A Treatise On The Principles And Practice Of Dock Engineering

228 DOCK ENGINEERING. pressures, but the area affected has been comparatively trifling, and it is tolerably certain that the intensity of pressure registered by a small anemometer can, in no wise, be considered representative of surfaces of indefinite extent. Eminent authorities are inclined to take this view, and Sir John Wolfe Barry, in his Presidential Address to the Mechanical Section of the British Association meeting, in 1898, pointed out that of two wind gauges of 300 and 1'5 square feet respectively, at the Forth Bridge, under the same conditions of wind and exposure, the larger registered a pressure of 38-7 per cent, less per square foot than the smaller, while of two other gauges with more greatly contrasted areas, at the Tower Bridge, the divergency amounted to over 70 per cent. Prior to the Tay Bridge disaster, in 1879, the recognised maximum allowance for wind pressure, in Great Britain, on exposed surfaces, was 40 Ibs. per square foot. Acting under the influence of public opinion, the Board of Trade, in 1880, raised the safe limit to 56 Ibs., at which figure—an undoubtedly excessive one—it now stands. The following table shows the ratio of wind pressure to velocity, as originally published by Smeaton in the Philosophien! Transactions of 1759, and as recently modified by Mr. W. H. Dines after a long and exhaustive series of experiments.* Taking the pressure, P, in Ibs. per square foot, and the velocity, V, in miles per hour, Smeaton and Dines’ formulæ are— P = -00492 V- and P = -003 V2, respectively :— TABLE XVIII.—Force of Wind in Lbs. per Square Foot. Velocity in Miles per Hour. 10 20 30 40 50 60 70 80 90 100 110 120 Smeaton, •5 20 44 7-9 12-3 17'7 241 31-5 39'8 49-2 59-3 70-8 Dines, . . •3 1-2 27 4-8 7’5 10'8 14-7 19-7 24-3 30-0 36-6 43-2 The connection, however, between velocity and pressure is one which cannot be exactly determined by a simple coefficient, and all such expres- sions must inevitably give results more or less erroneous, except within the narrow experimental limits upon which they are founded. To obtain immunity for an entrance from gales blowing from all points of the compass is, of course, a manifest impossibility, but something may be done towards minimising the effect of the more noxious winds. Advantage should be taken of any natural features—headlands, promontories, and the Iike—or even of moderately high ground in order to secure a leeward * Vide Engineer, Nov., 1897.