A Treatise On The Principles And Practice Of Dock Engineering

312 DOCK ENGINEERING. on the outside, of 20 vertical bulb-angle irons, eacli 7 by 3 inches, spaced about 2 feet apart. The plating between the framings varies in thickness from -J inch at the top to g inch at the bottom. The topmost member is arranged as an air chamber, and it also serves the purpose of a stiffening girder. The meeting surfaces of the gate, the sill and the jambs, consist of pièces of pitchpine, faced with strips of indiarubber, 2 inches wide and f inch thick, to secure watertightness. The method is apparently very effective, and the joint a perfectly durable one, as the author found from personal inspection. At the end of seven years the indiarubber, which is fastened by copper nails, was qiiite undeteriorated. The gate is swung on two hinges, having pins 4 inches diameter. A sluice at each side of the lowermost panel complétés the equipment of the gate. The success attending this type of gâte, of which the foregoing is probably the principal existing example in this country,* is sufficient to warrant its introduction on a larger scale. The main objections attending such a step are the necessity for a platform deep enough to contain the buoyancy chambers, and the possibility of some unseen obstacle preventing the gate from falling back to its full extent, and thereby endangering vessels passing over it. These disadvantages cannot be considered insuper- able. Special recesses might be formed in a comparatively shallow platform to receive the buoyancy chambers, and these would be kept clear of deposit by an efficient system of sluicing. An additional element of strength could be imparted to the gate by the adoption of a sill curved in plan, to which the turning axis would be tangential at its centre, as exemplified in the lower portion of a railway carriage door. This would entail somewhat longer hinges at the sides, in order to cover which, and the curved profile of the gate, the sill would also require to be curved in elevation—an objectionable arrangement for passages frequented by flat- bottomed vessels. Gates with two Leaves.—By far the more general method is that of gates in two symmetrical leaves, each a little longer than the semi-width of the waterway, meeting, when closed, at its centre line in such a way as to afford one another mutual support by pointing in the direction of the impounded water. Of this class of gate there are two varieties, representing distinct forms of construction, viz.:— (a) Those with horizontal girders. (3) Those with vertical girders. The first case represents the type most commonly met with in British ports. It is founded on the principle of the arch, and consists essentially of a series of horizontal ribs or girders. In timber structures, these are grouped more or less into “ cesses” throughout the height of the gate, the * The author is only aware of one other example, viz. :—A gate closing the entrance (35 feet wide) to a graving dock at Port Dinorwic, North Wales.