A Treatise On The Principles And Practice Of Dock Engineering

314 DOCK ENGINEERING. that they call for exceptional strength and careful construction. In sonie instances a ship caisson is employed for the purpose, especially when the circumstances are of rare occurrence. Strut Gates are auxiliary frames or shores which support the main ebb gates in their closed position and enable them to withstand a slight head on the outer face, and to resist the onset of waves at or about high-water level. They assist ebb gates to act to a certain extent as storm gates, and are accordingly found to be a useful adjunct in exposed situations (fig. 201). Stresses in Gates.—Proceeding now to an investigation of the stresses to which gates are subjected, it will be found on considération that the causes to which they are due may be ranged under five heads : — 1. The excess of water pressure on the inner side, or back of the gates, when closed. 2. The tension of chains, or the thrust of rams, during the operations of opening and closing. 3. Concussions and irregularities of movement in consequence of unseen obstacles and incomplete control of the motive power. In this connection it is to be noted that a strong current sometimes forms a very great part of the motive power. 4. Wind pressure and the impact of waves. 5. Collisions with passing vessels. Of these, the three last are of a more or less abnormal nature, and their magnitude cannot be estimated with any degree of exactitude or certainty. Nevertheless, they constitute very potent factors in the deter- mination of the life and stability of a gate. In boisterous weather, not only do external waves break against the outer face of a gate in a succession of shocks of varying intensity (the effect at high water being especially destructive), but even the water confined within the dock will often become considerably agitated, especially if there be any extensive area exposed to the action of the wind. This last named agent also exerts direct unbalanced pressure upon the surface of the gate above the water line, but as the unimmersed portion is, as far as possible, constructed in openwork, the result is minimised. During the operations of opening and closing, the gates are liable to jars and shocks from contact with sunken obstacles, from abrupt stoppages due to occasional fluctuations in hydraulic pressure, where such is employed, from the sudden impetus of wind, wave, and tidal current, and even from irregularities in, and silt accumulations upon, roller paths. If the tide be running out with any degree of swiftness, a rapid current is generated in narrow entrances, in which it is difficult with rams, and almost impossible with chains, to prevent the gates from striking the still with sorne force, instances of which have already been noticed. Collisions are occurrences more or less frequent during the time the entrance or passage is being worked. Accordingly, it is very essential that the open gate should be completely recessed beyond the face line of the