242 APPLIED MECHANICS 
Above the rollers is the top bearing, which is a steel casting 
chord, is placed on the top plate, to which it is bolted rigidly. The 
rocker plate is square in plan, its bearing surfaces being cylindrical, of 
radii equal to the side of the plate. A rocking motion is possible in 
any direction, and the bearing may be depended on to distribute the 
weight not only uniformly over the several rollers, but uniformly over 
the length of each roller. 
_a—(64+1'5) pat —(o+15) Span 
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The roller bearing provides for the longitudinal expansion and con- 
traction of the structure due to variations of temperature. Such 
provision is, however, only necessary at one end of the truss. At the 
other end a fixed bearing, or one which provides for rocking motion only, 
is provided. A form similar to that shown in Fig. 369, but without the 
rollers, may be used for the fixed end. Another form is shown in 
Fig. 368. 
215. Bridge Floors.—The floor of a bridge may be carried on the 
top of the main girders, or it may be attached to the bottom flanges of 
these girders. In the former case the traffic passes over the main girders, 
and the bridge is called a “deck” bridge; in the latter case the traffic 
passes between the main girders, and the bridge is then called a 
“through” bridge. 
216. Railway Bridge Floors—Cross Girders.—The weight of the 
bridge platform and the rolling train load is transmitted to the main 
girders by cross girders, which are usually shallow, plate web girders — 
spaced at intervals along the main girders, and placed transversely to 
them. Figs. 327, 328, and 329, p. 223, show the common means of 
attachment if the main girders are of the plate web type. 
Common methods of attaching cross girders to main girders of the - 
open web type are shown in Figs. 370, 371, and 372. Figs. 370 and 
371 apply to through bridges, and Fig. 372 to deck bridges. In Fig. 
370 the cross girder rests on the flange of the main girder directly, while 
in Fig. 371 it is slung below. The latter method has the advantage 
that the load is transmitted directly to the centre of the main truss, and 
does not tend to twist the flange. 
The minimum spacing of the cross girders should be from 7 to 8 feet, 
that is, not less than the distance apart of the driving axles of the 
heaviest locomotive crossing the bridge. This spacing, however, may be 
much increased in large spans. In any case, cross girders may only be 
attached to the main girders at panel points. 
Each cross girder must be capable of carrying its share of the dead 
load of the bridge platform, together with the heaviest live axle load 
which may come upon it. Cross girders are usually assumed to be freely 
supported at the ends. 
217. Rail Bearers.—Spanning between the cross girders, and placed 
directly beneath the rails, are longitudinal girders called rai bearers or — 
stringers. For a 4 feet 8} inches gauge these rail bearers would be spaced 
about 5 feet, centre to centre. The rail bearers carry the weight of the 
platform and the axle loads on to the cross girders, to which they are 
the rocker plate, which carries the top plate. The bolster, or the bottom — 
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