134 THE BEHAVIOR OF RIVETED JOINTS UNDER STRESSES. 



suring that permanency is being fully provided for. Not that iron and steel are not 

 enduring, but safety lies in the adoption of what appears to be low working unit 

 stresses, in order to avoid excessive local strains not contemplated in the design of 

 the structure. 



Referring specifically to the strength of those parts on which reliance is placed 

 in the design of a riveted joint, first comes the tensile strength of the plate, taken 

 as a whole. Next to this the strength of the steel between the rivet holes, that is, on 

 the net section. On the latter section the strength per unit of area is not the same 

 with different pitches. It may be greater or less than that accredited to the gross 

 section, per square inch. It is also modified according to whether the holes are 

 drilled or punched, and may be greater in one case or the other according to the 

 pitch of the rivets or, rather, the distance from rivet holes to rivet holes. It is not 

 likely that the strength with punched holes will be greater than with drilled holes 

 in practice, since very close pitched work is required to bring about such a result, 

 much closer than other considerations in a good joint render permissible. The 

 reason, however, that a punched plate may display greater strength than a drilled 

 one is found in the hardening of the steel by the punch and die at the sides of the 

 holes. 



The tensile strength on the net section of the plate is usually greater than on 

 a strip of uniform width, several inches in length. The increased area of metal on 

 each side of the center line passing through the rivet holes has a reinforcing effect 

 on the net section of the plate. This gain in strength is a substantial one in single- 

 riveted work, and in multiple riveting when the same pitch is maintained in the 

 different rows. 



The reinforcement, as natural to suppose, is greater in close-pitched than in 

 wide-pitched riveting. The reinforcement is at the sides of the holes, but if they are 

 very far apart there results a loss instead of a gain. The reinforcement is there- 

 fore not a fixed amount, but depends upon the proportions of the joint. It follows 

 that the strength of a single-riveted joint, whether butt or lap, will not, with 

 changes of pitch, remain proportional to the ratio of net to gross section of plate. 

 As the net section increases relative to the gross section there is a loss in the rein- 

 forcement in strength per inch of area on the net section. Instances have been met 

 in which one of these features practically compensated for the other and resulted 

 in furnishing joints of several pitches, each of substantially the same efficiency in 

 terms of the tensile strength of the solid plate. 



When the pitch of the riveting has been considerably increased, as witnessed in 

 butt joints with double covers, in which one strap is considerably wider than the 

 other, joints which fail by the rupture of the plate not infrequently show a dimi- 

 nution in strength on the net section. The plate tears apart at the outside row of 

 rivet holes, in detail. The presence of a few rivets, wide spaced, in the outside row 

 promotes tearing of the plate, the line of rupture starting at a rivet hole and reach- 

 ing an advanced stage before the plate at the middle of the pitch is separated. 



It will be understood from this that tables of proportions of riveted joints 



