74 



STRESSES IN A CANTILEVER TRUSS. 



Art. 55. 



cuts off the value of EF or R 2 on the vertical from E in the 

 force polygon. 



Using the external force diagram thus constructed and 

 beginning at joint 1 or 12 the stress diagram may be constructed. 

 The external forces in the force diagram must be laid off in 

 order proceeding around the truss diagram either clockwise or 

 counter-clockwise. Whichever direction is used in the begin- 

 ning must be adhered to throughout the solution. With this 

 system of notation it is not necessary to put arrows on the stress 

 diagram, as the order of the letters, beginning with the direc- 

 tion of any known force, indicates the direction of the other 

 forces, thus ABCDEFA proceeds around the truss clockwise 

 and gives the correct directions of all the external forces in 

 the stress diagram. The same is true of the stresses at any joint. 



It will be found that the stresses GH, LM, MN, NO, and 

 OP are each zero, and that EL=EN=EP and FM=FO=FP. 

 That GH and OP are zero is also apparent from 2 vert. comp. =0 

 at joints 3 and 11. 



The stresses will be altogether different for the wind on 

 the other side of the roof. 



The compression and tension members should be carefully 

 noted and marked on the truss diagram. 



55. Stresses in a Cantilever Truss. Fig. 52 shows a 

 bracket supporting two loads, and it is supposed to be held 



H= 16500 



Stress Diagram 



H = 16500 



e 

 4000 



Fig. 52. 



in such a way at B that there can be only a horizontal reaction 

 at that point. 



From 2 vert. comp. =0, the vertical component of the reac- 



