Discussion of session 7 - low-rise structures wind loading and flow field

Discussion of session 7 - low-rise structures wind loading and flow field

Journal of Wind Engineering and Industrial Aerodynamics, 29 (1988) 315-326 315 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Nether...

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Journal of Wind Engineering and Industrial Aerodynamics, 29 (1988) 315-326

315

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

7TH INTERNATIONAL

CONFERENCE ON WIND ENGINEERING

DISCUSSION OF SESSION

7

-

LOW-RISE STRUCTURES

WIND LOADING AND FLOW FIELD

Chairman:

Holmes,

J.D.

CSIRO Division of Building Research, PO Box 56, Hlghett,

Victoria 3190,

AUSTRALIA.

CHAIRMAN'S COMMENT:

T h i s S e s s i o n c o v e r e d a wide r a n g e o f p r o b l e m s o f l o w - and m e d i u m - r i s e buildings

involving

w i n d . The p a p e r s

wind damage and p l a s t i c and smoke e x t r a c t i o n wind p r e s s u r e exception

in the structure,

topics

as

and v e n t i l a t i o n

p r o b l e m s . However, a common t h r e a d was t h e u s e o f

distributions,

b o t h s t e a d y and u n s t e a d y .

With t h e

o f t h e n u m e r i c a l c o m p u t a t i o n s o f Mathews and Meyer ( s u r e l y

method o f i n c r e a s i n g

importance in the future),

obtained

experimentally.

of s c a l e

of turbulence

continued

deformation

included such disparate

in this

Discussions and ' d i s t o r t e d '

Session.

in earlier

these distributions Sessions,

a were

of the e f f e c t s

m o d e l l i n g i n wind t u n n e l s

were

316 DISCUSSION ON PAPER BY P.J. S A A T H O F F A N D W.H. MELBOURNE

COMMENT BY H. TIELEMAN I have similar data to that shown in Figures turbulent boundary two experiments

layer. The only significant

difference

in a

between the

is that I had a value of Lx/D of 7.0 instead of 2.6.

the pressure coefficients

If

do not change if Lx/D is changed between 2.6

and 7.0, the problem of integral-scale rise buildings

i, 2 and 3, obtained

distortion

for modelling of low-

is solved.

REPLY BY AUTHORS It is possibJe that the effect of scale decreases, beyond 2. However the results shown by Nakamura, Conference

cOMMENT

once values of Lx/D

Ohya and Ozono at this

show an effect of scale up to Lx/D equal to 10.

BY C. K R A M E R

Our experiments

have shown that when the leading edge is yawed relative

to the flow, the peak suction coefficient

decreases,

and the mean

suction increases to very high values.

REPLY BY AUTHORS Our study was limited to the effects of turbulence of 2-dimensional investigate pressures.

models normal to the flow.

intensity and scale

In the future we intend to

the effect of flow angle on the generation

of negative peak

317 DISCUSSION ON PAPER BY R.N.

MERONEY

COMMENT BY A. LANEVILLE P a p e r s and d i s c u s s i o n the effect

in earlier

of the scale

inconclusive. negligible;

of turbulence

I n some t e s t s , in o t hers

Sessions

it

this

of this

Conference concerning

on f l o w a b o u t b l u f f

effect

b o d i e s was

a p p e a r s t o he s m a l l o r

seems t o be r e l a t i v e l y

significant.

REPLY BY R.N. MERONEY A general placing

c o n s e n s u s seems t o be e m e r g i n g t h a t

t o o l a r g e a model i n an o t h e r w i s e

layer)will

result

due t o a d e c r e a s e

correctly

scaling

Whether t h i s

in turbulence

to integral

(~ 5~) o r l a r g e

It

intensity is also

or also

(i.e.

modelled shear

i n low p e a k and mean p r e s s u r e s .

changes is not always clear. are small

distorted

not yet clear

is solely scale

whether the errors

(> 25%).

CONNENT BY H. TIELENAN The r a t i o

surface

of i n t e gral

layer varies

duplicated

scale,

L, t o m i c r o s c a l e ,

as (~-~)~. u

i n t h e wind t u n n e l .

wind t u n n e l

However, t h i s As a r e s u l t ,

i s 150 ( a t a g e o m e t r i c s c a l i n g

the atmosphere it

i s 25 000.

shear

layers,

vortex

formation etc.,

field

on t h e model r e l a t i v e

to full

in t h e a t m o s p h e r i c

R e y n o l d s Number c a n n o t be

a typical ratio

I n t h e wind t u n n e l ,

o f t h e same o r d e r a s t h e l e n g t h s c a l e ,

1,

ratio

L/1 i n t h e

of 1/500),

whereas in

the microscale

~, a s s o c i a t e d and c o n s e q u e n t l y

c o u l d he

with separated alter

the pressure

scale.

REPLY BY R.N. NERONEY I agree that the lack of turbulent scales below the dimensions of the

318 shear

layers

separating

from m o d e l s i n w i n d t u n n e l s

contribute

to the confusion

concerning

and scales

on p e a k a n d mean p r e s s u r e s

the effect

may i n d e e d of t u r b u l e n t

intensity

on m o d e l s .

DISCUSSION ON PAPER BY E.H. /~ATHEWS AND J.P. NEYER

QUESTIONS BY S. ~JRAKAMI

Why was it regarded necessary to carry out an analysis of Reynolds Number dependency and how was it carried out wlth the k-s model?

REPLY BY AUTHORS:

i)

We were not sure of the Reynolds Number at which the full scale measurements were done. We therefore studied the influence of Reynolds Number on the pressure distribution.

li)

The turbulence properties and hence the turbulent viscosity change with wind speed.

CO~ENTS

BY A.P. ROBERTSON

I am a co-worker of the authors of the full-scale study referred to. The measurements were made with wind speeds between about 5 and 15 m/s (4 minutes mean at eaves height) over open country terrain.

The turbulence intensity of 0.14 in the numerical study is a little low. How sensitive are the numerical results to mean velocity profile and turbulence intensity?

Another effect

of wind speed in the full-scale

s t u d y was t h e c h a n g e o f

geometry caused by the lifting of the film plastic cladding as the wind

319

speed increases.

REPLY BY AUTHORS The effect of mean wind speed profile is expected to be small, as also shown in the paper by Toy and Tahouri. If the turbulence intensity is increased, the effective viscosity will increase. This will decrease the effective Reynolds Number and decrease the pressure coefficients. This should improve the agreement between the predictions and the full scale mesurements.

The e f f e c t

of p r o f ile

numerical

simulation.

d e f o r m a t i o n was n o t t a k e n

i n t o a c c o u n t in the

T h i s i s p r o b a b l y why o u r p r e d i c t i o n

point did not correspond with the full-scale

of separation

measurements.

COMMENT BY R.J. KIND The k-s model has had considerable difficulty in predicting different types of flow without adjustment of the empirical ~constants'.

REPLY BY AUTHORS We have used the empirical constants suggested for atmospheric turbulence by Dr Yeung and Professor Kot of the University of Hong Kong.

COMMENT AND QUESTION BY D. SURRY We h a v e r e c e n t l y that

considered

stronger

effect

an t h e a u t h o r s pressures

made wind t u n n e l m e a s u r e m e n t s on s i m i l a r by t h e a u t h o r s .

Our e x p e r i m e n t a l

comment o r t h e p o s s i b i l i t i e s

numerically?

to

d a t a show a much

o f R e y n o l d s Number t h a n do t h e a u t h o r ' s also

structures

calculations.

of p r e d i c t i n g

C

unsteady

320 REPLY BY AUTHORS Transition Reynolds Number phenomena are difficult to simulate. However the measurements of Toy and Tahouri appear to confirm our low Reynolds Number predictions.

Computations for unsteady flows can be carried out

using the large eddy simulation method shown by Murakami et al in Session 3.

COMMENTS BY H. SOCKEL In real flows you will have a separation bubble with a re-attachment point downstream.

I think your boundary conditions do not allow this

effect to occur. At the freestream boundary you impose the condition that the flow is prohibited from crossing it. Figure 3 then indicates a very high blockage ratio of about 25~.

REPLY BY AUTHORS The flow separation from the greenhouse,

as well as the separation

bubble and re-attachment point were predicted.

It is not true that the

zero gradient boundary conditions at outflow will not allow these effects

to

occur.

The caption for Figure 3 states that only the grid in the vicinity of the greenhouse

is shown. The actual flow domain that was simulated was

60 metres by 64 metres in dimensions. domain is less than 5~.

The blockage ratio for this flow

321 DISCUSSION OF PAPER BY P.N. GEORGIOU,

B.J. VICKERY AND D. SURRYY

QESTION BY C. DYRBYE The response of elasto-plastlc structures to wind loading shows a strong resemblance to the response to seismic actions. Could the concept of ductility used in earthquake engineering be applied in wind engineering.

REPLY BY AUTHORS The role of ductility in wind (drag) response is somewhat different to that in earthquake action. The post-yield deformation due to wind accumulates

slowly due to the many small excursions which are all in the

same direction,

due to the mean deformation.

there are large reversing excursions

In earthquake response

into the plastic range with large

energy dissipation.

COMMENT BY R. CIESIELSKI In the philosophy of limit states design, serviceability and strength limit states are considered separately.

In the former case,

structure should operate in the elastic range. limit states,

the

However for strength

the energy approach and ductility properties of the

material should be used, in my opinion.

REPLY BY AUTHORS Our a p p r o a c h c e r t a i n l y but the role earthquake

relies

of ductility

action

upon t h e a s s u m p t i o n t h a t

i s somewhat d i f f e r e n t

as discussed

in the reply

here,

ductility than for

t o C. D y r b y e .

exists,

DISCUSSION OF PAPER BY P.R. SPARKS,

M.C. HESSIG,

J.A. MURDEN AND

B.L. SILL

QUESTION BY G. REARDON I am concerned about the reliance on static load values for hurricane anchors. Our research has shown that such straps are likely to fail by fatigue from the gustiness of a hurricane. quality hurricane anchors'

Could it be that the

'poor

that failed during Hurricane Elena tore as a

result of fatigue cracking?

REPLY BY AUTHORS Most of the failures of hurricane anchors in H u r r i c a n e

Elena appeared to

be due to insufficient nailing in situations where one would have expected the load to exceed the capacity of the anchor. American manufacturers do not provide fatigue data on their anchors. However they use a factor of safety of 3 which should provide sufficient fatigue capacity if the anchors are sized by engineering calculations.

COMMENT BY G.R. WALKER The paper needs qualifying by stating that it only applies to traditlonal-type construction lacking detailed structural engineering input, and only up to wind speeds for which damage observations were made. With engineered construction,

roof slope, exposure,

security etc.

are automatically accounted for, and hence no longer variables, possible modes of failure are accounted for.

It is also important that

statements on probability of failure be consistent with known performance.

and all

323 REPLY BY AUTHORS One of the purposes of the paper was to point out the dangers of using observed damage to estimate wind speeds, We attempted to cover in our paper all types of construction,

including those meeting the

requirements of a modern wind loading code.

The roofing systems In the US are covered with asphaltic or fiberglass shingles. At present these do not have to resist the design suctions on low-pitched or gable-ended roofs in hurricane-prone areas. The shape of the roof therefore has an important bearing on the performance of these structures even when engineered. With regard to probability of failure, the main purpose, different

types

in the paper, was to show the relative risk for

of construction.

DISCUSSION OF PAPER BY Y. ~ATAKI t Y. IWASA~ Y. FUKAO AND A. OKADA

QUESTIONS BY R.J. KIND How was the large experimental air dome pressurised7 there substantial

In particular,

leakage or ventilation flow rate as in an actual

structure? This can have a large effect on pneumatic damping.

REPLY BY AUTHORS The f i e l d However, larger

m o d e l was p r e s s u r i s e d the

leakage

than that

entrances

ratio

(leakage/volume)

o f an a c t u a l

and windows.

i n t h e same way a s an a c t u a l

dome, b e c a u s e

of the field there

dome.

m o d e l was

w e r e many e x t r a

was

32,i DISCUSSION OF PAPER BY R.J. ROY AND J.D. HOLMES

COMMENT AND QUESTION BY D. SURRY The paper reminds us that maintaining similarity wherever possible is important.

I do not find it surprising that significant effects were

found for overall unsteady loads, but I would expect smaller effects for smaller tributary areas. Were any local pressure measurements made by the authors?

REPLY BY AUTHORS We agree that smaller effects should be observed for pressures and 3oads on smaller areas. However, we expect these would still be quite significant for bay or frame loads for which the ratio Luy/W is low.

Some further measurements of s c a l e

distortion on area-averaged panel

pressures and frame structural loads have been made at CSIRO. These have not yet been analysed or reported.

CONNENT AND QUESTION BY T. STATHOPOULOS I was very interested in this paper as it was a continuation of work presented by myself and Surry at the 6th International Conference. Concerning the errors in peak force coefficients

in Table 3, do these

include random variability inherent in peak values from single individual records?

REPLY BY AUTHORS All

the

values

was based and peak

quoted

were

on a s t a t i s t i c a l coefficients.

averaged

over

analysis

of

at

the

least

40 r u n s .

random errors

This of

both

number mean

325 QUESTION BY A. ROBINS Detailed and careful studies of this type, giving some indication of the variability

of actual

loads on buildings,

findings affect procedures

are very useful.

Wlll the

for wind tunnel work?

REPLY BY AUTHORS We h o p e t h a t

authors

of future

guidelines

and h a n d b o o k s f o r wind t u n n e l

testing will take account of the results in the paper when preparing their material. experimenters

Otherwise

themselves

it is necessary that individual wind tunnel consider the errors,

usually unconservative,

that may be caused when measuring wind loads with distorted model and turbulence

scallngs.

DISCUSSION OF PAPER BY TANAKA AND LEE

QUESTIONS i)

BY J. TEMPLIN

In Figure 2, a deviation of measured results from A.S.H.R.A.E. predictions

Is shown. Are the conditions

representations

of expected and real conditions?

imply the A.S.H.R.A.E. ii)

How do you predict buildings

simulated

calculation

in advance,

to provide

realistic

If so, this would

needs to be revised.

or measure,

leakage areas in real

input to the analytical

model?

REPLY BY AUTHORS i)

The A . S . H . R . A . E . real

buildings

calculation.

calculation

because it

d o e s n o t g i v e a good r e p r e s e n t a t i o n

e m p l o y s o n l y AH/Ao a s a p a r a m e t e r

This is shown by Figure 2 in the paper,

of

in the

in which our

326 analysis gives good agreement with measurement, A.S.H.R.A.E. ii)

equation,

but the

using AH/A o = 2.0, does not agree well.

We used a recommended value for typical opening per unit length of window frame to compute the leakage area.

QUESTIONS BY A. LANEVILLE How d o e s t h e m i x i n g p r o c e s s that

referred

test

section

to

in the flow in the test

in the A.S.H.R.A.E.

b a s e d on a s i m i l i t u d e

section

compare to

Code? A r e t h e d i m e n s i o n s

of

this

criterion?

REPLY BY AUTHORS The m i x i n g p r o c e s s

in the test

experiment.

However t h e s i z e

significant

effect

results

section

been investigated

o f t h e m o d e l was f o u n d n o t

on t h e r e s u l t s ,

of the analysis

has never

showed t h a t

to have a

as discussed

in the paper.

the effect

of the physical

dimensions

o f t h e m o d e l on t h e

location

of the neutral

negligible,

when t h e d i a m e t e r

is greater

t h a n 2 mm.

by

pressure

The

level

is