A Treatise On The Principles And Practice Of Dock Engineering

PRINCIPAL CONSTRUCTIVE FEATURES. 25S to support without settlement any weight which may be concentrated on a limited portion of its area. The excess weight of a large greenheart gate, over and above its flotation, may amount to as much as 50 tons, and this has to be divided between the pivot and, say, two truck wheels, so that the three points of contact are undergoing a stress équivalent to a pressure of nearly 600 feet of water more than the remainder of the platform area. The diäparity in pressure will be greatly accentuated for intermediate and outer gates at sucli times as when the lock happens to be dry ; and as caissons are frequently utilised as avenues for traffic, it is well to remember that the effect of any dead or moving load which they carry is transmitted direct to the platform below. The bedding and adjustment of the wheel tracks is then a matter for careful attention. 3. Side recesses for gates are usually curved in form and sufficiently deep to admit of the gate receding well beyond the face line of the side walls. in order to avoid concussion with passing vessels. A gate recess terminates in two returns, or quoins, called from their shape the hollow quoin and the square quoin respectively. The former receives the heelpost of the gate and, accordingly, is concave in plan, forming a circular segment. Combined with its curved junction with the side wall it may be described as a modi- fied Ogee or Cyma Recta. There are two types of hollow quoin. One, which finds favour in this country, provides a cylindrical surface in close contact with the heelpost for a considerable portion of its circumference. This design (fig. 177) entails very accurate and careful dressing, und is attended by the inevitable wear of the contiguous surfaces, resulting in leakage, though not to the extent which might be supposed. The alter- native plan (fig. 178), in vogue in Holland, is to limit the amount of water- tight contact to a narrow straight face, about 8 inches in width, the dressing and polishing of which, being a plane surface, is accomplished with greater facility than that of a cylindrical quoin. At the outer edge of the° quoin there is another close-fitting strip to prevent the passage of small floating objects. In both forms of quoin the friction of movement may be diminished by affording a slight play in the pivot, by which the gates revolve out of contact with the quoin. Hydrostatic pressure causes the surfaces to resume their watertight abutment. The joints of hollow quoins are preferably bedded in lead for a depth of 6 inches from the face. The