Bridges_notes

**Bridges **

 Bridges are one of the oldest structures because they connect portions of lands over a river, an ocean, a canyon, a valley, a road, or maybe a railway. They have arisen through the need of people of moving from one place to another, so men used their resources and their instincts and created structures that allowed the connection in an easy way. There is a variety of materials used to make bridges. Some of they are made with metals, others with concrete, bricks, wood. It basicly depends of the use and the traffic the bridge will have to hold up. The bridges are important for social and political of a place because provide connectivity to the city. Allow people to move from a point to another, and not only people; through them also the goods can be transported.

 There are several types of them, depending on land needs, but there exist four major bridges:

**The beam bridge: **

 It is a simple structure which consists in a rigid beam, and loads are dissipated through commonly vertical supports. In small beam bridges is common the use of wood, in other cases when beam bridges are longer, steel and stone are used, because this kind of bridge needs more resistance.



**The arch bridge: ** It consists in a bridge with semi circular shape, which is made of blocks cut to fit perfectly. This type of bridge resists heavy loads because the keystone, get vertical loads and then are distributed horizontally to the other elements of arch bridges. The marials used fort these bridges are stone, iron, steel and concrete.  **The suspension bridge: **  It Consists in a deck suspended by cables to avoid a river or any obstacle. As an improvement for this kind of bridge, there was the implementation of towers which support the loads of the hanging cables, so this helped to make possible to quarry the heavy loads. For suspension bridges steel cables are commonly used. **Cable-stayed bridge **: Its mechanism is basically the tension of the cables. They’re made of steel rope, post-tensioned concrete box girders, steel or concrete pylons. Any number of towers may be used and the tension on the cables must be transferred to the earth by the anchorages.