DESIGNING. 195 



to be afterwards readily assembled in the field, and at which 

 time only the riveting at the junction- or panel-points is neces- 

 sary. 



At this time the two favorite types are undoubtedly the Z bar 

 and the double-channel latticed column, both possessing in- 

 dividual advantages for structural purposes. 



Where the double channel is used, it should be so latticed 

 as to prevent individual weakness and that all parts should act 

 as a unit in the combined section. That there may not be a 

 tendency in the channels to bend between the points of bracing, 

 the distance / (Fig. 53) should be made to equal the total length 

 of strut multiplied by the least radius of gyration of a single 

 channel and the product divided by the least radius of gyration 



rL 

 of the whole section, or l = ~jy, 



where / = length between bracing; 

 L = total length of strut; 



r= least radius of gyration of a single channel; 

 R = least radius of gyration of the whole section. 



In practice the distance / is taken considerably less, the dis- 

 tance as determined by the formula being more or less used as a 

 guide. 



The Z-bar column, consisting of four "Z "-shaped bars riveted 

 to a web plate possesses so many structural advantages that for 

 building purposes it has had a wide popularity and extensive 

 use, and whether the tendency toward standardization will 

 render it obsolete remains to be seen, although this would seem 

 the case. 



In lengths ranging from 64 to 88 radii, from careful test, 

 an average ultimate resistance of 35,650 pounds was determined 

 for iron columns, and an assumption that ^ steel bars will have 

 some 20% higher value. Their great adaptability for making 

 connections with ether columns and members, their accessibility 

 for inspection, painting, and repair, the small number of rivets 



