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 Structures" 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.
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,
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 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 detailed 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.
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.
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 supplemented, 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.
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.
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.
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.
678
Paper No: 01-MF-04
First Author's Name: Mogens n. t'oder
Page: 1 of 8
The expression: "Ice Class" (hereafter IC) has been introduced as the simple way to define which amount of ice is or will be present on the structure
in question. ICs have been defined for both glaze and hard rime because
characteristics for these differ. ICG is for glaze deposits and ICR for rime
deposits (wet snow is here treated as rime).
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
ice
Air temperature [ ~ C]
Wind
speed
[m/s]
Droplet
size
Water
content
in air
Typical
storm
duration
any
any
large
flakes
medium
very
high
hours
hours
see
fig. 1
see
fig. 1
see
fig. 1
medium
medium
small
high
hours
medium
days
low
days
If you as an exception need to design for soft rime or wet snow (this is
normally not necessary), the standard suggest that you "on the safe side"
use the data for (hard) rime instead.
[ Precipitation icing
Glaze)
Wet
snow
- 10 < ta <0
0 < ta < +3
ICs are defined by a characteristic value, the 50 years-retum period of the
ice accretion on the reference collector. This reference collector is a 30
mm diameter cylinder of a length not less than 0,5 m, placed 10 m above
terrain and slowly rotating around its own axis, see ISO 12494, annex B,
chapter 3. ICs can be determined based upon:
I In-cloud icing
Glaze
see fig. 1
Hard
rime
Soft
rime
see fig. 1
see fig. 1
Meteorological and/or topographical data together with use of an
ice accretion model, or
Ice masses (weight) per m structural length, measured on site.
This means that a proper IC can be stipulated for certain sites, if one of the
above mentioned sets of information is available. If other than 50 yearsreturn period is needed, the National meteorologists should find the necessary data for those. It could be mentioned that for most Norwegian sites
the meteorologists have found 3 years-return period for ice mass to be
50% of 50 years-return period.
Also the basic information about influence of topography is mentioned.
This also helps the user (especially the more experienced one) to presume
which type and amount of ice accretion could be expected on the site in
question.
A very useful "tool" is height factor curve shown below. The shown variation of ice mass with height above terrain may not be correct in al cases, but
this variation could be object for more interest among scientists in the future. Such interest could lead to more reliable models or values for the
equation shown and by this afterwards give us a fine utility for tuning our
calculations.
H (m): heigth above terrain
250
200
150
100
50
0
I .....
..
". . . . . . . . . .
1
2
t
7i
Ice Classes for Glaze
Table 2 shows how glaze accretion has been defined. Glaze is designated
ICGx, where "x" tells about the amount of ice accretion. There are 5 standard classes and a possibility to define your own class with an ice accretion
higher than specified. It is fully possible to specify a higher value and this
will not give any problems in the remaining design work for glaze accretion
on a structure.
i
o.orHi
... i
0
[
i--
Mass of ice is always calculated as the cross sectional area of accreted ice
(outside the cross sectional area of object inside the ice) multiplied by density of the accreted ice. In practise the ISO 12494 makes it possible to find
the necessary information to design for any ice accretion on almost any
structure if you just know which IC you are dealing with.
3
4
5
Table 2 - - Ice Classes for glaze (ICG). Density of ice = 900 [kg/m 3 ]
i
I
6
7
8
Height factor: Kh
Figure 2 - - Typical variation of ice masses with the height above
terrain
In any case it is valuable to have something like the curve shown to use for
fitting simple information for ice accretions, see example of calculations in
concluding remarks which contains a result of investigations made by
Norwegian meteorologists for a real site.
Ice
Classes
IC
G1
G2
G3
G4
G5
Ice thickness
t
[ mm ]
10
20
30
40
50
1c6
]to be used for extreme ice accretions bigger than G5
10
0,6
1,7
3,4
;5,7
8,5
Masses for glaze, m lkg/m]
Cylinder diameter [mm]
30
100
300
1,1
3,1
8,8
2,8
6,8
18,1
5,1
11,0
28,0
7,9
15,8
38,5
11,3
21,2
49,5
In principle it is also possible to specify ICs in between the standard
classes, but it should not be necessary and is not recommended. It should
be noted that the definition for glaze classes is a specified ice thickness and
nothing else. Density is defined fixed as 900 [kg/m3], not because it is not
possible to vary, but because the natural variations for structural design are
of no practical importance.
Ice Classes
What is needed in designing for ice load is in principle rather simple:
Weights, dimensions and shapes. Based on these few data it is possible to
do the necessary design and calculation work. Of cause it has to be followed by other, detailed information, but they can be taken from already
known engineering science.
The accretion model for glaze is shown in figure 3 and is very simple, but
close to reality: a constant ice thickness around all possible cross sections
of profiles.
679
Paper No: 01-MF-04
First Author's Name: Mogens H. Foder
i3
Page: 2 of 8
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.
~
t
Wind direction
t
t
TypeA
t
Type B
,j/,. t
\/t
F i g u r e 3 m Ice accretion m o d e l 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.
I
i<
L
5
Type C
8t ._ max W
/--.
IC
_~
It
Type D
\~, t
--T
/I~,
iW~D
ji
t[<=
at .. i
_-
max ½ W
8t
_
max ½ W
j-,
,
lee C l a s s e s f o r R i m e
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 differently 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.
k
i-
i
Type E
-.
L
i., t
;
Type F
I t
v
,]
!
8t
-<-
I
I- ~
iI
'/
i
L
i'\ •
i max ½ "W
EZ 8t >!
1
I
L
i
.max½W
r
Table 3 ~ Ice Classes for rime (IC R)
Ice mass
Classes
m
IC
R1
[kg/m]
0,5
R2
0,9
R3
R9
1,6
2,8
5,0
8,9
16,0
28,0
50,0
RI0
to be used :or extreme ice accretions bigger than R9
R4
R5
R6
R7
R8
Figure 4 m Ice accretion m o d e l for R I M E
K~me o m m e t e r lmml lor object
d i a m e t e r = 30 mm
Ice
Density of rime l kg/m3] ;300
55
69
88
113
149
197
262
346
462
500
700
900
47
43
50
62
77
100
131
173
228
40
47
56
70
89
116
153
201
1268
56
71
90
ll7
154
204
269
358
303
D a t a for d e t e r m i n a t i o n of ICs
'
Now that ICs have been defined and a certain building site has been chosen, it is necessary" to find the most correct IC for the structure in question.
3 possible ways to use have been mentioned:
-
H o w to find the most c o r r e c t IC for the structure
Collecting existing experiences
Ice accretion
based on known
meteorological
data
Direct measurements of ice for
many years
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.
A good starting method if you have several
structures already placed in the area where
to build, or if you need to recalculate existing structures.
Additional information or assumptions
about the parameters usually are necessary.
Annex C and D in the standard describe the
method.
A (standard) measuring device must be in
operation in the area or in a representative
area during a long period. Annex B in the
standard describes the method.
In many cases an acceptable result can be achieved by combining 2 or all of
above mentioned methods. In practise it is rather easy to distinguish between the different ICRs, but may be it is more difficult with the ICGs. On
the other hand, olden more knowledge is available for glaze. When the
proper IC has been found the standard shows exactly how to proceed.
An example on the principle for use in calculations is shown in the standard, is table 4 below. The table shows vane lengths and widths for accreted rime in all ICRs for chosen profile dimensions up to 300 mm and
rime density of 500 [kg/m3]. All that sort of presentations in the standard is
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 occurring without any increase of object dimension perpendicular to wind direction. Beyond that point the accretion is growing also perpendicular to the
680
Paper N o 01-MF-04
First Author's Name" Mogens H. Foder
Page: 3 of 8
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] )
Types A and B
Cross sectional shape
Object width
IC
Ice
m
R1 '
~,g/m]
R2
0,9
R3
1,6
R4
2,8
R5
5,0
R6
8,9
R7
16,0
R8
28,0
R9
50,0
R10
I10
L
54
78
109
150
207
282
384
514
694
i
130
D
22
28
36
46
60
79
105
137
182
1300
Ice vanes dimension
L
D
L
D
iL
34
35
13
100 4
54
40
23
100 8
82
120
174
247
348
478
656
47
56
70
88
113
146
190
41
67
106
165
253
372
543
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.
D
300
300
100 14
300
104 24
300
114 42 3 0 0
129 76
300
151 136 300
181 217 317
223 344 349
Wind d i r e c t i o n , . _
l cemass m
per unit 1
~ \ L
P ' - ~
~
~
x sin c,
"~~'.-'--7
~
__L (round bar shown)
p ane
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.
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.
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 importance compared to all other, normal actions on the structure.
Table 5 m Ci_coefficients for glaze on b a r s .
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
[
,! 150mm
,I,150mm
V
direction
IC
Thickness
[mm]
G1
G2
G3
G4
G5
10
20
30
40
50
G6
to be used for extreme ice accretions bigger than G5
0,50
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
0,68
0,86
1,04
1,22
1,40
0,88
1,01
1,14
1,27
1,40
1,08
1,16
1,24
1,32
1,40
1,28
,31
1,34
1,37
1,40
1,48
1,46
1,44
1,42
1,40
1,68
1,61
1,54
1,47
1,40
1,88
1,76
1,64
1,52
1,40
> 300ram
r
I
~,/
i
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.
r
Figure 5 w Ice accretion model for rime, big objects
The principle for glaze accretions are that very smooth profile shapes (low
C-values without ice) become more rough and very rough shapes (high Cvalues without ice) become more smooth with glaze accretion. When object dimensions are very big the effect of glaze accretion is negligible.
cross sections. Again the equations in Annex A in ISO 12494 for big objects control the dimensions to be used.
681
Paper No: 01-MF-04
First Author's Name" Mogens H. Foder
Page: 4 of 8
Table 6 -
Ci-coefficients for glaze, ICG3, big objects
IC
Obiect
wRith
G3
[m]:
Ci coefficients for glaze, b ig objects
1.~ '3
Drag coefficients without ice = Co
1,75
0,75
1,00
1,25
1,50
1,54
1,04
1,14
1,24
1,34
1,44
0,961,081,201,331,45
1,57
210
310
>_+5,0
0,841,001,151,311,46
0,73
0,92
1,10
1,29
0,50
0,75
1,00
1,25
0,50
1,47
1,50
2,00
1,62
1,66
1,64
1,69
1,77
1,85
1,75
2,00
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 °)
~
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 - -
-coefficients for rime on bars
C i
m
R1
R2
R3
R4
R5
R6
R7
R8
R9
R1
[kg/m]
0,5
0,9
1,6
2,8
5,0
8,9
16,0
28,0
50,0
to be used
0,50
0,62
0,74
0,87
0,99
1,11
1,32
1,23
1,41
1,36
1,51
1,48
1,60
1,60
for extreme ice
1,40
1,47
1,53
1,60
1,48
1,52
1,56
1,60
1,57
1,58
1,59
1,60
bars
= Co
1,75
2,00
1,73
1,96
1,72
1,91
1,87
1,70
1,68
1,67
1,65
1,63
1,82
1,78
1,73
1,69
1,62
1,64
1,60
1,60
accretions bigger than R9
C i-cOefficient for rime, big objects
R5
[m]
0,50
_<0,3
0,5
1,0
1,5
2,0
2,5
1,11
3,0
4,0
>- 5,0
1,09
1,02
0,96
0,89
0,83
0,76
0,63
0,50
Drag coefficient without ice = Co
0,75
1,00
1,25
1,50
1,75
1,22
1,33
1,44
1,56
1,67
1,20
1,32
1,44
1,55
1,67
1,15
1,28
1,42
1,55
1,68
1,10
1,25
1,39
1,54
1,69
1,05
1,21
1,37
1,54
1,70
1,00
1,18
1,35
1,53
1,71
0,95
1,14
1,33
1,52
1,71
0,85
1,07
1,29
1,51
1,73
0,75
1,00
1,25
1,50
1,75
(0)= Fw (90 °) sin20
Forces on an inclined member
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 certain 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:
•
•
Table 8 - - Ci -coefficients for rime, ICR5, big objects
width
~
Action on lattice structures
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
IC Object
0
= 90 °
Figure 7 -
Ci coefficients for rime on
Drag coefficient without ice
0,75
1,00
1,25
1,50
1,07
1,29
1,51
0,84
1,13
1,33
1,52
0,94
1,37
1,53
1,03
1,20
1,27
1,41
1,54
1,13
1,22
1,33
1,44
1,56
IC Ice mass
F,~_(90 o) sin30
y
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:
2,00
1,78
1,79
1,81
1,83
1,86
1,88
1,91
1,95
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,
.
.
-
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 advanced computer program for calculating ice and wind actions on lattice
structures is necessary. We have for some years been using such programs 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. Statistically of cause it is too conservative to combine to different types of load
just by adding their full effect.
682
Paper No: 01-MF-04
First Author's Name: Mogens H. Foder
Page: 5 of 8
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.
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.
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"
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.
I n f o r m a t i o n from N o r w e g i a n M e t e o r o l o g i s t
Specification of iceaccretion in accordance with ISO 12494
T a b l e 9 m P r i n c i p l e s for c o m b i n a t i o n of wind actions and ice loads
50-years
ice (IC)
50-years
ice (kg/m)
ICR9
50
(% o f S0-years)
50
1,65 x
ICR9
92
50
Combina(Major load)
Wind action
Wind pressure
T (years)
Ice loads
Ice mass
T (years)
Height =
I (wind)
II (Ice)
k " q s0
~w" k - q s0
~)ice" m
Height =
200m
50
3
m
3
50
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 condition.
Factor k has values as shown in table 10.
k
ICR
0,40
0,45
0,50
0,55
0,60
R
R
R
R
R
:: . •
. , . : : :;ilj ~
.i~:
.i:..: . .
:
1
2
3
4
5
R 6
R 7
.:
i~:i~.. R 8
.:i ~:~.:ii ~:.::i; .. R 9
k
0,40
0,45
0,50
0,55
0,60
0,70
0,80
A n n e x A shows flowchart for a typical calculation procedure.
A n n e x B shows the table of contents for ISO 12494.
0,90
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 before 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.
683
Paper No: 01-MF-04
e°'°°°H
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 separate paper.
T a b l e 10 m F a c t o r for r e d u c t i o n of wind p r e s s u r e
1
2
3
4
5
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.
and qbware used to change actions and load from 50 years to 3 years
occurrence. The factor (Diceis 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.
G
G
G
G
G
• Factor K h (see f i g u r e 2 )
100m
~ice
ICG
1-year ice
Level in
mast
First Author's Name: Mogens H. Foder
Page: 6 of 8
Annex 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 ref. I S O 12494"
I
~.
Find ICGx or ICRx
}
I
~i MethodA: Collectingexistingexperience
~
-] MethodB: Icingmodellingby meteorologists
MethodC: Directmeasurementsfor manyyears
.
/Use
/
table 3
/Use
/
table 4
i
(
Q iCRx )
ICGx )
___>---
i2-----
I Profile or
big object
dimen/
Usefi, i /
gure 3
~ - Ice weights
(I.._
Big object
dimension
Profile dimension
/
Use figure 4 /
and table 5 - 7
Dr. m and iced dimensions
/Use
figure 5 /
and table 8 - 9
are calculated-~
Find drag coefficients
o
Use table 10
for bars
and 1115 for
big ob-
Use table
16 for
bars and
table 1725 for big
objects
~aCombine wind action " ~
nd ice load for dimen- i
sioning structure /
Calculate
" - ~ wind action
and ice load
Adjust drag coefficient on sloping I
elements for angle of incidence
684
PaperNo:01-MF-04
FirstAuthor'sName:MogensH. Foder
5;i':
Page: 7 of 8
Annex B
Table of Contents in ISO 12494
W i n d a c t i o n s on iced s t r u c t u r e s .................
S c o p e .........................................................................
1.1
1.2
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
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 .................................................
8.1
8.2
D e f i n i t i o n s .................................................................
8.3
8.4
8.2.1
8.2.2
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 .......................................................................
Ice a c t i o n .................................................................
Ice class (IC) ...........................................................
I n - c l o u d icing ..........................................................
P r e c i p i t a t i o n icing ...................................................
R e t u r n p e r i o d ..........................................................
R i m e ........................................................................
Symbols
.
9.1
9.2
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 for g l a z e .........................
D r a g c o e f f i c i e n t s for 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 ice l o a d s and w i n d a c t i o n s
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 ice load on g u y s
11.
F a l l i n g ice c o n s i d e r a t i o n s
Annex A (informative)
E q u a t i o n s u s e d in the I n t e r n a t i o n a l S t a n d a r d
E f f e c t s o f icing
Annex B (informative)
5.1
5.2
5.3
5.4
Static ice l o a d s ........................................................
W i n d a c t i o n on iced 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 ice .................................
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 icing
6.1
6.2
G e n e r a l ....................................................................
Icing t y p e s ...............................................................
Glaze
Wet snow
Rime
O t h e r t y p e s o f ice
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 terrain .........................
Icing on s t r u c t u r e s
S t a n d a r d M e a s u r e m e n t s for Ice A c t i o n s
Annex C (informative)
T h e o r e t i c a l m o d e l l i n g o f icing
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
Annex D (informative)
C l i m a t i c e s t i m a t i o n o f ice c l a s s e s b a s e d on w e a t h e r d a t a
Annex E (informative)
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 this s t a n d a r d ......................
G e n e r a l ....................................................................
Ice c l a s s e s ...............................................................
D e f i n i t i o n o f ice class, IC .......................................
G l a z e .......................................................................
G e n e r a l .......................................................................
G l a z e on lattice 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 on s i n g l e m e m b e r s ............................................
R i m e on lattice 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 ice v a n e s on the structure ................
Icing on m e m b e r s i n c l i n e d to the w i n d d i r e c t i o n .......
685
P a p e r No" 0 1 - M F - 0 4
First A u t h o r ' s Name" M o g e n s H. F o d e r
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