Art. 127. 



STRESSES IN SIMPLE TRUSSES. 



239 



Collecting the above results in the table on p. 238, carefully 

 noting the signs, the resultant maximum stresses are found : they 

 are symmetrical. The stresses for D 1 are all compressive. The stress 

 in Z> 2 is tensile except for load P 5 ; the maximum tensile stress in 

 D 2 occurs when the live load extends from the right support into 

 the panel of D 2 . There can be no compressive stress in Z> 2 because 

 P 5 is the only load producing compression and its stress is less 

 than the dead load tensile stress: the minimum stress is 12.7+ 

 3.8 8.9. The maximum compressive stress in D 3 occurs when 

 the live load extends from the right support into the panel of D 3 , 

 and the maximum tensile stress for the complementary loading. 

 In the second case the live loads P 4 and P 5 produce 7.6+3.8=11.4 

 tension ; the dead load compression being less than this, the result- 

 ant is 11.44.2=7.2 tension. D z is a counterbrace. 



Since the stresses in D 2 and D 5 are the same and likewise in 

 J> 3 and Z> 4 , the stresses of one kind may be found on the right of 

 the center and those of the other kind on the left. According to 

 the above table, P 5 should act for a maximum compression or a 

 minimum tension in Z) 2 ; P 5 and P 4 should act for a maximum 

 tension in D 3 ; P lt P 2 , and P 3 should act for a maximum compres- 

 sion in Z> 3 ; P t , P 2 , P 3 , and P 4 should act for a maximum tension 

 in Z> 2 : and all loads should act for a maximum in D 1 . The gen- 

 eral rule, for position of live load, is quite similar to that for a 

 solid beam (102). 



128. Position of Live Load for Maximum Chord Stresses. 

 Any load on a truss, supported at its two ends, will bend it down- 

 ward, thus producing positive bending moment at any section. 

 It follows that, in order to have a maximum bending moment at 

 any joint of a truss, as much load as possible should be on there 

 should be a full load, for live load. This rule is the same as for a 

 solid beam (102). Now since the stress in any chord member is a 

 maximum when the moment at a joint is a maximum, there must 

 be a full load for any chord stress. 



If the truss bends downward it is apparent that the top chord 



must be in compression and the bottom chord in tension. If in 



the example of the previous article, the chord stresses had been 



calculated, it would have been found that each panel load pro- 



duces stress of the same kind in each chord. 



Of course, the above rule can not be applied to continuous 

 trusses, cantilever trusses, and trusses with inclined reactions 

 like the three-hinged arch. 



