A Treatise On The Principles And Practice Of Dock Engineering

476 DOCK ENGINEERING. load. For example, the coefficient of the breaking weight of 8 to 1 concrete, uniformly loaded, may be taken at 10 tons for a unit beain (1 foot square section and 1 foot between supports). The intensity of water pressure due to a moderate head of, say, 35 feet is 1 ton per square foot. This means for a graving dock with only 60 feet width at floor level, a uniformly distributed load of 60 tons on the underside of the floor. To adequately sustain such a load in this way, the floor would need to be 38 feet thick. For, the breaking weight of the beam is calculable from the following formula :— D XV = — — x constant, and substituting the known values, with unit breadth, whence 4 = 19, and taking a factor of safety of 4, which is equivalent to twice ( ^4) the 'depth, the thickness of the floor becomes 38 feet. And this is for docks of the smallest class. For docks 80 feet and upwards in width, the thickness would be even more absurdly excessive. One simple consideration will dispose of the beam theory. There is hydrostatic pressure against the vertical faces of each extremity of the floor amply sufficient to neutralise any tension in the latter, and subject it entirely to a compressive stress. In other words, the floor must be treated as an arch, either actual or, in the case of flat floors, virtual. If we take a permissible compressive stress for concrete of 20 tons per square foot, and consider the real or imaginary arch to have a depth or thickness of 5 feet, the rise of the invert between the centre and sides of the dock is given by a slight modification of formula (90), explained in Chap. x. WZ r ~ 87’ where r is the rise, I is the span, W (=wZ) the total weight, and t the horizontal thrust at the centre. With unit breadth t = 20 x 5 = 100 tons, and 60 x 60 r = TTTioo = 4i feet’ and as this is to be measured to the centre line of the thickness of the arch, a flat floor only requires a maximum depth of 7 feet or so, which is a much more reasonable figure, and one which accords with results gained by experience. One practical observation is deducible from this conclusion—viz., that the joints in masonry floors, if flat, should radiate towards the imaginary centre of the invert. It is assumed that concrete floors will be constructed in a homogeneous mass, without joints.