A Treatise On The Principles And Practice Of Dock Engineering

STEEL GATES. 335 7 tons as the nett positive weight keeping the leaf down, and preventing its rising off the heel pivot, when the gates are open and the water level is not lower than the top of the buoyancy chamber. When the gates are closed and the dock is pumped dry, the upward lifting effort of the water on each leaf exceeds 34^ tons, owing to the fact that the clapping sill projects behind the body of the leaf, but even with the water level right up to the top of the gates, which would be a very remote contingency, the additional lifting force per leaf only amounts to about 9 square feet x 27^ feet x 64 Ibs. - 27240” = 7 tons nearly, making a total lifting force of 41| tons, or just equal to the weight of one leaf. But this lifting force could only exist when the gates were exerting an enormous thrust against the hollow quoin, and the friction would be amply sufficient to prevent the gates rising, even under such exceptional circuinstances. Owing to the water-bearing surface for the five lower compartments being the curved outer plating, while for the five upper compartments it is the straight inner plating, it is necessary to lay down two lines of pressure, and these are shown in fig. 273; but it will be noticed that they differ only very slightly from each other, and that the centres of the circular curves are practically coincident with a common radius of 53 feet 9 inches, say 54 feet. Also, because the ribs are not disposed according to zones of equal pressure, it will be necessary to treat each one separately, instead of dealing with the gate as a whole, as in the previous example. Space will not permit of our taking more than two cases, which, however, will be sufficient to indicate the method of dealing with the rest. First take the rib at a depth of 24 feet 3J inches, and deal with the section 5 - 5. The section of the rib is shown in fig. 274. Flat Plaie Fig. 274. The water pressure on the face of the rib is 2-08 x 24-3 x 64 = 3,240 Ibs., say. Hence, the résultant pressure: R = 3,240 x 54 = 175,000 Ibs., nearly, and since the direction of R is parallel to the centre of section, R = F.