Wind Engineering Studies on Tall Buildings: Transitions in Research Baskaran, B

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Wind engineering studies on tall buildings: transitions in research Baskaran, B. A.

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Wind Engineering Studies on Tall Buildings - Transitions in Research

by Appupillai Baskaran ANALYZED

8 4 1993 Appeared in MAR Building and Environment Volume 28, Number 1 p. 1-19, 1993 (IRC Paper No. 3130)

Reprinted with permission from Pergamon Press NRCC 35497 Building andEnvironmenf, Val. 28, No. 1, pp. 1-19, 1993 0j6&1323/93 55.00+0.00 Printed in Great Britain. Pergamon Press Ltd.

Wind Engineering Studies on Tall ~uildin~s-~ranGtionsin Research

APPUPILLAI BASKARAN*

Development of new building materials and advances in architectural concepts have led to light weight and more unconventional buildings. Consequently, unexpected windforces may act on these structures. Experimental results from wind-tunnel studies are considered as one reliable source for the wind loading information. An extensive literature survey has been conducted; this paper reviews some of the selected studies to show the research transitions in wind engineering studies of tall buildings. Ten dzferent stages and an establishedpattern have been identzfied in wind engineering studies of tall buildings.

1. INTRODUCTION entire building. Today most partitions are removable, and are therefore lighter and more.flexible. TALL buildings are the unique North American con- a Exterior wall detail was generally made of solid tribution to world architecture [I]. The ten tallest build- masonry or stone, with the opening a small percentage ings in the world are listed in Fig. 1 ; most of these are in of the total wall surface. In contrast, the modern glass two major American cities, New York and Chicago. Tall curtain wall systems significantly reduces structural buildings were constructed in response to social and econ- stiffness of the outer skin. omic needs of a particular place and time. Their unique- a Use of mechanical systems, HVAC facilities and pro- ness attracts tourists and increases a city's image. More vision for modern equipment such as computers are usefully, they satisfy the increasing office space demands noticeably increased in comparison to the past. of major cities. The external shape of the building is more irregular More and more developers are erecting tall buildings today than in the past, to maximize the working space in comparison to the past., Architects and engineers plan as well as to express the architectural dignity of one buildings of irregular shape with extremely light exterior building from others. skins or large H/B ratios. Any increase in building height increases the importance of wind loading. ~uildii~codes Along with the changes in construction, evaluation and wind standards are formulated to provide design methodologies have also changed. In this paper a sys- information for wind engineering practice. However, tematic attempt has been made to show the changes in they provide very little design guidance for wind loads wind engineering research on tall buildings by reviewing on buildings of unusual geometrical shapes or structural a few carefully selected studies. Wind effects on buildings properties. This inadequacy in building codes and stan- are mostly quantified by using wind tunnel measure- dards redirects designers and engineers to rely on wind- ments, and research transitions in wind tunnel studies are tunnel model tests for evaluating wind effects on very tall presented in the following section. Efforts are also made buildings. in identifying the various research stages in full scale Advances in construction technology and development monitoring of wind effects on buildings and this is pre- of modern building materials also enhance the growth of sented in Section 3. Section 4 summarizes ten different tall buildings and affect the conventional design pro- stages in wind engineering research, along with an estab- cedures. Thus there are significant differences in the con- lished pattern for tall building studies in the wind tunnel. struction of tall buildings today, in comparison to buildings constructed during the early thirties ; a few of these changes are grouped below : 2. REVIEW OF SELECTED WIND TUNNEL STUDIES A 20 feet column spacing was found to be adequate for office spacing half a century ago. Today, a mini- General reviews of wind effects on tall buildings in mum of about 40-50 feet is considered adequate. relation to structural design factors can be found else- Partitions were generally made of solid masonry from where [2, 31. Similarly, the research developments on tall floor to floor, adding considerably to the rigidity of the buildings are periodically compiled and presented by the Council on Tall Buildings and Urban Habitat. So far, the Council has released a five volume monograph on the *Institute for Research in Construction, National Research planning and design of tall buildings, published from Council of Canada, Montreal Road, Ottawa, Ont., Canada KIA 1978 to 1981 [4], followed by developments in tall build- 0R6. ings [5] and advances in high-rise buildings [6]. In this 2 A. Baskaran

Building (New York), the World Trade Center Towers (New York), the Sears Tower (Chicago) and the newest example, the Bank of China (Hong Kong). These studies may represent the design methodologies of 1930s, 1960s, 1970s and 1980s.

2.1 Empire State Building Dryden and Hill [q undertook the first significant wind tunnel study on the Empire State Building. A 1:250 model made of rolled aluminum plates 114 in. thick was constructed to represent the 1250 ft (381 m) high building. Both wind-induced pressures and overturning moments on the building were examined in a 10 ft. wind tunnel at the National Bureau of Standards. Pressure on the model was measured at three different elevations (36th, 55th and 75th floors) by connecting a pressure gauge to external holes with rubber tubing. In total there were 34 pressure taps on each floor level and the model was rotated through 180 degrees to study the effect of wind azimuth angle. The test was repeated at three wind speed levels : 40, 60 and 80 ft/sec (approximately 12, 18 and 24 m/s). Pressure coefficient distributions at three different levels are shown in Fig. 2, for two typical wind directions. Positive pressure was measured for the windward walls, whereas a more or less constant suction was found for other walls. The situation becomes more complicated when the wind arrives at an oblique angle to the buildings. In addition to the measurement of external pressure distributions, the base overturning moments were also 1 1. Sears Tower 443 Chicago 1974 1 measured and are presented in Fig. 3. Coefficients for two principal sway directions are shown. The measured moments are normalized by the velocity pressure, rep- - - 3. World Trade 415 New York 1973 resentative area and arm length which is taken as 4.4 ft. Center Souh and 2.0 ft. (model scale) for x and y directions, respectively. This study, which was the first of this kind, shows 4. Empire State 381 NewYork 1931 an appreciation of the effect of wind loads on the building 5. Central Plaza 372 Hong Kong 1992 design.

16. Bank olchlna 367 Chicago 1988 1 2.2 World Trade Center Towers The twin World Trade Center Towers of New York attracted significant attention from wind engineers 18. John Hancxldr 343 Chicaao 1968 1 before, and even after, their construction. The wind effect on the towers were examined at Colorado State Uni- versity (CSU) and confirmation tests were carried out at 10 Firs! tnteratale 310 Los Angeles 1~90 the National Physical Laboratory (NPL). Wind effects I on the plaza level environment were measured at the University of Western Ontario (UWO). This was the first major tall building project in which the simulation of natural wind turbulence was introduced. 1 1. Miglin Beitler 594 Chicago 19m 1 ( 2. Tour Sans Fin 400 Paris MA 1 2.2.1. Windloadon towers. A model of the twin towers, including the low-rise plaza level buildings and the sur- 1 3. Taiwan Tower 331 Koahsiuno t993 1 roundings, was tested at CSU with the shear flow tur-

-- bulence as a simulation of natural wind [8]. A geometric (Source: Engineering News Review - Nov. 15,1990) scale of 1 : 500 was used for the model simulation. About Fig. 1. The ten tallest buildings in the world. 250 pressure taps were connected to a scanivalve pressure measuring system. The distance between the towers was varied to provide a guideline for placing the twin towers paper, after an extensive literature survey, a few studies relative to each other. Pressure measured at CSU was have been selected and reviewed to emphasize the tran- confirmed by the NPL study. A static wind load of 55 sition of methodology employed for wind engineering psf (1 psf = 48 Pa) for the top 100 ft. and 45 psf for the studies of tall buildings. The most typical buildings of remaining portion of the tower was recommended from the century are considered, such as the Empire State the wind tunnel test results for the 100 year wind of Wind Engineering Studies on Tall Buildings A. Baskaran

Angle of face to wind Angle of faoe to wind Fig. 3. Measured overturning moment for x and y directions for different wind direction [7].

140 mph (62.6 m/s). Visco-elastic damping units were cal of the wind coming across the Hudson River from suggested to limit the maximum deflection to 318 in. (9.5 Jersey City, and mm) per story at the same bench-mark wind speed. Exposure 111-representing the Manhattan fetch, typical of the wind coming over heavily built-up ter- 2.2.2 Dynamic response of towers. The study at NPL rain. consisted of two parts: a pressure test, which more or All three exposure conditions were physically modelled less confirmed the results of CSU, and the prediction in the wind tunnel with the surrounding topography to of wind-induced dynamic response using a 1 : 400 scale a radius of 1600 ft. in order to include the local flow aeroelastic model [9]. The building model was con- characteristics. Such a precise terrain simulation was structed of a light timber frame covered with thin one of the novel points of this particular study. Isolated plywood, designed as a rigid body with proper mass pressure signals were collected by tubing with a scani- simulation and the required stiffness. Variable damping valve pressure transducer. Mean, RMS and peak pres- was prdvided by a system under the wind tunnel floor via sure coefficients were obtained based on the wind speed an,extended aluminum tube from the model connected at the top of the main towers. For the design of window to electromagnets (Fig. 4). panels and exterior cladding elements, gust factors were Its response to wind was observed in an idealized obtained. A summary of the measured peak factors is smooth flow and in two kinds of turbulent flow, homo- given in Fig. 6 which shows an average value of about geneous turbulence and shear turbulence, both created 4.5 for all building elements. The positive peak pressure by grids installed at the front end of the wind tunnel test factors were about 4 to 5, whereas the main peak suction section. Configurations of an isolated tower and twin factor was typically in excess of 7 for some locations. The towers were tested and it was concluded that the twin largest pressures, suctions and their fluctuations were towers were unlikely to undergo any adverse wind effect observed when the wind came from the SW quadrant, from aerodynamic instability for wind speeds below 100 which is over the Exposure I. mph (160 km/li or 45 m/s) on either configuration. How- Flow visualization and velocity measurement were car- ever, in order to limit amplitudes at the tower top to less ried out to establish the acceptable pedestrian level wind than 10 ft. for wind speeds up to 150 mph, a very high conditions. The flow visualization was performed by gen- damping (approximately 12% of critical) is required. erating smoke in the wind tunnel, whereas the velocity measurement was done by using a hot wire anemometer 2.2.3 Study of plaza level buildings. The wind engin- system. All three flow regimes as discussed in the previous eering study for the plaza level buildings consisted of section were considered. For each wind azimuth angle, two parts : measurement of wind-induced pressure on the 20 observation points were chosen at a full-scale elevation plaza level buildings for the design of exterior cladding, of 6 to 12 ft. Results indicate generally greater wind and pedestrian level environmental wind conditions speeds near the main towers. The passageways, especialy around the towers. These experiments were carried out between the U.S. Custom Building, the Towers and the at the Boundary Layer Wind Tunnel Laboratory of the Hotel building (see Fig. 5) show the highest mean speed University of Western Ontario [lo]. ratios, particularly for WNW to SSW winds. The peak Using a linear scale of 1 : 400, the four main buildings values of the wind speed ratio vary from 0.4 to 1.2 for and the surroundings were modelled. There were 45 pres- the positions examined. sure taps on each building model. The upstream terrain conditions of the site vary depending on the wind direc- 2.3 Sears Tower tion. As shown in Fig. 5, three different exposure con- To this date, the 443 m tall Sears Tower holds the ditions were simulated in the wind tunnel. They are : title of the world's tallest office building. The proposed Exposure I-representing the open water fetch, typi- Miglin-Beitler Tower (585 m) upon its completion will cal of the wind coming across the Upper Bay and move the Sears Tower to the second place [ll]. A corn- along the Hudson River ; prehensive wind engineering study was performed at the Exposure 11-representing the Jersey City fetch, typi- University of Western Ontario [12]. The Sears Tower Wind Engineering Studies on Tall Buildings - NPL 7'x 7' wind tunnel working section

- model constructed frame mvered with

2" dia aluminum tube coil springs provide It- required stiffness I

Fig. 4. General arrangement of the 11400 scale Model tested at NPL 191.

project comprised a study of local wind climate, measure- to examine further details of the upstream flow regimes. ment of wind-induced pressure loads and determination Based on this study, three flow conditions were identified of wind induced dynamic response of the building and and represented by power law exponents of 0.56, 0.40 the prediction of wind loads based on these results [13]. and 0.13. These conditions correspond to winds coming This particular project more or less established a pat- from the NE from NW or SW and from the SE, respec- tern for a wind engineering study of tall buildings and was tively. adapted for a number of tall buildings tested thereafter. Full scale wind data from six locations were used to These include : First National City Corporation Building, evaluate the probability of exceeding a given wind speed New York [14], John Hancock Tower, Boston [IS, 16, from a particular direction. The macro-scale spectra were 171, Columbia Seafirst Center, Washington [18, 191, and also established ;these provide the time domain variation the OUB Center, in Singapore [20]. of mean wind speed averaged over intervals of time long enough, compared to time scales associated with tur- 2.3.1 Local wind climate study. Local wind climate at bulent velocity fluctuations. The effective cycling rate for the site of the Sears Tower was established based on two the Chicago area was found to be 0.11 cycles/hour ; i.e., approaches. First, the meteorological data from surface the number of events becomes about 960 per annum. and upper level observations in the Chicago area were This was based on the frequencies associated with the used to establish the general wind climate of the area. macro-scale variations in velocity spectra. Second, using a 1: 2000 scale topographical model of the 2.3.2 Pressure study. Two different models with the Chicago area in the wind tunnel, details of the wind linear scales of 1: 400 and 1: 2000 were fabricated to condition for the site were measured. For the topo- evaluate the wind-induced external pressure distribution graphical modelling, the surrounding area extended over on the Sears Tower. The 1:2000 model was used for a circle with a full scale radius of 400 m centered approxi- finding the scale effect on the measured pressures and mately at the tower site. Two types of upstream terrain, also to correlate the local wind statistics influenced by open water and urban terrain, were considered. Vertical the local topography. Detailed pressure measurements profiles of mean and rms wind speeds at the site were were performed using the 1: 400 model with 183 pressure established by normalizing with the gradient wind speed. taps. The model was tested for various wind directions A 1: 400 scale wind tunnel study was also performed using all three exposures discussed in the previous A. Baskaran

9. NORTH

Fig. 5. Three kinds of exposure conditions based on wind directions [12].

section. A typical distribution of pressure for a west wind 2.3.3 Aeroelaslic study. A multi-degree of freedom condition is presented in Fig. 7. The figure contains the aeroelastic model of the Sears Tower was constructed to mean and dynamic pressure values, typically at four levels a scale of 1: 400. The model was mounted on a flexible on the building. Irregular building shapes, which are base designed to represent the rotational flexibility of the often introduced in modem tall buildings, affect the wind foundation. The model consisted of seven rigid floor pressure distribution; this is evident from the figure, plates, a base plate, and columns to simulate the building which shows a significant change in pressure distribution stiffness. Including the three degrees of freedom at the from one level to another. This emphasizes the need for base, the model has a total of 24 degrees of freedom. At wind tunnel testing of unusual buildings. the full scale height of 1165 ft., the top floor acceleration At the time of this project, the importance of fluc- was monitored. Structural damping was assumed to be tuating wind loads on buildings were better realized and 0.5 and 1.0% of critical. Measurements were carried out incorporated in the design procedure through peak in three different flow regimes which are developed in the factors. These factors are very useful for the design of wind climate study. window glass panels and other exterior cladding elements Figure 10 shows a typical aeroelastic response with which are subject to the fluctuating wind gusts. Cal- two damping values for a benchmark gradient wind speed culated factors for selected tap locations are shown in of 100 mph. Results are shown for different wind azimuth Fig. 8. The measured results suggest a peak factor of angles tested. Discontinuities occur due to the changes in approximately 3.5 to 4 whereas the negative peak suction the upstream terrain conditions both for the mean and factors were sometimes found to exceed 10. Figure 9 dynamic response. The dynamic response was found to compares the mean pressure coefficients at various levels be preliminary in the fundamental sway modes of of the building obtained from 1: 2000 and 1: 400 models. vibration. Increasing the damping from 0.5% to 1.0% This comparison confirms that the scale effect, if any, is generally causes buffeting response for all wind direc- negligibly small. Thus the pressure results obtained from tions. This may be due to the turbulence action of wind. the 1 :400 model were extensively used with the topo- As shown in Fig. 11 the measured mean base moment graphical wind speed data obtained from the 1:2000 coefficients from the aeroelastic test agreed well with the model for the wind load predictions. For the design, a calculated values from the pressure study and this has peak pressure of 25 psf and 60 to 70 psf for peak suction been found true for the three exposure conditions and were recommended by considering a 100 year return two building sway motions considered. In the case of the wind. pressure study, the measured mean pressure values were Wind Engineering Studies on Tall Buildings

/-GTI 2=opEl

MPOSURE 3 u

HOUSE

EXPOSURE 3 MPOSURE 3

55.7 24.3 TOWER 0 -+ -+ 266 AZIMUTH 144

u NOTATION: 3-4.0 X-Y x wall level lave. hourly extreme - mean! RMS pressure Fig. 6. Peak pressure factors on the Plaza buildings for different wind directions [12].

integrated over the building surface and the base The wind tunnel flow regime was established based on moments were taken at the level of 105 ft. below the plaza records from the U.S. National Weather Record Center level. Comparisons of this nature provide confirmation in Ashveille, N.C. These observations were taken at the of the different measurement techniques and may reveal John F. Kennedy Airport, N.Y., during the period of the experimental errors or uncertainty, if any. 1960 to 1969. The prevailing wind in the New York area is westerly, particularly in winter months. However, four 2.4 First National City Corporation Building different upstream roughnesses were established for the The study of the FNCC [14] is considered to be unique wind tunnel testing. because of the building's height, unusual geometrical shape and the installation of Tuned Mass Damper 2.4.1 Aeroelastic study. Only two fundamental sway (TMD) system to suppress its possible dynamic motion. modes of vibrations were modelled in the aeroelastic The construction site was a heavily built-up area with study. Any contributions of the torsional mode and high turbulence intensity. The study consists of the higher sway modes of vibration were neglected. A build- following: the wind climate at the project site, the pres- ing model was fabricated using a scale of 1 : 500. It con- sure study, the aeroelastic study of the tower, and the sisted of seven lumped masses interconnected with elastic pedestrian level wind environment. columns. The natural periods of vibration in two sway A. Baskaran

Level 7

Level 6

L~w!4

Level 2

------...---CP I~PI+ C~rms -.-.-*- I~PI + aCprm, ...... """- ...... r -- Dynamic pressure

Fig. 7. Variation of external pressure for a West wind on the Sears Tower [12].

modes were 6.66 and 6.25 seconds. Measurement of the structural damping and aerodynamic damping compon- wind-induced response was carried out with three different. The latter can be evaluated from the autocorrelation ent values of structural damping: 0.5, 1.0 and 2.0% of function that can be obtained from the model free critical. Figure 12 shows the dynamic response of the vibration. For FNCC, the TMDs were added to reduce building for different gradient wind speeds with a damp- the peak acceleration values and it was found that a ing ratio of 0.020. It is evident that the correlation combination of 0.5% structural damping and 1.0% between the x and y displacements was very small and TMD damping would suppress the peak acceleration any increase in the wind speed increased the response. down to an acceptable level for human comfort. A similar approach was also followed for the wind tunnel study of 2.4.2 Pressure study. A rigid model equipped with 147 the John Hancock Tower, in Boston [15]. In addition, taps was constructed using a geometric scale of 1: 500 for the determination of pedestrian level wind environ- and pressure was measured under simulated wind con- ment, the local wind condition was also observed at eight ditions to evaluate wind-induced external pressure loads. different locations. The results were then integrated with Results were normalized by the reference dynamic pres- the statistics of reference wind climate and predictions of sure at the gradient height. These coefficients were then local extreme wind conditions were made for various integrated with the wind climate statistics of the site to seasons. obtain the peak exterior wind-induced pressures and suctions for a given return period. Examples are given in 2.5 Bank of China building Fig. 13, which shows the pressure contours for a return When completed, the Bank of China building will period of 50 years. The largest suction was found to be become the tallest structure in Hong Kong and also the about 35 psf (1 psf = 48 Pa). The maximum pressure of tallest building outside of North America. Its unusual about 25 psf was predicted on the south face of the geometry and the local high incidence of typhoon winds building, whereas all other walls have more or less the pointed to the need for a wind engineering study. An same suction. Comparison of the mean base moments extensive study of typhoon conditions in Hong Kong has obtained by integrating the pressure data with those from been reported elsewhere [21, 221; the following infor- the aeroelastic test gave good agreement. mation is gathered from Davenport et al. [23]. First, the wind records were synthesized to obtain the 2.4.3 Other studies. Another interesting feature of this profile of the hourly mean wind speed. The Hong Kong study is the use of Tuned Mass Dampers (TMDs) in the wind climate can be divided into two types of winds: building. In building, the total damping consists of the those associated with typhoons and those which are free Wind Engineering Studies on Tall Buildings

0.0 8.0 TAP- 530 6.0

4.0

20 - e ' 0 90. a-Mmum angle (-) a-muth angh (-) 3n o -2.0 40

-4.0 -4.0 lmln

4.0 4.0

8.0 4.0

Fig. 8. Peak pressure factors at selected taps on the Sears Tower [12].

from typhoons. The structural design for safety and was not enough to cover the high loads caused by an strength is always governed by typhoon winds, typically unusual building configuration. For the final design of about 48 m/s, whereas the occupants' comfort and ser- cladding and other external elements, the code has been viceability are designed based on the non-typhoon wind generally used, except at those locations where it was climate of around 28 m/s. Both values are considered at exceeded by the wind tunnel predictions. the gradient height and they correspond to a return Another approach taken in this study was the use of period of 50 years. the force balance technique developed by Tschanz [24] Second, a 1 : 500 pressure model was tested at the UWO for the measurement of wind loads and for response to predict the wind loads for various return periods. The prediction. It is a simple approach compared with con- largest suction for the 100 year wind was about 6.6 kPa ventional aeroelastic modelling, as it does not include occurring on a joint corner of the building. Generally, the details of the structural dynamic properties. Con- the east exposure has higher peak suction values than struction of simpler models reduces the model cost. Also other exposures. This is not only because of the prevailing the structural properties are not vitally important during wind direction but is also due to the unusual building initial design of the building. A comparison of the results shape. The 50 year suction of 5.9 kPa was observed, as obtained by using the new force balance technique to the opposed to the Hong Kong building code value of 5.3 kPa. conventional aeroelastic testing is shown in Table 1. This is a case in which the conservativeness of the code Base bending moments calculated from the Hong

Table 1. Comparison of the results derived from aeroelastic modelling and force balance technique [24]

Moments (50 yr.) Acceleration (100 yr.) (lo6 kN-m) (milli g)

X Y T X Y T

Force balance method 5.18 4.86 0.28 6.8 5.5 8.4 Aeroelastic technique 3.42 3.00 3.16 5.1 4.4 10.6 Hong Kong Code 14.6 10.2 - - - A. Baskaran

? 1 :400 scale model IPressure ----I Suction

o 1 :2000 scale model

Level 7

b A Cp - scale Level 4 17 0 0.5 1.0 1.5

Level 2 C, WEST WIND NORTH - EAST WIND a = 180' a - 45O Fig. 9. Comparison of mean pressure coefficients obtained with 1 : 2000 and 1 : 400 scale models [12].

Kong Code are also included as a reference. In general, ments on pressures, aeroelasic response and design loads the moments obtained from the aeroelastic model study respectively. are smaller than those derived by the force balance technique. However, the major structural components were 3.1 Empire State Building designed to meet the requirements of the code. Full scale measurement of wind-induced pressure on the Empire State Building was camed out by Rathbun [26]. In this experiment, one anemometer, 30 mano- 3. FULL SCALE STUDIES AND meters, 28 cameras with operating mechanisms, 22 exten- COMPARISON WITH WIND TUNNEL someters, 1 collimator with its target and 1 plumb-bob RESULTS were used. Pressure signals were measured at 10 stations The only way to verify the wind tunnel test results is on each of three floors using manometer boards and flash to compare them with the behavior of the real buildings. cameras. As mentioned previously, Dryden and Hill [7] Since this information can be obtained only after con- performed wind tunnel measurements for the same build- struction of the structure, it cannot be used during the ing configuration. However, no attempts were made by design stage of that building. However, full scale data Rathbun to compare his full scale values with the wind have vital importance for the validation of physical mod- tunnel test results, presumably because they appeared to elling and numerical simulation. Unfortunately, full scale agree very little. measurements are relatively costly and they may often In 1969, Dalgliesh [27] made some comparisons using provide obscure outputs, which do not allow straight- the results of the above two studies ; an example is shown forward comparison, due to various reasons. Thus, only in Fig. 14. Only few points (solid points) are available a few studies have been made so far [25]. from the full scale study. Generally speaking, the wind Some of these rare and yet important measurements tunnel values are higher than the full scale data and this are summarized here. The buildings considered are: may be due to the differences in reference pressure used. Empire State Building, Commerce Court Building and More seriously, Dryden and Hill assumed that the wind the Allied Bank Plaza. These three buildings may typ- flow would be uniform at 200 ft. or more above ground ically represent the construction of the 1930s, the 1960s and based on this assumption they used an aeronautical and the 1980s, respectively. Moreover, their full scale type wind tunnel for the measurement. The results could data were used for validating the wind tunnel measure- have been, of course, significantly different if one con- Wind Engineering Studies on Tall Buildings 11

Exposure 1 -+( Exposure 3 Expowre 1 -q +Exposure 3 r I' Expowre 2 -1

Azimuth angle a (degrees) Arimuh angle a (degrees) Fig. 10. Typical variation of aeroelastic response with azimuth angle [12].

siders the boundary layer wind tunnel profile, which var- simultaneously at 32 points on the building. The building ies with height. In any event, the change in wind direction internal pressure was also measured at one point and causes significant changes in pressure reading for both used as the reference for the calculation. Pressures were cases. The agreement seems to be better when the wind collected for all points at a sampling rate of 120 samples is normal to the building walls rather than when the wind per minute over a period of 5 minutes. strikes the building at an angle. Extensive comparisons between full-scale results and Another comparison was attempted by Davenport [28] wind tunnel experiments were reported in [29, 301 and in terms of the overturning base moments as a result of [31]. Figure 16 depicts the mean and rms pressure mea- the recent boundary layer wind tunnel tests on the Empire sured at two different levels of the building. The solid line State Building model ; the comparison was carried out indicates the full-scale estimates and the open circles are using the base balance technqiue. A model of the building from the wind tunnel model data. The mean pressure machined from a stiff foamed plastic was mounted on a coefficients are in better agreement than the rms values, sensitive high frequency balance to measure the base particularly for the south winds. These discrepancies were shears, moments and torques. Figure 15 compares the attributed to the fact that winds from the south had measured values of mean moment coefficients with their not been frequent enough or strong enough to provide full scale counterparts. The agreement between them is sufficient reliable rms data. remarkable. This provides a very important full scale Wind tunnel studies of the Commerce Court building confirmation of a model test. were first carried out in the UWO and this study included measurement of mean and fluctuating pressures, dynamic 3.2 Commerce Court Building response of a two degree of freedom aeroelastic model Full scale measurements were undertaken by Dalgliesh and a synthesis with metrological data [32]. Later the and other members of the Division of Building Research, National Aeronautical Establishment of the National National Research Council Canada, during the period Research Council of Canada also performed an extensive 1973-1980. Surface wind-induced pressure was measured wind tunnel study on the Commerce Court building. A. Baskaran

0.5

0.4

0.9

0.2

0.1

-0.1

-0.2 - from pressure measurements -0.3 Aemelastic results: o exposure 1 -0.4 A exposure2

-0.5 0 exposure3

Fig. I I. Comparison of mean base bending moment obtained from the aerolastic and pressure measurements [12].

There are many significant differences between the NRC stiffness, which was based on several practical con- study and that of the UWO [33, 34, 351. The pressure siderations. This resulted in a frequency scale of 1 : 53 study at NRC concentrated on occurrence of the peaks rather than a full-scale value of 1 : 58. Other noticeable [31] for the design of cladding elements and glass and factors from the full-scale measurements were the highest window panels. Also, in the aeroelastic study [31], seven 5 min mean reference speed of 33 m/s and the largest mass levels were chosen by placing one at each of the five peak pressure difference of 640 N/m2. For the displace- instrumented levels in the full-scale building and then ment, the building experienced about 220 mm at its 234 m dividing the remaining level into two modules to make level along with a peak acceleration of 10 to 15 milli g. all the modules approximately of the same height. More- over, taking advantage of the 9 m x 9 m, NRC wind 3.3 Allied Bank Plaza tunnel, models were fabricated using a geometric scale of Full scale observation of the top floor acceleration of the 1 : 200 in comparison to the 1 : 500 scale of UWO. Allied Bank Plaza in Houston, Texas has been reported by To demonstrate the measured aeroelastic response, Halvarson and Isyurnov [36] as a comparison with the wind Fig. 17 compares the model and full scale acceleration tunnel test results. The measurement was done using two power spectra of the first mode for two typical wind kinemetric Model VM-1 accelerometers, with a range of 0.1 directions (North-South and East-West). In general, the mg to 1.0 g. The measurement was not successful in the agreement is quite satisfactory. However, in the North- beginning, when the wind speed was in the range of 35 to South acceleration, sharper peaks and greater fall-off of 45 mph. However, when the area was later hit contribution by the second mode are evident in the model by a tropical storm with wind gusts of 56 mph and also than in the full-scale results. This may be due to the mode by Hurricane Alicia, with the fastest mile speed of Wind Engineering Studies on Tall Buildings

12 N 11 X RMS Response b

0 40 80 120 160 200 240 280 320 3M) Azimuth (degrees) 4 2 E Y RMS Response Smchrral damping t ratio 5, =- 0.020 "g - - - 125 mph ---lOOmph -...... 75 mph - 50mph

Azimuth (degrees) Fig. 12. Variation of aeroelastic rms x and y response with 4 wind speed and wind direction for different values of structural damping [14].

1 90 mph, some data were obtained. The response at the completed building. With this in mind, the agreement 71st floor level is compared with the wind tunnel test between full scale observation and wind tunnel data is results in Fig. 18. A possible fluctuation of wind speed acceptable. This comparison shows that the code values and wind yaw angle is expected to be less than 2.5 m/s are conservative and overestimate both the moment and and 5", respectively. shear, typically by a factor of two. In addition to the direct comparison of the wind- induced parameters, it would be useful if the design wind loading criteria were validated. This was attempted for 4. SUMMARY OF TRANSITION IN this building using the acceleration measurement. The RESEARCH lateral force acting on each floor can be estimated by the product of the weight at each floor, the top floor peak Review of the wind tunnel studies and full scale acceleration and the mode shape factor, normalized at measurements are presented in the previous sections. As the top floor. Using this concept, the estimated base shear mentioned before, only a few studies were selected as and moment from the wind tunnel test results can be representative of the major changes in the research compared (Table 2) with the observations, recorded dur- approach. A summary for the transitions of wind engin- ing Hurricane Alicia. The recommended values of the eering study of tall buildings can be listed as follows : Houston Building Code (C, = 1.4 assumed) are included Appreciation of wind loads in design for comparison. In general the agreement was satis- Pressure measurements using aeronautical wind tun- factory. The full scale monitoring program started after nel the structure was competed ; however, the interior con- Measurements using aeroelastic models struction and windows still remained to be finished. The Full scale measurements of wind pressures calculation, on the other hand, assume a fully occupied, Better simulation of turbulence conditions r -

14 A. Baskaran

-. - Wind Engineering Studies on Tall Buildings

75th FLOOR 75th FLOOR

36th FLOOR 36th FLOOR

Fig. 14. Comparison of the mean pressure coefficients measured in a model and full scale study of Empire State Building [27].

Integrating with local wind climate data was completed to mark the occasion of the Paris exhi- Introduction of peak factors in structural design bition in 1889 [28]. On the other hand, research for the Cross checking of force (pressure) and response wind effects on tall buildings started only during the Full scale measurements of dynamic response design of the Empire State Building. Introduction of high frequency force balance Static wind loads are evaluated by fabricating and testing scale models in aeronautical wind tunnels. Once These stages may overlap and the above grouping is not the building has been erected, full scale measurements in chronological order ;rather it identifies major changes are carried out to validate the results obtained from the in the research activities. Appreciation of wind loads in aeronautical wind tunnels. These comparisons sig- design started nearly 100 years ago, when the Eiffel Tower nificantly helped the wind engineering community to

NORTH - SOUTH EAST - WEST ciy cix

a - AZIMUTH (degrees)

0-0 Full scale measurement, Rathbun (1940) - Wind tunnel measurement, Davenport (1988) Fig. 15. Comparison of the base overturning moment for the Empire State Building as measured full scale and in the wind tunnel [28].

BAE 28:l-0 16 A. Baskaran

-1 .o -1 .o West North East South West West North East South West Wind direction Wind direction Fig. 16. Comparison of the pressure coefficients measured and full scale study of Commerce Court Building [29].

overcome the misconception of uniform velocity field tunnel results. This will help in validating the frequency around buildings. and fluctuating nature of wind conditions in the wind A new area of research simulates wind turbulence con- tunnel simulations. To reduce the design and model cost $itions in the wind tunnel by constructing atmospheric of the aeroelastic testing, the high frequency balance tech- boundary layer wind tunnels. The surrounding top- nique was introduced in the Bank of China project. Dur- ography is also considered and included in the study ing the course of all these processes, the wind tunnel of the World Trade Center. Metrological wind climate results are also compared with values from the Building records are integrated with the wind tunnel results for codes and wind standards in order to transfer the new the probabilities method of design. information to the end users by updating the codes and In the early 1980s, pressure measurements and aero- standards [37, 381. elastic measurements were regarded as equally important At present a majority of wind engineering studies on for the evaluation of wind effects on tall buildings and tall buildings follow a pattern as shown in Figure 19. the results from the measurements have been compared Complete analysis of the wind effects on buildings can be for cross checking the experimental techniques. Next, the obtained by following the four-fold experimental full scale'dynamic wind effects on the Commerce Court approach, namely, local wind climate study, aeroelastic Building were monitored and compared with the wind modelling, pressure measurements and wind environ-

Iv' N-s E-W I

0 0.2 0.4 0.6 0 0.2 0.4 Frequency (Hz) Frequency (Hz) Fig. 17. Comparisons of the model and full-scale acceleration power spectrum [30]. Wind Engineering Studies on Tall Buildings

FunTseale maw-

01 """""""'~ 0300 0600 1000 1400 1800 GMT 2200 01 00 0500 0900 1300 CDT hme - Augus118.1983 Fig. 18. Peak resultant accelerations on the plaza top level and comparisons with the wind tunnel data [36]. mental study. The local wind climate conditions can be engineering research activities on tall buildings, three obtained from the meteorological data of the location main areas are in-progress, as listed below : and can also be used to determine probability distribution of the wind speed ; this information will help in Time domain treatment of wind loads probabilities methods of design. However, the meteoro- Computer modelling of wind effects logical data may not represent the local surroundings, Winds induced internal pressures which cause most of the turbulence effects on such buildings. Effects of wind on tall buildings have usually been The aeroelastic measurements or base balance tech- analyzed in the frequency domain because of its station- niques are vitally important in identifying wind-induced ary random characteristics over a considerably long per- dynamic effects on buildings. Damping evaluation and iod of time as opposed to the earthquake response cal- top floor accelerations will provide better serviceability culations which are usually done in the time domain. For criteria for a building. Pressure measurements are equally places where earthquake and wind have equal magnitude, important, since they are useful not only for the design a common approach will not only make the design pro- of structural elements and cladding, they also play a cess economical, it also helps the designer in selecting major role in the energy calculations for buildings. the optimum conditions. Studies have been initiated to Finally, the pedestrian level velocity measurements will represent wind load conditions in the time domain. provide information on the local wind environmental Advancements of computer software and hardware conditions and help in town planning. technology provide a new direction for analyzing engin- No doubt, the state of the art for tall buildings will be eering problems. The field of wind engineering is gaining different tomorrow from today. Currently, in the wind significant momentum in computer modelling processes.

Table 2. Comparison of the base moment and shear for the Allied Bank Plaza as measured in full scale and in a wind tunnel [36]

Wind tunnel Full scale Houston (Alicia) Code 100 yrs 50 yrs

First mode Base shear (kips) 5,600 4,900 4,500 12,500 Base moments (ft - K) x lo6 3.5 3.1 2.7 7.1 Second mode Base shear (kips) 4,200 3,500 3,800 9,500 Base moments (ft .K) x lo6 2.6 2.2 1.8 5.4 18 A. Baskaran

Attempts have been made in predicting wind flow con- WlND ENOWmNO STWVOFTAU BUILDINGS ditions around buildings and in computing the wind- induced effects on buildings [39]. Research efforts have also been extended in modelling time dependent wind flow conditions around buildings in super computers [40] by applying sophisticated numerical algorithms [41]. Until recently less emphasis has been placed on wind- induced internal pressures, particularly for tall buildings. Internal building pressure is not only caused by external wind pressure ;the mechanically operated ventilation systems in the building and the stack effect developed by the thermal difference across the envelope also contribute to anplw the changes in internal pressure. The effect of these fac- Smy d.nOFnoh. tors on tall buildings were reviewed by Tanaka [42] and Top mW Tanaka and Lee [43]. Field monitored results [44] on a .-Im# 20 storey building show that the wind and stack effect has predominant influence on the pressure difference across the envelope. Developments in wind tunnel -r measurement techniques and comparison of the internal pressure coefficient with those of North American Stan- dards are respectively reported in [44] and [45,46]. DESION OFTUBWLDIMGS Acknowledgement-It is indeed a great pleasure to acknowledge Fig. 19. Established pattern for wind engineering study of tall Dr Hiroshi Tanaka for providing continuous advice during the buildings. course of this project.

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