CHAP. XII.] METHODS COMBINED. 189 



So we go through the truss and find the strains in every 

 piece. Heavy lines in the strain diagram denote compression. 

 We see at once that for this position of the weight, all the 

 upper flanges in span A B are compressed, the last lower flange 

 F 7 is also compressed, and all the other lower flanges are ill 

 tension. At the point of application of the weight P 4 , the two 

 diagonals 3 4 and 4 5 are in tension, and either side the}' alter- 

 nate in strain as far as C or diagonal 8 9. Diagonals 8 9 and 9 10 

 are both tension, and then the strains alternate to support D. 

 All the upper flanges of the right half are tension and increase 

 towards the middle. All the lower are compression and like- 

 wise increase towards the middle. 



If we go through the whole truss from A to D, the last diago- 

 nal 1 5, 1 6 should evidently pass exactly through E 2 , thus check- 

 ing the accuracy of the construction. The diagonal 6 7 crosses 

 the force line, thus causing the strain in the lower flange to 

 change from tension in F x 5 to compression in F t 7. The point 

 of inflection, therefore, falls to the right of diagonal 5 6. 



The reaction at B diminishes greatly the strain which would 

 otherwise take effect in 7 8 and E 8 ; while the reaction at C 

 reverses the strain which would otherwise take effect in 9 10 

 and diminishes E 10. We recommend the reader to follow 

 through carefully the strain diagram, Fig. 86 (a). 



A series of figures similar to Fig. 86 (a) (in the present case 

 seven separate figures) will give completely the strains due to 

 the rolling load. A table may then be drawn up containing 

 the strains due to dead load, and the maximum strains due to 

 live load in every piece, and the total maximum tension and 

 compression in every piece may then be found. [Com/pare 

 Art. 12, Fig. 7.] 



For the supports fixed, instead of B and C tipping, the pro- 

 cess is precisely similar, except that we have to make use of 

 the formulae of Art. 122. The reaction at A will then be 

 somewhat less than in the present case ; the inflection point is 

 therefore found further from the right support B ; it may be 

 even to the left of diagonal 5 6, in which case (see Fig. 86, a) 

 we should have tension in upper flange E 6. The reaction at 

 B would then be still positive, but greater than E E 1} while C 

 would be negative and no longer equal to B, and D would be 

 positive. We should thus have 7 8 tension and E 8 tension ; 

 F 7, as before, compression, 8 9 compression, and 9 10 com- 



