Acceleration Thresholds of Vertical Floor Vibration According to Human Perception Levels

Transcription

1 Acceleration Thresholds of Vertical Floor Vibration According to Human Perception Levels Min Jung Lee, a, Ja Ock Cho, b, Sang Whan Han, c The College of Architecture, Hanyang University, Seoul, Korea, a mjrabbi@hanyang.ac.kr, b juliettoscar@naver.com, c swhan@hanyang.ac.kr ABSTRACT Floor vertical vibration in residence buildings are typically induced by jumping and walking activities of occupants. For controlling the floor vibration induced by occupants, several criteria have been developed mostly based on human perception tests under heel drop impacts. The objective of this study is to propose acceleration thresholds of floor vertical vibration due to heel drop and walking according to four different perception levels; imperceptible, slightly perceptible, distinctly perceptible, and strongly perceptible. For this purpose, shaking table tests were conducted under vertical vibrations generated for heel drop impacts and walking activities with various combinations of frequencies, damping ratios and acceleration amplitudes. Korean twenty subjects were tested. The acceleration threshold obtained by this study using Korean subjects was significantly different from that obtained by former study using American subjects. Test subjects were more sensitive to walking induced vibrations than heel drop-induced vibrations. Furthermore, the effect of damping ratios (2~0%) on acceleration threshold was small. KEYWORDS: Floor vibration, Shaking table, Perception level, Walking, Heel drop. Damping ratio. INTRODUCTION In modern constructions long span floor systems have been widely used due to the development of high strength materials and design methods. Since these systems become more flexible, floor vibration could annoy more the occupants in buildings. Many countries do not have their own criteria for vertical floor vibration. They adopt criteria developed by other countries such as ISO 263-2, CSA, AISC Design Guide Series, and ATC. Most criteria have been developed based on human perception tests under heel drop-induced transient vibrations. Since people in each country have different life styles and the common floor systems in each country also vary, the human perception level on the same floor vibration could be different country to country. This study is to develop criteria on floor vibrations in Korea. For this purpose this study conducted human perception tests using the shaking table. Twenty Korean subjects were tested having different sex, ages, and social and economic backgrounds under heel drop- and waling-induced vibrations generated with various combinations of different damping ratios, peak acceleration amplitudes, and frequencies. Each subject exposed 26 vertical vibrations sequenced in random order. After exposing the subject to each vibration, the subject was asked to classify the vibration into one of four perception levels; imperceptible, slightly perceptible, distinctly perceptible, strongly perceptible. This procedure was also used by Reiher and Meister (946) and Wiss and Parmelee(974). From the test results, accelerations thresholds were proposed for four different perception levels under heel drop impacts and walking activities. The thresholds were compared with those obtained by Wiss and Parmelee (974) and those in the AISC Design Guide Series (997). Furthermore this 44

2 study investigated the effect of different vibration sources (heel drop and walking) and different damping ratios. 2. BACGROUNG OF THE STUDY Reiher and Meister (946) conducted human perception tests using ten subjects. Steady state vibrations lasting about 5 minutes were used which had frequencies of 5~70Hz and amplitude of ~.02mm. Based on the test results, they proposed acceleration thresholds for floor vibrations. Lenzen (966) modified the acceleration thresholds proposed by Reiher and Meister (946) based on heel drop tests using 46 floor systems. It is noted that heel drop impact produces transient floor vibrations. The vibration thresholds in Figure are about 0 times greater than those proposed by Reiher and Meister (946). It was reported that floor damping and mass played an important role on the transient floor vibrations. Wiss and parmelee (974) conducted a shaking table test to provide acceleration thresholds for different perception levels under vertical transient vibrations. Forty subjects were tested with a various combinations of vibration input signals with frequencies of 2.5~25Hz, amplitudes of ~2.5mm, and damping ratios of ~0%. The signals were generated for one foot fall impact with a duration ranging from 0.33~5 second using a mathematical model. Allen and Rainer (976) conducted vibration tests using 42 long span floor systems under heel drop impacts. The proposed criteria were adopted in CSA standard (989) shown in Figure (c). Allen and Murray (993) observed that walking induced vibration could be more annoying to the occupants in residence building, and proposed acceleration thresholds for walking vibration for broad applications. The AISC design guide series (997) and ATC design guide (999) adopted these criteria as shown in Figure (d). Acceleration (%gravity ) 00 0 Strongly Perceptible Distinctly Perceptible Slightly Perceptible Not Perceptible Acceleratoin (%gravity) ' 00 0 Strongly Perceptible Distinctly Perceptible Slightly Perceptible Not Perceptible Acceleration (%gravity ) 00 0 (c) Walking vibration (2% Damping) ( 6% Damping) ( 3% Damping) Acceleration (%gravity) ' 00 0 (d) Rhythmic activity Shopping Office, Residence Figure. Acceleration threshold; Lenzen (966), Wiss and Parmelee (974), (c) CSA (989), (d) AISC Design Guide (997) 442

3 3. FHUMAN PERCEPTION TEST 3. Shaking table test The high performance-shaking table in the Korean Institute of Standards was used which had a plan dimension of.2mx.2m. Rigid chair was specially made and installed on the shaking table as shown in Figure 2. Since sitting position, more reliable data could be obtained from human perception tests using the shaking table. It is also noted that Koreans are mostly sitting on the floors in houses. It is also important to construct similar environments with actual residence buildings since test subjects could be affected by test environments. In this study, wall panels with a height of 2.5m were constructed on the three sides of the table and carpets were placed on the table surface as shown in Figure 2. A picture was attached on the front wall and one light bulb was installed below the picture to notify the end of each vibration signal to the test subjects. Little noise was detected in the testing lab during operating the shaking table. Noise masking was used, which maintained the difference between noise and sound pressure by 30~40 db throughout the test. Playing a solo piano music made a sound pressure. A subject was wearing headphones for listening to the music throughout the test. 3.2 Test signals Two types of vibration input signal were used, which represented vibrations induced by heel drop and walking. Total number of vibration signals experienced by each subject was 26 which had different frequencies, peak accelerations, damping ratios, and types of source as follows ; Frequencies ( f n ) 3, 4.5, 7.5, 9Hz, 2 Peak accelerations( a P ): 0.~7%gravity, 3 Damping ratios (ξ ): 0.02, 0.05, 0., 4 Source types: walking, heel drop. Figure 3 shows the combinations of parameters (frequency and peak acceleration amplitude) used for generating vibration signals. For generating the input floor vibration acceleration signals, two way flat plate slabs were used. Acceleration response histories of the floor under either heel drop or walking were obtained by using commercial FEM software MIDAS GEN The floor dimensions were determined to have four different fundamental frequencies. The heel drop impact was applied at the center of the floor. Normal walking activities with a frequency of 2Hz were applied diagonally. The weight of a person was assumed as 700 N. Figure 4 shows the generated acceleration response histories under heel drop and walking activities. Side wall Front wall Shaking table Chair Picture Side wall Head phone Peak Acceleration(%gravity ) ' Figure 2. Testing room Figure 3. Combination of parameters 443

4 Acceleration (m/s 2 ) sec Acceleration (m/s 2 ) ' Time (sec) Time (sec) Figure 4. Acceleration histories ( f n =4.5Hz, ξ =0.02, a P =) due to : heel drop impact walking 4. TEST RESULT Based on the acceleration data for each perception level, a regression analysis was conducted to provide the mean acceleration threshold for each perception level. Figure 4 shows the acceleration thresholds for different perception levels under heel drop impacts and walking activities. This figure shows that the subjects were more sensitive to walking-induced vibrations than heel dropinduced vibrations. Acceleration thresholds under heel drop impacts are higher than that under walking activities by 5~30% except for perception level imperceptible. For perception level imperceptible minor difference in acceleration thresholds was detected. As shown in Figure 4, the difference in acceleration thresholds due to heel drop impacts and walking activities becomes more significant as the perception level is higher. Figure 5 shows the effect of damping ratios on the acceleration thresholds for four perception levels under heel drop impacts and walking activities. The effect of damping ratio seems negligible for both heel drop- and walking-induced vibrations. It is noted that, in this study, damping ratios of 0.02, 0.05, and 0. were tested. Thus, for damping ratios beyond the range of 0.02~0.0, this observation may not be valid. The acceleration thresholds obtained from this study were compared with those obtained from Wiss and Parmelee (974), and the AISC steel design guide series (997) as shown in Figure 6. Acceleration thresholds obtained from this study using Korean twenty subjects are lower than those obtained by Wiss and Parmelee (974). It can be seen that acceleration threshold specified in the AISC steel design guide series (997) lies between perception levels slightly perceptive and imperceptive obtained in this study. A ccelera tio n (% g ra v ity ) Heeldrop vibration Walking vibration Strongly perceptible Distinctly perceptible Slightly perceptible Not perceptible 0 00 Figure 4. Comparison of acceleration threshold under heel drop impacts and walking activities (ξ =0.02) 444

5 Acceleration (% gravity) ζ = 0.0 ζ = 0.05 ζ = 0.02 Acceleration (% gravity) Figure 5. Evaluation of damping ration effect; heel drop walking vibration A cceleration (% gravity) This study Other criterion Strongly perceptible Distinctly perceptible Slightly perceptible Not perceptible 0 00 Acceleration (% grabity ) Frequncy (Hz) Figure 6. Comparison perception levels under heel drop impacts ( =2%); () Wiss and Parmelee (974), AISC Steel Design Guide series (997) 6. CONCLUSIONS This study proposed acceleration thresholds according to four perception levels under heel drop impacts and walking activities. For the purpose, the shaking table test was conducted using twenty Korean subjects having different ages, sex, and social backgrounds. This study observed that subjects were more sensitive to walking-induced vibrations than heel drop-induced vibrations even though both vibrations had the same peak acceleration amplitude. This phenomenon was more prominent as vibrations become more severe. This leads to lower acceleration thresholds for walking-induced vibrations than heel drop-induced vibrations. Acceleration thresholds for different perception levels did not vary with respect to different damping ratios. This finding holds for both heel drop- and walking-induced vibrations. It is observed that acceleration thresholds for a given perception level obtained from this study is much lower than Wiss and Parmelee (974). This indicates that human perception on floor vibrations could vary country to country. The acceleration thresholds in the AISC Steel Design Guide series (997) lies between acceleration thresholds of perception levels imperceptive and slightly perceptive obtained by this study. 445

6 REFERENCES Allen, D. E. and Murray, T. M., 993. Design criterion for vibrations due to walking. Engineering Journal, AISC, 4 th Qtr, pp Allen, D. E. and Rainer, J. H., 976. Vibration criteria for long span floors. Canadian Journal of Civil Engineering, National Research Council of Canada, Vol.3, No.2, pp Applied technology council, 999. Minimizing floor vibration. ATC Design guide, Redwood City, CA. CSA, 989. Canadian Standard CAN3-S6.-M89 : Steel Structures for Buildings Limits States Design : Appendix G. Guide for Floor Vibration, Rexdale, Ontario. International Standard Organization, 989. Evaluation of Human Exposure to Whoe-Body Vibratoin- Part 2 : Continuous and Shock-Induced Vibration in Buildings. ISO 263-2, pp Lenzen, K. H., 966. Vibration of Steel Joist Concrete Slab Floors. Engineering Journal, AISC, Vol.3, No.3, pp Murray, T., Allen, D. E., and Ungar, E. E., 997. Floor vibration due to human activities. Steel design guide series, AISC, Chicago, IL, pp Reiher, H. and Meister, F. J., 946. The effect of vibration on people. Proc., 7 th Conf. Of the Int. Association for Computer Methods and Advances in Geomechanics, Cairns, pp Wiss, J. F. and Parmelee, R. A., 974. Human perception of transient vibrations. Journal of the Structural Division, ASCE, Vol. 00, pp