198 



FRAMEWORK 



chords, and the main struts and ties are commonly made of two angles 

 placed back to back, forming a T-section, the connections being made 

 by means of plates. The upper chord should preferably be made of 

 unequal legged angles with the short legs turned out. Sfib-struts and 

 ties are usually made of one angle. Flats should not be used. Where 

 a truss member is made of two angles placed back to back, the angles 

 should always be riveted together at intervals of 2 to 4 feet. 



Trusses that carry heavy loads or that support a traveling crane 

 or hoist, are very often made with a lower chord composed of two chan- 

 nels placed back to back and laced or battened, and are sometimes made 

 with channel chord sections throughout (see Fig. 175). 



When the purlins are not placed at the panel points of the truss 

 the upper chord must be designed for flexure as well as for direct stress. 

 The section in most common use for the upper chord, where the purlins 

 are not placed at the panel points, is one composed of two angles and a 

 plate as shown in (c) Fig. 96. 



(a) 



Trusses may be fastened to the columns by means of a plate as 

 shown in (a) Fig. 94, or by means of connection angles as shown in 

 (b) and (c). The first method is to be preferred on account of the 

 rigidity of the connection, and the ease -with which the field connection 

 can be made. 



