The strands of the cable splay out from the saddle to a very large number of attachments into the concrete.  This type of anchorage may be used when the ground is not good enough for a buried anchorage.  In good rock, the saddle and the attachments are in a cavity in the ground.  The anchorages of the Clifton bridge are in tapering tunnels that are filled with masonry which acts as wedges.

The next diagrams show (not to scale) how saddles carry the cables at the tops of towers.

The deck has also to possess enough local rigidity in bending and torsion to prevent undue flexure as vehicles pass.  Locally, around each vehicle, it acts as a beam with rather diffuse supports, namely the hangers for some distance in each direction.  This same rigidity must be sufficient to help in the task of preventing undesirable amplitudes of oscillation.  Some hangers are provided with small devices that help to damp oscillations in them.

Before the invention of steel, many suspension bridges were based on wrought iron eye-bars, resting on masonry towers.  The invention of the steel wire cable, spun in place, changed everything.  Though the great Brooklyn bridge had masonry towers, steel became the normal material for a long period, until concrete became a popular alternative.  The Humber bridge, one of the longest, has towers of concrete.  Decks are almost always made of steel, usually in the form of a steel truss or an aerodynamic box girder, which can be much lighter than a truss.

Here are some pictures of suspension bridges.

SevernBig.jpg (244380 bytes) ForthRoadSep2003BS.jpg (227907 bytes) Clifton7X.jpg (78869 bytes) GW4.jpg (26191 bytes) HerefordSuspB.jpg (394314 bytes) SellackBoatBig.jpg (328383 bytes)

For more pictures please click here.

Advantages of suspension bridges

The main sustaining members, the cables or chains, are purely in tension, and are not required to be rigid, so they can be only as thick as needed to resist the tension.  The towers are almost purely in compression, so their design is  relatively simple.

Disadvantages of suspension bridges

They are only as rigid as the deck structure, which in older structures was usually of truss construction.  This makes them generally unsuitable for railway traffic.  Great attention is required in the design stage to deal with aerodynamic loads and, in the smaller sizes, periodic loads applied by pedestrians.  During construction, the cables and towers may be susceptible to wind induced oscillations.  The anchorages must sustain very strong horizontal forces as well as vertical ones.  Constructing the cables or chains across the gap can be a lengthy process.

Functions of the Parts

The towers hold up the cables.  They have to be rigid enough to act as struts between the downward forces from the cables and the upward forces from the foundations.  But modern cables are fixed to the towers at the saddles: there is no sliding.  So the towers have to be flexible enough to allow for changes in length due to live loads and temperature.

The anchorages have to hold the ends of the cables against the enormous tension, either by sheer weight, or by taking the tension into the ground.  At the time of construction, they have to include means of adjusting each strand to the correct tension.

The cables hold up the deck and the traffic, via the hangers or suspenders.  They have to be strong enough to do this without undue stretching.  They have to withstand vibration and be resistant to corrosion and wind-borne dust.

The deck has to be as light as possible, but rigid enough to prevent a dip as each vehicle passes.  It is very difficult to make a suspension deck stiff enough so that a railway locomotive doesn't spend the whole crossing in trying to climb out of a valley.  The deck must also be stable in winds of any possible direction and magnitude, whether steady or gusting.

Click here to download a program that helps you to design a model suspension bridge.

Click here for a list of the longest suspension spans.

For more information please see the main pages about suspension bridges.

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Arch    Beam    Cable-Stayed    Cantilever    Suspension    Truss