The Viaduct Works' Handbook

29 The cost of these girders is generally a trifle more per ton than of parallel girders, in consequence of the iron cutting to waste, and the extra expense of forging and bending the iron; there are obvious objections to the kind of floor here used, although often adopted. A very elegant form of arched girder bridge is one recently made at Crumlin upon the design of E. Woods, Esq., C.E., an engraving of which is given on the accompanying page ; the form of arch, when loaded, being a parabolic curve; it is to carry a public carriage road 19 feet 6 inches wide, and two footpaths each 4 feet wide, over a river. The span is 105 feet. There are two main girders, and their depth at centre is sufficient to allow of a horizontal brace or stay between them, at such height above the floor of the bridge as to admit of the passage under it of loaded vehicles. The main girders are of what is called the bow and string truss principle, and are placed 22 feet apart, centre to centre. The roadway is between, and the footways outside, the main trusses; the floor is carried upon transverse girders, which are suffi- ciently long to project on each side the main girders in the form of cantilevers, and are attached to the main girders at the points of intersection of struts and ties at tension bar, which are arranged at intervals of 10 feet 6 inches apart; upon these transverse girders is laid a floor for the roadway, consisting of joists of wood 12 inches deep, 6 inches thick, placed 3 feet apart, and covered close with 21-inch diagonal planking in two layers, which is again covered with 3 inches of asphaltum, and upon this the road metalling, from 4 inches to 5 inches thick; the floor for footway is composed of 5-inch planking. The footways are protected on each side by orna- mental cast-iron parapets, the base and top of which are finished off with moulded polished teakwood of massive section. The total weight of ironwork in this structure is about 60 tons. All the holes for rivets were most accurately drilled, and the workmanship throughout of a superior class. The cost of ironwork, taking the iron at £18 per ton, f. o. b. an English port, for such a bridge, is about £10 5s. per foot run. Suspension Bridges are not much used, chiefly because a moving load produces more or less undulation and vibration, resulting in injury to the roadway and other parts of the structure, and this, in large spans, becomes so serious as to involve the necessity of