Bridges are truly one of the more amazing wonders of human engineering. Below are three of the main types of bridges that are used today.
Arch bridges were one of the first types of bridges ever built. Arches are designed with an odd number of stones, with the top-middle one, called the keystone, the stone on which the bridge’s weight rests. The stones of these bridges have slanted sides so that they fit tightly against each other. The downward force of the bridge’s weight spreads out to the stones on either side of the keystone. One of the really clever things about an arch bridge is that a load makes it stronger: an arch made of bricks or stones will pack together tightly when a load is placed on it, making a more solid structure. Today, mostarch bridges are made of steel or concrete, and they can span up to 800 feet.
Beam bridges are the simplest kind of bridge. A beam bridge is usually supported by a pier(a raised structure) at each end, with a beam stretching across them. Theweight of the beam pushes down on the piers, and the weight of the vehicles crossing 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. This is why beam bridges rarely span more than 250 feet.
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 usesteel wires and cables.
A steel wire only 1/10th of an inch thickcan 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. Ateach end of the bridge, side cables stretch out to the abutments(the place where the bridge is built into the ground). A suspension bridge can span up to 7,000 feet, which is a greater distance than any other type of bridge.