THE IMPROVEMENT OF RIVERS. 



On the lower Seine was another system largely composed of needle dams, but these 

 were replaced between 1878 and 1888 by dams of much higher lift, closed by gates or 

 curtains, only three dams being now closed by needles. These support lifts of 9.1 feet, 

 9.3 feet, 10.5 feet. 



On the Moldau, in Bohemia, a system is now under construction which includes 



needle dams, and which has been briefly described 

 in another chapter. 



Other examples are to be found in France, 

 Russia, and other countries. 



Calculations for Trestles. The trestle of a 

 needle dam is practically a cantilever girder or 

 truss, supporting a load at its end. It may thus 

 be treated as an ordinary truss with tension and 

 compression members and stiffening braces. 



If BADC (Fig. 14) represent a trestle sup- 

 porting a load P from the needles, A B will be in 

 tension and AC in compression, while ADC will 

 support the footway or track and the other mem- 

 bers, as EF and CH, bind the whole together. 

 These last must be made strong enough to support blows from the drift, etc., and are 

 usually made of the same section as AB. 



To find the strains in AB and AC, taking moments about C we have 



Pxd 



FIG. 14. 



Pxd^ABxCJ, or AB- 



CJ 



(tension). 



Similarly for AC, taking moments about B, we have 

 PXe=ACxBK or AC 



PX e 

 . . (compression). 



Then if / = length of AC in feet and r = the radius of gyration of its section in 

 inches, the ultimate strength per square inch may be found from the usual formula, 



50000 



i + 



24000 X r 1 



Many needle trestles have been built with a second diagonal brace from B to D. 

 In this case the strains will be divided between BA, AD, DC, AC, and BD, and may- 

 be found from equations of moments or by graphics. It is preferable, however, to 

 use the simpler form of trestle, as it requires fewer pieces, and these, having in conse- 

 quence to be made stronger, will resist shocks and wear better. 



The omission of a continuous axle from B to C for all trestles except those of 

 Boul6 and Camerd dams, also appears desirable, as trestles have been in successful 

 use for many years without axles, and the use of them complicates and adds to the 

 ironwork. 



