CHAPTER II 



DOUBLE INTERSECTION TRUSSES 



Double Intersection Trusses are made up of two or 

 more simple trusses and the influence diagram for any 

 member is found by first drawing the influence diagram 

 for each simple truss and then connecting these diagrams 

 so as to form one diagram. This is shown in the fol- 

 lowing examples. 



Top Chord of Whipple Truss. The Whipple truss shown 

 in Fig. 20 is made up of two simple trusses as indicated 

 by the full and dotted lines of the truss diagram. 



The chord U^Ui is a part of the top chord of both 

 trusses. Considering it as a top chord member of the 

 truss shown by the full lines, the center of moments is 

 at 1/4 and the length of the cut stringer is L 4 Le. The 

 influence diagram DAFBD is constructed according to 

 the general rule. When the member UzU* is considered 

 as a part of the top chord of the dotted truss, the center 

 of moments is at Ls, the length of the cut stringer is Z/oI/7 

 and the influence diagram is DA'F'B'D'. This diagram 

 is constructed upon the line DB of the first diagram by 

 making D'C' equal s'. Now a load at Z/2 does not affect 

 the members of the dotted truss and hence the ordinate 

 ef in the influence diagram for the truss shown by full 

 lines is the correct moment for a unit load at L 2 . A load 

 at La by similar reasoning has for its true ordinate e'f 

 in the influence diagram for the dotted truss. For a 



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