74 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[MAaoH, 



■walls ; while it is not impossible that, from unequal settlements in the bed 

 of the river, combined with strains at the tops of the towers, the latter may 

 be wholly overturned.* 



From the estimate which has been made of the weight, (850 tons) on 

 either of the pairs of saddles, and the horizontal pressure (57 tons) which 

 may take place before the saddles will move, it may easily be computed by 

 the parallelogram of forces, that in either of the two towers on one side of 

 the centre of the bridge, the diagonal which represents the resultant of the 

 forces would make towards the river, an angle of 3° 49' nearly, with a verti- 

 cal line passing through the centre of the saddle, and that the value of the 

 resultant for one tower only, is 429 tons. The direction of this resultant, if 

 produced, would fall within the base of the tower and therefore the latter 

 may seem to have suflicient stability ; but if, in the tower, a line be drawn 

 from the top of a saddle to the foot of the opening, towards the river, at its 

 middle point, that line will make an angle of 22° 45' with the resultant just 

 mentioned ; therefore, multiplying the pressure in the direction of that re- 

 sultant by the cosine of this angle we should have 395 tons for the pressure 

 thrown obliquely on that side wall of the tower. Such a pressure, being but 

 slightly counteracted by the tenacity of the walls on the other sides of the 

 tower, would be sufficient to cause the wall to bulge towards the river, and 

 would prostrate the fabric in ruins. 



Under the enormous pressure which the saddles have to support, it may 

 be presumed, that these have scarcely moved from their places since the 

 construction of the bridge, having experienced only the slight agitations 

 produced by the small number of foot passengers who have hitherto been on 

 it at one time, and it may be readily admitted that, as yet, the strains on the 

 pier-heads have produced no effects which appear to be detrimental to the 

 stability of the bridge. ]3ut the case will be different should even such 

 strains be long continued ; and no one can, without dismay, contemplate the 

 probable consequences of failure should such a bridge he frequently subject 

 to the sudden rushes on it of bodies of people, and the rapid movement of 

 numerous trucks and other carriages, laden with baggage, proceeding from 

 the main terminus of the several stations and roads leading from Kent, Sus- 

 sex, and the south-western counties, which it is proposed to establish oppo- 

 site to Hungerford, and for which it is imagined this bridge may serve as a 

 viaduct. 



Melancholy instances of the fatal consequences attending the failure of 

 these graceful but treacherous structures, are too frequently taking place. 

 Mr.Trollope relates.t that while he was inthe^Vest of France, a suspension 

 bridge over the river Dordogne gave way, at the time that a heavy van was 

 conveying, to the place of their destination, criminals who had been con- 

 demned to the gallics, when the unhappy persons, being unable to disengage 

 themselves, were all drowned ; and an account of a still more lamentable 

 occurrence is given in the Civil Engineer and Architect's Journal, for Jan. 

 1S47, page 31. The suspension bridge recently completed at Jingurutchy 

 near Jessore, and the iron-work for which had been obtained from England, 

 gave way at a time when 5 or 600 persons had assembled on it to witness a 

 poojah, and when three boats were passing under it ; the sad accident was 

 caused, as in the case of the Yarmouth catastrophe, by the whole crowd 

 suddenly rushing to one side of the bridge ; and it is reported that 100 per- 

 sons were killed or drownded. 



But a case which must be considered as one of the highest importance on 

 account of the lesson which it offers with respect to the instability of sus- 

 pension bridges when the great strains to which they, above all other bridges, 

 are liable are not duly counteracted, is that of the Pont des Invalides, which 

 was constructed across the Seine, between the years 1824 and 182u, and 

 which immediately upon the centering being removed gave way, so as to 

 render its entire removal a measure of necessity. 



The span of the bridge was about 557 feet, and the deflexion of the chains, 

 in ithe middle, nearly 33 feet. The dimensions of these were more than 

 sufficient to enable them to resist the tension to which they would be sub- 

 ject ; but the chains were supported on the tops of four Egyptian columns, 

 (of which the two at each extremity of the bridge, were connected by iron 

 braces,) and, descending abruptly from thence on the land side, they passed 

 down a deep pit formed in masonry, to which, at the bottom, they were 

 firmly attached. 



* The piers rest upon the natural gravelly bed of the river, like those of Westminster 

 hridge! surrounded by sheet piling driven, it is said, tifteen feet; an expedient which 

 has not, however, prevented the subsidence and ruin of the two niaiu piers of Westmin- 

 ster bridge. Though the towers of Hungerford bridge were erected iu cofler-dams, there 

 is no piling undemeatb. 



t A Summer la Western France, vol. 3, p. S59. 



If mere strength, unaccompanied by equilibrium, could have ensured sta- 

 bility, this bridge ought to have stood, a monument of elegance. But though 

 here, as in Hungerford Bridge, there was an effort made, by permitting the 

 chains to slide on the tops of the piers, to produce a compensation for an 

 excess of pressure on the bridge, yet experience has shown that such com- 

 pensation does not take place ; and it is evident that, in consequence of the 

 great friction, the horizontal pressure towards the river, is allowed to take 

 etTect at the tops of the supporting piers or columns ; that pressure, (a — a' 

 above.) combined with the normal or vertical pressure of the chain, produced 

 a resultant force which overturned the piers.* 



It is remarkable that the project for the bridge was sanctioned by the ap- 

 proval of a commission constituted of engineers, in the department des 

 Ponts et Chaussces, and that its construction was superintended by two en- 

 gineers specially appointed for the purpose, as well as by the distinguished 

 projector himself. It is painful also to reflect, that the failure of the bridge 

 pressed so heavily on that talented individual as to cause his premature 

 death. 



The writer of the article in the Civil Engineer and Architect's Journal, 

 endeavours to show, that the dangers which may be apprehended from 

 vibrations, undulations, sudden additions of weight, and rushes of people, are 

 rather imaginary than real, and treats this question as one purely statical, 

 considering the bridge as a rigid, solid, inflexible structure. But it is clear 

 that in all the preceding cases, as well as those of North Yarmouth and 

 Broughton, the catastrophes that occurred, arose from the dynamical effects 

 of oscillations and undulations ; these the writer entirely rejects, on account 

 of the difficulty of investigating and determining mathematically the effects 

 of the forces, which ought to be taken into account, from the mutual action 

 of the flexible bodies on each other. There is no doubt that it is extremely 

 difScult to investigate these dynamical effects, and to assign precise values 

 for them ; but are we therefore to reject them altogether, in determining the 

 strength which should be given over and above that which is required to 

 sustain the statical pressure, and are we to make no allowance whatever for 

 the additional strains which the dynamical effects produce ? It might as well 

 be said, that no considerations, with respect to the action of winds and 

 waves of the sea, ought to be made in erecting Piers, Light-houses, &c., be- 

 cause we cannot estimate exactly their dynamical amounts." 



The above is Sir Howard Douglas's rejoinder to the remarks made by ua 

 upon his former paper in this Journal. As might he expected of two dis- 

 putants who set out, open to conviction, and determined that the discussion 

 shall never digress to topics foreign to the question, there remains now but little 

 difference of opinion between us. We think that he has made out a strong 

 prima facie case for the necessity of determining, by actual experiment, 

 whether the " shifting saddles" be really efficient for their proposed object. 

 At the same time, we still think the horizontal friction overrated : it is diffi- 

 cult to suppose that the rolling friction of wooden cylinders can be less than 

 that of iron cylinders, and " the experiments of Coulomb on iron axles," in- 

 cluded (we presume) the effect of rubbing friction, and therefore are not 

 strictly analogous to the case before us. 



The fact is we are contending iu the dark. To make anything like an ac- 

 curate estimate of the friction of the Hungerford Bridge saddles, we must 

 not content ourselves with the confessedly inadequate experiments which 

 have been hitherto made, but ought to make a direct experiment upon the 

 Bridge itself. When the consequences of the failure of a Jletropolitan 

 Bridge are considered, it will not seem unreasonable to ask that this inex- 

 pensive investigation should be undertaken. Even supposing it ascertained 

 that the saddles move with the greatest facility when the Bridge has its ex- 

 treme load.t the mere fact of satisfying the pobUc mind would be ample re- 

 compense for the trouble and cost incurred. Not only ought the Bridge to 

 be secure, but every one ought to be satisfied that it is secure. As far as 

 we know, Hungerford Bridge has never been in any way proved by heavily 

 loading it. 



By analogy, from what little is known of rolling friction from former ex- 

 periments, it certainly still seems to us that the statical effect of the friction 

 of the saddles would be small. But friction, hke other forces, may be of the 

 nature of impact. A crowd suddenly running from the side spans to the 



* This description of JM. Naviers" bridge is taken from an accurate plan in my posses- 

 sion, showing, in detail, the construction and dimensions of its several parts. The true 

 causes of its destruction were, the inequality of the angles formed by the chains, with the 

 vertical lines, at the towers, and the insufficiency of these at the abutmentsj both of 

 which delects exist iu considerable degrees in Hungerford bridge. 



t If this were found nut to be the case, many mechanical contrivances might be Sug- 

 gested for diiuinishing the horiaontal strain on the piers. 



