APPENDIX.] 



PIVOT SPAN. 



375 



Of course, for this condition of things the ends must always 

 be bolted down. 



It is sometimes customary to raise the ends by an apparatus 

 for that purpose, after closing the draw, until the proper pro- 

 portion of the dead load takes effect also as a positive reaction. 



We can easily find the strains in this case also by adding 

 the numbers in the last horizontal line of our table for bridge 

 shut, with their proper signs, and taking half the results for a 

 new line for dead load strains. The resulting strains can 

 then be found precisely as in the table of Art. 8 (Appendix). 

 We must also find the strains for bridge open as above, and then 

 take the greatest strains of each kind from these two tables. 



In this case the strains would be differently distributed. 

 Flange E will be always in tension, A and K always in com- 

 pression ; the compression in B C and D will be somewhat 

 greater than above, and the tension in the same flanges less. 

 The reader can easily deduce the strains for this case from the 

 two preceding tables. 



If the truss may act as a girder over four fixed supports, 

 we should, in order to be certain of the maximum strains, 

 make the calculation for this case also, using the formulae of 

 Art. 122. This is unnecessary, however, if the supports B and 

 C can never sink far enough to strike* the turn-table, or be im- 

 peded in their motion. 



4:th. STKAINS IN THE DIAGONALS. 



We may find the strains in the diagonals also for each 

 weight separately, both for bridge open and shut ; and a pre- 

 cisely similar method of tabulation will give the strains. 



It will here be found preferable to make a series of dia- 

 grams, as illustrated in Fig. 86, Art. 124, for each weight and 

 its own system of triangulation. We obtain thus the diagonal 

 strains, and at the same time check the results obtained for the 

 flanges above. 



If we wish to calculate the diagonals, it will be better to find 

 the resultant shear acting upon the diagonal, and multiply it by 

 the secant of the angle the diagonal makes with the vertical. 



