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Advantages
& Disadvantages of Types of Bridges
By Laurie Brenner
While bridges come in all
sizes and shapes, nearly all of them represent one of five types or variations
of the basic bridge types.
Typical bridge designs
include beam, truss, girder, suspension, arch, cable and cantilever.
Forces that come into play
in bridge design and engineering include compression, tension or stretch, deck
flexibility, torsion or twisting and shear, the force that stresses the bridge
materials laterally across the bridge deck.
One of the oldest arch
bridges still in use today, a testament to its Roman engineering, includes the
Ponte dei Quattro Capi bridge which dates back to 62 B.C., and spans half of
the Tiber River in Rome Italy.
Beam, Truss and Girder
Bridges
The beam, truss and girder
bridges work simply, much like laying a plank between two banks.
Piers or posts on either end
support a flat bridge deck that spans the gap between the posts.
The bridge deck consists of
beams, like hollow box girders, an open frame or truss that spans the posts or
supports on either end.
The bridge deck must
withstand compression above and tension from below.
Most of the covered bridges
found in New England, represent these types of bridges made from wood.
Economical because of the
abundance of wood, beam bridges are not as strong as steel and require constant
maintenance.
The Arch Bridge
While the type of bridge
plays a role in how well it’s constructed and stands up to wear, tear and
weathering, the materials in the bridge also play a part in its longevity.
One of the oldest engineered
bridges, the arch bridge, supports a deck built above two abutments that serve
as the curved arch.
Made from masonry and stone,
the arch design prevents any one area of the bridge receiving too much tension.
With abundant building
materials, arch bridges are durable and strong, requiring little to no
maintenance. Its drawback is that masonry and stone don’t have great tensile
strength.
Suspension Bridges
The first suspension bridges
date back to the 15th century and typically span waterways, because the bridge
deck requires little to no access from below to build.
Tall pillars support this
bridge, evenly spaced across the span as needed, from which massive wires on
either side sweep from pillar to pillar.
From these sweeping wires
suspenders hang vertically to hold up the bridge deck.
The tension in the cables
and the compression from the pillars work together to cancel out the force of
gravity, making them strong and efficient.
These bridges can span great
distances once the pillars are in place, but they are costlier to build,
require extensive upkeep and the bridge decks can move and twist when exposed
to fierce winds.
The Brooklyn Bridge in the
state of New York and the Golden Gate Bridge in San Francisco both represent
suspension bridges.
Cantilever Bridges
Cantilever bridges offer a
way to build a continuous bridge across multiple supports to effectively
distribute the load evenly.
A portion of the bridge
provides an anchor that supports a bridge deck that extends to either side of
the support that requires precise counterbalance engineering.
The advantage of building
this bridge design comes during the construction phase.
Cantilever designs cost less
to build because of their uniformity and they don’t require temporary supports
during construction, which helps to speed up the process.
But cantilever bridge
designs do require precise engineering because the counterbalance weights can
affect their strength if incorrect, especially if contractors build the
segments slightly differently.
As a
journalist and editor for several years, Laurie Brenner has covered many
topics in her writings, but science is one of her first loves. Her stint as
Manager of the California State Mining and Mineral Museum in California's gold
country served to deepen her interest in science which she now fulfills by
writing for online science websites. Brenner is also a published sci-fi author.
She graduated from San Diego's Coleman College in 1972.
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