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Push + Pull = Bridge?

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Push + Pull = Bridge?
Reading Level

	edHelper's suggested reading level:	grades 4 to 5
	Flesch-Kincaid grade level:	5.94

Vocabulary

	challenging words:		suspension, therefore, compression, spans, foundation, entire, roadway, span, design, towers, opposite, mortar, pressing, ideal, remain, however
	content words:		Golden Gate Bridge, San Francisco

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Push + Pull = Bridge? By Trista L. Pollard

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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 have been around for thousands of years. What makes these structures special is how they are able to support their weight and the weight of vehicles traveling across their spans. So what helps bridges to stand over running water?
 
²     A special set of forces helps to support bridges all over the world. Force is defined as the push or pull on objects. Compression and tension are the push and pull that keep bridges 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 these forces are not balanced, the structure will become weak and may fall. Now, let's talk about bridges.
 
³     There are three types of bridges called 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 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 will also have frames or larger beams called girders to give extra support.

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