Bridges+notes

=== The bridge is not a human invention opposite to what people believe. The first bridges we can find are located in nature, made accidentally with stones or wood (fallen trees). Bridges can be defined as "a link or connection between two objects - usually places either side of an obstacle, such as a river, chasm, or estuary.” ===

===There are three major types of bridges: beam bridges, arch bridges and suspension bridges. The type of bridge used depends mainly on the size of the obstacle and how far it is from one side to the other. ===

===The first type of bridge is the Beam bridge. This kind of bridge consist in a rigid horizontal structure supported by two piers, one at each end. The most common material used in beam bridges construction are steel and concrete. There are different types of beam bridges, determined by the design, location and composition of the truss. The technology involved in the construction of this kind of bridges doesn't need to be very advanced. In fact, there are what we call "Military bridges" that are made of steel and can be deployed very quickly over an obstacle without using big and complicated machines. ===

===The second type of bridge is the Arch bridge. This kind of bridge consists in a semicircular structure with abutments on each end. The most common material used in arch bridges construction are brick, stone and concrete. "Most of arch bridges that we can find in the present time, were built long time ago, because after girders, arches are the second oldest bridge type and a classic structure." Since Romans or Greeks, arch bridges are used to cross an obstacle. The only disadvantage of this kind of bridge is that it is not very useful to cross long distances, because it is too rigid and its characteristics do not allow it to resist weight if the bridge is too long. ===

===The third and last type of bridge is the Suspension Bridge. A suspension bridge is conformed by two or more towers that support two big cables that are strung across the river, and attached to these cables there are others a bit smaller that support the deck or roadway. The most common material used in suspension bridges construction are concrete, steel, and iron. Because of their characteristics, this kind of bridges are used for covering or crossing long distances like from an island to another or from a mountain to another. And this is exactly the reason why suspension bridges are the ones that have a more complex process of construction and require the use of specialized tools and machines. The only disadvantage that these bridges have is that because of their nature they get affected by wind and they move a bit more than the other two types of bridges . ===