A Treatise On The Principles And Practice Of Dock Engineering

DOCK ENGINEERING. 310 unsuitable design for localities in which there was any important change in tidal level, and it is chiefly in such places that gates are required. Classification of Gates.—Gates may be most efficiently classified as— (a) Those consisting of a single leaf. {ß) Those having double leaves. In the former case the axis of rotation may be either horizontal or vertical ; in the latter, it is necessarily vertical. Single Leaf Gates.— A. single leaf gate with a vertical axis can only be advantageously employed for a very narrow waterway. When swung back to allow a passage for vessels, it occupies a side recess of considerable extent, rendering the entrance or lock unduly long and correspondingly expensive. Such a gate is rarely, if ever, constructed for dock work, and is almost entirely confined to canals. The following conclusion, voted by the International Navigation Congress sitting at Brussels in 1898, sums up the advantages and disadvantages in a clear and concise manner. “ Single gates, turning on a pivot, claim the attention of engineers. Notwithstanding the lengthening of the lock which they involve, they are not more expensive than mitred gates; they are subject to less strain, cause less loss of water, and are more easily adjusted, repaired, and replaced ; and their working is simpler and more regular. Nevertheless, the great expenditure of water, and the increase in the period of locking, resulting from the elongation of the chamber, are inconveniences which, as regards the lower gates, counterbalance and even outweigli the advantages mentioned above.”* A single leaf gate, however, with a horizontal axis, is capable of much wider application. It turns upon a hinge or pivot, fixed slightly below the level of the sill of the entrance. When open, it lies prone upon a platform, below and outside the sill, so contrived that no part of the gate in this position projects above the sill level. The process of closing consists in raising the outer edge of the gate until it is vertically over the pivot. When this is done, the gate has a bearing against the two side quoins and against the face of the sill. The raising may be effected by means ofa suitable attachment of wire ropes or chains, leading from the topmost member to a winch or other winding apparatus on the quay, but the action can be aided to a considerable extent by the formation of watertight com- partments within the gate, the flotation power of which reduces the external lifting force required. Messrs. Clover, Clayton & Oo., of Birkenhead, have a gate constructed on this principle at one of their private graving docks. It is illustrated in figs. 247 to 249.1 It closes an entrance of rather more than 40 feet mean width and its height is 27 feet 7 inches. The framing consists, on the inside, of four horizontal tiers of bulb-angle iron, ranging from 6 by 3 inches to 9 by 3 inches, with a lowermost tier of 10 by 6 inches bulb tee iron; and * Proceedings, p. 638. + Vide Brodie on “ Dock Gates,” Min. Proe. L.E.S., vol. xviii.