Wind engineering studies on tall buildings: transitions in research

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Wind engineering studies on tall buildings: transitions in research

Baskaran, B. A.

S e r

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National Research

Conseil national

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council ~anacia

de recherches Canada

no,3130

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Wind Engineering Studies on

Tall Buildings

-

Transitions in

Research

by

Appupillai Baskaran

Appeared in

Building and Environment

Volume 28, Number 1

p. 1-19, 1993

(IRC Paper No. 3130)

Reprinted with permission from

Pergamon Press

NRCC 35497

ANALYZED

Building andEnvironmenf, Val. 28, No. 1 , pp. 1-19, 1993 Printed in Great Britain.

0j6&1323/93 55.00+0.00 Pergamon Press Ltd.

Wind Engineering Studies on Tall

~uildin~s-~ranGtions

in Research

APPUPILLAI BASKARAN*

Development of new building materials and advances in architectural concepts have led to light weight and more unconventional buildings. Consequently, unexpected wind forces 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,

TALL buildings are the unique North American con- tribution to world architecture [I]. The ten tallest build- ings in the world are listed in Fig. 1 ; most of these are in two major American cities, New York and Chicago. Tall buildings were constructed in response to social and econ- omic needs of a particular place and time. Their unique- ness attracts tourists and increases a city's image. More usefully, they satisfy the increasing office space demands of major cities.

More and more developers are erecting tall buildings in comparison to the past., Architects and engineers plan buildings of irregular shape with extremely light exterior skins or large H/B ratios. Any increase in building height increases the importance of wind loading. ~ u i l d i i ~ codes and wind standards are formulated to provide design information for wind engineering practice. However, they provide very little design guidance for wind loads on buildings of unusual geometrical shapes or structural properties. This inadequacy in building codes and stan- dards redirects designers and engineers to rely on wind- tunnel model tests for evaluating wind effects on very tall buildings.

Advances in construction technology and development of modern building materials also enhance the growth of tall buildings and affect the conventional design pro- cedures. Thus there are significant differences in the con- struction of tall buildings today, in comparison to build- ings constructed during the early thirties ; a few of these changes are grouped below :

A 20 feet column spacing was found to be adequate for office spacing half a century ago. Today, a mini- mum of about 40-50 feet is considered adequate. Partitions were generally made of solid masonry from floor to floor, adding considerably to the rigidity of the

*Institute for Research in Construction, National Research Council of Canada, Montreal Road, Ottawa, Ont., Canada KIA 0R6.

and are therefore lighter and more.flexible.

a Exterior wall detail was generally made of solid masonry or stone, with the opening a small percentage of the total wall surface. In contrast, the modern glass curtain wall systems significantly reduces structural stiffness of the outer skin.

a Use of mechanical systems, HVAC facilities and pro- vision for modern equipment such as computers are noticeably increased in comparison to the past. The external shape of the building is more irregular today than in the past, to maximize the working space as well as to express the architectural dignity of one building from others.

Along with the changes in construction, evaluation methodologies have also changed. In this paper a sys- tematic attempt has been made to show the changes in wind engineering research on tall buildings by reviewing a few carefully selected studies. Wind effects on buildings are mostly quantified by using wind tunnel measure- ments, and research transitions in wind tunnel studies are presented in the following section. Efforts are also made in identifying the various research stages in full scale monitoring of wind effects on buildings and this is pre- sented in Section 3. Section 4 summarizes ten different stages in wind engineering research, along with an estab- lished pattern for tall building studies in the wind tunnel.

2. REVIEW OF SELECTED WIND TUNNEL STUDIES

General reviews of wind effects on tall buildings in relation to structural design factors can be found else- where [2, 31. Similarly, the research developments on tall 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 planning and design of tall buildings, published from 1978 to 1981 [4], followed by developments in tall build- ings [5] and advances in high-rise buildings [6]. In this

1

1. Sears Tower 443 Chicago 1974

1

- -

3. World Trade 415 New York 1973

Center Souh

4. Empire State 381 NewYork 1931

5. Central Plaza 372 Hong Kong 1992 16. Bank olchlna 367 Chicago 1988

1

18. John Hancxldr 343 Chicaao 1968

1

10 Firs! tnteratale 310 Los Angeles 1 ~ 9 0

I

1

1. Miglin Beitler 594 Chicago 19m

1

(

2. Tour Sans Fin 400 Paris M A

1

1

3. Taiwan Tower 331 Koahsiuno t993

1

- -

(Source: Engineering News Review

-

Nov. 15,1990)

Fig. 1. The ten tallest buildings in the world.

paper, after an extensive literature survey, a few studies have been selected and reviewed to emphasize the tran- sition of methodology employed for wind engineering studies of tall buildings. The most typical buildings of the century are considered, such as the Empire State

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 exter- nal holes with rubber tubing. In total there were 34 pres- sure 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 build- ings. In addition to the measurement of external pressure distributions, the base overturning moments were also 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- resentative area and arm length which is taken as 4.4 ft. and 2.0 ft. (model scale) for x and y directions, respec- tively. This study, which was the first of this kind, shows an appreciation of the effect of wind loads on the building design.

2.2 World Trade Center Towers

The twin World Trade Center Towers of New York attracted significant attention from wind engineers 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 the National Physical Laboratory (NPL). Wind effects 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.

2.2.1. Windloadon towers. A model of the twin towers, including the low-rise plaza level buildings and the sur- roundings, was tested at CSU with the shear flow tur- bulence as a simulation of natural wind [8]. A geometric scale of 1 : 500 was used for the model simulation. About 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 relative to each other. Pressure measured at CSU was confirmed by the NPL study. A static wind load of 55 psf (1 psf = 48 Pa) for the top 100 ft. and 45 psf for the

remaining portion of the tower was recommended from the wind tunnel test results for the 100 year wind of

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 less confirmed the results of CSU, and the prediction

of wind-induced dynamic response using a 1 : 400 scale

aeroelastic model [9]. The building model was con-

structed of a light timber frame covered with thin plywood, designed as a rigid body with proper mass simulation and the required stiffness. Variable damping was prdvided by a system under the wind tunnel floor via an,extended aluminum tube from the model connected to electromagnets (Fig. 4).

Its response to wind was observed in an idealized smooth flow and in two kinds of turbulent flow, homo- geneous turbulence and shear turbulence, both created by grids installed at the front end of the wind tunnel test section. Configurations of an isolated tower and twin towers were tested and it was concluded that the twin towers were unlikely to undergo any adverse wind effect from aerodynamic instability for wind speeds below 100 mph (160 km/li or 45 m/s) on either configuration. How- ever, in order to limit amplitudes at the tower top to less than 10 ft. for wind speeds up to 150 mph, a very high damping (approximately 12% of critical) is required.

All three exposure conditions were physically modelled in the wind tunnel with the surrounding topography to a radius of 1600 ft. in order to include the local flow characteristics. Such a precise terrain simulation was one of the novel points of this particular study. Isolated pressure signals were collected by tubing with a scani- valve pressure transducer. Mean, RMS and peak pres- sure coefficients were obtained based on the wind speed at the top of the main towers. For the design of window panels and exterior cladding elements, gust factors were obtained. A summary of the measured peak factors is

given in Fig. 6 which shows an average value of about

4.5 for all building elements. The positive peak pressure factors were about 4 to 5, whereas the main peak suction factor was typically in excess of 7 for some locations. The largest pressures, suctions and their fluctuations were observed when the wind came from the SW quadrant, which is over the Exposure I.

Flow visualization and velocity measurement were car- ried out to establish the acceptable pedestrian level wind conditions. The flow visualization was performed by gen- 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

University of Western Ontario [lo]. Hotel building (see Fig. 5) show the highest mean speed

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-

tion. As shown in Fig. 5, three different exposure con- 2.3 Sears Tower

ditions were simulated in the wind tunnel. They are : To this date, the 443 m tall Sears Tower holds the

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

Wind Engineering Studies on Tall Buildings

-

NPL 7'x 7' wind tunnel

working section

-

model constructed frame mvered with

It-

2" dia aluminum tube coil springs provide

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- ment of wind-induced pressure loads and determination of wind induced dynamic response of the building and the prediction of wind loads based on these results [13].

This particular project more or less established a pat- tern for a wind engineering study of tall buildings and was adapted for a number of tall buildings tested thereafter. These include : First National City Corporation Building, New York [14], John Hancock Tower, Boston [IS, 16, 171, Columbia Seafirst Center, Washington [18, 191, and the OUB Center, in Singapore [20].

2.3.1 Local wind climate study. Local wind climate at the site of the Sears Tower was established based on two approaches. First, the meteorological data from surface and upper level observations in the Chicago area were used to establish the general wind climate of the area. Second, using a 1 : 2000 scale topographical model of the Chicago area in the wind tunnel, details of the wind condition for the site were measured. For the topo- graphical modelling, the surrounding area extended over a circle with a full scale radius of 400 m centered approxi- mately at the tower site. Two types of upstream terrain, open water and urban terrain, were considered. Vertical profiles of mean and rms wind speeds at the site were established by normalizing with the gradient wind speed.

A 1 : 400 scale wind tunnel study was also performed

to examine further details of the upstream flow regimes. Based on this study, three flow conditions were identified and represented by power law exponents of 0.56, 0.40 and 0.13. These conditions correspond to winds coming from the NE from NW or SW and from the SE, respec- tively.

Full scale wind data from six locations were used to evaluate the probability of exceeding a given wind speed from a particular direction. The macro-scale spectra were also established ; these provide the time domain variation of mean wind speed averaged over intervals of time long enough, compared to time scales associated with tur- bulent velocity fluctuations. The effective cycling rate for the Chicago area was found to be 0.11 cycles/hour ; i.e., the number of events becomes about 960 per annum. This was based on the frequencies associated with the macro-scale variations in velocity spectra.

2.3.2 Pressure study. Two different models with the linear scales of 1: 400 and 1: 2000 were fabricated to evaluate the wind-induced external pressure distribution on the Sears Tower. The 1:2000 model was used for finding the scale effect on the measured pressures and also to correlate the local wind statistics influenced by the local topography. Detailed pressure measurements were performed using the 1 : 400 model with 183 pressure taps. The model was tested for various wind directions using all three exposures discussed in the previous

9.

NORTH

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

section. A typical distribution of pressure for a west wind condition is presented in Fig. 7. The figure contains the mean and dynamic pressure values, typically at four levels on the building. Irregular building shapes, which are often introduced in modem tall buildings, affect the wind pressure distribution; this is evident from the figure, which shows a significant change in pressure distribution from one level to another. This emphasizes the need for wind tunnel testing of unusual buildings.

At the time of this project, the importance of fluc- tuating wind loads on buildings were better realized and incorporated in the design procedure through peak factors. These factors are very useful for the design of window glass panels and other exterior cladding elements which are subject to the fluctuating wind gusts. Cal- culated factors for selected tap locations are shown in Fig. 8. The measured results suggest a peak factor of approximately 3.5 to 4 whereas the negative peak suction factors were sometimes found to exceed 10. Figure 9 compares the mean pressure coefficients at various levels of the building obtained from 1 : 2000 and 1 : 400 models. This comparison confirms that the scale effect, if any, is negligibly small. Thus the pressure results obtained from the 1 :400 model were extensively used with the topo- graphical wind speed data obtained from the 1:2000 model for the wind load predictions. For the design, a peak pressure of 25 psf and 60 to 70 psf for peak suction were recommended by considering a 100 year return wind.

2.3.3 Aeroelaslic study. A multi-degree of freedom aeroelastic model of the Sears Tower was constructed to a scale of 1 : 400. The model was mounted on a flexible base designed to represent the rotational flexibility of the foundation. The model consisted of seven rigid floor plates, a base plate, and columns to simulate the building stiffness. Including the three degrees of freedom at the base, the model has a total of 24 degrees of freedom. At the full scale height of 1165 ft., the top floor acceleration was monitored. Structural damping was assumed to be 0.5 and 1.0% of critical. Measurements were carried out in three different flow regimes which are developed in the wind climate study.

Figure 10 shows a typical aeroelastic response with two damping values for a benchmark gradient wind speed of 100 mph. Results are shown for different wind azimuth angles tested. Discontinuities occur due to the changes in the upstream terrain conditions both for the mean and dynamic response. The dynamic response was found to be preliminary in the fundamental sway modes of vibration. Increasing the damping from 0.5% to 1.0% generally causes buffeting response for all wind direc- tions. This may be due to the turbulence action of wind. As shown in Fig. 11 the measured mean base moment coefficients from the aeroelastic test agreed well with the calculated values from the pressure study and this has been found true for the three exposure conditions and two building sway motions considered. In the case of the pressure study, the measured mean pressure values were

Wind Engineering Studies on Tall Buildings

u

HOUSE EXPOSURE 3

/-GTI

2 = o p E l MPOSURE 3 MPOSURE 3 55.7 24.3 TOWER 0

-+

2 6 6 AZIMUTH 1 4 4

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 moments were taken at the level of 105 ft. below the plaza level. Comparisons of this nature provide confirmation of the different measurement techniques and may reveal the experimental errors or uncertainty, if any.

2.4 First National City Corporation Building

The study of the FNCC [14] is considered to be unique because of the building's height, unusual geometrical shape and the installation of Tuned Mass Damper (TMD) system to suppress its possible dynamic motion. The construction site was a heavily built-up area with high turbulence intensity. The study consists of the following: the wind climate at the project site, the pres- sure study, the aeroelastic study of the tower, and the pedestrian level wind environment.

The wind tunnel flow regime was established based on records from the U.S. National Weather Record Center in Ashveille, N.C. These observations were taken at the John F. Kennedy Airport, N.Y., during the period of 1960 to 1969. The prevailing wind in the New York area is westerly, particularly in winter months. However, four different upstream roughnesses were established for the wind tunnel testing.

2.4.1 Aeroelastic study. Only two fundamental sway modes of vibrations were modelled in the aeroelastic study. Any contributions of the torsional mode and

higher sway modes of vibration were neglected. A build-

ing model was fabricated using a scale of 1 : 500. It con-

sisted of seven lumped masses interconnected with elastic columns. The natural periods of vibration in two sway

N Level 7 Level 6 L ~ w ! 4 Level 2

-

---...---

CP

_{I ~ P I}

+ C ~ r m s

-.-.-*-

."""-

I ~ P I

+ 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 wind-induced response was carried out with three differ- ent values of structural damping: 0.5, 1.0 and 2.0% of critical. Figure 12 shows the dynamic response of the building for different gradient wind speeds with a damp- ing ratio of 0.020. It is evident that the correlation between the x and y displacements was very small and any increase in the wind speed increased the response.

2.4.2 Pressure study. A rigid model equipped with 147 taps was constructed using a geometric scale of 1: 500 and pressure was measured under simulated wind con- ditions to evaluate wind-induced external pressure loads. Results were normalized by the reference dynamic pres- sure at the gradient height. These coefficients were then integrated with the wind climate statistics of the site to obtain the peak exterior wind-induced pressures and suc- tions for a given return period. Examples are given in Fig. 13, which shows the pressure contours for a return period of 50 years. The largest suction was found to be about 35 psf (1 psf = 48 Pa). The maximum pressure of about 25 psf was predicted on the south face of the building, whereas all other walls have more or less the same suction. Comparison of the mean base moments obtained by integrating the pressure data with those from the aeroelastic test gave good agreement.

2.4.3 Other studies. Another interesting feature of this study is the use of Tuned Mass Dampers (TMDs) in the building. In building, the total damping consists of the

structural damping and aerodynamic damping compon- ent. The latter can be evaluated from the autocorrelation function that can be obtained from the model free vibration. For FNCC, the TMDs were added to reduce the peak acceleration values and it was found that a combination of 0.5% structural damping and 1.0% TMD damping would suppress the peak acceleration down to an acceptable level for human comfort. A similar approach was also followed for the wind tunnel study of the John Hancock Tower, in Boston [15]. In addition, for the determination of pedestrian level wind environ- ment, the local wind condition was also observed at eight different locations. The results were then integrated with the statistics of reference wind climate and predictions of local extreme wind conditions were made for various seasons.

2.5 Bank of China building

When completed, the Bank of China building will become the tallest structure in Hong Kong and also the tallest building outside of North America. Its unusual geometry and the local high incidence of typhoon winds pointed to the need for a wind engineering study. An extensive study of typhoon conditions in Hong Kong has been reported elsewhere [21, 221; the following infor- mation is gathered from Davenport et al. [23].

First, the wind records were synthesized to obtain the profile of the hourly mean wind speed. The Hong Kong wind climate can be divided into two types of winds: 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 2 0

-

e

'

0 90.

a-Mmum angle (-) a-muth angh (-)

3

n o -2.0 4 0 -4.0 -4.0 l m l n 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 strength is always governed by typhoon winds, typically about 48 m/s, whereas the occupants' comfort and ser- viceability are designed based on the non-typhoon wind climate of around 28 m/s. Both values are considered at the gradient height and they correspond to a return period of 50 years.

Second, a 1 : 500 pressure model was tested at the UWO to predict the wind loads for various return periods. The largest suction for the 100 year wind was about 6.6 kPa occurring on a joint corner of the building. Generally, the east exposure has higher peak suction values than other exposures. This is not only because of the prevailing wind direction but is also due to the unusual building shape. The 50 year suction of 5.9 kPa was observed, as opposed to the Hong Kong building code value of 5.3 kPa. This is a case in which the conservativeness of the code

was not enough to cover the high loads caused by an unusual building configuration. For the final design of cladding and other external elements, the code has been generally used, except at those locations where it was exceeded by the wind tunnel predictions.

Another approach taken in this study was the use of the force balance technique developed by Tschanz [24] for the measurement of wind loads and for response prediction. It is a simple approach compared with con- ventional aeroelastic modelling, as it does not include the details of the structural dynamic properties. Con- struction of simpler models reduces the model cost. Also the structural properties are not vitally important during initial design of the building. A comparison of the results obtained by using the new force balance technique to the conventional aeroelastic testing is shown in Table 1.

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 -

-

-

?

1 : 400 scale model I Pressure ----I Suction o 1 : 2000 scale model b A Cp

-

scale 0 0.5 1.0 1.5

1

7

C, WEST WIND Level 7 Level 4 Level 2

NORTH

-

EAST WIND

a = 180' a

-

4 5 O

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, the moments obtained from the aeroelastic model study are smaller than those derived by the force balance tech- nique. However, the major structural components were designed to meet the requirements of the code.

3. FULL SCALE STUDIES AND COMPARISON WITH WIND TUNNEL

RESULTS

The only way to verify the wind tunnel test results is to compare them with the behavior of the real buildings. Since this information can be obtained only after con- struction of the structure, it cannot be used during the design stage of that building. However, full scale data have vital importance for the validation of physical mod- elling and numerical simulation. Unfortunately, full scale measurements are relatively costly and they may often provide obscure outputs, which do not allow straight- forward comparison, due to various reasons. Thus, only a few studies have been made so far [25].

Some of these rare and yet important measurements are summarized here. The buildings considered are: Empire State Building, Commerce Court Building and the Allied Bank Plaza. These three buildings may typ- ically represent the construction of the 1930s, the 1960s and the 1980s, respectively. Moreover, their full scale data were used for validating the wind tunnel measure-

ments on pressures, aeroelasic response and design loads respectively.

3.1 Empire State Building

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- meters, 28 cameras with operating mechanisms, 22 exten- someters, 1 collimator with its target and 1 plumb-bob were used. Pressure signals were measured at 10 stations on each of three floors using manometer boards and flash cameras. As mentioned previously, Dryden and Hill [7] performed wind tunnel measurements for the same build- ing configuration. However, no attempts were made by Rathbun to compare his full scale values with the wind tunnel test results, presumably because they appeared to agree very little.

In 1969, Dalgliesh [27] made some comparisons using the results of the above two studies ; an example is shown in Fig. 14. Only few points (solid points) are available from the full scale study. Generally speaking, the wind tunnel values are higher than the full scale data and this may be due to the differences in reference pressure used. More seriously, Dryden and Hill assumed that the wind flow would be uniform at 200 ft. or more above ground and based on this assumption they used an aeronautical type wind tunnel for the measurement. The results could have been, of course, significantly different if one con-

Wind Engineering Studies on Tall Buildings 11

Exposure 1 -+( Exposure 3

r

Expowre 1

-q

+

Exposure 3

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- ies with height. In any event, the change in wind direction causes significant changes in pressure reading for both cases. The agreement seems to be better when the wind is normal to the building walls rather than when the wind strikes the building at an angle.

Another comparison was attempted by Davenport [28] in terms of the overturning base moments as a result of the recent boundary layer wind tunnel tests on the Empire State Building model ; the comparison was carried out using the base balance technqiue. A model of the building machined from a stiff foamed plastic was mounted on a sensitive high frequency balance to measure the base shears, moments and torques. Figure 15 compares the measured values of mean moment coefficients with their full scale counterparts. The agreement between them is remarkable. This provides a very important full scale confirmation of a model test.

3.2 Commerce Court Building

Full scale measurements were undertaken by Dalgliesh and other members of the Division of Building Research, National Research Council Canada, during the period 1973-1980. Surface wind-induced pressure was measured

simultaneously at 32 points on the building. The building internal pressure was also measured at one point and used as the reference for the calculation. Pressures were collected for all points at a sampling rate of 120 samples per minute over a period of 5 minutes.

Extensive comparisons between full-scale results and wind tunnel experiments were reported in [29, 301 and [31]. Figure 16 depicts the mean and rms pressure mea- sured at two different levels of the building. The solid line indicates the full-scale estimates and the open circles are from the wind tunnel model data. The mean pressure coefficients are in better agreement than the rms values, particularly for the south winds. These discrepancies were attributed to the fact that winds from the south had not been frequent enough or strong enough to provide sufficient reliable rms data.

Wind tunnel studies of the Commerce Court building were first carried out in the UWO and this study included measurement of mean and fluctuating pressures, dynamic response of a two degree of freedom aeroelastic model and a synthesis with metrological data [32]. Later the National Aeronautical Establishment of the National Research Council of Canada also performed an extensive wind tunnel study on the Commerce Court building.

Fig. I I. 0.5 0.4 0.9 0.2 0.1 0 -0.1 -0.2

-

from pressure measurements

-0.3 _{Aemelastic results:}

o exposure 1

-0.4

A exposure2

-0.5 0 exposure3

Comparison of mean base bending moment obtained from the aerolastic and pressure measure- ments [12].

There are many significant differences between the NRC study and that of the UWO [33, 34, 351. The pressure study at NRC concentrated on occurrence of the peaks

[31] for the design of cladding elements and glass and window panels. Also, in the aeroelastic study [31], seven mass levels were chosen by placing one at each of the five instrumented levels in the full-scale building and then dividing the remaining level into two modules to make all the modules approximately of the same height. More- over, taking advantage of the 9 m x 9 m, NRC wind tunnel, models were fabricated using a geometric scale of

1 : 200 in comparison to the 1 : 500 scale of UWO. To demonstrate the measured aeroelastic response, Fig. 17 compares the model and full scale acceleration power spectra of the first mode for two typical wind directions (North-South and East-West). In general, the agreement is quite satisfactory. However, in the North- South acceleration, sharper peaks and greater fall-off of contribution by the second mode are evident in the model than in the full-scale results. This may be due to the mode

stiffness, which was based on several practical con- siderations. This resulted in a frequency scale of 1 : 53

rather than a full-scale value of 1 : 58. Other noticeable factors from the full-scale measurements were the highest

5 min mean reference speed of 33 m/s and the largest peak pressure difference of 640 N/m2. For the displace- ment, the building experienced about 220 mm at its 234 m level along with a peak acceleration of 10 to 15 milli g.

3.3 Allied Bank Plaza

Full scale observation of the top floor acceleration of the Allied Bank Plaza in Houston, Texas has been reported by Halvarson and Isyurnov [36] as a comparison with the wind tunnel test results. The measurement was done using two kinemetric Model VM-1 accelerometers, with a range of 0.1 mg to 1.0 g. The measurement was not successful in the beginning, when the wind speed was in the range of 35 to

45 mph. However, when the area was later hit by a tropical storm with wind gusts of 56 mph and also by Hurricane Alicia, with the fastest mile speed of

Wind Engineering Studies on Tall Buildings 12 11 _{X RMS Response} N _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 71st floor level is compared with the wind tunnel test results in Fig. 18. A possible fluctuation of wind speed and wind yaw angle is expected to be less than 2.5 m/s and 5", respectively.

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 this building using the acceleration measurement. The lateral force acting on each floor can be estimated by the product of the weight at each floor, the top floor peak acceleration and the mode shape factor, normalized at the top floor. Using this concept, the estimated base shear and moment from the wind tunnel test results can be compared (Table 2) with the observations, recorded dur- ing Hurricane Alicia. The recommended values of the Houston Building Code (C, = 1.4 assumed) are included for comparison. In general the agreement was satis- factory. The full scale monitoring program started after the structure was competed ; however, the interior con- struction and windows still remained to be finished. The calculation, on the other hand, assume a fully occupied,

completed building. With this in mind, the agreement between full scale observation and wind tunnel data is acceptable. This comparison shows that the code values are conservative and overestimate both the moment and shear, typically by a factor of two.

4. SUMMARY OF TRANSITION IN RESEARCH

Review of the wind tunnel studies and full scale measurements are presented in the previous sections. As mentioned before, only a few studies were selected as representative of the major changes in the research approach. A summary for the transitions of wind engin- eering study of tall buildings can be listed as follows :

Appreciation of wind loads in design

Pressure measurements using aeronautical wind tun- nel

Measurements using aeroelastic models Full scale measurements of wind pressures Better simulation of turbulence conditions

14 A. Baskaran I I I I r

-.

-

Wind Engineering Studies on Tall Buildings

75th FLOOR 75th FLOOR

0

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 Introduction of peak factors in structural design Cross checking of force (pressure) and response Full scale measurements of dynamic response Introduction of high frequency force balance These stages may overlap and the above grouping is not

in chronological order ; rather it identifies major changes in the research activities. Appreciation of wind loads in design started nearly 100 years ago, when the Eiffel Tower

was completed to mark the occasion of the Paris exhi- bition in 1889 [28]. On the other hand, research for the wind effects on tall buildings started only during the design of the Empire State Building.

Static wind loads are evaluated by fabricating and testing scale models in aeronautical wind tunnels. Once the building has been erected, full scale measurements are carried out to validate the results obtained from the aeronautical wind tunnels. These comparisons sig- nificantly helped the wind engineering community to

NORTH

-

SOUTH EAST

-

WEST

c i y c i x

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].

-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 around buildings.

A new area of research simulates wind turbulence con- $itions in the wind tunnel by constructing atmospheric boundary layer wind tunnels. The surrounding top- ography is also considered and included in the study of the World Trade Center. Metrological wind climate records are integrated with the wind tunnel results for the probabilities method of design.

In the early 1980s, pressure measurements and aero- elastic measurements were regarded as equally important for the evaluation of wind effects on tall buildings and the results from the measurements have been compared for cross checking the experimental techniques. Next, the full scale'dynamic wind effects on the Commerce Court Building were monitored and compared with the wind

tunnel results. This will help in validating the frequency and fluctuating nature of wind conditions in the wind tunnel simulations. To reduce the design and model cost of the aeroelastic testing, the high frequency balance tech- nique was introduced in the Bank of China project. Dur- ing the course of all these processes, the wind tunnel results are also compared with values from the Building codes and wind standards in order to transfer the new information to the end users by updating the codes and standards [37, 381.

At present a majority of wind engineering studies on tall buildings follow a pattern as shown in Figure 19. Complete analysis of the wind effects on buildings can be obtained by following the four-fold experimental approach, namely, local wind climate study, aeroelastic 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)

Wind Engineering Studies on

Tall

Buildings FunTseale maw- 0 1 " " " " " " " ' ~ 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 obtained from the meteorological data of the location and can also be used to determine probability dis- tribution of the wind speed ; this information will help in probabilities methods of design. However, the meteoro- logical data may not represent the local surroundings, which cause most of the turbulence effects on such build- ings.

The aeroelastic measurements or base balance tech- niques are vitally important in identifying wind-induced dynamic effects on buildings. Damping evaluation and top floor accelerations will provide better serviceability criteria for a building. Pressure measurements are equally important, since they are useful not only for the design of structural elements and cladding, they also play a major role in the energy calculations for buildings. Finally, the pedestrian level velocity measurements will provide information on the local wind environmental conditions and help in town planning.

No doubt, the state of the art for tall buildings will be different tomorrow from today. Currently, in the wind

engineering research activities on tall buildings, three main areas are in-progress, as listed below :

Time domain treatment of wind loads Computer modelling of wind effects Winds induced internal pressures

Effects of wind on tall buildings have usually been analyzed in the frequency domain because of its station- ary random characteristics over a considerably long per- iod of time as opposed to the earthquake response cal- culations which are usually done in the time domain. For places where earthquake and wind have equal magnitude,

a common approach will not only make the design pro- cess economical, it also helps the designer in selecting the optimum conditions. Studies have been initiated to represent wind load conditions in the time domain.

Advancements of computer software and hardware technology provide a new direction for analyzing engin- eering problems. The field of wind engineering is gaining 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

WlND E N O W m N O S T W V OFTAU BUILDINGS anplw Smy d.nOFnoh. Top mW .-Im#

-r

DESION O F T U BWLDIMGS

Fig. 19. Established pattern for wind engineering study of tall buildings.

Attempts have been made in predicting wind flow con- 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 sys- tems in the building and the stack effect developed by the thermal difference across the envelope also contribute to the changes in internal pressure. The effect of these fac- tors on tall buildings were reviewed by Tanaka [42] and Tanaka and Lee [43]. Field monitored results [44] on a 20 storey building show that the wind and stack effect has predominant influence on the pressure difference across the envelope. Developments in wind tunnel measurement techniques and comparison of the internal pressure coefficient with those of North American Stan- dards are respectively reported in [44] and [45,46].

Acknowledgement-It is indeed a great pleasure to acknowledge Dr Hiroshi Tanaka for providing continuous advice during the course of this project.

REFERENCES

1. A. C. Erickson and A. Steele, The tall buildings: Is America's past East Asia's future?, Proc. 4th International Conf. on Tall Buildings, Hong Kong, 2-7 (1988).

2. B. S. Taranath, Structural Analysis and Design of Tall Buildings. McGraw-Hill(1988). 3. A. Coull and B. Stafford-Smith, Tall Buildings, Pergamon Press (1966).

4. Monograph on planning and Design of Tall Buidings, Council on Tall Buildings and Urban Habitat, Lehigh university, Bethlehem, Pennsylvania, U.S.A., 1978 to (1981).

5. Developments in Tall Buildings, Council on Tall Buildings and Urban Habitat, Lehigh university, Bethlehem, Pennsylvania, U.S.A. (1983).

6. High-Rise Buildings : Recent Progress, Council on Tall Buildings and urban Habitat, Lehigh univer- sity, Bethlehem, Pennsylvania, U.S.A. (1986).

7. H. L. Dryden and G. C. Hill, Wind pressures on a model of the Empire State Building. Bureau oJ

Standards Journal of Research, 10,493-523 (1933).

8. L. S. Feld, Superstructure for 1,350 feet, World Trade Center. Civ. Engng, ASCE, 6 6 7 0 (1971). 9. R. E. Whitebread and C. Scrnton, An investigation of the aerodynamic stability of the World Trade

Center. New York (Part I), NPL Aero Rept. 1156, National Physical Laboratory (July 1965). 10. A. G. Davenport, N. Isyumov, D. J. Fader and C. F. P. Bowen, Study of wind effect on World Trade

Center, New York; Exterior Pressure on Plaza Buildings and Airflow in Plaza. BLWT-6-70, University of Western Ontario (1970).

11. ASCE, World's tallest tower approved in Chicago. Civ. Engng, ASCE, 18 (1990).

12. A. G. Davenport, N. Isyumov and T. Jandali, A Study of wind effects for the Sears project. BLWT- 5-71, University of Western Ontario (1971).

13. A. G. Davenport, On the Statistical Prediction of structural performance in the wind environment. Meeting Preprint 1420, ASCE National Structural Meeting, Baltimore, Maryland (1971).

14. N. Isyumov, J. D. Holmes, D. Surry and A. G. Davenport, A study of the wind effect for the First National City Corporation project. New York, U.S.A., BLWT, University of Western Ontario (1976). 15. D. Surry, H. Tanaka and A. G. Davenport, The effect of proposed tuned mass damper system on the wind induced response of the John Hancock Tower, Boston, BLWT, University of Western Ontario (1976).

16. A. G. Davenport, D. Surry and H. Tanaka, A study of wind induced response of the John Hancock Tower, Boston, BLWT-SS2-76, University of Western Ontario (1975).

17. D. Surry, R. B. Kitchen and A. G. Davenport, Wind induced exterior pressures on the John Hancock Tower. Boston, BLWT-SS2-76, University of Western Ontario (1976).

18. J. B. Skilling, T. Tschanz, N. Isyumov, P. Loh and A. G. Davenport, Experimental studies, structural design and full scale measurements for the Columbia Seafirst Center. Building Motion in Wind, ASCE,

1-22 (1986).

19. C. J. Keel and P. Mahmoodi, Design of viscoelastic dampers for the Columbia Center building.

Building Motion in Wind, ASCE, 67-82 (1986).

20. S. Sasaki, M. Shiko and Y. Atsukawa, Superstructure of OUB center. Singapore, Second Century of

the Skyscrapers, Van Nostrand Reinhold Co., 975-987 (1988).

21. G. R. Lythe, D. Surry and A. G. Davenport, Wind

proses

over selected sites in the Hong Kong area. BLWT-SS22-81, University of Western Ontario (1981).

22. P. N. Georgiou, M. J. Mikitiuk, D. Surry and A. G. Davenport, Wind climate for Hong Kong. BLWT-SS2-84, University of Western Ontario (1984).

Wind Engineering Studies on Tall Buildings

A. G. Davenport, N. Isyumov, G. R. Lythe, L. E. Robertson, A. Steckley and D. Surry, The wind engineering study for the bank of China. Proc. 4th International Conference on Tall Buildings, Hong Kong, 143-147 (1988).

T. Tschanz, The base balance measurement technique and applications to dynamic wind loading of structures. Ph.D. Thesis, Faculty of Engineering Science, University of Western Ontario (1982). A. G. Davenport, Perspectives on the full scale measurement of wind effects. J. Ind. Aerod. 1,23-54 (1975).

C. J. Rathbun, Wind forces on a tall building. Trans. ASCE (105) Paper No. 2056, 1-82 (1940). W. A. Dalgliesh, Experiences with Wind Pressure Measurements on a Full-scale Building, Proc.

Technical Meeting concerning Wind Loads on Buildings and Structures. NBS Bldg. Sc. (30) Gaithers- burg, Md., 61-71 (1979).

A. G. Davenport, The response of supertall buildings to wind. Second Century of Skyscraper, Van Nostrand Reinhold Ltd., New York, 705-725 (1988).

W. A. Dalgliesh, Comparison of Model/Full-scale Wind Pressure on a High-rise Building. J. Ind.

Aerod. 1, 55-66 (1975).

W. A. Dalgliesh, Comparison of the model and full scale test of the commerce court building in Toronto. Proceedings of International Workshop on Wind Tunnel Modelling Criteria of Techniques in

Civil Engineering. National Bureau of Standards, Gaithersburg, 515-527 (1982).

W. A. Dalgliesh, J. T. Templin and K. R. Cooper, Comparisons and wind tunnel and full-scale building surface pressures with emphasis on peaks, Wind Engineering. Proc. 5th International Conference on

Wind Engineering, July 1979 (Ed. by J . E. Cermak (1, 553-565 (1980).

A. G. Davenport, M. Hogan and N. Isyumov, A study of wind effects on the commerce court tower. BLWT-7-69, University of Western Ontario (1969).

J. T. Templin and K. R. Cooper, Design and performance of a multi-degree-of-freedom aeroelastic building model, J. Wind Engng Ind. Aerod. 8, 157-175 (198 1).

J. T. Templin and K. R. Cooper, Torsional effect on the wind-induced response of a high-rise building. Presented 4th U.S. International Conference on Wind Engineering Research, Seattle, Wash~ngton, 2 6 29 (1981).

W. A. Dalgliesh and J. H. Rainer, Measurements of wind induced displacements and accelerations of a 57-storey building in Toronto, Canada, Proc. 3rd Colloquium on Industrial Aerodynamics,

Buildings Aerodynamics, Part 2, Aachen, Germany, 1, 67-78 (1978).

R. Halvarson and N. Isyumov, Comparison of the predicted and measured dynamic behavior of Allied bank plaza. Building Motion in Wind, ASCE, 2 3 4 1 (1986).

M. R. Wiliford and A. J. Fitzpatrick, The integration of structural analysis and wind tunnel testing for the New Hong Kong and Shanghai Banking Corporation headquarters in Hong Kong-Part I.

Proc. 3rd International Conf. on TUN Buildings, Hong Kong, 243-249 (1984).

A. G. Davenport, D. Surry and G. R. Lythe, The integration of structural analysis and wind tunnel testing for the New Hong Kong and Shanghai Banking Corporation headquarters in Hong Kong- Part 11, Proc. 3rd International Con$ on Tall Buildings, Hong Kong, 256256 (1984).

A. Baskaran, Computer simulation of 3D turbulent wind effects on buildings, Ph.D. thesis, Concordia University, Montreal, P.Q., Canada (1990).

F. Baetke, Numerische Berechnung der Turbulenten umstrongum eines kubisher korpers, Ph.D. Thesis, Technische Universitat Munchen, Germany (1986).

S. Murakarni, A. Mochida and K. Hibi, Three dimensional numerical simulation of air flow around a cube model by means of a large eddy simulation, J. Wind En,qng Ind. Aerod., 25, 291-305 (1987). H. Tanaka, Building internal pressures and air infiltration, Proceedings of 4th Canadian workshop on

Wind Engineering, Toronto, Canada, 88-1 12 (1984).

H. Tanaka and Y. Lee, Stack effect and building internal pressure, Proceedings of 7th International

Conf. on wind engineering and industrial aerodynamics, Aachen, Germany, July 1987,293-302 (1988).

Y. Lee, H. Tanaka and C. Y. Shaw, Distribution of wind and temperature induced pressure difference across the walls of a twenty storey compartmentalized buildng, J. Wind Engng Ind. Aerod. 10, 287- 301 (1982).

T. Stathopoulos and H. D. Luchion, Transient wind induced internal pressures, J. Struct. Engng, ASCE, 115,7, 16501-1514 (1987).

A. G. Davenport and D. Surry, The estimation of internal pressures due to wind with application to cladding pressures and infiltration, published in Proc. Wind Pressure Workshop, Belgium (1984). T. Stathopoulos and R. Kozutsky, Wind-induced internal pressures in buildings, J. Struct. Engng, ASCE, 112,9,2012-2026 (1986).