Vibrations of Tall Buildings Caused by Wind Pressure.
During the recent storm in Chicago when the velocity of the wind as observed at the United States Signal Service station was as high as 84 miles an hour, observations were taken of the vibrations of the Monadnock and Pontiac buildings by W. L. Stebbins, civil engineer. An exceedingly sensitive level in which one division on the scale shows a variation of five seconds was set up on the eleventh floor of the Monadnock in Mr. Stebbins’ office, and the time of the vibrations was observed to be about two seconds. A 14-oz plump-bob was then hung in the north air well, the point of suspension being on the sixteenth floor, and the plumb-bob itself vibrating just above a drawing board placed on the second floor. The curve here traced was a circle >4 in. in diameter. The plumbbob was then suspended in the south air well extending from the seventeenth to the second floors, and the observed curve was elliptical, the major axis being north and south and 7-16 in. in length, and the minor east and west with a length of 3/8 in. In the Pontiac building the same plumb-bob hung in the freight elevator shaft from the fourteenth floor gave an elliptical curve with a major axis % in. long and extending east and west, the north and south axis being 3-16 in. Observations were also taken in the Monadnock building with a transit sheltered so as to be unaffected by the wind. The one taken in the northwest corner showed a vibration of % in. and that in the southwest corner an average vibration ofj^ in. with a maximum of )4 in.
The Monadnock building is 400×67 ft. and 182 ft. high, the largest dimension being in the north and south line. The north half is built of heavy masonry, while the south half, being of more recent construction, is a steel framework with light walls. It may be noticed that the greatest vibration of the south half was in the direction of the length. The Pontiac is of steel frame construction and is 100 x 68 ft. and 175 feet high, the length being in the same direction as that of the Monadnock. The wind during the storm was northeast.