ISO 12494:2001

INTERNATIONAL ISO
STANDARD 12494
First edition
2001-08-15
Atmospheric icing of structures
Charges sur les structures dues à la glace
Reference number
ISO 12494:2001(E)
©
ISO 2001

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ISO 12494:2001(E)
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ISO 12494:2001(E)
Contents Page
Foreword.iv
Introduction.v
1 Scope .1
1.1 General.1
1.2 Application .1
2 Normative references .2
3 Terms and definitions .2
4 Symbols .3
5 Effects of icing .4
5.1 General.4
5.2 Static ice loads.4
5.3 Wind action on iced structures .4
5.4 Dynamic effects .4
5.5 Damage caused by falling ice.5
6 Fundamentals of atmospheric icing .5
6.1 General.5
6.2 Icing types .6
6.3 Topographic influences .9
6.4 Variation with height above terrain.9
7 Icing on structures.10
7.1 General.10
7.2 Ice classes.10
7.3 Definition of ice class, IC .11
7.4 Glaze .11
7.5 Rime .12
7.6 Rime on lattice structures.18
8 Wind actions on iced structures .19
8.1 General.19
8.2 Single members .20
8.3 Angle of incidence.27
8.4 Lattice structures.28
9 Combination of ice loads and wind actions.29
9.1 General.29
9.2 Combined loads.29
10 Unbalanced ice load on guys .30
11 Falling ice considerations.31
Annex A (informative) Equations used in this International Standard .32
Annex B (informative) Standard measurements for ice actions .35
Annex C (informative) Theoretical modelling of icing.39
Annex D (informative) Climatic estimation of ice classes based on weather data .48
Annex E (informative) Hints on using this International Standard .51
Bibliography.55
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ISO 12494:2001(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO
member bodies). The work of preparing International Standards is normally carried out through ISO technical
committees. Each member body interested in a subject for which a technical committee has been established has
the right to be represented on that committee. International organizations, governmental and non-governmental, in
liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical
Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 3.
Draft International Standards adopted by the technical committees are circulated to the member bodies for voting.
Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this International Standard may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights.
International Standard ISO 12494 was prepared by Technical Committee ISO/TC 98, Bases for design of
structures, Subcommittee SC 3, Loads, forces and other actions.
Annexes A to E of this International Standard are for information only.
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ISO 12494:2001(E)
Introduction
This International Standard describes ice actions and can be used in the design of certain types of structures.
It should be used in conjunction with ISO 2394, and also in conjunction with relevant CEN standards.
This International Standard differs in some aspects from other International Standards, because the topic is poorly
known and available information is inadequate. Therefore, it contains more explanations than usual, as well as
supplementary descriptions and recommendations in the annexes.
Designers might find that they have better information on some specific topics than those available from this
International Standard. This may be true, especially in the future. They should, however, be very careful not to use only
parts of this International Standard partly, but only as a whole.
The main purpose of this International Standard is to encourage designers to think about the possibility of ice
accretions on a structure and to act thereafter.
As more information about the nature of atmospheric icing becomes available during the coming years, the need for
updating this International Standard is expected to be more urgent than usual.
Guidance is given as a NOTE, after the text for which it is a supplement. It is distinguished from the text by being in
smaller typeface. This guidance includes some information and values which might be useful during practical
design work, and which represents results that are not certain enough for this International Standard, but may be
useful in many cases until better information becomes available in the future.
Designers are therefore welcome to use information from the guidance notes, but they should be aware of the
intention of the use and also forthcoming results of new investigations and/or measurements.
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INTERNATIONAL STANDARD ISO 12494:2001(E)
Atmospheric icing of structures
1 Scope
1.1 General
This International Standard describes the general principles of determining ice load on structures of the types listed
in 1.2.
In cases where a certain structure is not directly covered by this or another standard or recommendation, designers
may use the intentions of this International Standard. However, the user should always consider carefully the
applicability of the standard (recommendation) to the structure in question.
The practical use of all data in this International Standard is based upon certain knowledge of the site of the
structure. It is necessary to have information about the degree of “normal” icing amounts (= ice classes) for the site
in question. For many areas, however, no information is available.
Even in such cases this International Standard can be useful, because local meteorologists or other experienced
persons should be able to, on the safe side, estimate a proper ice class. Using such an estimate in the structural
design will result in a much safer structure, than designing without any considerations for problems due to ice.
CAUTION It is extremely important to design for some ice instead of no ice, and then the question of whether
the amount of ice was correct is of less importance. In particular, the action of wind can be increased considerably
due to both increased exposed area and increased drag coefficient.
1.2 Application
This International Standard is intended for use in determining ice mass and wind load on the iced structure for the
following types of structure:
� masts;
� towers;
� antennas and antenna structures;
� cables, stays, guy ropes, etc.;
� rope ways (cable railways);
� structures for ski-lifts;
� buildings or parts of them exposed to potential icing;
� towers for special types of construction such as transmission lines, wind turbines, etc.
Atmospheric icing on electrical overhead lines is covered by IEC (International Electrotechnical Commission)
standards.
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ISO 12494:2001(E)
This International Standard is intended to be used in conjunction with ISO 2394.
NOTE Some typical types of structure are mentioned, but other types might be considered also. Designers should think in
terms of which type of structure is sensitive to unforeseen ice, and act thereafter.
Also, in many cases only parts of structures should be designed for ice loads, because they are more vulnerable to unforeseen
ice than is the whole structure.
Even if electrical overhead lines are covered by IEC standards, designers may use this International Standard for the mast
structures to overhead lines (which are not covered by IEC standards) if they so wish.
2 Normative references
The following normative documents contain provisions which, through reference in this text, constitute provisions of
this International Standard. For dated references, subsequent amendments to, or revisions of, any of these
publications do not apply. However, parties to agreements based on this International Standard are encouraged to
investigate the possibility of applying the most recent editions of the normative documents indicated below. For
undated references, the latest edition of the normative document referred to applies. Members of ISO and IEC
maintain registers of currently valid International Standards.
ISO 2394:1998, General principles on reliability for structures
ISO 4354:1997, Wind actions on structures
3 Terms and definitions
For the purposes of this International Standard, the following terms and definitions apply.
3.1
accretion
process of building up ice on the surface of an object, resulting in the different types of icing on structures
3.2
drag coefficient
shape factor for an object to be used for the calculation of wind forces in the along-wind direction
3.3
glaze
clear, high-density ice
3.4
ice action
effect of accreted ice on a structure, both as gravity load (= self-weight of ice) and as wind action on the iced
structure
3.5
ice class
IC
classification of the characteristic ice load that is expected to occur within a mean return period of 50 years on a
reference ice collector situated in a particular location
3.6
in-cloud icing
icing due to super-cooled water droplets in a cloud or fog
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ISO 12494:2001(E)
3.7
precipitation icing
icing due to either
a) freezing rain or drizzle, or
b) accumulation of wet snow
3.8
return period
average number of years in which a stated action statistically is exceeded once
NOTE A long return period means low transgression intensity (occurring rarely) and a short return period means high
transgression intensity (occurring often).
3.9
rime
white ice with in-trapped air
4 Symbols
C Drag coefficient of an iced object 1
i
C 1
Drag coefficient for large objects (width � 0,3 m)
0,3
C Drag coefficient of an object without ice 1
0
D Diameter of accreted ice or total width of object including ice mm
F Wind force N/m
w
H Height above terrain m
k
Factor for velocity pressure from wind action 1
K Height factor 1
h
L Length of ice vane measured in windward direction mm
m
Mass of accreted ice per meter unit length kg/m
m Ice mass for ice on large objects kg
W
T Return period year
t
Ice thickness mm
t Air temperature
�C
a
W Width of object (excluding ice) perpendicular to wind direction mm
Angle of incidence between wind direction and the objects longitudinal axis °
�
3
Density of ice kg/m
��
Angle of wind incidence in a vertical plane °
�
� exposed panel area
1
Solidity ratio:
total panel area within outside boundaries
� � 1
Increased value of ��caused by icing to be used in calculations
Factor of combination 1
�
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ISO 12494:2001(E)
5 Effects of icing
5.1 General
The general effects of icing are the increased vertical loads on the iced structure and increased wind drag caused
by the increased wind-exposed area. The latter can lead to more severe wind loads than without icing.
NOTE This clause describes the way the ice loads act on a structure, and this should enable designers to understand the
background and to use this International Standard, even in cases which are not mentioned here.
5.2 Static ice loads
Different types of structure are more or less sensitive to varying aspects concerning ice action, and some examples
on this are as follows.
a) Tensioned steel ropes, cables and guys, etc., are generally very sensitive to ice action, consequently tension
forces in such elements can increase considerably in an iced condition.
b) Slender lattice structures, especially guyed masts, are sensitive to the increased axial compression forces from
accreted ice on the structure.
c) Antennas and antenna structures can easily be overloaded by accreted ice, if this has not been foreseen. In
particular, small fastening details are weak when increased load is added on top of other actions, because the
ice may easily double the normal load.
d) “Sagging of ice” on non-structural elements can be harmful. Non-structural elements such as antennas and
cables, may be exposed to unexpected ice load because the ice sags downwards and covers or presses on
the elements. The ice action on these elements can then be substantially greater than the ice load normally
accreted on them.
e) The load of accreted ice can easily deform or damage envelope elements (claddings, etc.), and damage also
might occur if the ice has not fallen off before forces have grown too great.
5.3 Wind action on iced structures
Structures such as masts and towers, together with tensioned steel ropes, cables, mast guys, etc., are sensitive to
increased wind drag caused by icing.
Wind action on iced structures may be calculated based on the same principles as the action on the ice-free
structure. However, both the dimensions of the structural members and their drag coefficients are subject to
changes. Therefore, the main purpose of this International Standard is to specify proper values for
� dimensions and weight of accreted ice,
� shapes of accreted ice, and
� drag coefficients of accreted ice.
5.4 Dynamic effects
A significant factor influencing the dynamic behaviour of a structure is its natural frequencies.
Normally the natural frequencies of a structure are decreased considerably if the structure is heavily iced. This is
important in connection with dynamic investigations because the lower frequencies normally are the critical ones.
In addition, the change in cross-sectional shape due to the accreted ice may require dynamic investigations to be
made. For example, the eccentric cross-sectional shape of ice on a cable or guy can cause aerodynamic instability
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ISO 12494:2001(E)
resulting in heavy oscillations (e.g. galloping). Also, fully iced mast or tower sections can introduce vortex shedding,
resulting in cross wind vibrations.
Shedding of ice from a structure can cause severe dynamic effects and stresses in the structure, depending on the
type of structure and the amount and properties of the ice. Such dynamic effects should be investigated if the
structure in question is sensitive to those actions. For a guyed mast, the shedding of ice from heavily iced guys may
introduce severe dynamic vibrations and should be considered; see clause 10.
NOTE This phenomenon has caused total collapses of very tall, guyed masts.
5.5 Damage caused by falling ice
When a structure is iced, this ice will sooner or later fall from the structure. The shedding of ice can be total or
(most often) partial.
Experience shows that ice shedding typically occurs during increasing temperatures. Normally, accreted ice does
not melt from the structure, but breaks because of small deflections, vibrations, etc. and falls off in fragments.
It is extremely difficult to avoid such falling ice, so this should be considered during design and when choosing the site
for the structure.
Damage can occur to structural or non-structural elements (antennas, etc.) when ice from higher parts fall and hit lower
elements in the structure. The height of falling ice is an important factor when evaluating risks of damage, because a
greater height means greater dynamic forces from the ice. A method of avoiding or reducing damage from falling ice is
the use of shielding structures.
NOTE See also 5.2 d) about “sagging of ice” and clause 10 about unbalanced ice on guys, and clause 11 on
considerations on ice falling from a structure.
6 Fundamentals of atmospheric icing
6.1 General
The expression “atmospheric icing” comprises all processes where drifting or falling water droplets, rain, drizzle or
wet snow in the atmosphere freeze or stick to any object exposed to the weather.
The accretion processes and resulting types of ice are described in this clause. The more theoretical explanation of
the processes is given in annexes C and D.
NOTE Unlike other meteorological parameters such as temperature, precipitation, wind and snow depths, there is
generally very limited data available about ice accretions.
The wide variety of local topography, climate and icing conditions make it difficult to standardize actions from ice accretions.
Therefore local (national) work has to be done, and such work should be based upon this International Standard (see annex B).
It is urgent to be able to undertake comparisons between collected data and to exchange experiences, because this will be a
way to improve knowledge and data necessary for a future comprehensive International Standard for atmospheric icing.
Detailed information about icing frequency, intensity, etc. should be collected.
The following methods may do this.
� A: collecting existing experiences.
� B: icing modelling based on known meteorological data.
� C: direct measurements of ice for many years.
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ISO 12494:2001(E)
Method A is a good starting one, because it makes it possible to obtain quickly information of considerable value. However, it
will be necessary to have different types of structures established on proper areas, to be able to collect sufficiently broad
information on ice frequencies and intensities. Therefore experienced people in those fields should be consulted, e.g.
telecommunication and power transmission companies, meteorological services and the like with in-service experience. The
method can be recommended as the first thing to do, while awaiting results from Method C.
Method B usually demands some additional information or assumptions about the parameters.
The principles of icing modelling are presented in annexes C and D.
For Method C standardized measuring devices must be operating in the areas representative of the planned site or at the actual
construction site.
It is important that measurements follow standardized procedure, and such a procedure is described in annex B.
Measurements should be taken for a sufficient long period to form a reliable basis for extreme value analysis. The length of the
period could be from a few years to several decades, depending on the conditions.
However, shorter series can be of valuable help and can also be connected to longer records of meteorological data, either
statistically or (better) physically, in combination with theoretical models.
6.2 Icing types
6.2.1 General
Atmospheric icing is traditionally classified according to two different formation processes:
a) precipitation icing;
b) in-cloud icing.
However, a classification may be based on other parameters, see Tables 1 and 2.
The physical properties and the appearance of the accreted ice will vary widely according to the variation in
meteorological conditions during the ice growth.
Besides the properties mentioned in Table 1, other parameters, such as compressive strength (yield and crushing),
shear strength, etc., may be used to describe the nature of accreted ice.
The maximum amount of accreted ice will depend on several factors, the most important being humidity,
temperature and the duration of the ice accretion.
A main preconditions for significant ice accretion are the dimensions of the object exposed and its orientation to the
direction of the icing wind. This is explained in more detail in clause 7.
Table 1 — Typical properties of accreted atmospheric ice
Type of ice Density Adhesion and General appearance
cohesion
3
Colour Shape
kg/m
Glaze 900 strong transparent evenly distributed/icicles
Wet snow 300 to 600 weak (forming) white evenly distributed/eccentric
strong (frozen)
Hard rime 600 to 900 strong opaque eccentric, pointing windward
Soft rime 200 to 600 low to medium white eccentric, pointing windward
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ISO 12494:2001(E)
NOTE 1 In practice, accretions formed of layers of different types of ice (mentioned in Table 1) can also occur, but from an
engineering point of view the types of ice do not need to be described in more detail. Table 2 gives a schematic outline of the
major meteorological parameters controlling ice accretion.
A cloud or fog consists of small water droplets or ice crystals. Even if the temperature is below the freezing point of water, the
water droplets may remain in the water state. Such super-cooled droplets freeze immediately on impact with objects in the
airflow.
Table 2 — Meteorological parameters controlling atmospheric ice accretion
Type of ice Air temperature Wind speed Droplet size Water content in air Typical storm duration
m/s
�C
Precipitation icing
Glaze (freezing any large medium hours
� 10 � t � 0
a
rain or drizzle)
Wet snow any flakes very high hours
0 � t �� 3
a
In-cloud icing
Glaze see Figure 1 see Figure 1 medium high hours
Hard rime see Figure 1 see Figure 1 medium medium days
Softrime seeFigure1 seeFigure1 small low days
NOTE 2 When the flux of water droplets towards the object is less than the freezing rate, each droplet freezes before the
next droplet impinges on the same spot, and the ice growth is said to be dry.
When the water flux increases, the ice growth will tend to be wet, because the droplets do not have the necessary time to
freeze, before the next one impinges.
In general, dry icing results in different types of rime (containing air bubbles), while wet icing always forms glaze (solid and
clear).
Figure 1 gives an indication of the parameters controlling the major types of ice formation.
The density of accreted ice varies widely from low (soft rime) over medium (hard rime) to high (glaze).
NOTE The curves shift to the left with increasing liquid water content and with decreasing object size.
Figure 1 — Type of accreted ice as a function of wind speed and air temperature
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ISO 12494:2001(E)
6.2.2 Glaze
Glaze is the type of precipitation ice having the highest density. Glaze is caused by freezing rain, freezing drizzle or
wet in-cloud icing, and normally causes smooth evenly distributed ice accretion.
Glazemayresult also informationof icicles;inthis casetheresultingshapecanberather asymmetric.
Glaze can be accreted on objects anywhere when rain or drizzle occurs at temperatures below freezing point.
NOTE Freezing rain or drizzle occurs when warm air aloft melts snow crystals and forms rain drops, which afterwards fall
through a freezing air layer near the ground. Such temperature inversions can occur in connection with warm fronts, or in valleys
where cold air may be trapped below warmer air aloft.
The surface temperature of accreting ice is near freezing point, and therefore liquid water, due to wind and gravity, can flow
around the object and freeze also on the leeward side.
The accretion rate for glaze mainly varies with the
...