DESIGNING. 217 



I 2? 



as 1.56, then = ; = 14.7, and from the table (Gordon's formula) 

 their ultimate strength is approximately 21,320 Ibs.; their per- 

 missible unit strength therefore is - =4265 Ibs.; then the 



7060 

 area of the metal in the section required would be -7- =1.65 



square inches; the area of section for each of the assumed channels 

 13 given as 1.55, hence for the two the combined area is 3.10, 

 which is greater than required, but since it is economical con- 

 structurally to make all sections as near alike as possible, when 

 differences are small, the same size section may be employed 

 throughout in each of the pannels. The lower horizontal member 

 having a length of 30 feet and its radius of gyration being the 

 same (1.56), its length, divided by its least radius of gyration = 

 19.3, which from the table gives an ultimate strength of ap- 

 proximately 16,170 Ibs., and dividing by 5, = 3234 Ibs. permissible 



/TOO 



unit stress; the area of the metal required is =2.1, and 



3 34 



since the pair of channels selected have an area of 3.1, they would 

 be satisfactory throughout for compression members. Scaling 

 the lengths and multiplying by the unit weights of rods, channels, 

 and main post, the additional weight at the foot of the column 

 is found to be 7075 Ibs. Neglecting the slightly increased stress 

 which would be found by multiplying this weight by the secant 

 of the angle, and adding the weight directly, the total compres- 

 sion in the lower section of the column is 325,257 Ibs., or 162.6 

 tons. 



When a horizontal strut or tie is used at the base of the tower, 

 its weight must also be included. 



The horizontal reaction H at the foot of the post = W tan 0, 

 or 325,257X0.14 = 45,536 Ibs. 



TT 



The stress in each tie is sec ft, or in this square tower frame 



