Suspension Bridges
Suspension Bridges are perhaps the most dramatic bridge type, spanning vast distances and with monumental towers to carry the suspension cables across the water. They are also probably older than many people imagine, dating back thousands of years - think of Indiana Jones et al crossing deep gorges in Asian jungles!
The principle is remarkably simple too, with the two members (Cable and deck) spanning the river counteracting each other working in a similar way to a cantilever truss, where the forces are carried back to the towers, and converted into vertical forces. In order for the structure to work, however, the cables have to be anchored securely into the riverbank on either side so as to prevent the towers falling inwards. Cable Stayed Bridges may look similar, but the cables are straight rather than curved, and so each balance their pair making the bridge a true cantilever structure.
Most of the major crossings built in the UK from 1960 to 1980 were Suspension Bridges - the Forth Road Bridge, Humber Bridge and Severn Bridge being the three that immediately come to mind. However, as can be seen from the list below Suspension Bridges have been constructed in this country for nearly 2 centuries.
Early Suspension Bridges
The first Suspension Bridges built used iron chains in place of the spun steel cables used today. The earliest is Thomas Telfords Menai Suspension Bridge of 1826, where the 'cables' consist of a number of identical cast-iron chain links, not like a padlock or bike chain, but straight rods flared into roundels at either end and bolted together through these points. Several links ran parallel, alternating position at each connection, and with suspension cables - or iron rods - descending from each connecting point to support the road deck below.
This style of construction remained in common use up to the end of the 19th century, with the first suspension bridge to use steel wire being the Brooklyn Bridge of 1883 in New York. It was, needless to say, some time before this 'new' technology reached our shores!
James Dredge
A different variation on the Suspension Bridge was devised by James Dredge of Bath. his bridges used the old type chain system for the cables, but instead of continuous cables across the channel, one of the parallel links was dropped at each hanger point, so that the central section of the deck had no suspension cable at all. The most famous example is probably the Bridge of Oich near Loch Ness.
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