Physics of an Arch Bridge
An arch bridge is able to stand, owing to the laws of physics. It balances the force from above with the force from below, using a block weight to support the weight of the opposing forces. One of the most famous arch bridges is the Bridge of Sighs in Italy, which is used as a walkway for prisoners going to court.
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History
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The oldest existing arch bridge is the Mycenaean Arkadiko Bridge in Greece, dating back to 1300 B.C. Early bridges were constructed out of brick and stone. However, it was Roman engineers that took full advantage of the arch design, and build many bridges with concrete. But it wasn't until the Industrial Revolution that other people began to build arch bridges, to make best use of the structure's natural strength.
Forces
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Original arch bridges consist of a series of stones, built into the shape of a semicircle. A keystone is positioned in the middle, at the top of the arch. It makes each stone press against each other, to transfer the force along the bridge's curve. The weight or load of the bridge is carried through the stones, and pushes against the supports at each end known as abutments. The abutments prevent the bridge collapsing, since they stop the force of the load spreading out. The abutments provide resistance, and consequently oppose the downward force of the bridge. The upward push balances the arch and it can stand.
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Test the Physics
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Get a piece of cardboard measuring approximately 11 inches, and bend it into an curve. Position it on a table and press down in the middle; it collapses. Now, place a stack of books 4 inches apart. Bend the cardboard into an arch and wedge each end between the books. The books act as abutments and keep the arch upright. Press down on the top and it does not collapse.
Famous Bridges
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A prime example of a Roman arch bridge is the Pont du Gard aqueduct near Nîmes, France. It was built more than 2,000 years ago, and consists of two tiers of arches. The top tier is held together only by mortar, while the other stones are held in place solely by their own forces. Modern arch bridges are constructed using steel or prestressed concrete, enable the arches to have a wider span. The New River Gorge in West Virginia spans 1,700 feet, in comparison with most bridges, which span between 200 and 800 feet.
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References
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