Bridges_notes

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.

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. 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.
 * TYPES OF BRIDGES **


 * The beam bridge **

This is the earliest and simplest kind of bridge. The design is as simple as a single rigid 'beam', resting on supports at either end and unsupported in the middle. The weight of the beam, and of any traffic on it, is carried directly to the ground by the supports, often called 'piers' in the trade. The beam need not be of any particular shape and there are no other elements besides the piers to help dissipate the load. Hence the piers take the full weight of the load and are said to be in 'compression'. This means that they are being squashed by the forces at the top and bottom, and must be built from materials that can resist such forces without crumpling.





Materials for a beam 

The degree to which bridge components can withstand these opposing forces depends largely upon the material from which that bridge is made. Stone is very strong under compression and is thus ideal as a material for the piers, but it is not strong under tension, cracking easily if given a sudden blow. In addition, stone slabs are very heavy and therefore difficult to manoeuvre into position.



By contrast, wooden tree trunks or cut planks form a very strong beam when laid across a stream, despite their comparative lightness. Wood is able to carry both compression and tension equally well. Of course, wood is not as hard wearing as stone and needs replacing relatively often.


 * //The world's longest bridge (but with no exceptional individual span) crosses Lake Pontchartrain in the United States. In total, the bridge is 38 kilometres long between the banks. //**



Disadvantages of a beam bridge

All beams tend to 'sag' between the piers and 'hog' over the piers themselves. This results from the downward forces of the load and the upward forces at the pier supports. The greater the length or the load, greater the tendency towards sagging and hogging. The longer a beam is, the weaker it becomes. The greater length gives more weight and more leverage for that weight - increasing the 'bending moment'

To make a beam bridge as strong as possible, it is often necessary to reinforce it with 'girders'. The strength of a girder depends on its depth, not its width. Then more material is added where it's needed – at the top and bottom, where the tension and compression act.


 * <span style="color: #591c0d; font-family: Tahoma,Geneva,sans-serif; font-size: 110%;">Truss Bridges **

<span style="color: #591c0d; font-family: Tahoma,Geneva,sans-serif; font-size: 110%;">These bridges are composed of several joined elements. At the beginning, these bridges were made of wood with iron tensile rods, but modern ones are made of metals such as wrought iron, steel or even concrete.

<span style="color: #591c0d; font-family: Tahoma,Geneva,sans-serif; font-size: 110%;">Each design dissipates the main load through different combinations of 'struts' in compression and 'ties' in tension. As a vehicle passes over the bridge, a particular member of the lattice may go from being in compression to being in tension, so the material chosen for a truss must be able to take both kinds of force.

<span style="color: #591c0d; font-family: Tahoma,Geneva,sans-serif; font-size: 110%;"> <span style="color: #591c0d; font-family: Tahoma,Geneva,sans-serif; font-size: 110%;">


 * <span style="color: #591c0d; font-family: Tahoma,Geneva,sans-serif; font-size: 110%;">The Cantilever **

<span style="color: #591c0d; font-family: Tahoma,Geneva,sans-serif; font-size: 110%;">This is the idea behind the 'cantilever' bridge design. The part of the beam near to the pier support is so strong that it can support the slender <span style="color: #591c0d; font-family: Tahoma,Geneva,sans-serif; font-size: 110%;">But, in the end, for all the tricks you might try, a beam bridge can't escape the fact that it will sag in the middle. There is simply nowhere else for the loads to dissipate. Most of these bridges are two cantilever arms extending through the obstacle from contrary sides, meeting at the center with a suspended span, supported only by the two beams.

<span style="color: #591c0d; font-family: Tahoma,Geneva,sans-serif; font-size: 110%;"> <span style="color: #591c0d; font-family: Tahoma,Geneva,sans-serif; font-size: 110%; line-height: 0px; overflow: hidden;">