64 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[Feb. 



3rd. Cables form a system less rigid than chains of -wroHght iron do, so 

 that the horizontal oscillations of the roadway are more considerable in the 

 former than in the latter case. 



I believe that I have not withheld any of the objections urged against the 

 emplovment of iron wire, nor vieakcned those I have presented. I shall now 

 examine them in order. 



First objection. — They offer a much greater chance for oxidation. 



It is certain that if we expose to alternations of dryness and humidity a 

 bar of iron and a certain number of isolated wires, the sum total of their 

 individual sections being equal to that of the bar, the surface attacked will 

 be far greater in the wires, and in them the oxidation will be most rapid. 



In confining ourselves to this general /ac/, without reference to any of the 

 means employed by art for retarding this oxidation, it will be well to examine 

 if even this inconvenience of the more rapid destruction of the cables, is not 

 more than counterbalanced by the advantages which they present. 



It is very evident that if the cables remain only forty years without being 

 renewed, while the chains may last for sixty or one hundred years, we must 

 calculate what will be the amount at the end of forty years of the sum saved 

 by the use of iron wire instead of bar iron. 



To render this more plain, I will give an example. I suppose that a given 

 suspension bridge requires 200,000 kilogrammes (441,090 lb.) of iron, 

 (which was nearly the quantity for the bridge of Roche Bernard). 



The expense of the system of suspensions . . . 300,000 francs. 



According to note (1) to replace the iron wire there 

 must be used 454,545 kilogrammes of wrought iron, 

 which would cost 454,545 



Saving in favour of iron wire, 154,545 francs. 



Now this sum put at interest, will amoimt at the end of twenty-three 

 years, to 475,506 francs, and supi)Osing that the cables must be entirely 

 renewed at this time, there will still remain a surplus of 175,596 francs, 

 which will be more than sufficient to produce at the end of another twenty- 

 three years, a new capital equal to the cost of tlie system of suspension. 



In the case which we have considered, caljles of iron wire lasting but 

 twenty-three years will then be preferable to chains of indefinite duration. 



Tlie s\ipposition that isolated wires will last twenty-three years without 

 the necessity of being renewed, is not without foundation — and we shall 

 produce a fact which strongly tends to confirm it. 



M. Mongolfier, jun., having learned that a grating of iron wire from the 

 church of St. Martin's at Paris was about being taken down, after having 

 remained forty years without any repair, had the curiosity to prove these 

 wires, after having carefully ascertained their number, and he was convinced 

 that they bad lost but one-fifth of (heir entire strength.* 



Tliis loss of strength is not sufficient to require a complete renewal of the 

 system of cables. 



But the most determined opponents of the use of iron wire confess that 

 cables do not afford such facilities for oxidation as detached wires. The 

 greasy substance which covers them afl'ords a powerful preventive to rust ; 

 their union preserves them in tlie interior, more or less, from moist air ; the 

 ligatures are a still further obstacle to its introduction; and fiually, the careful 

 superintendence which should be given, are all reasonable motives for hoping 

 that the effects of oxidation may be diminished in a remarkable manner. 



It may be olijected that experience has not fully confirmed the opinion, 

 however probable it may be, that cables arc less susccptilde of the attacks of 

 oxidation than iron wires. I confess that no one fact can as yet, incon- 

 testably, prove the justice of this opinion, but there are several which we can 

 ])roduce, dipable of giving much strength to it. 



The bridge of Tournon has been in existence eleven years, but no very 

 considerable trace of oxidation has manifested itself, at least, to my obser- 

 vation, ujion the surface of the cables ; and if there existed any in the 

 interior, it would not have failed to show itself by a brownish stain upon the 

 outside of the paint which covers them. 



A bridge of iron wire was built at Brest, in 1520; the cables, exposed to 

 the salt air, which attacks iron with so much energy, should have undergone a 

 remarkable deterioration in the space of three years. M. Trotte de la Roche, 

 Chief Engineer, who, on account of the ])lans adopted for the port, was 

 obliged to dismount it, took the pains, at the invitation of M. Inspector 



* The increase of oxidatinn is not as rap'd as would be supposed from the 

 first observations made — for the first layer of rust which covers the surface of 

 a bar of iron, instciul of favouring this riNirlntiun, pruvcs a coating which is 

 an obstacle to it. 



General Lamblardie, to prepare a process verlul of the state in which be 

 found the cables. 



It appears from the T^rocess verbal, 1st. That the continuous ligature which 

 covered the cables was slightly attacked, but that by the first scratch of the 

 file, the oxidized portion was removed. 2nd. That the exterior wires of the 

 cables showed slight traces of oxidation, but that the slightest scratch of the 

 file caused them to disappear. (M. Trotte de la Roche supposes that the 

 oxide was only deposited upon the wires of the cables, and that it came from 

 the ligatures.) 3rd. That the interior wires were perfectly untouched. 



Eight years is a short space of time, but if we consider that the effects of 

 oxidation probably continue to decrease, we may conclude that they are not 

 so very rapid, and that the fears entertained upon this point are greatly 

 exaggerated. 



An observation has been made, which is worthy of remark ; it is, that in 

 chains the surface of the bars which is attacked by oxidation proves the 

 portion of them offering the most resistance, while in a cable the interior 

 portions have the same strength with the others. 



Second objection. — The imperfection of the present process for manufac- 

 turing the cables does not allow of an equal tension in all of the wires, so 

 that when the cable is raised to its place, the wires under most tension are 

 overstrained by many pounds, while those under the least tension do not 

 draw at all. 



This last objection is a serious one, and cannot be absolutely done away 

 with, that is to say, it is impossible to prove that this defect does not deserve 

 the most serious attention ; but we can employ, in defence of iron wire 

 negative arguments, or in other words we can prove that it is not possible to 

 resolve the problem of equal tension in a more perfect manner by the system' 

 of chains than by that of cables of iron wire. 



We must, in the first place, distinguish carefully between bridges of large 

 or small span ; in the latter, where the tension requires only a section of the 

 chains equal to that of 4 to 8 bars of about 0-05 metres to 0-OG m. (2 to 

 2-1 in.) in diameter,* (dimensions beyond which the quality of tlie iron be- 

 comes considerably deteriorated,) we can establish on each side of the bridge 

 two or four separate chains in one or two layers, each chain being made of a 

 single bar only ; in these two cases the problem of equal tension is perfectly 

 resolved, and althougli in the second each suspension rod bears upon two 

 chains which cannot have exactly the same curvature, the holding plate of 

 the rods will always bear upon the two chains, which will then support 

 equally their share of the whole weight of the bridge. 



By establishing three or even four layers, we can form an excellent system 

 of 16 chains, each made of a single bar; but these 16 chains present only a 

 total section of less than 44,000 millimetres (97,041 lb.), corresponding to a 

 tension of 352,000 fcilogrammes, (776,329 lb.) that is to say, to bridges of 

 medium span ; but if we pass to bridges of such a span that the tension in- 

 creases to more than a million of kilogrammes, it will be necessary that the 

 chains should be composed of 48 or even 64 bars, that is, 24 or even 32 bars 

 on each side. 



Let us consider the last hypothesis, which applies to the case of the bridge 

 of Roche Bernard. 



It is impossible to employ the simple system of suspension roils resting 

 upon a couple of chains, of single bars, and arranged in layers, for we would 

 then have 10 of tliese layers, one above the other, which, beside the incon- 

 venience presented by a considerable height, would allow of the attachment 

 of suspension rods only at every sixteen intervals upon the same chain. 



Here then it is necessary to employ a more complicated system, namely to 

 form the chains of several bars fastened together by a single bolt — in this 

 case I would reduce the number of chains to eight, and form tliem of eight 

 bars, fastened by one bolt. 



We can double the number of chains, and so reduce to four the bars in 

 each, by making each rod rest, by means of plates, upon two chains at once, 

 but if the two chains forming the couple are not in the same plane, the upjier 

 plate of the rods will bear only upon one of the chains — for it must remain 

 parallel to the iilane of tlie four bars — and one half of the system will sup- 

 port nothing ; this disposition is too faulty to be adopted — this is my o]iinion 

 in the hypothesis of eight bars to each chain and fastened by one bolt. 



Whatever may be the manner of forming the eye at the end of the bar, 



* I reason on the supposition of the use of round iron, of which I need nol^ 

 prove the superiority over square iron, that is hammered again aflcr being 

 re-heated to a cherry red. 



t I suppose it would not be desirable to emjjloy more than fuur l.iyers— 

 this number is alreaily considerable and troublesome in the passage over the 

 towers and in the moorings. 



