CHANOINE WICKET DAMS. 



233 



the same when the wicket is upright as when it is lowered, the actual difference being 

 small. 



Thus in Fig. 16, according to this supposition, the direct load on the chain will be 

 P . sec ft, where P = the total water pressure on the wicket, and fi = the angle between 

 the chain and the direction of P. The water should be assumed at pool level, or above, 

 with no water below, so as to provide for the most unfavorable condition. This load is 



FIG. 16. 



usually a maximum on the weir of a dam, as its wickets are lowered first, and by the 

 time the pass is reached the difference between the upper and lower pools has been much 

 reduced. The crab, however, should have considerably more than the theoretical 

 power required, and the trestle should be designed accordingly, since the wickets have 

 sometimes to be pulled out of deposits of sand or mud. 



To find the strains in the trestle, assume the line' of pull to pass through D and 

 resolve it there into two components, Q and R, along AD and DC. 



Taking moments about C, we have 



Q X d = AB X CJ, or AB = 



CJ 



(tension). 



Taking moments about B, we have 



Qxd 



Q X d = AC X BK, or AC = (compression). 



The upward pull on the anchorage at B where 7- = angle of inclination on AB 

 strain in AB X cos ?. 



As mentioned in the calculations for needle trestles, it will be found that a con- 

 siderable excess of strength must be supplied for stiffness, and experience can prove 

 the only guide in such matters. On the Meuse dams, with a head of about 7^ feet, the 



