Chat. VIII.
495
arc cast with flanchcs at the sides, 4 inches in depth, and arc screwed together, and, beinglaid according to the curvature of the ribs, and secured firmly at the abutments, form anarch in themselves, and thus in some degree relieve the ribs of the weight they wouldotherwise sustain. As there is only one rib in the middle of the bridge, others are placed
Fig. 489.
Bl'lLDWAS BRIDGE.
at right angles, 4 inches in depth, cast with flanchcs, which serve to support the 18 feetplates that extend across. The suspending ribs are 18 inches in depth, and, exclusive ofthe moulding, 2§ inches in thickness. The bearing ribs are 15 inches in depth, and 2.^inches in thickness, and each is cast in three pieces, of about 50 feet; the braces arc 6 by 3inches. The principal king-posts are 10J by 4.^ inches. The springing plates are 3 feetbroad, and 3 inches thick, cast with openings to save metal. The uprights against theabutments are 4.J inches square. The strongest uprights in the railing are 3 inches square,and those between them 1 inch ; they are placed 6 inches from centre to centre, and theirheight above the road is 4 feet 9 inches.
In each spandrill are three circular arches, formed with hard bricks, which are con-cealed by a covering of iron plates, an inch in thickness. The abutments are carefullybuilt of squared masonry, filled in with rubble ; the foundations are laid upon the solidrock.
The whole weight of iron is 1731 tons, and the cost was about 6000/., including the abut-ments, the masonry of the wing-walls, (which have a curve both on the plan and vertically,)and the towing path, which is about 10 feet above the level of low water.
Sunderland Iron Bridge, over the Wear, was formed in part out of another contrived by thecelebrated Thomas Payne, which was put up at the Yorkshire Stingo, Lisson Grove, andafterwards carried back to Rotheram ; it was opened to the public in the year 1796, andwas cast at Rotheram , under the superintendence of Mr. Thomas Wilson. It is composed ofone arch, the segment of a circle, the chord line being 236 feet, and the versed sine 34 feet;the height from the surface of the ordinary low water to the soffite of the arch is 100 feet, thespringing line is 60 feet above the level of low water, the width of the roadway is 32 feet,
Fig. 490.
SUNDERLAND BRIDGE.
and the whole is carried upon six ribs, each composed of 125 small frames, about 2 feet inlength and 5 feet in depth, in the direction of the radius. In each of these frames are threepieces 4 inches square, following the curve of the arch, and connected, in the direction ofthe radius, by two other pieces 4 by 3 inches. In each side of the larger pieces is a grooveJ inch in depth and 3 inches broad, in the middle of which is a hole opposite each cross-piece.