.-7'J NOTE TO ART. 124:. [APPENDIX. 



Concero a section through the girder at, say, the centre of 

 flange A. It cuts 4 pieces, but, since the weight P t acts only 

 through its own system of diagonals, only three are strained. 

 The point of moment for A is then at 6, the intersection of the 

 other two strained pieces. The strain, then, in A x by its lever 

 arm = 7.415 x 10. The lever arm of A is 6.965 ; hence 



A x 6.965 = 7.415 x 10, 

 or A =+ 10.64 tons compression, 



because the upward reaction acting with 6 as a centre of rota- 

 tion tends to compress A. 



This strain evidently acts through both A and B, since both 

 these flanges are included by the two diagonals of the system 

 for P! ; hence also, B = + 10.64 tons. 



For flanges C and D, since 7 8 is the strained diagonal, 8 is 

 the centre of moments. The same reaction acts now with the 

 lever arm 30 to cause compression, and P x acts with the lever 

 arm 20 to cause tension. We have then 



C x 8.955 = + 7.415 x 30 - 10 x 20, 

 or C = D = 2.5 tons compression. 



Now we come to the centre span, and must carefully observe 

 the following points. Since D has been found to be compres- 

 sion for P : , we see at once that the whole upper flange for the 

 span A B is for this weight in compression. Diagonal 8 9 is 

 therefore in tension. Were there no vertical strut at B, this 

 would cause compression in 910. But brace 910 cannot by 

 construction take compre^on. The strained pieces cut by a 

 section through E are then E, B 11 and K, which give us the 

 centre of moments at B for strain in E. Observe, that were 

 it not for the vertical, we should have had 10 for the centre 

 of moments ; or, with the vertical, had D been found tension, 

 8 9 would have been compression ; there would then have been 

 no strain in the vertical, that being incapable of tension, and 

 diagonal 9 10 would have been strained, thus giving us also 10 

 for the centre of moments. Attention to the above is necessary 

 in order to properly pass from the span A B into the middle 



r-pilM. 



We have then for strain in E 



* 



E x 10 = 7.415 x 40 - 10 x 30, or E = - 0.34, 



