DESIGNING. 



215 



The vertical gravity load at the top having been estimated 

 at 958,000 Ibs. for the four-post tower, the weight at the top of 

 each column is 239,500 Ibs. 



W sec 



The vertical reaction of this load through each post is sin 0, 



4 



or 239,500X1.01. This stress is compression and must be com- 

 bined with the maximum compression at the foot of the column 



,P-R 



P-R 



p-R =1748.51 



_P-R^ 

 1748.51 



C P- R; 



P-R 



P=5079.79 



P= 5079. 79 



P-R 



. 1748.51 

 P-R=1748.51 



FIG. D. 



FIG. E. 



and produced by the wind, graphically found to be 78,682 Ibs.; 

 hence the maximum compression from the combined gravity 

 and wind stress and exclusive of the weight of the post itself 

 and dependent tower members is 318,182 Ibs., or 159 tons. It 

 is generally considered good practice to limit the length of any 

 section of main posts to 100 times its least radius of gyration, 

 and in such case an 8-inch rectangular column, whose radius 

 of gyration is r=2.3i(r 2 = T V^ 2 ), might be used in length as 

 great as 23 feet. Having determined upon 18 feet as the longest 



section of the tower column, =7.7, and from the table according 



to the Gordon formula, by interpolation, the ultimate strength 

 for such length is taken as 32,220 Ibs.; with a factor of safety 



