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Types of BridgesArch
There
are different types of bridges. Which type to use is based on the
purpose, the location, the environment, the length of the span,
the materials available and also the cost. Here are descriptions
of the basic types of bridges. Arch
bridges typically span distances between 200 to 800 feet. In others,
the roadway of the bridge is suspended from the arch not supported
by it.
Cable-Stayed
Bridge
Cantilever
Bridge Covered
Bridge Movable
Bridges
Suspension
Bridge Information
from NOVA. Truss
Bridge
References:
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Arch
bridges are one of the earliest types of bridges. China built stone
arch bridges in very early times. The Romans also used stones to
build the arches. Many are still standing. Most modern day arch
bridges are built from steel or concrete. Arches are very strong
bridges. The weight does not push straight down on the bridge but
it is carried outward along the curve of the arch to the supports
at each end which are known as abutments. The abutments carry the
load and keep the arch and the roadway above it in place.
In
some arch bridges, the roadway is on top of the arch.
A
beam bridge is the simplest type of bridge. It is also known as
a girder bridge. It is a horizontal beam supported by piers at each
end. A beam bridge is a good design to use to span short distances
that are not more than 250 feet because the farther apart the piers,
the weaker the bridge becomes. Therefore, support columns are used
along the bridge if the beam bridge is long. A beam bridge is usually
built with concrete and steel. 
Cable-stayed bridges have cables attached to the towers. Cable-stayed
bridges are used for medium span between 500 and 2,800 feet.
In
the radial pattern, cables extend from several points on the road
to a common point at the top of the tower.
In
a parallel pattern, cables extend from several points on the road
to points at different heights along the tower, running parallel
to one another.
A
cantilever bridge has two arms that sticks out from each pier. One
arm is anchored down on the shore. The other arm projects toward
the center. A section is built in the center that rests on the ends
of the two arms. The Cantilever bridge can be used to span great
distances.
Covered
bridges were built to cross streams of moderate distance. The roof
protected the wooden planking of the road from rain and snow.
They
are needed over rivers, canals or harbor entrances. These bridges
are low but they must allow the passing of boats. Bascule is a moveable
bridge which has one or two sections which are raised when a boat
wants to go through. 
Suspension
bridges are used to span distances from 2,000 to 7,000 feet. They
are longer than the other types of bridges. A suspension bridge
uses huge main cables which extend from one end of the bridge to
the other end. The cables rest on the top of the high towers and
are secured at each end by anchorages.
A
truss bridge is like a beam bridge except it has triangular shape
patterns known as trusses which make the bridge stronger. The trusses
are commonly made from straight, steel bars. They are rigid. They
prevent movement when they are pushed.