222 



PRACTICAL STRUCTURAL DESIGN 



When reciprocal figures do not close there is either another mem- 

 ber required or the work of the draftsman is poor. 



A B 



6 H 



Fig. 141. Pratt Deck Truss. 



The diagrams for the Pratt truss, shown in Fig. 140 an3 Fig. 

 141 are readily followed. 



In Fig. 142 and Fig. 143, of bowstring trusses, the student 

 should observe how nearly uniform the stress is in the chord for 

 each panel. The web stresses are very small, and, as the arch 



Fig. 142. Through Bowstring Truss and Girder Stringer. 



more nearly approaches a parabola the lower will be the stresses 

 in the web members and the more nearly equal will be the stress 

 in each panel length of the chords. The student should make 

 diagrams for Pratt, Howe, Whipple and Warren trusses with 

 parallel chords and compare these with the same framing of web 



Fig. 143. Deck Bowstring Truss. 



members in bowstring trusses of the same span, carrying the 

 same loads. 



