— 111 — 
To ascertain whether it needs consideration, find the stress on the post with one 
train only upon tho bridge, reaching from the most remote end of the span to the 
foot of the post considered, and under the supposition of an equal distribution of tho 
チ rain load between the trusses ; tlieu proportion the post to resist this stress accord- 
ln g to tlie method to be explained iu Chapter XI, and to oue half the section thus 
f °und add the value of A in the last equaiion. If the sum exceed the area of oue of the 
post channels required to resist the maximum live and dead load stresses when both 
tracks are partially covered by the assumed moving loads, then the post section is to 
bö increased accordingly. 
The vibration rods should be proportioned to resist the transferred load stress 
Us 出 g an intensity of five tons, or to resist the sum of the transferred load stress and 
the wind stress under thirty pounds pressure, using an intensity of seven and a lialf 
tons. If the formei* give the greater section, then tlie strut should be proportioned 
ド resist the transferred load stress using the intensity given in Table YIII, bufc, if not, 
^ should be proportioned to resist tlie sum of the transferred load stress and the 
wind stress uuder thirty pounds pressure using tho intensity given in Table IX. It 
姐 ust not be forgotton that the effect of initial tension is to be allowed for in propor- 
tioning both vibration rods and intermediate struts. 
In double track bridges without vertical sway bracing the trusses will j>robably 
act nearly independently, but of this oue cannot bo certain, bo it may be well to 
calculate tlie formula for the bending effect on tlie upper lateral struts duo to the 
transferred load under tho assumption of equal distribution between trusses, and 
a Pl% it to a practical case. * 
Let the notation bo tho shihg as in Fig. 3., but let s have fclio samo signifioa- 
tiou as in Fig. 2., then the bending moment upon the strut will be 
亚 =“[ 2 ㈣ _ s ] 
f tlie distance between centres of gravity of strut channels bo h and tlie intensity 
of working compressive stress be six tons, the area required for one channel to 
resist bending will be 
^ M Wa [2{a + b) — s] 
z — 顶一 m o+ り， 
Let us take the case of a 20, panel and assume a 十み = 12, / 各 = 1, and s == 7, we 
will then have the following data 、 • 
W = 21.2 
a = 4.8 
b t= 7.2 
h = 1.0 
and s = 7.0 
which substituted above gives 
A 21.2 x 4.8 x 17 . 1 . 
A = ザ =12 square inches, 
