[Fig. 147] shows cross-section of a truss-girder bridge, 120 feet span, carrying a single line of railway over a river. The cross-girders are placed at 10-foot centres to correspond to the vertical members of the main truss-girder. Longitudinal plate-iron rail-girders are riveted in between the cross-girders, and the entire floor is covered with curved wrought-iron ballast plates, as shown. The rails are carried on longitudinal timbers, which are bolted on to the rail-girders. Angle iron brackets, riveted on the top of the cross-girders, keep the rail timbers in position and gauge.
In each of the above examples, where longitudinal rail timbers are adopted, flange rails are shown, as many engineers prefer to have a continuous bearing for the rails on bridges, in case of rail fracture. There is nothing, however, to prevent the chair road being laid on longitudinal timbers, and for this purpose the writer has used chairs of the ordinary pattern, specially cast with side lugs to grip the timber, as shown in
[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 floorway 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 foothold. 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.
BRIDGE CARRYING THE D. W. AND W. RAILWAY (LOOP LINE) OVER AMIENS STREET, DUBLIN. [To face p. 144.