Trustworthy, bureaucrat, administrator
5,275
edits
Changes
formatting and belated proof reading
The strength and arrangement of the abutments will be ascertained from an inspection of Figs. 10, 11, and 12, which are respectively longitudinal, horizontal, and transverse sections. The foundations are entirely surrounded with sheet piling, which encloses an area 66 ft. long by 84 ft. 9 in. wide and 19 ft. 6 in. deep. This space is filled with 1650 cubic yards of concretion, and forms the foundation on which the abutments are constructed. The level of the ground is 3 ft. above the surface of this mass of concrete, and 14 ft. below the springing of the main ribs, to which height the face of the abutment is built in solid brickwork 8 ft.. thick. The arrangement of the moulded stone course beneath the springing and the skewback is shown Fig.10, the brickwork behind the skewback being set in cement, and bonded with iron, and convenient recesses are left beneath for the reception of the holding-down bolts, which secure in their places the cast-iron shoes in which rest the rounded ends of the main ribs. From the top of the skewback to formation level the abutment has merely to retain the earth contained within the face and wing walls, and the thickness is gradually decreased from 8 ft. to 2 ft. 7½ in. The face of the abutment is strengthened by concrete backing, increasing from a thickness of 1 ft. 6 in. at formation level to 33 ft. at the foot of brickwork (Fig. 10); and three rows of horizontal arches transfer the thrust from the face of the abutment to the wing walls, as shown. These latter have a thickness of 7 ft. 2 in. at the base, gradually decreasing as the pressure of the retained earth diminishes, to 2 ft. 7½ in. at formation level, and they are tied together with four 2½ in. diameter bolts. Externally the abutments present a symmetrical though not highly ornamental appearance; they are of brickwork, with stone mouldings, and parapets.
There are four main ribs placed4 placed 4 ft. 11 in. and 6 ft. apart, so that the centres of the ribs coincide with the position of the line of permanent way. The general construction of the main ribs is shown in Fig. 10, and in cross section, Fig. 1, as well as in detail, Figs. 2, 3, and 7. They are 4 ft. in depth in the centre, increasing to 4 ft. 9 in. at springing, with a top and bottom flange 1 ft. 3½ " in. wide, and 2 in. thick, which also is the strength of the web. Nine segments each 22.81 ft. long, with the intrados curved to a radius of 260 ft., complete the rib. The construction of the end segment is shown in Figs. 2 and 3, where it will be seen to terminate with a rounded lintel, curved with a radius of 2 ft. 5½ in. and strengthened transversely and longitudinally with ribs and feathers. This rounded end fits into a curved shoe (Figs. 2, 3), which is held down to the abutment by seven 2 in. diameter bolts 6 ft. long. The shoe is 3 ft. in breadth, corresponding to that of the main rib, which is widened out as shown in the plan, and 6 ft. long over its bed plate, the thickness of metal averaging 2½ in. Both shoe and heel of main rib were cast to as nearly a true fit as could be obtained, and afterwards the surfaces were faced, and ground one on the other, so that extreme accuracy of contact was obtained. It is found, however, that the girders do not turn at all upon these joints, but rise and fall in the centre with the variations of temperature. Horizontal wrought-iron girders 2 ft. deep, and of section shown (Fig. 16), rest on the top of the spandril-filling, bearing at one end, on the abutments, 22 ft. above the springing, and dying away on the main ribs, at a point 18 ft. from the centre of the bridge. The upper and lower flanges, however, are continued until they meet the corresponding girder on the other side. These girders are of ordinary construction, with a constant cross sectional area throughout of 8¼ square inches. The thickness of web is <span style="vertical-align:super;font-size:x-small">7</span>/<span style="vertical-align:sub;font-size:x-small">16 </span> in., and the top and bottom angle irons are 3½ in. x 3 in. x ½ in.
Stiffeners of the construction shown are placed 8 ft. apart, at the joints of the web-plate, which are made good with ½ in. covers 1 ft. wide by 1 ft. 4 in. deep. Intermediate T-iron stiffeners are also placed at intervals of 8 ft. The cover plates of the bottom flange are placed on the inside of the girder, so that the web has to be notched, and the angle irons cranked, to accommodate the extra thickness. This is done to preserve a perfectly flush surface on the under side, and all rivets have countersunk heads for the same purpose.
The spaces between the under side of the horizontal girders and the main ribs are filled in with cast-iron standards, as shown in Fig. 10, and in details, Figs. 7 and 15. The standards are placed 4 ft. apart, from centre to centre ; they have a cruciform section, and vary in size from 12 in.x 6 in., 1¾ in. thick, to 9 in. x 6 in., 1¼ in. thick. In each case they are cast in 4 ft. lengths, the joints being made with 1¼ in. diameter bolts mid-way between the standards, as shown in detail, Fig. 7. On the lower side they are secured to the main arched rib by 1 in. diameter bolts, placed 12 in. apart, and at top they are fastened to the horizontal wrought-iron girder by ¾ in. bolts, 8 in. apart, which alternate with the rivets in the bottom flange of the girder. Horizontal struts of the construction shown in Pigs. 5 and 6 are placed between the main ribs, and bolted thereto. The distance between them varies front 16 ft. 6 in. to 8 ft. 4 in., the space being governed by the length of the spandril standard. These struts are formed from two channel-iron rolled beams, 4½ in. deep, ½ in. thick, and 1½ in. wide across time top and bottom tables, placed back to back, and rivetted together, except at the ends, where they are opened out sufficiently to admit one web of the spandril standard, while the ends are turned back to bear against the other web, to which they are fastened by two 1¼ in. diameter bolts. Fig. 1 shows the method of vertical cross-bracing between the main ribs, adopted for this bridge. It consists of a series of cast-iron struts of the section shown, the top and bottom horizontal members being circular, and 4 in. in diameter, and hollow to admit of the passage of a 1¾ in. diameter bolt, which secures them to the main ribs. There are two sets of struts to each segment, or eighteen altogether in the whole length. At the top and bottom of these struts tie rods, 1¾ in. in diameter, extend diagonally from rib to rib, forming a through system of horizontal bracing throughout the bridge. The spandril standards are tied together vertically by diagonal bracing rods 2 in. wide by ¾ in. thick, and horizontally by bars of the same scantling, which do not cross each other, but are turned round at a distance a little short of the centre of each bay, and are bolted together by 1¼ in. diameter bolts passing through iron distance pieces which are suspended from the platform Overhead, Figs. 13, 14. The wrought-iron girders underneath the roadway are also similarly braced, horizontally and vertically, in each case with tie rods 2½ in, by <span style="vertical-align:super;font-size:x-small">5</span>/<span style="vertical-align:sub;font-size:x-small">8 </span> in., and struts formed of two tee irons 151<span style="vertical-align:super;font-size:x-small">5</span>/<span style="vertical-align:sub;font-size:x-small">8 </span> in. by 3¼ in. by <span style="vertical-align:super;font-size:x-small">9</span>/<span style="vertical-align:sub;font-size:x-small">16 </span> in. are bolted horizontally to the bottom flange of the girders, as well as to the top of the spandril filling, as shown in Fig. 15.
It is to this complete system of bracing throughout the structure that the bridge owes its lateral stiffness, the width being so small as compared with the length, that the greatest care was necessary in designing this part of the work. Upon the road-way girders balks of timber, 13 in. square, are laid and bolted to the top flange at frequent intervals. Into these longitudinal timbers, cross beams of similar scantling, and 4 ft. apart, are tenoned in the method shown, Figs. 8 aud. 9, and upon them a close planked flooring, 3 in. thick, is spiked. The ballast which is laid over the platform to a depth of 9 in., is prevented from falling over the sides of the bridge by cast- iron facias 12 in. high, and panelled on the out-side, which run the whole length of the bridge. On these facias the handrailing is secured, and is of the design shown in the perspective sketch. Short pipes, 3 in. in diameter, and about 6 in. long, are passed through the flooring, and carry off such drainage as may accumulate on the ballast. These pipes are placed in three rows, transversely, and at intervals of about 20 ft. Before erection, all the girders were tested, and each segment of the main ribs was proved separately on the concave as well as the convex side, a load of about 70 tons being applied to the centre of each without causing them to show any permanent set, and only a deflection of about 0.08 in.
