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The quest for travel has always been one that man has tried to conquer.   First, he traveled on foot, and then developed methods of transportation such as boats, but one of the first obstacles that he had to overcome was getting across rivers, waterways, and canals.   Today, bridges have become a part of everyday life; hence without them, people would have a much more difficult time going from one place to another.   In the United States alone, there are more than half a million bridges (History of Bridges 1).   Although bridges have been in existence since ancient times, they were not first made by man.   Nature actually made the first bridges by toppling logs across streams.   This was considered the first beam bridge as the flat rocks were used to support the log.   Then, the water eroded the rocks and eventually a stone bridge was formed.   The ancient Romans built arched bridges because they needed roads and bridges to connect the sections of their empire.   The original swing bridge, from which the suspension bridge was produced, was a simple vine that swung people over the water.   From these beginnings, men created four main types of bridges.
To begin with, beam bridges, also known as girder bridges, are one of the easiest and simplest types of bridges to build.   In ancient times, people used to make these bridges out of granite.   Beam bridges can be made by putting a wooden plank over an area needing to be crossed.   To get across larger areas, piers or supports can be fabricated up from the river bed to hold up a couple of beams.   The piers of the girder bridge have constant pressure on them. Therefore, the beams, which are usually made out of concrete or steel, have a lot of strength so they do not crumble due to the heavy load.   Many beam bridges are made of pre-stressed concrete.   This concrete has steel rods inside of it, which help resist tension.   When the weight pushes down on a bridge, the top of the beams are pressed together creating compression, while...