252 APPLIED MECHANICS | 
Wohler’s éestnaae: have been proposed to take this fact into account, * 
Either of the two following may be used :— 
f=safe working stress in the bar in tons per square inch, 
minimum stress — 
maximum stress. 
r= the ratio 
Claxton Fidler’s Formula (slightly modified in form).—For the flanges — 
of girders up to 100 feet span and for all web members, f == for 
ib “f : 
tension members, and, f= ar for compression members. 
-7 
For the flanges of girders over 100 feet span, f= er 
members, and, f= for compression members. 
ve 
13-3r 
The Launhardt-Weyrauch Formula.— 
f= 5( 1+ r) for tension members, 
f= 43(1 + 5 ) for compression members. 
The gross area of compression members is to be taken, and suitable 
allowance made for the tendency of long struts to buckle. : 
The working stresses found by either, or both, of these formule — 
are tabulated on the stress sheet. 
Cross Sections of Members.—Using the safe working stresses as found — 
above, design the members of the main truss in the following order :— 
Tension Boom.—Find the necessary area at the centre, and determine 
the section there. Set out a diagram similar to the lower part of Fig. 
331, p. 227, showing the variation in the force in the boom throughout 
its length, and show on this diagram the worth of each element in the 
boom section, using the safe working stress in each panel in turn, after 
deducting the area lost through rivet holes. This diagram determines 
the number and length of the boom plates. 
Compression Boom.—Make a similar diagram for the compression 
boom, using, however, the gross area of the section. In the present 
design there is no need to make any allowance for buckling. 
Joints in Booms.—Arrange for suitable grouped joints in the booms. 
The proposed lengths of plates should be shown on the diagrams. They 
must of course be convenient from a practical point of view. Design 
the riveted joints and find their efficiencies. They must equal in strength 
the plates which they connect. The safe working shear stress may be _ 
taken as 0°8 of the safe working stress in the bar. The safe working 
bearing stress may be double that of the shear stress. 3 
Diagonal Ties.—Find a suitable section, distributing the load over 
one, two, or four bars as may appear necessary. Design the riveted joint — 
in the end of the tie. If the stress reverses in a tie it becomes a strut, 
and it must also be considered as such. 
* For a full discussion of this subject the student is referred to Professor 
Claxton Fidler’s treatise on ‘‘ Bridge Construction,” 
for tension 
