21-04-2014, 02:24 PM
Acceleration Indexes for Human Comfort in Tall Buildings—Peak or RMS?
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Abstract
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 Application of Research Information
A large amount of information has been compiled by researchers aimed at predicting human
response to motion in tall buildings. But several issues which must be addressed, and there are
difficulties to be overcome, before this information can be applied. Of these, that of the waveform
is now addressed through specific examples.
Perhaps best defined is the threshold of motion perception, which has been studied by a number
of researchers (Soliman 1963; Chen & Robertson 1972; Chang 1973; Irwin 1978; CTBUH 1981;
Kanda et al. 1994). These studies utilized “moving room” experiments, in which subjects were
placed in a room subjected to simple harmonic motion of varying amplitude. The subjects were
asked at what point they were able to sense that the room was moving, and probability
distributions fit to the responses allowed prediction of the motion amplitude which can be sensed
by various percentages of the population. The results of several studies are compiled in Figure 3.
Note that the frequency of motion is apparently a significant parameter, with more-or-less
consistent results obtained from completely independent studies.
Qualitative Arguments
There appear to be no documented studies at this time which address the dependence of
waveform or peak factor on human response to vibration, at least in a context directly applicable
to horizontal motions in tall buildings. How, then, is the selection of an appropriate measurement
index to be made? It can only be done on a qualitative or practical basis. Qualitative, or intuitive,
considerations are discussed in this section. Practical issues, associated with the technical
feasibility of implementing these indices, are addressed in the following section.
Historically, most criteria for human response to vibration of various natures have been based on
the rms index. This includes ISO 6897 and ISO 2631 (in which the “effective acceleration” is
defined as the rms value), and ANSI S3.18 and S3.29, as well as Hansen et al. The reasons for
this are not well documented, however, and the simple fact that rms has been used should not
necessarily be inferred as having a rational basis. It is of interest to note, however, that Irwin
(1978) stated “accelerations much in excess of the suggested average [rms] magnitudes will occur
for short periods but these higher levels, briefly experienced, are not considered to have any great
contribution to the memory of the storm...Short periods of higher acceleration which occur during
the worst 10 minutes of the storm occurrence are accounted for in the rms value of the vibration
of the structure for the storm peak.”
Technical Arguments
There are several arguments from a technical standpoint that favor the use of the rms index. These
center around the fact that the rms index is more straightforward to measure and/or predict, in
either analytical or wind-tunnel studies, and is therefore more likely to result in uniformity among
predicting agencies.
Dynamic response predictions are most often made from model tests in which the power spectral
density of the aerodynamic loading is measured, to which theoretical random vibration techniques
are applied to obtain the mean square or rms response. This is performed separately for the x, y
components of motion (generally torsion is also included but only two components are considered
here for simplicity). This procedure is well understood and all wind tunnel laboratories utilize
basically the same methodology (differences will occur due to the equipment used, the physical
wind model, proximity model, means of accounting for nonideal mode shapes, etc., but these are
incidental to the issue of concern).