294 



MAXIMUM SHEAR IN A PRATT TRUSS. 



Art. 149. 



will come at 7 unless some loads come on or go off the span. Even 

 in the latter case, the effect upon the shear in 5-7, and therefore 

 upon the stress in 5-6, will be slight. Hence we may always test 

 for positions giving maximum stress in a web member of any 

 panel (by equation (88) ) with a load at the right hand panel 

 point of the panel. 



148. Particular Cases of the Application of the Criterion 

 for Maximum Web Stresses. Equation (88) applies to any 

 simple truss with a single system of web-bracing. In Fig. 200; 



When c=0, 5-6 is vertical and we have Pratt bracing. 

 When c=p, 6-7 is vertical and we have Howe bracing. 

 When c=y 2 p, we have the Warren type of bracing. 

 When the chords are parallel i= oc and equation (88) be- 



becomes =Cr2 (89) 



In this case the chords carry no shear and therefore equation (89) 

 is the criterion for maximum shear in any panel. 1 It will be 

 noted that for parallel chords the criterion for maximum web 

 stresses is independent of G . Equation (89) applies, in Fig. 200, 

 to 6-7, because 6-8 is horizontal. 



When i equals some multiple of a panel length, equation 

 (88) is simplified. 



149. Maximum Shear in a Pratt Truss. This is a particu- 

 lar case under equations (88) and (89). The criterion for maxi- 



C 1 

 mum shear then, in any panel, is^=G 2 . In general, this equa- 



2' / 



Fig. 204. 

 lion is satisfied, as shown for (88), when there is a load P at the 



- Equation (89) may easily be derived independently of equation 

 (88). 



