68 



STRESSES IN THE SIMPLEST TKUSS. 



Art. 50. 



of P. Transferring the directions of Z> 1 and D 2 to the truss dia- 

 gram, we find they act toward the joint. 



Laying out R^ and drawing L and D 1 parallel to their direc- 

 tions in the truss diagram, the force triangle for joint 1, deter- 

 mines the stresses L and D^. The directions are counter-clock 

 wise as determined by R l - J transferring these to the truss, D^ 

 acts toward the joint and L away from the joint. 



,t 



Truss O/ayram 



Treating joint 3 in like manner, it is found that each one of 

 the stresses has been determined twice. To avoid this unneces- 

 sary labor, the force polygons (triangles in this case) may be 

 drawn in one diagram, called a stress diagram, 1 as shown. 



Starling with any known force, in the stress diagram, the 

 direction in ivhich any stress acts at a certain joint, can be deter- 

 mined by going around the force polygon for that joint. Since 

 the lines are common to two joints, the directions which they 

 show will be opposite to each other ; if these directions are toward 

 the two joints in the truss diagram, the stress is evidently com- 

 pressive, and if they are away from them, the stress is tensile 

 (7). Thus are the stresses fully determined. 



Tensile and comprcssive stresses may be distinguished from 

 each other by means of arrows in the truss diagram, or by lines 

 of different colors, or by any other convenient sign. The usual 

 signs used are + and (as shown in Fig. 48), which have noth- 



1 Stress diagrams are also called Cremona diagrams, Maxwell diagrams, 

 and reciprocal figures. 



