210 



FRAMEWORK 



The American Bridge Company's specifications for lattice bars for 

 single and double lacing are shown in Fig. 104. 



Maximum Distance C for given thickness of bar. 



Single lacing should make an angle of not less than 60 degrees 3 

 and double lacing, riveted at the center, not less than 45 degrees with 

 the axis of the member. These specifications are standard. 



- The properties of angles, I beams and channels, and of Z-bar, 

 Larimer and Gray columns are given in the manufacturers handbooks. 

 The moment of inertia of two channels placed back to back and laced, 

 as in (a) or (b) Fig. 98, about an axis parallel to the webs and through 

 the center of gravity of the section, is given by the formula 



I = 2 I' + 2 Ad 2 



where /' = moment of inertia of one channel about an axis through 

 its center of gravity and parallel to the given axis, A = the area of one 

 channel, and d = distance from the center of gravity of one channel 

 to the center of gravity of the column. The lacing is omitted in find- 

 ing the moment of inertia and the area of the section. The moment of 

 inertia of the column about an axis perpendicular to the webs is equal 



