A building moves with the music

In my capacity as a consulting acoustical engineer for 39 years, I’ve been involved with several building vibration problems. So Peter Irwin’s excellent Quick Study article on vortices (PHYSICS TODAY, September 2010, page 68 ) was a real pleasure to read.

While forces “even greater than those caused by earthquakes,” as Irwin puts it, can threaten a building’s integrity, much lower force levels can still create interesting real-world problems. Low-frequency sound (infrasonics) from the vortices can be significant, both at the building generating them and downwind in other buildings. Low-frequency building movement can cause mechanical problems and induce nausea in occupants. The amount of force, especially if it is in phase with the building’s resonant frequency, doesn’t need to be very much to induce motion and related issues.

Some years ago I worked on a problem in an established 40-story office building in Manhattan. Elevators were binding and sticking in their shafts and people on upper floors were complaining about feeling nauseous. The problem was recent; the building had existed for years without those issues. Something had clearly changed.

The cause was traced to the establishment of an aerobics dance class held on an upper floor. The open area, with its concrete floor, was rented out for exercise and dance classes. About 30 people, all moving to music, stepping on and off stools in synch, were enough to move the building to the point where the elevators were sticking and the office occupants were experiencing motion sickness. So for a tall building, it doesn’t take a lot of fluctuating force, whether from vortices or other sources, to have very real effects.

The solution was to install a floating floor, a concrete pad on appropriate vibration isolators, to decouple the dancers’ energy from the rest of the building.