Transcript Types of Bridges - Midwest Central CUSD #191 / Homepage

Types of Bridges
Source:
http://www.brantacan.co.uk/bridges.htm
Arch Bridge
Introduction:
The essence of an arch is that ideally there
should be no tendency for it to bend,
except under live loads. It should be purely
in compression, and for that reason it can
be made of materials such as, masonry,
cast iron and concrete, that perform poorly
in tension.
Arch Bridge Questions
Why must an arch be curved?
In order to balance the downward forces, the
forces from the side must not be exactly in
line: the angle between them, repeated
throughout the arch, is the reason for the
curvature.
Why is a true arch bridge not convenient?
Must have a support system as it is being built
until the keystone is placed.
Arch Bridge Pics
Arch Bridge Pics
Arch Bridge Pics
abutment
keystone
Arch Bridge
Advantages: The entire arch is in
compression. The compression is
transferred into the abutments, and
ultimately resisted by tension in the ground
under the arch. The absence of tension in
the arch means that it can sustain much
greater spans than beams can achieve,
and it can use materials that are not strong
in tension, such as masonry and cast iron.
Disadvantages: An arch cannot stand
until it is complete.
Beam Bridge
Introduction:
Like the arch, the beam is in principle very
simple. As with the arch, we see the
spreading of the force in the ground, but
this time the ground has to support only
the weight of the beam: there is no
horizontal thrust. The nature of the ground
is therefore not quite as critical as it is for
an arch.
Beam Bridge Questions
Why build an arch bridge when you can
build a beam bridge?
The fact is that forces within a beam can be
far greater than they are in an arch of the
same weight and span. The reason is that
the beam is pulled down at all points by its
local weight, which tends to make it bend.
Beam Bridge Pics
Beam Bridge Pics
Beam Bridge
Advantages: Because a beam rests simply on
the supports, the effects of thermal expansion
and movements of the ground are fairly easily
sustained. In principle, a beam may be built
away from the final position, and lifted swiftly into
place with minimal disruption of traffic or
navigation.
Disadvantages: This means that a beam
contains forces which are much larger than the
load, and it needs to be relatively massive
Cantilever Bridge
Introduction:
The cantilever is rather like a bracket, projecting
out into space. The two forces almost always act
in opposite directions. A cantilever is really a
large bracket, held rigidly at one end. The
largest cantilever bridges are made of steel,
though medium sized ones are sometimes in
pre-stressed concrete. Ancient ones in Asia were
made of wood.
Cantilever Bridge Questions
How many cantilevers do most bridges
have?
Almost all cantilever bridges have only two.
Cantilever Bridge Pics
Cantilever Bridge Pics
Cantilever Bridge Pics
Cantilever Bridge
Advantages: Building out from each end
enables construction to be done with little
disruption to navigation below. The span can be
greater than that of a simple beam, because a
beam can be added to the cantilever arms.
Cantilever bridges are very common over
roads. Because the beam is resting simply on
the arms, thermal expansion and ground
movement are fairly simple to sustain.
Disadvantage: Like beams, they maintain their
shape by the opposition of large tensile and
compressive forces, as well as shear, and are
therefore relatively massive.
Cable-Stayed Bridge
Introduction:
The cable-stayed bridge is related to the
cantilever bridge. The cables are in
tension, and the deck is in
compression. The spans can be
constructed as cantilevers until they are
joined at the centre.
Cable-Stayed Bridge Pics
Cable-Stayed Bridge
Advantages: The two halves may be
cantilevered out from each side. There is no
need for anchorages to sustain strong horizontal
forces, because the spans are selfanchoring. They can be cheaper than
suspension bridges for a given span. Many
asymmetrical designs are possible.
Disadvantages: In the longer sizes, the
cantilevered halves are very susceptible to wind
induced oscillation during construction. The
cables require careful treatment to protect them
from corrosion.
Suspension Bridge
Introduction:
With the deck high above the floppy cables, this
looks unstable, and it is. This construction can
be used only for spans that are short enough for
a stiff deck to transmit lateral forces to the
anchorages. The cable of a suspension bridge is
in tension, enabling it to be much narrower and
cheaper than an arch of the same span.
Suspension Bridge Question
What is the function of the following parts of
a suspension bridge?
Towers, Anchorage, Hangers, Deck
The functions of the various parts are easy
to understand.
The towers hold the cable up.
The anchorages pull the cable outwards
and downwards.
The hangers connect the deck to the main
cable.
And the deck is there to carry the traffic.
Suspension Bridge Pics
Suspension
Bridge Pics
Suspension Bridge
Advantages: The main sustaining members, the
cables or chains, are purely in tension, and are
not required to be rigid, so they can be only as
thick as needed to resist the tension. The
towers are almost purely in compression, so
their design is relatively simple.
Disadvantages: They are only as rigid as the
deck structure, which in older structures was
usually of truss construction. This makes them
generally unsuitable for railway traffic.