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Bridge
Engineering
Bridge types
deck: the bridge surface on which traffic moves
abutments: heavy supports at the ends of a bridge, which transfer
the thrust from an arch or strut to the bedrock or earth below.
pier: a heavy column or pillar which holds up a bridge
girder: a large beam, usually created by bolting or welding
together steel plates
Suspension bridge
Suspension
bridges involve a continuous girder hung from cables supported by
towers erected on piers. They are a very old bridge form,
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although, in their
modern incarnation, they are widely used where very long spans are needed.
The introduction of steel wire cables in the 1830s greatly extended the
potential of suspension bridges. At both ends of the bridge, large anchors
are placed (usually underground) to hold the ends of the main cables.
The main cables are stretched from anchor to anchor over the tops of the
towers, passing over a saddle which allows the cable to slide as loads
move on the bridge. Smaller hanger cables are used to hang the girder
from the main cables. While other bridge types are supported by piers
or abutments, here the girder and bridge deck hang suspended from the
main cables. Due to the flexibility of the cables, long suspension bridges
are prone to vibrate in high winds, much like the string of a guitar or
violin.
Examples in the Minneapolis Riverfront District
Existing bridges:
•Hennepin Avenue Bridge
Lost bridges:
•First and second Hennepin Avenue bridges
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Arch Bridge
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arch bridge is based on the ancient concept of spanning an opening
with a |
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curved
structural member. The arch transmits the load from the bridge deck to
the abutments on both sides of the span and thus to the ground below.
Early arch bridges were built of stone blocks wedged together to form
the arch. Short modern arch bridges may use wood or concrete, while longer
arch spans are built of steel. Since the arch requires no central support,
it can be used to bridge long open spans. The arch can be either above
or below the bridge deck. The arch pushes downward and outward against
its abutments, which must be heavy to resist the thrust. Since the abutments
transfer both horizontal and vertical forces from the bridge deck, arch
bridges can only be used where the ground or foundation is solid and stable.
The curved arch structure offers a high resistance to bending forces.
Construction of an arch bridge is difficult, since the structure is unstable
until the two sides of the arch are joined. Until recently, most arch
bridges were built with elaborate wooden falsework as temporary supports
below each span.
Closed
spandrel deck arch bridge |
Arch
bridges can be constructed with the deck above the arch (a deck
arch bridge), or the deck can be hung from a segment of the arch
which rises above |
the deck (a
through arch or tied arch bridge). In a deck arch bridge, the space between
the bottom of the arch and the deck can be solid (a closed spandrel deck
arch) or open with supporting vertical members (an open spandrel deck
arch, such as that illustrated at the beginning of this section).
Examples in the Minneapolis Riverfront District
Existing bridges:
•Stone Arch Bridge (St. Paul, Minneapolis,
and Manitoba Railway Bridge)
•Third Avenue Bridge
Lost bridges:
•Third Hennepin Avenue main channel
and east channel bridges |
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Truss bridge
A truss is a simple, rigid skeletal structure, usually based on a triangle-shaped
frame. A truss bridge uses a series of triangles in some sort of superstructure
to transfer the load from the deck to the piers. Since the elements of
a truss are subject only to tension (stretching) or compression (pushing)
forces and cannot handle bending forces, truss bridges are typically best
used for straight alignments. A truss bridge can support heavy weights
and span long distances, but it requires a fair amount of vertical room
to accommodate the truss structure.
An
under-truss or deck truss bridge has the truss structure under the deck.
There are a number of types of truss bridge. First, the bridge deck
can pass either over or under the skeletal truss structure. In a
through-truss bridge, the deck is located under (or through) the
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Through-truss bridge |
truss structure.
An under-truss
or deck truss bridge has the truss structure under the deck.
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Under-truss (deck truss) bridge
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In addition, there are a variety
of specific truss structures, some of which have been used in the Minneapolis
Riverfront District.
| Pratt
truss: a simple
truss structure with vertical compression members and diagonal tension
members. Except for the end sections, the diagonal members all slant
down toward the center of the span. Since these members are subject
to tension forces only, they can be thinner, allowing for a more
economical design. Patented by Caleb and Thomas Pratt in 1844, this
was among the most common American bridge types for the ensuing
decades and was built in both wood and metal. There are many variations
on the basic Pratt truss. |
Pratt truss bridge |
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| Howe
truss: this truss structure is
the reverse of the Pratt truss. Here, the diagonal members all slant
toward the closest bridge end, so they are subject to compressive
forces. This design necessitates large steel members, rendering
it an uneconomical choice for steel construction. This truss type
was patented by WIlliam Howe in 1840. |
Howe truss bridge |
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Examples in the Minneapolis Riverfront District
Existing bridges:
•Boom-Nicollet Island pedestrian bridge
(Wisconsin Central Railroad bridge)
•Merriam Street bridge
•Portion of current Nicollet Island
(Burlington Northern Santa Fe) Railroad bridge
crossing main river channel
Lost bridges:
•Minneapolis Western Railroad Bridge
•Lower (10th Avenue South) bridge
•Third Hennepin Island – east
bank bridge
•First and second Plymouth Avenue (Upper)
bridges
•Second Hennepin Avenue east channel
bridge
•First Nicollet Island (St. Paul and
Pacific) Railroad bridge |
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| Beam
bridge (or girder bridge) |
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In its simplest form, this is a beam (such as a board or log) laid across
supports or piers. The beam must be able to take the heaviest load which
may be placed on it, and the weight of the beam (plus its load) pushes
straight down on the piers. The load on the beam causes the upper edge
of the beam to be pushed together (compression) and the lower edge to
be stretched (tension). Many modern beam bridges are composed of beam
girders (typically I-beams or box girders) supported on piers. I-beams
are simpler and less expensive to fabricate, but box girders (which
are literally a long, box-shaped member) are better suited to handling
twisting forces (such as would be found in curved bridges) and longer
spans. Other modern beam bridges use pre-stressed concrete beams, which
combine steel’s ability to handle tension with concrete’s
strength under compression. Since the strength of a beam bridge depends
largely on the close spacing of piers, this bridge type is generally
ill-suited to long spans, unless many beam bridges are linked end-to-end
in a “continuous span.”
Examples in the Minneapolis
Riverfront District
Existing bridges:
•Hennepin Avenue east channel bridge
•Nicollet Island (Burlington Northern
Santa Fe) Railroad bridge
Lost bridges
•Minneapolis and St. Louis Railroad
Bridge
•First Street South Bridge
•First and second Hennepin Island-east
bank bridges
•First Hennepin Avenue east channel
bridge
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Concrete
box girder bridge
This modern bridge type uses box girders (see above) made not of steel
plates but of concrete reinforced with steel bars. Beams of this type
take advantage of the ability of concrete to handle compression loads
and the ability of steel to handle tension loads. The box girder segments
allow long gaps to be spanned between supporting piers.
Online bridge resources
http://www.pbs.org/wgbh/nova/bridge/build.html
http://www.pbs.org/wgbh/buildingbig/bridge/basics.html
http://www.matsuo-bridge.co.jp/english/bridges/basics.shtm
http://travel.howstuffworks.com/bridge.htm
http://www.co.multnomah.or.us/bridge/bridge_types/bridge_types.html
http://www.mnhs.org/places/nationalregister/bridges/bridges.html
http://bridgecontest.usma.edu/
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