Arch bridges were one of the first types of bridge. The Romans used arches in their bridges and aqueducts as well as in their triumphal arches and other great architecture. See a picture of Constantine's Arch. Arches are designed with an odd number of stones, with the top, middle one being the keystone on which the bridge's weight rests. These stones have slanted sides so that they fit tight against each other. The downward force of the bridge's weight spreads out to the stones on either side of the keystone. Arches used in bridges also have supports at each end of the arch called abutments. These supports keep the stones in the arch in place even when weight is put on top of them.
Suspension bridges use a combination of strong cables and tall, solid towers to achieve a balance of forces. The roadway of a suspension bridge is suspended from the tower and held up by incredibly strong cables. Most suspension bridges today use steel wires and cables, because steel is very strong. A steel wire only 1/10th of an inch thick can hold more than 1,000 pounds of weight without breaking! To make the cables used in bridges, thousands of strong steel wires are twisted and bound together to make them even stronger. The Brooklyn Bridge in New York is one of the most famous bridges, now not so much for its size as its beauty. Built in 1883, it was the first great suspension bridge and a feat of engineering. At the time, it was the longest suspension bridge ever built (though several larger ones have been built since). The stone and steel that Brooklyn Bridge was built with combined to make it both solid and graceful.
Learn more about another famous suspension bridge, the Golden Gate Bridge, located in California.
Beam bridges are the simplest kind of bridge. A beam bridge is usually supported by a "pier" at each end, with a beam stretching across them. The weight of the beam pushes down on the piers, and the weight of cars or anything that will cross the bridge pushes down on the beam. Both the beam and the piers must be strong enough to support the weight that will be put on them. Beam bridges are often built over highways, rivers, and valleys. However, they cannot stretch across very long distances, since they are only supported by the piers at the ends of the beam, with nothing supporting the center. To make longer bridges, lots of beam bridges can be linked together in a row; each beam is still supported by a pier at each end.
Truss bridges utilize the strength of triangles to spread out force. A web system of sturdy beams between two girders forms a zigzagging triangle pattern, stiffening the bridge and spreading out weight so that no one section bears all the weight at that spot. Trusses are also used to support other types of bridges, like beam or suspension bridges.
To learn more about different types of bridges, check out this site.
In general, bridges are designed by two different types of people. Architects are responsible for how a bridge looks while engineers are responsible for how the bridge works and is supported. There are a lot of things to consider when designing a bridge. Physics is a field of science that studies matter, energy, forces, and more. People who design bridges need to understand some important physics concepts in order to make a bridge that will be strong, stable, and secure over time.
Some physics concepts that are considered when bridges are designed are compression and tension. These are both forces that happen every day on bridges and can act differently on different materials. Compression presses down or shortens an object. Try putting a marshmallow on the counter and pressing down on it with the palm of your hand. The force from your hand compressed the marshmallow! Tension stretches out or lengthens an object. When you stretch a rubber band, a toy slinky, or pull two ends of a string tight, that's tension.
In addition to being able to support lots of weight, a bridge must also be able to stand up to lots of different types of weather, like high winds and sometimes even earthquakes!
Sometimes architects and engineers design special bridges that combine two or more types of bridges together to make the best possible bridge for a location. They have to be very careful to make sure the bridge design will still be stable and strong enough. Bridge designs must be tested quite well before a bridge is ever built.
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