A bridge is a structure that spans horizontally between supports, whose function is to carry vertical loads. Generally speaking, bridges can be divided into two categories: standard overpass bridges or unique-design bridges over rivers, chasms, or estuaries. The prototypical bridge is quite simple—two supports holding up a beam—yet the engineering problems that must be overcome even in this simple form are inherent in every bridge: the supports must. The first bridges were made by nature — as simple as a log fallen across a stream. It provides passage over these barriers and is a critical part of any transport infrastructure. The concept of bridging two points has existed for thousands of years, evolving from simple. Fixed bridges are by far the most common structures which carry the traveling public (both vehicular and pedestrian) over roadways, railways, waterways, and valleys.
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