On the Adaptation of Suspension Bridges to sustain the passage of Railway Trains. By C. VIGNOLES, C.E., F.R.S. THE following observations are submitted to the British Association as appearing to possess sufficient interest for discussion, from the circumstance of differences of opinion amongst civil engineers having thrown doubt upon the feasibility of applying the principle of suspension to the purpose of railway transit. But the practical success in America of this principle on a large scale may be quoted as an example in its favour, and is a striking set-off against the failure in this country, which occurred upwards of five-and-twenty years ago, under circumstances which have militated against any attempt to repeat the experiment. Some debate on this question took place in the Institution of Civil Engineers of London, at a meeting last spring, from which many engineers were absent; and as the subject was on the intended application of a suspension bridge to carry a railway across a navigable river in the North of Ireland, a further inquiry may not be wholly uninteresting at a meeting held in the Irish capital, where many engineers and other practical and scientific men may be present, and who, not having had a previous opportunity of joining in the inquiry, may be disposed to propound their opinions. A further reason for bringing the subject forward, and one which will naturally create a more extended interest in the discussion, is, that the recent events in India cannot fail to produce, among the remedial measures to be applied, a general and a more rapid extension of railways, even to the most remote parts of our Asiatic dominions, and in the course of this extension many rivers of great breadth must be bridged. It is desirable to condense the matter into a few salient and important points, and it may be generally assumed that the whole inquiry is comprised under the following heads, viz. 1st. The maximum load to pass the bridge. 2nd. The velocity of the train. And these being given, there are then to be determined 3rd. The strength of the chains. 4th. The rigidity of the platform; which having been duly provided for, the additional considerations will be as to 5th. Prevention of undulation, vibration, and oscillation. 1st. Maximum load to pass the Bridge. This load may be taken as equal to the weight of the locomotive engine and tender, and of as many carriages as will extend on a single line of railway along the platform of one whole opening between the suspension piers; to the consideration of such a single line the inquiry may be confined. The length of the train, and consequently the weight on the platform of the bridge, will therefore be in proportion to the span or opening. The weight of an engine and tender may be taken, speaking roundly, at one ton per lineal foot of the railway over which they pass, and the weight of loaded carriages at half a ton per lineal foot. For a bridge with a clear opening of 400 feet, the weight of a train extending the whole length of the platform would average little more than half a ton per lineal foot; but as it has been generally customary to compute the insistent load on railway bridges at one ton per lineal foot of single line, this weight will be the one assumed. 2nd. Velocity of the Train.-It would be opening too wide a field upon the present occasion to inquire into or to attempt to solve the complex problem of what additional gravitating effect is produced upon railway structures by the percussive action of trains moving at different velocities. It must be admitted, in limine, that we have not at present sufficient justification to recommend that railway trains should be allowed to pass over the platforms of suspension bridges except at moderate speed; nor, as a matter of every-day practice, should the locomotive engine be allowed to act, except slowly, while passing over such a bridge. With these limitations of speed, and of action of the driving-wheels, of the locomotive, the resistance to weight which must be provided for in a railway suspension bridge, need not be more than to meet the maximum load above assumed, namely, one ton per lineal foot of the platform, in addition to the weight of the platform itself, of the chains and their accessories, and of the suspension-rods, all of which are matters of strict calculation dependent upon the span. 3rd. Strength of the Chains.-The mathematical theory of suspension bridges has been so fully entered into by the best foreign and English authors, more particularly by the French, amongst whom M. Navier is the most distinguished, that little need be said now, except to give the best admitted formulæ for calculation. There is so little practical difference in the form of the curve which the chain of a suspension bridge assumes when freely suspended without a load, and when fully loaded, that is, the difference in form between a catenary and a parabola, that the most esteemed writers on this subject have, by common consent, agreed to consider the curve of the chain of such a bridge to be a parabola rather then a catenary, on account of the very much greater simplicity of the mathematical calculations. Perhaps it may not be irrelevant to enter very briefly into this. When a heavy chain, freely suspended from two fixed points, is acted on by the force of gravity only, the form of curve which it assumes is called the catenary. If, however, the chain be loaded with weights, distributed in such a manner that for each unit of length (ex. gr. for each foot), measured along the horizontal tangent at the lowest point of the curve, the weights should be equal to each other, the effect of such a distribution is to cause the curve of the chain to approach in form to another curve called the parabola. If the distributed weights become so great that the weight of the chain may be neglected in comparison with them, the form which the curve assumes in this case is accurately that of the parabola. In most, if not all ordinary cases, the weight of the chain is, however, never inconsiderable in relation to that of the platform and of the testingload together; and consequently the form of the chain is never exactly that of the parabola, though it approaches more nearly to this curve than to the catenary; so near, that for all practical purposes it may be considered to have attained that form, viz. of the parabola. In the case where the curve of the principal openings has a chord, say for instance of 424 feet, and a versed sine of 294 feet, or the proportion between the chord and versed sine of between 14 and 15 to 1, the two curves (catenary and parabola) passing through the points determined by these conditions approach so near to each other in form, that their greatest distance, measured in a vertical line intersecting both of them, is only 06 (3)5th) of an inch. 4th. The Rigidity of the Platform.-This is perhaps the most important point of the subject, and has probably hitherto been least considered, and, strictly speaking, the novelty of the inquiry is confined to this alone. In all the earlier examples of suspension bridges, the object of the engineer appears to have been to construct the platform as light as possible. In many instances this was carried to a most dangerous extent; even in the case of the great suspension bridge over the Menai Straits, the platform has been repeatedly damaged by storms of wind, which twisted it as if made of pasteboard. The late Mr. Rendel was the first engineer who perceived the mistake which had been hitherto committed in this respect. When the suspension bridge at Montrose had been destroyed about twelve or fourteen years ago, he reconstructed the platform and stiffened it by bracings so effectually that it has since remained uninjured. This principle of strengthening the suspended platform was carried out to a greater extent by the writer of these observations at the bridge over the Dnieper at Kieff, in Russia, and the successful resistance of this well-braced platform to the effect of hurricane winds, and to vibration, oscillation, and undulation, has been very remarkable. The desideratum is, that the platform of a suspension bridge intended to sustain a railway train should be made as stiff as possible; and the first natural consideration is, how is this stiffness or rigidity to be best obtained? The mode in which this has been effected in the great Niagara suspension bridge, is on the system of a deep trellis frame,-in fact, a timber tube, the sides of which are of lattice-work, the railway passing on the top. It is generally understood, and a print published at the time seems to confirm this, that the original intention of Mr. Stephenson was to have added suspension chains for supporting the tubular platform of the Britannia Bridge, although that intention was subsequently abandoned, and the tubes made sufficiently stiff not to require their assistance. Another great point in this discussion seems to relate to the adapting of suspension bridges for passing railway trains in localities and under circumstances where fixed bridges could not be erected except at an unjustifiable expense, or not at all, from the onerous conditions naturally or judicially imposed. According to the locality, timber or iron may be best suited for constructing the platform, the platform being made as deep and as stiff as possible, and thus becoming a girder held up by suspension chains; and the stiffness being augmented by the increased depth of framing, it will be advisable that the rails should be attached thereto as high up as practicable. But the weight of the platform must be kept within reasonable limits, to avoid too great an increase in the sectional area and weight of the chains, which would otherwise become necessary; and further precautions have to be taken as regards the distribution of the load on the platform, and to guard against oscillation and undulation, for all which due consideration must be given as to the proper breadth of the platform. The weight of the platform of an ordinary suspension bridge was formerly scarcely more than 36 lbs. to the square foot of the area of the whole platform; the present weight of the Menai Bridge platform, after having been strengthened, is about 38 lbs. to the square foot; the weight of the platform of the Montrose Bridge, as reconstructed by Mr. Rendel, is 41 lbs. to the square foot; and the weight of the platform of the Kieff Bridge is 49 lbs. to the square foot, including the two footpaths which are corbelled out from the main part of the framing; but the weight of that part of the platform between the chains, and which sustains the roadway, is about 60 lbs. to the square foot. The ordinary test-load for a suspension bridge was about 62 lbs. to the square foot; the proof-load put upon the Kieff Bridge was really about 84 lbs. to the square foot. Now a railway-load passing over a suspension bridge being taken at one ton per foot forward, the weight per square foot upon the platform will vary as the breadth of the bridge: if the bridge be 20 feet, the passing load will be one cwt., or 112 lbs. to the square foot; if 27 feet wide, it will be 83 lbs.; and if 30 feet wide, 75 lbs. to the square foot. The Kieff Bridge is 52 feet wide, and therefore a passing load of one ton per lineal foot spread over this area, is only 43 lbs. per square foot, whereas the test-load was 84 lbs. to the square foot, which is about double what would have been the weight of the heaviest railway train; or taking 42 feet, exclusive of footpaths, the railway-load would have been 52 lbs. per square foot, or less than two-thirds of the test-load, which, it may be remarked, has remained on forty-eight hours without the platform showing any deflection visible to the eye, although some deflection really took place. It appears therefore most undoubted, that suspension bridges of modern construction may be perfectly adapted to sustain the passage of railway trains, and that the chief consideration has to be given to the character and dimensions of the platform; and as a general rule I would suggest, that notwithstanding the advantage to be gained by depth, this should not be carried too far, more especially if the lattice-girder system be adopted, as it presents too much surface to the wind, and thus induces increased lateral oscillation. Also, that the breadth of the platform for a single line should not be less than 25 feet, in order to spread the load and reduce the insistent weight per square foot of platform. It might be interesting to establish a comparison of the expense of various descriptions of platform, but this would lead too much into detail, and the materials for this purpose have yet to be collected. Still, as a contribution, and by way of illustration, the present opportunity may be taken to state the cost of the platform of the Kieff Bridge, already mentioned as so remarkably stiff, and capable of sustaining the transit of a railway train. In a length of 12 feet of the whole breadth of 524 feet of the platform, the quantity of materials was as follows: Timber, 600 cubic feet .... £150 0 0 30 0 0 Total.... £180 0 0 for a length of 12 feet, or £15 per lineal foot of the whole breadth of the platform, which is something less than six shillings per square foot of a platform such as that at Kieff (of which the drawings were shown). 5th. Prevention of Undulation, &c.-The effects upon a suspension bridge of passing loads and of strong winds, cause vibration, oscillation, and undulation. Of these, the undulation is considered to be the most serious. The vibration may be assumed as produced by what may be called the percussive action of the passing load, and when the platform is not sufficiently stiff, and the passing action is irregular over the surface, as, for instance, by the impetuous rush of a drove of cattle, or of a multitude of people, oscillation and undulation ensue; the first producing a lateral swing of the platform, the latter arising from the bending of the platform in its longitudinal direction. The remedy for vibration and oscillation is provided by a sufficiency of stiffness, not to say absolute rigidity, in the platform, which will also, to a certain extent, counteract the propagation of the undulation, but not entirely. The experience, however, of four years on the Kieff Bridge, has proved that the mode adopted in that construction of disposing the suspension rods alternately (in the manner shown on the exhibited drawings) has completely counteracted the undulation; and many very heavily-laden carriages together, -artillery, cavalry, and large bodies of troops,-have been continually passed over the platform of this bridge without the slightest undulatory or oscillating motion having been produced. We are hence enabled to infer, without looking to improvements in detail, which will naturally be introduced, that a platform so constructed and so suspended as the one at Kieff, is capable of sustaining the passage of railway trains at a moderate velocity, and within a reasonable cost of construction; and taking the example of the wire bridge in America, and of this wroughtiron chain bridge in Russia, it may be legitimately concluded, that the adapting of suspension bridges to railway purposes is perfectly practicable. The extent to which this application may be made can scarcely be defined à priori, but the writer ventures, from his own experience, to state his opinion, that where the span of the required bridge must exceed 300 feet, the suspension principle should be adopted for the sake of economy. It would be extending these observations far beyond the bounds assigned to such meetings as these, to go further into the details, and therefore, however tempting the opportunity, we must abstain from entering upon the subject of the modern mode of obtaining foundations and forming riverpiers, which mode would greatly influence any selection between a fixed or a suspension bridge. Neither must we even touch upon the choice between the wire-rope and the wrought-iron plate chain, as the means of suspending the platform, though it is obvious that where the span becomes very large, the superior lightness of the wire is a great inducement to decide the preference for it over the wrought iron. The proportion between the chord and the versed sine of the curve of the suspending chain is another point of the highest interest, as relating to the questions of more or less oscillation, and of increase or decrease in the amount of tension, as this proportion varies. It is sufficient to have brought the general subject of the practicability of adapting suspension bridges to sustain the passage of railway trains before the Mechanical Section of the British Association; and it is to be hoped that this opportunity will not pass away without engineers and the other scientific and practical men now assembled, bringing their judgement and experience to an examination of this very important question. On Electro-Chemistry. By Professor W. A. MILLER, M.D., F.R.S. IN reporting upon the recent progress of electro-chemical research, the author stated that the inquiries made of late years in the field of electrochemistry were characterized rather by modifications of the laws previously admitted, than by any fundamental additions to the existing stock of knowledge upon the subject. Faraday's observations on the exceptional conducting power of solid sulphide of silver, and one or two other substances when heated, had been traced, by the researches of Beetz and Hittorf, to true electrolytic decomposition, which is rendered possible by the somewhat viscous condition produced by heating these bodies. The true electrolytic nature of the decomposition was proved, first by the rise in conducting power, occasioned by rise of temperature (whereas in metals the effect is exactly the reverse); and secondly by the effects of polarization observed upon the electrodes between which such bodies are placed. Allusion was then made to the experiments by Bunsen on the insulation |