The Viaduct Works' Handbook

33 diminution in size of the parts so acceptable as in the chain or cable, subject only to tension. The following is an estimate of a suspension bridge of the most improved kind, consisting of three spans, 400 feet, width of road 20 feet, and two footpaths equal to 10 feet, ratio of depth to span 1 to 10; strain allowed on iron with bridge loaded 84 pounds per foot, or 400 tons on each span in addition to the weight of the bridge, is on charcoal iron wire in cables 8 tons per inch, best wrought- iron, 5 tons tension and 4 tons compression. The platform of the bridge is of i-inch plate iron strengthened by angle iron with cross girders of wrought-iron ; the parapet is of wrought-iron rivetted to the wrought platform, forming with it and the longitudinal beams a horizontal girder of great lateral stiffness to prevent oscillation ; at the same time considerable vertical stiffness is given to the platform by attachment of the longitudinal girders and bracing. The floor plates, previously asphalted, are intended to be covered with wood paving; the footpaths of stone paving laid on the plates in a similar way. The quantities of iron are as follows :— Tons. Best charcoal cable ...................250 Beams and flooring plates of the platforms . 387 Cast saddles to attach to cable . • 40 Parapet, including angle iron and sundries . 272 Cast-iron towers .............................110 1059 @ £25 = £26,475 or about £22 per foot run. If plain wire is used instead of cables the cost of manufacture is greatly reduced, and the material is made up in small bundles, and therefore more convenient for transport in a foreign country. Swing Bridges.— Under this heading we include such bridges as are required to cross navigable rivers, canals, &c., where it is necessary that they should open to allow of the passage of ships and other river traffic. There are several ways in practice of ac- complishing this object, each having its own merit. The accom- panying illustration represents one recently made at Crumlin to carry a public carriage road 18 feet wide, and one footpath 4 feet 6 inches wide, over a river, giving a clear way for the passage of vessels on c