SKYSCRAPER FORUM

17 - SWAYING BUILDINGS

ID# 17.1
From: George Dunbar ([email protected]) - 7 Mar 1998
It seems to be a New York "myth" that the Empire State Building sways in the wind. What is the proof of this?
I once read that the inventors of the gyroscope took their invention to the top of that building to test its effectiveness and found no "sway". A gyroscope is a very sensitive instrument which is intended for the measurement of movement. Since reading that report, I have wondered about the truth of swaying skyscrapers. Any comment?.......or proof?
I will appreciate any information.
- ID# 17.2 (reply to #17.1) - 7 Mar 1998
  It is certain that high structures, such as skyscrapers, tend to sway at least to some extent in high winds. The swaying is naturally not as emphasized as in radio masts etc. less rigid structures, especially when compared to older high-rises, such as the building in question, the Empire State Building, which sways only about 5 millimeters at the top. In those days it was customary to make the frame very rigid to counter the wind loads. In that sense you are, practically, correct about the non-swaying of that building. Later, however, the frames have become less rigid, and, for example, the Sears Tower in Chicago was known for its annoying tendency to sway and creak -- a characteristic partly, undoubtedly, leading to Sears relocating elsewhere from the high-rise... (On the issue.) ED.
  - ID# 17.5 (reply to #17.2)
    From: Brian Frew ([email protected]) - 16 Jun 2000
    Maybe you could help me ... I saw a documentary that discussed a building (I believe in New York) that uses a 400 ton moving block to counter wind sway, and would like to get more information on this building. I would especially like to know how the building was equipped with this block, and how it is kept in motion.
    - ID# 17.6 (reply to #17.5) - 16 Jun 2000
      The building was the Citicorp Center (today Citigroup C.). As far as I know, the large concrete block is on an oiled plate (to lessen movement friction) and dampens the movements of the building simply by its inertia (ie. in this case, its tendency to stay stationary regardless of the movements of the tower). When the tower starts to tilt to one side, the 360-ton block slides to the other, thus decreasing the amount of sway. The same goes for the counter-sway to the other direction. ED
      (13 October 2002: The plate itself is actively tilted by computer-controlled machinery to counter the sway.)
  - ID# 17.3 (reply to #17.1)
    From: Donald Friedman ([email protected]) - 3 Apr 1998
    Speaking as a structural engineer...
    All building move when loaded. Short, squat building just move such a small amount that no one notices or cares. Skyscrapers, by definition, are not short and squat, so their movements are measureable, calculatable, and sometimes perceptible to people on high floors.
    - ID# 17.4 (reply to #17.3)
      From: - - 23 Sept 1998
      It's not just very tall skyscrapers that sway in the wind. A friend of mine works in on the top floor of an office block in Cardiff, Wales which is a mere 15 stories (though very thin, which probably helps) and has often watched his light fittings sway from side to side as they hang vertically downwards while the building moves in strong winds . . .