353 



BRIDGE. 



BRIDGE. 



351 



In calculating the strength of lateral trusses, reference must be had 

 to the principle upon which they are designed, and the motives which 

 may determine the choice of any particular system are so numerous 

 that it would be dangerous here to lay down any abstract rules on the 

 subject, further than to say that the simpler a truss is, and the fewer 

 the number of its parts, the better will it be fitted for its work, both 

 in so far as its resistance to the efforts it is intended to support, or its 

 susceptibility of repair, are concerned. The object aimed at in 

 designing a truss is to form artificially a rigid beam, through which the 

 weight is ultimately resolved into a vertical pressure upon the points 

 of support ; and the parts of the truss must be so put together as to 

 allow the various efforts of compression and tension to balance one 

 another in a permanently safe manner. Some of the timber bridges 

 upon the Utica and Syracuse railway afforded excellent illustrations of 

 the mode of putting together trusses of this description, and the 

 reader is referred to Weale's work on ' Bridges,' 8vo, London, 1843, 

 for an account, and illustrations of them ; but the lattice, or Thorn's 

 bridges, and the Long's bridges, as they are called in America, are the 

 boldest specimens of this mode of spanning large openings yet 

 attempted. The formula adopted by the French engineers for calcu- 

 lating the resistance of the former bridges may here be quoted, as this 

 mode of construction has found so little favour in the eyes of our own 

 engineers that no work upon the subject can be referred to. It is to 

 M. Garella (in ' les Annales des Ponta et Chaussees,' for 1842) that 

 we are indebted for the formula in question, which is (in metres) 



2P = 



( y 



36'a 



and in it2P= the safety load ; R= a co-efficient of the strength of 

 timber; c= the united thicknesses of the whaling pieces; b'= the 

 height of the truss ; 6" = the distance, vertically, between the 

 whales ; and a = the half span. The formula! for calculating the 

 strength of ordinary trusses are reserved for the articles CARPENTRY 

 and TRUSS. 



Amongst the timber bridges in which the load is supported by a 

 framing under the line of the roadway, by means of which framing the 

 load ig thrown upon the abutments without the intervention of any 

 tie-beam, the most remarkable are perhaps the bent timber bridges 

 erected upon the system first introduced into England by Messrs. 

 Green, of Newcastle. In these bridges the principal portion of the 

 weight is thrown upon an arched rib, formed of thin planks, bent flat, 

 and fastened to one another with treenails, thus producing a beam of 

 great strength and elasticity ; but the very elasticity of the beams is an 

 element of their destruction, for on every occasion of their changing 

 their form, the various parts of which they are composed become more 

 or less opened, and thus allow moisture to penetrate to their interior. 

 It has thenco happened that, in railway bridges especially, where the 

 movements are unusually great, the arched ribs of thin planks, and 

 also, it may be added, of square timber, rapidly decay. In a treatise 

 by Colonel Emy, ' Sur la Charpenterie,' Paris, 1837 ; and in a brochure 

 by Colonel Ardant, ' Sur les Charpentes a grand Portee,' Paris, 1840 ; 

 the various conditions affecting arched timber beams are elaborately 

 discussed ; and in Krafft's work ' 1'Art de la Charpente,' Paris, 1805, 

 will be found a hill description of the German and Swiss bridges, con- 

 structed upon the various systems hitherto adopted, when it has been 

 impossible to introduce tie-beams. 



Metallic bridges, which are not built upon the suspension principle, 

 are either of cast or of wrought iron, or of a combination of iron and 

 wood work. As cast iron is better adapted to resist efforts of com- 

 pression than efforts of tension, it is preferable to use it only in 

 positions where it is exposed to the former kind of action, and there- 

 fore it is that habitually cast-iron bridges are of the form of arcs of 

 circles, and composed of a number of voussoirs. With spans inferior 

 to 24 or 30 feet, however, straight cast-iron girders are often used to 

 carry the roadway of bridges ; their dimensions of course being ascer- 

 tained by the rules commonly adopted in calculating those for ordinary 

 cast-iron beams. Mr. Eaton Hodgkinson has given some empirical 

 rules upon this subject, which may be briefly stated to be, that the 

 area of the bottom flange should be as nearly as possible six times that 

 of the top one ; and that the breaking weight in the centre (of a girder 

 uniformly loaded and supported at both ends) may be ascertained as 

 follows : Calling the sum of the area of the bottom flange multiplied 

 by the depth of the beam a ; the length of the bearing between the 



ABTS AND SCI. DIV. Vf L, It. 



points of support, b ; both of which are expressed in inches ; then 



x ~ T^Ta ' tne sa ^ e l iu * ^ or a ra il wa y bridge, however, is only ith of 

 1028 



the breaking weight thus ascertained. Mr. Beardmore 'states that the 

 above formula may be applied to ordinary wrought-iron beams by 

 making the co-efficient 1500 instead of 1028. 



When the opening of a bridge exceeds the dimension above quoted, 

 it is necessary to resort to a cast-iron .arched beam ; or to one or other 

 of the descriptions of wrought-iron girders lately introduced upon the 

 lines of railway, which have modified the arts of construction in this 

 country, perhaps quite as much as they have altered the relations of 

 society. Wherever there is a sufficient clear space under the bridge to 

 allow the traffic to be carried on uninterruptedly, it is preferable, 

 within certain very wide limits of span, to use cast-iron arches ; aud 

 the very remarkable bridges of Southwark, Newcastle High Level, 

 Laira, Nevers, Tarrascon, &c., may be referred to as illustrating the 

 principles and the practical details of such works. In these cases, all 

 the action upon the cast-iron arches resolves itself into one of com- 

 pression, which is precisely the most favourable condition for the use 

 of this metal ; but as it is essential in a cast-iron bridge to keep the 

 springing of the arch above the water-line, it very frequently happens 

 that it is impossible to keep the whole of it below the line of roadway, 

 or the line of the application of the live weight. In such cases it is 

 necessary either to suspend the roadway from a cast-iron arch rising 

 above the roadway itself, which will render inevitable the introduction 

 of a horizontal tie ; or to resort to the use of framed beams, lattice, or 

 tubular girders. The bridge over the Regent's Canal of the North- 

 Westeni Railway is a good illustration of the former system ; the 

 Newark Dyke, the Boyne and the Britannia bridges, are respectively 

 illustrations of the three systems of wrought-iron bridges above- 

 mentioned. It would be preferable to describe the principles which 

 are considered to apply to these various systems under the head of 

 girders trianyular, lattice, and box; or the reader may refer for an 

 investigation of their conditions, to the works of Messrs. Edwin 

 Clarke, Fairbairn, Hodgkinson, Tate, Latham, Humber, Moseley, &c., 

 wherein their respective merits are fully and elaborately examined. 

 And it may suffice here to say that, up to an opening of 150 feet, any 

 of those three systems may be adopted in bridges having the support- 

 ing beams above the roadway, according to local considerations of 

 economy; above 150 and under 300 feet span, the choice is limited to 

 the lattice and plate-iron tubular bridges ; whilst above 300 feet span 

 the tubular bridges only are admissible. When the latter extra- 

 ordinary dimensions are attained, the stability of the structure can 

 only be ensured by the use of deep tubes,' consisting of an upper and a 

 lower boom, calculated to resist the respective efforts of extension and 

 of compression to which all girders are exposed ; and either of close- 

 plated sides, as in the Britannia, or of open framework, as in the Salt- 

 ash bridge. In the former of these colossal structures, both the 

 upper and the lower booms were composed of a series of rectangular 

 cells ; whereas in the Saltash Bridge the upper boom was composed of 

 a large elliptical tube connected with the tie girders by means of a 

 series of suspension rods. It is asserted that by this arrangement a 

 considerable economy is produced in the iron-work of the bridge, 



A very efficient system of trussing, known under the name of the 

 Bowstring girder, has been applied in some of the modern railway 

 bridges which do not exceed 120 feet span. The principle upon which 

 it is based consists, as its name implies, in a curved boom, with a 

 tension-chain at the feet, solidly attached to the boom by a lateral 

 framing ; the roadway is carried in these bridges directly upon the 

 tie-chains, so that the other parts of the framing act as parapets. A 

 very good example of this description of bridge is to be seen upon the 

 Bow branch of the Blackwall Railway. The advantage of this system 

 consists in its admitting of the nearest practical approach, in the 

 outline of the girder, to the form of a solid of the greatest resistance 

 with the least amount of material, and in thus diminishing the perma- 

 nent load of the whole structure. 



Until the recent introduction of the plate-iron girders, the only mode 

 by which a roadway could have been carried across a very wide span, 

 when it was impossible to construct a cast-iron arch beneath, either on - 

 account of the deficient headway, or on account of the difficulties in 

 the way of fixing centres, was by the application of the system of 

 chain or suspension bridges, first applied on a large scale by Telford 

 upon the Holyhead road, the scene of the greatest achievements of 

 modern engineering. In suspension-bridges, the roadway is suspended 

 by vertical rods to chains of flat iron links, or to iron-wire ropes, which 

 hang from cradles or saddles carried by masonry or iron piers, and are 

 continued on the opposite side to what are called the anchoring-wells. 

 The chains in such cases have permanently to support an effort com- 

 [xjunded of, 1, their own weight ; 2, of the weight of the suspension- 

 rods, which varies with the lengths of the latter ; and 3, of the weight 

 of the roadway, which may be considered to be uniform in the length 

 of the span. But in addition to this permanent effort, it is necessary 

 to take into account the rolling load, the action of wind upon the 

 whole structure, and, as in the case of all iron bridges, by the way, the 

 effect of variations in the temperature upon the various parts of the 

 work. Local circumstances, also, may render it necessary to modify 

 the general system of suspension-bridges; for at times it may be 

 :lesirable to dispense with the piers and back chains, whilst in others 



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