Minimal but disturbing, building vibration can be dampened
Since the 1970s, floor slabs have become thinner and lighter, and column spacing has increased, with typical spans of 40 feet rather than 25 feet. As a result, newer buildings often do not dampen vibrations as well as older ones. Vibration can now be a source of anxiety for occupants.
Vibration may be caused by nearby traffic or subways, building mechanical systems, or simply normal human activity, such as walking across the floor. This vibration does not jeopardize the building’s structural integrity. Yet, while the actual movement may be minimal, human perception of motion is significant. Vibration can be disturbing to building occupants.
Recent trends in office design have aggravated the problem. More and more offices are laid out as open plans, with the absence of full-height, floor-to-ceiling partitions. In addition, many modern offices have lighter loadings than before, with lightweight furniture and the so-called “paperless office” requiring fewer heavy file cabinets. Consequently, there is less live loading in the building and less damping.
In the past, structural engineers designed first for strength and then used established procedures based on slab stiffness and the percentage of damping to check for vibration acceptability or serviceability. One leading authority in the field of perceptible vibration now holds that it may be preferable to design for serviceability first, and then verify strength.
Some guides to serviceability design can be found in American Institute of Steel Construction (AISC) publications, most notably Design Guide 11: “Floor Vibrations Due to Human Activity.” Some tips:
- Typical walking speed is 1.25 to 1.5 Hz, or steps per second. Floor frequencies should be kept well above the range of 2.5 to 3 Hz, the second harmonic for walking frequencies.
- When evaluating a floor for vibration perceptibility, the designer should assume lower floor loadings than are required in designing for structural strength. Thus, if the structural requirement calls for loadings of 100 pounds per square foot, half that amount should be used when checking serviceability.
Another approach is the judicious use of concrete reinforcing steel. This method was used successfully in the construction of a mechanical room on a 300-foot span above an operating room of a hospital. Although the original design resulted in a floor natural frequency of 3 Hz, a frequency of 6 Hz was needed to isolate the vibration of the mechanical equipment. Doubling the amount of concrete reinforcing steel doubled the stiffness of the floor and diminished the vibration, without increasing the cost of the structural steel or reducing the floor-to-ceiling height. Another alternative is to use a damping compound in the concrete, which increases the natural damping of the concrete and cuts the amplitude of vibrations by a factor of four.
It is also possible to remedy the vibration situation once a building is constructed, although it is more costly. One method is adding damping arms to the underside of the structure. A damping arm dissipates vibration through friction. It’s an effective but expensive measure. Partitions can be added but could interfere with usable space.
A practical and inexpensive approach is to increase the floor loading within the building. In a building in midtown Manhattan, there were complaints about vibration on some floors. Occupants perceived the vibrations as transient — of short duration. This was attributed to foot traffic. The conclusion, confirmed by tests, was that there was too little damping in the building. Because the building had a raised floor, acoustical engineers suggested adding weight under the floor to increase damping.
Once the weight was placed under the floor, the floor movement decreased and the vibration dissipated. Moveable items, such as heavy planters, placed strategically on the floor can serve the purpose well.