The Engineer. 169
Total Page：16
File Type：pdf, Size：1020Kb
March 10, 1882. THE ENGINEER. 169 obtained or considered, though a most important element. hesitation to put a lighted lamp into the middle of a powder barrel. STEEL FOR STRUCTURES. Now the comparison of these four sets of figui'es for a We wish to emphasise the assertion that the accidents that have Amongst engineers and constructors there seems still to happened, or which may happen, are due entirely to insufficiently number of different specimens or makes of steel is not very careful engineering. be a good deal of difference of opinion as to what should easily made so that the best relation of the four values is Sufficient has also been done in the practical application of elec be the limits of strain which may be visited upon steel in clearly conceived. It is desirable that they should be so tricity to lighting purposes to indicate the directions in which engi structures, and more especially such structures as bridges collated as to express in one quantity or index the value neers and electricians must work in order to arrive at the most of the material for work under tension and in one quantity economical and most complete systems of electric lighting plant. and roofs. A paper on steel for structures was read Much of the progress which has been made toward a large use of before the Institution of Civil Engineers on the 1st inst. the value for work under compressive strain. Having electricity for the purpose has been effected by engineers and prac by Mr. Ewing Matheson, the value of which was greatly brought all the values down to a pair of indices in this tical machinists who have turned their attention to the subject. enhanced by the important discussion which it provoked way, the relative values of iron and of steel could be so The electrician alone has moved slowly, and the engineer would then and on the 8th inst. The chief object of the paper simply and clearly shown that the demonstration of the move even more slowly, but the combined labours of the members uncer- desirability for relaxing the official rules would be a com of these two professions have brought about a sufficiently wide was to draw attention to the fact that though the application of electricity for lighting purposes to enable us now to tainties and want of uniformity in steel, which hindered paratively simple matter. take stock of the information which has been obtained by the work its application to shipbuilding and boiler-making purposes, As far as we know, the best attempt to express in this thus carried out. have been removed, steel is nevertheless but little used, in way the structural value of a given material was made by In considering the selection and arrangement of a set of electric this country at least, for bridge building, and hardly at all Mallet in his work on the “ Physical Conditions Involved light plant, the first question is the character or kind of light to be in the Construction of Artillery, ” and in a paper in the employed ; secondly, the kind of electrical generator most suited for roofs or buildings. In stating his case, which may be for this light ; thirdly, the kind of motive and means of transmis termed steel versus iron, Mr. Matheson resorted to a series “ Proceedings ” of the Institution of Civil Engineers, sion from motor to machine; and fourthly, the apparatus and of propositions as the most convenient way of eliciting vol. xviii., on “The Coefficients Te and Tr of Elasticity material for the distribution and regulation of the current between opinions on the various points. In these propositions he and of Rupture of Wrought Iron. ” These coefficients and at generator and lamp. If we speak, then, first of the kind of referred to the certainty with which plates and bars of all expressed the balance in any material between strength light, we have the choice of three kinds—that is to say, we have and toughness, or the work done by an extending the choice of lamps of three kinds, viz.: (1) the arc lamps, (2) in forms are now rolled of steel, to the advantages in candescent lamps, and (3) semi-incandescent lamps. respect of sizes and weights as compared with iron, to the and compressing force on an elastic body at the point Of these lamps, then, experience has only confirmed the reasons superior mechanical properties of steel, to the question of where its elasticity becomes permanently impaired and for selecting the arc lamp as the most suitable and the most manipulation, and to others leading to the proposition that at the further point where rupture occurs. These co economical for lighting large spaces, and, except in ornamental structures of steel are superior to those of iron, but that efficients express the value, then, of a structural material design, little now remains to be done in the improvement of these by two indices, either for tensile or for compressive resist lamps other than in the perfection of the carbons employed. The there are at present only a few instances in which its use incandescent lamp will probably remain permanently the best for offers any pecuniary advantages, and that this limit to ance, and the formulae at length and their application will general indoor illumination. It may, however, be found upon the application of steel is partly due to official rules and be found illustrated in The Engineer for the 8th October, further experience that it is adapted also for street lighting pur PE poses. Two things remain to be done in its improvement, viz., partly to exigencies of design. On all these propositions, 1880. In the simplest form the formula for Te = —and and more, Mr. Matheson enlarged, especially from the the perfection of the carbons and contacts, and the mechanical arrangements in processes of manufacture, so as to produce it at a point of view of the constructor ; but the question to which FE' . minimum cost. The experience gained with these lamps is at particular attention is drawn is what is considered the T r — 0 , in which P and P' = elastic and ultimate 2 ’ present too small to speak with certainty as to their length of anomalous character of the rules of the four official bodies life. Many of the earlier ones lasted for comparatively a few by whom the maximum strain which shall be visited on strength respectively in pounds, and E and E' the elastic and hours, but improvement has been so rapid that the later ones the parts of structures is limited, and the properties of the ultimate extension respectively in feet. Thus for a material have an average life of from 700 to 1000 hours. Some of the material determined by certain stipulations. Of these we have having an elastic strength of 15 "3 tons, an elastic extension makers guarantee this, others do so by guaranteeing renewals of 0"0143in. in a length of 1ft., an ultimate strength of over and above a certain percentage, on condition, of course, that first, the Admiralty demanding that the steel used for the lamps are worked properly. There is no reason why, as ships shall have an ultimate tensile strength of not less 24"06 tons, and total extension of 2-216in. in a foot, experience is gained in the manufacture of these lamps, and with than 26 tons, or more than 30 tons per square inch, with Te = 20-579, and the current regulated in accordance to the design of the lamp, the an elongation of 20 per cent, in 8in. By Lloyd ’s rules the _ Tr = 4978T ; average light of a lamp should be limited to a thousand hours. It minimum and maximum ultimate tensile strengths are While for a material having an elastic strength = 14'22 is quite probable that the average life may be extended to two 1 ton higher, or 27 and 31 tons, the other figures remaining tons, elastic extension = 0"0288in., ultimate strength = thousand hours, or to even a longer period, and that the first cost 42'3 tons, and total extension = 0"67in. in one foot, of a lamp will be far less than the most sanguine expect now. the same; while the Liverpool Underwriters raise the It is, perhaps, unnecessary to say more of the semi-incandescent limit to from 28 to 32 tons. The French Admiralty rules Te = 38 "22 and lamp than that it does not seem to command itself to public favour, demand higher minimum strength than any of the English Tr = 2693 although the light is steady and of moderate intensity. This may bodies, and they apportion the strength according to the To arrive at the values of the same materials for work be on account of its cost. Referring now to the electric generator, section. Thus it is only when fin. in thickness is reached under compression it is only necessary to substitute the which is usually a dynamo-electric machine, a selection has to be compression for the tensile strengths, and the corresponding made in accordance with the selection which has been made from that the minimum of 28 tons is allowed, no maximum among the different lamps. It is, therefore, necessary to take into con being prescribed ; but the minimum increases inversely elastic and total compressions. From the preceding examples sideration the special requirements of the chosen lamp—for example, as the thickness of the plates or bars, and for plates relating to tensile strains and extensions it will be seen arc lamps may be chosen, such as Crompton ’s, as used at King ’s above |-in.