504

HISTORY OF ENGINEERING.

Book I.

Fig. 503. CRAIG ELLACHIE.

niB

Tewkesbury Bridge , of one arch of cast-iron, was constructed by Mr. Telford over theSevern, at about \ a mile above its confluence with the Avon ; the bed of the river wasfound by boring to consist of alluvial matter to a great depth.

The masonry of the abutments was laid 6 feet under low water level, upon strong woodenplatforms, surrounded by sheet piling about 10 feet in depth ; it was carried up to 16 feetabove low water, which, allowing 3 feet of masonry to receive the cast-iron springing plates,determined the lower edge of the large ribs to be 13 feet above low water level, the water,in the time of floods, never rising more than 18 feet.

The span of the arch is 170 feet, and its rise about one-tenth of the span, or 17 feet; thebreadth, measured across the soffite, is 27 feet; it consists of six main ribs, each 3 feet deep,the thickness of the two outer ones being inches, and that of the four interior 2 inches.They were all cast in lengths of 22 feet, and are connected at each joint by grated cross-plates, 3 feet in depth, and 2 inches in thickness, to which they arc severally screwed.

The whole of the ribs thus united in one arch are placed upon springing plates 3 feet inbreadth, and 4 inches in thickness, embedded in the stone abutments; their upper edgesare covered with grated plates l inch in thickness, and upon the six ribs is a perpendicularlozenge framing to support the roadway bearers, which are connected by wrought-iron rodsor cross ties, 1^ inches in diameter, passing through cast-iron tubes with flanches adjusted tothe lozenge frames, and retaining them in a perpendicular position, while the wrought-ironbolts, being screwed to the outer ribs of the bridge, prevent them from bending outwards,and the interior of the spandrills are further secured by diagonal braces. The roadwaybearers support plates 4 feet 6 inches in breadth, and 1 inch in thickness, with flanches4 inches in depth at each juncture, by which they are screwed together. Upon the roadplates are skirtings to retain the gravel of the footpaths, which are also adapted to receivethe bars of the side railing.

The whole is cast from No. 2 Shropshire iron of the best quality, and the work was com-pleted'in April, 1826. The cost of the iron-work was 4500/., and that of the masonry, em-banked approaches, and land arches, 10,000/.

Manchester and Birmingham Railway _At Fairfield Street is a bridge whose span in a

straight line is 128 feet 9 inches, with a versed sine of 12 feet; the width, measured fromface to face, is 31 feet.

Fig. 501.

FAIRFIELD STREET BRIDGE.

Six ribs of iron abut on as many independent walls, e&ch of which are 5 feet 10.^ inchesthick, and project before each other 12 feet 11 inches. The transverse sections show theconstruction of the piers, the faces of which arc rusticated ; an ornamental stone parapet andcornice crown this viaduct, and present a novel and agreeable character.

This bridge, executed after the designs of Mr. George Watson Buck, is remarkable forits acute angle, which is about 24^ degrees. The weight of the iron work employed inthe six ribs was 540 tons, and the whole was admirably screwed together; the remainderof the viaduct is formed with brick arches of 45 feet span. Iron ribs have enabled theengineer to cross rivers and roads in situations where arches of masonry or brick couldnot have been introduced : in the present instance, the level of the viaduct would not havepermitted a rise sufficient; but wherever they can be introduced, there is no doubt that aconsiderable portion of the expense of iron structures would be saved. For obliquebridges iron constructions are particularly well adapted, and, as in this instance, beau-tiful in their effect; timber has been employed, as we shall hereafter see, for the same pur-