Push + Pull = Bridge?

Push + Pull = Bridge?

Push + Pull = Bridge?
Reading Level

	edHelper's suggested reading level:	grades 6 to 7
	Flesch-Kincaid grade level:	6.44

Vocabulary

	challenging words:		cable-stayed, clapper, equalize, pontoon, standing, underside, cantilever, tidal, ideal, entire, bridges, posts, massive, compression, remain, goal
	content words:		Ancient Romans, Golden Gate Bridge, San Francisco

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Push + Pull = Bridge?
By Trista L. Pollard

¹     You see them everywhere. When you travel on those long family vacations, you even cross a few of them. What are they? They are bridges. Bridges are structures that cross rivers and lakes. They are one of the earliest forms of architecture that have existed around the world. What makes these structures special is how they are able to support their weight and the weight of vehicles and people traveling across their spans. Why were bridges built, and what helps bridges to stand over running water?

²     Bridges were built for one practical reason -- to cross waterways to get to the other side. These structures which are elevated or raised must support their own weight and the weight of people and vehicles. There is a special set of forces that helps to accomplish this goal. Force is defined as the push or pull on objects. Compression and tension are the push and pull that help bridges to remain standing. Compression is a force that squeezes objects together. Think about the s'mores you made at camp last summer. After you combined the marshmallows, chocolate, and graham crackers, you squeezed everything together to make your ideal camp snack. Tension is the force that stretches or pulls objects apart. This is the opposite of compression. Compression and tension cause objects to become shorter or longer. If an object is being compressed (like your marshmallow), it becomes shorter. If an object is being pulled (like chewing gum), it is under tension and therefore becomes longer. Together, tension and compression balance to help structures like bridges remain standing. When the roadway of a bridge is being compressed, the underside of that roadway is in tension. If these forces are not balanced, the structure will become weak and may fall. Now, let's talk about bridges.

³     Bridges are designed to remain standing regardless of the changes in outside forces. These forces include high winds, tidal and river currents, and shifting in the earth's surface like earthquakes. Bridges are classified as three types: beam, arch, and suspension. Beams are horizontal pieces of wood, metal, or stone that are used for support. You usually see wooden beams in your floor at your home. They are held up by columns or posts. Beam bridges have a horizontal piece that stretches across a stream or ditch. The earliest and simplest form of a beam bridge could be a huge tree branch stretching across a small stream. The weight of the vehicles or people traveling on the bridge compresses or pushes down on the beam. As the top of the beam is pushed, the bottom of the beam is stretched or in tension. Beam bridges may also have larger beams called girders, piers, and trusses to give extra support. Trusses are frames made from beams that equalize tension and compression in a structure such as a bridge. Some examples of beam bridges are pontoon, clapper, cantilever, and cable-stayed bridges.

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