The suspension bridge
A suspension bridge is one where the road deck is suspended from cables that are strung across the river (or whatever the obstacle happens to be). There is no support in the centre of the span and the deck hangs below the supports rather than resting upon them.
The earliest suspension bridges were simply ropes thrown across a narrow gorge or river, from which people could hang as they crawled across. Later, wooden footways were added between the ropes. Donkeys and carts, however, found such swinging, dipping footways very difficult to manage. It was simply not possible to take a horse-and-carriage or a heavy load safely across these kinds of suspension bridge.
© structurae.de (Per Wåhlin)
The modern development of the suspension bridge dates from as recently as 1800, when an American judge called James Finlay used towers to elevate the cables and trusses to stiffen the deck. The significance of the towers was that they made it possible for the deck itself to remain perfectly flat for horses and carriages to cross; the significance of the trusses was that they prevented the deck from swaying. This was the first rigid suspension bridge.
Nowadays, we use steel cables, rather than the iron links used by Finlay, but the design remains the same.
The basics of the design
In all modern suspension bridges, the roadway hangs from massive steel cables, which are draped over two towers and secured into solid concrete blocks, called 'anchorages', at both ends of the bridge.
The vehicles push down on the roadway, as on any beam bridge, but because the roadway is suspended by 'hangers' from the two main cables, the cables transfer the entire onto the tops of the two towers. The two towers are constantly in compression, and transfer the forces to the ground on which they are built. The anchorages are usually attached to solid rock on either bank, and absorb the tension in the main cables.
© structurae.de (Per Wåhlin)
Early suspension bridges used massive iron chain links - rather like those in a bicycle chain - from which the road deck was hung. The famous Clifton Suspension Bridge seen above still has such iron chain links.
Despite being one of Brunel's earliest and most famous bridge designs, Clifton was actually built after his death. His design was accepted in 1830, but only completed in 1864, by a group of loyal colleagues who wished to create a fitting memorial to Victorian Britain's most flamboyant entrepreneur.
© structurae.de (Cotswold Images)
It wasn't until the 'air-spinning' of steel wire cables became possible, from about 1870, that the modern suspension bridge really flourished.
Stronger than iron for less weight, steel could therefore take the weight of much longer bridges - sometimes up to 150,000 tonnes.
© structurae.de (Jean-Marc Morand)
This made enormous bridge lengths possible, and the Brooklyn Bridge in New York was the proof: 50 percent longer than any bridge that preceded it, Brooklyn Bridge is still in operation today, carrying 150,000 vehicles and 3000 pedestrians into and out of Manhattan daily.
Since the end of the nineteenth-century, the world record for the largest bridge span has gone from 500 metres to nearly 2000 metres, but no other type of bridge has come close to challenging the suspension bridge for sheer length.
Typical style of a rope bridge.
Clifton suspension bridge, showing typical suspension bridge towers and deck.
Golden Gate suspension bridge.
Clifton suspension bridge – tourist information.
Clifton suspension bridge in past times.
Brooklyn Bridge, New York.