— 120 — 
The width of the top chord or batter brace plate is dependent upon the depth of 
tho channels, as the transverse distance between the centre lines of rivets, which 
attach the channels to tlio plate, should never be less than the depth of tlie channels : 
the chord packing often demands tho use of wider plate. The least widths and thick- 
nesses are given in Chapter YI 
In pony trusses tho channels are spread apart and the width of plate increased 
to give lateral stiffness to tlio truss. To proportion tho top chord or batter brace for 
a given stress, assume the depth of the channels and divide the length of panel or 
batter brace by it, both dimensions being expressed in the same unit. 
Referring to Table VIII， look down tlio column marked “Ratio of L to D ’’， 
until the ratio jusfc found is reached, tlio number to the right, in the first of 
tlie three columns, is the intensity of working- stress to be used. The three columns 
are for the three cases, — both ends fixed, one end fixed and one end hinged, and 
both ends hinged, marked 隨 面 , 恩 ®, and ©㉘ respectively. Then, to find tlie area 
of the top chord or batter brace, divide tho stress given on tho diagram by the intonsity 
of working- stress taken from the table ; from tlio quotient subtract the area of tho 
top plate, and divide the remainder by two : tho final quotient will be the area of 
each channel. This calculation should bo made with both the stress in the panel 
nearest the middle of the span and that in the end one, or, in long spans, that in tlie 
ono next to the end. If, then, with the depth of channel assumed, it be found that 
there is, in the table of channel sections employed, a light channel that will not bo 
much too heavy for the end, and a heavier one suitable for the middle of the chord, 
all right: if not, another trial must be made, with a channel of a different depth. 
The greater tlie depth of channel, the less the ratio of length of strut to diameter, 
and consequently tho greater the intensity of working-stress, and tho less the sec- 
tional area required: so, generally speaking, it is well to use tlie lightest and deepest 
channels possible, unless the saving|in section be small, when it will be more econo- 
mical, for other reasons, to use tlio next smaller depth. These reasons will be given 
in Chapter XYL The dimensions of the channels and plate should be written 
on the diagram of stresses as shown on Plate XV. 
The sizes of the post channels are to be found in a similar manner to the one 
just described, with these two exceptions, — that the column for two hinged ends is 
to be used, and that there is no plate. 
After all tho posts are proportioned, the light ones should be tested according 
the method explained m Chapter IX to see if they are strong enough to resist tho 
combined effects of loads and wind pressure. If not the aections slioultl be increased 
so as to make them strong enough. 
In high double intersection bridges where tlie diagonals are halved, and con- 
nected by pins passing through the middle of the post channels, as shown in Plate 
I, tho posts may be proportioned for half-length with both ends hinged ; but in this 
case tlie counters must extend to tlio ends of tho span, although there bo no stress 
in some of them, for the purpose of preventiug the posts from moving laterally at 
tlie middle. 
All lateral and portal struts, also tho intermediate struts in double track bridges, 
