JOINTS AND CONNECTIONS 189 



beyond the plates is formed into a head by means of presses in 

 the shop or by hammers in the field. 



A rivet should never be used in tension when it is possible to 

 avoid so using it. In very exceptional cases a rivet may have to 

 be so used and then the allowable tensile stress should not exceed 

 8000 Ibs. per sq. in. The body of the rivet when used in tension 

 may be amply large, but the thickness of the head must be inves- 

 tigated to determine whether it will be sheared by the pull on 

 the rivet, this shearing being on a circle having a diameter equal 

 to the diameter of the body of the rivet. The head of the rivet 

 must be thick enough to withstand the shear. 



The reason for not using rivets in tension is that the unequal 

 heating and cooling during the process of fabrication of a member 

 which is rivetted together sets up in the rivet expansion and con- 

 traction stresses of unknown amount. It has happened many 

 times that rivet heads were snapped off when cooling and they 

 snap off sometimes in extremely hot weather and in extremely 

 cold weather. With such stresses existing in rivets it is mani- 

 festly dangerous to further impose on them a direct tensile stress, 

 for the rivet heads may be on the verge of snapping off and any 

 slight additional load may cause them to go. It is best to use 

 bolts in joints in which rivets would be subjected to tension when 

 field rivets are driven and if shop driven rivets are under tension 

 the stress should be very low. Shop driven rivets are pressed 

 into place and all the conditions in the shop make it likely that 

 the work is uniform. It is impossible, however, to secure proper 

 conditions in the field, for the heat cannot be controlled and many 

 rivets are burned. They are thrown through the air and driven 

 after some cooling has taken place. The hammering may be 

 uneven and the rivets may not be hot enough when driven to 

 be forced to completely fill the hole. 



Rivets are assumed to act entirely in shear and all computa- 

 tions for rivetted joints are based on this assumption. There 

 can be no doubt that friction is a big factor in rivetted joints, 

 the rivets in shrinking drawing the plates together and holding 

 them in contact so that friction between the plates assists the 

 shearing strength of the rivets. The assistance obtained from 

 friction is neglected in computations and merely increases the 

 factor of safety of the joints. 



Rivets may fail by bending. This effect, however, is not 



