How do Suspension Bridges work?

On the Severn Bridge, the two main cables act a bit like a washing line.  The tension in a washing line supports the weight of the clothes that are pegged to it.  In the same way, the tension in the main cables supports the weight of the deck and traffic.  The bridge deck is hung from the main cables using wire hangers (rather than clothes pegs).  And because the main cables are held up by the towers, the weight of the whole bridge is carried down through the towers, on to the underlying foundations.

If you put something heavy on a washing line, it will sag at that point.  With a suspension bridge, the road is supported by a stiffening girder, which spreads out the weight of the traffic, so avoiding excessive sag under an exceptional load.  If you hang something on a washing line away from the centre, the point will not only sag but it will also move towards the nearest end (try it!).  Similarly, as a heavy load travels over a suspension bridge, it will not only dip downwards at the point of the load, it will also move longitudinally towards the nearest tower.

If you stand on the walkway of the Severn Bridge, you can feel it moving as the traffic travels over it.  If you stand by one of the towers and watch the expansion joint, you can sometimes see the whole bridge moving as the weight of the traffic travels across.  We should not worry that the bridge moves.  It is meant to do this.  This is how it absorbs the weight of the traffic and transfers it into the main cables.

Diagram showing the main loads in a suspension bridge

Diagram showing the main loads in a suspension bridge

The tension in the main cables carries the whole weight of the bridge deck and the traffic. This tension is resisted by the anchorages at each end, just as the tension in a washing line is resisted by whatever it is tied to at each end.  And because the main cables are held up by the towers, the weight of the whole bridge is transferred through the towers to the ground.

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