Chap. VIII.
Fig. 493 voussoirs of Sunderland bridge.
expense saved in the braces, ties, and bolts; the circles placed in the spandrills are alsoill-adapted to support the roadway, thepressure they receive being unequal.
Iron Bridge over the Witham, atBoston in Lincolnshire , executed fromthe designs of Mr. Rennie, has one ^arch, with a span of 85 feet, and a riseof 5 feet 6 inches; its breadth is 36feet. It is composed of eight ribs, eachconsisting of eleven frames, of a depth,in the direction of the radius, of 3 feet.
At the joints cast-iron gratings are in-troduced across the arch which connectthe frames together, in a similar wayto those practised in the aqueduct atPontcysyte. In the direction of thecurves are two pieces of iron 7 by 4} 2inches, and these are connected in eachframe in the direction of the radiusby pieces 4 by 3 inches. Upon theback of the ribs pillars 4 by 3 inchesare perpendicularly placed to supportthe roadway. The rise of the arch isonly one-fifteenth of its span; theframes are nearly four times the lengthof those at Sunderland, and being con-nected with cast-iron gratings insteadof wrought-iron is a decided improve-ment ; but the pieces in the frames,which are in the direction of the ra-dius, are perhaps not sufficientlystrong, in proportion to the main piecesin the direction of the curve.
Two Cast-iron Bridges over the Avon ,at Bristol, built after the designs ofMr. Jessop, each consisting of a singlearch of 100 feet span ; the rise 12 feet6 inches, or one-eighth, the breadth30 feet. There are six ribs, eachcomposed of two pieces, meeting in the middle, and connected crosswise by nine cast-ironties, dovetailed and wedged into the ribs, their cross sections being T-shaped. The abut-ments that receive the ribs are laid in courses following the direction of the radius, and onthem are plates 32 feet in length, 2 feet 4 inches in breadth, and 4 inches in thickness ;each plate has five apertures 5 feet long, and 20 inches in width. The ribs in the directionof the radius are 28 inches in depth and 2 inches in thickness, and have each 80 apertures
K k
SECTION THROUGH SUNDERLAND BRIDGE.