GRAPHIC STATICS 211 



is shown on the right of the load line. The loads are set off verti- 

 cally to scale and when the construction of the stress (reciprocal) 

 diagram is completed the lengths of the lines are measured by 

 the same scale and the amounts of the stresses found. 



The character of the stress in each member must be determined 

 and this can only be done by some study and by following the 

 loads. It is a help, at first, to make a free-hand diagram for each 

 joint and place arrow heads on each line, remembering that if 

 the frame is to be in equilibrium it must be possible to start at 

 one joint and follow the lines clear around the figure to the start- 

 ing point. The student is advised to make the figures now to be 

 described, free hand, and move the pencil as he follows the 

 directions. 



The kind of stress is to be found. Start with the joint between 

 B and D, sketching it on the stress diagram. Here there is a 

 vertical load of 2000 Ibs., from B to D on the reciprocal (stress) 

 diagram. Move from D to E and the arrow point is seen to point 

 to the joint. The stress is compressive. Move from E to C. 

 The arrow point is towards the joint, so the stress is compressive. 

 Move from C to B. The arrow point is towards the joint and the 

 stress is compressive. 



Take the joint at the foot of the truss, between A and B. This 

 should have been taken first, but the second joint gave the best 

 illustration for a beginner. Start from B on the load line and 

 move to C. This is towards the joint, so the stress is compressive. 

 Move from C to F on the load line. This is away from the joint, 

 so the stress is tensile. 



It has been determined that the stress in DE, in D'E', in CE, 

 and in C'E', is compressive. A vertical member connects the 

 upper and lower joint. Assuming this member to be cut, will 

 the joints be spread apart or will they close up? If they will be 

 spread apart, then the action of the forces at the joints will cause 

 tension in the member. If they will close up, then the member 

 will l>e in compression. Applying this reasoning, it is seen that 

 the stress in E E' is tensile. The character of stresses in all 

 members of trusses may be determined by such reasoning. 



The stress in the vertical member may, however, be deter- 

 mined by the first method. On the load line take the load of 2000 

 Ibs. between D and D'. Go from D' on the load line to E' and as 

 the arrow point is towards the joint the stress is compreseive. 



