WIND PRESSURES AND WATER TOWERS.
THE use of stand-pipes and water towers of great height in water supply service makes the consideration of the wind pressures they are likely to be subjected to in America one of special interest and import, particularly in view of some recent disastrous experiences with stand-pipes which have been chronicled in our columns. Let it be said, then, at the outset that, in the design of all high structures erected in this country exposed to the pressure of wind, an allowance for wind pressure should be made of fiftysix pounds per square foot. This allowance will be ample, but not excessive, and every structure should be so designed as to be able to resist such wind pressure without the factor of safety of the structure being exceeded by the strain so imposed. If this pressure be deemed by some unjustifiably high, let them remember that, according to Trautwein, the wind-gauge at Girard College, Philadelphia, broke, some years ago, under a strain of forty-two pounds per square inch. On the authority of Dr. Daniel Draper, director of the Meteorological Observatory, Department of Public Works, New York city, we know that the gauge in Central Park registered on February 26 of last year, a wind pressure of 37.5 pounds per square foot. The eminent bridge engineer, lately deceased, C. Shaler Smith, C. E., in a paper read before the American Society of Civil Engineers, speaks of actual wind pressures even in excess of the figure given by us as the highest to be considered, but the evidence in regard to the existence and duration of higher wind pressures than fifty-six pounds per square foot is not such as to make it incumbent to provide for higher pressures in design.
Furthermore, it may be stated, Mr. Baker, the engineer of the Firth Bridge, who has conducted most precise and exhaustive experiments on wind pressures in one of the most exposed locations of Great Britain, and who has given the whole matter of wind-pressure most painstaking consideration, concludes as the result of his investigations, that the pressure of fifty-six pounds per square foot, recommended by a committee of the English Board of Trade as the maximum wind pressure to be provided for, is in excess of anything likely to be realized.
We know that it is the growing practice of the most able engineers in this country to allow for a pressure of fifty pounds per square foot in the design of all high structures and others exposed to the action of the wind, and we consider the allowance of fifty-six pounds to be still better practice. In figuring up the pressure corresponding to a given velocity of wind, it is satisfactory, in ordinary practice, to adopt the formula:
P=1005V2 in which Prepresents the pressure of wind, in pounds per square foot, and v the velocity of the wind in miles per hour.
In figuring up the total wind pressure on an exposed structure, which the structure must be designed to withstand, the following rules maintain : If the structure is rectangular or square in shape, with flat surfaces, multiply the area in square feet, of the diametrical vertical section, by the highest wind pressure per square foot (viz., fifty six pounds); if the structure is circular in external shape multiply the diametrical vertical section by half the highest wind pressure (viz., by twenty-eight pounds); if octagonal, by seven-tenths and if hexagonal, by seventy-five-hundredths, the highest wind pressure (viz., respectively by thirty nine and by forty-two pounds per square foot).