Wind pressures on structures

Abstract

There is a general feeling among engineers that the values of the pressure exerted by the wind commonly used in the design of engineering structures are too high. The experiments on which these values are based were carried out many years ago by relatively inaccurate methods and on models not resembling actual structures. The success of modern wind-tunnel methods in the field of aero-nautics suggests their use for the determination of wind pressures on models of structures, and the present paper describes the results of some experiments on a model in the 10-foot wind tunnel of the Bureau of Standards. The paper is divided into two parts, Part I being a general discussion of the subject. A general survey is given of the effects of a uniform and steady wind, and some discussion of the allowances to be made for the gustiness of the wind. Typical values of the coefficient of wind pressure are given for various types of models. Part II describes experiments on a square base prism 24}^ inches high, the base being 8 inches square, at speeds up to 70 miles per hour. The pressure was meas-ured at 70 stations on one face and 49 stations on the top for wind directions varying from 90° to one face to 90° to the opposite face by 15° steps, the ground being represented by a fixed platform and in a second series of tests by the tunnel floor. The chief conclusions of interest to structural engineers are as follows: 1. The greatest average pressure against the building occurs when the wind blows normal to a face and is equal to 1.5 times the velocity pressure; that is, 22 lbs. /ft. 2 for a true speed of 76 miles per hour (100 miles per hour indicated by a Weather Bureau Robinson type anemometer). 2. The average decrease in pressure over the roof is about 0.84 times the ve-locity pressure; that is, 12.4 lbs. /ft. 2 for a true speed of 76 miles per hour. 3. The maximum moment about a vertical axis is equivalent to a horizontal displacement of the resultant force of 0.077 times the width of the building.