1842.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



227 



the Journal, vol. iv, for 1841, p. 79, the specific gravity of cast iron is stated 

 as 7-82 r, which, taliing water as unity, gives 460 J lb. as the weight of one 

 cubic foot of cast iron. It is also observed that if the gravity be different, 

 the strength must be reduced in proportion ; another authority gives 455i lb- 

 as the weight of a cubic foot of cast iron. Both of these weights are suffi- 

 ciently near for practical purposes. The weight of metal in a structure may 

 be found by multiplying the length in feet by the number of square inches in 

 the section, and by 3-2 (the weight in lb. of a bar one foot long and an inch 

 square), which will give the weight in lb. ; or find the number of cubic 

 inches, and divide the same by 3-7J, which will also give the weight in lb. 



In the Second Part of the Transactions of the Rojal Society, 1840, is a 

 paper " On the Strength of Cast Iron Pillars," by Eaton llodgkinson, Esq., 

 from which I extract the following. The ratio of strength is constant in 

 pillars whose length is 30 times their diameter, up to 120 times, but not in 

 shorter lengths. In the experiments, when the pillars broke, it was generally in 

 the middle, as if the weight had been apphed transversely. The theory of the 

 comparative strength of pillars is that they vary inversely as the square of 

 the length : e. y. a pillar half the length of another of like area, in section 

 would have four times the strength. In the experiment the pillars broke 

 with I to i the crushing weight. The crushing weight of cast iron is 49 tons, 

 and its tensile power about an eighth of that amount. If the crushing 

 weight of cast iron be taken at 1.000, wrought iron would be l'74o. 



The rule as given by Barlow for the stitfuess of cast iron beams is that it 

 is as the cube of the depth if the weight be in the middle, and if distributed 

 J of that weight would be borne with an equal degree of deflection. 



In the above extracts sufficient is developed of the theory of the strength 

 of iron for the present purpose of illustrating these notes. 



In Weale's work on bridges, we learn that iron was introduced as a main 

 constituent of a bridge in 1775, by Thomas Fainolls Pritchard, of Shrews- 

 bury ; also that the principles that govern a bridge of carpentry are the same 

 as for a bridge of iron, and in cast iron bearing bridges the construction 

 must be considered as partaking more of the beam than the arch, although 

 there are some important works in which the properties of the arch appear 

 to have been mainly depended upon by the engineer. In the celebrated 

 Sunderland Bridge, 236 ft. span, the versed sine being nearly tliat of a semi- 

 circle, the principle of the arch is fully carried out, small portions of the arc 

 being in frame work, like the pend stones or voussoirs of a stone bridge. 

 In Southwark Bridge, London, of three arches, the centre 240 ft. span, and 

 the side arches 210 ft. each, with a versed sine of 24 ft., the ribs of the 

 arches are in long lengths, and 61 ft. in depth. This bridge contains 3115 

 tons of iron: the centre arch aloue contains 1665 tons. Up to 1830 there 

 were only two cast iron bridges erected iu France — the Pont des Arts, of a 

 similar construction to Southwark Bridge, and the Pont de Austerhtz. The 

 reason of this material not being more extensively used in that country up 

 to 1830, was stated to be the great expense of iron, and the uncertainty of 

 casting large pieces; although the same writer states, in favour of the use 

 of this material, that it is adapted to bridges of large spans, and that it is as 

 durable as stone, less expensive, more easily constructed, and, as compared 

 with wooden bridges with abutments of stone, is one half less expense. A 

 new construction of iron bridge has been invented by M. Polonceau, and 

 erected at Nantes, in the Bridge of Eidre, and the Carrousel Bridge of Paris, 

 drawings of which are given in vol. ii. of the Journal, p. 79 : they are called 

 Polonceau bridges after their inventor ; see vol. iv. of the Journal, p. 92, 

 whence the above account has been condensed. Tlie Carrousel Bridge was 

 completed in 1836; it has three arches, centre 187 ft. span, rise 16.1 ft ; 

 two side arches 156 ft. span, and rise 15^ ft.; width of piers 13 ft. The 

 principle of construction is a wooden arc on the laminated principle incased 

 with cast iron, the transverse section being elliptic and cast in 22 lengths 

 and from its shape has been called the " tidjular rib." From the preceding 

 notes sufficient has been shown that cast iron can be used in bridges to 

 the extent of 300 ft. span. When the circular arc is used similar 

 to Southwark Bridge, engineers are agreed that -^ of the chord line 

 is a sufficient rise, and that -^ of the span is sufficient width for the piers, 

 the strcngtii of the abutments being regulated merely as retaining walls. 

 But they are not agreed whether greater depth should be given to a rib at 

 the hauuches and diminish towards the crown, or that the greatest depth 

 should be at the crown and diminish towards the abutment. 



Provided that the principle of a beam girder, brestsummer, or joist be 

 considered as the true theory of construction, a beam parabolic on the upper 

 edge and horizontal or level on the lower edge, and reduced at each end to 

 one-third of its depth in the centre, will be equally strong as if its form was 

 parallel throughout. On the London and Birmingham Railway a bridge is 



constructed under the Hampstead road, where the lower edge of the beam is 

 cambered vvith a versed sine of 3 ft. ; the upper edge is also circular, being 

 drawn from two centres, the extreme depth being 2 ft. in the centre, and 

 9 in. at each end : the span is 25 ft. ; sectional area in middle 67] superficial 

 inches. There is another bridge on the same railway crossing Park-street, of 

 two openings of the same span ; the beams have a sectional area of 56J iu., 

 and are horizontal at top, and cut out in the centre to tlie form of an arch, 

 the spandrils being filled in solid. From the above two bridges a fair com- 

 parison may be instituted, and the inference drawn that no advantage is 

 gained in material in adopting the arch, over the girder ; and 1 apprehend 

 also that the only advantage gained by the two centred or bent girders, 

 whose elevation is a " phase" like tlie new moon, is in the extra depth acting 

 in a perpendicular or ordinate line, and not on a radius line with either of 

 the arcs. It may be also inferred that if the bridge be of largo span and the 

 arched form used, it will be necessary for calculating the breaking weight of 

 the bridge to take the whole depth from tlie chord line, including the depth 

 of the roadway bearer, and deducting therefrom the openings between the 

 spandril fillings. Indeed no advantage is gained in coi.tiuing the principal 

 weight of iron used in the structure to the arch formed at bottom, over the 

 equal distribution of the weight over the roadway bearer, and filling in of 

 the spandrils. By this means the insistent weight, or the weight of the 

 materials composing the structure, is more equally diffused over the whole 

 erection. In farther confirmation of the above remark, that no advantage 

 is gained in adopting the arch over the girder, I may observe that in the 

 arched form all above the piers are cast iron, which rest upon bed frames or 

 dead plates laid on the piers, and that the ribs are strengthened longitudinally 

 by fillets, mouldings, flanches, &c., and that when the structure is of more 

 bays or spaces than one, the different ribs in each arch are united by 

 junction pieces on the piers in the direction of each rib. The ribs are also 

 strengthened sideways with vertical diagonal frames or cross stays and dis- 

 tance pillars, and with horizontal diagonal braces and the roadway plates, so 

 that the whole forms a complete square piece of framework, and mav he 

 compared to the American trellis bridge, with a circular piece cut out of 

 it to form a headway betweeu each pier. 



But to return to the principle of construction of iron bridges, as adopted 

 by various engineers on the railways in England. I will give a description 

 of the different kinds of bridges which have been constructed of this ma- 

 terial. The principle of the bow and string suspension, as employed by Mr. 

 Leather in the bridges of Hiinslet and Monk Bridge at Leeds, (the latter 

 being 112 ft. span, the former 152 ft. span,) has been adopted on the Thames 

 Junction Railway, and on the London and Birminghara line tlie same principle 

 is applied to a bridge over the Regent's Canal at Caindeu Ton n, the roadway 

 being on the chord line, and the ties of the bow strong bars of wrought iron : 

 the span is 50 ft. On the same line, the load from Baniiuiy to Lutterworth 

 is carried over the railway on a bridge, with a span of 61 ft., in this case 

 the road is above the bow. and the ties are distended below the chord line : 

 the distance between the under side of the bow and tie bar is 4 ft., which is 

 filled in with studding pieces or saddles, about the san.e scantling as the 

 spandril filling of a bridge of similar span ought to be on the common con- 

 struction. The same plan has been used on the Manchester and Leeds road 

 as the bridge over the Regent's Canal ; in Ibis bridge 288 tons of wrought 

 iron, and 155 tons of cast iron were used : a similar biidge is on the North 

 Midland line, at Derby. This kind of bridge is used where headway under 

 the bridge is an object. On the Birmingham and Gloucester line, at Chel- 

 tenbaiii, a bridge is built on the above principle, where both the rib and the 

 tie at the level of the roadway are cast iron, the lower one being suspended 

 at a distance of 4 ft. 3 in. from the upper, with strong iron rods about the 

 same distance apart ; the tie or bottom rib has also a rise or versed sine of 

 IS in. On this line of railway 654 tons of cast iron, and 150 tons of wrought 

 iron were used in bridges. On the Great Western Railway parallel gurders 

 were mostly used. In a bridge over the I'addington Canal 37 ft. span, the 

 extreme depth of girder is 22 in., and breadth of flanch at top and bottotu 

 % in., of a uniform thickness of 2{ in. ; the weight of iron in the bridge is 

 41i tons. A similar bridge to the last was erected over the Grand Junctioa 

 Canal, in which above 73 tons of iron were used ; the dimensions are as iu 

 the last example, except the span and depth of girder; the latter is 26 in. deep. 

 The bridge over the Uxbridge road, containing 165 tons of iron, is on the 

 principle of a framed floor of joists : the railway crosses tl e high road vety 

 obUquely, say 15', at the junction of a cross load ; oi.e girder is used similar 

 to a trimming joist, with seven others weighing about 30 tons, tenoned 

 through it and bearing upon it. This girder weighed 9.\ tons, and was 26 in. 

 iu extreme depth, with flanch at top and bottom 11 in. broad, of a uniform 



