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Mechanics magazine
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- N.S. 22=91.1869
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- 1869
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- Universitätsbibliothek Chemnitz
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- Universitätsbibliothek Chemnitz
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Mechanics magazine
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Band N.S. 22=91.1869
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- Titelblatt Titelblatt -
- Register Index to vol. XXII I
- Register Illustrations to vol. XXII III
- Ausgabe July 2, 1869 1
- Ausgabe July 9, 1869 19
- Ausgabe July 16, 1869 37
- Ausgabe July 23, 1869 55
- Ausgabe July 30, 1869 73
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- Ausgabe Sep. 24, 1869 219
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- Ausgabe Oct. 15, 1869 273
- Ausgabe Oct. 22, 1869 291
- Ausgabe Oct. 29, 1869 309
- Ausgabe Nov. 5, 1869 327
- Ausgabe Nov. 12, 1869 345
- Ausgabe Nov. 19, 1869 363
- Ausgabe Nov. 26, 1869 381
- Ausgabe Dec. 3, 1869 399
- Ausgabe Dec. 10, 1869 415
- Ausgabe Dec. 17, 1869 433
- Ausgabe Dec. 24, 1869 451
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THE MECHANICS’ MAGAZINE. 363 Nov. 19, 1869. THE MECHANICS’ MAGAZINE. LONDON: FRIDAY, NOVEMBER 19, I860. LONG SPAN BRIDGES. V N allusion was made in our leading article of last week to the fact that engineers are not so prone to indulge in the erection of bridges of large spans as was the practice some years ago. Experience has since demonstrated that when the bearings between the piers reach a certain dimension, the superstructure becomes more expensive than the substructure. It is not generally known that but for the fear of introducing an awkward curve in the line, the bridge over the Menai Straits might have occupied the site of the Swilly instead of the Britannia Rock, and its present maximum spans would have been reduced by nearly 100ft. But the engineers o the old school had a mortal dread of steep gradients and sharp curves, and considered their absence cheaply purchased by any other sacrifice incurred in the shape of embank ments, cuttings, tunnels, bridges, and per manent works of any description. With their limited ideas of one mile radius for the curves, and one in a hundred for the gradients, what would they have thought of engines being manufactured and almost able to “turn in their own length?” and as for gradients, “going up a ladder” is a mild expression for their climbing capabilities! As with the largest span bridge in England so with that in the sister country. The Boyne Viaduct has a maximum span of 260ft., and carries the Dublin and Drogheda Railway over the valley of that river at some distance from the town. It is now well understood that there was not the slightest necessity for erecting so expensive a bridge, and that the line could have been taken, to more advantage in every sense, nearer the town, where the valley narrows. There is no disguising the truth that when iron was introduced as a material eminently adapted for the construction of some of the most prominent of the permanent works of a railway, there was a pardonable rivalry among the members of the profession with respect to erecting bridges of large span. Telford led the way both at home and abroad; Stephenson and Brunei followed, and Fairbairn and others brought up the rear. It is clear that every engineer con sidered it incumbent upon him to prove what he could do with iron, and many structures of that material, hideous in appearance, with proportions in mockery of science, and a distribution of metal betraying an utter and reckless ignorance of the knowledge of strains and strength of materials, attest to the pre sent day the manner in which some of the profession displayed their ability. To such a pitch did their incompetency attain that, after many fatal proofs of it, a Royal Com mission was appointed to inquire into the “Application of iron to railway structures.” The whole subject was then thoroughly sifted, the aid of the mathematician was called in to supplement that of the engineer, and the result was the establishment of certain laws and regulations which, if they could not instruct the ignorant, could, at any rate, protect the public from the consequences of their ignorance. Admitting that bridges of large spans have been erected in positions in which it has been subsequently demonstrated there was no absolute necessity for them to occupy, it must not be understood, on the other hand, that the necessity for a bridge of large span may not arise. There are manifestly .numerous localities which roads and railways are ultimately destined to traverse possessing physical features incapable of being sur mounted by any other means. To be of any value, railway intercommunication between any two points must be continuous. The chain must be perfect. The absence of one link is fatal to the whole. Should a ravine, a mountain pass, a chasm, or a hiatus of any kind intervene along the route, it must be bridged, and the size of the span must obviously depend upon the number of piers or supports that it is possible to erect on the sides and bottom of the gulf. This brings us to the question of the maximum span that it is possible to obtain by any known principle of scientific construction, and just at the present time it has a peculiar interest bearing upon the proposed international bridge over the Channel. We are not about to advocate any particular scheme or discuss the merits of any proposed method for accomplishing the transit between the two shores. Our readers will find full informa tion upon this point in a paper published in our recent numbers.* The only method to which our article has any reference is that of M. Boutet, but -were we inclined to advocate the project, we have not the slightest evidence which would enable us to form any opinion of its theoretical or practical feasibility. In the following investigation no notice will be made of stone arches, as their maximum span does not exceed 200ft., although that principle, being the oldest, has the first claim to consideration. It was, in fact, the original principle proposed by Stephenson upon which to execute the bridging of the Menai Straits. There is no necessity to refer to the reasons why that design was abandoned, because it had to do with other considerations, which reflected not the shadow of doubt respecting the practicability of the project. Neither is there any necessity for our present purpose to advert to the scheme in detail. It is sufficient to state that the maximum span in cast iron was 360ft. So far as theoretical construction is concerned there is no limit to the span of an arch or suspension bridge, but, in spite of this, the former type has almost become obsolete for spans exceeding 200ft., and the latter has never yet been successfully applied to railway traffic. By railway traffic we do not mean the drawing of carriages, or the crawling of a locomotive, over a bridge, but the passage of an express train or limited mail, at full speed. Here we notice the difference between theory and practice. Stephenson considered that if he increased his cast-iron arch to a span of 4G0ft., as he contemplated, the rise of the crown due to expansion and contraction would present a difficulty that he would be unable to provide for. This was no doubt an erroneous impression, as he would have found had not the contingencies imposed by the Admiralty compelled him to abandon the arch principle altogether, and seek for a solution of the novel engineering problem in the adoption of different means. Without inquiring into the relative suitability of cast and wrought iron for the construction of arches, it may be taken for granted that 500ft. will represent the practical limit to the span of an iron arch. The next type that deserves consideration is that in which the horizontal strains are resisted by the various members of the struc ture itself, and a vertical pressure alone transmitted to the supports ; and the Menai Bridge certainly ought to be mentioned first. The question therefore is, what is the limit to the span of a tubular bridge ? All bridges which are self-containing with respect to the strains exerted upon them come under the denomination of beams, and the limit of their * Vide the paper *■ International Communication,” by Perry F. Nursey, read before the Society of Engineer and published in the Mechanics' Hasazine of October 22 and 29. spans is based upon the fact that the strength is as the square, and the weight as the cube of their lineal dimensions. The weight of a tubular or any solid-sided girder increases far more rapidly with an increase of span than that of the lattice or open web type, in consequence of the large amount of metal required as stiffening irons. About 21 per cent, of the whole material in the Menai Bridge was absorbed in stiffening the sides. The question of determining the maxi mum possible span of girders becomes very complicated, because by the employment of steel instead of iron much larger spans could be obtained. At the same time there are no practical examples upon which to base a reli able calculation. It will be safer, then, in the present instance to deduce our conclusions from what has been already accomplished with a material that is trustworthy than to hazard conjectures respecting what might be done with another that has not yet been tried. The ultimate span reached by steel would not probably exceed that attained by iron by more than 30 per cent. A tubular girder would not practically exceed the limit of 600ft. as the maximum span, and if of steel, might reach 800ft. There is very little doubt that, with the exception of Dr. Fair bairn, the great advocate for solid-sided girders, no engineer would ever again employ the tubular system. A glance at the recently erected railway bridges of large spans points out unmistakably that the open web prin ciple has superseded the older and the more cumbrous one. Larger spans would be prac ticable upon the open web or lattice prin ciple than upon that of its predecessor. A lattice girder would probably reach a span of 800ft. if constructed of iron, and of 1,000ft. if steel were employed. Whatever particular form of horizontal girder might be adopted, it may be safely laid down that the maximum span attainable with any material would not be less than 1,000ft., or, in round numbers, the fif h part of a mile. It must not be con sidered that we are advocating the economy of bridges of five spans to a mile; we are simply demonstrating their possibility. The only system to be now alluded to is the suspension, which, so far as mere length of span is concerned, decidedly occupies the first place. Arguing analogically from the size of the existing spans of suspension and horizontal girders, and calculating propor tionally, the limit to the span of a suspension girder would be 1,700ft. The maximum limit, under any circumstances, might be put at 2,000ft., or about double that attainable by the other type of bridge. Besides these ordi nary principles of construction there arc others, such as those of Von Rupert, M. Boutet, and other engineers. With respect to these it should be borne in mind that while on the one hand there are unfortunately no practical data to be guided by, yet there is no valid reason why it may not be possible to achieve larger spans than have hitherto been attempted. The proper line of argu ment to adopt, and method of testing any of these novel schemes to be employed, is to follow the plan pursued in the case of the Britannia Bridge. Those who believe in the practicability of spans of nearly a mile in length should have a model constructed to an actual reduced scale of the bridge which is to serve the purpose. It should be broken, put together, and broken again, until the best proportions were thus experimentally arrived at. Calculations of the breaking weight and the strains upon the various parts could thus be accurately worked out, and the truth of theoretical formulae either verified by em pirical results or modified as might be neces sary. This is the only plan by which to satisfy professional men and judges Of such matters. The mere drawing of a design upon paper, and the results of mathematical investigations alone, are not sufficient to jus tify any confidence in a novel principle of engineering construction.
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