WIND TUNNEL STUDIES AND THEIR VALIDATIONS WITH FIELD MEASUREMENTS FOR A SUPER-TALL BUILDING

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1 The Seventh Asia-Pacific Conference on Wind Engineering, November 8-12, 2009, Taipei, Taiwan WIND TUNNEL STUDIES AND THEIR VALIDATIONS WITH FIELD MEASUREMENTS FOR A SUPER-TALL BUILDING Q.S. Li 1, Jiming Xie 2, Alex To 3, L. H. Zhi 1 1 City University of Hong Kong, Hong Kong, bcqsli@cityu.edu.hk, Lunhazhi@cityu.edu.hk 2 RWDI Inc. Canada, Jiming.Xie@rwdi.com 3 Ove Arup and Partners, Hong Kong, alex.to@arup.com ABSTRACT This paper presents selected field measurement results of wind effects on a super-tall building in Hong Kong during the passage of Typhoon Hagupit in September For comparison purposes, the building s dynamic responses were re-calculated based on the high-frequency force-balance (HFFB) model data obtained in the design stage of the building and the newly identified structural dynamic properties from the field measurements. This provided an excellent opportunity to evaluate the accuracy of the model test results and the adequacy of the techniques used in wind tunnel tests. The comparison of the wind tunnel predictions and the full-scale measurements revealed that the measured acceleration data were consistent with those obtained in the model tests. KEYWORDS: WIND TUNNEL TEST, FIELD MEASUREMENT, WIND-INDUCED VIBRATIONS, STRUCTURAL DYNAMICS CHARACTERISTICS 1. Introduction A super-tall building, which has a height of about 420m, is located in Central, Hong Kong Island. The superstructure is an 88-storey office tower with five additional basement levels going down to -32mPD. It is the tallest building in Hong Kong. The sitting of the tall building is very close to the seashore in an active typhoon generating area. This super-tall building may be subjected to severe wind forces induced by typhoons. All these facts made it necessary to investigate the performance of the super-tall building under typhoon conditions. At the design stage of the tall building, detailed wind tunnel studies were carried out for evaluating its wind-induced responses. The wind tunnel studies included a 1:3000 scale topographic model to determine the detailed local wind conditions at the project site. The information obtained from the topographical model tests was then used in the boundary layer wind simulations for the 1:400 scale high-frequency force-balance (HFFB) model tests. The predicted wind-induced building responses, including design wind loads and building accelerations, were then used in the structural design. A main concern for tall buildings in Hong Kong is the impact of typhoons, in particularly the building s motion in terms of occupants comfort level. Although wind tunnel simulations were believed to provide good indications on wind-induced responses, this concern did not go away until the new structure successfully experienced the first typhoon hit (Cermak, 2003). To further investigate the building s wind-induced responses at full-scale and to verify the wind tunnel predictions, City University of Hong Kong installed a wind and movement

2 monitoring system including anemometers, accelerometers, GPS and pressure sensors on the building and successfully measured the building s responses during the passages of several typhoon in 2007 and The building s dynamic properties were also identified based on the measurements. Figure 1 shows the roof plan, the coordinate system in reference to the north and the locations of the installed accelerometers (denoted by A1, A2, A3 and A4). This paper presents selected field measurement results of wind effects on the super-tall building during the passage of Typhoon Hagupit in September Based on the measured dynamic properties, the building s wind-induced responses were re-calculated based on the HFFB model data obtained at the design stage of the building. Comparisons were made between the field measured acceleration responses and those from the wind tunnel test to evaluate the accuracy of the model test results and the adequacy of the HFFB technique used in wind tunnel tests. Figure 1: Building plan 2. Introductions of the Super-Tall Building The general footprint of the super-tall building is about 57m x 57m, while at the roof level this area is reduced to 39m x 39m. The gross floor area of the office tower is about 180,000 square meters with typical floor-to-floor height 4.2m. The structural system consists of a central reinforced concrete core wall linked by steel beams and outriggers to eight exterior composite mega-columns. Two secondary columns are located at each corner of the building to support the gravity load. Composite slabs are used as floor slabs, comprising 460mm deep steel secondary beams spanning from the core wall to the 900mm deep primary girders spanning between the mega-columns. Four sets of outrigger and belt truss systems are built to stabilize and strengthen the external steel frame onto the core wall. The core wall measures 29m by 27m at its base with a maximum wall thickness of 1.5m. The wall is made of Grade 60 reinforced concrete. The core houses the primary building functions, including the elevators, stairs, toilets and mechanical rooms for containing building services facilities. 3. Description of HFFB Method Used in the Study HFFB method provides a convenient approach in wind tunnel testing for determining wind-induced response of tall buildings. HFFB is also referred to as High-Frequency Base Balance (HFBB) in some literature because most of its applications are with the force balance being set at the model base. HFFB is basically a method that combines the experimental and analytical approach. In a mathematic model that describes the wind-induced structural response, only the wind

3 excitation part is unknown while the structural dynamic part can be determined analytically using available structural analysis tools such as finite-element method. The extent of wind excitations is sensitive to the local wind environment, the building geometry, the aerodynamic interactions with adjacent structures and potentially the structural motions. Due to these complicated factors, the wind excitations are difficult to be determined analytically. For most buildings, the aeroelastic effects are less significant because the building deflections are normally very small compared with the building horizontal dimensions and the building mass is often very high. With aeroelastic effects being neglected, the wind excitation can be determined by measuring a rigid model (i.e., a HFFB model) in a simulated wind environment with modeled surroundings. The measurements consist of mean wind loads and background dynamic loads. By including these loads in the equation of motion for modal analysis with random-vibration theory, the structural resonance response can be calculated and the total wind loads are therefore determined. Figure 2: Measured normalized spectra of accelerations Since its original development about 27 years ago (Tschanz 1982), HFFB method has been improved significantly to deal with more complex structures. The HFFB method used in the current study was developed by Xie and Irwin (1998). One of the advantages with this method is to deal with non-linear mode shapes for complicated turbulent wind profiles. In the original HFFB method, one of the main assumptions is that the building sway mode shape has to be linear, so that the generalized wind loads used in the equation of motion can be directly obtained from the measurement of the overturning moment on the test model. ~ z Pt () pzt (,) (,) H dz 1 H pztzdz M B = = = H H H ( 1 ) where H is the building height; the ratio (z/h) represents an idea linear mode shape; p(z,t) is the horizontal wind loads at elevation z; and MB is the measured overturning moment. This assumption soon became an interesting research topic and many studies were conducted to assess its acceptance in the response prediction (Holmes 1987, Boggs 1989, Xu 1993, Yip 1995, Xie 1998, Chen 2005). In these studies, a range of mode shape nonlinearity was considered by assuming idealized gust wind profiles with a range of power law exponent constants. As a result of these studies, a number of schemes for mode shape corrections were proposed. In general, different mode shape correction factors are needed for different response components.

4 For a building in a typical urban setting, the real situation could be much more complicated than the idealized wind profiles. With aerodynamic interactions and wake effects generated by adjacent structures, the gust wind profile tends to be very complicated. In some cases, the wind gust at the upper portion and the lower portion of a building can be in an opposite phase. Aware of these complications, Xie and Irwin (1998) proposed a new analytical framework that determines the generalized force for nonlinear mode shapes directly, so that the needs for gust wind profile assumptions and the mode shape correction factors can be eliminated. The key step of this method is to identify the equivalent gust wind pressure distributions from the simultaneously measured overturning moments and base shears. Although a representative pressure distribution for torsion cannot be identified in the same way as for shears, a rational choice is to assume the pressure distributions for torsion have a similar shape as the weighted pressure distributions of shears. This implies that the exterior torsional loads are induced by the offset of the exterior shears. With this method, the generalized force for the j-th mode is given by = ( Υ + Λ ) + ( Υ + Λ ) My t M ( jmy jmy ) ( jmx jmx ) ~ P () t F () t F () t j jfx jfx x jfy jfy y () x () t Mz () t + Υ + Λ + Υ + Λ + ΥjMz h h r (2) where F x and F y = measured base shears in two orthogonal directions; M y and M x = measured base overturning moments in two orthogonal directions; M z = measured base torque; Υ j[.] and Λ j[.] = contribution factors as a function of mode shapes and building properties; h = building height; and r = typical radius of gyration used for normalizing torsional mode shapes. Various validation studies have been undertaken for this method by using wind tunnel experiments as well as conducting analytical studies (Xie and Garber 2008). The validation results confirm that this improved HFFB technique is adequate for engineering applications. The comparison with in-situ measurements presented in this paper can be considered as a further validation study of this method. 4. Results and discussion Based on the measured acceleration response spectra during Typhoon Hagupit in September 2008, the fundamental building natural frequencies were identified to be about 0.142Hz for both x- and y- directions, as shown in Figure 2. Meanwhile, the Random Decrement Technique was employed to determine amplitude-dependent damping properties, as shown in Figure 3. The damping ratio curves (damping ratio versus vibration amplitude) presented in the figures demonstrate non-linear energy dissipation characteristics of the building. Similar to the damping measurements from other super-tall buildings (Li et al., 1998, 2005a, 2005b, 2008), the overall tendency is observed to be that the damping ratios increase with increase in amplitude. From the measurements of damping, it appears that damping values of 1.0% of critical are reasonable for wind-resistant analysis of the super-tall building for serviceability consideration. The wind-induced responses of the building were re-calculated based on the HFFB model data obtained at the design stage of the building and the newly identified structural dynamic properties based on the field measurements. During Typhoon Hagupit, the recorded mean wind velocities from three anemometers mounted at a mast set up at the roof (anemometer height 420.6m above ground) varied between 14m/s and 27m/s at the mean incident wind direction around 120 degrees clockwise

5 from the north. According to the local wind climate statistics, this represents the annually maximum storm event. The building maximum deflection measured from the GPS installed on the building roof was 16cm, about 0.4% of the building width. During the storm, the time series of building accelerations at elevation of 400m were recorded and the peak maximum accelerations were determined for every 10 minute interval. By sorting the accelerations based on the reference mean wind speed at the anemometer height, the comparison of the measured building accelerations at 400m height with those predicted from the wind tunnel test are plotted in Figure 4 for x- and y- directions, respectively. It can be seen that the predicted peaks approximately equal to the averaged values of the measured peaks. This is controversy to the practical expectation that the wind tunnel results should represent an envelope of peak responses. The plots in Figure 5 of simultaneous accelerations in x and y directions also indicate that the measured peak accelerations exceeded the envelope specified by the wind tunnel results. Damping ratio(%) Damping ratio(%) Amplitude(milli-g) Amplitude(milli-g) (a) Direction X (b) Direction Y Figure 3 Variation of damping ratio with vibration amplitude (a) Direction X (b) Direction Y Figure 4 Comparison of accelerations Figure 5 Simultaneous accelerations at 21m/s

6 Figure 6 Components of acceleration To understand the differences, the spectral analysis was conducted on the measured accelerations. The results indicate that the measured accelerations actually contained significant amount of high frequency components. Given the sampling rate of 20 samples per second, the identifiable frequency range was up to 10Hz. Within this frequency range, the spectrum area associated with the fundamental modal response was found to be less than 50% of the total area. As shown in Figure 6 of the y-direction acceleration at 21 m/s, the ratios of the low frequency background portion, the fundamental resonance portion, and the high frequency portion are approximately 0.06:0.46:0.48. Within the 48% of high frequency portion, significant amount was due to noise near 10 Hz. The contribution for the higher order modes was about 7% in the spectrum. Therefore, if only the fundamental response is considered, the corresponding acceleration could be about 68% of the total in magnitude (= ). With this fundamental mode correction factor being applied, the peak accelerations caused by the fundamental modal responses were extracted from the measurements and the comparison with the wind tunnel predictions are provided in Figure 7. (a) Direction X (b) Direction Y Figure 7 Comparison of accelerations after correction for fundamental modes The comparison of the wind tunnel results with the full- scale measurements shown in Figure 7 indicates that the measured acceleration data are consistent with those obtained from the HFFB model test with which only the fundamental modal responses and background contributions were included. If the higher order modal responses are considered, the total accelerations should be increased by about 6%, which is in a common magnitude of higher order mode effects identified by RWDI for similar super-tall buildings using high-frequency pressure integration test method (Xie and Irwin, 1998; Xie, 2008).

7 5. Conclusion remarks This paper presents selected dynamic responses measured from the highest tall building in Hong Kong based on a wind and movement monitoring system during the passage of Typhoon Hagupit in September, With the measured dynamic properties, the building s wind-induced responses were re-calculated based on the HFFB model data obtained at the design stage of the building. A comparative study of the wind tunnel predictions and the full-scale measurements were carried out and it was found that the measured acceleration data were consistent with those obtained from the HFFB model test. This illustrated that the HFFB wind tunnel test can provide reasonably accurate predictions of the wind-induced vibrations of the super-tall building under typhoon condition. Acknowledgement The work described in this paper was partially supported by a grant from the Research Grants Council of Hong Kong Special Administrative Region, China (Project No: CityU ). References Cermak, J. E. (2003). Wind-tunnel development and trends in applications to civil engineering. Journal of Wind Engineering and Industrial Aerodynamics, 91: Li, Q. S., Fang, J. Q., Jeary, A. P., Wong, C. K. (1998). Full scale measurement of wind effects on tall buildings. Journal of Wind Engineering and Industrial Aerodynamics, 74-76: Li, Q.S., Xiao, Y.Q., Wong, C.K. (2005a). Full-scale monitoring of typhoon effects on super tall buildings," Journal of Fluids and Structures, 20: Li, Q.S., Xiao, Y.Q., Wong, C.K., Hau, S.K., Jeary, A.P.(2005b) Serviceability of a 79- storey tall building under typhoon condition Proceedings of The Institution of Civil Engineers, Structures and Buildings, 158(4): Li, Q.S., Xiao, Y.Q., Wu, J.R., Fu, J.Y., Li, Z.N. (2008) Typhoon effects on super-tall buildings," Journal of Sound and Vibration, 313 : Tschanz, T. (1982), The base balance technique for the determination of dynamic wind loads, J. Wind Eng. Ind. Aerodyn., 13: Xie, J., Irwin, P.A. (1998). Application of the force balance technique to a building complex."journal of Wind Engineering and Industrial Aerodynamics, 77-78, Xie, J., Haskett, T., Kala, S., Irwin, P.A. (2007), Review of rigid model studies and their further improvements, Proc. 12 th Int. Conf. on Wind Eng., Cairns, Australia. Xie, J. (2008). Progress of wind tunnel techniques in practical applications."proc. 4th International Conference on Advances in Wind and Structures (AWAS'08), Jeju, Korea, May Xie, J., Garber, J. (2008), HFFB technique and its validation studies, Proc. 4 th Int. Conf. on Advances in Wind and Structures (AWAS 08), Jeju, Korea.