DESIGN OF STRUCTURES 253 
_ Example. Bar 32. Maximum load, 151°7 tons. Safe working 
stress, 5°9 tons (Claxton vegatd or 6°2 tons (Launhardt-Weyrauch). 
Try four bars 11 inches wide by } inch thick. 
Net area (less one rivet hole) = 25°3 square inches. 
Actual stress in tension = 151°7 + 25°3=6 tons per square inch. 
Number of rivets required, 15 of 3 inch diameter. 
Minimum efficiency, 90°4 per cont. (tearing at second row, and shear- 
ing at first). Equivalent stress, 6°1 tons per square inch. 
Compression Members.—Allowance may be made for the lengths of 
_ the struts by the following modification of the Rankine-Gordon formula. 
Using the notation of Art. 160, p. 166, f= aA! + a(z) } 
Secondary flexure in a strut braced as that shown in Fig. 384, p. 254, 
need not be considered. 
To find the number of rivets in the end of the strut, the safe working 
stress for shear may be taken as 0°8 of the safe working stress in the 
bar, and the safe bearing pressure as double the safe shear stress, 
Example. Bar 22. Maximum load, 95°3 tons. Calculated length, 
8°45 feet. Equivalent length, 8°45 x 1:6=13°5 feet. Assume a section 
consisting of two B.S. channels, No. 19, 23°55 Ibs. per foot-run, to each 
of which is riveted a plate 10 inches x } inch (see Fig. 384, p. 254). 
Area of section =23°8 square inches. Minimum I= 286, and k&?=12, 
both in inch units. 
Safe stress, 4°5 tons per square inch (Claxton Fidler). 
» » 5D 5 »» (Launhardt-Weyrauch). 
95:3 (195 < 12)" _ 
Actual stress = 55 ant + 36000 x 12 = 4-3 tons per square inch. 
Safe stress in single shear on rivets =0°8 x 4°5=3°6 tons per square 
inch. Number of rivets required = 43. 
Gussets.—These should be thicker than the members which they 
unite, and of suitable shape to allow of good connections. The number 
of rivets through the ties and struts has already been determined. The 
number of rivets through the gussets into the boom is found as follows. 
The dead load which a gusset adds to the boom is determined from the 
dead load stress diagram, and the live load which it adds is found from 
the maximum shear force diagram. From these the minimum and 
maximum stresses and their ratio can be obtained. The safe working 
shear stress may be taken as eight-tenths of the safe working com 
pressive stress, as found by the formule on p. 252. 
‘Where a cross girder is attached to a gusset the additional load which 
it adds, including dead, live, and wind loads, must be compounded with 
that from the struts and ties, allowing for the fact that part of it will be 
in a direction perpendicular to the flange. The safe working stress can 
thus be determined, and then the number of rivets required. There will 
be local bending moments on the gussets and the rivets in them in 
almost every case, and it is well to err on the side of liberality when 
designing them. 
. Outline Drawing of Main Girders.—This can now be made. The 
centre lines are first set out, and the sections of the members shown upon 
them. Next the riveting is arranged, care being taken to get uniform 
