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jueves, 9 de febrero de 2012

 BUILDING BIG :BRIDGES BASICS
There are more than half a million bridges in the United States, and you rely on them every day to cross obstacles like streams, valleys, and railroad tracks. But do you know how they work? Or why some bridges are curved while others are straight? Engineers must consider many things -- like the distance to be spanned and the types of materials available -- before determining the size, shape, and overall look of a bridge.
Since ancient times, engineers have designed three major types of bridges to withstand all forces of nature.
 
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