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bearing at either end, to the under side of the girder, at a distance of about 11 feet short of the centre. This intermediate space of 22 feet has horizontal truss-bars passing beneath, and the horizontal and oblique bars are secured by bolts or pins 3 inches in diameter, passing through projecting saddles beneath the lower flange of the girder. At their upper extremities, these bars pass through sockets cast upon the girders, and are keyed through them. Each set of trussbars consists of four bars 6 inches wide and 1 inch in thickness. Another bridge of similar construction and dimensions is constructed to carry the York and Scarborough railway over the river Ouse at York.
It is worth while to refer to the great defect of these compound constructions, as it points directly to the superiority of homogeneous fabrics, and, moreover, involves an error in principle which should always be borne in mind in designing works of the kind here referred to. This defect consists in the difficulty, or rather impossibility, of making the two kinds of iron-cast and wrought-act fully together in bearing the load. The strength of cast iron depends upon its rigidity; for although it possesses the property of elasticity, this cannot be tasked with safety, and it is well known that repeated deflections will often destroy a casting which has withstood previous pressures with apparent impunity. Malleable iron, on the other hand, applied in the form of truss-bars to castiron girders, is intended to act by the application of its tensile strength, but the effect of this can only be secured when it becomes active before the cast girder has suffered any dangerous deflection. It is, therefore, indispensable that the adjustment of the length of the bars during all changes of temperature shall be strictly preserved-a condition which is physically impracticable by any known form of construction or arrangement of parts.
This defect was submitted to a lamentably fatal proof in the failure of the largest bridge of this kind, erected over the river Dee, near Chester, and on the line of the Chester and
Holyhead railway. This bridge, which crosses the Dee at an angle of 48°, consists of three spans or bays, each 98 feet wide in the clear, the three series of girders forming the bridge being supported on two abutments of masonry, one at either end, and two intermediate piers. The width of the bridge is formed by four of these girders, placed parallel to each other, in two pairs, one roadway or railway being supported between each pair of girders, and formed of 4-inch planking laid upon transverse balks of timber, which rest upon the bottom flange of the girders. The girders are secured transversely from moving outward or away from each other by tension-bars, fitted at the ends to dovetailed sockets, cast upon the girders. The entire bridge thus comprises twelve girders, each having a clear span of 98 feet, and a total length of 109 feet; that is, including a bearing at each end of 5 feet 6 inches in length. Each of these girders, 109 feet long, is composed of three castings, or lengths, having an uniform vertical depth of 3 feet 9 inches. The dimensions of the section are as follow: vertical rib, or web, 2 inches thick; top flange, 7 inches wide and 11⁄2 inch thick; bottom flange, 2 feet wide and 2 inches thick. The sectional area of the top flange, including the moulding, is equal to 14 square inches; of the bottom flange, including the moulding, 66 square inches; and of the rib, 80 square inches; making a total uniform sectional area of 160 square inches. The joints of the three castings in each girder, secured by wrought-iron bolts passing through flanges, are strengthened by additional cast-iron joint plates, 3 feet deep at the centre, over the joint, and 13 feet in length, bolted to and scarped over the top flanges of the castings, over a length of 6 feet 6 inches upon each: dovetailed bosses, cast upon the lower flanges, are also secured with clips of wrought iron. The total depth of the girders, at each joint, is thus increased to 6 feet 9 inches. Similar plates, of half the length of those over the joints, are also bolted over the ends of each compound girder; and the vertical inclination of the truss-bars,
from the top of the girder at each end to the bottom of it at the joints, is thus increased to about 6 feet. The malleableiron truss-bars are arranged in sets of four each, one set on each side of the girder, each bar being 6 inches wide and 14 inch thick, put together in lengths or long links, similar to those used for suspension bridges, and secured by bolts at the joints of the girders, passing through the cast-iron girder and the eight wrought-iron bars. The upper ends of the bars are secured with wrought-iron keys, driven through the bars and the casting, so as to tighten them well up in their position. By the great length of the girders, and the comparatively small depth thus afforded for the trussing, the action of the bars is reduced to nearly a horizontal direction, and their power to avert deflection in the girders is thus much diminished. Besides this, it must be remarked that the sectional area of the bars is much less when compared with the total length of each girder than in all smaller structures onthis principle; and the relative effect of any increase of temperature in extending their length, and thus reducing the effectiveness of their assistance, is similarly augmented. The cause of the failure of one of these girders, which occurred on the 24th of May, 1847, was variously ascribed to a passing train having got off the rails, and to an undue loading of the bridge with additional ballasting; but the inherent weakness of all such combinations of wrought and cast iron in bridges, subjected not only to the action of a dead or merely insistent weight, but to the vastly increased momentum of a rapidly passing and vibrating load, is too apparent to allow of any constant safety in such structures.
We may therefore conclude, that in this last bold experiment, the principle of compound cast-iron girders, trussed with malleable-iron bars, was fully tested to its utmost limits: and the great necessity of seeking a safer construction for bridges, in which the minimum of depth should be equally attained, opened a field for great experiments in engineering construction.
Malleable Iron-its Manufacture into Plates and Bars of different Sections-i'he application of Iron Plates in the formation of Steam Boilers -and of Plates and Bars in building Ships, Caissons, &c.'
THE duties of the engineer, as imposed in the highest services of his profession, are admitted to involve a constant encounter of difficulties, in order, on the one hand, to surmount natural obstructions of the most formidable character, and, on the other, to adapt such materials of construction as are within his command with economy and success. But the exercise of his genius, thus demanded in bold and discreet design and the skilful application of means, becomes yet more severe when required in the devisal of remedies for failure, by which energy and invention are so liable to have been chilled and prostrated. On this account the name of Robert Stephenson, in its association with the daring experiment described in the first section, and the gigantic design so successfully realised at Conway and the Menai Straits, stands forth as that of one of the greatest among the illustrious of English engineers.
Before proceeding to the description of Tubular Bridges and Tubular Girder Bridges, as composed of malleable-iron plates and frames, we shall find it interesting to refer to other structures formed of these materials, and the previous use of which will help us to understand the history of their application to the purpose of bridge-building.
The manufacture of iron into the forms of plates, and of bars of varied section, is effected by a process of rolling between pairs of rollers, by which any required degree of lamination may be effected in the production of plates, and an infinite variety of sectional forms given to bars of the ductile metal. This invention, in its modern applications, is due to Mr. Henry Cort, of Southampton, who obtained two patents for his improvements in the iron manufacture. The first of these patents is dated January 17, 1783, and the invention is
entitled "a method and process of preparing, welding, and working various sorts of iron, and of reducing the same into uses by machinery, a furnace, and other apparatus." The second patent is dated February 13, 1784, and is entitled "a new mode and art of shingling, welding, and manufacturing iron and steel into bars, plates, &c., of purer quality, in large quantities, by a more effectual application of fire and machinery, and with greater yield than by any method before attained and put in practice." These inventions are described in the 3rd vol. of the "Repertory of Arts" for the year 1795, and from which the following extract from the patentee's specification is quoted. After describing his process of puddling, Mr. Cort states," The whole of the above part of my method and process of preparing, manufacturing, and working of iron, is substituted instead of the use of the finery, and is ny invention, and was never before used or put in practice by any other person or persons. The iron so prepared and made may be afterwards stamped into plates, and piled or broke, or worked in an air furnace, either by means of pots or by piling such pieces, in any of the methods ever used in the manufacture of iron from coke fineries without pots. But the method and process invented and brought to perfection by me is to continue the loops in the same furnace, or to put them into another air furnace or furnaces, and to heat them to a white or welding heat, and then to shingle them under a forge-hammer, or by other machinery, into half-blooms, slabe, or other forms; and these may be heated in the chafery, according to the old practice; but my new invention is to put them again into the same or other air furnaces, from which I take the half-blooms, and draw them under the forge-hammer, or otherwise, as last aforesaid, into anconies, bars, half-flats, small square-tilted rods for wire, or such uses as may be required. And the slabe, having been shingled in the foregoing part of the process to the sizes of the grooves in my rollers, through which it is intended to be passed, is worked
me through the grooved rollers, in the manner in which