There are many ways of forming the floor or deck of a girder bridge intended to carry a railway over a road or stream. In some cases it will be imperative to have a thoroughly water-tight floor to prevent rain-water percolating through to the roadway below; while in others, such as bridges over streams, and secondary roads, this special provision will not be necessary, and a lighter and more economical floorway can be adopted. A strong wrought-iron or steel-plate flooring, with its corresponding filling and ballasting, means not only so much additional cost in the flooring proper, but also so much additional dead weight to be carried by the main girders.
[Fig. 132] is a sketch of rolled joist-iron I-girders and timber floor frequently adopted for small farm roads and cattle creeps of 10 or 12 feet span. A beam of timber is fitted in between the two rolled joist-irons, and the three pieces securely fastened together with strong iron bolts placed about 3 feet apart. These small compound girders rest on bearing-plates of wrought or cast
iron, and are held together and to gauge by tie-rods, as shown. The rails are spiked or bolted down on to the timber beams, and the flooring formed of strong planking.
[Fig. 133] shows an arrangement of plate girders for a 16-foot opening over a stream. The girders are placed immediately under the rails, and are tied together by plate-iron cross-bracing the same depth as the main girders. The flooring consists of 4-inch planking laid with ¾-inch spaces, on which are laid longitudinal rail-bearers 14 inches wide by 7 inches thick.
[Fig. 134] is a sketch of a somewhat similar arrangement for a lattice-girder bridge, 45 feet span, carrying a single line of railway over a river. The main girders are tied together by lattice-work cross-bracing. The floorway consists of 5-inch planking, laid with ¾-inch spaces, on which is placed the 14 feet by 7 feet longitudinal rail-bearers. Plate-iron outside brackets are riveted to the main girders to carry the ends of the planking and light tube-iron parapet.
[Fig. 135] illustrates an example of trough girders, constructed to carry a double-line railway over a country road 25 feet wide, where the space from under side of girder to rail-level is small. The girders are constructed in pairs, with short, shallow cross-girders at 3 feet 6 inch centres, riveted in between them to carry longitudinal timbers on which the rails are laid. Bottom plates, 5/8 inch thick, unite the two girders for the length of their bearing on the abutments, and a similar plate, 9 inches wide, unites them at the centre; the remainder of the span is left open to prevent the lodgment of rain-water. Three strong tie-rods are placed to keep the girders to gauge. Curved wrought-iron ballast-plates are used between the running-rails, and plank flooring forms the rest of the covering.
