BRIDGE. 



it may be necsaMry to introduce a series of spans. In the latter cases, 

 the chains may either be attached to the respective piers, or they may 

 be carried over a series of saddles; and, in fact, there are so many 

 reason* for modifying the detail* of then bridges, that they can only 

 with difficulty be brought under any general rake. 



According to Navier {who** work, ' Memoire sur les Pont* Bus- 

 oeodn..' Paris, 1830. i* still the text-book upon this subject), the 

 formulas which may be considered to represent the maximum tension 

 in the direction of the are., and the horUontal tension at the top of 



the piers are T J / ^ 1 ; and <J = -% ; in which F i* the 



total nMrtimnn load upon the arc ; <f, the span ; and /, the versed sine. 

 From thete formula- it appears, that the greater the versed sine the 

 lea* strain in there upon the piers, or upon the chains themselves ; but, 

 at the same time, the length of the chain is necessarily increased, and 

 the horizontal vibrations of the whole system are also rendered more 

 dangerous, whilst the transverse vibrations are likewise increased. 

 For these reasons, the ratio of the versed sine to the span of sus- 

 pension-bridges is usually made to vary between the maximum of 17. 

 to the minimum of 1-25 ; and in the Fribourg Bridge, the largest span 

 hitherto attempted, the ratio is M4. The latter ratio is the one most 

 commonly adopted. In all cases, and whatever be the material of the 

 chains, the suspension-rods are placed at equal distances horizontally. 



There is a remarkable difference in the theory and practice of 

 ng>iti and of Continental engineers, with respect to the manner of 

 resisting the vibrations to which the floors of suspension-bridges are 

 exposed. English engineers, indeed, consider that the weight and 

 rigidity of the whole structure are desirable conditions; whereas 

 foreign engineers make the several parts as light and as elastic as 

 possible. So many accidents have happened to suspension-bridges con- 

 structed on either of these principles, that it would be impossible to 

 aay which of them is the more correct; but it certainly would appear 

 that when drawn iron suspension-rods are used instead of wire ones, it 

 i* very desirable to keep the roadway as stiff aa it can be made, for if 

 any undulations of great amplitude, such as have been described by 

 Mr. Provi* in his account of the Meuai Bridge, should occur, the rods 

 are very likely to break. Great stiffness may be given by trussing the 

 hand-rail ; by carrying the platform through the piers ; and by intro- 

 ducing a system of cross-bracing under the roadways. These pre- 

 cautions, however, may be adopted upon either the English or the 

 foreign systems above mentioned. 



One of the moat important parts of a suspension-bridge ia the 

 anchoring of the end of the chain, so as to interfere as little aa pos- 

 sible with the contraction and expansion of its metal, and yet to resist 

 the tension. Great precautions require to be taken, especially in the 

 tying down well*, to protect the chains from oxidation ; and it may 

 be interesting to add, that some of the worst accidents to suspension- 

 bridges have arisen from the destruction of the sunk part of the back 

 chains, precisely from this cause. This was the case with the bridge 

 at Angers, which fell in consequence of the rusting of the wire-ropes in 

 the wells ; and so much are the said wire-ropes exposed to this par- 

 ticular danger, that it would appear to be desirable to exclude them 

 entirely from that part of the work. In the open air, or in positions 

 where they can be easily examined, however, iron-wire ropes present 

 many advantages over the solid bar-iron chains or suspension-rod*. 



The wire-ropes used on the Continent consist, in fact, simply of 

 annealed wire* in close juxtaposition, bound round at interval* so as 

 to prevent any longitudinal or lateral motion. They were originally 

 introduced by M. Seguin, and described by him in a ' Me'moire sur les 

 Pont* en Fil de Fer,' Paris, 1826; and have been successfully applied 

 in an infinite number of cases, amongst others, in the Fribourg and the 

 Niagara bridge* : the latter is even used to carry a railway. A descrip- 

 tion of the Fribourg Bridge will be found in the ' Annalcs des Pont* et 

 Chauacees ' f or 1885 ; a description of the great bridge, since fallen, of 

 La Roche Bernard was published by M. Leblanc in 1841; and a 

 description of tbo Niagara Bridgu is to be found in Weale's ' Papers 

 and Practical Illustrations of Public Works, 1856,' to which the reader 

 i* referred for details of the mode of forming the ropes, and of calcu- 

 lating their resistance. In the ' Annales das PonU et Chausaee* ' for 

 1842 there i* also to be found a description of an ingenious system of 

 forming the cobles and suspension-rmU of hoop-iron instead of wire 

 which had been shortly before applied by Messrs. Flachat and Petiet 

 in the construction of a bridge at Suresnes, near Paris ; and there are 

 to be found in the *amo valuable publication, especially of the perioc 

 between 1832 and 1849, a aerie* of interacting notice* upon the various 

 description, of .lupeosion bridges, and the accident* to which they an 

 spaced. The latter have, unfortunately been very numerous of late 



Eand there certainly ia at present, in consequence of theae acci 

 * tendency in the public estimation to undervalue this very 

 and economical method of bridge-building. Kveii chain BUB 

 pension-bridge* have abared th disfavour of the iron-wire ones ; and 

 in Pan* especially, they have been pulled down with a moat imperia 

 diaregard for expense ; but certainly, if due precautions be observed 

 there can be no reason for the sudden revulsion in public 

 which has taken place with respect to suspension-bridges of all kind* 

 and the UM of the woven-wire rope* especially would seem t obviate 

 many of the objection* which practically apply to the ordinary 

 parallel stranded rope* used by the French engineers. 



BRIDQR. 







The " Dredge's Suspension-bridge*," which were very favourably 

 received in England a few yean since, have been even more unfor- 

 unate than the other modifications of this style of bridge- They were 

 irincipally remarkable for the attempted application in them nf the 

 heoretical principle that the strength, and consequently the weight, of 

 ho chain in any part of its length should diminish in propori : 

 he strain likely to be thrown upon it, and also for the peculiar ii 

 >osit ion of the suspension-rods. Such a system, however, is defic 

 ts powers of resistance to cither horizontal or transverse waves, pre- 

 cisely at the point where their action is the greatest ; and the conse- 

 quence has been that, in the small bridges of the Regent's Park for 

 DHtance. the platforms have been obliged in many cases to be Burn 

 iy an intermediate framing. 



It must be observed that, whatever be the descripti 

 employed in a suspension-bridge, the greatest danger to which that 

 class of structures is exposed arises from regular impulses repeated at 

 isochronous intervals. Occasionally the gusts of wind in storms occur 

 thus; and the regular cadence of a body of troops passing over a 

 suspension-bridge will also excite a vibration of a very dangerous 

 character. In the case of the Pesth Bridge, however, the dimensions 

 ;iven to the various parte of the work have been sufficient to enable 

 -hat structure to resist even the passage of troops in large bodies, in 

 the hurry of a fierce fight. 



The French government do not allow greater permanent weights 

 than those quoted below to be applied to the respective materials 

 named in the table; and they require that the dimensions of the 

 various parts of the works should be calculated so as to present the 

 sectional areas corresponding with the safety loads thus indicated. 







Chain cabin, safe load, per inch superficial . . . 17,073 lb. 

 Iron wire cables safe load, per Inch luperScial . . Si, CIO 



Vat hoop iron cables, safe load, per inch ttipeifldal . 19,913 ,, 



Rope bridges are so seldom used that it may suffice to refer to Sir 

 Howard Douglas's work on Military Bridges for the details connected 

 with them. Skew bridges, being only practical varieties of ordinary 

 bridges, will be noticed under that head. [SKEW BKIDGE.] 



Hitherto nothing has been said in this article about the mode of 

 executing the foundations of bridges, for the reason that the same 

 principles applying to those portions of all such buildings, it was con- 

 sidered desirable to notice first the modes of forming the super- 

 structures, because they, in some cases, affected the details of the 

 foundations intended to carry them. Now, the foundations of a bridge 

 are required to resist two kinds of effort ; namely, the vertical pressure 

 of the load ultimately brought upon them, and t!ie lateral thrust of the 

 arches. Moreover, they must be executed in such a manner as to be 

 able to resist any tendency on the part of external agencies to under- 

 mine the natural surfaces upon which they rest : this latter consideration 

 applie.- with particular force to the foundations of bridges when erected 

 in running water. 



The powers of a foundation to resist the lateral displacement 

 superstructure, depend, it may here bo observed, upon the dead weight 

 of that superstructure and the resistance of the bearing surface to 

 motion, either through the adhesion of the material, or through the 

 friction it may produce. This part of the investigation woul.l Iv better 

 treated under the head of FRICTION, and therefore the subsequent 

 remarks refer only to the resistance of foundations to vertical loads. 



The vertical pressure must of course be resisted by distributing the 

 weight it represents over a base of practically incompressible materials. 

 Of the natural soils there may be considered to be three great divisions, 

 ao far as regards their powers of resistance to efforts of compression ; 

 and they respectively require to be treated according to their i 

 of resistance. The fin-t di visions is composed of the bard 



rocks, which can only be worked by the pick or by the use of gunpow- 

 der. The second is composed of sands, gravels, loams, clays, and other 

 such easily worked materials, but which are practically incompressible 

 when prevented from spreading laterally. The third is compoaed of 

 the light earthy soils, of peat, mud, or other substances which arc 

 almost of a semi-fluid nature ; and it is precisely with the latter that 

 thf greatest difficulties are encountered, at the same time that they are 

 the most frequently to be met with in the beds and banks of rivers. 

 Aa the difference* between the various rlinsrs of foundations are so dis- 

 tinctly marked, and are so important in their practical bearings, the 

 greatest care must be exercised in ascertaining the conditions actually 

 prevailing in any case under investigation ; and as it may at times 

 happen that the top or upper bed of any particular situation may rest 

 UJMIII a softer and highly compressible substratum, it is essential that 

 the observations upon the foundations of an intended bridge should be 

 carried down to a very considerable depth, by means of abaft*, wells, 

 or boring*. It may nlo happen that the natural stratum of any par- 

 ticular puce may be able to resist the decomposing action of the air so 

 long aa the stratum itself is undisturbed ; but that directly the outer 

 surfaces are removed, as will often be the caw when excavations are 

 made for the purpose of receiving new foundations, the inner i 

 of the stratum will begin to decay. In fact, very careful examination* 

 must be made of the geological nature and the mechanical structure of 

 the materials about to be built upon, and the load brought upon them 

 must be proportioned to their least powers of resistance. The 1- i in- 

 tools must in such cases be skilfully and liberally applied, in order to 



