498
HISTORY OF ENGINEERING.
Book I.
12 inches square, separated by bars 3 inches broad, excepting opposite the cross-ties, wherethe solid metal is 12 inches in breadth. In the middle, where the ribs meet, are flanches2 inches thick and 8 inches broad, which are connected by 3-inch cast-iron screw bolts.Between the ribs and the bearers of the roadway are perpendicular supports, the horizontalsection of which is T-shaped. The road is carried upon cast-iron plates, and protected by*railing of the same metal.
Iron Bridge over an Arm of the Sea at Bonar , in the county of Sutherland, in Scotland ,under the direction of Mr. Telford, consists of one arch 150 feet span, and rises 20 feet;it is 16 feet in width, and has four ribs. The springing plates are laid in the directionof the abutments, and are 16 feet in length, 3 feet in breadth, and 4 inches in thickness;they are all cast with sockets and shoulder pieces to receive the ribs; in each plateare three apertures, 3 feet in length and 18 inches in width. Each rib is composed of fivepieces, 3 feet in depth in the direction of the radius, and inches in thickness. In theribs are triangular apertures formed by the pieces in the direction of the radius, and theSt. Andrew’s cross or diagonals between them, but all parts are of equal dimensions.Where the ribs join a cast-iron grating passes across the whole arch, upon which are castjoggles to receive the ends of the ribs, which are Handled and fixed in the gratings withcast-iron screw bolts. The ribs are preserved in their vertical position by covering thewhole with grated Handled plates, secured together and to the top of the ribs by cast-ironscrews and pins. The spandrills are filled in with lozenge-work, each triangular form beingcast in one frame, with a joggle above and below’, which pass into the sockets formed in thetop of the ribs, and in the bearers of the roadway. Where these lozenges meet in themiddle of their height each has a cast-iron tie, which passes by a notch from each side,and meets in the middle of the breadth of the arch, where they are secured by forelocks.Towards the abutments the lozenges are halved, in order to suit the inclined surface. Thecovering plates are cast in a reticulated form, the holes or apertures being larger on theunder surface than the upper, the better to support the materials of the roadway.
From the experiments made by Dulong and Petit it seems that the temperature ofiron is nearly equable, between 32° and 212°; the rate of expansion increasing with thetemperature : it is therefore most important that due allowance should be made for themetal’s expansion and contraction wherever it is introduced. 'Hie arches of the SouthwarkBridge arc said to rise and fall an inch within the range of our atmospheric tempera-ture : in most of the first constructed French bridges, sufficient attention was not paidto this quality of the metal, and hence the cause of many of the failures where it was em-ployed.
In the construction of an iron bridge it is highly important that the distinction betweenties and struts should be well understood ; and that cast should not be made use of, wherewrought-iron alone can be efficient. M. Duleau, who made several experiments on themalleable iron of Perigord , found that its elasticity was greatly afFected by the variousweights to which he subjected it, and that there was a point at which it would not againrecover its form ; and he concluded that a bar of wrought-iron might be safely straineduntil the extension, at the point of the greatest strain, is equal to of its originallength, without losing its elasticity, and that the load upon a square inch which pro-duces this extension is 8540 pounds. In some of his experiments, however, he foundthe extension was three times this without permanent loss of elasticity ; and thus hasthe art of construction in iron been latterly directed by the French engineers, who haveconsequently been very successful. Navier, Rondelet, and others, have calculated the re-sistance of iron to compression and tension, and upon their data the use of iron is applied.Tlie greatest load they found a bar of malleable iron whose length was 11 *8 feet andscantling 1*21 inches square would bear without doubling was 4400 pounds; and uponthese results their iron-bridge-builders proceed in proportioning their metal.
Southwark Iron Bridge , over the Thames, at London , was commenced after the designsof Mr. Rennie on the 23d of September, 1814, and completed in April, 1819. The wholewas cast at the iron works at llotheram, in Yorkshire , and weighed 5780 tons; the ex-penses, amounting to about 800,0007, were defrayed by a joint-stock company.
There are three arches, the middle one spans 240 feet, and has a rise of 24 feet; itis composed of eight ribs strongly secured by diagonal braces, each in the direction ofthe radius, being 6 feet in depth at the top of the arch, and gradually increasing to 8 feetwhere it rests upon the abutments. The two other arches each span 210 feet. Themasonry of the abutments is constructed of stone from Bramley Fall and "Whitby ; avertical bond was adopted, running through every two courses at intervals; the masonryof the piers was carried up in the same manner, with horizontal and vertical courses to thespringing of the arches, where they radiated to receive the iron-work. The piers are24 feet in breadth, 56 feet 6 inches in length, and 60 feet high from the bed of the river tothe top of the parapet; the length of the starlings is 74 feet 6 inches. The middle archsettled at the vertex after the centre was struck 1| inches ; the entire width of the soffite is44 feet 4 inches. The foundations of the piers are laid 10 feet below the actual bed of the