River Crossings

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По введению в специальность

«Мосты и тоннели»

Санкт-Петербург

I

River Crossings

Streams and rivers have always been obstacles in the way of man’s continuous travel. If the stream is narrow he can jump across it, but if he is carrying a load it is easier to lay a plank and walk over. That plank represents the simplest type of bridge, and it is out of that simple bridge that all the rigid forms of river crossings have been developed.

The main obstacle to traveling is not a river but a marsh. A river can be crossed in a boat, but you cannot use a boat in a marsh.

The points where rivers have always been crossed is where the ground is firm. So the first thing to realize is that from the oldest times men chose a river crossing not necessarily where the river was narrowest or shallowest but where the banks firmest. If the banks were firm and the river narrow, so much the better, but marsh had to be avoided.

Bridges differ in design because the nature of the ground on either side of the river, and the width and depth of the water, vary widely. Each type of bridge has its own particular advantages and disadvantages. It may be interesting merely to look at a fine bridge, but is much more satisfying to find out why some bridge has been made in its particular form.

Дата добавления: 2015-08-27