iso 12494

Proceedings of the Eleventh (2001) International Offshore and Polar Engineering Conference Stavanger, Norway, June 17-22, 2001 Copyright 2001 by Tile blternational Society of Offshore and Polar Engineers ISBN 1-880653-51-6 (Set); ISBN 1-880653-52-4 (Vol. I); ISSN 1098-6189 (SeO

ISO 12494 "Atmospheric Icing of Structures" and How to Use It

Mogens H. Foder RAMBOLL

Copenhagen, Denmark

Abstract Now, after many years the new ISO 12494: "Atmospheric Icing of Struc- tures" has been finished and is ready for use. As it is the first standard, where all issues about ice and dimensioning for ice have been collected in the same standard, it differs in its substance from "normal" constructional standards (codes of practice) for actions on structures. Therefore, it may be necessary to introduce the use of it for meteorologists, designers and other interested engineers as well as other users.

This paper explains how the structural designer or engineer should use the ISO 12494 and point out the most important facilities for this use. The types of actions specified are ice mass as well as wind action from wind load on the iced structure. The standard has been prepared in such a way that it invites to use small "calculation tools" which very much facilitates the use of information and improves the understanding of the whole structure of the standard.

The standard could be used also even if a National Standard of icing already exists, because more or less of the content could be adopted by the National Standard without any problems or contradictions. The ISO 12494 could e.g., be used for preparing icing maps for countries or part of countries, as National Bodies often want this.

Keywords: Icing; structural design; ice actions; calculation of loads; combination of loads.

Introduction As it is the first time a standard include all necessary information for dimensioning structures for both glaze and rime, a guidance for its use may be appropriate, and this paper might be a start.

The definitions of Ice Classes for both glaze and rime as well as the principle for using the standard including examples of the most needed tools is presented and commented. The steps through the whole dimensioning process are shown and how this process needs connection to information of icing data, given in the standard. For practical use of cause it is necessary to have the ISO 12494 itself as only few examples from its content are shown in this paper, but it is possible to use the standard in a very constructive way when designing for atmospheric ice.

Brief description of ISO 12494. In 1986 a working group ISO/TC 98/SC 3/WG 6 with representation from all countries which showed interest in participating was established with the purpose to work out an international standard for ice actions on structures: ISO 12494 "Atmospheric Icing of Structures". As said in the title, the aim was very broad and should include all necessary basic information about icing itself, because such information was found needed to make the whole subject understandable for the user,

The standard is therefore very different in content compared to same type of standards for wind actions, snow actions etc. For wind and snow loads we know sufficient to be able to work out very' precise and de- tailed codes of practise for actions from those types of loads, but this is not the fact for ice load. It is our intention and hope that the content during coming years should be more like the other standards for actions on structures, but this might need a rather long period of gaining more experience and information of details in icing.

A lot of information in the standard are guidance, and that is to underline the uncertainties connected to the specific figures presented. Never the mind, we need to have those data for being able to do the necessary calculation for structures. We hope that (all) meteorologists in the future will help improving data reliability by concentrating for just some of their research on those matters which in particular are wanted updated or supple- mented, in short: the content of most figures and tables. Because of that we have proposed e.g. a standard for measurements of ice actions (Annex B in the ISO) so as much as possible of new research can be made useful for future revisions of the ISO 12494.

Basic nominal ice load information. In chapter 6 in the standard all basic information about ice is gathered. The "not so experienced user" will there find any information needed about the subject itself: ice. He can understand the difference between glaze and rime, precipitation and in-cloud icing, hard rime and soft rime etc. By reading here he also can understand, why it is important that he know both which type of ice but also which amount of ice he has to foresee on the structure in question.

Paper No: 01-MF-04

678

First Author's Name: Mogens n. t'oder Page: 1 of 8

Table 1 is an example from the standard, which gathers some basic information on the types of ice accretion. Figure 1 is a guideline to predict the likelihood for the type of ice accreted on the site in question.

Table 1" Meteorological parameters, controlling atmospheric ice accretion

Type of Air tempe- ice rature [ ~ C]

[ Precipitation icing

Wind Droplet Water speed size content [m/s] in air

Typical storm duration

Glaze) - 10 < ta

~ t

t t t

Figure 3 m Ice accretion model for glaze

Now, by using ICGx arid the model shown in figure 2 it is possible to calculate masses and dimensions needed, and in table 2 glaze masses are given for the cylinder dimensions 10, 30, 100 and 300 ram. Accreted glaze can easily be calculated for all other object dimensions. The 30 mm diameter has been included because it is the recommended diameter for standard measurements, see later. The shown model for glaze accretion can be used for all object dimensions, but for practical use the effect on structure dimensions is insignificant, when object dimension is around or above 5000 mm in cross section, so object size has been limited to < 5000 mm in cross section.

lee Classes for Rime Rime in this standard has to be understood as "hard rime". In the same way as for glaze, a model for accreted rime has defined the amount of rime in different ICRs. However, the model itself has been constructed quite differ- ently compared to the model for glaze because the nature of forming those types is very distinct. For rime accretions the ice mass has been defined constant in every ICRx and ice dimensions vary with both object/profile type and dimension. The table 3 below shows the definitions of ICRx, which have been numbered from ICR1 to ICR9, and as for glaze: ICR10 may be used for extreme rime accretions exceeding the defined classes.

Table 3 ~ Ice Classes for rime (IC R)

Ice Ice mass

Classes m

IC

R1

R2

K~me ommeter lmml lor object d iameter = 30 mm

Density of rime l kg/m3] - '

R3

R4

R5

R6

R7

R8

R9

R I0

[kg/m] ;300 0,5 55

0,9 69

1,6 88

2,8 113

5,0 149

8,9 197

16,0 262

28,0 346

50,0 462

500 47

56

71

90

l l7

154

204

269

358

700

43

50

62

77

100

131

173

228

303

900

40

47

56

70

89

116

153

201 1268

to be used :or extreme ice accretions bigger than R9

In addition to profile cross section, the density of rime is a variable in the model for rime. This is necessary because the density in practice may vary within a broad spectre, and this variation results in rather different results. The effect can be seen by comparing rime diameters in table 3 for different values of rime density.

The rime accretion model in figure 3 is valid only for object/profile dimensions up to 300 mm. For bigger cross sections the model changes, see later. The model shows the chosen principle of accretion: Rime is building up in windward direction and in the horizontal plane. Until an accreted vane length of W or lAW (see different types of profile), the accretion is occur- ring without any increase of object dimension perpendicular to wind direction. Beyond that point the accretion is growing also perpendicular to the

wind direction, but at a slower rate than in the windward direction. In this way it is now possible to calculate all rime vane dimensions by means of rather simple equations, see Annex A in ISO 12494.

Wind direction

TypeA ,j/ t ,.

i< L

Type B \ / t

I 8t ._ max W / - -- . I -

5 C _~

Type C \~, t

t [

based on this density and adjustment for correct density has to be done, see Annex A in ISO 12494.

Table 4- Ice dimensions for vane shaped accreted ice on bars, types A and B (Valid only for in-cloud icing. Density of ice - 500 [kg/m3] )

Cross sectional shape Types A and B

Object width I10 130 i 1300 IC Ice Ice vanes dimension

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 42 300 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 used for extreme ice accretions bigger than R9

Now the principle for the rime accretion model is clearly shown: Because of the constant ice mass in ICRs, the rime dimensions are decreasing as profile dimension is increasing, and up to ICR3 and ICR7 ice accretion has not changed object widths 100 mm and 300 ram. This is in fine agreement with the effect observed in practise. The rime dimensions will vary slightly with the type of profile used, and this effect will be controlled by the correct use of equations, see Annex A in ISO 121494.

Model for rime accretion on big objects Of cause profile dimensions cannot be limited to 300 mm cross section. When object dimension increases 300 ram, the obtained rime vane length for 300 mm is kept constant, and then only rime masses still grow, but not vane lengths and widths.

This model is valid up to object dimensions of 5000 mm, and beyond this dimension, rime accretion might be neglected or the same result as for 5000 mm might be used, if it seams reasonable for the structure in question. For objects of that size, rime accretion would normally be of almost no impor- tance compared to all other, normal actions on the structure.

Figure 5 shows the model for rime accretion, where only 2 different types of object shape have been found necessary to introduce: flat or circular

='- Wind d i rec t ion

[

,I,150mm ,! 150mm

V > 300ram

r I

~,/ i

r

Figure 5 w Ice accretion model for rime, big objects

cross sections. Again the equations in Annex A in ISO 12494 for big objects control the dimensions to be used.

Accreted rime on members inclined to wind direction In "real life" structural members (profiles etc.) cannot always be situated in a plane, perpendicular on the icing wind direction. It must therefore be possible to operate with all inclinations compared to wind direction.

Figure 6 below shows how this correction should be done for masses and dimensions. The vane dimensions given or calculated in accordance with this standard must always be measured in the horizontal plane and in windward direction of the icing wind.

Wind d i rect ion , . _ ~ \ L x sin c,

l cemass m P ' - ~ "~~' . - ' - -7

per unit 1 ~ ~ ~

__L (round bar shown)

p ane

Figure 6 m Calculations for inclined members

Wind actions on iced structures An important parameter for calculating wind actions is drag coefficient (hereafter C-value). The standard has given an easy understandable principle for finding a c-value for any iced situation, but only for a single member, e.g. a bar, a profile etc. The values in the standard should be used unless the user has more reliable values from other sources. By doing more research on these subjects in the future the values in the standard could still be improved and thus increasing reliability.

Glaze accretion The table 5 and 6 below show C-values for ICGs on bars/profiles and for glaze on big objects for ICG3. In the standard similar tables are shown for big objects and all ICGs.

Table 5 m Ci_coefficients for glaze on bars .

IC Thickness [mm]

G1 10 G2 20

G3 30 G4 40

G5 50

G6

0,50

0,68 0,86 1,04 1,22

1,40

C. coefficients for glaze on bars D~-ag coefficients without ice .= C O

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

0,88 1,08 1,28 1,48 1,68 1,88 1,01 1,16 ,31 1,46 1,61 1,76 1,14 1,24 1,34 1,44 1,54 1,64

1,27 1,32 1,37 1,42 1,47 1,52 1,40 1,40 1,40 1,40 1,40 1,40

to be used for extreme ice accretions bigger than G5

It can be seen that all you need to know beside ICs is the C-value for the profile in question without ice, and this value can be found in the technical literature for all wanted cross sections.

The principle for glaze accretions are that very smooth profile shapes (low C-values without ice) become more rough and very rough shapes (high C- values without ice) become more smooth with glaze accretion. When object dimensions are very big the effect of glaze accretion is negligible.

Paper No: 01-MF-04

681

First Author's Name" Mogens H. Foder Page: 4 of 8

Table 6 - Ci-coefficients for glaze, ICG3, big objects

IC

G3

Obiect Ci coefficients for glaze, b wRith Drag coefficients without [m]: 0,50 0,75 1,00 1,25 1,50

1,04 1,14 1,24 1,34 1,44 1.~ '3 0 ,961 ,081 ,201 ,331 ,45

210 0 ,841 ,001 ,151 ,311 ,46 310 0,73 0,92 1,10 1,29 1,47 >_+5,0 0,50 0,75 1,00 1,25 1,50

ig objects 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

Rime accretion Almost the same principle is used for rime accretion. C-values for profile dimensions up to 300 mm are shown in table 7 below, and table 8 shows an example for big objects and ICR5.

Table 7 - - C i -coefficients for rime on bars

IC Ice mass 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

Ci coefficients for rime on bars Drag

0,75

0,84 0,94 1,03 1,13 1,22 1,32 1,41 1,51 1,60

coefficient without 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 for extreme ice accretions bigger than R9

As for glaze, there is a table for each ICR in the standard, so use of the standard does not necessarily mean a lot of calculating. Most of the figures you need for further calculating can just be taken from the tables. It is allowed of cause to interpolate between the values given, if you so wish, but be aware of the fact that improving those figures does not mean a more reliable calculation as sucht

Table 8 - - Ci -coefficients for rime, ICR5, big objects

IC Object width

R5 [m] _< 0,3 0,5 1,0 1,5 2,0 2,5 3,0 4,0

>- 5,0

0,50 1,11 1,09 1,02 0,96 0,89 0,83 0,76 0,63 0,50

C i-cOefficient for rime, big objects Drag coefficient without 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

Wind angle incidence As for ice accretion itself you also need to be able to find wind action on elements sloping to the wind direction. Therefore following allowance shown in figure 7 for calculating forces on inclined members is used.

By using the simple equations from figure 7 it is now possible to calculate any resulting force from ice mass and wind action on any normal,

single bar or profile or big massive object. It is also possible to use the principle for single bars even if several single bars form the structure. In that case the total structure load can be found as the sum of all single bar's load, but if the structure is a real lattice structure this method is much too conservative.

,-- Wind direction

Fw (90 ) y

- '~ = 90

/ F,~_ (90 o ) sin 30 ~ 0 ~ (0)= Fw (90 ) sin20

Figure 7- Forces on an inclined member

Action on lattice structures Ice mass on a lattice structure may with good approximation be found as the total sum of ice masses of all single members, but more precisely should allowance be given for overlaps of ice in joints of profiles, or shorter profile lengths than theoretical should be used.

Wind load however, should be found in principle in the same way, you normally use for lattice structure without ice accretion. There is several methods for that, and some National Codes of Practice recommend a cer- tain model to be used. Unless there is reliable information about the wind direction for the ice accretion situation and the highest wind speed and direction is the same for the two, the following principle must be used: For rime accretions the ice vane should for not horizontal members be

placed in a plane, perpendicular to the direction of the dimensioning wind.

Because of the iced members some parameters in the calculation model must be changed:

Wind area exposed shall be increased in accordance with the dimensions found for the iced members in the standard.

C-value shall be adjusted in accordance with the C-values found for the iced members in the standard.

If the model includes use of"structural panels" and solidity ratio these parameters also must be changed:

. Solidity ratio shall be increased with the ratio: total iced exposed area/ total un-iced exposed area.

. Increased solidity ratio will decrease wind load on all leeward placed panels of the structure.

- If nothing else is specified, it is allowed for ICRs (but not for ICGst) to use one class lower ice accretion on all leeward placed panels in the structure.

If every aspect should be taken care of in the optimal way a rather ad- vanced computer program for calculating ice and wind actions on lattice structures is necessary. We have for some years been using such pro- grams with success.

Combination of ice loads and wind actions An extremely important, but often forgotten part of calculating is the question of how to combine the different types of actions on the structure. Sta- tistically of cause it is too conservative to combine to different types of load just by adding their full effect.

Paper No: 01-MF-04

682

First Author's Name: Mogens H. Foder Page: 5 of 8

The standard has given a rather precise answer to that question. The table 9 below shows how to combine wind and ice with each of the 2 actions as the major one. The table 10 shows the factor for reducing 50 years wind pressure, when this is combined with a heavy ice load (3 years) at the same time.

If some national Codes of Practise give rules for these combinations, of cause they overrule this standard. But if you do not find anything about combining those two types of loads the method below is recommended.

Table 9 m Principles for combinat ion of wind actions and ice loads

Combina- Wind action Ice loads (Major load) Wind pressure T (years) Ice mass T (years)

I (wind) k" q s0 50 ~)ice" m 3 II (Ice) ~w" k - q s0 3 m 50

~ice and qbw are used to change actions and load from 50 years to 3 years occurrence. The factor (Dice is used to reduce 50 years ice to 3 years ice, and from to day's experience a value between 0,3 and 0,5 could be recommended.

The factor ~w should be taken from national codes for the possible decrease of wind action for simultaneous variable actions. The factor k should be used to decrease wind pressure because of reduced probability for simultaneous 50 years wind action combined with heavy icing con- dition.

Factor k has values as shown in table 10.

Table 10 m Factor for reduct ion of wind pressure

ICG k ICR 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 ~:.: :i; .. R

0,70

7 0,80

8 0,90

9 1,00

It can be seen that it is assumed most unlikely that you will get maximum wind speed together with glaze and lower ICRs accretions. However, the higher ICR the more likely is the situation where you at the same time can get maximum wind speed and much ice accretion. This is partly because that type of ice accretion can remain in the structure for very long time be- fore it melts or in other way disappears. In some areas this ice accretion can stay for several months.

Concluding remarks The new ISO 12494 has already proved its value as en helpful tool for the designing engineers dealing with the difficult subject: Actions from ice load on structures.

To make the full benefits of such a "design tool" a close co-operation between meteorologists and engineers are necessary. At best the engineers should tell the meteorologists which information or data they need for their calculations and the meteorologists should try to find them by including the subjects into their research.

This co-operation has for some years with success taken place between the Norwegian meteorologists and us as designers of big telecommunication masts for the greatest mast owner in Norway.

To illustrate how some of the design work can take place below a normal procedure for a calculation of ice load and wind action on the ice load is shown for an approx. 200 m high guyed mast in the middle of Norway"

In format ion from Norwegian Meteoro log is t Specification of iceaccretion in accordance with ISO 12494 Level in mast Height = 100m Height = 200m

50-years ice (IC) ICR9

1,65 x ICR9

50-years ice (kg/m)

50

92

1-year ice Factor K h - (% of S0-years) (see f igure2) 50 e '~H

50 e 'H

As can be seen, the specification is extremely simple but contains much more information that at first realised. In level 100 m IC9 is estimated as a proper value. In level 200 m ICR10 is found necessary and the value estimated to

be 65% more than IC9. 1-year ice accretion is estimated to be approx. 50% of 50 years

value (more than the recommended 30% in the standard!). Height factor is estimated to e'6H instead of the recommended

value of e 'ill . This value is important because the equation is used to "smoothen out" the 2 single load values to a continuous load with an even variation with height.

To show how these matters look like in practice some photos of light and heavy rime accretions on masts and guy ropes are enclosed as sepa- rate paper.

Annex A shows flowchart for a typical calculation procedure.

Annex B shows the table of contents for ISO 12494.

Paper No: 01-MF-04

683

First Author's Name: Mogens H. Foder Page: 6 of 8

Annex A

F lowchar t of ca lcu la t ion procedure ref. ISO 12494"

I I Find ICGx or ICRx ~. ~i Method A: Collecting existing experience

} ~ -] Method B: Icing modelling by meteorologists

Method C: Direct measurements for many years .

/Use / /Use / table 3 table 4

i ( ICGx )

___>--- I Profile or

big object dimen-

Q iCRx )

i2----- Profile dimension Big object dimension / U sefi, i / / Use figure 4 / /Use figure 5 /

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

~- calculated-~ (I.._ Ice weights Dr. m and iced dimensions are

Find drag coefficients

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

~aCombine wind action "~ nd ice load for dimen- i

sioning structure /

Calculate " -~ wind action

and ice load

Paper No: 01-MF-04

684

First Author's Name: Mogens H. Foder 5;i':

Page: 7 of 8

Annex B

Table of Contents in ISO 12494

.

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

Scope .........................................................................

General ....................................................................

Appl icat ion ............................................................. Normat ive references .................................................

Def in i t ions .................................................................

Accret ion ................................................................

Drag coeff ic ient ......................................................

Glaze .......................................................................

Ice action .................................................................

Ice class (IC) ...........................................................

In-c loud icing ..........................................................

Precipitat ion icing ...................................................

Return per iod ..........................................................

R ime ........................................................................

Symbols

Effects o f icing

Static ice loads ........................................................

Wind action on iced structures ................................

Dynamic effects ......................................................

Damage caused by fal l ing ice ................................. Fundamenta ls o f atmospher ic icing

General ....................................................................

Icing types ...............................................................

Glaze

Wet snow R ime

Other types o f ice

Topograph ic inf luences ........................................... Var iat ion with height above terrain ......................... Icing on structures

General ....................................................................

Ice classes ...............................................................

Def in i t ion of ice class, IC .......................................

Glaze ....................................................................... General .......................................................................

Glaze on lattice structures ..........................................

R ime ........................................................................ General .......................................................................

R ime on single members ............................................

R ime on lattice structures ........................................

General .......................................................................

The direct ion o f ice vanes on the structure ................

Icing on members incl ined to the wind direction .......

Wind act ions on iced structures .................

8.1

8.2

8.2.1

8.2.2

8.3

8.4

General ......................................................

Single members .........................................

Drag coeff ic ients for glaze .........................

Drag coeff ic ients for r ime ..........................

Ang le of incidence .....................................

Latt ice structures ....................................... Combinat ion of ice loads and wind actions

9.1

9.2 10.

General ......................................................

Combined loads ......................................... Unba lanced ice load on guys

11. Fal l ing ice considerat ions

Annex A (informative)

Equat ions used in the International Standard

Annex B (informative)

Standard Measurements for Ice Act ions

Annex C (informative)

Theoret ica l model l ing of icing

Annex D (informative)

Cl imat ic est imation of ice classes based on weather data

Annex E (informative)

Short introduct ion about using this standard ......................

Paper No" 01-MF-04

685

First Author 's Name" Mogens H. Foder Page: 8 of 8

MAIN MENUPREVIOUS MENU---------------------------------Search CD-ROMSearch ResultsPrint

iso 12494

Documents