— 124 — 
Jjet W = tlie load applied at tlie bearing of iwo a butt in f? track stringers : 
it includes one half of the greatest combined engine anti car 
loads that can be concentrated at a panel point, the allowance 
for shock, one half the constant track load per lineal foot 
multipliod by the panel length, and the total weight of one 
stringer. 
祕 = tlie weight of floor beam per lineal foot 
心 = the perpendicular distance between central planes of trusses. 
x = the distance from the central plane of the nearer truss to tho 
point of the beam considered ； 
then for any point between the stringers fclio moment is given almost exactly by the 
equation 
M =W ( 1.8 5 ^ J w I s 
and for any exterior point by the equation 
M= Wx + [b — x) 
For a floor beam of a double track bridge tho moment at any point between tlie inner 
track rails is given approximately by the equation 
M=W { ^ — 9.8 8 ) + itc h 2 
Untier the outer rails it will be given approximately by the eo nation 
Af= JF ( i - 1 8.5 8 ) + ^wb^) 
and for any exterior point exactly by tlie equation 
M=2W x + 
The area of a flange for a floor beam is determined by substituting the value of M 
ill one of the equations 
A M A M A 
A = YI orA 1 = +A" 
which were given for girders. 
The economic depth for floor beams of single track bridges will vary from for 
short panels to for long ones : for double track bridges the corresponding ratios will 
probably be found to be and 1^. 
Tlie values of IF for tlie different panel lengths are given approximately in the 
following table. 
Pan. Length. 
w 
Pan. Leugtli. 
w 
1 C ， 
13. 84 toils 
18 ， 
19* 79 tons 
II ， 
14. 85 ,, 
19 ， 
20.43 ，， 
12 1 
】 5-79 
2 C f 
21.20 „ 
げ 
16. 67 ,, 
w .95 ，， 
17.36 » 
22.62 „ 
15 ， 
18. 06 „ 
23 ， 
23.26 ,， 
i 6 r 
18.73 „ 
2" 
24. 00 „ 
11 ， 
19.28 „ 
24.64 ，， 
