BRIDGES

What they are?? BRIDGES - “Across gulfs and rivers, between peoples and countries, bridges break down separation and foster connectedness”. A bridge is a structure built for the purpose of providing a passage over an obstacle, such as a valley, road, canyon, body of water, or any other physical obstacle. Thanks to bridges, humans solve the problem of crossing geographic obstacles, allowing them to cut down the effort and time needed to do so. The design of a bridges vary depending on his function, nature of the terrain, material used to make it and culture of the place where is located. There are five main types of bridges: Beam Bridges Beam bridges are horizontal beams supported at each end by piers. This is the earliest and simplest kind of bridge, they were mostly made up of wood, but now at days, they are made of metal. A bridge beam in stone is not ideal, because despite they are very good under compression; stone is not strong under tension, cracking easily. Compare to the stone, the wood works a lot better, because it is able to carry compression and tension just as well. Most simple beam bridges have been made from wooden planks, laid on stone pier supports. Beam bridges made from steel, are very stable, strong, and resist equally compression and tension, without the need of replacing it as often as wood. The beam bridges can´t be too long, because the weight on the top of the beam pushes straight down on the piers at either end of the bridge. Depending on the legth of the roadway above the obstacle, the bridge could comprise one or more beams.

**viaduct**
Bridge composed of several beams that crosses a very high valley.

**simple-span beam bridge**
Bridge whose deck is composed of only one continuous beam.

**multiple-span beam bridge**
Bridge whose deck is composed of several juxtaposed beams, each one supported at each end.  The longer the bridge, the weaker. The world's longest beam bridge is Lake Pontchartrain Causeway in southern Louisiana in the United States, at 23.83 miles (38.35 km), with individual spans of 56 feet (17 m). Cantilever Bridges ===  These types of bridges are built using horizontal beams that are supported on only one end, called cantilevers. Best cantilever bridges use two cantilever arms extending from opposite side of the obstacle to be crossed, meeting at the center; this allows a shape light and delicate in the center where it tends to span, and thick and strong in the opposite sides where it needs to be. ===

[[image:http://farm1.static.flickr.com/72/198370469_af0fb5e7cc.jpg]]
Arch Bridges Arch bridges have abutments at each end and the beam curved in the form of a semicircle; this shape allows getting the sagging forces away from the middle, and as a result the weight of the bridge rest into the abutments at either side, evading the span. Arches need a lot of material to the sides, and tiny portion of material in the middle, to help the partial dissipation of vertical forces horizontally.

=== The traditional arch bridge is made from a series of blocks (can be rocks, but mostly bricks), cut to fit perfectly together. The first civilizations to use the arch were the Greeks, and later on, the Romans. The earliest known arch bridge is called ArkadikoBridge, and was built by the Greeks. But Romans take to arch to another level. They were the first civilization to build large scale bridges, which allows them to expand his Empire trough traveling efficiently. === ===One of the problems of an arch bridge comes from his construction. Building sturdy foundation halfway across a river is very difficult, and need the help of the caissons and cofferdams. Also, the piers built mid-stream are an obstruction for the flow of the river and the traffic which used it. === The arch has gone through big changes across the history, incorporating new designs and materials. As a result, now days we have beautiful reincarnations of the traditional arch bridge made of bricks. ===The Sheikh Rashid Saeed Crossing is currently being built by Dubai. When completed, it will be the largest arch bridge in the world. ===

Suspension Bridges === In a suspension bridge cables allows the deck to hang from towers that are attached across the obstacle (river or any other geographic difficulty) to caissons or cofferdams which are implanted deep into the floor. This type of bridge hangs below the supports rather than resting upon them. === === The earliest suspension bridges were made of ropes, from which people could hang as they crawled across. These kind of suspension bridge were not safe, even though later, wooden footways were added between the ropes; they didn´t allows to cross horses, donkeys, or heavy package. When James Finlay used towers to elevate the cables, the deck remain perfectly flat, allowing the cross of horses and carriages. === Even though since then, modern suspension bridge´s stability has improve wisely, they are susceptible to the phenomenon of resonance, making dangerous vibrations. Advanced computer simulators are vital tools to keep our giant bridges spans safe and stables. <span style="font-family: 'Tahoma','sans-serif'; line-height: normal;">The longest suspension bridge in the world is the 12,826 feet (3,909 m) Akashi Kaikyo Bridge in Japan. Cable-Stayed Bridges ===  Like suspension bridges, cable-stayed bridges hung the deck, but in this case, from diagonal cables that apply force towards the towers as well as vertically. These allow l ess cable required and towers holding the cables proportionately shorter. ===

This type of bridge is a lot more stable, because tension in the steel cables, is extremely high and effectively stabilize the towers from both sides.
The longest cable-stayed bridge is the Sutong Bridge over the Yangtze River in China.

<span style="font-family: 'Verdana','sans-serif'; font-size: 18pt; line-height: 18pt; margin: 0cm 0cm 10pt 36pt;">How bridges can influence life in a place (city, town, nation) <span style="font-family: 'Verdana','sans-serif'; font-size: 12pt; line-height: 18pt; margin: 0cm 0cm 10pt;">Transportation, economy, time of traveling

** Bridges are the perfect tool to connect. Thanks to them, men have been able to get across a lot of geographic obstacles in a shorter time and with less effort. Romans were example of the benefits that bridges has given to humans. They expand his Empery through the use of bridges. ** ** The transportation of food and traded goods bring economic benefits, doing the cost of travel very lower, and allowing social and financial cohesion between different towns, cities, or even countries. Also bridges increased opportunities to share intellectual, political and religious ideas. **  ** Now days, cities becomes more and more complex, and bridges( in big and small scale) have become crucial for distribution networks, for example, to control traffic, to created passes for **  pedestrian trough high ways,   <span style="font-family: 'Arial','sans-serif'; font-size: 12pt;"> as a landscape compositor, etc.

<span style="font-family: 'Arial','sans-serif'; font-size: 18pt;">Project of west 8 for China, called: garden of 10000 bridges. **<span style="font-family: 'Tahoma','sans-serif'; font-size: 18pt;">Cantilever: ** <span style="color: #333333; font-family: 'Verdana','sans-serif'; font-size: 12pt; line-height: normal; margin: 0cm 0cm 10pt;">1. Any rigid structural member projecting from a vertical support, esp. one in which the projection is great in relation to the depth, so that the upper part is in tension and the lower part in compression. **<span style="color: #7b7b7b; font-family: 'Verdana','sans-serif'; font-size: 12pt;">2. **//<span style="color: #333333; font-family: 'Georgia','serif'; font-size: 12pt;">Building Trades, Civil Engineering: //<span style="color: #333333; font-family: 'Verdana','sans-serif'; font-size: 12pt;"> Any rigid construction extending horizontally well beyond its vertical support, used as a structural element of a bridge (cantilever bridge), building foundation, etc.

**<span style="color: #333333; font-family: 'Arial','sans-serif'; font-size: 12pt;">Caissons **<span style="color: #333333; font-family: 'Arial','sans-serif'; font-size: 12pt;">: <span style="color: #333333; font-family: 'Verdana','sans-serif'; font-size: 12pt;">a structure used in underwater work, consisting of an airtight chamber, open at the bottom and containing air under sufficient pressure to exclude the water.
 * <span style="color: #333333; font-family: 'Arial','sans-serif'; font-size: 12pt; line-height: 115%;">Cofferdams: **<span style="color: #333333; font-family: 'Verdana','sans-serif'; font-size: 12pt; line-height: 115%;"> a watertight enclosure placed or constructed in waterlogged soil or under water and pumped dry so that construction or repairs can proceed under normal conditions.

<span style="color: #333333; font-family: 'Verdana','sans-serif'; font-size: 12pt; line-height: 0px; overflow: hidden;">﻿media type="file" key="Bridge alejandro.wma" width="300" height="300"