A Treatise On The Principles And Practice Of Dock Engineering

332 DOCK ENGINEERING. erroneous. The variation from the truth is greatest in the case of the topmost panels, and the approach to accuracy increases with the depth. No account is taken, generally speaking, of the support derived from the fixture of the ends, nor, in cambered gates, of that due to the curvature, any excess of strength in these respects being set off against possible loss from corrosion or decay. Calling the shorter unsupported length of the panel 1, and d the depth of the centre of the panel below the surface, the maximum bending moment . wdl2 Then, if t be the thickness of the plate and f the safe maximum fibre stress, the moment of resistance is^—; and, equating, whence ft2 wdl2 T= ~ t (67) In the foregoing expression the unit is the foot. It will be, perhaps, more convenient to express t and l in inches. Giring tc its numerical value, the expression then becomes— t=1\/jf ..........................................................(68) Mr. Ivan C. Boobnoff, naval architect of the Imperial Russian Navy, proposes to calculate the thickness of plating for ships by a similar formula, deduced in a rather more elaborate manner— These are theoretical thicknesses. There is in practice a minimum of I inch for iron and steel and 3 inches for wood, beyond which it is not safe to go, on account of the exceptionally rough usage to which the panels are subjected and their liability to corrosion and decay. Practical Illustrations.—It will be useful at this stage to take an actual pair of gates and see how far their construction conforms to the theoretical requirements of the preceding formulæ. Examples of both wood and iron gates have been selected for this purpose, as representing two widely distinct types, the main dimensions of the entrances which they close being, as far as possible, alike, in order that a certain comparison may be instituted between them. For the plans and particulars relating to the metal gates the author is indebted to the courtesy of Mr. J. M. Moncrieff, of Messrs. Sandeman & Moncrieff, Newcastle-upon-Tyne. Case 1. — Wooden Gates.—A pair of gates at Liverpool, each leaf consisting of a series of curved horizontal ribs, built in two voussoirs with connecting pieces, as shown by the drawings in figs. 266, 267, and 268. With the