244 APPLIED MECHANICS 
attached. Fig. 373 shows the method of connecting the rail bearers to 
the cross girders. For a further illustration of rail bearers, see Fig. 386, 
p. 256. Rail bearers are designed in a similar manner to the cross girders. 
218. Floor Plating.—Upon the double system of girders, made up 
of the cross girders and rail bearers, is placed the deck proper, which 
consists either of flat or buckled floor plates, and upon which the ballast 
rests, The thickness of these plates varies with their area and the weight 
supported, but is usually about 2 inch. 
219. Ballast and Sleepers.— Ballast consisting of broken stone 
asphalt is spread over the floor plating to a depth of 3 inches. Above 
this and under the sleepers at least 4 inches of hard ballast is placed. 
The sleepers are of pine, 9 feet long, 10 inches wide, and 5 inches _ 
thick, spaced at about 3 feet centre to centre. If head room is limited, 
the sleepers may be placed longitudinally and bolted down directly to the 
floor plating above the rai! bearers. This method has the disadvantage 
that it breaks the continuity of the permanent way system. 
To prevent the ballast spreading, vertical plates, called ballast guards, 
are fitted (see Fig. 386, p. 256). Provision must also be made to confine 
the ballast at the end of the bridge. Suitable drainage arrangements to 
carry off water from the bridge floor are also necessary. 
220. Trough Floors.—Instead of flat or buckled plates, trough sec- 
tions of various forms may be employed. 
Fig. 374 shows a common form. In this $ \ 
case rail bearers are unnecessary, the troughs 
running longitudinally, and resting upon the e « 0 «© « « @ 
cross girders, to which they transmit the load. Fic. 374 
With plate web girders, the cross girders them- a 
selves may also be dispensed with, and the troughs are then laid trans- 
versely and attached directly to the main girders. 
The troughs are designed as beams, the total moment of resistance of 
those which actually bear the load being equated to the bending moment 
upon them. Dimensions and moments of resistance of trough flooring 
are given in the various makers’ catalogues. 
221. Widths of Railway Bridges.— With a single line of rails, the 
clear width between the parapets should be 15 feet. A double line 
running between two main girders requires 26 feet, the distance between 
the roads being 6 feet. 
222. Road Bridges.—The flooring takes much the same form as that 
of railway bridges. It must, however, be capable of carrying a live load, 
consisting of heavy traction engines and other vehicles, anywhere upon 
the surface of the road. The various forms of trough flooring are very 
suitable. Very small bridges may even be made without main girders, 
longitudinal troughs carrying the load from abutment to abutment. 
The widths of road bridges correspond to those of the roads which 
they serve. 
223. Example of American Practice.—Fig. 375 illustrates the details _ 
of a type of bridge common in America. The line diagram near the top — 
of the figure shows a portion of the truss, and details of the joints at 
A, C, D, and E are shown to a larger scale. It will be observed that the 
tension members are eye-bars, which are connected to the other members 
by pin joints. 
