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Bridge
Basics
A bridge is a structural
system
that is designed to transfer loads safely to the ground.
There are three major types of bridges:
• Beam bridge
• Arch bridge
• Suspension bridge
A main difference is the distance each bridge can cross in a single
span.
Span: The distance between two bridge supports.
Compare the Spans: A beam bridge may span a
distance
of up to 200 feet, while a modern arch bridge can safely span up to
1,000 feet. A suspension bridge, the most spectacular of all
bridge technologies, is capable of spanning up to 7,000 feet or more.
The Beam Bridge
A beam bridge is a rigid horizontal
structure resting on two piers, one at each end. The weight of
the bridge and its loads are directly transferred downward to the
ground through the piers. Beam bridges are often strengthened by
a truss.
Truss: A triangular support framework that is
lightweight and allows the wind to pass through.
Pier: Vertical structures (columns) anchored
to the
ground.
The Arch Bridge
An arch bridge is a semicircular
structure
with abutments on each end. The weight of the bridge and its
loads are directly transferred along the arch and into the
abutments.
Arch bridges are always in compression. The force of compression pushes
the load outward along the curve of the arch towards the
abutments. The abutments keep the arch structure from spreading
out.
Compression: Forces that push or squeeze.
Abutment: A support on each end of an arch
that
directly receives the weight of the load. Abutments keep the arch
from spreading. Since they are always in compression, abutments
are usually made of brick or concrete.
The Suspension
Bridge
Suspension bridges are built using
large
main cables strung over two main towers. The roadway (or deck) is
then suspended from these main cables with vertical secondary cables,
called suspenders.
Because the main cables are always in tension, they are made of
high-tensile steel. These main cables must be firmly embedded
into huge concrete anchorages at each end of the bridge to support the
weight of the deck and its load.
The decks of most suspension bridges are strengthened by trusses.
Because of the long distances that suspension bridges can span, they
are the most economical of all bridges.
Deck: The roadway or surface of intended use.
Tensile Strength: The ability of a material to
withstand stretching forces (tension) without parting. Steel has
a very high tensile strength.
Tension: Forces that stretch.
Dead Load: The weight of the bridge itself.
Live Load: The weight of the cars, trucks,
people,
snow, birds, etc. that the bridge is intended to support.
Specialized
Bridge
Types
Cable-Stayed
Suspension Bridge
Instead of large main cables, a
cable-stayed suspension bridge has many smaller cables connected to the
tower and secured directly to the deck. Cable-stayed suspension
bridges are being built more often because they are economical and
beautiful.
Cantilever
Beam
Bridge
A beam is said to be cantilevered
when it
projects outward, weighed down or otherwise supported at one end.
A cantilever bridge is generally made from cantilevered beams at each
end of the span that are joined in the center. Cantilever bridges
require large amounts of materials, and are very expensive to build and
maintain.
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