Bridges_notes



BRIDGES: A bridge is a structure built to span a valley, road, body of water, or other physical obstacle, for the purpose of providing passage over the obstacle. Designs of bridges vary depending on the function of the bridge and the nature of the terrain where the bridge is constructed. 


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TYPES OF BRIDGES: **

There are three major types of bridges:

__ The beam bridge __ : A beam bridge is basically a rigid horizontal structure that is resting on two piers, one at each end. The weight of the bridge and any traffic on it is directly supported by the piers. The weight is traveling directly downward. The force of compression 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.

__ The arch bridge: __ An arch bridge is a semicircular structure with abutments on each end. The design of the arch, the semicircle, naturally diverts the weight from the bridge deck to the abutments. Arch bridges are always under compression. The force of compression is pushed outward along the curve of the arch toward the abutments. The tension in an arch is negligible. The natural curve of the arch and its ability to dissipate the force outward greatly reduces the effects of 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. As we just mentioned, the shape of the arch itself is all that is needed to effectively dissipate the weight from the center of the deck to the abutments. As with the beam bridge, the limits of size will eventually overtake the natural strength of the arch.

__ The suspension bridge: __ A suspension bridge is one where cables (or ropes or chains) are strung across the river (or whatever the obstacle happens to be) and the deck is suspended from these cables. Modern suspension bridges have two tall towers through which the cables are strung. Thus, the towers are supporting the majority of the roadway's weight. The force of 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 firmly entrenched. The supporting cables, running between the two anchorages, are the lucky recipients of the tension forces. The cables are literally stretched from the weight of the bridge and its traffic as they run from anchorage to anchorage. The anchorages are also under tension, but since they, like the towers, are held firmly to the earth, the tension they experience is dissipated. 




 * 1. Are all bridges alike? **

 Wood, brick, stone, cast iron, wrought iron, mild steel, high-tensile steels, alloy 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, torsion, vibration; contraction, corrosion, erosion, expansion, fatigue, 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.  They are connecting two objects, usually places, either side of an obstacle, such as a river, chasm, or estuary.   A span is the distance between two 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, for instance, is likely to span a distance 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, the pinnacle of bridge technology, is capable of spanning up to 7,000 feet (2,100 m). What allows an arch bridge to span greater distances than a beam bridge, or a suspension bridge to span a distance seven times that of an arch bridge? The answer lies in how each bridge type deals with two important forces called compression and tension: Compression is a force that acts to compress or shorten the thing it is acting on and tension is a force that acts to expand or lengthen the thing it is acting on.
 * What are they made of?
 * What are they connecting?
 * How is the span between their piers or towers?

  **<span style="font-family: 'Tahoma','sans-serif'; font-size: 12pt; mso-fareast-font-family: 'Times New Roman';">2. What social and/or political benefits can bridges provide to places? **<span style="font-family: 'Tahoma','sans-serif'; font-size: 10pt; mso-fareast-font-family: 'Times New Roman';">

<span style="font-family: 'Tahoma','sans-serif'; font-size: 10pt; mso-fareast-font-family: 'Times New Roman';"> As far back as we can see in history, human beings have used new technology to solve problems and ease their physical burdens. The distinctiveness of humans as a species is defined by their use of tools, and bridges are technological tools that aim to solve the problem of crossing an obstacle in such a way as to cut down the effort and time needed to do so. The better a bridge is, the less attention the user will need to pay it.
 * <span style="font-family: 'Tahoma','sans-serif'; font-size: 10pt; mso-fareast-font-family: 'Times New Roman';">Why were the bridges built?

<span style="font-family: 'Tahoma','sans-serif'; font-size: 10pt; mso-fareast-font-family: 'Times New Roman';"> 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. This means that, in economic terms, the cost of travel and trade falls and the financial benefits of increased social cohesion and sharing resources rise. Other longer-term payoffs from easier travel, which is crucially dependent on good bridges, come as a result of increased opportunities to share ideas – intellectual, political and religious. 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: #333333; font-family: 'Arial','sans-serif'; font-size: 9pt; mso-ansi-language: EN; mso-fareast-font-family: 'Times New Roman';">
 * <span style="font-family: 'Tahoma','sans-serif'; font-size: 10pt; mso-fareast-font-family: 'Times New Roman';">Did they provide benefits? To whom? Why?