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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.

Beam bridge


The beam bridge...
consists of a horizontal beam supported at each end by piers. The weight of the
beam pushes straight down on the piers. The farther apart its piers, the weaker
the beam becomes. This is why beam bridges rarely span more than 250 feet.

Sneak a peek at the forces that act on beam bridges!

Check out other types of beam bridges!



Cantilever bridge:
Firth of Forth

The truss bridge...
consists of an assembly of triangles. Truss bridges are commonly made from a
series of straight, steel bars. The Firth of Forth Bridge in Scotland is a
cantilever bridge, a complex version of the truss bridge. Rigid arms extend from
both sides of two piers. Diagonal steel tubes, projecting from the top and
bottom of each pier, hold the arms in place. The arms that project toward the
middle are only supported on one side, like really strong diving boards. These
"diving boards," called cantilever arms, support a third, central span.

Check out the forces that act on truss bridges!



Ancient Roman aqueduct

The arch bridge...
has great natural strength. Thousands of years ago, Romans built arches out of
stone. Today, most arch bridges are made of steel or concrete, and they can span
up to 800 feet.

Catch a glimpse of the forces that act on arch bridges!

Check out how arch bridges are built!



Suspension bridge:
Golden Gate Bridge

The suspension bridge...
can span 2,000 to 7,000 feet -- way farther than any other type of bridge! Most
suspension bridges have a truss system beneath the roadway to resist bending and
twisting.

See how forces act on suspension bridges!

Check out another type of suspension bridge!

Now that you've mastered the bridge basics, test your bridge-building skills in
the Bridge Challenge!


 



 

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