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DESIGN OF STRUCTURES 249 
tl » spacing of the cross girders. Fig. 383 shows an outline of the 
truss as decided upon. The depth at the centre is 12 feet 6 inches, at 
the ends 7 feet 6 inches, and there are twenty panels, each 7 feet 6 inches 
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Fig. 383. 
Outline of Main Girders and Cross Section.—Draw an outline of the 
_ bridge, showing the centre lines of the various members. Draw also an 
‘approximate cross section at the centre, showing the cross girders, rail 
bearers, flooring, etc. See Fig. 386, p. 256. Reference to an existing 
bridge is here desirable. 
Lengths of Members.—Calculate the lengths of the centre lines of all 
' the — of the main girders and tabulate them. See stress sheet, 
: Travelling Load Ejffects—Determine the maximum bending moment 
and shearing force diagrams, and also the equivalent uniform load due to 
the specified travelling load. The tracing paper method described in 
_ Art. 106, p. 98, may be used. 
' When the travelling load crosses the bridge the bending moment 
_ rises to a maximum of 4800 foot-tons, the equivalent uniform load being 
256 tons. The maximum shear force occurs at the ends of the span, and 
_ is 133 tons. 
_-~Weight of Bridge.-—The dead load due to the weight of the bridge 
and its floor must be estimated as closely as possible, basing the calcula- 
tion if possible on an existing design. The following will indicate the 
_ method. 
_ The area of bridge floor covered with ballast will be taken as 
150 feet x 114 feet (see cross section, p. 256). The weight of the bridge 
floor, exclusive of cross girders, is therefore— 
Tons. 
150 feet of permanent way, including rails, chairs,and sleepers 11°0 
3 inches of broken stone asphalt @ 140 lbs. per cubic foot 27:0 
4 inches of ballast under sleepers @ 120 __,, 36 . 380°8 
5 inches of ballast around sleepers @ 120 __,, - . 801 
8 inch flooring plates, ballast guards, and fixings ; CER (ip | 
Timber foot paths * ‘ : 3 ; : : . 38 
Rail bearers (rolled steel joists @ 57 lbs. per foot-run) .° of 
Total . : . 128°2 
___ There are 20 panels and 21 cross girders. The dead load per cross 
_ girder is therefore 6°4 tons. Taking the maximum live load on a cross 
girder to be the heaviest axle load 17°4 tons, and the equivalent uniform 
load as 1} times this, and doubling this latter figure to reduce it to a 
