124 



PRACTICAL STRUCTURAL DESIGN 



when all the panels carry equal loads, the truss then being sym- 

 metrically loaded. Instead of starting from the middle panel 

 and working to the ends, the coefficients may be obtained as 

 follows: Count each panel load = 1. One-half the number of 

 panel loads will be the reaction (expressed as fractional coefficients). 

 From one end reaction subtract 1 successively at each panel 

 joint and thus obtain the coefficient for each member in the 



Load on 



Bottom Chord 



Fig. 79 



following panel. The coefficient for the chord in any panel is the 

 sum of the coefficients of the diagonals between that panel and the 

 end support. 



The weight (W) carried by each truss member is equal to the 

 coefficient of the member multiplied by the unit panel load, P. 

 Let I = length of panel, center to center of verticals. 

 d = depth of truss, center to center of chords. 

 t = length of diagonal, center to center of chords. 



then t = VP + d 2 . 



Wt 

 Stress in diagonals = r- 



, . Wl 



Stress in chords = r 

 a 



Stress in verticals = W 



