1840.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



IDS 



SUSPENSION BRIDGES. 



WITH AN ENGRAVING, PLATE X. 



Lord Western's Letter lo Lord Melhourne, descriptive of a Suspension 

 Bridge built across the Amn, at Bath, by Mr. Dredge, a resident of 

 thai city, upon an entirely novel principle. 



My dear Lord, — Having heard that Government is about to expend 

 a further sum of money on the reparation of tlie Menai Bridge, which 

 is said to be in a perilous state, I cannot refrain from entreating your 

 attention to tlie vast improvement that has been made in the con- 

 struction of suspension bridges by Mr. Dredge, of Bath. During a 

 recent residence of two months in that city, I have had an opportunity 

 of seeing often the bridge that has been built by him across the Avon ; 

 it is a beautiful structure, and at once commands admiration of its 

 beauty and confidence, in its stability; I have communicated with him 

 frequently aljout it, and altogether the consequence has been so strong 

 an impression upon my mind of the vast and immeasurable superiority 

 of the principle on which it is built over anything that has hitherto 

 been attempted, that I have been led into this somewhat extraordinary 

 intrusion upon your Lordship on a matter with which I may be, I own, 

 justly considered to have no very intimate or scientific acquaintance ; 

 such however is the simplicity of the work, that I will not hesitate to 

 attempt some account and explanation of it, in the hojie of drawing 

 your attention in the first instance, which, if I accomplish, you will be 

 led, ■ 'hink, to give it a closer examination, which will produce 

 eventually as strong a conviction in its favour on your mind as it has 

 produced upon mine. 



Mr. Dredge's statements of the superiority of the power of his sys- 

 tem over the established ])lan of structure certainly at first astonislied 

 me ; he has indeed proved by trials, in the [jresence of very many 

 persons, a superiority of strength to the extent of at least 15ll per 

 cent. These were made upon small models of bridges formed severally 

 on the present and on his new principles, each out of the same quan- 

 tity of iron, but he carries his calculations of the accumulating power 

 derivable from size and extent over and above the 150 per cent, shown 

 upon the small models to such a degree, that I will not venture to state 

 it; but if he should be called upon, in the way I trust sooner or later 

 he will be, to exhibit his system before your Lordship and the public, 

 he is confident he can mathematically and practically establish any of 

 the statements he may make, and I have little doubt he will bo found 

 to be correct. He insists on the possibility of reconstructing the iron 

 work of the Mcnai-ljridge at a less sum than the superfluous iron 

 would sell for, so nuich less is requisite than was there used, and he 

 pledges liimself to the power of the bridge, if the irons are altogether 

 altered and reconstructed on his principle to be capable of supporting 

 on transit 1,0UU tons. The Menai-bridge is believed to have cost near 

 150,U0U/., and to have consumed in its construction above "2,000 tons 

 of iron, and to be declared only capable of sustaining 733 tons on tran- 

 sit. Before I submit to your Lordship a detail of some practical ex- 

 periments Mr. Dredge has made justificatory of the declarations he 

 thus ventures to put forth, I will endeavour to give some explanation, 

 imperfect though I am sensible it must be, of tlie fundamental princi- 

 ple upon which his mighty fabric is erected ; I must give it merely as 

 it has struck my unlearned common sense, and which it has, from its 

 simplicity, with a force so irresistible that it makes me believe I fully 

 understand it ; in aid of my endeavour I have given a few drawings 

 on an annexed sheet, I conceive the grand foundation may be said to 

 be the rendering the chains strongest and indeed very much the 

 strongest at the base, tapering them by regular degrees to the centre, 

 where they come at last, in fact, to a cipher, from the ciphei' com- 

 mence therefore their size, weight, and strength, which regularly in- 

 crease by degrees quite up to its base, which base you know in a sus- 

 pension bridge is the towers of masonry on which the chains are hung ; 

 in truth, it is the application of that principle horizontally which is so 

 obviously necessary in all perpendicular erections, of superior size and 

 strength at the base, and tapering away to a cipher on its ultimate 

 summit; as for example the obelisk, the pyramid, the church spire, 

 and which principle he shows to be as effective horizontally applied as 

 it is in the perpendicular; indeed, it maybe said to be far more eftec- 

 tive, as it has to support iu so diflBcult a position, comparatively with 

 the perpendicular, its own intrinsic weight, and a heavy transit load 

 besides. The manner in which the chains of his bridge are formed to 

 render them stronger at the base is shown in plate. Fig. 2 ; and Fig. 4 

 is a section of one of the main chain of the Menai-bridge ; these are 

 the same size throughout, creating thereby an enormous intrinsic and 

 superfluous weight, exceeding that which it has to sustain on transit, 

 and this it is which constitutes the grand vice of the present system, 

 and which sooner or later Mr. Dredge's must supersede. Mr. Dredge's 

 bridge may be well imagined by supposing a church spire laid hori- 



zontally, and met by another of equal dimensions at the point, as re- 

 presented at Fig. 8. 



There is anoflier figure by which the principle may be more clearly 

 shown; it is the bracket; two brackets meeting at their extreme 

 points give a very satisfactory idea of it, as in plate, Fig. 9. Every 

 body knows that the bracket tapering from its base will bear hori- 

 zontally a great weight, but if it was the same size from the base to 

 its extremity, though it might continue to be called a bracket, it would 

 hardly sustain itself if it was any considerable length.* I have to re- 

 mark now upon another most important peculiarity in Jlr. Dreilge's 

 bridge, and that is the diagonal direction of the road suspending rods, 

 instead of perpendicular, and forming, therefore, as it unquestionably 

 does, a powerful contributary effect to the support of the whole, and 

 this is also most easily capable of direct practical proof. There is 

 still a further point of difference and advantage in Mr. Dredge's bridge, 

 which appears to me to be equally simple and as proveable, and which 

 also essentially contributes to increase its aggregate power and se- 



Piiritv that is, its horizontal action or pressure, which is also made 



obvious by a simple -and familiar figure representing one half of a 

 brido-e : suppose a straight rod of any given length, fasten a cord at 

 one end of it, and thence to the top of a wall, place the other end to 

 that at which the cord or chain is fastened against the wall, at such a 

 distance below the top of the wall as will render the position of the 

 rod horizontal, and it must be plainly seen that the rod is supported 

 as well bv its compression against the wall at one end, as by its cord 

 of suspension at the other, see Fig. 10. Thus every component part 

 of the structure is brought harmoniously to work and in perfect unity 

 of action towards the grand object. I will now advert again to the 

 Menai-bridge, and show further in essential points the difference be- 

 tween that and indeed most other suspension bridges, and Mr. Dredge's. 

 The actual intrinsic strain at the centre of the Menai-bridge according 

 to " Dreivry," page 1(37, amounts to 1,S7S tons, and at each extremity 

 1,943 tons. This vast intrinsic weight operates its own destruction, 

 increasing its self-destructive power as it increases in length; thus it 

 becomes vibratory, and upon a gale of wind blowing upon its broad- 

 side, it has a swing or pendulous motion ; this I have felt myself in 

 passing it, the wind blowing strong at the time. 



On the other hand, as I have observed before, upon Mr. Dredge's 

 principle, the strain and weight only commence at the centre, increas- 

 ing as the strength of the bridge increases up to the base, and of 

 course its ability to sustain it; this dilierence between these two sys- 

 tems may be readily imagined. By supposing a ton of iron formed 

 into a bar of equal dimensions from one end to the other, as is shown 

 in Fig. 7, and fixed into a wall, it will hardly support itself, still less 

 any additional load ; if extended to any considerable length it will not 

 support itself; on the other hand, make the same weight of iron into 

 a taper form, as in Fig. (5, and it will support its own weight to any 

 extent, and a heavy extrinsic weight in addition ; Ixit further than 

 this, if the parallelequal-sized bar is cut away by one-half, (see dotted 

 line in Fig. 7,) it will then support itself and an extrinsic weight in 

 addition. The reason is obvious; it has discharged itself of that, 

 which was altogether superfluous and therefore noxious in the ex- 

 treme, being wholly destructive of power to carry any extrinsic 

 weight. In this figure is a singularly accurate exemplification of the 

 vice'of the Menai-bridge, and others built upon the same principle, 

 and the obvious good sense of Mr. Dredge's. Thus his genius has 

 led him, by the simplicity and perspicuity of his conceptions, to effect 

 a discovery which, I firmly believe, will turn out of great national 

 importance, the recognition of which by the country will, i am sure, 

 be felt by him as the "highest possible reward. Having thus endea- 

 voured to show the simple principle on which Mr. Dredge's system is 

 founded, I proceed to give you some account of some experiments he 

 has made practically substantiating the truth of it, prefacing them, 

 however, with a brief description of the expense and particulars ol 

 the Victoria bridge across the Avon, built in ISSlJ, and which has proved 

 itself equal to its inventor's most sanguine expectations ; its cost was 

 1,IJ50/., its span is 15U feet, andonly 21 tons of iron were consumed in 

 its construction, which, at 20/. per ton, is only 420/. ; the great expense, 

 therefore, was on the masonry and the timbers supporting the plat- 



•' It may be remarked that there is not a strict similarily between Uio 

 common Ijracket and the bridge, inasmuch as the platform or horizontal line 

 is. in the former, above and in the latter, below ; there is, however, no real 

 difference. Tlie ]io«er of the bracket is compounded of suspension and com- 

 pression, that is, suspension from the fulcrum, and compression against the 

 tulcrum. In the case of the cummon bracket, the horizontal line which is 

 uppermost, being fixed or fasteneil securely to the fulcrum, performs the 

 suspenson part of the work, the arch or diagonal line below the compression, 

 attaching itself to the fulcrum widiout fastening; the case of the liridge is, 

 lioHever, only so far dillerent. that the arched line does the suspension part, 

 and the horizontal the cumpressiou. 



2 D 



