18-17.] 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



369 



RAILWAY SUSPENSIOX BRIDGE. 



CWilh an Engraving, Plate XVJIF.) 



At the last meeting of the Institution of Meclianical Engineers, held at 

 ' Birmingham, a paper, by Mr. A. E. Cowper, of the London Works, was 

 read, " On an Improved Suspension Bridge for carrying a Railway, and for 

 other purposes" 



In bringing before the Institution of Mechanical Engineers a paper on a 

 peculiar form of bridge, I ought, perhaps, to apologise for introducing matter 

 which may by some of our friends be thought to belong more strictly to our 

 rivil brethren ; but possibly before the conclusion of the paper, I shall have 

 anticipated any objection which might have been made on that score by 

 showing, in point of fact, that I have only been explaining a piece of boiler- 

 maker's work, and wliich may certainly he considered to be far enough re- 

 moved from civil engineering. The object of the present paper is to call the 

 attention of engineers, and railway directors generally, to a mode which I 

 have invented of constructing suspension bridges in such a way that they 

 shall not be thrown out of shape, or in any way distorted, by the weight of 

 a passing load, whether it consists of a railway train or only of the ordinary 

 traffic of a common road. It is well known that suspension bridges are de- 

 cidedly less costly than any stone bridges, and we may add than most iron 

 bridges, when the span is at all above the length of an ordinary girder; and 

 although many persons have turned their attention to them, particularly 

 with regard to their use on railways, I am not aware that any suspension 

 bridge has ever been made, or proposed, that was at all competent to carry 

 the weight of a railway train in motion, or, in other words, that should be 

 safe as a railway bridge. My attention was particularly called to suspension 

 bridges by the proposal of carrying a railway over the Hungerford-bridge, or 

 over a bridge placed alongside of it j and it appeared to me that the weight 

 of a passing train would so move and distort the oiiaina as to cause the road 

 very soon to get out of order, if not actually to give way ; and I then schemed 

 the plan of making a chain of such depth as to include any alteration in the 

 curve of the strain that might take place. 



The curve which the chains of an ordinary suspension bridge takes is well 

 known to be a catenary, or rather a curve between a catenary and a para- 

 bola; it would be a true parabola if all the weight were in the platform, and 

 8 true catenary if all the weight were in the chain. As, however, the difference 

 between thetatenary and the parabola is very slight indeed in that portion 

 which would be used for a bridge, we may assume it to be a catenary for all 

 practical purposes. Now, on loading an ordinary suspension bridge with 

 even a small weight, it at once assumes a different curve (unless the weight 

 be equally distributed over the bridge,) and if the weight be large, it will 

 assume a very different curve ; so much, indeed, will the form be altered as 

 to injure or strain the material of which the platform or road is composed. 

 Now, it is evident that, if the road has to distribute the weight, it must be a 

 very strong and stiff beam, or, in fact, a girder of the full length of the 

 bridge; and the strength of this girder would very nearly be equal to cairy- 

 ing a quarter of the weight of the load in the centre; it is, therefore, evident 

 that the plan of forming a stiff platform or road for a railway suspension 

 bridge, although by no means impossible, must be at least half abandoning 

 the suspension principle, and be the cause of greater outlay. The plan of 

 keeping the road in shape, by distributing any weight that might come upon 

 it, by means of strong diagonal ties, was the first idea that I had ; but it will 

 be found by calculation that these diagonals would have to be very strong, 

 and of considerable height, thereby causing the total depth of the bridge to 

 be much greater. But the plan on which I propose to construct suspension 

 bridges capable of carrying railway trains without being in any way injured 

 thereby, is simply to construct the chain of such depth as to include the 

 curve of strain when the weight is placed on the bridge in the most unfa- 

 vourable positions. With this object I construct the chains of boiler plate 

 of considerable depth — say three or four feet, or more — and rivet the whole 

 well together without any moveable joints, or separate links, and at the top 

 and bottom edges of the chains (I still call them chains, that I may be clearly 

 understood) I rivet or otherwise attach bars, either flat, half-round, or angle 

 iron, so as to give an accumulation of metal at those parts, and at the same 

 time to render the edges of the chains perfectly secure against any tendency 

 to rip or tear. 



In the engraving,fig. 4, it will be observed that there are two chains, each 4 feet 

 deep, which support the ends of cross wrought-iron girders, in the position 

 of sleepers, each chain being composed of four boiler-plates, rivetted together 



No. 123.— Vol. X.— December, 1S17. 



in pairs, each plate being three-eighths thick, and at the top and bottom 

 edges there are securely rivetted strong angle irons. The suspension bars 

 hang between the two pairs of plates forming the chain, and are supported 

 by a small saddle, which bears on the top edges of them. The ends of the 

 cross wrought-iron girders are firmly secured to a light rib of boiler-plate, 

 which runs along each tide of the bridge, as shown in the cross section of 

 the bridge ; the lower ends of the suspension-bars are secured to the ends of 

 the girders, with means of adjustment, so that the road may be trimmed 

 perfectly level when the bridge is fixed. There are also light diagonal ties 

 introduced, as shown in fig. 3, for more perfectly staying the road to the chains, 

 particularly in case of the breaks being applied whilst the train is passing 

 over the bridge. The rails, either of the ordinary form placed in chairs, or 

 of that form commonly called the bridge-rail, are supported on balks of tim- 

 ber scarphed together, which run longitudinally throughout the bridge, and 

 these are supported by short balks of timber running from girder to girder, 

 immediately under the first. There are a series of diagonal ties placed in 

 the platform, as shown in plan, fig. 3. These act as a means of stiffening 

 the platform, and preventing any vibration or shaking of the parts. There 

 are also diagonal ties or stay-rods, by which the bridge is prevented from 

 moving or swinging sideways. They are attached to the piers, and are very 

 similar to some used by Mr. Brunei, senior, in a bridge at the Isle of Bour- 

 bon. 



The engraving shows a bridge 200 feet span, having the cross 

 girders eight feet from centre to centre, and the chains four feet deep, which 

 depth has been arrived at by actual experiment ; the weight of the road from 

 one line of rails and the train is one ton per foot run, and the weight of a 

 train of locomotives I have assumed at one ton per foot run, and this is 

 allowing some margin for the continued growth of locomotives ; and I have 

 taken as a proof load, two tons per foot run ; thus the weight of the load, 

 or disturbing cause, will be just double the weight of the bridge. I find the 

 greatest distortion of the curve strain takes place when the bridge is only 

 half loaded — i. e., from one end to the centre ; the curve then approaches 

 the bottom of the chain, very nearly in the centre of the loaded half, and 

 approaches the top of the chain in the centre of the unloaded half, whilst at 

 the piers it approaches the top at the loaded end, and the bottom at the 

 unloaded end, as shown by the dotted lines in fig. 2. Again, if the same 

 load he placed in the centre of the bridge (covering one-half of the 

 length), the curve of strain will approach the bottom of the chain in the 

 centre, and will approach the top of the chain at very nearly one.fifth 

 from each pier, whilst at the piers it will be near the centre of the 

 chain, but rather above it. Take one more case, and we shall have disposed 

 of all the heavy disturbing tendencies — viz., that of the ends loaded, and the 

 centre left unloaded ; the curve of strain will then approach the top of the 

 chain in the centre, and the bottom of the chain at about one-sixth from 

 each pier, whilst at the piers the strain will be slightly above the centre. I 

 may add that, when the bridge is fully loaded throughout, the curve of strain 

 is in the centre of the chain, throughout its length. I propose to call bridges 

 made on this plan, " Inverted-Arch Bridges." 



Photogenic Experiments. — M. Claudet, in a paper lately read at the 

 Acade'inie des Sciences, Paris, containing an account of various photogenic 

 experiments, states that the solar spectrum is endowed with three diflTcreut 

 photogenic actions, which correspond with three groups susceptible of 

 being attributed to the three groups of red, yellow, and blue rays. These 

 three actions have distinct characters ; each of the radiations has the 

 effect of fixing the vapours of mercury in Daguerreotype plates, but they 

 are in other respects so different that they cannot mingle or assist each 

 other; on the contrary, they destroy each other. The effect commenced 

 by the blue rays is destroyed by the yellow and red rays, and that which 

 is produced by the red rays is destroyed by tiie yellow. The effect of the 

 yellow rays is destroyed by the red, and that of the last two is destroyed 

 by the blue rays. These changes appear to indicate that the chemical 

 compound which covers the plate remains always the same under the 

 various influences, and that there is no separation or isolation of the con- 

 stituent principles. By a proper application of this theory, it will be pos- 

 sible to efface any image upon a plate, and yet leave it in such a slate as 

 to receive a new impression. 



48 



