STRUTS AND BRACING 2 1 1 



to twice the moment of inertia of one channel, which may be found in 

 the table of properties of channels given in the handbooks. 



Having the moment of inertia I, the radius of gyration of the 

 column is given by the formula 



r = 



With channels placed back to back and laced, the radii of gyra- 

 tion about the two axes are equal when the clear distance is equal to 

 about 3 inches for 5~in. channels, and 10 inches for 15-in. channels. A 

 common rule is to space the channels about eight-tenths the depth. 

 With channels placed with backs out and laced, the radii of gyration 

 about the two axes are equal when the clear distance is about equal to 

 5 inches for 5-in. channels, and 13 inches for 15-in. channels (see 

 Cambria Steel, 1903 Edition, p. 217). 



The moment of inertia of a 4-angle laced column, about an axis 

 perpendicular to the lacing and through the center of the post, is given 

 by the formula 



where I' = moment of inertia of one angle about an axis through its cen- 

 ter of gravity and parallel to the given axis, A = the area of one angle 

 and d = the distance from the center of gravity of the separate angles 

 to the center of gravity of the column. The moment of inertia about 

 the other axis is found in a similar manner. 



STRUTS AND BRACING. Have struts are very commonly 

 mde of four angles laced, made in the same way as the 4-angle posts, 

 Fig. 100. Eave struts made of single channels are more economical, 

 and are equally as good as the laced struts for most cases. End rafters 

 are commonly made of channels. The sides, ends, upper and lower 

 chords are commonly braced as shown in Fig. 81. The bracing in the 

 plane of the lower chords should preferably be made of members cap- 

 able of taking compression as well as tension. The diagonal bracing 

 in the plane of sides, ends, and upper chords is commonly composed of 

 rods. Initial tension should always be thrown into diagonal rods by 



