A Treatise On The Principles And Practice Of Dock Engineering

STORM GATES. 313 intermediate spaces being faced with planking. In the larger gates, the cesses are not continuons from one end of the leaf to the other, but are intersected by verticals which divide each leaf into a series of voussoirs. In iron gates the horizontal members are single girders, continuous throughout, with intermediate connecting pieces, or stiffeners, and plating. There are at least two continuous vertical members in both kinds of gate—the heel-post, or axis of rotation, set in the hollow quoin of the ■entrance, and the mitre-post, forming the abutment at the outer end of the leaf. In timber gates the horizontal ribs are tenoned into and between these two main verticals, and for small gates they are sufficient. But for medium sized leaves of arched form, ranging from 30 to 40 feet in length, an additional vertical called the middle-head is economically introduced, dividing the leaf into two voussoirs. For larger entrances still, the middle-head can be duplicated, the two posts being distinguished as the heel-middle-head and the mitre-middle-head, according to their respective positions. In extreme cases, where the length of the leaf reaches from 50 to 60 feet, three intermediates will be required. In the second type the method of construction is reversed, and the principle of the beam adopted. There are only two continuous horizontal members, one at the sill level, forming a watertight abutment, and the other at the summit of the gate. Between these are set a series of verticals at regular intervals from heel-post to milre-post. The intervening space is made good with planking or plating, as the case may be, the thrust upon which is transmitted by the verticals to the upper and lower transoms, and these, accordingly, receive the whole hydrostatic pressure in a ratio to be determined later. For curved timber gates the verticals may, in certain cases, be arranged in contiguity as the voussoirs of an arch, but the necessity for having them in such close contact is remote, and the system is more generally characteristic of flat gates, such as are in evidence at Dunkirk, on the North Sea Canal and elsewhere. For the sake of offering some basis of comparison of the merits of the two systems, it may be remarked that the vertical type is more readily adaptable to the accommodation of large sluice openings in the gate itself, as these can be arranged between the verticals without impairing the strength of the framing. On the other hand, the horizontal system has obvious advantages in respect to the more effective distribution of the material, and, in the case of wooden gates, at any rate, it undoubtedly represents the soundest and most economical form of construction. Storm Gates.—A class of gates differing in function, rather than in mode or form of construction, is that known as storm or flood gates. They are employed in entrances subjected periodically to floods or to extraordinarily high tides accompanied by cyclones and tempestuous weather. During such periods it is often necessary to exclude part of the tidal water from a dock, and the gates consequently point in the opposite direction to those used for impounding water. From the nature of their duties it is evident