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Environmental Earth Science Archive


Cable Stay vs Suspension Bridges


2/1/2005

name         Carter
status       student
grade        6-8
location     SD

Question -   What kind of bridge out of beam cable stay arch and
suspension holds the most weight? How much weight do each of them hold?
-----------------
Well, it depends on the distance you have to span.  To span a medium
distance (500 to 2500 feet), a cable-stayed bridge takes less cable than a
suspension bridge and the deck can be constructed out of precast pieces that
can be fabricated on land.  (The longest cable-stayed bridge in North
America is the new Cooper River Bridge in Charleston, SC, which spans 1546
feet.)  So, if you define strength as the weight carried per pound of cable,
the cable-stayed bridge is stronger under these conditions.

On the other hand, for very long spans (2500 to 7000 feet) a suspension
bridge is more efficient.  At some point, the towers for a cable-stayed
bridge become too tall and have to carry too much weight, making them
impractical.  For example, the longest suspension bridge in the world, the
Akashi Kaikyo Bridge in Japan spans 6432 feet and has towers that rise 710
feet above the roadway.  The Cooper River Bridge's towers are approximately
375 feet tall at the roadway.  So, the Akashi Kaikyo's towers are less than
twice as tall as the Cooper River's, even though its span is more than four
times longer.

As far as the total weight these bridges can carry, I am not sure.  But, each
of the main cables on the Akashi Kaikyo Bridge has a tensile strength of 138
million pounds, which is equal to 1720 fully loaded tractor-trailers or
25,800 Chevrolet Suburbans.  The Cooper River Bridge has 128 cables that can
each carry more than one million pounds.

Andy Johnson
=====================================================
How big is big?  Meaning what are the limits?
Frank Lloyd Wright proposed a building one mile high.  Usually there is some
piece of the puzzle that imposes the limits. It has been said, structural
engineering is the task of getting the loads and forces down into the ground.
Sears Tower was possible in Chicago, because of the quality and presence of,
reasonably close to the ground surface,  sound rock.  Similarly, there could
be designs for either type bridge of comparable lengths, heights etc.  the
foundation may pose the limit.  Construction materials and techniques may
limit portions of the design.
So there is no easy answer to your question.
Another point, President Kennedy issued a challenge to put a man on the moon.
Humans had to design and create the materials to perform the task.  So,
saying something is not possible, or has a limit, also could be wrong.  So
far man has been able to conquer problems.

Here is a web site describing suspension bridges, note the advantages and
disadvantages.  http://en.wikipedia.org/wiki/Suspension_bridgeSuspension
bridge - Wikipedia, the free encyclopedia

James Przewozink
=====================================================



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