by justin miller
TRANSCRIPT
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Why We Need Bridges
Bridges make it possible to cross from one shoreof a river to another. In earlier times bridgesallowed trade between farmers and the
movement of goods across a natural divide. Early man often placed stepping stones in the
narrow sections of small streams as a way to getto the other side. It is easy to see how a bridge
might develop using stepping-stones as a basefor logs or planks. The earliest bridge wasprobably a toppled tree that had fallen across anearby stream.
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Are Bridges Always over Water?
No they can be over other roads, railways,
and deep gulley's. They are not only for cars
and people. Wooden trestle bridges wereessential to the passage of the railway
through the mountains in western Canada.
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HowA suspension Bridge Is Made
This type of bridge has cables suspended between towers plusvertical suspender cables that carry the weight of the deck below,upon which traffic crosses. This arrangement allows the deck to belevel or to arc upward for additional clearance. Like other suspensionbridge type this type often is constructed without false work.
The suspension cables must be anchored at each end of the bridge,since any load applied to the bridge is transformed into a tension inthese main cables. The main cables continue beyond the pillars todeck-level supports and further continue to connections withanchors in the ground. The roadway is supported by verticalsuspender cables or rods called hangers. In some circumstances thetowers may sit on a bluff or canyon edge where the road mayproceed directly to the main span otherwise the bridge will usuallyhave two smaller spans, running between either pair of pillars andthe highway which may be supported by suspender cables or mayuse a truss bridge to make this connection. In the latter case therewill be very little arc in the outboard.
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Cantilever Bridge
Cantilever bridges are built with many
materials in different ways. Others just have
arms and a roadway or arms a roadway andcables that work like a cable stayed bridge.
Cantilever bridges are built in a way similar to
beam and arch bridges depending on how the
bridge is designed.
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Why We Have Interstates
We need interstates so we can make it places
that we normally cant. We also need them so
we can make it to places quicker instead ofgoing in main streets and driving 25 to 30
mph. The Interstate System currently has
approximately 210,000 lane miles of
pavement. The interstate system also has55,000 bridges on them.
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Facts about Interstates