194 PRACTICAL STRUCTURAL DESIGN 



while the plate can carry the whole. At the line of the two rivets 

 one-half (three-sixths) of the stress is carried by shear while the 

 remainder of the plate can carry five-sevenths of the load. At 

 the line of the three rivets all the stress can be carried by shear 

 while the plate has area enough remaining to carry four-sevenths 

 of the load. Therefore, by this arrangement of rivets it is neces- 

 sary to deduct only the width of one hole, whereas if the rivets 

 are arranged as in Fig. 113 it will be necessary to deduct the width 

 of three holes. 



When an arrangement is made such as that shown in Fig. 114 

 the splice plates will be a little longer and possibly a little thicker 

 than the plates used in an arrangement such as that shown in 

 Fig. 113. This is, however, offset by the fact that when material 

 is added to overcome the cross-sectional area cut out by holes, 

 the material is added to the area of the member throughout 

 its whole length. The saving effected by a design like Fig. 114 

 is two rivet holes when compared with the design shown in 

 Fig. 113. 



The efficiency principle is applicable only to splices and in 

 connections of rivetted trusses. There are many cases in which 

 the efficiency principle must be disregarded, except as it affects 

 the sizes of rivets used. 



A rivetted joint fails by shearing of the rivets or by the metal 

 between the rivets giving way. The distance between centers 

 of rivets is termed the pitch. It is fixed partly by considering 

 the shearing strength of the metal. It is fixed partly by arbitrary 

 specifications. It is fixed partly by the requirement that the 

 section between rivet holes must be fully as strong as the rivet. 



Referring to the rivets shown in Fig. 113 and Fig. 114. The 

 plate if it tears will tear at the weakest point. This may be on 

 the vertical line joining centers of the rivets and may be on a 

 zigzag line joining centers of adjacent lines of rivets. Experi- 

 ments apparently indicate that rupture is as likely to occur on 

 the zigzag line as on the vertical line, the cross-sectional net 

 area determining this matter. By net area is meant the area 

 measured between edges of holes. Some specifications require 

 that the net area on the zigzag line exceed the square area by 

 30 per cent, but general rules should never be followed, except 

 when required by specifications. It is poor policy for designers 

 to follow arbitrary rules when they are competent to investigate, 



