DESIGNING. 209 



which divided by the area of the whole shape A , = 23.88, gives 

 the neutral axis of the girder, 



So6.8 

 c = ~ =21.2 inches; 



\)* 



and the moment of inertia, 



the modulus of rupture is 



/ 8658.2 



and the safe resisting moment, 



RX S = 408.4 X 10,000 = 4,084,000 inch- pounds, 



and to withstand a maximum bending moment at the post of 

 3,270,612 foot-pounds. 



An angle with dimensions 6X4X J would have been closer to 

 the requirements, but the difference in weight is small, and since 

 the circular girder also sustains, in this case, a small wind stress 

 beside and is the most important member of the structure, the 

 1/16 thicker angle will be retained, and it is also well to reenforce 

 the girder with stiffening angles over the point of support as 

 shown in Figs. 51, 56, 57. 



Wind Stress in the Girder. The wind stress in the girder 

 is produced by the action of the wind upon the sides, and is 

 usually considered as being exerted upon the diametral plane of 

 the cylinder and acting as a cantilever beam, the formula for 



o c ^./ 

 which has heretofore been given as extreme fibre stress 5= , 



where A = height of cylinder ; 



r = its radius in inches; 



/ = the thickness of the shell. 



