A Treatise On The Principles And Practice Of Dock Engineering

DOCK ENGINEERING. 3°8 certainly produce a nasty, festering wound, difficult to heal. There are- sundry precautions which may be adopted to minimise the mischief caused to gates by marine vermin. They have already been dealt with in the chapter on Materials of Construction. 6. Strength.—Another respect in which timber gates have an advantage over iron gates is their more solid construction and consequent greater ability to stand the peculiarly rough usage to which dock gates are unavoid- ably subjected. Entrances sometimes have to be closed in the face of a strong outflow of water, and at such times there is a tendency for the- gates to strike the sill with considérable force, in spite of the restraint of check chains and springs. Occasionally, moreover, the leaves do not reach the sill simultaneously, and the top part of the leaf, meeting with no sup- port, is jerked violently forward. An instance is on record where, in the absence of a check chain, the topmost outer corner of a gate at Birkenhead was projected momentarily some 10 or 12 feet out of plumb.* The leaf then recoiled, and, fortunately, mitred fairly with its neighbour without further mishap; but the shock must have been tremendous, and nothing save the elasticity and flexibility of a wooden frame, with broad tenoned joints, could possibly have withstood the strain. As another instance of the almost disastrous nature of some of the conditions to which a gate may be subjected, mention may profitably be made of a serious accident which quite recently befell a pair of wooden gates at Liverpool, closing a passage 90 feet wide between two adjoining docks. One of these is a half-tide dock, in which the water is allowed to fall with the tide for some hours after high water. The passage gates were carefully mitred at the turn of the tide, and attention was directed to them until a steadily increasing head of some 15 or 18 inches of water was registered. At this point, being night-time, they were left, apparently secure. Unfortunately, by some carelessness or oversight, water for levelling purposes was run off from the inner dock at too rapid a rate, and the accumulated head was dissipated, with the result that the gates parted. Shortly afterwards, when the sluices were closed and at a time when the tide was ebbing fast, the gates came together again, probably with some impact, certainly imperfectly, and in such a way as to cause nipping between the outer edge of one mitre- post and the inner edge of the other. The falling tide soon produced a fresh head of 4 feet or so, at which point the foully mitred gates yielded with a loud crack. The alarm being raised, immediate steps were taken to avert any further evil conséquences. The gates were found to be badly strained, and one leaf had to be taken into the graving dock for repairs. Despite the resistance of the connecting straps, the topmost ribs were torn out of the heel-post, and the upper portion of the latter was so split as to need splicing with new timber. The nipped edge of the mitre-posts were also badly detruded. However, the damage was soon made good at a moderate cost, and though the incident, at first sight, demonstrates * Tilig is the authentie statement of an expert eyewitness.