Ғ. Н.Қисметова Ағылшын тілі грамматикасы
UNIT 21 THE HISTORY OF BRIDGE AND TUNNEL BUILDING
жүктеу/скачать 3.76 Mb. Pdf көрінісі
|
| UNIT 21
THE HISTORY OF BRIDGE AND TUNNEL BUILDING BRIDGES 1. One of the outstanding statesmen once said in his speech, There can be little doubt that in many ways the story of bridge-building is the story of civilization. By it we can readily measure an important part of a people s progress." Great rivers are important means of communication for in many parts of the world they have been, and still are, the chief roads. But they are also barriers to communication, and people have always been concerned with finding ways to cross them. 2. For hundreds of years men have built bridges over fastflowing rivers or 141 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 жүктеу/скачать 3.76 Mb. Достарыңызбен бөлісу:
|