A Treatise On The Principles And Practice Of Dock Engineering

334 DOCK ENGINEERING. Taking the ultimate compressive stress of greenheart at 8'5 tons per square inch, it is evident that a minimum depth of some 18 inches of solid rib is needed to withstand the 147 tons compression. This, however, is the critical value, when the material is tested to breaking point, and as it is inadvisable to take a less factor of safety than 10, 180 inches, or 15 feet, in depth is actually required. As a matter of fact, in the gate in question the total depth of solid rib amounts to over 20 feet, so that the factor of safety adopted lies between 13 and 14—a by no means excessive value for a wooden gate, having regard to the duties which it is called upon to perform. In the foregoing calculation no account has been taken of the connecting pieces, for reasons which have already been given. They can only be looked upon as affording a reserve of strength for contingencies. Allowing a working stress of 1J tons in the outermost fibres of the green- heart planking, the thickness of the bottom panel is deduced from t = 1-25 = 305 inches. V3x11 This is an ample allowance, for it takes no account of the fixture of the ends, and as the stress on the other panels is much less, a uniform thickness of 3 inches has been adopted throughout. Case II. —Steel Gates.—A pair of gates for a graving dock on the River Blyth, constructed in mild steel, with greenheart heel and mitreposts and clapping sill, as shown by the drawings in figs. 269 to 273. Data : — Ft. Ins. Width of waterway, between fenders, . . 60 0 „ „ between faces of walls, . 61 0 Span of gates (between heel-post centres), . 64 6 Rise of versed sine of gates,* ... 10 9 Height of gate above dock sill, ... 26 6 Depth of lowest compartment in body of gate, in order to allow ample room for rivetters, not less than .... . . 2 0 Width of leaf at each end, for reasons of access, ....... 1 9 Width of leaf at centre, in order to ensure line of pressure passing within the gate, . 3 9 Dealing witli the question of buoyancy in the first place, the horizontal sectional area of one leaf, as measured from plan, is about 108 square feet, so that the displacement, with the five lowermost compartments formed into a buoyancy chamber, is roughly, Il-A feet depth x 108 square feet area x 64 Ibs. , — 1 — = 34-1 tons, while the actual weight of one leaf complète = 41| tons nearly, leaving * See footnote, p. 333.