full-scale measurement on steel chimney stacks

8/16/2019 Full-scale Measurement on Steel Chimney Stacks

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342

1 4 5 m - -

1 4 0 m

U n l i n e d

chtmney_

3 s t a r t

h e l i c a l

c a b l e s

C o m p l e t e d c h i m n e y

3 s t a r t h e l i c a l

s t r o k e s

w i d t h 0 , 1 D

p i t c h

5 D

h i n t : r i c k w o r k

t 5 M p / m

h e i g h t w a l l - t h i c k n e s

( m ) ( r a m ) i

0 / 5

3 0

5 J . l ~

~

i s ~

5 ~

i

3 5 m - - - 2 5 / 3 5 2 4

3 5 ~ 4 Z 5 2 4 ,

[ 4 7 5 / 5 2 5 2 2

5 2 5 / 5 7 5 2 0 i

5 F , 5 / 6 5 1 8 . i

7 5 16 I

75

- / . 8 5 1 4 l

7 m

Fig.1. Details of th e 145-m high steel c him ney stack.

T h u s it w a s e v i d e n t t h a t th e h e l i c a l ca b l e s w e r e n o t v e r y u s e f u l in r e d u c i n g

t h e a m p l i t u d e s o f t h e o s c i l l a t i o n s . B e f o r e i t w a s p o s s i b l e t o d e v i s e m o r e e f -

f e c ti v e m e t h o d s o f p r e v e n t i n g t h e o s c i l la t io n s , t h e c h i m n e y s t ac k c o l l a p s e d .

T h e c o m b i n e d m a s s -d a m p i n g f a c to r ,

M 6 si p

D 2 , w a s v e r y l o w . T h e e q u i -

v a l e n t m a s s p er u n i t l e n g t h M f o r t h e f u n d a m e n t a l b e n d i n g m o d e c a l c u l a t e d

o n t h e a s s u m p t i o n o f a p a r a b o li c m o d e o f o s c i l l a t io n i s M = 1 4 3 k p m -2 s : .

U s i n g t h e l o g a r i th m i c d e c r e m e n t 5 s = 0 . 0 3 f o r t h e s t r u c tu r a l d a m p i n g , t h e

m a s s - d a m p i n g f a c t o r i s a p p r o x i m a t e l y 1 . 9 . T h i s l o w v a l u e is i n t h e r a n g e o f

i n s t a b i l i ty [ 1 , 2 ] .

T h e a s su m e d l o g a r i t h m i c d e c r e m e n t i s b a s ed o n e x p e r i e n c e f r o m o u r o w n

m e a s u r e m e n t s o n f r e e - st a n d in g w e l d e d - s te e l c h i m n e y s t a ck s , a n d o n t h e f a c t

t h a t in t h is c a se t h e f o u n d a t i o n p r o v i d e d a su b s t a n ti a l c o n t r i b u t i o n t o t h e

t o t a l d a m p i n g . T h i s is e v i d e n t f r o m t h e r e c o r d s o b t a i n e d in t h e s t u m p t e s ts ,

c a rr ie d o u t a f te r c o l l a p s e o f t h e s t a ck .

I t s h o u l d b e o f i n t e r e s t t h a t t h e c r o s s - w i n d o s c i l l a t i o n s o f t h e s t a c k a t t h e

c r it ic a l w i n d - s p e e d w e r e a p p r o x i m a t e l y c o n s t a n t o v e r a l o n g t i m e . A c c o r d i n g -

l y t h e c a l c u l a ti o n o f a f l u c t u a t i n g a e r o d y n a m i c c o e f f i c i e n t is p o s s i b l e w i t h i n

c e r t a in li m i t s. R e f e r r in g t o W . L a n g e r [ 3 ] , t h e g e n e r a l i z e d a m p l i t u d e o f t h e

s t e a d y - s t a t e o s c i l la t i o n s , t a k i n g t h e a ir d e n s i t y a s 0 . 1 2 5 k p s 2 m -2 a n d t h e

S t r o u h a l n u m b e r a s 0 .2 , m a y b e w r i t t e n a s

c o d 2 A

Y o

2 5 M * 6 s

7 9 m


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where A =Jd ~)r f(~) d~ is the generalized area a n d M *

= f m e ) f

(~)d~ the

generalized mass. The assumption that the first bending mode is parabolic in

the case of the above ment io ne d chimn ey gives A = d × h /3 = 280 m 2 and

from Fig .l , M* = 4,080 kp m -~ s 2 . The ref ore with Yo = + 1.2 m the fluctu-

ating aerody namic coefficient is abo ut 0.12 (based on the log decreme nt of

0.03). This value is in agree ment w ith th e results o btai ned b y full-scale mea-

surements on the television tow er at Hamburg [4] at small vibrati on ampli-

tudes.

2 . 2 R e bu i l d i ng and f ul l- sc a le m e as ur e m e n t s

For the c omple ted ch imney with a mass ratio (lining/without lining) of

abou t 3 : 1, a natural frequency of 0.27 Hz was predicted. From the st um p

test it was evident tha t the m ove men t of the fo und ati on block (3,800 Mp

weight) for the natural frequen cy is negligible. The e ffec t is less than 4%.

Because of the increase of the mass as well as the log decrem ent of the

lined chimne y, the mass-damping par ame ter also increases. With M = 429 kp

m-2s 2 and 5s = 0.06 (estima ted) the above -men tioned para mete r is 11.5. The

increase o f the paramete r relative to th e value for the unlined chi mney is

evident. However, because it is no t possible with present knowled ge to set

accurate criteria for instability due to vortex shedding at transcritical Re ynolds

numbers, aerodynamic devices to prevent cross-wind oscillations must be

proposed. To optimize the aerod ynamic devices for the rebuilding of the

chimney, wind-tunnel tests were carried out. Based on the test results and

the s ubsequent re comm endat ions of Scruton [ 5], a 3-start helical strake system

with strakes of width 0.1D (600 mm), of pitch 5D (30 m) and extending from

the 140-m level down to the 79-m level, was fitted to the surface of the com-

pleted chimney. The thickness of the strakes (mou nte d on insulator pins) is

5 mm. The completed chimney stack is shown in Fig.2.

2 . 2 .1 M e as ur e m e n t s o f t he s tr uc tu r a l p r ope r t i e s

After rebuilding of the chi mney, the natural freq uency and damping were

measured by the cable releasing me tho d (44-mm top deflection).

Chimney vibrations were measured using accelerometers mou nte d in ortho-

gonal pairs, with one axis orient ed along the NS direct ion and th e oth er along

the EW direction. The natural frequency of the vibration pick-ups was 4.5 Hz at

0.65 critical damping. The signals from the inst ruments were received at a

co mm on ground-level recordin g panel. Fig. 3 shows sample accelerati on and

strain records.

It should be no ted t hat during these tests agen tle breeze of 4 m/s was

blowing. Also the still-air values of 5 s would probably have been smaller than

those actually measured because of the positive aerod ynamic dampin g caused

by t he helical strakes.

The measur eme nt of th e e ffective dampin g gave a log decr eme nt 8 s = 0.1.

The predicted natural freque ncy agreed well with the experiment, in which

0.288 Hz was measured. Moreover the first harm onic values were measured


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Fig. 2 . C om ple ted ch im ney wi th h e l ica l s trakes .

a s 1 . 3 H z a n d 8 s = 0 . 0 8 . T h e c o r r e s p o n d i n g b e n d i n g d e f l e c t i o n w a s a b o u t

-+ 2 m m . I t s h o u l d b e n o t e d t h a t t h e a c c e l e r o m e t e r o u t p u t F i g . 3 ) s h o w s a

r e la t iv e l y l ar g e c o n t r i b u t i o n f r o m t h e f i r s t h a r m o n i c . T h e r e a s o n i s t h e d i f -

f e r e n t s e n s i t iv i t y o f t h e a c c e l e r o m e t e r f o r 0 . 2 8 8 H z a n d 1 . 3 H z o s c i l l a t i o n s .

2 2 2 Vibrati on and win d meas urem ents over a long peri od

T h e v i b r a t i o n s o f t h e c o m p l e t e d c h i m n e y s t a ck w e r e m e a s u r e d b y m e a n s

o f s t r a i n g a u g e s i n s t a l l e d a t t h e 3 5 - m l e v e l . S a m p l e s t r a i n r e c o r d s a r e s h o w n

i n F i g . 4 .

W i n d - sp e e d a n d d i r e c t i o n m e a s u r e m e n t s w e r e t a k e n a t th e 3 0 - m l e ve l s o m e

d i s t a n c e t o t h e w e s t o f t h e c h i m n e y s t a c k w h e r e t h e p r e v a i l in g w i n d d i re c -

t i o n s w e r e c le a r o f su r f a c e o b s t r u c t i o n s . T h e i n s t r u m e n t s i n s ta l l ed w e r e a c u p

a n e m o m e t e r a n d d i r e c t io n v a n e f i t t e d w i t h p o t e n t i o m e t e r .


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N S

E-W

(N -S ) Stroin gouges output

(E- W) Stroin gouges output .. ~

5trgg.

~L-~+_4],6mm ,.,~ .....

3 4 5

s e c

Time

F ig . 3 . S a m p le a c c e l e r a t i o n a n d s t r a in r e c o r d s c a b l e r e le a s in g te s t ) .

E~W

30 sec

I

E - W

N - S

F i g .4 . S a m p l e s tr a i n r e c o r d s w i n d - i n d u c e d v i b r a ti o n s ) .

The investigations of the behaviour of the completed chimney stack re-

vealed tha t the addition of strakes suppressed the vibrations significantly. At

the critical wind speed of about 8 m/s based on fundamental frequency of

0.288 Hz) the maximum amplitude at the chimney top was + 3 to 4 cm ap-

proximately . The corresponding alternating stresses were negligible.

At higher wind speeds, chimney vibrations were observed both across the

mean wind direction nearly westerly) and in wind. Both types of vibrations

appeared to be fairly random in character, with peak amplitudes increasing

with wind speed and with in tensity of gustiness. The funda mental vibration

mode p redominated in the response of the chimney stack.


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In the case of extreme wind conditions at the 140-m level: exp one nt for

the wind profile 0.182, 10-min mean wind speed 23.3 m/s to 24.8 m/s, gust

expo nent 0.0835 and max. gusts 30.1 m/s to 32.5 m/s) the m axi mum deflec-

tion at the top was ab out + 13.4 cm + 0.0223 X D).

Based on the experimental data obtained from the full-scale measurements,

maximum amplitudes at the chimney top of about 20 to 30 cm must be ex-

pected at the ma xim um wind speed of 44 m/s wind speed for design). That

is 0.16 to 0.14 times the static deflec tion 1.24 m) calculated with the drag-

force coeffic ients Cw = 0.7 f or the u nst rak ed parts of the chi mne y and Cw =

1.3 for the s traked parts of wid th 0.1 D. The dyna mic pressure is 123.5 kp/ m 2,

3. Dyn amic characteristics of some oth er steel chimn ey stacks

Six different welded-steel chimney stacks were tested for structural damp-

ing. Details of the chimneys investigated are given in Table 1.

T A B L E 1

D y n a m i c c h a r a c t e r i s t ic s o f s t e e l c h i m n e y s t ac k s .

N o . H m )

H D

t m m ) F l u e N o . L i n in g T e s t a ± m m ) ~ s N H z )

1 4 7 5 6 . 6 6 1 g u n i t e c a b l e 2 0 . 0 0 . 0 4 6 0 . 4 5

2 4 0 6 6 . 7 8 1 - - c a b l e 1 9 . 0 0 . 0 1 2 0 . 7 3

3 4 0 1 5 . 4 5 3 - - c a b l e 1 : 7 0 . 0 5 1 1 . 2 5

4 7 4 2 4 . 6 8 1 - - c a b l e 1 0 . 0 0 . 0 1 2 0 . 6 6

5 4 0 2 8 . 6 5 1 - - c a b l e 1 0 . 0 0 . 0 1 2 1 . 1 7

6 3 6 1 7 . 3 1 0 1 g u n i t e c a b l e 3 . 8 0 . 1 5 0 1 . 0 0

The thickness of the gunite lining outside chimney 1 is 100 mm. In the

case of chi mne y 2 there are fou r diameters 1.3 m fr om 0 to 7.3-m level, 0.8

m from 7.3 to 12 m, 0.7 m from 12 m to 21 m and 0.6 m from 21 to 40 m).

To prevent vortex-induced oscillations, three hydraulic a utomot ive shock-

absorbers KONI typ e 90--1090) were installed betw een the stack and a

separate stru cture at th e 18-m level. In plan, the dampers are located 120 °

apart. The log decr eme nt o f this system is 0.365.

Chimn ey 6 is framewor k-su ppor ted from 0 to 13.4 m) and the log de-

creme nt is noticeab ly higher than the values for the other unlined chimneys.

Stack 4 was fit ted on t he r oof o f a reactor-house at t he 57.15-m level. At

the predicted critical wind speed of about 9 to 10 m/s, cross-wind oscillations

with maxim um deflections of + 0.46 D + 1.4 m) at the top of the chimne y

were observed.

e f e r en c e s

1 C . S c r u t o n a n d A . R . F l i n t , W i n d - e x c i t e d o s c i l l a t i o n s o f s t r u c t u r e s , P r o c . I n s t . C iv . E n g . ,

2 7 A p r i l 1 9 6 4 ) 6 7 3 - - 7 0 2 .


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2 L . R . W o o t t o n , T h e o s c i l l a t i o n s o f l a rg e c i r c u l a r s t a c k s i n w i n d . P r o c . I n s t . C i v. E n g ., 4 3

A u g u s t 1 9 6 9 ) 5 7 3 - - 5 9 8 .

3 W . L a n g e r , Q u e r s c h w i n g u n g e n h o h e r s c h l a n k e r B a u w e r k e m i t k r e i s f ~ r m i g e m Q u e r s c h n i t t ,

3 9 4 . M i t t e i l u n g a u s d e m I n s t i t u t f u r L e i c h t b a u , D r e s d e n , D D R , S . 1 8 4 / 1 9 7 , 1 9 6 9 .

4 H . R u s c h e w e y h , B e i t r a g z u r W i n d b e l a s t u n g h o h e r k r e i s z y l i n d e r l ih n l i c h e r s c h l a n k e r

B a u w e r k e i m n a t t t r l i c h e n W i n d b e i R e y n o l d s z a h l e n b i s R e = 1 .4 × 1 07 , D i s s e r t a t i o n T H

A a c h e n , 1 9 7 4 .

5 C . S cr u t o n , R e p o r t o n t h e v o r t e x - e x c i t e d o sc i l l a t io n s o f t h e p r o p o s e d T h y s s e n c h i m n e y

f o r C o w p e r a n l a g e H O I - S e h w e l g e r n , 2 8 t h F e b r u a r y 1 9 7 2 , u n p u b l i s h e d p a p e r .

full-scale measurement on steel chimney stacks

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