148 RESISTANCE OF MATERIALS 



as above, but since in this case there are two rivets for each strip 

 of length c, the efficiency against rivet shear is 



_ird*q 

 ~2chp 



Similarly, for a single-riveted butt joint with two cover plates 

 the efficiency of the joint against tearing of the plate is 



o d 



e = - 

 c 



and against rivet shear is 



_ 

 ~ 



For a double-riveted butt joint with two cover plates the efficiency 

 against tearing of the plate is 



c-d 



e = , 



c 



and against rivet shear is 



chp 



The average efficiencies of various types of riveted joints as used 

 in steam boilers are given in Fig. 105. 



91. Boiler shells. In designing steam-boiler shells it is customary 

 in this country to determine first the thickness of shell plates, by 

 the following rule : 



To find the thickness of shell plates, multiply the maximum steam 

 pressure to be carried (safe working pressure in lb./ in. 2 ) by half the 

 diameter of the boiler in inches. This gives the hoop stress in the 

 shell per unit of length. Divide this result by the safe working stress 

 (working stress = ultimate strength, usually about 60,000 lb./in. 2 , 

 divided by the factor of safety, say 4 or 5), and divide the quotient 

 by the average efficiency of the style of joint to be used, expressed 

 as a decimal. The result will be the thickness of the shell plates 

 expressed in decimal fractions of an inch. 



Having determined the thickness of shell plates by this method, 

 the diameter of the rivets is next found from the empirical formula 



