Suspension, Beam and Arch Bridges
Various kinds of bridges exist, but the most commonly used bridge types are suspension, beam and arch bridges. Different types of bridges were developed to create new and modern looks, but also to span longer distances.
A suspension bridge, such as the Golden Gate Bridge in San Francisco, California, suspends the bridge’s deck using ropes or chains. Suspension bridges can span longer distances than other kinds of bridges because the towers hold the deck. The greater the number of towers, the longer the deck. In order to help stabilize suspension bridges, truss systems are used. The truss system helps the deck resist bending, twisting and swaying. According to Design Technology, an educational materials website, the longest bridge, as of 2010, is England’s Humber Bridge, which spans 4,624 feet; however, suspension bridges can span up to 7,000 feet theoretically.
According the London Borough of Barking and Dagenham's online bridges project, a beam bridge is “one of the simplest forms of bridge.” This is because a beam bridge can consist of nothing more than a log crossing a stream. The “beam” part of a beam bridge is what would be called a “deck” on suspension bridges, which is the part of the bridge which spans the gap. On suspension bridges, the cables take the brunt of the weight. However, on beam bridges, the beam takes the brunt of the weight and not the pillars. Longer beam bridges need to have pillars to help strengthen the bridge; however, the longer the distance between pillars, the less strength the beam bridge will have. This is why beam bridges are not usually used to span distances greater than 250 feet.
Arch bridges are very similar to beam bridges except that the arches of the bridge, rather than the beam, take the brunt of the weight. This feature helps make arch bridges capable of spanning longer distances than beam bridges, but not as long as suspension bridges. Arch bridges are ancient technology, and were used by the Romans. These bridges are usually made of either concrete or steel and usually span no more than 800 feet.