Urban and Civil

Nov 29, 2013 (4 years and 6 months ago)

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Bridge History

Long before there were freeways, highways, or even roads, people needed to
cross rivers and other obstructions. This is the purpose of a bridge. The first
bridges were probably logs tossed across narrow streams. From these simple
beg
innings, has evolved the marvelous bridges we
see today.

Bridge Lingo

Span

This is the distance a bridge covers. In the case of our log bridge, it’s the
distance from one side of the river to the other side.

Force
-

When you stand on a bridge,

you are applying a force to the bridge.
Your weight is a force. The more you weigh, the larger the force that is applied
to the bridge. There are four types of forces.

Compression and Tension

There are two types of forces that act on a bridge

tensi
on and compression. Tension is a pulling force. Compression is a
pushing force.

Shear

A shear force is a force that shears or cuts through.

Torsion

This is a force that causes twisting.

Beam

A beam spans a distance using both tension and compressio
n. Our log
bridge is an example of a beam

the simplest style of bridge.

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Cable

A Cable is a long wire. For the earliest rope bridges they used vines,
but today they make cables out of steel. A cable works great in tension but
terrible in compression
. For example, it’s easy to pull on a cable, but have you
ever tried to push a cable?

Types of Bridges

Beam Bridge

This is the simplest bridge. A beam
spans across a distance and creates a bridge. The
beam bends in the center and this causes
compres
sion in the top of the beam and tension in the
bottom of the beam.

Cantilever Bridge

A cantilever bridge is similar to a beam bridge. The
difference is the cantilever beams are secured at one end and hang free at the
other.

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A cantil
ever beam carried forces opposite a standard beam. The tension is on
the top and the compression is on the bottom.

Arch Bridge

Arches have been around since
ancient times. The graceful curves of an arch
also carry the load outward in bo
th directions to
the abutments. All the parts of an arch bridge
are in compression and are leaning on each
other. The famous Italian artist Leonardo da
Vinci once said, “An arch consists of two weaknesses which, leaning on one
another, become a strength.

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Span

Member

Verti
cal

Member

Diagonal

Member

Lower chord

Upper Chord

Member

Panel

Panel

Panel

Panel

Joint

Truss Bridge

Trusses can be used in
many places. Trusses can be used in
roofs of houses, to support power lines,
and also as bridges. The first trusses
appeared in the middle ages. A truss bridge usually consists of two trusses side
by
side. The parts of a truss are shown on the diagram below:

The members of a truss are assumed to carry only tension or compression. Here
is an example of a loaded truss. The red members are in tension and the blue
members are in compres
sion.

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Some common trusses are shown below:

Have you noticed

that all of the trusses shown are made of triangles? This is
because a triangle is very strong. Trian
gles are even stronger than squa
r
es even
though a square has an extra
member. This is because where
Warren Truss

King Post Truss

Queen Post Truss

Pratt Truss

Howe Truss

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triangle, there will always be a second member, at some angle, helping to brace
the load. Here is an example of this:

Suspension Bridge

Indian
a

Jones
has

been known to cross rivers on a
Thousands of years ago in South
America, Asia and Africa these bridges
got their start. The Brooklyn Bridge
was the first steel suspension bridge. The cables of a suspension bridge hang
f
rom the towers. These cables hang naturally in a catenary shape. The cables
are in tension and the towers are in compression.

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Cable Stayed Bridge

Cable Stayed Bridges
are similar to suspension Bridges. The
difference is the cables atta
ch directly from the

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Bridge Materials

The materials used to build a bridge are just as important as
the design of the bridge. Steel is a very strong material but it requires special
welding.

Wood is much easier to work with but it is not as strong.

Steel is equally strong in tension and compression. However because
wood has grain it does not have the same properties in every direction. For
instance wood is much stronger parallel to it’s
grain.

Bridge Member Geometry

The way you orient your members is just as
important as what they are made from. For example consider the two beams,
which one is stronger?
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The one on the

left is stronger. This is because it has a higher
moment of
inertia
. Moment of inertia describes the resistance to turning.