— 22ß 一 • 
them have been ascertained as follows : 一 
Let 
TF】 = weight per foot of the iron-work of the bridge, 
下 ド 2 = average weight per foot in height of one bent of falsework and the timbe l，s 
whose weight it supports, 
= wind pressure per square loot, 
A = area per lineal foot which the two trusses present to the wind (it is genera レ 
]y about live or six square foot), 
A* =： tlie average area subject to wind pressure per foot in height on one bei 山 
and its share of longitudinal bracing, 
l = panel length, 
c：t Co, etc. = horizontal distance between centre lines of inclined posts 1 ⑽ - 
sured along the caps, 
d = depth of truss, 
cl lt d ：9 etc- = lieiglits of the different tiers commencing at the top, 
h =» vertical distance between centre of chord and upper cap of bent, 
and 
0 = the angle which the inclined posts make with the vertical ; 
then 
2，Al = pressure on trusses at each panel point, 
2^A f d\ = pressure on upper tier, 
pA’d‘ 2 = pressure on second tier from top, 
])A’da= pressure on third tier from top, 
and the stresses F ti F 3 , etc., in tlie inclined posts of tlie first, second, and third 
tiers respectively, will bo given by tlio equations, 
^ =^[,<4 + / t + + 令] +(空+ 孕 )_， 
F 2 =^L(4 + /t + rfl + d \ + £Siif^r\ + 
■ [孕 +姊 1 + 夺)卜 
= ^- Ö T^(4 + h + d i + d 2 + rf 3 ) + £I£L±ii±i2in + 
— ‘ “ か ^^); P ， 
F4 = &C. + &C. 
These formulae are obtained under the supposition that the inclined posts 似 
not aided by the vortical ones, which supposition is necessary in order to avoi^ 8,111 
bigaifcy : it would be correct, were the falsework on the verge of overturning. I“h e 
tiaiber bo green, the error thus made upon the side of safety is advantageous ; 
if tlio timber be dry and of good quality, ifc is permissible to make a slight retlucfci 011 
ia the size given by Table XXIV. In applying the table, find the size of sq ual0 
imber required for a stress Fi and length di sec 〇 , that for a stress and length 
do sec 0 , etc” then take the greatest of these sizes. 
