The Engineering and Mining Journal raises the question whether steel, which is becoming so popular a substitute for wrought iron, will, when it is subjected to continuous strain in suspension bridges and other similar structures, do as well as iron has proved that it can. Recent tests of sections from the cables at Fairmount Park, Philadelphia, and at Niagara Falls show that long use has not materially changed the structure. The Journal says: "It is a serious question, and one which time only can completely answer, whether steel structures will prove as uniformly and permanently reliable as wrought iron has proved itself to be. In other words, whether the fibrous texture of wrought iron can be equaled in this respect by the granulated texture of steel or ingot iron. In this connection it is interesting to note that the fibrous texture referred to is imparted to wrought iron by the presence in it of a small proportion of slag from the puddling furnace, and that this can be secured in the Bessemer converter also if desired. The so-called Klein-Bessemerei, carried on at Avesta in Sweden for several years past, produces an exclusively soft, fibrous iron by the simple device of pouring slag and iron together into the ingot mould.
This requires however a very small charge (usually not more than half a ton), and a direct pouring from the converter, without the intervention of a ladle, which would chill the slag."
The effect of the introduction of slag would seem to be to retrace the steps usually taken in producing steel, viz., to separate the iron from its impurities, and then to add definite quantities of carbon and such other ingredients as are found to neutralize the effects of certain impurities not fully removed.
The most intelligent engineers, after ascertaining by exhaustive physical tests what they need, present their "requirements" to the iron and steel makers, whose practical experience and science guide them in the protracted metallurgical experiments necessary to find the exact process required. The engineer verifies the product by further tests, and by practical use may find that his "requirement" needs further modifications. As a result of all this care, some degree of certainty is secured as to what the material may be expected to do.
No doubt the chemical composition of the slag used at Avesta was known and met some equally well known want in the iron, and thus the result arrived at was one which had been definitely and intelligently sought.
An important factor in selecting material for the cables of suspension bridges is its true elastic limit. By this term we mean the percentage of the total strength of the material which it can exert continuously without losing its resilience, i.e., its power to resume its former shape and position when stress is removed. Now, in the case particularly of steel wire as commonly furnished in spiral coils, the curve put into the wire in the process of manufacture seriously diminishes this available sustaining power.
For it is evident that it would be unsafe to subject these cables at any time to a stress beyond their elastic limit. If, e.g., a snowstorm or a great crowd of people should load a bridge beyond this limit, when the extra weight was removed the cables could not bring the bridge back to its normal place, and the result would be a permanent flattening and weakening of the arch.
By a process invented and patented by Col. Paine, the wire in the New York and Brooklyn bridge was furnished straight instead of curved. Now, if a short piece of common steel wire is taken from the coil, and pulled toward a straight position, and then released, it springs back into its 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.