wind engineering canadian.pdf

8/11/2019 Wind engineering canadian.pdf

1/20


2/20


3/20

2 A Baskaran

11. SearsTower 443 Chicago 1974

3 World Trade 415 New York 1973

Center Souh

4.

mpire

State

381 NewYork 1931

5. Central Plaza 372 Hong Kong 1992

16

ank

olch lna 367 Chicago 1988

18

John

Hancxldr

343 Chicaao

1968

1

10 Firs

tnteratale 310

LosAngeles 1 ~ 9 0

1

1.

Miglin Beitler

594 hicago

9m 1

2. Tour Sans Fin 4 Paris

M A

13

Taiwan

Tower

331

Koahsiuno t993 1

(Source:

Engineering

News Review

-

Nov

15,1990)

Fig

1 The ten tallest buildings in the

world

paper, after a n 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 Trad e Center Towers

New York), the Sears Tower Chicago) and the newest

example, the Bank of China Hong Kong ). These studies

may represent the design methodo logies of 1930s, 1960s,

1970s an d 1980s.

2.1

Empire State Building

Dryden an d Hill

[q

nder took the first significant wind

tunnel study on the Empire State Building. A 1:250

model m ade of rolled a luminum plates 114 in. thick was

constructed to represent the 1250 ft 381 m) high building.

Both w ind-induced pressures and overturning moments

on the building were examined in a 10 ft. wind tunnel a t

the National B ureau 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 approxim ately 12, 18 and 24 m/s).

Pressure coefficient distributions at three different

levels are sho wn in F ig. 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 a t 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 fo r

two principal sway directions are shown. The m easured

moments are normalized by the velocity pressure, rep-

resentative area a nd a rm length which is taken as 4.4 ft.

an d 2.0 ft. model scale) for x and y directions, respec-

tively. This study, which was the first of this kind, shows

an ap preciation of th e 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, an d even after, their construction. T he wind effect

on the towers were examined at Colorado State Uni-

versity CSU) an d 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. Ab out

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

5

psf for the

remaining portion of the tower was recommended from

the wind tunnel test results for the 100 year wind of


4/20

Win d Engineering Studies on Tal l uildings


5/20

A

Baskaran

ngle of f ce

to

wind ngle of

f oe

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 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

W W

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 [l l] . 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


6/20

Wi nd Engineering Studies on allBuildings

PL 7 x

7'

wind tunnel

working

section

model constructed

frame mvered with

It

2 dia aluminum tube

coil springs provide

required stiffness

Fig 4

eneral arrangement of the

11400

scale Mode l 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.

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 hese 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 .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 :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


7/20

A Baskaran

9

ORTH

Fig.

5.

Three kinds of exposure conditions based on win 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 :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


8/20

Wi nd Engineering Stud ies on Tall Buildings

HOUSE

EXPOSURE 3

/ GTI

= o p E l

MPOSURE

MPOSURE3

55 .7 24.3 TOWER

-

6 6 AZIMUTH 1 4 4

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 Cit y Corpora tion 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. build-

ing model was fabricated using a scale of 500. t con-

sisted of seven lumped masses interconnected with elastic

columns. The natural periods of vibration in two sway


9/20

A Baskaran

N

Level7

Level 6

L ~ w

Level

2

-

--------...---

CP

I ~ P IC ~ r m s

-.-.-*-

. -

I~ P IaCprm

.......

......

Dynamic

pressure

r

Fig

7 Variation of external pressure for a West wind o n 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

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. 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 o 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


10/20

Wind Engineering Studies on Tall uildings

0.0 8.0

TAP 53

6.0

4.0

20

-

e

0

90

a Mmum

angle

a muth angh

2.0

4

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 typh oon winds, typically

about 48 m/s, whereas the occupants comfort and ser-

viceability are designed based on the non-typhoon wind

climate of around

8

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

UW

to predict the wind loads for various return periods. The

largest suction for the 100 year wind was ab out

6.6

kPa

occurring on a joint corner of the building. Generally,

the east exposure has higher peak suction values than

other e xposures. This is not o nly 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 Hon g Ko ng 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 tun nel predictions.

Another approach taken in this study was the use of

the force balance technique developed by Tschanz [ 4]

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 a re not vitally imp ortant d uring

initial design of the building. A com parison 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)

Y T Y

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


11/20

A Baskaran

?

1

4 cale model

ressure

- - - - I uction

o 1 000 scale model

Cp

scale

0 0 5 1 0 1 5

7

C

WEST WIND

evel

evel

4

Level 2

NORTH

EAST WIND

= 180

5 O

Fig 9 Comparison of mean pressure coefficients obtained with

: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 a t 10 stations

on each of three floors using manometer boards and flash

cameras. As mentioned previously, Dryden and Hill

[ ]

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-


12/20

Win d Engineering Studies on Tall uildings

Exposure -+ Exposure

r

xpowre

q

xposure

I

xpowre 2

1

Azimuth angle

a

degrees)

Arimuh anglea

degrees)

Fig

10 Typical variation of aeroelastic response with azimuth angle

[12]

siders the bou ndary layer wind tunnel profile which var-

ies with height. In any event the change in wind direction

causes significant changes in pressure reading fo r both

cases. The agreement seems to

e

better when the wind

is normal to the building walls rather th an when the w ind

strikes the building at a n angle.

Anothe r comparison was attempted by D avenpo rt [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. 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. Th e agreement between them is

remarkable. This provides a very important full scale

confirmation of a model test.

3.2

Comm erce Court uilding

Full scale measurements were undertaken by Dalgliesh

and other mem bers of the Division of Building Research

National R esearch Council Canada during the period

1973-1980. Surface wind-indu ced pressure was measu red

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 a t a sampling rate of 120 samples

per minute over a period of

5

minutes.

Extensive comparisons between full-scale results and

wind tunnel exp eriments were reported in [29 301 an d

[31]. Figure 16 depicts the mean and rms pressure mea-

sured at two d iffere nt evels 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 w inds. 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 d ata .

Wind tunnel studies of the Commerce Cou rt building

were first carried out in the U WO and this study included

measurement of mean an d 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 Cana da also performed a n extensive

wind tunnel study on the Commerce Court building.


13/20

A Baskaran

Fig. I.

0.5

0 4

0.9

0.2

0.1

0

0.1

0.2

rom pressure measurements

-0.3

Aemelastic results:

o

exposure 1

0.4

A exposure2

0.5

exposure3

Comparison of mean base bending moment obtained from the aerolastic and pressure measure-

ments [12]

There a re many significant differences between the N R C

study and that of the UWO [33 34 351. The pressure

study a t NR C 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 on e a t each of th e five

instrumented levels in the full-scale building and then

dividing the remaining level into two m odules to make

all the modules approximately of the same height. More-

over, taking advantage of the m m, NR C wind

tunnel, mo dels were fabricated using a geom etric scale of

200 in comparison to the 500 scale of UW O.

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-S outh and East-West). In general, the

agreement is quite satisfactory. However, in the N orth-

South acceleration, sharper peaks a nd greater fall-off of

contribution by the second mode are evident in the model

than in the full-scale results. This may be d ue to the mod e

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 abou t

220mm

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 to p floor acceleration of the

Allied ank Plaza in Ho uston, Texas has been reported by

Halvarso n and Isyurnov [36]as a comp arison with the wind

tunnel test results. The measurement was done using two

kineme tric Mod el 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 an d also

by Hurricane Alicia, with the fastest mile speed of


14/20


12

11

N

X

RMS Response

b

40 80 120

160

200 240 28 320

3M

Azimuth

(degrees)

E

Y RMS Response

Smchrral damping t

ratio 5

0.020

g

125 mph

OOmph

....... 75

mph

0mph

Azimuth (degrees)

Fig.

12.

Variation of aeroelastic rms x and

y

response with wind speed and wind direction for different

values of structural damping

[14]

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

SUMM RY

OF

TR NSITION

IN

RESE RCH

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


15/20

4

A Baskaran


16/20


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

Introduction of peak fa ctors 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 an d the above grouping is not

in chronological order

;

ather it identifies major changes

in the research activities. Appreciation of wind loads in

design started ne arly 100 years ago, wh en the Eiffel Tow er

was completed to mark the occasion of the Paris exhi-

bition in 889

[28]

On the other hand, research for the

wind effects on tall buildings started only during the

design of the Em pire 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 t o 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 )

ind 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]

B E

28:l 0


17/20

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

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

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].


18/20

Wind Engineering Studies on

Tall

uildings

FunTseale

maw

1

' ~

0300 0600 1000 1400

1800

GMT

2200 01 00 0500 9

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

;

his 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

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


19/20


20/20

Wi nd 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 W ind Load s 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 Worksh op 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-freedomaeroelastic

building model, J Wind Engng Ind. A erod.

8,

157-175 198 1).

J. T. Templin and K. R. Cooper, Torsional effect on the wind-induced response of a high-rise building.

Presented4th

U S

International Con ference on Win d Engineering Research, Seattle, Wash~ngton, 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, 2341 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 C onf. 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. A erod. 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 Wo rkshop, Belgium 1984).

T. Stathopoulos and R. Kozutsky, Wind-induced internal pressures in buildings, J Struct. Engng,

ASCE, 112,9,2012-2026 1986).

wind engineering canadian.pdf

Documents