Bridges help us get from place to place. They are built
where there is water, mountains, or where land is hard to get
across. It would take us a lot longer to get someplace if we
didn't have bridges.
We learned about three types of bridges:beam,arch, and
suspension. We also learned that the type of the bridge that is
built depends on the person building the bridge. A bridge can be
very simple or very complicated. The way the bridge is built
depends on the material used,how much it will way,the size, and
how wide it is.
Most bridges are beam or girder bridges. The beam bridge is
made of flat pieces of concrete or metal and is held at the end
by supports called piers. The weight of the beam pushes straight
down on the piers. If the piers are very far apart, the beam can
get very weak and unsteady and the bridge may not hold up.
That is why beam bridges are usually not longer than the size of
a football field.
The arch bridge is very strong. It looks like half a circle. Arch
bridges used to be made of stone. The problem was that until
the last block was in place, these bridges were unstable and
could fall down. The Romans used wooden frameworks to
support the arches until the keystone could be put in place. The
keystone put even pressure over the arch so it wouldn't fall
down. Today, arch bridges are made of steel and concrete. They
can span up to 800 feet which is the size of three football fields!
Venice,Italy has many arch bridges since there is so much water
surrounding the city. The most famous is the Bridge of Sighs. It
is the bridge that prisoners used to go across on their way to
The longest bridges in the world are the suspension bridges.
Enormous towers of steel or concrete are set into the foundation
and steel cables hang between the towers to hold up the road. It
can be almost a mile between towers in the longest suspension
bridges. Suspension bridges cross the deepest canyons and
water. Strong winds can make a suspension bridge move back
and forth unless they are reinforced. The most famous
suspension bridge that swayed back and forth and then finally
fell down was the Tacoma Narrows Bridge. The longest
suspension bridge in the world is the Akashi Kaikyo in Japan
which spans 6,570 feet.