Discussion of session 4 - wind loading of tall buildings (a)

Journal of Wind Engineering and Industrial Aerodynamics, 13 (1983) 209--213

209

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands DISCUSSION

OF SESSION 4 - WIND LOADING OF TALL BUILDINGS

Rapporteur:

Professor J. A. Peterka Fluid Mechanics and Wind Engineering Department of Civil Engineering Colorado State University

(a)

Program

DISCUSSION ON PAPER BY R. A. EVANS AND B. E. LEE COMMENT BY R.H. Scanlan Please comment on the possibility of fluid-structure at

the resonant amplitudes

interaction

occurring

of the model - (in view of the possibility

of large

amplitudes with a "soft" model). AUTHOR'S

REPLY - B. E. Lee

The models used in the study carbon-fibre expected under

plates

with

presented

here

to represent a stiff, rigid system.

test

have

been

The ratio of aerodynamic

Tower building

been

constructed

The displacement

checked with a linear displacement

been shown to be compatible with the corresponding buildings.

have

a foam infill to prevent panel vibration

movement

damping to structural

of

from

and may be the

models

transducer and have of the

full

scale

damping for the Arts

in a strong wind, 25 m/s at 84 m, is of the order of 1%.

COMMENT BY J. SAUNDERS Could Dr. Lee elaborate on the substantial force

(or moment)

spectrum,

spectrum,

Int.

on

as

reported

in

the

London

Conference Heathrow,

K.

1977).

REPLY - B. E. Lee

The advantage of the resonant model testing method is amplification

of the system results

to noise ratio of the measurement Nicolet

the displacement

function which depends

Conf. on Wind Effects on Buildings and Structures,

J. Eaton ed., Cambridge Univ. Press, AUTHOR'S

admittance

of the

I'm pleased to see he has used the technique which we

have had to use at Monash University, (Fourth

in measurement

given the difficulty of measuring

and the shape of the mechanical

the damping measurement.

improvement

FFT

Computing

the resonant response

in a considerable

system.

We have

that

the

improvement

made

mechanical

in the signal

extensive

use

of

a

Spectrum Analyzer which has enabled us to characterize

spectrum with sufficient

accuracy to yield a satisfactory

model load spectrum.

DISCUSSION

ON PAPER BY A. P. JEARY AND B. R. ELLIS

COMMENT BY DR. H. RUSCHEWEYH You showed in your presentation a full-scale

only 6 1/2 min each. 6

a diagram with different response

building which showed poor stationarity.

Our experience with such full scale measurements

1/2 min is too short;

0167-6105/83/$03.00

spectra of

The integration

it must be - depending on the natural

© 1983 Elsevier Science Publishe~ B.V.

time was is

that

frequency of the

210

building - i/2 hour to i

hour

increased

time?

the integration

to

get

stationary

results.

Have

you

also

AUTHOR'S REPLY - A. P. Jeary The slide used was an old one used only to demonstrate involved.

The attainment

the sort

of small confidence bands on analysis

of

problem

is something

to

which I attach great importance and we do indeed use long record lengths. The 200 criteria used is T = f ~ where ~r is the damping ratio and fr is the frerr quency of the resonance being considered. Thus for 1 H z and 1% damping the record

length

required

is

20,000 records,

or just over five hours.

In fact

non-stationarity

of data over periods of this length mean that ensemble averag-

ing

are

techniques

lengths

needed

for

more

detailed

in excess of 200 hours are needed.

experiment)

studies.

Details of

In this case record

this

(the

Arts

Tower

have been published before.

COMMENT BY R. H. LEICESTER Of the five examples given in Table 2 of the paper, measured

structural

code method. loads

and

Do I correctly

interpret

the associatedmember

this

to

structural

mean

that

structural

member

analysis procedure?

REPLY - A. P. Jeary

Yes, that is exactly the interpretation. the answer

is required.

Firstly,

However,

that

the

factor

a little amplification

there is a confidence

tion of response and another on the calculation possible possible

the

stresses were twice as great as that predicted

by an engineer using a reasonable AUTHOR'S

three indicate that

response was more than twice that predicted by the Canadian

to

interval on the estima-

of expected response,

so it

is

of 2 could reduce to less than one, but it is also

that it is really greater than 4.

Secondly,

general not taken in the way that designers

expect.

loads in buildings The individual

are in

loads taken

in any member may be much larger (or smaller). COMMENT BY JUN KANDA I would like to ask Dr. Jeary about frequency estimation 100%.

The natural frequency

ing ratio. compared

to the design value.

designed

Another

forces should have

for wind forces.

different

natural

if amplitudes

the order of design value) and if sites are limited then the estimation

small

amplitudes

factor could be that buildings

Therefore

error should be significantly

as

as well as the damp-

The plots in Figure 1 might be obtained based on

for strong earthquake those

is dependent on the amplitude

errors as large

designed

frequencies

from

are limited (e.g. to

(e.g. to Japan

or

U.

K.)

reduced.

AUTHOR'S REPLY - A. P. Jeary It is quite right that natural frequencies

reduce with increasing

amplitude

and so measurements of natural frequency under earthquake loading would tend to show a larger variation

from expected values.

However,

our

measurements

have

211

shown

frequency

amplitudes excited

changes not to be particularly

expected

measurements.

design calculations ferent

Additionally

predicting

from what (later)

based

largely

we have direct experience

a value of natural

frequency

that inclusion of measurements

of natural frequencies

on

wind-

of cases of the

significantly

it was found to be by measurement.

would lead to a larger variance

DISCUSSION

large over the entire range of

for wind loading and Figure 1 is

dif-

I think therefore

under earthquake

loading

than that shown in Figure I.

ON PAPER BY N. ISYUMOV AND M. POOLE

COMMENT BY H. KAWAI You said in your paper that the spectral peak at the 0.05 corresponds

to that of the longitudinal

the quasi-static

theory,

to

the

turbulence

turbulence.

only the lateral component of

torque when wind direction does not contribute

reduced

frequency

However,

according

of to

turbulence

contributes

e is 0 ° while the longitudinal

component of

as shown in equation

(4).

I

spectral peak at the reduced frequency at 0.05 corresponds

think

that

the

to the spectral peak

of the lateral component of turbulence. AUTHOR'S REPLY - N. Isyumov As seen from equation turbulence

(4) of the paper, both the u'

and

v'

components

of

to the torque. Of course in some cases v' predominates d C_ (~) due to the large values of dT ~ Possibly there is some confusion in notation

contribute

which

regarding

has led to the above question.

Nevertheless,

the importance of the v' component

peak around 0.05 is the combined approaching

Dr. Kawai is correct

and I agree that the broad spectra

effect of both the u' and v' components

of the

flow.

COMMENT BY H. KAWAI You mentioned four

faces

that the correlation between the torque

were

relatively

the correlation must be large.

small.

If the quasi-static

components

from

the

theory is applicable,

Do you have a comment?

AUTHOR'S REPLY - N. Isyumov The contributions as

mentioned

Quasi-static tions

in

to the torque from various faces are not equal.

the

paper

where

in the load unbalance are mainly caused by the on-coming

~ >

40 °

and

~ = i0 ° in Figure 9).

dominate

the

fluctua-

turbulence. (see

the

ambient turbulence.

It

rectangle

In any event, the quasi-static

mates are suggested only as a measure of the portion of the load unbalance to

fact,

certain faces dominate the total dynamic torque.

theory applies reasonably well in situations

does not work as well when wake induced fluctuations for

In

estidue

It is clearly not a reliable model in the general

case where wake turbulence plays a role.

212

COMMENT BY A. LANEVILLE Did the authors find a vertical correlation in their

measurements

on

the

longitudinal side near the wake region? AUTHOR'S

REPLY

-

N. Isyumov

Measurements to-date have largely been confined to the torques at particular levels.

Nevertheless,

some data obtained between levels suggest that the co-

variance between levels varies in an expected manner where the is

dominated

by turbulent fluctuations

dynamic

torque

contained in the approach f~ow.

Simi-

larly there is some coherence at the Strouhal frequency and at the higher quency

peak,

which

from

fre-

subsequent point pressure and velocity measurements

seems to be due to exposure of the trailing edge of a square or rectangle ~ 90 ° .

These measurements are preliminary,

with

however.

COMMENT BY A. KAREEM Synthesis of torque indicated graphically interface

coherence

that

the

the

has been ignored, assuming the inputs from all four faces

to be independent random processes. gests

in your figures suggests that

antisymmetric

My experience in a similar

exercise

nature of vortex shedding contributes

cantly to the overall torque arising from the two side

faces

sug-

signifi-

(for ~

=

0 ° ).

Such contribution is quite sensitive to the approach flow characteristics, the overall torque spectra in the urban situation has higher

energy

than

e.g. the

open country flow. AUTHOR'S REPLY - M. Isyumov Direct measurements of the component torques from each half face, each face, combinations

of various faces, and the total torque per unit height were made.

Consequently no assumptions of independence were necessary

and

the

presented

results do correctly include all inherent correlations.

While not presented in

this paper, we have calculated the inter-half

full

matrices.

In

face

tion of the lift force and its point of action, not

strong.

Nevertheless,

particularly unbalanced

and

face

covariance

situations where the dynamic torque comes largely from fluctuaare

there is some coherence at the Strouhal frequency,

in situations where the pressure

the interface correlations

dynamic

torque

comes

distributions due to vortex shedding.

principally This coherence

from (-)

is quite strong between the half back faces for a 2 x 1 rectangle with the wind normal

to

the

broad face.

Similarly,

there is significant coherence between

the rear half of the side face and the adjacent half of the back face. Regarding the comment that vortex shedding contributes overall

torque

not the case.

significantly

Pressure fluctuations at the Strouhal frequency

are

quite well along the side faces, consequently their contributions small.

to

the

arising from the two side faces, our results show that this is correlated

to torque are

The dominant torque exciting mechanism due to vortex shedding

in

fact

213

comes

from

the

back face when there is a strong negative

correlation

between

the two half faces. The paper length limitation precluded terrains.

urban exposures. becomes

the inclusion of torque data for other

We find that the general trend remains similar for both suburban and As expected,

somewhat

more

the

broad-band

vortex in

shedding

contribution

rougher terrain.

paper is the effect of large flow assymmetries which can built-up

city terrains.

our shopping

to

torque

Not examined

in this

be

found

in

highly

This very important mechanism of load unbalance

is on

list of things to do next.

COMMENTS BY T. MIYATA Regarding do

not

the quasi-steady

assumption

for the torsional motion of a body,

understand why it can succeed in describing

the case of bluff sections behaviour,

in

the reattachment

which

the

separated

The distribution

faces

the motion, which results

depend

on

from that derived from the quasi-steady AUTHOR'S

REPLY

-

shear

point of the separated

due to the body motion. will

the torsional flow

governs

of pressure acting on the

side

in quite a different

assumption.

the

sur-

pattern

What are your comments?

N. Isyumov on

will depend on the body motion and would be quite different large motion occurs,

for example for bridges.

The

the

side

faces

in situations where

objective

of

this

is to study buildings where the body motions are relatively

where aeroelastic

I In

shear flow may vary rapidly

I agree with Dr. Miyata that the pressure distribution

however,

loading.

effects are not expected to play an important

paper,

small and

role.

COMMENTS BY P. JACKSON In Figure 4 the pressures in

phase,

at the peak frequencies

of spectra 3 and 4 must be

as they do not contribute much to the torque.

However,

in Figure 7

spectra 3 and 4 seem to reinforce each other as there is then a large contribution

to

torque

from the side faces.

the side of the rectangle AUTHOR'S

REPLY

-

I generally

Do you have any ideas why the flow down

is so different

from that down the square?

N. Isyumov

concur with Dr. Jackson's

observations

although I do not

regard

the difference between the square and the rectangle with ~ = 0° to be great.

I

do not have a full explanation

a

detailed

knowledge

for

the

of the fluctuating

difference

as

pressure field.

it

would

tion is that the extent of the separation bubble is approximately both shapes.

This would lead towards a better correlation

torque.

I

needed.

some

do not believe that this is the entire explanation

that further detailed

study of the separation bubble and its

explana-

the same

of pressures

the windward and leeward half faces of the square and hence of

require

One plausible

for

between

cancellation and believe

intermittency

is