Acceleration Indexes for Human Comfort in Tall Buildings—Peak or RMS

Traditionally, the effect of vibration on human comfort has been evaluated using the rms value of acceleration. More recently, a widely used criteria in North America has utilized the peak value of acceleration. The differences between these two methods are examined, and the importance of making a rational selection of the appropriate motion index for future standards is emphasized. It is proposed that, until additional research in this field demonstrates otherwise, the rms value be adopted as the best available index. Introduction Human response to wind-induced vibration in tall buildings has traditionally been evaluated by the acceleration in the horizontal plane. Two different measures of acceleration have been used: the peak value which occurs during a period of time—say 20 to 60 minutes—or the rms value averaged over this same period. These distinctions have arisen because of the different waveforms, or acceleration signatures, which must be addressed. Characteristic signatures are shown in Figure 1. All of these signatures can be characterized as narrow-band vibration at the same frequency, with differences in the envelope uniformity. The first signature of Figure 1 is harmonic (sinusoidal) vibration. Harmonic acceleration of this nature does not occur in real buildings, but it has been widely used in laboratory “moving room” experiments aimed at determining human thresholds of perception of small vibrations, or tolerance of larger vibrations, under controlled conditions. The uniformity of this signature can be characterized, on a first-order basis, by the ratio of peak value to rms value. This ratio, commonly referred to as the peak factor and designated g, is 2 for sinusoidal signatures. The second signature was recorded from an accelerometer in a wind-tunnel model of a tall building, measuring the crosswind vibration while “locked-in” to vortex shedding forces. The signature is nearly harmonic but slightly more random, with a correspondingly higher peak factor; g = 2.0. Acceleration of this type is rare in typical buildings, but could occur in special structures such as tall slender towers. The third signature was obtained from an accelerometer in the same model under the same conditions, but it was oriented parallel to the wind direction. This motion is characteristic of tall building motion in the alongwind direction. It is more random than the above and has an even higher peak factor; g = 3.1. The fourth signature is another idealized case which does not occur in practice. Conceptually it represents the transient response to a uniform impulse train. Signatures of this nature are