JOINTS AND CONNECTIONS 199 



In proportioning the web member carrying tension the area 

 of one rivet hole is deducted from the area of the member. In 

 the web member subjected to compression the whole area is taken, 

 for the rivet is assumed to fill the hole. The resulting direct 

 stresses are shown in the figure. Provided the rivets are driven 

 on the line of the center of gravity of the member the ascer- 

 tained direct stresses will be realized. If the rivets are driven 

 to one side of the axis of the center of gravity an eccentricity 

 will be developed which may very seriously increase the stress. 

 Where angles are used to resist direct 

 stress, and connected through one leg 

 only, the gauge line for the rivets should 

 be set in as close to the back of the 

 angle, or as near to the center of gravity 

 axis as possible. This matter is of funda- 

 mental importance and yet it is habitu- 

 ally disregarded in structural work. 



The rivet clearance for machine driving is shown in Fig. 116. 

 It is customary to use so-called "standard gauges" for angles, 

 pitching the rivets from the back of the angle a distance some- 

 what greater than the half width of the leg. In the case of the 

 web members shown in Fig. 115 the dimension D would be }$ 

 in. Adding to this the thickness of the outstanding leg, we obtain 

 1J ins. as the permissible gauge of these angles. This coincides 

 exactly with the center of gravity axis of the angles and if the 

 rivets were so placed the fiber stress due to eccentricity of the 

 line of rivets would be entirely eliminated. The method to use 

 in figuring the stress due to eccentricity in the line of rivets is 

 given in detail in another chapter. 



To avoid eccentricity as much as possible in a group of rivets 

 the forces should act through the center of gravity of the group. 

 The best way to obtain the center of gravity is by using the 

 method of moments, as shown in Fig. 117. 



Having found the center of gravity of a group of rivets find 

 the axis through the center of gravity of the member connected 

 to the plate or angle forming the connection. Extend this axis 

 through the group of rivets. If it does not pass through the 

 center of gravity of the group it can be replaced by a force equal 

 in amount and parallel, which will pass through the center of 

 gravity. There will be a moment developed equal to the load 



