— 116 — 
The sizes of the lateral and vibration rods can bo found from the last mentioned 
table by looking in the column headed u Intensity of Working Stress = 7.5 tons ” 
in tlie same maimer as explained for counters. 
If the panel length correspond with the one given in Table I, or if it do not differ gre- 
flifcly tlioröfroui, tliGro need l)o no Ctilculutioiis lnfidö for stresses in tlio laterril systems 
and sway bracing ; because the dimensions of all the struts and rods for these systems 
are given in Table XIII, In that table the dimensions in the columa marked Pan. 
1 M are the sections respectively of the portal vibration rods (if any), the lower portal 
struts ( if any ), the end lower lateral rods, and the lower lateral strut at the free 'end. 
Those in the second column are the sections respectively of the upper portal 
struts, tlie upper lateral rods, the lower lateral rods of the second panel, and the 
lower lateral strut at the first panel point. Those in the other columns are res- 
pectively the sections of the upper lateral strut, the upper lateral rods, the vibration 
rods ( if any ), the intermediate strut ( if any) the lower lateral rods and and tlie lower 
lateral strut. Thus the portal rods, lower portal struts, end lower lateral rods, and 
end lower lateral strut are assumed to belong to the first panel ； the upper portal 
struf, end upper lateral rods, second paucl lower lateral rods and tlie lower lateral 
strut at the first panel point to belong to the second panel ; the end tipper lateral 
strut, the vibration rods and intermediate strut attached to the first pair of posts, 
the lower lateral rods of the third panel and the lower lateral strut at tlie second 
panel point to belong to the third panel etc. etc. Spans abovo one hundred and 
fifty feet in length have vertical sway bracing. 
If the counter stresses be largo, it is preferable to use double counters : sometimes 
botli single and double counters are employed in tlie same truss. Where there is an 
odd number of panels, the centre diagonals should be made double and adjustable. 
Tho number of main diagonals per panel is generally two ; but, if the sections 
bccoAe so great as to necessitate excessively large chord pins, it is bettor to employ 
four ; placing two inside, and two outside, of the top chord and posts. Tho widths 
of tlie main diagonals should, for tlie sake of appearance, increase from the centre 
of tlie bridge to the ends. For the same reason, it is well to have all the chord bars 
of tlie same, oi neaily the same, depth ; the correct area of section being obtained 
f oi. each paucl by varying the thickness and the number per panel. In large 
bridges it is peimissiblo to reduce the depth of the chord bars towards tlie ends of 
the span in or(kr to economize on the pins. It is also permissible, when there are 
several chord bars in tlie samo panel, to employ depths varying bv a quarter of an 
inch, provided that the bars of smaller depth be placed on the inside. 
If tlie bottom chord contain a channel strut, it will be necessary to proportion 
this member before deterniiiimg the 11111 aber and sizes of tho bars, which is accom- 
plished by subtracting from the total section required the effetive area of tho trebs of 
the channels, and using the remainder as the section required for tlie bars. 
In order that tlie strut may never be subjected to more than the stress assigned 
to it, each pin liole should be elongated towards the nearer end of tlie span a cer- 
tain amount which can be determined by tlie following method. 
Let A = the effective area of the strut webs in the middle panel, or panel 
