HUMAN BODY RESPONSE TO LOW FREQUENCY NARROW- BAND RANDOM BUILDING MOTIONS

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1 Wind Engineering, November 8-12, 2009, Taipei, Taiwan HUMAN BODY RESPONSE TO LOW FREQUENCY NARROW- BAND RANDOM BUILDING MOTIONS Marianne N. Michaels 1, Kenny C.S. Kwok 2, Peter A. Hitchcock 3 1 Research Assistant, The Hong Kong University of Science and Technology (HKUST), Clear Water Bay, Hong Kong, wtmnm@ust.hk 2 Professor, School of Engineering, University of Western Sydney, Australia, k.kwok@uws.edu.au 3 Associate Director, CLP Wind and Wave Tunnel Facility, HKUST,wtpete@ust.hk ABSTRACT The natural sway of a building in the wind can potentially have an effect on the well-being and comfort of its occupants, especially at the upper levels of the building. Empirical results from a series of motion simulator experiments suggested that the comfort level and effects to well-being of 578 volunteers, subjected to bidirectional narrow-band random motion, was dependent upon increasing frequency of oscillation, acceleration and duration. Evidence of frequency dependence of motion perception was in agreement with previous researchers. As the frequency increased, test subjects who became nauseated, fatigued and/or developed a headache also increased, with 0.25 Hz and 0.50 Hz being the more provocative frequencies. Test subjects who had become annoyed, developed a headache, and/or had difficulty concentrating in the test conditions at 0.50 Hz was more than double of that for the 0.16 Hz test conditions. Over 50% of test subjects felt discomfort in 22 of the 24 test motion conditions (excluding the control condition). Building occupant complaints were also highlighted, with results showing that 60% of test subjects would complain to building management, and this would in turn influence a test subjects decision (as home buyers) to purchase or occupy upper levels of tall buildings. KEYWORDS: ENGINEERING, WIND-INDUCED TALL BUILDING MOTION SIMULATOR, WIND- INDUCED BUILDING VIBRATION, OCCUPANT COMFORT, TALL BUILDING, WIND MOTION Introduction With a population of nearly 7 million, the people of Hong Kong occupy more than 39,000 buildings, of which more than 7,000 of them are skyscrapers - the highest number in the world [Home Affairs Department of the Government of the HKSAR (2009)]. Of the top 100 residential high-rise structures in the world, 36 are located in Hong Kong [Emporis.com (2009)]. The natural sway of a building in the wind potentially has an effect on the well-being and comfort of the occupants who live or work within these high-rise structures. Human perception levels, due to building motions in the low frequency range below 1 Hz, are dependent upon various physiological and psychological parameters. Human response to vibration and comfort is subjective and individualistic, leading to a wide range of occupant responses. As some occupants react by feeling unwell, suffering symptoms similar to that of sea sickness, the equanimity of others may be adversely affected, with some possibly objecting to even small amounts of unexpected motion, and leaving them with a disconcerting feeling about the structural integrity of the building [Smith (1988), Stafford Smith et al. (1991), Bachmann (1995)]. During strong wind events, such as typhoons and monsoons, some occupants, especially those who are more sensitive to motion or those who are paying a premium to occupy the upper levels of buildings, may complain to the building management

2 Wind Engineering, November 8-12, 2009, Taipei, Taiwan about the motion [Denoon (2000)], and new home buyers may even choose not to occupy the upper levels of tall buildings. In addition, employers in high-rise structures may notice a decrease in motivation, productivity and cognitive ability from their employees, due to their staff becoming affected by the motion [Smith (1988)]. There are challenges with using a motion simulator in replicating a realistic tall building environment elements such as fear, anxiety and alarm, normally present in fullscale are not present in a motion simulator. However, volunteers who are tested for occupant comfort in a motion simulator can be tested under various conditions that are either impractical or impossible to test in field studies, and the results obtained allow the engineer to gain insight and infer how the population may respond in a full-scale building. A motion simulator is an effective tool with which to test volunteers in a carefully controlled environment, as it is not feasibly possible to test large numbers of volunteers in full-scale [Denoon (2000), Burton (2006)]. The objectives of this investigation are to characterize the human body s response (i.e. comfort and well-being) during exposure to low frequency narrow-band random motion, and to determine its dependence on frequency of oscillation, magnitude of acceleration and duration of exposure. Research was also conducted to gauge if an occupant would complain about the effects of these main study parameters. Methodology Under carefully controlled laboratory conditions, a series of occupant comfort tests were conducted using the wind-induced tall building motion simulator at the CLP Power Wind/Wave Tunnel Facility (WWTF) at The Hong Kong University of Science and Technology (HKUST). Guidelines are specified in International Standard ISO 6897 (1984), against which the acceptability of low-frequency horizontal motion of buildings subjected to wind forces may be evaluated. These guidelines specify recommended upper limits of satisfactory magnitudes of acceleration for frequencies in the range of to 1.0 Hz. For this study, three frequencies of bi-directional random motion were chosen: 0.16, 0.25 and 0.50 Hz. Based upon these three frequencies, ISO recommended limits were used to determine equivalent criteria in terms of peak acceleration, as shown in Table 1. Additional tests were also conducted at peak accelerations with magnitudes of 30% greater than and less than the ISO recommended accelerations. However, at 0.16 Hz, the third level of acceleration (+30% of ISO) of 20.4 milli-g could not be tested due to the physical limitations of the motion simulator. Table 1: Tested Frequencies of Oscillation and Peak Accelerations Frequency (Hz) -30% below ISO 6897 Limits of Acceleration recommended by ISO 6897 (milli-g) +30% above ISO To ascertain potential duration effects on test subjects, three separate durations were also evaluated for each motion condition, consisting of: 10 minutes, 30 minutes, or 60 minutes. Each test motion duration consisted of the same 10 minutes of simulated motion ; with the 30 and 60 minute motion conditions repeated either three or six times, respectively. It was important for all test subjects to complete the exit questionnaire at the same time after they

3 Wind Engineering, November 8-12, 2009, Taipei, Taiwan had experienced the test motion condition. Therefore, an additional time delay of no motion was added to the beginning of each 10, 30, and 60 minute test motion conditions, consisting of either 52, 32 or 2 minutes, respectively; resulting in a overall total duration of 62 minutes for each test motion condition, as shown in Figure 1. Figure 1: Representative Samples of 10, 30, and 60 Minute Duration Test Conditions

4 Wind Engineering, November 8-12, 2009, Taipei, Taiwan A sample of 578 men and women, aged between 16 and 65, were chosen randomly from an international demographic to participate in the motion simulator study. Approximately 61% of the test subjects were drawn from the general population of Hong Kong, while the remaining 39% came from within the academic community (e.g. consisting of staff, students and faculty members) at HKUST. Seventy percent of the test subjects were male and aged between 16 and 29 years. Each test subject was assigned to only one of 25 different test conditions, with the order determined using a double Latin square randomization design. The interior of the motion simulator test room recreated a work/home environment and was configured to accommodate eight rigid stools. Audio-visual equipment, consisting of four LCD TV screens mounted on four rigid tables (stations) located within each corner of the room, accommodated two test subjects per TV station, as shown in the schematic diagram of Figure 2. All movable objects were secured and the window masked to avoid potential external visual cues. An air-conditioner maintained thermal comfort inside the motion simulator, and masked potential audio cues. A DVD player/recorder simultaneously played an English or Chinese movie (with subtitles) and recorded all test sessions using a CCTV camera mounted in the top corner of the simulator room. Upon completion of each session, test subjects were required to complete an English/Chinese questionnaire. Figure 2: Schematic Diagram of the Motion Simulator Test Room Results and Discussion Test subjects were asked how they first perceived motion during their motion simulator test, with 82% of the test subjects indicating that they had felt the motion. Of those 82% of test subjects who felt the motion, approximately 78% chose their head as the primary location on the body where motion perception occurred. Test subjects were seated on

5 Wind Engineering, November 8-12, 2009, Taipei, Taiwan a stool, with no backrest, allowing the head and body to respond freely to the simulated motion, as previous research suggested that with this configuration the head pivots at the shoulders [Ishimoto and Otsuka (1932), and Chang (1981)]. It has also been suggested by [Kojima et al. (1972)] that humans respond to the head acceleration rather than the floor acceleration, thereby accentuating perception of motion. The percentage of test subjects perceiving motion increased as the frequency increased, demonstrating frequency dependence of motion perception [Chen and Robertson (1972), Irwin (1981a), Goto (1983), Kanda et al. (1990), and Burton (2006)]. The perception of motion variable was further collapsed into two categories ( perceived the motion and not perceived ), with 96% of all test subjects in the perceived the motion category. A Pearson s Chi-Square test was conducted and a highly statistically significant association was found between acceleration (χ 2 =159.1, (df=8, N=578), p<.001) and frequency (χ 2 =148.6, (df=3, N=554), p<.001) and perception of motion. In terms of motion perception, some peoplee have the tendency to become habituated to motion (i.e. become accustomed and stop noticing the motion after perceiving it) as highlighted by [Irwin (1981b), Smith (1988)]. Additional questions related to perception of motion asked if the test subjects had stopped noticing the motion during their test session and if so, the time that it took for them to become accustomed to the motion after it had commenced. Of the 47% of test subjects who stopped noticing the motion, 68% of those claim to have taken less than 10 minutes to become accustomed to the motion, while the remaining test subjects claimed that it took longer than 30 minutes. The effects on well-being that were examined in the current investigationn included the self-assessed responses of nausea, fatigue, dizziness, annoyance, headache and difficulty concentrating. Results indicatedd that with increasing acceleration, the percentage of test subjects who became nauseated also increased within the 0.16 Hz and 0.50 Hz conditions; became dizzy within the 0.25 Hz and 0.50 Hz conditions; became annoyed within the 0.50 Hz conditions; and developed a headache within the 0.25 Hz conditions. As shown in Figures 3-5, the 0.25 Hz and 0.50 Hz test conditions were shown to be the more provocative frequencies. Furthermore, the number of testt subjects who had become annoyed, developedd a headache, and/or had difficulty concentrating in the test conditions at 0.50 Hz was more than double of that for the 0.16 Hz test conditions. Figure 3: Frequency versus the Percentage of Test Subjects experiencing Nausea (left) and Fatigue (right)

6 Wind Engineering, November 8-12, 2009, Taipei, Taiwan Figure 4: Frequency (left) and Duration (right) versus the Percentage of Test Subjects becoming Annoyed Figure 5: Frequency versus the Percentage of Test Subjects developing a Headache (left) and Difficulty Concentrating (right) Excluding the control condition, over half of the test subjects indicatedd they had felt uncomfortable in 22 of 24 test motion conditions. Moreover, increasing the duration did not seem to increase discomfort. It is possible that some test subjects could have experienced a learning effect or acclimatization, as highlighted by [Lauder (1971), Irwin (1981b and 1983)], as test subjects within the 30 and 60 minute test conditions were exposed to the exact same 10 minutes of repeated motion (i.e.. the same waveform was repeated) and had perhaps simply adapted to the motion [Smith (1988), Denoon (2000), Burton (2006)]. It is also of interest to note that within this investigation, discomfort would not necessarily lead a person to complain. Contrary to expectations, test subjects who had indicated that the simulated motion they experienced was comfortable were just as likely to complain, with a Chi-square testt revealing a highly significant association. Sixty-two percent of test subjects claiming that they were comfortable (χ 2 = (df=1, N=186), p<.001), and nearly 70% of those who claimed that they were uncomfortable (χ 2 = 7.53 (df=1, N=352), p<.05), would complain. Complaints from respondents in the general population survey conducted by [Burton (2006)] amounted to 2.3%, far less than test subjects within this investigation. However, those respondents in [Burton (2006)] did not actually experience motion prior to being asked if they would complain. Therefore, such high percentages within this investigation may be attributed to how much the preceding simulated motion that the test subjects had just experienced had affected them. Of those test subjects who claimed that they

7 Wind Engineering, November 8-12, 2009, Taipei, Taiwan would complain, over 60% indicated that potential wind-induced building motion would influence their decision to purchase real estate, as shown in Figure 6. The potential for a tall building to experience significant wind-induced building motion, would influence the decision of approximately 60% of both men and women (equally) to purchase property located on the higher floors of a building. These percentages increased with increasing frequencies of oscillation, which is also shown in Figure 6.. Figure 6: Comfort and Complain (left) and Frequency of Oscillation (right) versus Percentage of Test Subjects Who Would be Influenced to Purchase Property Conclusions The findings from a series of motion simulator experiments suggested that the comfort level and well-being of 578 human test subjects subjected to bi-directional narrow-band random motion was dependent upon increasing frequency of oscillation, acceleration amplitude and duration. Results showed that the percentage of test subjects perceiving motion increased as the frequency increased, demonstrating frequency dependence of motion perception. With an increasing frequency, effects on well-being, including fatigue, difficulty concentrating, annoyance and nausea also increased. Results indicated that the number of test subjects who had become annoyed, developed a headache, and/or had difficulty concentrating in the test conditions at 0.50 Hz was more than double of that for the 0.16 Hz test conditions. Additionally, it was shown that over 50% of test subjects felt discomfort in 22 of the 24 test motion conditions (excluding the control condition). Evidently, habituation occurred for some test subjects. Building motion was found to have the potential to influence a person s decision to purchase property. It was also shown that even comfortable test subjects were just as likely to complain to building management due to wind-induced building motion and this may then influence their decision to purchase/occupy the upper stories of tall buildings. Acknowledgements The authors gratefully acknowledge the enthusiastic contributions of all members of HKUST, Dr. Melissa Burton, and staff of the CLP Power Wind/Wave Tunnel Facility, especially David Leung and Andrew Wong. A special thanks to all the volunteers who patiently participated in this research. This research was approved by the Human Subject

8 Wind Engineering, November 8-12, 2009, Taipei, Taiwan Panel of the HKUST Committee on Research Practices, and funded by the Research Grants Council of Hong Kong (project CA04/05.EG01). References Bachmann, H. (1995). Vibration Problems in Structures. Basel, Switzerland: Birkäuser Verlag. Burton, M. D., (2006). Effects of Low Frequency Wind-Induced Building Motion on Occupant Comfort, PhD Thesis, The Hong Kong University of Science and Technology. Chang, F. K., Yamada, M., and Reed, J. W., (1981), Motion Perception and Tolerance in Planning and Environmental Criteria for Tall Buildings, Monograph of Council for Tall Buildings and Urban Habitat, PC- 13, Chen, P.W. and Robertson, L.E., (1972). Human Perception Threshold of Horizontal Motion", Journal of Structural Division, ASCE, Vol. 98, No. 8, Denoon, R.O., (2000). Designing for Wind-Induced Serviceability Accelerations in Buildings, PhD Thesis, The University of Queensland. Emporis7/2009. (07/01/2009). "High-rise Statistics." Goto, T., (1983). Studies on wind-induced motion of tall buildings based on occupants' reactions, Journal of Wind Engineering and Industrial Aerodynamics, Vol. 13, Home Affairs Department of the Government of the Hong Kong Special Administrative Region (7/2009). International Organization for Standardization (1984). Guidelines for the Evaluation of the Response of Occupants of Fixed Structures, Especially Buildings and Offshore Structures, to Low-Frequency Horizontal Motion (0.063 to 1.0 Hz) ISO , International Organization for Standardization, Geneva, Switzerland. Irwin, A., (1981a). Perception, comfort and performance criteria for human beings exposed to whole body pure yaw vibration and vibration containing yaw and translational components, Journal of Sound and Vibration, Vol. 76. no. 4, Irwin, A.W., (1981b). "Evaluation of Human Response to Pure and Combined Forms of Low Frequency Vibration at Low and High Acceleration Amplitudes", Proceedings of a Meeting of the United Kingdom Informal Group on Human Response to Vibration, Edinburgh, 9-11 Sept., pp Irwin, A.W., (1983). Relative Influence of Noise and Whole-Body Vibration On the Response of Humans, Internoise 83, Proceedings on International Conference on Noise Control Engineering, Edinburgh, July, Vol. II, pp Ishimoto, M. and Otsuka, M., (1932). Determination de 18 limite perceptible des secousses, Bulletin of Earthquake Research Institute Tokyo, No Kanda, J., Tamura, Y. and Fuji, K., (1990). "Probabilistic Perception Limits of Low-Frequency Horizontal Motions", Conference with International Participation, Serviceability of Steel and Composite Structures Proceedings, Pardubice, Czechoslovakia, Kojima, N., Goto, T. and Yamada, M. (1972). "Human perception and the swaying state of the body (Part 1)." Summaries of Technical Papers of Annual Meeting, Planning and Environmental Engineering, Architectural Institute of Japan, Tokyo, Japan, Lauder, J. (1971), Comfort Aspects of Structural Behaviour A Critical Review of Literature, MSc Thesis, The University of New South Wales, Sydney, Australia. Smith, J. W. (1988). Vibration of Structures, Applications in Civil Engineering Design. London: Chapman and Hall. Stafford Smith, B., and Coull, A. (1991). Tall Building Structures. New York: Wiley.