Bridges+Notes


 * =====** What are bridges? **A bridge is a structure built to help to cross a valley, road, body of water, or any other obstacle, for the purpose of providing passage over the obstacle. Designs of bridges vary depending on the function of the terrain where the bridge is going to be constructed. =====

> The supporting cables, running between the anchorages are the recipients of the tension. The cables are literally stretched from the weight of the bridge and its traffic as they run from anchorage to anchorage. The anchorages are under tension too, but since they, like the towers, are held firmly to the earth, the tension they experience is dissipated.
 * ===__**Tipe of Bridges**__: ===
 * 1) **__Beam bridge__**: This is the earliest and simplest kind of bridge: a fallen tree lying over a stream. Is basically a horizontal structure that is resting on two piers, one at each side. Its weight and any traffic on it is directly supported by the piers. The weight is traveling directly downward. The compression force manifests itself on the top side of the beam bridge's deck (or roadway). This causes the upper portion of the deck to shorten. The result of the compression on the upper portion of the deck causes tension in the lower portion of the deck. This tension causes the lower portion of the beam to lengthen.
 * 1) **__Arch bridge__**: It is a semicircular structure with abutments on each end. The design of the arch, naturally diverts the weight from the bridge deck to the abutments, and they're always under compression. The force of compression is pushed along the curve of the arch toward the abutments, practicly there's no tension because the natural curve of the arch and its ability to dissipate the force greatly reduces the tension on the underside of the arch. The greater the degree of curvature (the larger the semicircle of the arch), however, the greater the effects of tension on the underside.
 * 1) **__Suspension bridge__**: Its one where cables (or ropes, or chains) are strung across the obstacle and the deck is suspended from these cables. Modern suspension bridges have two towers through which the cables are strung. Thus, the towers are supporting the majority of the roadway's weight. The compression pushes down on the suspension bridge's deck, but because it is a suspended roadway, the cables transfer the compression to the towers, which dissipate the compression directly into the earth where they are entrenched.


 * **__What are they made of? __**

 Wood, brick, stone, cast iron, wrought iron, mild steel, high-tensile steels, aluminium, steel-reinforced concrete, pre-stressed concrete, glass-reinforced plastic. These are some of the materials that are available for bridge building.  Bending, compression, impact, oscillation, pressure, tension, vibration; contraction, corrosion, erosion, friction, rain, river flow, sea-water, scouring, temperature changes, tidal flow, turbulence, waves, wind erosion, wind gusts, wind pressure. These are some of the stresses that bridge materials must withstand, in a variety of combinations.
 * **__What are they connecting? __**

 They are connecting usually places, either side of an obstacle, such as a river, chasm, or estuary.


 * **__How is the span between their piers or towers? __**

 A span is the distance between the bridge supports, whether they are columns, towers or the wall of a canyon. The span depends on the type or bridge (Beam bridge, Arch bridge or Suspension bridge). A modern beam bridge is likely of up to 200 feet (60 meters), while a modern arch can safely span up to 800 or 1,000 feet (240 to 300 m). A suspension bridge, is capable of spanning up to 7,000 feet (2,100 m)



 Human beings have used new technology to solve problems and ease their physical burdens, and bridges are technological tools that aim to solve the problem of crossing an obstacle in such a way as to shortening the effort and time needed to do so.
 * **__Why were the bridges built? __**

 Some of the benefits of bridges are obvious: supplies of food and traded goods can get across an obstacle or through difficult terrain in a shorter time. In economic terms, the cost of travel and the financial benefits of increased social cohesion and sharing resources rise. Today bridges allow easy travel across major rivers and estuaries, over the new obstacles of motorways and railway lines, and between neighbouring islands. International trade and travel depend on shipping and air routes, but efficient distribution networks depend on bridges.
 * **__<span style="color: #800080; font-family: Tahoma,Geneva,sans-serif;">Did they provide benefits? To whom? Why? __**



BRIDGES SUMMARY.

<span style="color: #800080; font-family: Tahoma,Geneva,sans-serif;">BRIDGES: WHAT THEY ARE, HOW THEY ARE AND WHY. <span style="color: #800080; font-family: Tahoma,Geneva,sans-serif;">Bridges are technological implements used to connect two places and make easier the communication, transportation, and traveling.

<span style="color: #800080; font-family: Tahoma,Geneva,sans-serif;">There are three different types of bridges, the beam bridge, the arch bridge and the suspension bridge. Each one of them have their own characteristics, for example, the beam bridge is the oldest type of bridge and it’s basically a rigid horizontal structure laying over two piers on each side of it, which are supporting all the weight. The arch bridge has a semicircular structure supported on each side by abutments which are always under compression, minimizing the tension in the arch. And finally, we have the suspension bridge, where the cables, ropes or chains are the most important part. All the cables end in an anchorage system which takes all the tension and transmit it to the columns.

<span style="color: #800080; font-family: Tahoma,Geneva,sans-serif;">Usually, bridges are made of wood, brick, stone, cast iron, wrought iron, mild steel, high-tensile steels, aluminium, steel-reinforced concrete, pre-stressed concrete, glass-reinforced plastic.

<span style="color: #800080; font-family: Tahoma,Geneva,sans-serif;">· Advantages & Disadvantages

<span style="color: #800080; font-family: Tahoma,Geneva,sans-serif;">BEAM BRIDGE Advantages: <span style="color: #800080; font-family: Tahoma,Geneva,sans-serif;">Disadvantages:
 * <span style="color: #800080; font-family: Tahoma,Geneva,sans-serif;">Good for short spans;
 * <span style="color: #800080; font-family: Tahoma,Geneva,sans-serif;">Wide distances can be spanned by resting beams on piers.
 * <span style="color: #800080; font-family: Tahoma,Geneva,sans-serif;">Can be expensive, even for fairly short spans, because of the need to use steel. A cheap material for beam bridges is concrete, but concrete on its own, is not strong enough to stand up to high forces of tension (pulling), so concrete needs to be reinforced with steel mesh;
 * <span style="color: #800080; font-family: Tahoma,Geneva,sans-serif;">Very expensive for long spans, because of need for [|piers] to support the long beams;
 * <span style="color: #800080; font-family: Tahoma,Geneva,sans-serif;">It is not always possible to build support piers.

<span style="color: #800080; font-family: Tahoma,Geneva,sans-serif;">ARCH BRIDGE Advantages: <span style="color: #800080; font-family: Tahoma,Geneva,sans-serif;">Disadvantages: <span style="color: #800080; font-family: Tahoma,Geneva,sans-serif;">SUSPENSION BRIDGE Advantages: <span style="color: #800080; font-family: Tahoma,Geneva,sans-serif;">Disadvantages:
 * <span style="color: #800080; font-family: Tahoma,Geneva,sans-serif;">Good for short spans;
 * <span style="color: #800080; font-family: Tahoma,Geneva,sans-serif;">Inexpensive, with local building materials such as stone and brick, being used in the past. Today, concrete is more likely to be used.
 * <span style="color: #800080; font-family: Tahoma,Geneva,sans-serif;">Wider distances can be spanned by building more arches.
 * <span style="color: #800080; font-family: Tahoma,Geneva,sans-serif;">Uses a lot of material, even on short spans.
 * <span style="color: #800080; font-family: Tahoma,Geneva,sans-serif;">Can cover very long spans;
 * <span style="color: #800080; font-family: Tahoma,Geneva,sans-serif;">The steel cable used is cheaper than steel girders.
 * <span style="color: #800080; font-family: Tahoma,Geneva,sans-serif;">A very flexible bridge, so not used for railways. As trains are heavier than cars or lorries, their weight is less spread out across the bridge.



<span style="color: #800080; font-family: Tahoma,Geneva,sans-serif;">Bridges help people to communicate, travel, work and so many other things that they’re one of the most important technological advances from the earlier times of history. Thanks to them, people can get food, cross very dangerous rivers, travel comfortably and the time required to cross the valley, river or anything gets shorter and people reaches the other side faster.

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