UNIT 21 THE HISTORY OF BRIDGE AND TUNNEL BUILDING

deep and rocky canyons. Early man probably got the idea of a bridge from 
a tree fallen across a stream. From this, at a later stage, a bridge on a very 
simple bracket or cantilever principle 1 was evolved. Timber beams were 
embedded into the banks on each side of the river with their ends 
extending over the water. These made simple supports for a central beam 
reaching across from one bracket to the other. Bridges of this type are still 
used in J apan, and in India. A simple bridge on the suspensioli principle 2 
was made by early man by means of ropes, and is still used in countries 
such as Tibet. Two parallel ropes suspended from rocks or trees on each 
bank of the river, with a platform of woven mats laid across them, made a 
secure crossing. Fur ther ropes as handrails 
were added. When the 
Spaniards reached South America, they found that the Incas of Peru used 
suspension bridges made of six strong cables, four of which supported a 
platform andtwo served as rails.
3. All these bridges made possible crossings only over narrow rivers. The 
type of temporary floating bridge, he pontoon bridge, has been used for 
military purposes; military engineers can construct a temporary bridge on 
this principle, able to cany all the heavy equipment of a modern army, in 
an extremely short time.
The idea of driving wooden piles into the bed of the river in order to 
support a platform was put into practice 3,500 years ago. This is the basis 
of the trestle' or pile bridge which makes it possible to build a wider 
crossing easier for the transport of animals and goods.
4. With the coming of the railway in the 19th century there was a great 
demand for bridges, and the railways had capital for building them. The 
first railway bridges were built of stone or brick. In many places long lines 
of viaducts were built to carry railways; for instance, there are miles of 
brick viaducts supporting railways to London.
The next important development in bridge-building was the use of
iron and, later, steel. The first iron bridge crossed . the river Severn in 
Great Britain.
The idea of a drawbridge, a bridge hinged so that it can be lifted by 
chains from inside to prevent passage, is an old one. Some 
Leningrad 
bridges were built on this principle.
A modern bridge probably demands greater s k ill from designer and 
ш er than any other civil engineering project. Many things should be 
en into consideration, and these may vary widely according to local 
conditions. In deciding what type of bridge is most suitable the 

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