— 108 — 
\ 
The reloasoil weight V on the windward post passes down tlio leeward post, nroducing 
a stress equal to f on each channel and making the total wind stress on one channel 
It is only for light posts that this stress need be considered, because when it is to be 
combined with the live and dead load stresses both C and V must be calculated for a 
pressure of only thirty pounds per square foot. The method of providing for this 
■wind stress is clearly indicated in Chapter VI. 
All the formulae for vertical sway bracing, except that for the stress in GH, may 
be made applicable to tlio portal bracing by putting for d the length of tlio batter 
brace, for / the perpendicular distance between coutre lines of upper and lower portal 
struts, for J J/ the pressure on one lialf of the batter brace and for P one fourth of the 
sum of all the pressures concentrated nt windward and leeward panel points of the 
upper lateral system. 
If P e be tlio pressure concentrated at the leeward hip, the stress on the upper 
portal strut will be given by the formula 
C = j{P + F) — F+ P - Pe 
It must not be forgotten that the stresses on all portal vibration rods must be increased 
for initial tension or the rods bo proportioned by using Table VI, and that the stress 
on each portal strut is to be increased by the sum of the components in the 
direction of the length of the strut of the initial tensions in all -the rods meeting at 
one of its ends. 
In the case of a deck bridge with a curved track thereon, the centrifugal force 
of the greatest panel load will affect the vertical sway bracing in the same manner 
as does tlio wind pressure; so for P must be put P 屮 i (T ド 7 + 穴 i ), the last two 
quantities having the same signification as in Chapter YIII. 
Tlio portal bracing will also be affected by tlie centrifugal force of the whole 
train, so for P must bo put one half of the greatest reaction at one end of the span, 
clue to the combined wind pressure affecting the upper lateral system, and the cen- 
trifugal force of the whole train, or 
ギ un+w) +-^- 3 £ 1 , 
and for P e one half of a panel loading ot wmd pressure, centrifugal force of car load and 
centrifugal force of engine excess or i (TF 3 + W 7 -f E x ) 
When there is no verticul sway- bracing, stiffness is obtained by the use of 
knee braces or brackets AB, CD, Fig 2, making angles of forty-five degrees with the 
vertical. 
Let tlie notation be as shown in Fig. 2., F being as before the relief of weight 
at F. P in this case is the sum of the pressures at H and G. Taking the centre 
of moments at E gives. 
