______http://www.paper.edu.cn THE STRUCTURAL DESIGN OF TALL AND SPECIAL BUILDINGS Struct. Design Tall Spec. Build. 12, 67–82 (2003) Published online in Wiley InterScience (www.interscience.wiley.com). DOI:10.1002/tal.213

FIELD MEASUREMENTS OF WIND EFFECTS ON THE TALLEST BUILDING IN †

Q. S. LI1*, Y. Q. XIAO1,2, C. K. WONG1 AND A. P. JEARY3 1Department of Building and Construction, City University of Hong Kong, Kowloon, Hong Kong 2School of Civil Engineering, Harbin Institute of Technology, Harbin, 3School of Building and Construction Science, University of Western Sydney, NSW, Australia

SUMMARY Central Plaza has a height of approximately 374 m and is 78-storeys tall. It is the tallest structure in Hong Kong and was the highest reinforced-concrete building in the world when it was built several years ago. This paper describes some results obtained from the full-scale measurements of wind effects on Central Plaza. The field data such as wind speed, wind direction and acceleration responses have been measured during the close passage of several typhoons in recent years. Detailed analysis of the field data is conducted to investigate wind effects on the tall building. The full-scale measurements are compared with the wind tunnel results. The amplitude-dependent characteristics of damping that were obtained by using the random decrement technique are investigated on the basis of the field measurements. Copyright  2003 John Wiley & Sons, Ltd.

1. INTRODUCTION Hong Kong is close to the most active typhoon-generating area in the world, with tropical cyclones formed in the western Pacific Ocean or in the several times each year. This makes an accurate determination of design wind loading for high-rise structures of vital importance for Hong Kong. Recently, many super-tall buildings (building height greater than 350 m) have been or are being built in Hong Kong and mainland China in response to expanding economies. As most codes for structural design are made for usual tall buildings, the current codes are not guaranteed to cover the design of super-tall buildings. It is thus necessary to investigate wind effects on such tall buildings. Although there have been many advances in wind tunnel testing and numerical simulation techniques, many critical phenomena can still only be investigated by full-scale experiments. The most reliable evaluations of dynamic characteristics and wind effects are obtained from experimental measurements of a prototype building. Field measurements of wind effects on buildings and structures date back about 100 years. However, practical difficulties associated with operation and maintenance and problems with instrumentation reliability led to many of the early field measurements to be unreliable. Furthermore, very-low-frequency activity often introduces bias and nonstationarity into data records. More modern techniques to avoid such problems are available and are being used in isolated cases. During the past two decades a revolution in data handling and collection has made enormous strides possible in full-scale measurements of wind effects on tall buildings. However, reliable full-scale measurement of wind effects on super-tall buildings is still very limited, in particular under typhoon conditions. Hence, there is a need for building up such a database. Central Plaza, which has a height of approximately 374 m and 78 storeys, is the tallest structure in

* Correspondence to: Q. S. Li, Department of Building and Construction, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong. E-mail: [email protected] † This article was originally published online as an EarlyView article under the journal’s previous title of The Structural Design of Tall Buildings, ISSN 1062-8002. The DOI of the online article is unchanged but the article should now be cited under the journal’s new title, The Structural Design of Tall and Special Buildings. ISSN 1541-7794.

Copyright  2003 John Wiley & Sons, Ltd. Received February 2002 Accepted March 2002 中国科技论文在线 http://www.paper.edu.cn 68 Q. S. LI ET AL.

Figure 1. The building site

Hong Kong and was the tallest reinforced-concrete building in the world when it was built several years ago. The basic planform of the building is essentially triangular. This tall building is located in Wanchai North commercial district in the heart of Hong Kong’s expanding business centre. Figure 1 shows a diagram of the Hong Kong area, and the point represents the location of the building. The siting of the building is very close to the seashore and on the lee slope of extremely hilly terrain in a typhoon area; therefore, this tall building may be subjected to extreme horizontal wind forces under typhoon conditions. This fact makes a study of the wind effects on this tall building of particular importance. Therefore, full-scale measurements of wind effects on this building under Typhoon conditions were conducted in order to provide important validation of design procedures and an assurance of acceptable behavior of this high-rise structure as well as to evaluate the accuracy of wind tunnel test technologies. Some selected results are presented in this paper. Long-term field measurements of wind effects on several tall buildings in the vicinity of Central Plaza are also being made.

2. MEASUREMENT INSTRUMENTATION Two accelerometers were installed at the elevation of the 73th floor of the building to provide measurement of the accelerations. The accelerometers were placed orthogonally, as shown in Figure 2. Acceleration responses are continuously acquired and digitized at 20 Hz and were amplified and low- pass filtered at 10 Hz before digitization. Two rotating cup-type anemometers were installed on the 75th floor of Central Plaza by the Hong Kong Observatory Department. However, these mechanical cup-type anemometers are too slow to offer the frequency resolution of 20 Hz that we need for the study of turbulence characteristics. For this reason, two Gill-propeller-type anemometers were installed on the masts erected on top of a tall building nearby (approximately at an elevation of 350 m from the ground). These Gill-propeller-type anemometers produce analog output voltages proportional to the wind speed and wind direction at the height equivalent to the top-floor level of Central Plaza. The data outputs of the present field measurements include acceleration responses (two channels), wind speeds (two channels) and wind directions (two channels), all of which were measured simultaneously. Figure 3 shows samples of the measurements. Meanwhile, the wind velocity data

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Figure 2. The top view and orientation of the building &, locations of accelerometers. Note; there are two accelerometers, on the level of the 73rd floor, pointing as indicated by the arrows

recorded by the Hong Kong Observatory Department on the 75th floor were also analysed for the purposes of wind velocity calibration.

3. SPECTRAL ANALYSIS OF WIND SPEED AND ACCELERATION RESPONSE The measurements of wind action and wind-induced vibration of this tall building during the passage of Typhoon Sally were made in September 1996. As reported by the Hong Kong Observatory (1996), Typhoon Sally developed as a tropical depression about 1300 km east of on 5 September 1996. Moving west-northwestwards, it intensified over water and attained typhoon strength on the 7 September. Typhoon Sally entered the South China Sea the next morning and moved rapidly towards the coast of western province, People’s Republic of China. After traversing Leizhou peninsula and province on 9 September, Typhoon Sally moved into northern and dissipated over land the next day. Figure 4 shows the route of Typhoon Sally (Hong Kong Observatory, 1996). Some 27 hours of data recorded during the passage of the typhoon were analysed and some results are presented in this paper. The maximum instantaneous wind speed measured from the two Gill-propeller-type anemometers during the passage of Typhoon Sally is 30 m sÀ1, suggesting that the strength of Typhoon Sally was moderate, since it did not hit Hong Kong directly, as shown in Figure 4. The first 15-hour wind velocity record was used for spectral analysis and the obtained spectrum is presented in Figures 5; a sampling number of 1080000 corresponding to a continuous record of 15 hours was adopted for the spectral analysis presented in this figure. In the first 4-hour period Typhoon Sally blew approximately parallel to one accelerometer axis (direction 2 in Figure 2); that is, the measurement axes of acceleration responses were along-wind and cross-wind during this period. Figures 6(a) and 7(a) present the corresponding acceleration response spectra measured in direction 1 (cross-wind direction) and direction 2 (along-wind direction), respectively. In order to examine the participation of the various modes of vibration, the logarithmic plots of the acceleration spectra in the two directions are presented in Figures 6(b) and 7(b). These spectra of wind action and

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Figure 3. Samples acquired from the measurements Acc., acceleration; Ch., channel

acceleration were all obtained from a direct analysis of the anemometer and accelerometer output data that were measured simultaneously. Figure 5 shows the normalized spectrum of wind speed calculated on the basis of the measured data with a long sample (15-hours) in Typhoon Sally. The spectrum estimated using the Von Karman spectrum is also presented in Figure 5 for comparison purposes. The Von Karman type spectrum was calculated on the basis of the turbulence integral scale determined from the full-scale measurements. It

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Figure 4. Route of Typhoon Sally can be seen that the shape of the power spectral density of wind speed that was measured well above the central district of Hong Kong Island agrees fairly well with the Von Karman spectrum. The value of the turbulence integral scale determined based on the data measured during the 15-hour record period under typhoon conditions is 300 m. It can be seen in Figures 6 and 7 that the wind-induced response of this building is primarily in the two fundamental sway modes of vibration, but higher modes are also clearly present, especially as shown in the logarithmic plots. Figure 6(a) shows that the power spectrum of acceleration in direction 1 (cross-wind direction) has a dominant peak at the first-sway mode, which is actually the fundamental

Figure 5. Power spectral density of wind speed during a 15-hour recording period

Copyright  2003 John Wiley & Sons, Ltd. Struct. Design Tall Spec. Build. 12, 67–82 (2003) 中国科技论文在线 http://www.paper.edu.cn 72 Q. S. LI ET AL.

Figure 6. Power spectral density of acceleration in direction 1 (see Figure 2): (a) acceleration response spectrum; (b) logarithmic plot of acceleration response spectrum

sway mode in direction 1. The same phenomena was found in accelerometer 2 (in direction 2, along- wind direction). The measured natural frequencies of the first sway mode in direction 1 and direction 2 are 0Á253 Hz and 0Á244 Hz, respectively (see Figures 6 and 7). The corresponding natural frequencies

Figure 7. Power spectral density of acceleration in direction 2 (see Figure 2): (a) acceleration response spectrum; (b) logarithmic plot of acceleration response spectrum

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Figure 8. Probability density function of wind speed. Note: V-Vmean, wind speed minus mean wind speed; Vrms, standard deviation of wind speed

calculated by the structural engineers from a computational dynamic analysis at the design stage of this building are 0Á208 Hz and 0Á206 Hz (Ho and Surry 1989). There is about 17% difference between the calculated and measured natural frequencies for the two fundamental sway modes. It is maintained that the 17% difference between the calculated or measured natural frequencies for the two fundamental sway modes is attributable to several reasons, including the fact that the effective mass of the building is less than that assumed at the design stage or the effective stiffness of the building is higher than that determined at the design stage owing to the contribution of nonstructural components. As a result, the measured natural frequencies for the two fundamental sway modes are larger than those calculated.

4. PROBABILITY ANALYSIS OF WIND SPEED AND ACCELERATION RESPONSE Knowledge of the probabilistic nature of wind speed and the corresponding wind-induced vibrations of tall buildings is important and required in the design of tall buildings and high-rise structures. The shape of a probability density function (PDF) can be characterized by its third and fourth central moments (3 and 4) relative to the standard deviation value (). For a normal or Gaussian 3 3 distribution, the skewness coefficient, Sk = 3/ , and the kurtosis coefficient, Ku = 4/ , are equal to 0 and 3, respectively. Sk < 0 corresponds to skewness to the left, and Sk > 0 corresponds to skewness to

Table 1. Characteristics of wind speed in the first 4-hour record duration

À1 V (m s ) v/VSk Ku 15Á122Á5% 0Á0637 2Á967

Note: V, mean wind speed; v, standard deviation of wind-speed fluc- tuation; v/V, turbulance intensity; Sk, skewness coefficient; Ku, kurtosis coefficient.

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Figure 9. Probability density functions of acceleration response in (a) direction 1 and (b) direction 2. Note: Ch., Channel; A-Amean, acceleration minus mean acceleration; Arms, standard deviation of accelerations

the right. Ku > 3 represents a distribution more peaked at the centre than the Gaussian, and Ku < 3 characterizes distributions flatter at the centre than the Gaussian. The word ‘kurtosis’ represents the curvature of PDF.

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Table 2. Wind conditions and characteristics of acceleration responses in directions 1 and 2

À1 V (m s ) Sk1 Ku1 Sk2 Ku2 15Á1 À0Á0407 3Á8691 À0Á0048 4Á2373

Note: V, mean wind speed; Sk, skewness coefficient; Ku, kurtosis coefficient; subscript 1 and 2 refer to direc- tions 1 and 2, respectively.

The probability distributions of wind speed were obtained from direct analysis of the outputs of the anemometers. A sampling number of 288 000 corresponding to record duration of the first 4 hours was adopted for the probability analysis in this paper. Figure 8 shows a typical PDF of wind speed. A normal PDF is also plotted in this figure for comparison purposes. Table 1 presents the mean wind speed and turbulence intensity in this record duration. V is the mean wind speed and v/V is the turbulence intensity, v represents the standard deviation of wind-speed fluctuation. It can be seen from Figure 8 that the PDF of the wind speed in this record duration approximately follows a normal distribution. Meanwhile, as shown in Table 1, the values of skewness and kurtosis (Sk =0Á0637, Ku =2Á967) are close to those of the normal distribution, illustrating that, in engineering applications, taking the probability distribution of wind speed as normally distributed would be reasonable. It has been recognized that the serviceability of tall buildings is affected mainly by excessive acceleration experienced at the top floors in strong windstorms, which may cause discomfort to the occupants. Building acceleration is most appropriate for establishing checking procedures for structural serviceability requirements. This has made it necessary to design tall buildings that will not exceed the specified acceleration response level. Therefore, knowledge of the probability distribution of acceleration response based on measured field data from tall buildings is particularly useful in tall building design. The probability density function of acceleration responses, which were calculated from the 4-hour record, is shown in Figure 9. Table 2 presents the wind condition for measuring the acceleration responses and the calculated statistical parameters of the acceleration responses. It can be seen from Figure 9 that the PDFs of acceleration in the along-wind and cross-wind directions are distinctly different from the normal distribution in the regions of both tails. The actual probability distributions are much larger than those described by the normal distribution in the regions of both tails. This indicates a much higher probability for the occurrence of larger acceleration than that predicted by a Gaussian PDF. The probability distribution in the region of the tails has a significant effect on the estimation of peak acceleration, which is required and important for structural serviceability assessment. The estimation of peak acceleration responses derived from the Gaussian distribution could cause a large error. It is worth noting that the practical importance lies more in the maximum and minimum value of acceleration responses in the region of the tails than those around the mean. This is because of the fact that the peak acceleration is an important indicator of discomfort felt by occupants in tall buildings. From the results presented in Table 2, it is also clear that the Gaussian distribution is not suitable for describing the probability distribution of acceleration response, because the kurtosis coefficients determined from the measured field data are distinctly different from 3Á0.

5. WIND-INDUCED VIBRATIONS A typical example of locus of the acceleration responses measured during Typhoon Sally is shown in

Copyright  2003 John Wiley & Sons, Ltd. Struct. Design Tall Spec. Build. 12, 67–82 (2003) 中国科技论文在线 http://www.paper.edu.cn 76 Q. S. LI ET AL.

Figure 10. Locus of the building acceleration

Figure 10. It can be seen from the figure that the cross-wind response of this building (direction 1) is less than the magnitude of the along-wind response (direction 2) for this case. The relationship between the root mean squared (rms) of the measured acceleration response averaged over a 1 hour period and the mean wind speed measured from the Gill-propeller-type anemometers is shown in Figures 11 and 12. It appears that both components of acceleration increased monotonically with mean wind speed during the passage of Typhoon Sally. No significant vortex shedding was found for this slender structure. The cross-wind response is caused mainly by turbulence

Figure 11. Relation between wind and acceleration response in direction 1: (a) root mean squared (RMS) acceleration (Acc.) of channel 1 (ch. 1); (b) logarithmic plot

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WIND EFFECTS ON CENTRAL PLAZA 77

Figure 12. Relation between wind and acceleration response in direction 2: (a) root mean squared (RMS) acceleration (Acc.) of channel 2 (ch. 2); (b) logarithmic plot

buffeting, plus the contributions of coupling between the along-wind and cross-wind acceleration components and so on. For the data presented in Figures 11 and 12, the regression curves of acceleration response for each direction are expressed by:

 2?93 A1 ˆ 0?0000121V mg 1†

in direction 1, and

 2?51 A2 ˆ 0?0000462V mg 2†

in direction 2. The measured data during Typhoon Sally show that the standard deviation of acceleration response in direction 1, A1, is proportional to the wind-speed equivalent at the top of the building raised to the power of 2Á93, and that the standard deviation of acceleration response in direction 2, A2, is proportional to the wind-speed equivalent at the top of the building raised to the power of 2Á51.

6. COMPARISON WITH WIND TUNNEL MEASUREMENTS It is always useful to compare model test results with actual performance, in particular, for a super-tall building such as Central Plaza. Such comparisons are scarce and have rarely been made for a super-tall building under typhoon conditions and in the region of complex topography. Wind tunnel tests for the tall building were conducted in the Boundary Layer Wind Tunnel Laboratory (BLWTL) at the University of Western Ontario (UWO) in 1989 before the building was constructed (Ho and Surry, 1989). The wind engineering study carried out at UWO included the

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Figure 13. Comparison of field measurements with wind tunnel test data: (a) azimuth angle T = 78°; (b) azimuth angle = 117°

determination of overall wind loads and dynamic responses of this building. The wind-induced vibration of the tall building was investigated through the force balance model tests conducted in the BLWTL at UWO. As introduced in Ho and Surry (1989), the force balance wind tunnel tests were carried out at a

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geometric scale of 1:500. Measurements were made of the mean and dynamic components of the model forces at the foundation level of the building. The model of the wind tunnel tests reproduced the topography and all major buildings around the project site. The two translational accelerations and the rotational accelerations near the top of the building (at a full-scale height of 298 m above the foundation level) were determined based on the wind tunnel tests. Ho and Surry (1989) gave a detailed introduction about the wind tunnel tests. By examining the measured wind velocity data it was found that during the passage of Typhoon Sally the wind direction in the first 4-hour duration varied mainly around 60°–80°, with a mean wind direction of 78°. The rest of the time, wind direction varied in the range of 113°–125°, with a mean wind direction of 117°. This implies that the wind directions during these two periods can be taken as constant (78° and 117°, respectively). Thus, it is reasonable to assume the field acceleration data were measured under wind-attack angles of 78° and 117°, respectively, during the two record periods. Figure 13 shows a comparison between the full-scale measurements and the model test results at different wind speeds for these two azimuthal sectors. In Figure 13, the wind tunnel data were extracted from Ho and Surry (1989) and the field data were measured during the passage of Typhoon Sally, except those corresponding to wind speeds greater than 25m sÀ1, which were estimated from Equations (1) and (2). Basically, the agreement between the two sets of data is satisfactory, especially for the data measured in direction 2, as shown in Figure 13, although the model measurements in direction 1 are conservative when the wind speed at top of the building is greater than 25 m sÀ1.

7. AMPLITUDE-DEPENDENT DAMPING CHARACTERISTICS OF THE TALL BUILDING The determination of damping ratios is very important in exactly estimating responses of high-rise structures at the design stage. Damping is found as a nonlinear parameter and increases with increasing amplitude. Hart and Vasudevian (1975) reported varying damping values for amplitudes that increased under earthquake excitation. This was one of the earliest reported investigation of amplitude- dependent damping of buildings. Over the past three decades significant measurements for structural damping have been made throughout the world (e.g. Fang et al., 1999; Jeary, 1986; Li et al., 1998, 2000; Tamura and Suganuma, 1996). However, our literature review reveals that the amplitude- dependent damping contained in the literature concerns mostly mid-rise buildings, in the vicinity of 20 storeys or shorter. Thus, there is a serious scarcity of damping data for high-rise buildings taller than 20 storeys, especially for super-tall buildings such as Central Plaza. The measured acceleration data can be used to obtain the dynamic characteristics of the building (damping, natural frequencies, etc.). The modified random decrement technique that we have developed was employed to evaluate the damping in this building. As pointed out earlier (Jeary, 1992, 1996; Li et al., 1999), the random decrement technique represents a quick and practical method for establishing nonlinear damping characteristics. As discussed in Section 3, the response of the building is dominated by the fundamental sway mode in each direction; however, these modes are coupled, and, in order to obtain the damping estimates, the fundamental mode responses were band-passed with a 4096-pole filter before processing the random decrement. The damping estimates for the two accelerometers were obtained and a typical example of damping curves based on the acceleration data measured in direction 2 is shown in Figure 14. Information on amplitude-dependent damping obtained from Central Plaza should be very useful, since similar measurements are still very limited for such a super-tall building. The damping curve [damping (per cent) against amplitude] shown in Figure 14 clearly demonstrates nonlinear energy

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Figure 14. Variation of damping ratio with amplitude of the building

dissipation characteristics in the building. It is clear that the damping increases with increase in amplitude. The values of damping estimated and shown in Figure 14 comprise both structural damping and aerodynamic damping. As yet there is no widely accepted method available for evaluating damping ratios of buildings prior to construction. In the wind tunnel tests at the design stage, Ho and Surry (1989) predicted the loads and response of this building for cases of structural damping assumed to be 1%, 1Á5% and 2%. From Figure 14, it appears that the assumption of a value of 1Á0% is reasonable, at least as far as an amplitude appropriate to the serviceability criterion for this building is concerned, as the damping curve presented in Figure 14 was measured in moderate wind conditions. Continuing research is aimed at determining the characteristics of this curve at higher amplitudes and, in particular, at ascertaining the onset of a high-amplitude plateau value of damping.

8. CONCLUDING REMARKS This paper presents some results measured on a 78-storey building under typhoon conditions. Based on the spectral analysis of data measured from relatively long samples during Typhoon Sally (15 hours), it was found that the wind-induced response of the building is primarily in the two fundamental sway modes of vibration, but higher modes are also present. The measured natural frequencies of the first- sway and the second-sway modes are 0Á244 Hz and 0Á253 Hz, respectively. There is a 17% difference between the natural frequencies for the first two sway modes calculated in the design stage and the measured values. This may be attributed to the fact that the effective mass of the building is less than that assumed at the design stage, or the effective stiffness of the building is higher than that determined at the design stage owing to the contribution of nonstructural components. As a result, the measured natural frequencies for the two fundamental sway modes are larger than those calculated. The Von Karman type spectrum is found to be able to describe the energy distribution fairly well for wind speeds above the central district in Hong Kong Island. The average value of the turbulence integral scale is 300 m during a 15-hour record period for Typhoon Sally.

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WIND EFFECTS ON CENTRAL PLAZA 81

The results of probability analysis show that, strictly speaking, the PDF of wind speed is not a normal distribution but is not very different from a normal distribution. Thus, taking the fluctuations of wind speed as normally distributed would be reasonable. However, it was found that the PDF of accelerations in the regions of both distribution tails are distinctly different from those of the normal distribution. The actual probability distributions are much larger than those described by the normal distribution for both tails. This indicates a much higher probability for the occurrence of larger accelerations than that predicted by a Gaussian PDF. Therefore, taking acceleration responses as a normal distribution would result in considerable error in estimating peak acceleration response. Wind-induced acceleration responses were found to be monotonically increasing with wind speed atop the building. No significant vortex shedding was found during the passage of Typhoon Sally for this slender structure. The measured field data show that the standard deviation of acceleration response in direction 1 is proportional to 2Á93 powers of wind speed atop the building, to 2Á51 powers in direction 2. The field-measured acceleration responses have been compared with the wind tunnel results obtained in the BLWTL at UWO. The measured acceleration data are broadly consistent with those obtained in the force balance model study, especially for the data measured in direction 2, although the model measurements in direction 1 are conservative when the wind speed atop the building is greater than 25 m sÀ1. Information on structural damping obtained from the building is extremely useful, since similar measurements are still very limited for such super-tall buildings. The measured damping demonstrates amplitude-dependent characteristics and increases with increasing amplitude. It is concluded that the assumption of the critical damping ratio as 1Á0% at the design stage for wind-resistant design of this building appears reasonable for the serviceability condition of the building.

ACKNOWLEDGEMENTS The work described in this paper was fully supported by a grant from the Research Grant Council of Hong Kong Special Administrative Region, China (Project No. CityU 1054/98E, CityU Reference No. 9040448), which is gratefully acknowledged.

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Li QS, Liu DK, Fang JQ, Jeary AP, Wong CK. 2000. Damping in buildings: its neural network and AR model. Engineering Structures 22: 1216–1223. Tamura Y, Suganuma S. 1996. Evaluation of amplitude-dependent damping and natural frequency of buildings during strong winds. Journal of Wind Engineering and Industrial Aerodynamics 59: 115–130.

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