C. J. Burgoyne
Dept of Civil Engineering, Imperial College of Science
and Technology, London, UK
Over the centuries, many materials
have been used as structural tension members; in Ancient Greece, flax ropes,
stressed, it is believed, to about 30 Tonnes by means of a Spanish Windlass,
were used to prestress the hulls of the Triremes used to maintain Athenian Sea
power.
With the advent of wrought iron,
and later steel wire, most of the alternative materials have fallen into
disuse, due to the efficiency with which steel wire can be made and anchored.
Tension members today are made almost exclusively of steel wire.
However, steel suffers from a
number of drawbacks; most importantly, it corrodes and it is heavy. Other
problems in some applications are caused by its electrical conductivity and its
magnetic properties. Large amounts of money are spent getting round these
problems, since until recently, there has been little alternative.
About 20 years ago however, a range
of new polyaramid materials was developed. These are usually known by the names
of Kevlar (a trade name of Du Font), Twaron, (a trade name of Enka) or Tecnora
(a trade name of Teijin). These materials are produced in the form of fine
filaments, which are then given a surface coating and supplied as tows, each
containing about 1000 filaments loosely twisted together.
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