A Treatise On The Principles And Practice Of Dock Engineering

3’6 DOCK ENGINEERING. (a) Magnitude.—The weight of a cubic foot of fresh (distilled) water is 1,000 ozs. av. or 624 Ibs. The weight of a cubic foot of salt water depends upon its impurities, which vary in different localities. Within the limits of the British Isles, it ranges from 1,000 to 1,025 ozs., and will be taken here at its maximum value of 64 Ibs., and symbolised, when necessary, by the letter w. The intensity of water pressure on the back of a gate increases from zero at the surface level to an amount at any other level due to the height of the column of water above it. Thus, at a depth of 10 feet below the surface, the pressure is 10 m = 640 Ibs. per square foot. When there is no water on the front of the gate this represents the resultant intensity of pressure at that level. If the height of the water behind the gate be designated h, then the mean intensity of pressure is the total resultant pressure per lineal foot of gate, and P _ ^2 2' (45) If there be a height, hv of water at the front of the gate, it is manifest that this expression must be modified into r, - ^ - ^’ - ?(“->.,■). • (46) (Æ) Line of Action.—The centre of pressure is situated at the level of the centre of gravity, D, of the triangle, ABC, representing the distri- bution of pressure (fig. 250). That is to say, the line of action is horizontal, at one-third of the height of the water. When there is water on only one side of the gate, this applies to the resultant pressure also. With water on both sides, the resultant pressure will act at a point determined by the centre of gravity of the quadrilatéral, A C F D (fig. 251), formed by deduct- ing the lesser triangle of pressure from the greater. If the gate be curved, the line of action will be normal to the curve. Fig. 250. Fig. 251. (y) The Sense is obvions, being always towards the gate. 2. The Jieaction of the Mitre-posts is a force perpendicular to the plane of their abutting surfaces, and, therefore, also perpendicular to the longi- tudinal axis of the waterway. Even with perfectly fitting gates, it would not