A Treatise On The Principles And Practice Of Dock Engineering

RESULTANT WATER PRESSURE. 315 waterway. Even when this point is carefully attended to, it is impossible to avoid chance contact, and the abrading action can only be neutralised by the provision of stout and ample fendering. Perhaps the best form of gate to suit these conditions is that with a straight inner face, and when the gate is segmentai, it is desirable that the fendering should be arranged so as to form a cliord to the segment. This gives a straight lead to shipping, and prevents the arch voussoirs from receiving a pressure from the quarter in which they are least fitted to resist it. For all these and other varieties of stress, more or less intermittent in character, uncertain in direction and unknown in amount, provision can only be made in a crude and wholesale manner by the employment of a high factor of safety. It is not too much to say that the actual strength of a gate should be at least ten times, and, in certain cases, as much as twenty times, in excess of its calculated requirements under normal statical con- ditions. This factor of safety attains its higher values in the case of wooden gates, where the material has a wide range of strength. The resist- ance of iron and steel can be estimated with greater exactitude, and therefore admits of a closer approximation. Statical Forces.—The only statical forces are those called into action by the excess of water pressure on the back of the closed gate, and by its own weight, and it is inevitable to limit the calculations for the stability of gates to a considération of these simple elements. Calculations are some- times carried out to a theoretical nicety, which, however ingenious and interesting, is of questionable expediency in view of the wide margin of safety ultimately adopted. Inordinate detail in calculation entails two evils ; it not only involves a waste of time, but leads to an exaggerated view of the accuracy and importance of the result. In investigating, therefore, the internal stresses, caused by external agency, an attempt must be made to steer a middle course between the Scylla of useless refinement and the Charybdis of superfluous strength. When a pair of dock gates is closed and the water within the dock is at a higher level than that outside, the horizontal external forces at work are four in number :— 1. The resultant pressure of the water against the back of the gate. 2. The mutual reaction of the mitre-posts. 3. The reaction of the hollow quoins on the heel-posts. 4. The reaction of the sill against the bottom of the gate. The conditions, in fact, are those of a loaded vault closed at one end. It will, perhaps, be preferable to consider primarily the joint effect of the first three forces, and then to estimate the modification caused by the fourth force, which does not in any way affect the relationship existing between the other three. The forces being symmetrical for each half of the gate, it will only be necessary to deal with a single leaf in each case. 1. Resultant Water Fressure.—This force is completely defined, since it is known in magnitude, line of action and sense.