Evaluation of a floor system for potential annoying

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Evaluation of a floor system for potential annoying vibration requires careful estimation of the weight supported by the floor on a typical day. A fully loaded floor will never be a problem; most occupant complaints are received when the problem floor is slightly loaded. The design dead load for mechanical equipment and ceiling should never be used, nor should the design live load. An estimate of the real mechanical loading (for instance, 2 psf not 5 psf as may be used for strength design) and ceiling is required. Recommended live loads in the Floor Vibrations design guide are 11 psf for office live loading (not 50 psf as used for strength design), 6 psf for resi- dences, and 0 psf for shopping malls. Figure 23. Recommended peak acceleration for human comfort for vibrations due to human activities (International Standards Organization [ISO], 2631-2: 1989)
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SYSTEMS PAGE 24 Frequency is the rate at which a floor vibrates and is expressed in cycles per second (Hz). Floor systems general- ly have a frequency between 3 and 20 Hz. For a typical steel framed 30 ft by 30 ft office building bay, the fre- quency will be in the 5-8 Hz range. Frequency is a function of span (the longer the span, the lower the frequen- cy) and weight supported (the heavier the floor and the supported contents, the lower the frequency). Thus, a floor constructed using normal weight concrete will vibrate at a lower frequency than the same floor constructed with lightweight concrete. When the frequency is above 15 Hz, as occurs in very short spans (say less than 15 ft), floor vibrations are generally not felt. Damping is energy loss due to relative movement of floor components or fixtures on the floor. Damping causes a freely vibrating floor system to come to rest and is usually expresses as a percent of critical damping. Critical damping is the amount of damping required to bring a vibrating system to rest in one-half cycle. Damping for floors is usually between 2 percent and 5 percent. The lower value is for floors supporting few non-structural com- ponents, like for open work areas and churches. The larger value is for floors supporting full-height partitions. A typical office floor with movable, half-height partitions has about 3 percent damping. Particular attention should be given to office floors with open spaces, no fixed partitions, and light loads. This sit- uation is what results in problem floors if the design is not done correctly. Also, floors with high design loads (say 125 psf) and light actual loads (say less than 15 psf) do not have the same amount of damping as floors designed for normal office loading (say 50 psf). In this case, a lower estimate of damping should be used (e.g., 1-2 per- cent). The design of floors supporting rhythmic activities, dancing, aerobics, etc. require consideration of the entire structure, not just the supporting floors. These activities introduce very high energy levels into the structure and can cause annoying floor motion quite some distance from the activity area. Aerobics on the 60 th floor of a build- ing have caused excessive floor motion twenty floors below. When a rhythmic activity floor is located above
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