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THE BRIDGES of
THE TYNE
The Science of
the Bridge
Man has
frequently copied nature in his search for scientific progress.
The first
bridges were those formed naturally by tree trunks lying across
streams, and, later on, the early inhabitants of the earth made
primitive bridges by dragging fallen trees into suitable
positions. The longer bridges required for the wider streams
were made by resting tree trunks or stone slabs on rows of piles
of loose stones, and these piles were the forerunners of our
modern " columns " or " piers." In the same
way nature, in the form of the spider's web, was copied by man
in the earliest suspension bridges, which were made of twisted
ropes of canes, vines or strong creeping plants and were slung
from trees.
The natural
arches of caverns gave man his first ideas of the arch as a
means of bridging a space. The first man-made arches were built
of rough stones and filled in with earth, for at that time
mortar had not been invented. In later times tools were used and
more suitable building materials discovered, and arches were
then built of hewn stone, brick, cast-iron, steel, and
reinforced concrete (or artificial stone strengthened with bars
of steel).
A great advance in the science
of bridge-building was made in the eighteenth century when two
Swiss brothers built timber bridges of 200 feet span. These
bridges were built up of frameworks of timber cut to size and
trussed, that is, secured together by bolts; and it is from
these bridges that modern truss or framework bridges have been
developed.
Modern steel
bridges are commonly of beam construction, that is, made up of
steel beams resting on masonry supports. The beam may be made up
of steel bars riveted together to form a truss or framework, or
it may be made of steel plates and angle-bars riveted together
to form a " plate-girder."
A girder is far stronger than a
solid rectangular beam containing the same weight of steel. The
purpose of a truss or a girder is to resist bending.
A steel column
opposes " crushing " and " buckling " and,
if the same weights of steel are used, a large hollow column is
more effective than a smaller solid column.
A tie-rod,
however, has to resist " pulling " forces and is
therefore generally solid. There is, of course, no tendency to
buckle in tie-rods, and they are therefore sometimes made of
strips of plate steel.
The new Tyne Bridge was designed
by Messrs Mott, Hay, and Anderson, M.M.Inst.C.E., of London, and
was built by Messrs. Dorman, Long and Co., Ltd., of
Middlesborough. Mr. R. Burns Dick, of Messrs. Cackett and Burns
Dick, Newcastle upon Tyne, has been responsible for the
architectural treatment of the bridge.
The main span of
the bridge is a steel arch, The shape of this arch is a
parabola, and clever mathematicians have found that this shape
is the best for carrying a heavy load without producing
excessive strains and stresses in the steelwork of the bridge.
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