Bridges_notes

=BRIDGES=

A bdirge is a link or connection between two objects - usually places either side of an obstacle, such as a river, chasm, or estuary. Across gulfs and rivers, between peoples and countries, bridges break down separation and foster connectedness.

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.


 * ==The beam bridge==

This is the earliest and simplest kind of bridge: a fallen tree lying over a stream, perhaps, or a log lodged across a narrow chasm? We still come across these when walking in the hills, and there is no way of dating the first time some hominid straightened or stabilised such a 'beam' and thereby became the world's first bridge engineer.



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 span or the load, the greater the tendency towards sagging and hogging.



To create beams that have the effect of being very deep but don't weigh much, ingenious nineteenth-century bridge designers in the United States added supporting latticework, or a 'truss' to the bridge's beam. There are many subtle variations on the truss, with the name of each variation immortalising its inventor.




 * ==The arch bridge==

The arch is really a beam curved to form a semicircular shape, which is prevented from straightening and spreading sideways by strong 'abutments' at either end.



One disadvantage of the arch is that it needs firm support from the sides. If there are no abutments or banks to build against, the arch will spread and collapse.


 * ==The suspension bridge==

A suspension bridge is one where the road deck is suspended from cables that are strung across the river (or whatever the obstacle happens to be). There is no support in the centre of the span and the deck hangs below the supports rather than resting upon them.



One example of disadvantages of suspension bridges is the Tacoma Narrows Bridge that unfortunately, it displayed rather too much grace and flexibility, becoming known as 'Galloping Gertie' for the wave-like motion of its deck, even in light winds.

n the cable stay version of the suspension bridge, the deck is hung from diagonal cables that exert a force towards the towers as well as vertically. This makes the tension in the steel cables extremely high, and hence they are very stiff. In addition, the cables effectively stabilise the towers from both sides.



In summary, it looks like the future has two faces, as far as bridges are concerned:

 * 1) The mass-produced bridge built to standardised designs using cheap and readily available materials like concrete and steel. 'Functionality' is the keyword where quantity is concerned, and 'quantity' is the keyword when a typical stretch of new motorway can require building a new bridge every mile or so. We can expect to see dull but dependable beam bridges across our roads and railways for a good while yet.
 * 2) The 'one-off' iconic bridge, such as those commissioned for Millennium projects throughout the world. Many cities like the idea of building an innovative bridge to symbolise their modern outlook and their openness to certain ideas of the future. These 'one-off' bridges are often designed as much for their form as for their function, and public competitions are frequently held to choose a suitably impressive-looking addition to the city skyline. Eco-friendly planners away from the City also like the idea of Nature being complemented by high-tech, low-impact bridge designs.