To answer this question, we must turn to prehistoric times, since always and everywhere man had to find a way to cross the streams and rivers that he met on his way.
Probably, nature itself provided the first bridge for man when a tree fell across a stream. A person could easily copy it. Probably, just such wooden bridges were used for a long time, before some prehistoric engineer came up with the idea of pouring stones in the middle of the stream and throwing logs from them to the banks.
So it turned out a simple girder bridge with one imperfect support. The next step in building a bridge over a wide, shallow stream was to build a few pillars and connect them with logs or stone slabs. Two logs were stacked side by side and crossbars were laid on them as a flooring. The result was a wooden girder bridge, very similar to those still being built across small streams in the countryside. Larger girder bridges are now built on iron beams, and the most powerful ones on steel beams.
Bridge spans should not be too long, but where the necessary supports can be built, a bridge of any length can be built. Therefore, many long railway viaducts are just girder bridges.
Any bridge has two main parts – the superstructure and the supports on which it rests.
The bridge piers must be strong, because if they sink or are washed away by water, the entire bridge may collapse. Today, engineers usually strive to install bridge piers as deep as possible, and this often involves a huge amount of excavation. For example, during the construction of the Eads Bridge across the Mississippi in St. Louis (Missouri), the supports were dug 40 meters below the water level, and for the bridge across the bay between San Francisco and Oakland – even 70 meters!
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