RAILWAY CONSTRUCTION. 145 



Fig. 148. Chairs of this form have a very firm hold on the 

 longitudinal timber, and the side lugs check any tendency of 

 the splitting or opening of the wood when putting in the spikes 

 or screw bolts. 



Fig. 149 shows cross-section of a plate-girder over-line 

 bridge, 32 feet span, carrying a private road, 12 feet wide, over 

 a double-line railway. The road traffic being small, the floor- 

 way was constructed of creosoted planking carried on rolled 

 I-iron cross-girders placed at 3 feet 8 inch centres, and riveted 

 to the main girders. The horse-tread track was provided with 

 a second layer of planking, laid transversely, to take up the 

 wear, cross battens, 4 inches by 2 inches, being placed at 

 12-inch centres, and sand spread between to give good foot- 

 hold. A light lattice-work parapet was bolted on to the top of 

 the main girders. 



Fig. 150 gives cross-section of a plate-girder over-line bridge, 

 30 feet span, carrying a private road, 20 feet wide, over a double- 

 line railway. The main girders are tied together by lattice- 

 work bracing, spaced at 7-foot centres. Curved wrought-iron 

 plates are laid across from girder to girder, and butt against a 

 narrow horizontal plate, which forms part of the upper boom. 

 The curved plates are riveted on to the top of girder, and .form 

 a continuous iron floor, or deck, from side to side of the bridge. 

 Upon this iron floor is laid an ordinary asphalte roadway. The 

 outside girders are made deeper, and carry an ornamental cast- 

 iron parapet. In some bridges of a similar construction, the 

 roadway is formed of creosoted wooden block paving, on a 

 foundation of asphalte. 



Fig. 151 shows cross-section of a plate-girder over-line bridge, 

 28 feet span, carrying a public road, 35 feet wide, over a double- 

 line railway. The main girders, 2 feet 4 inches deep, are placed 

 at 5 feet 2 inch centres, and are tied together by plate-iron 

 cross-bracing 2 feet deep. Jack-arches of brickwork, 9 inches 

 thick, are built in between the main girders, the haunching 

 being filled in with concrete. The entire surface is covered 

 over and made watertight with asphalte, on which is laid the 

 metalling of the roadway. The outside girders are made 

 considerably deeper, and have strong cast-iron-plate parapets 

 bolted on to the top booms. There is no doubt that jack-arching 

 of brickwork or concrete makes a very strong and permanent 

 floorway, but its dead weight is very great, and its adoption is 



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