122 



PRACTICAL STRUCTURAL DESIGN 



In Fig. 78 (6) two panels have been added and it is assumed 

 the panels are loaded equally, the loads being concentrated at 

 the joints. The end panels not only carry the reactions from the 

 original middle triangle but their own loads in addition. The 

 diagonal ties ef are more heavily stressed on this account than are 

 the ties cd, the stress being tension, for the whole load is sus- 

 pended at the end points /. This increased tension throws on the 



Load an Bottom Chord 



Load on Too Chord 



middle of the upper chord the added load of the end panels in 

 addition to the load carried by the middle triangle, so the middle 

 panels of both upper and lower chords are more heavily stressed 

 than the end panels. 



The coefficient for ad and cd = \. When the unit load, 1, at 

 the next joint is at c the coefficient for ce = | + 1 = f , but if the 

 load is at e the coefficient for ce = \. The coefficient for ef is the 

 sum of the coefficient for dc and the unit load on the line ce. The 

 above explanations are based on the load being on one chord only. 

 When there are loads on both chords there should be two sets of 

 coefficients written down and their sum used. Using the positive 

 sign (+) to indicate compression and the negative sign (-) to indi- 

 cate tension, the algebraic sum is meant when the stresses are 

 opposite in kind. To check what has been stated, note that there 

 are three panel joints carrying equal loads, so f of the total load 



