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Scientific American Supplement, No. 520, December 19, 1885 online

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former curve; but if a short piece of the straight-furnished wire that
was put into this bridge is bent, and then released, it springs back
toward its straight position.

It is easy to see that if a curved wire is pulled straight, there must
occur a distention of the particles on the inside of the curve and a
compression of those on the outside. The inside is in fact strained past
its elastic limit before _any_ stress comes upon the outside. Hence,
after the wire has been pulled straight, the elastic limit of only a
portion of it can be taken into the account in calculating the load that
can safely be put upon it. In the case of curved steel wire pulled
straight, its ultimate strength was found to be only about 90 per cent.
that of similar wire furnished straight by this process. The superior
ductility of iron wire in some measure compensates for the distention of
the particles on the inside of the curve, and that is a reason why it has
heretofore been used for suspension bridges. But with straight steel wire
there is no such distention, and its _entire elastic limit_ is available.
This elastic limit is 66 per cent. of the ultimate strength, and,
besides, that ultimate strength is 10 per cent. greater than that of
similar curved wire. Thus if we have a curved steel wire large enough to
sustain 1,000 lb. without breaking, a similar straight wire, such as
those in this bridge, will hold up 1,100 lb., and 66 per cent. of this
1,100 lb = 720 lb.

The elastic limit of curved wire has never been determined, since any
stress that will cause it to reach a straight line is beyond the elastic
limit of the inside of its sectional area. That of curved iron wire has
been estimated at 40 per cent. of its ultimate strength, which is about
half the ultimate strength of curved steel wire; that is, it would be
unsafe to put more than 40 per cent. of 500 lb. - or 200 lb. - upon a
curved iron wire when a _straight_ steel one can sustain 720 lb. without
injury. In the New York and Brooklyn bridge the cost of a sufficient
amount of such iron wire as is used in all other suspension bridges would
have been some $200,000 greater than that of the straight steel wire
which was used. At five per cent., this effects an annual saving in
interest of $10,000.

There must, too, be a considerable saving in the current expense for
painting and care, to say nothing of the more neat and elegant appearance
of the less bulky steel. And as the whole area of the section of these
wires is subjected to an even strain that is always far within the
elastic limit, there is no danger of a change of structure under that
stress.

It is highly probable - although Col. Paine has been too busy to work up
the matter - that piano wire made in this straight method could be drawn
up to and kept at pitch, without approaching very near the elastic limit.
In that case not only would they seldom if ever require tuning, but
probably all along the tone would be more satisfactory. And there would
not be those exasperating periods when the pitch is not quite perfect,
but yet is not far enough out to make it seem worth while to send for a
tuner.

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Online LibraryVariousScientific American Supplement, No. 520, December 19, 1885 → online text (page 9 of 9)