How to Use ISO 12494 Atmosperic Icing for Structures

8/9/2019 How to Use ISO 12494 Atmosperic Icing for Structures

1/8

Proceedings of the Eleventh 2001) International Offshore and Polar Engineering C onference

Stavanger, Norway, June 1 7-22, 2001

Copyright 2001 by Tile blternational Society of Offshore an d Polar Engineers

ISB N 1-880653-51-6 Set); ISB N 1-880653-52-4 Vol. I); ISSN 1098-6189 SeO

I S O 1 2 4 9 4 A t m o s p h e r i c I c i n g o f S t r u c t u r es a n d H o w to U s e I t

M o g e n s H . F o d e r

R A M B O L L

C o p e n h a g en , D e n m a r k

A b s t r a c t

N o w , a f te r m a n y y e a r s t h e n e w I S O 1 2 4 9 4: " A t m o s p h e r i c I c i n g o f S t ru c -

tures" has been f ini shed and i s ready for use . As i t i s the f i rs t s tandard,

wh e re a l l i s sue s about i c e a nd d i me ns i on i ng fo r i c e ha ve be e n c o l l e c t ed i n

t he s a m e s t a nda rd , i t d if f e r s i n i ts subs t a nc e f rom "norm a l " c on s t ruc t iona l

s t a nda rds ( c ode s o f p ra c ti c e ) fo r a c t i ons on s t ruc t u re s. T he re fore , i t ma y be

ne c e s sa ry t o i n t roduc e

th e u s e o f i t

fo r me t e oro l og i s t s , de s i gne r s a nd o t he r

i n t e re s te d e ng i ne e r s a s w e l l a s o t he r use r s .

T h i s pa pe r e xp l a i ns how t he s t ruc tu ra l de s i gne r o r e ng i ne e r shou l d use t he

ISO 12494 a nd po i n t ou t the mo s t i mpor t a n t f a c i l it i e s fo r th i s use . T he

t ype s o f a c ti ons spe c i f i ed a re i c e ma ss a s we l l a s wi nd a c t i on f rom w i nd

l oa d on t he i c e d s t ruc t u re. T h e s t a nda rd ha s b e e n pre pa re d i n suc h a wa y

t ha t i t i nv it e s t o use sma l l " c a l c u l a t i on t oo l s " whi c h ve ry mu c h fa c i li t at e s

t he use o f i n forma t i on a nd i mp rove s t he unde rs t a nd i ng of the who l e s t ruc -

t u re o f t he s t a nda rd .

T he s t a nda rd c ou l d be use d a l so e ve n i f a Na t i ona l S t a nda rd o f i c i ng a l-

r e a dy e x i s t s , be c a use more o r l e s s o f t he c on t e n t c ou l d be a do pt e d by t he

Na t i ona l S t a nda rd wi t hou t a ny prob l e ms or c on t ra d i c t i ons . T he ISO 12494

c oul d e .g . , be use d fo r p re pa r i ng i c i ng ma ps fo r c ount r i e s o r pa rt o f c oun-

t r ie s , a s Na t i ona l B o di e s o f t e n w a nt t h i s.

K e y w o r d s :

Ic ing; s t ruc tura l des ign; i ce ac tions; ca lcula t ion o f loads; com-

bi na t i on o f l oa ds .

I n t r o d u c t i o n

As i t is t he f i rs t ti me a s t a nda rd i nc l ude a l l ne c e s sa ry i n forma t i on fo r d i -

me n s i on i ng s t ruc tu re s fo r bo t h g l a z e a nd r i me , a gu i da nc e fo r i ts use ma y

be appropria te , and thi s paper might be a s ta r t .

T he de f i n i ti ons o f I c e C l a s se s fo r bo t h g l a z e a nd r i me a s we l l a s the p r i nc i -

p l e fo r us i ng t he s t a nda rd i nc l ud i ng e xa mpl e s o f t he mos t ne e de d t oo l s i s

p r e s e n t e d a n d c o m m e n t e d . T h e s t e p s t h ro u g h t h e w h o l e d i m e n s i o n i n g p r o -

c e s s a re shown a nd how t h i s p roc e s s ne e ds c onne c t i on t o i n forma t i on o f

ic ing da ta , give n in the s tandard. Fo r prac t ica l use o f cause i t i s nece ssary to

ha ve t he ISO 12494 i t s e l f a s on l y f e w e xa m pl e s f rom i ts c on t e n t are shown

in thi s paper , but i t i s poss ible to use the s tandard in a very const ruc t ive

wa y whe n de s i gn i ng fo r a t mosphe r i c i c e .

B r i e f d e s c r i p t i o n o f I S O 1 2 4 9 4 .

I n 1 9 8 6 a w o r k i n g g r o u p I S O / T C 9 8 / S C 3 / W G 6 w i t h r ep r e s e n ta t

a l l c ount r ie s w hi c h sh owe d i n t e re s t i n pa r ti c i pa ti ng w a s e s t a b l i she

t he purpo se t o w ork ou t a n i n t e rna t i ona l s t a nda rd fo r i c e a c t ions o

t u re s : ISO 12494 "At m osphe r i c I c i ng o f S t ruc t ure s " . As s a i d i n th

a i m wa s ve ry b roa d a nd shoul d i nc l ude a l l ne c e s sa ry ba s i c i n form

a bout i c i ng i t s el f , be c a use suc h i n form a t i on wa s fou nd ne e de d t o

who l e sub j e c t unde rs t a nda b l e fo r t he use r,

T h e s t a n d a r d i s t h e r e f o r e v e r y d i f f er e n t i n c o n t e n t c o m p a r e d t o

t y p e o f s t a n d a r d s f o r w i n d a c t io n s , s n o w a c t i o n s e tc . F o r w i n d

l o a d s w e k n o w s u f f i ci e n t t o b e a b l e t o w o r k o u t v e ry ' p r e c i s e a n

t a i l e d c od e s o f p ra c t i s e fo r a c t i ons f rom t h ose t ype s o f l oa ds , bu

not t he f a c t fo r i c e l oa d . I t i s our i n t e n t i on a nd hope t ha t t he c on

d u r i n g c o m i n g y e a r s s h o u l d b e m o r e l i k e th e o t h e r s t a n d a r d s f o

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

e xp e r i e n c e a nd i n form a t i on o f de t a i ls i n i c i ng .

A l o t o f i n forma t i on i n t he s t a nda rd a re gu i da nc e , a nd t ha t i s t o un

t he unc e r t a i n t i e s c onne c t e d t o t he spe c i f i c f i gure s p re se n t e d . Ne ve

mi nd , we n e e d t o ha ve t hose da t a fo r be i ng a b l e t o do t he ne c e s sa

l a t ion fo r s t ruc tu re s . W e hop e t ha t ( a l l) me t e oro l og i s t s i n t he fu t u r

he l p i m prov i n g da t a r e l i a b il i ty by c on c e n t ra t i ng fo r j us t some of t

s e a rc h on t hose ma t t e r s whi c h i n pa r t i c u l ar a re wa nt e d u pda t e d o r

me nt e d , i n shor t : t he c on t e n t o f mos t f i gure s a nd t a b le s . B e c a use

ha ve p ro pose d e .g . a s ta nda rd fo r me a sure m e nt s o f i c e a c t ions (An

t he ISO ) so a s muc h a s pos s i b l e o f ne w re se a rc h c a n be ma d e use

fu t ure r e v i s i ons o f t he ISO 12494 .

B a s i c n o m i n a l i c e l o a d i n f o r m a t i o n .

In c ha p t e r 6 i n t he s t anda rd a l l ba s ic i n form a t i on a bou t i c e i s ga t h

"no t so e xp e r i e nc e d use r" wi l l the re f i nd a ny i n forma t i on ne e d e d

sub j e c t i t s e lf : i c e. He c a n unde rs t a nd t he d i f f e re nc e be t we e n g l a z e

rime, prec ipi ta t ion and in-c loud ic ing, hard r ime and soft r ime e tc

re a d i ng h e re he a l so c a n unde rs t a nd , wh y i t is i mpor t a n t t ha t he k

whi c h t ype o f i ce bu t a lso whi c h a mo unt o f ic e he ha s t o fo re se e o

s t ruc ture in quest ion.

6 7 8


2/8


3/8

~

t t t

F i g u r e 3 m I c e a c c r e t i o n m o d e l f o r g l a z e

Now , by us i ng IC Gx a r i d the mod e l sho wn i n f igure 2 i t is pos s i b l e to c a l -

c u l a t e ma sse s a nd d i me ns i ons ne e de d , a nd i n t a b l e 2 g la z e ma sse s a re g i ve n

for the c y l i nde r d i me ns i ons 10 , 30 , 100 a nd 300 ra m. Ac c re t e d g l a z e c a n

e a s i ly be c a l c u l a t e d fo r a ll o t he r ob j e c t d i me ns ions . T he 30 mm d i a me t e r

ha s be e n i nc l ude d be c a use i t i s t he r e c omme nde d d i a me t e r fo r s t a nda rd

me a sure me n t s , s e e la t er . T he show n mod e l fo r g l a z e a cc re t i on c a n be use d

for a l l objec t dimensions , but for prac t ica l use the e ffec t on s t ruc ture di -

me ns i ons i s i ns ign i f ic a n t , whe n ob j e c t d i me ns i on i s a round or a bove 5000

mm i n c ros s s e ct i on , so ob j e c t s i ze ha s be e n l i m i t e d t o < 5000 m m i n c ros s

sec t ion.

l e e C l a s s e s f o r R i m e

R i me i n t h i s s ta nda rd ha s t o be unde rs t ood a s "ha rd r i me " . In t he s a me wa y

a s fo r g la z e , a mode l fo r a c c re t e d r i me ha s de f i ne d t he a moun t o f r i me i n

d i f fe re n t IC R s . How e ve r , t he m ode l i t s e l f ha s be e n c ons t ruc t e d qu i t e d i f f e r-

e n t l y c ompa re d t o the mod e l fo r g l a z e be c a use t he na t u re o f fo rmi ng t hose

types i s very di s t inc t . For r ime accre t ions the ice mass has been defined

c ons t a n t i n e ve ry IC R x a nd i c e d i me ns i ons va ry wi t h bo t h ob j e c t / p rof i l e

t ype a nd d i me ns i on . T he t a b l e 3 be l ow shows t he de f i n i t ions o f IC R x,

whi c h ha ve be e n num be re d f rom IC R 1 t o IC R 9, a nd a s fo r g l a z e : IC R 10

ma y be use d fo r e x t re me r i me a c c re t i ons e xc e e d i ng t he de f i ne d c l a sse s .

T a b l e 3 ~ I c e C l a s s e s f o r r i m e ( I C R )

I c e I c e m a s s

C l a s s e s m

I C

R 1

R 2

K ~ m e o m m e t e r l m m l l o r o b j e c t

d i a m e t e r = 30 m m

De ns i t y o f r i me l kg / m3] - '

R 3

R 4

R 5

R 6

R 7

R 8

R 9

R I 0

[kg/m] ;300

0 ,5 55

0 , 9 69

1,6 88

2 ,8 113

5 ,0 149

8 ,9 197

16 ,0 262

2 8 , 0 3 4 6

5 0 , 0 4 6 2

5 0 0

47

56

71

9 0

l l 7

154

2 0 4

2 6 9

3 5 8

7 0 0

43

50

62

77

100

131

173

2 2 8

303

9 0 0

40

47

56

70

89

116

153

201

1268

t o be use d : o r e x t re m e i c e a c c re t i ons b i gge r t ha n R 9

In addi t ion to profi le c ro ss sec t ion, the den si ty of r ime i s a variable in the

mode l fo r r ime . T h i s i s ne c e s sa ry be c a use t he de ns i t y i n p ra ct i c e ma y v a ry

wi thin a broad spec t re , and thi s varia t ion resul t s in ra ther di ffe rent resul t s .

T he e f fe c t c a n be s e e n by c o mpa r i ng r i me d i a m e t e r s i n ta b l e 3 fo r d i f fe re n t

va l ue s o f r i me de ns i t y .

T he r i me a c c re t i on mod e l i n f igure 3 i s va l i d on l y fo r ob j e c t / p rof i l e d i me n-

s i ons up t o 300 mm . F or b i gge r c ros s s e c t i ons t he mode l c ha nge s , s e e l a t er .

T he m ode l sho ws t he c hose n p r i nc i p le o f ac c re t ion : R i m e i s bu i l d ing up i n

wi ndw a rd d i re c ti on a nd i n t he hor i z on t a l p l a ne . Unt i l a n a c c re te d va ne

length o f W o r lAW (see di ffe rent types o f profi le ) , the accre t ion i s occur-

r i ng wi t hou t a ny i nc re a se o f ob j e c t d i me ns i on p e rpe ndi c u l a r t o wi nd d i re c -

t i on . B e y ond t ha t po i n t the a c c re t i on i s g rowi ng a l so pe rpe ndi c u l a r t o t he

wind di rec t ion, but a t a s lower ra te than in the windward di rec t ion.

wa y i t i s now poss i b l e to c a l c u l at e a ll r i me va ne d i me n s i ons b y me

ra t he r s i mpl e e qua t ions , s e e Anne x A i n ISO 12494 .

W i n d d i r e c ti o n

T y p e A

,j/ t

,.

i< L

T y p e

B \ / t

I 8t ._ ma x W

/ - - - . I -

5

C _~

Ty pe C \~ , t

t


4/8

based on this densi ty and adjustment for cor rect densi ty has to be done, see

Anne x A in I S O 12494 .

T a b l e

4 -

I c e d i m e n s i o n s f o r v a n e s h a p e d a c c r e t e d i c e o n b a r s ,

t y p e s A a n d

B

( V a l i d

o n l y f o r i n - c l o u d i c i n g . D e n s i t y o f i c e - 5 0 0

[kg/m3] )

C r o s s s e c t io n a l s h a p e T y p e s A a n d

B

O b j e c t w i d t h I 1 0 1 3 0 1 3 0 0

I C I c e

I c e v a n e s d i m e n s i o n

m L D L D L D iL D

R1 ' ~ ,g/m ] 54 22 34 35 13 100 4 300

R2 0 ,9 78 28 54 40 23 100 8 300

R3 1,6 109 36 82 47 41 100 14 300

R4 2 ,8 150 46 120 56 67 104 24 300

R5 5 ,0 207 60 174 70 106 114 42 30 0

R6 8 ,9 282 79 247 88 165 129 76 300

R7 16,0 384 105 348 113 253 151 136 300

R8 28 ,0 514 137 478 146 372 181 217 317

R9 50 ,0 694 182 656 190 543 223 344 349

R10 to be u sed f o r ex t r em e ice acc r e t i ons b igge r t han R9

Now the p r i nc ip l e f o r t he r im e acc r e t i on m ode l i s c l ea rl y shown: B ecause

o f t he cons t an t i ce m ass i n I CRs , t he r im e d im ens ions a r e dec r eas ing a s

prof i le d imension is increasing, and up to ICR3 and ICR7 ice accret ion has

no t changed ob j ec t w id ths 100 m m and 300 r am . Th i s i s in f ine ag r eem e n t

with the ef fect observed in pract ise . The r ime dimensions wi l l vary sl ight ly

with the type of prof i le used, and this ef fect wi l l be control led by the cor -

rect use of equat ions, see A nne x A in ISO 121494.

M o d e l f o r r i m e a c c r e t i o n o n b i g o b j e c t s

Of cause p r o f i l e d im ens ion s cann o t be l im i t ed to 300 m m c r oss sec t i on .

When ob j ec t d im ens ion i nc r eases 300 r am , t he ob t a ined r im e vane

l eng th f o r 300 m m i s kep t cons t an t , and t hen on ly r im e m asses s t i l l

g r ow, bu t no t vane l eng ths and wid ths .

Th i s m ode l i s va l id up t o ob j ec t d im ens ions o f 5000 m m , and be yond t h i s

d im ens ion , r im e acc r e t i on m igh t be neg l ec t ed o r t he sam e r e su l t a s f o r 5000

mm might be used, i f i t seams reasonable for the st ructure in quest ion. For

ob j ec ts o f t ha t s i ze , r im e acc r e t i on wou ld no r m al ly be o f a lm os t no im por -

tance compared to al l o ther , normal act ions on the st ructure.

F igu r e 5 shows t he m ode l f o r r im e acc r e ti on , wher e on ly 2 d i f f e r en t t ypes

of object shape have been found necessary to in t roduce: f la t or c i rcular

= - W i n d d i r e c t i o n

[

, I ,150mm

,

1 5 0 m m

V > 3 0 0 r a m

r I

~ / i

r

F i g u r e

5 w

I c e a c c r e t i o n m o d e l f o r r i m e , b i g o b j e c t s

cross sect ions. Again the equat ions in Annex A in ISO 12494 for big ob-

j ec t s con t r o l t he d im ens ions t o be u sed .

A c c r e t e d r i m e o n m e m b e r s i n c l i n e d t o w i n d d i r e c t io n

In "real l i fe" st ructural members (prof i les etc .) cannot always be s

a plane, perpendicular on the ic ing wind di rect ion. I t must therefo

sible to operate wi th al l incl inat ions compared to wind di rect ion.

F igu r e 6 be low shows how th i s co r r ec t i on shou ld be done f o r m

and d im ens ions . The vane d im ens ions g iven o r ca l cu l a t ed i n ac

wi th t h i s s t anda r d m us t a lways be m easu r ed i n t he ho r i zon t a l p

in w indwar d d i r ec t i on o f t he i c i ng wind .

W i n d

d ir e c t i o n , . _ ~ \ L x sin c,

l c em a ss m P ' - ~ " ~ ~ ' .- ' - - 7

p e r un i t 1 ~ ~ ~

__L ( r ound ba r shown

p a n

F i g u r e

6 m

C a l c u l a t i o n s f o r i nc l i n ed m e m b e r s

W i n d a c t i o n s o n i ce d s t r u c t u r e s

An impor tant parameter for calculat ing wind act ions is drag coef f

(hereaf ter C-value) . The standard has given an easy understandabl

ple for f inding a c-value for any iced si tuat ion, but only for a sing

ber , e .g . a bar , a prof i le e tc . The values in the standard should be u

l e ss the u se r has m or e r e l i ab l e va lues f r om o the r sou r ces . By do in

research on these subjects in the future the values in the standard c

be improved and thus increasing rel iabi l i ty .

G l a z e a c c r e t i o n

The t ab l e 5 and 6 be low show C- va lues f o r I CGs on ba r s /p r o f i l

f o r g l aze on b ig ob j ec t s f o r I CG3 . I n t he s t anda r d s im i l a r t ab l e s

shown f o r b ig ob j ec t s and a l l I CGs .

T a b l e 5 m C i _ c o e f f i c i e n t s f o r g l a z e o n b a r s .

I C

T h i c k n e s s

[ m m ]

G1 10

G 2

2 0

G 3

3 0

G 4

4 0

G 5 5 0

G 6

0 ,50

0 ,68

0 ,86

1,04

1 ,22

1 ,40

C . c o e f f i c i e n t s f o r g l a z e o n b a r s

D~-ag coef f ic ients wi th out ice = C O

0,75 1 ,00 1 ,25 1 ,50 1 ,75

0 ,88 1 ,08 1 ,28 1 ,48 1 ,68

1,01 1,16 ,31 1,46 1,61

1 ,14 1 ,24 1 ,34 1 ,44 1 ,54

1 ,27 1 ,32 1 ,37 1 ,42 1 ,47

1 ,40 1 ,40 1 ,40 1 ,40 1 ,40

to be used for ext reme ice accret ions bigger than G5

I t can be seen that a l l you need to know beside ICs is the C-value

prof i le in quest ion wi thout ice, and this value can be found in the

l i terature for a ll wanted cross sect ions.

The pr inciple for g laze accret ions are that very smooth prof i le sha

C- va lues w i thou t i ce) becom e m or e r ough and ve r y r ough shapes

va lues w i thou t i ce) becom e m or e sm oo th wi th g l aze acc r e ti on . W

ject d im ension s are very big the ef fect of g laze accret ion is negl ig

6 8 1


5/8

T a b l e 6 - C i - c o e ff i c ie n t s for g l a z e , I C G 3 , big objec t s

IC

G 3

Ob iect C i coef f i c i en ts f or

glaze, b

wRith

Drag coe f f ic ien ts wi thou t

[m]: 0 ,50 0,75 1,00 1,25 1,50

1,04 1,14 1,24 1,34 1,44

1 .~ 3 0 , 9 6 1 , 0 8 1 , 2 0 1 , 3 3 1 , 4 5

2 1 0 0 , 8 4 1 , 0 0 1 , 1 5 1 , 3 1 1 , 4 6

310 0,73 0,92 1,10 1,29 1,47

>_+5,0 0,5 0 0,75 1,00 1,25 1,50

i g ob jec t s

ice = Co

1,75 2,00

1,54 1,64

1,57 1,69

1,62 1,77

1,66 1,85

1,75 2,00

R i m e a c c r e t io n

Alm ost the same princip le is used for r ime accretion. C-value s for profile

dimensions up to 300 mm are shown in table 7 below, and table 8 shows an

example for big objects and ICR5.

T a b l e 7 - - C i - coef f i c i en t s f or r im e on bars

I C I c e m a s s

m

[kg /m] 0 ,50

R1 0 ,5 0 ,62

R2 0 ,9 0 ,74

R3 1 ,6 0 ,87

R4 2 ,8 0 ,99

R5 5 ,0

1,11

R6 8,9 1 ,23

R7 16,0 1,36

R8 28,0 1,48

R9 50,0 1,60

C i

coef f i c i en t s f or r im e on

b a r s

D r a g

0,75

0,84

0,94

1,03

1,13

1,22

1,32

1,41

1,51

1,60

coef f ic ien t wi thou t ice

1 ,00

1,07

1,13

1,20

1,27

1,33

1,40

1,47

1,53

1,60

1,25 1,50

1,29 1,51

1,33 1,52

1,37 1,53

1,41 1,54

1,44 1,56

1,48 1,57

1,52 1,58

1,56 1,59

1,60 1,60

= Co

1,75 2,00

1,73 1,96

1,72 1,91

1,70 1,87

1,68 1,82

1,67 1,78

1,65 1,73

1,63 1,69

1,62 1,64

1,60 1,60

R1 to be used fo r ex t reme ice acc re t ions b igge r than R9

As for glaze, there is a table for each ICR in the standard, so use of the

standard does not nece ssarily mean a lot of calculating. Mo st of the f igures

you need for further calculating can just be taken from the tables. I t is a l-

lowed o f cause to interpolate between the values given, if you so wish, but

be aware o f the fact that improving those f igures does not mea n a mor e re-

l iable calculation as sucht

T a b l e 8 - - C i - coef f i c i en t s f or r im e , ICR5 , b ig ob jec t s

I C O b j e c t

wid t h

R5

[m]

_< 0, 3

0 ,5

1,0

1,5

2,0

2,5

3 , 0

4,0

>- 5,0

0 , 5 0

1,11

1,09

1,02

0 ,96

0 ,89

0,83

0 ,76

0,63

0 ,50

C i-cOefficient f or r im e , b ig ob jec t s

Drag coe f f ic ien t wi thou t ice = Co

0,75 1,00 1,25 1,50 1,75 2,00

1,22 1,33 1,44 1,56 1,67 1,78

1,20 1,32 1,44 1,55 1,67 1,79

1,15 1,28 1,42 1,55 1,68 1,81

1,10 1,25 1,39 1,54 1,69 1,83

1,05 1,21 1,37 1,54 1,70 1,86

1,00 1,18 1,35 1,53 1,71 1,88

0,95 1,14 1,33 1,52 1,71 1,91

0,85 1,07 1,29 1,51 1,73 1,95

0,75 1,00 1,25 1,50 1,75 2,00

W i n d a n g l e i n c i d e n c e

As for ice accretion i tself you also need to be able to f ind wind action on

e lemen ts s lop ing to the wind d i rec t ion . T here fo re fo l lowing a l lowance

shown in f igure 7 for calculating forces on inclined mem bers is used.

By us ing th e s imple equ a t ions f rom f igu re 7 i t is now poss ib le to ca lcu -

la te any re su l t ing fo rce f rom ice mass and wind ac t ion on any no rmal ,

s ing le ba r o r p ro f i le o r b ig m ass ive ob jec t . I t i s a l so poss ib le to

p r inc ip le fo r s ing le ba rs even i f seve ra l s ing le ba rs fo rm the s t ru

tha t case the to ta l s t ruc tu re load can be found as the sum o f a l l

bar 's load, but if the structure is a real la t t ice s tructure this meth

much too conse rva t ive .

, -- W i n d d i re c t i o n

F w ( 9 0 )

y

- ' ~ = 9 0

/

F,~_ 90 o sin30

~ 0 ~ (0 )= Fw (90

F i g u r e

7 -

F o r c e s o n a n i nc l i ne d m e m b e r

Act ion on la t t i ce s t ruct ures

Ice mass on a la t t ice s tructure may with good approxim ation be fo

the total sum of ice masses of a ll s ingle members, but mo re precis

should allo wanc e be given for overlaps o f ice in joints of profiles ,

profile lengths than theoretical sh ould be used.

Wind load however, should be found in principle in the same way

norm ally use for la t t ice s tructure without ice accretion. There is se

methods fo r tha t, and some Na t iona l Codes o f P rac t ice recommen

tain model to be used. U nless there is reliable information about th

direction for the ice accretion si tuation and the highest wind speed

rection is the same for the two, the following principle must be use

For r ime accretions the ice vane should for not horizontal me

placed in a plane, perpendicular to the direction of the dimen

wind.

Because o f the iced mem bers some pa ramete rs in the ca lcu la t ion m

mus t be changed :

Wi nd area exposed shall be increased in accordance with the

sions found for the iced mem bers in the standard.

C-valu e shall be adjusted in accordance with the C-values fo

the iced mem bers in the standard.

I f the mode l inc ludes use o f"s t ruc tu ra l pan e ls" and so l id i ty ra t

pa ramete rs a l so mus t be changed :

. Solidity ratio shall be increased with the ratio: total iced expo

total un-iced expo sed area.

. Increased solidity ratio will decrease wind load on all leewar

panels o f the structure.

- If nothing else is specif ied, i t is a l lowed for ICR s (but not fo

to use one class lower ice accretion on all leeward placed pan

structure.

I f eve ry a spec t shou ld be taken ca re o f in the op t ima l way a ra t

vanced co mpu te r p rogram fo r ca lcu la t ing ice and wind ac t ions

s t ruc tu res i s necessa ry . W e have fo r some yea rs been us ing suc

grams wi th success .

Co m b inat ion o f i ce loads and wind act ions

An extrem ely important, but often forgotten part of calculating is

t ion of how to com bine the different types of actions on the struct

t is t ically of cause i t is too con servative to combine to different typ

just b y adding their full effect .

6 8 2


6/8

T he s t a nda rd ha s g i ve n a r a t he r p re c i se a nsw e r t o tha t que s t ion . T he t a b l e 9

be l ow show s how t o c om bi ne w i nd a nd i c e wi t h e a ch of the 2 a c t ions a s t he

ma j or one . T h e t a b l e 10 show s t he f a c t o r fo r r e duc i ng 50 ye a r s wi nd pre s -

sure , whe n t h i s i s c ombi ne d w i t h a he a vy i c e l oa d (3 ye a r s ) a t t he s a me

t ime.

I f some na t i ona l C ode s o f P ra c t i s e g i ve ru l e s fo r t he se c ombi na t i ons , o f

c a use t he y ove r ru l e t h i s s t a nda rd . B ut i f you do no t f i nd a ny t h i ng a bout

c o m b i n i n g t h o s e t w o t y p e s o f l o ad s t h e m e t h o d b e l o w i s r e c o m m e n d e d .

T a b l e

9 m

P r i n c i p l e s f o r c o m b i n a t i o n o f w i n d a c t i o n s a n d i c e l o a d s

C o m b i n a - W i n d a c t io n I c e l o a d s

( M a j o r l o a d) W i n d p r e s s u r e T ( y e a r s ) I c e m a s s T ( y e a r s )

I (wi n d) k" q s0 50

~ ) i c e "

m 3

II ( I c e ) ~w" k - q s0 3 m 50

~ i c e a nd qbw a re use d t o c ha nge a c t ions a nd l oa d f rom 50 ye a r s t o 3 ye a r s

oc c ur re nc e . T he fa c t o r

( D i c e

i s use d t o r e duc e 50 ye a r s i c e t o 3 ye a r s i c e , a nd

f rom t o da y ' s e xpe r i e nc e a va l ue be t we e n 0 ,3 a nd 0 ,5 c ou l d be r e c om-

m e n d e d .

T h e f a c t o r ~w s h o u l d b e t a k e n f r o m n a t i o n a l c o d e s f o r t h e p o s s i b l e d e -

c r e a s e o f w i n d a c t i o n f o r s i m u l t a n e o u s v a r i a b l e a c t io n s . T h e f a c t o r k

s h o u l d b e u s e d t o d e c r e a s e w i n d p r e s s u r e b e c a u s e o f r e d u c e d p r o b a b i l i ty

f o r s i m u l t a n e o u s 5 0 y e a r s w i n d a c t i o n c o m b i n e d w i t h h e a v y i c i n g c o n -

di t ion.

Fa c t or k ha s va l ue s a s sho wn i n t a b l e 10 .

T a b l e 1 0 m F a c t o r f o r r e d u c t i o n o f w i n d p r e s s u r e

I C G k I C R k

G 1 0,40 R 1 0,40

G 2 0,45 R 2 0,45

G 3 0 ,50 R 3 0 ,50

G 4 0,55 R 4 0,55

G 5 0,60 R 5 0,60

6: . . i: . . : . . : R

R

. , . : : : ; i l j ~ . : i~:i~.. R

.i~: .:i ~:~.:i ~:.: i; .. R

0 ,70

7 0,80

8 0 , 9 0

9

1 , 0 0

I t ca n be s e e n t ha t i t i s a s sume d mos t un l i ke l y t ha t you w i l l ge t ma x i mu m

wi nd sp e e d t oge t he r wi th g l a z e a nd l owe r IC R s a c c re t ions . How e ve r , t he

h i ghe r IC R t he mo re l i ke l y i s t he s i t ua ti on wh e re you a t t he s a me t i me c a n

ge t ma xi mum wi nd spe e d a nd muc h i c e a c c re t i on . T h i s i s pa r t l y be c a use

t ha t t ype o f i c e a c c re ti on c a n re ma i n i n the s t ruc t u re fo r ve ry l ong t i me b e -

fore i t me l ts o r i n o t he r wa y d i s a ppe a r s . In som e a re a s t h is i c e a c c re t i on c a n

s t a y fo r s e ve ra l mont hs .

C o n c l u d i n g r e m a r k s

T he n e w IS O 12494 ha s a l r e a dy p rove d i ts va l ue a s e n he l pfu l t oo l fo r the

de s i gn i ng e n g i ne e r s de a l i ng w i t h t he d i f fi c u l t sub j e c t: Ac t i ons f rom i c e

load on s t ruc tures .

T o m a ke t he fu l l be ne f i t s o f suc h a "de s i gn t oo l " a c l ose c o-ope ra t i on be -

t we e n me t e oro l og i s t s a nd e ng i ne e r s a re ne c e s sa ry . At be s t t he e ng i ne e r s

shoul d t e l l t he me t e oro l og i s t s whi c h i n forma t i on o r da t a t he y ne e d fo r t he i r

c a l c u l a ti ons a nd t he m e t e oro l og i s t s shou l d t ry t o fi nd t he m by i nc l ud i ng t he

subjec t s into the i r research.

T hi s c o-ope ra t i on ha s fo r some ye a r s wi t h suc c e s s t a ke n p l a c e be t

N o r w e g i a n m e t e o r o l o g is t s a n d u s a s d e s i g n e r s o f b ig t e l e c o m m u n i

m a s t s f o r t h e g r e at e s t m a s t o w n e r i n N o r w a y .

T o i l l us tr a t e how some o f t he de s i gn wo rk c a n t a ke p l a c e be l ow a

p roc e du re fo r a c a l c u l at i on o f i c e loa d a nd wi nd a c t ion on t he i c e

s h o w n f o r a n a p pr o x . 2 0 0 m h i g h g u y e d m a s t i n th e m i d d l e o f N o r

I n f o r m a t i o n f r o m N o r w e g i a n M e t e o r o l o g i s t

S p e c i f i c at i o n o f i c e a c c r e t i o n i n a c c o r d a n c e w i t h I S O 1 2 4 9 4

L e v e l i n

m a s t

H e i g h t =

1 0 0 m

H e i g h t

=

2 0 0 m

5 0 - y e a r s

i c e ( I C )

I C R 9

1,65 x

I C R 9

5 0 - y e a r s

i c e ( k g / m )

50

92

1 - y e a r i c e F a c t o

( %

o f S 0 - y e a r s ) ( s e e f

50 e '~

50 e '

A s c a n b e s e e n , th e s p e c i f i c a ti o n i s e x t r e m e l y s i m p l e b u t c o n t a i

more i n forma t i on t ha t a t f i r s t r e a l i s e d .

In l e ve l 100 m IC 9 i s e s t i ma t e d a s a p rop e r va l ue .

I n l e v e l 2 0 0 m I C R 1 0 i s f o u n d n e c e s s a r y a n d t h e v a l u e e s t

b e 6 5 % m o r e t h a n I C 9 .

1 -ye a r i c e a c c re t i on is e s t i ma t e d t o be a pprox . 50% of 50 y

v a l u e ( m o r e t h a n t h e r e c o m m e n d e d 3 0 % i n t h e s t a n d a r d ) .

He i g h t f a c t o r i s e s t i ma t e d t o e ' 6H i ns t e a d o f t he r e c o mm

va l ue o f e ' il l . T h i s va l ue i s i mpo r t a n t b e c a use t he e qu a t i o

t o " s m o o t h e n o u t " t h e 2 s i n g l e l o a d v a l u e s t o a c o n t in u o u s

wi t h a n e ve n va r i a t i on wi t h he i gh t .

T o s h o w h o w t h e s e m a t t e rs l o o k l i k e in p r a c ti c e s o m e p h o t o s o f

a n d h e a v y r i m e a c c r e t io n s o n m a s t s a n d g u y r o p e s a r e e n c l o s e d

ra t e pa pe r .

A n n e x A sh ows f l ow c ha r t fo r a typ i c a l c a l c u l a ti on p roc e dure .

A n n e x B sho ws t he t a b l e o f c on t e n ts fo r ISO 12494 .

6 8 3


7/8

A n n e x A

F l o w c h a r t o f c a l c u l a t i o n p r o c e d u r e r e f . I S O 1 2 49 4

I I

Find ICGx or ICRx ~ . ~i MethodA: Collectingexistingexperience

} ~ -] MethodB: Icing modellingby meteorologists

Method C: Directmeasurements or many years

/ U s e / / U s e /

table 3 table 4

( ICGx )

_ _ _ > - - -

I Profile or

big object

dimen-

Q iCRx )

i2

rofile di-

mension

Big object

dimension

/ U efi, i / / Use figure 4 / / U s e figure 5 /

gure 3 and table 5 - 7 and table 8 - 9

~ - c a l c u l a t e d - ~

I.._

I ce we ights D r . m and i ced d im ens ions

are

Find drag coeff ic ients

o

Use ta- Use table

ble 10 16 for

for bars bars and

and 11- table 17-

15 for 25 for big

big ob- objects

Adjust drag coefficient on sloping I

elements for angle of incidence

~aCo mbin e wind action " ~

nd ice load for dimen- i

s i o n i n g s t r u c t u r e

/

Calculate

" - ~ wind action

and ice load


8/8

A n n e x B

T a b l e o f C o n t e n t s i n I S O 1 2 4 9 4

1.1

1 .2

3 .1

3 . 2

3 . 3

3 . 4

3 . 5

3 . 6

3 . 7

3 . 8

3 . 9

5 .1

5 . 2

5 . 3

5 . 4

6 .1

6 . 2

6 . 2 . 1

6 . 2 . 2

6 . 2 . 3

6 . 2 . 4

6 . 3

6 . 4

7 .1

7 . 2

7 . 3

7 . 4

7 . 4 . 1

7 . 4 . 2

7 . 5

7 . 5 . 1

7 . 5 . 2

7 . 6

7 . 6 . 1

7 . 6 . 2

7 . 6 . 3

S c o p e . . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. .

G e n e r a l . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . .

A p p l i c a t i o n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

N o r m a t i v e r e f e r e n c e s . . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . ..

D e f i n i t i o n s . . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . .

A c c r e t i o n . . . . . . . .. . . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . ..

D r a g c o e f f i c i e n t . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

G l a z e . . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . .

I c e a c t i o n . . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. .

I c e c l a s s ( I C ) . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . .

I n - c l o u d i c i n g . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . .

P r e c i p i t a t i o n i c i n g . . . . . . . .. . . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . .

R e t u r n p e r i o d . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

R i m e . . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .

S y m b o l s

E f f e c t s o f i c i n g

S t a t i c i c e l o a d s . . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. .

W i n d a c t i o n o n i c e d s t r u c t u r e s . . . . .. . . .. . . .. . . .. . .. . . .. . . .. . . .

D y n a m i c e f f e c t s . . . . . . . . .. . . . . . . . .. . . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . .

D a m a g e c a u s e d b y f a l l i n g i c e . . .. . . .. . . .. . .. . . .. . . .. . . .. . .. . . .

F u n d a m e n t a l s o f a t m o s p h e r i c i c i ng

G e n e r a l . . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . .

I c i n g t y p e s . . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . .

G l a z e

W e t s n o w

R i m e

O t h e r t y p e s o f i c e

T o p o g r a p h i c i n f l u e n c e s . . .. . . .. . . .. . .. . . .. . . .. . .. . . .. . . .. . .. . . .. . .

V a r i a t i o n w i t h h e i g h t a b o v e t e r r a i n . . .. . .. . .. . . .. . .. . .. . .. . .

I c i n g o n s t r u c t u r e s

G e n e r a l . . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . .

I c e c l a s s e s . . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .

D e f i n i t i o n o f i c e c l a s s , I C . . . . . . . . . .. . . . . . . . .. . . . . . . .. . . . . . . .. . . .

G l a z e . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .

G e n e r a l . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . .

G l a z e o n l a t t i c e s t r u c t u r e s . . . . . . .. . . . . . . . .. . . . . . . .. . . . . . . .. . . . . . . .. .

R i m e . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . ..

G e n e r a l . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . .

R i m e o n s i n g l e m e m b e r s .. . . . . . . .. . . . . . . . .. . . . . . . .. . . . . . . .. . . . . . . .. .

R i m e o n l a t t i c e s t r u c t u r e s . . . . . . .. . . . . . .. . . . . . . .. . . . . . .. . . . . . . .. . .

G e n e r a l . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .

T h e d i r e c t i o n o f i c e v a n e s o n t h e s t r u c t u r e . . .. . .. . .. . .. . ..

I c i n g o n m e m b e r s i n c l i n e d t o t h e w i n d d i r e c t i o n .. .. .. .

W i n d a c t i o n s o n i c e d s t r u c t u r e s . . . .. . .. . .. . ..

8 .1

8 . 2

8 . 2 . 1

8 . 2 . 2

8 . 3

8 . 4

G e n e r a l . . . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . .

S i n g l e m e m b e r s . . . . . . . . . .. . . . . . . . .. . . . . . . .. . . . . . . .. . . .

D r a g c o e f f i c i e n t s f o r g l a z e . . . .. . .. . . .. . .. . . .. . .. .

D r a g c o e f f i c i e n t s f o r r i m e . . . .. . .. . . .. . .. . . .. . .. . .

A n g l e o f i n c i d e n c e .. . . . . . .. . . . . . . .. . . . . . .. . . . . . . .. . . .

L a t t i c e s t r u c t u r e s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

C o m b i n a t i o n o f ic e l o a ds a n d w i n d a c t i o

9 .1

9 . 2

10.

G e n e r a l . . . . . . . . .. . . . . . . . .. . . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . . .

C o m b i n e d l o a d s . . . . . . . .. . . . . . . .. . . . . . . .. . . . . . . .. . . . . . .

U n b a l a n c e d i c e l o a d o n g u y s

11 .

F a l l i n g i c e c o n s i d e r a t i o n s

A n n e x A ( i n f o r m a t i v e )

E q u a t i o n s u s e d in t h e I n t e r n a t i o n a l S t a n d a r d

A n n e x B ( i n f o r m a t i v e )

S t a n d a r d M e a s u r e m e n t s f o r I ce A c t i o n s

A n n e x C ( i n f o r m a t i v e )

T h e o r e t i c a l m o d e l l i n g o f ic i n g

A n n e x D ( i n f o r m a t i v e )

C l i m a t i c e s t i m a t i o n o f ic e c l a ss e s b a s e d o n w e a t h e r d a t a

A n n e x E ( i n f o r m a t i v e )

S h o r t i n t r o d u c t i o n a b o u t u s i n g t h i s s t a n d a r d . . . . . . . . . . . . . . . . . . . .

Documents

How to Use ISO 12494 Atmosperic Icing for Structures