A bridge goes over water or land so people can cross.
A bridge joins two places that are far apart.
Some bridges are small but some are really long.
Some bridges are for walking across or riding a bike.
Some bridges are for cars or trains.
A bridge is a structure that carries a road, path or railway across a waterway, a gap between cliffs or hills, roads, or other obstacles.
A simple log bridge ©Getty
From the earliest times, people have made bridges, the simplest being moving a fallen or chopped down tree across a creek, or moving stones across a shallow stream to make a walkway.
This kind of bridge is the simplest of a group called beam bridges.
There are different kinds of bridges, each constructed in different ways. However, there are four key basic components that are used in combination or alone: beams, arches, trusses and suspensions. These variations are used according to the distance between the two supports, or span.
Tension and Compression
A bridge has to deal with two forces: tension and compression.
Tension: think of pulling a rope, the rope gets tight or taut: that's tension.
Compression: think of pushing two ends of a spring.it shortens: that’s compression.
Either force can make a bridge buckle or break, so engineers design a bridge so that these forces are either transferred from a weak area to a strong one, or spread out evenly over a greater area so that no one spot gets more than another.
Arch bridge
As its name suggests, it is an arch shape under the road or path. It has a support at each end of the arch, or of each arch if there are more than one. Arches spread the compression through to the supports, or abutments. There is little tension because the curve reduces it by spreading it outwards.
An ancient arch bridge in Japan, still strong and in use today. ©Getty
Ancient Greeks and Romans made stone arch bridges over 2,000 years ago, and because of the curve, the stonework often did not need mortar.
A single arch bridge over River Meles in Turkey, built in 850 BCE, is one of the oldest existing bridges.
Today arch bridges are still made, but modern materials allow engineers to vary the basic arch design in many ways, including the size. The 1741 metre long Chaotianmen Bridge in China is the largest arch bridge in the world.
Beam bridge, also called a girder bridge:
A beam bridge is a very simple bridge structure, consisting of a horizontal beam supported at each end. The weight of traffic bears downward onto the supports. The longer the beam is, the weaker the bridge is. For that reason, beam bridges are not more than 76 metres in length.
To make the supports longer so that the beam can be higher and longer, engineers add a triangular latticework of metal, called a truss, over or under the beam, which helps spread the tension and compression. This is a called a truss bridge.
A truss bridge ©Getty Images
A triangle is the strongest shape, which is why the trusses are triangles.
Suspension bridge
The bridge is suspended from strong cables from a tower at each end. The towers take most of the weight because compression pushes down on the bridge and travels up the cables to the towers, which then transfer it into the earth. The cables that run from the towers to the anchors, which are either huge concrete blocks or solid rock, take the tension. Suspension bridges can span long distances and even withstand earthquakes.
The Golden Gate Bridge in San Francisco, USA, is a suspension bridge. It is one of the most famous bridges in the world and was named one of the Modern Wonders of the World.
The Golden Gate Bridge is one of the most famous bridges in the world. ©Getty Images
Japan's Akashi-Kaikyo Bridge, also called the Pearl Bridge, spans the Akashi Strait between Kobe and Awayi Island. At 1, 991 metres, it is the longest suspension bridge in the world.
Cable-stayed bridge
A cable-stayed bridge is a variation of the suspension bridge. However, they do not need the big anchorage, nor do they need two towers, and they use less cable.
The cables run from different spots on the roadway to a single tower that bears the weight.
A cable-stayed bridge in Portugal. ©Getty Images
The cables are generally in either a fan or a harp design.
Some famous bridges
The Great Belt Bridge links the eastern and western parts of Denmark, crossing a wide body of water called Storebaelt. There are two sections of the bridge that meet on a small island called Sprogo. The East Bridge is 6,790 metres long and the West Bridge, a combined road and rail bridge, is 6,611 metres long.
New York's Brooklyn Bridge spans the East River and connects the island of Manhattan with Brooklyn.
When it opened in 1883, it was then the longest suspension bridge in the world.
As well as carrying vehicles, it has a wide walkway for pedestrians and cyclists.
The Brooklyn Bridge in New York ©Getty Images
Sydney Harbour Bridge is the largest, but not longest, steel arch bridge. It spans the harbour of Sydney, Australia, and, together with the Opera House, is one of Australia's most famous landmarks.
The top of the bridge is 134 metres above Sydney Harbour. The bridge was opened in 1932.
A famous view of Sydney Harbour Bridge and the Sydney Opera House. ©Getty Images
Read more about the Sydney Harbour Bridge on kidcyber’s page about Sydney.
The Millau Viaduct spanning the valley of Tarn river in southern France is the tallest vehicle-carrying bridge.
The speed limit on the cable-stayed bridge had to be reduced because cars kept stopping so people could admire the view from the bridge.
The Millau Viaduct in France ©Getty Images
A bridge can join two countries or continents
The Oeresund Bridge connects the countries of Sweden and Denmark.
It is 16.4 km long, stretching between the cities of Malmo in Sweden and Copenhagen in Denmark, with a four lane road and two rail tracks.
The Oeresund Bridge ©Getty Images
The Bosphorus Bridge joins two continents. Istanbul, in Turkey, is separated by the Bosphorus Strait, so that one part of the city is in Asia and the other in Europe.
The Bosphorus Bridge ©Getty Images
When the Bosphorus Bridge was completed in 1973, it was at that time the 4th longest suspension bridge in the world.
It’s a good idea to get information from more than one source!
Read more about bridges
http://travelerspress.com/amazing-and-famous-bridges/
https://easyscienceforkids.com/all-about-bridges/