yo STRESSES IN BRIDGE; TRUSSES 



The maximum chord stresses shown on the left of (c) are equal 

 to the sum of the live and dead load chord stresses. The minimum 

 chord stresses shown on the right of (c) are equal to the dead load 

 chord stresses. 



The maximum and minimum web stresses are found by adding 

 algebraically the stresses in the members due to dead and live loads. 



Since the diagonal web members in a Pratt truss can take tension 

 only, counters must be supplied as C7 3 L 1 2 in panel L 1 2 L 3 . The tensile 

 stress in a counter in a panel of a Pratt truss is always equal to the 

 compressive stress that would occur in the main diagonal web member 

 in the panel if it were possible for it to take compression. Care must 

 always be used to calculate the corresponding stresses in the vertical 

 posts. 



Graphic Resolution. The stresses in a Warren truss due to dead 

 loads are calculated by graphic resolution in Fig. 47. The solution is 

 the same as for ceiling loads in a roof truss. The loads beginning with 

 the first load on the left are laid off from the bottom upwards. The 

 analysis of the solution is shown on the stress diagram and truss and 

 needs no explanation. 



From the stresses in the members it is seen (a) that web members 

 meeting on the unloaded chord have stresses equal in amount but op- 

 posite in sign, and (b) that the lower chord stresses are the arithmetical 

 means of the upper chord stresses on each side. 



The live load chord stresses may be obtained from the stress dia- 

 gram in Fig. 47 by changing the scale or by multiplying the dead load 

 stresses by a constant. 



The live load web stresses may be obtained by calculating the left 

 reactions for the loading that gives a maximum shear in the panel (no 

 loads occurring between the panel and the left reaction), and then con- 

 structing the stress diagram up to the member whose stress is required. 

 In a truss with parallel chords it is only necessary to calculate the stress 

 in the first web member for any given reaction since the shear is con- 

 stant between the left reaction and the panel in question. 



