A Treatise On The Principles And Practice Of Dock Engineering

350 DOCK ENGINEERING. Dock Caissons. The primary meaning of the word caisson appears to be a box or ehest (Fr. caisse), but its use has been extended, in maritime engineering, to include all hollow structures, not being gates, used to close entrances or passages. Generally speaking, though not universally so, the horizontal axis of a caisson is a straight line, differing in this respect from gates, the leaves of which usually meet at an angle. Any absolute distinction, how- ever, between gates and caissons is difficult to draw, owing to the extreme variety of types in both classes. Stresses in Caissons.—The stresses induced in a horizontally-framed caisson in situ are those incurred by a series of beams uniformly loaded and supported at each end. It is only necessary to find the proportion of hydrostatic pressure on each beam and then to consider it as a uniformly distributed load. The bending moment at the centre will be where I O is the length of the unsupported portion. The moment of resistance will be afd, where / is the maximum permissible unit-stress in either flange, with area a, and d the distance between the centres of gravity of the flanges. Equating the two moments, and noting that a and d are the only variables, we get , wP ad = 8/’ in which, by selecting a value for the width of the caisson, we détermine the corresponding sectional area in the flanges of the horizontal beams of which it is composed. Where the horizontal members are, however, discontinuous, and the external thrust is sustained by a series of verticals, it will be necessary to provide a substantial transom at the top capable of taking one-third of the total pressure. The distribution of stress in this case has already been investigated in connection with gates constructed on identical lines. Apart from the resistance of a caisson to lateral pressure, it is further- more necessary to take into account its conditions of stability under the upward pressure of the water. This upward pressure, which is equal in amount to the weight of the volume of water displaced by the caisson, may cause its derangement and ultimate capsisal, if not properly provided for. In every floating body, there are two points which determine the stability of its position. One is the centre of gravity (G, fig. 322) of the body itself, and the other, the centre of gravity (B) of the fluid displaced, otherwise designated the centre of buoyancy. These may have any number of positions relative to one another, but as long as they remain in the same vertical line the equilibrium is complète. If, however, after a slight displacement of the body, the points are no longer in the same vertical, it is manifest that there is a couple, W x (W = weight of displaced fluid ,•