ISO 12494:2017
(Main)Atmospheric icing of structures
Atmospheric icing of structures
ISO 12494:2017 describes the general principles of determining ice load on structures of the types listed in this clause. In cases where a certain structure is not directly covered by this or another standard or recommendation, designers can use the intentions of this document. However, it is the user's responsibility to carefully consider the applicability of this document to the structure in question. The practical use of all data in this document is based upon certain knowledge of the site of the structure. Information about the degree of "normal" icing amounts (= ice classes) for the site in question is used. For many areas, however, no information is available. Even in such cases, this document 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. ISO 12494:2017 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. This document is intended to be used in conjunction with ISO 2394. NOTE Some typical types of structure are mentioned, but other types can also be considered by designers by thinking in terms of which type of structure is sensitive to unforeseen ice, and act thereafter. Also, in many cases, only parts of structures are to 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 can use this document for the mast structures to overhead lines (which are not covered by IEC standards) if they so wish.
Charges sur les structures dues à la glace
General Information
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Standards Content (Sample)
INTERNATIONAL ISO
STANDARD 12494
Second edition
2017-03
Atmospheric icing of structures
Charges sur les structures dues à la glace
Reference number
ISO 12494:2017(E)
©
ISO 2017
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ISO 12494:2017(E)
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© ISO 2017, Published in Switzerland
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ISO 12494:2017(E)
Contents Page
Foreword .v
Introduction .vii
1 Scope . 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
6.2.1 General. 6
6.2.2 Glaze . 8
6.2.3 Wet snow . 8
6.2.4 Rime . 8
6.2.5 Other types of ice . 9
6.3 Topographic influences . 9
6.4 Variation with height above terrain .10
7 Icing on structures .11
7.1 General .11
7.2 Ice classes .11
7.3 Definition of ice class, IC .12
7.4 Glaze .12
7.4.1 General.12
7.4.2 Glaze on lattice structures .12
7.5 Rime .13
7.5.1 General.13
7.5.2 Rime on single members.15
7.6 Rime on lattice structures .18
7.6.1 General.18
7.6.2 Direction of ice vanes on the structure .19
7.6.3 Icing on members inclined to the wind direction .19
8 Wind actions on iced structures .20
8.1 General .20
8.2 Single members .20
8.2.1 General.20
8.2.2 Drag coefficients for glaze .21
8.2.3 Drag coefficients for rime .23
8.3 Angle of incidence .27
8.4 Lattice structures .27
9 Combination of ice loads and wind actions .28
9.1 General .28
9.2 Combined loads .28
10 Unbalanced ice load on guys .29
11 Falling ice considerations .30
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ISO 12494:2017(E)
Annex A (informative) Formulae used in this document.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 .50
Annex E (informative) Hints on using this document .53
Bibliography .57
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ISO 12494:2017(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.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www. iso. org/d irectives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www. iso. org/p atents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO’s adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT) see the following URL:
ww w .iso. org/iso / foreword. html.
The committee responsible for this document is ISO/TC 98, Bases for design of structures, Subcommittee
SC 3, Loads, forces and other actions.
This second edition cancels and replaces the first edition (ISO 12494:2001), of which it constitutes a
minor revision. The changes made are the following:
— 8.1, line 2, replaced “ISO 4355” by “ISO 4354”;
— 8.3, Figure 7, revised the right figure;
— 9.1, line 2 ,9.2, line 2 to 4, replaced “exceedence” by “exceedance”;
— 9.2, line 11, replaced “to day’s” by “today’s”;
— Clause 10, line 15, replaced “5.3” by “5.4”;
— A.2, Table 3, line 1, replaced “the glaze mass” by “the mass of the ice, glaze or rime”;
— A.2, Table 3, line 2, replaced “the glaze thickness” by “the thickness of the ice, glaze or rime”;
— A.2, Table 3, line 4, replaced “the glaze density” by “the density of the ice, glaze or rime”;
— A.2, Table 3, line 4, replaced “r” by “γ”;
— A.2, Table 3, line 1 to 4, moved before Table 3,
— B.3.2, c), replaced “see Table 2 and 2.3” by “see Table 1 in 6.2.1”;
— B.3.3, line 5, replaced “definitions 3.1 and 3.2” by “definitions B.3.1 and B.3.2”;
— B.3.3, line 6, replaced “Table 4 or 5” by “Table 3 or 4”;
-3 -3
— C.3, paragraph 6, line 4, replaced “0,7 cm ” by “0,7 g cm ”;
— E.4, b), line1, replaced “ICGx” by “ICRx”.
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ISO 12494:2017(E)
Annexes A to E of this document are for information only.
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ISO 12494:2017(E)
Introduction
This document 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 document 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 document. This may be true, especially in the future. They should, however, be very careful
not to use only parts of this document partly, but only as a whole.
The main purpose of this document 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 document 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
document, 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 12494:2017(E)
Atmospheric icing of structures
1 Scope
This document describes the general principles of determining ice load on structures of the types listed
in this clause.
In cases where a certain structure is not directly covered by this or another standard or recommendation,
designers can use the intentions of this document. However, it is the user’s responsibility to carefully
consider the applicability of this document to the structure in question.
The practical use of all data in this document is based upon certain knowledge of the site of the
structure. Information about the degree of “normal” icing amounts (= ice classes) for the site in question
is used. For many areas, however, no information is available.
Even in such cases, this document 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.
This document 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.
This document is intended to be used in conjunction with ISO 2394.
NOTE Some typical types of structure are mentioned, but other types can also be considered by designers
by thinking in terms of which type of structure is sensitive to unforeseen ice, and act thereafter.
Also, in many cases, only parts of structures are to 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 can use this document for the
mast structures to overhead lines (which are not covered by IEC standards) if they so wish.
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ISO 12494:2017(E)
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 2394:2015, General principles on reliability for structures
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— IEC Electropedia: available at http:// www .electropedia .org/
— ISO Online browsing platform: available at http:// www .iso .org/ obp
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
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 1 to entry: A long return period means low transgression intensity (occurring rarely) and a short return
period means high transgression intensity (occurring often).
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ISO 12494:2017(E)
3.9
rime
white ice with in-trapped air
4 Symbols
C drag coefficient of an iced object 1
i
C drag coefficient for large objects (width >0,3 m) 1
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
α angle of incidence between wind direction and the objects longitudinal axis °
3
γ density of ice kg/m
θ angle of wind incidence in a vertical plane °
τ
exposed panel area
solidity ratio:
1
total panel area within outside boundariies
increased value of τ caused by icing to be used in calculations 1
¢
τ
ϕ factor of combination 1
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ISO 12494:2017(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 Clause 5 describes the way the ice loads act on a structure, and this can enable designers to understand
the background and to use this document, 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 document 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
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ISO 12494:2017(E)
cause aerodynamic instability 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 document
(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 12494:2017(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 shall be operating
...
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