74° APPLIED MECHANICS 
Of the two factors 2x2 in expression (2), one is for two rivets 
and the other is for two sections of each rivet, the rivets being in 
double shear. Experiment has shown that the strength of a rivet in 
double shear is twice that of the same rivet in single shear. The 
Board of Trade, however, only allow a load on a rivet in double 
shear equal to 1:75 times the load allowed on the same rivet in single 
shear. 
If each cover strap carries half the load on the joint, it is obvious 
that each should have a thickness equal to 
4t, but in practice the straps in an ordinary 
butt joint have a thickness equal to 4. 
Fig. 84 shows a treble riveted butt joint 
with two cover straps, in which the pitch 
of the rivets in the outer rows is twice the 
pitch of the rivets in the other rows. In 
this form of joint, and in all joints where 
there are fewer rivets in the outer rows 
than in the others, there is another way 
in which the joint may fracture in addi- 
tion to those already considered, viz. the 
outer row of rivets may shear, and at the 
same time the plate may tear between the rivets of the next row. 
Considering a strip of the joint equal in width to the greatest 
pitch p. 
Resistance to tearing between rivets of outer rows=(p—d)t/,. . (1) 
» shearing =a x 5 x f= OY, pide <! auiesaiaraee 
Resistance to shearing of rivets in outer row and tearing between 
rivets of next row = 7 x 2f,+(p—2d)tfy ee ee (8) 
Resistance to crushing=5dif, . . . 1... we eee 4) 
These four expressions yield three equations to determine p and d if 
all the stresses are given, and this is more than sufficient. 
Ronit a7 
Equating (1) to (3) d= af A Eee se (5) 
Boe te 12tf, 
quating (1) to (2) and substituting from (5) p= Tbe 6d. 
7, s 
Equating (2) to (4) and substituting from (5) f,=/,. 
Since the safe crushing stress is always greater than the safe tensile. 
stress, it is evident that, with the proportions deduced above, the joint 
will have an excess of crushing strength. 
The combined. resistance to tearing of the two cover straps is 
2(p —2d)t,f:, where 7, is the thickness of each cover strap. Equating 
this to (p — d)t/; , the resistance of the plates to tearing, ¢, = 3 _ oy 
P _ 
practice this would be made hd Making p = 6d, then t,= 28 
8(p — 2d) 
