MODEL CODE FOR
STEEL CHIMNEYS
THE CICIND CHIMNEY STANDARD
Revision 2 – September 2010
Copyright CICIND 1999, 2002, 2010
ISBN 1-902998-16-2
CICIND Steel Chimney Model Code
page i
CICIND
Model Code for Steel Chimneys
Revision 1 – December 1999
Revision 2 – September 2010
TABLE OF CONTENTS
FOREWORD .................................................................................................................................. 1
0. INTRODUCTION ..................................................................................................................... 1
0.1 General .................................................................................................................................. 1
0.2 Appendices and Commentaries ............................................................................................. 1
0.3 Philosophy............................................................................................................................. 2
1. SCOPE........................................................................................................................................ 2
2. FIELD OF APPLICATION ..................................................................................................... 2
3. REFERENCES .......................................................................................................................... 2
4. NOTATIONS, UNITS AND DEFINITIONS .......................................................................... 3
4.1 General .................................................................................................................................. 3
4.2 Subscripts-Superscripts ......................................................................................................... 4
4.3 Units ...................................................................................................................................... 4
4.4 Definitions............................................................................................................................. 4
5. BASIS OF DESIGN AND SAFETY FACTORS .................................................................... 4
5.1 General .................................................................................................................................. 4
5.2 Reliability differentiation ...................................................................................................... 4
5.3 Partial Safety Factors ............................................................................................................ 5
5.4 Cross-wind Effects ................................................................................................................ 5
6. MATERIALS ............................................................................................................................. 5
6.1 General .................................................................................................................................. 5
6.2 Structural Steels .................................................................................................................... 5
6.3 Stainless and Alloy Steels ..................................................................................................... 8
7. ACTIONS (EXTERNAL AND INTERNAL) ......................................................................... 8
7.1 Permanent Load .................................................................................................................... 8
7.1.1 Dust load (temporary load) .......................................................................................................... 8
7. 2 Wind ..................................................................................................................................... 8
7.2.1 General ......................................................................................................................................... 8
7.2.2 Wind speed................................................................................................................................... 9
7.2.3 Wind load in the direction of the wind ....................................................................................... 13
7.2.4 Vortex shedding ......................................................................................................................... 16
7.2.5 Ovalling ..................................................................................................................................... 20
7.2.6 Increase of wind effects by nearby structures ............................................................................ 21
7.2.7 Damping ratio ............................................................................................................................ 22
7.2.8 First and second natural frequencies .......................................................................................... 23
7.2.9 Passive dynamic control............................................................................................................. 23
7.2.10 Special chimney designs for damping ...................................................................................... 24
7.3 Earthquake loading ............................................................................................................. 24
7.4 Thermal Effects ................................................................................................................... 24
7.5 Explosions ........................................................................................................................... 24
7.5.1 External explosions .................................................................................................................... 24
7.5.2 Internal explosions ..................................................................................................................... 24
7.6 Internal Effects governing the Chimney Design ................................................................. 24
7.6.1 High temperature flue gases ....................................................................................................... 24
7.6.2 Fire ............................................................................................................................................. 25
7.6.3 Chemical effects......................................................................................................................... 25
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CICIND Steel Chimney Model Code
8. DESIGN OF STRUCTURAL SHELL .................................................................................. 26
8.1 Minimum Thickness ........................................................................................................... 26
8.2 Required Checks ................................................................................................................. 26
8.3 Carrying Capacity of Shell ................................................................................................. 26
8.3.1 Load factors and load combinations ........................................................................................... 26
8.3.2 Second order effects ................................................................................................................... 27
8.3.3 Biaxial stresses ........................................................................................................................... 27
8.3.4 Stability ...................................................................................................................................... 27
8.4 Serviceability of Shell ........................................................................................................ 29
8.5 Fatigue check ...................................................................................................................... 29
8.5.1 Basic principles ........................................................................................................................... 29
8.5.2 Fatigue strength ........................................................................................................................... 29
8.5.3 Influence of high temperatures ................................................................................................... 31
8.6 Allowance for Corrosion .................................................................................................... 31
8.6.1 External corrosion allowance ..................................................................................................... 36
8.6.2. Internal corrosion allowance ...................................................................................................... 36
9. DESIGN DETAILS ................................................................................................................. 37
9.1 Connections ........................................................................................................................ 37
9.1.1 General provisions ..................................................................................................................... 37
9.1.2 Bolted connections ..................................................................................................................... 37
9.1.3 Welded connections ................................................................................................................... 39
9.2 Flanged Connections .......................................................................................................... 40
9.3 The support at the Base ...................................................................................................... 41
9.3.1 Anchor bolts ............................................................................................................................... 41
9.3.2 Grouting ..................................................................................................................................... 41
9.3.3 Temperature effects .................................................................................................................... 41
10. STEEL LINERS .................................................................................................................... 41
11. CONSTRUCTION ................................................................................................................ 41
11.1 General.............................................................................................................................. 41
11.2 Structural Shell ................................................................................................................. 42
11.3 Structural Flanges and Opening Reinforcement ............................................................... 42
11.4 Stiffening Rings ................................................................................................................ 42
11.5 Base Plate ......................................................................................................................... 42
11.6 Straightness....................................................................................................................... 42
11.7 Erection Tolerance ............................................................................................................ 42
12. SURFACE PROTECTION .................................................................................................. 42
13. OPENINGS............................................................................................................................ 42
14. GUYED AND STAYED CHIMNEYS ................................................................................ 43
14.1 Stayed Chimneys .............................................................................................................. 43
14.2 Guyed Chimneys .............................................................................................................. 43
15. PROTECTION AGAINST LIGHTNING .......................................................................... 43
16. ACCESS LADDERS ............................................................................................................. 43
17. AIRCRAFT WARNING LIGHTS ...................................................................................... 43
DISCLAIMER
This CICIND Model Code is presented to the best of the knowledge of its members as a guide only.
CIC1ND is not, nor are any of its members, to be held responsible for any failure alleged or proved to be
due to adherence to recommendations or acceptance of information published by a Model Code or in any
other way.
Office of the Secretary:
CICIND. Preussenstrasse 11, 40883 Ratingen, Germany
Telephone: +49 (0)2102 896 840 / Fax: +49 (0)2102 896 842
Email: secretary@cicind.org
Web: www.cicind.org
CICIND Steel Chimney Model Code
page 1
FOREWORD
When it was formed in 1973, the “Comité International des Cheminées Industrielles” (CICIND) adopted as
a major goal the harmonization of national codes for the design of industrial chimneys. As a means to this
end, a subcommittee was appointed in 1981, charged with drafting a proposal for a model code for steel
chimneys which reflected the current “state-of-the-art” and a consensus of views, internationally. This
document was published in 1988, with Commentaries being published the following year.
Since 1988, the science and technology of chimneys has advanced and in 1995, CIC1ND appointed a
committee to revise the Model Code, recognizing current best international practice and knowledge.
The 2010 revision of the Model Code refers to the wind loads, both in along-wind and across-wind
direction. The revision resulted from the findings, that the surface roughness of the chimney site and its
surroundings should be taken into account. Corresponding amendment had been introduced in the revised
Model Code.
The committee comprises:
J. Roberts
B.N. Pritchard
Max Beaumont
Michael Beaumont
G. Berger
J. Bouten
R. Ghermandi
S. Ole Hansen
G. Pinfold
R.M. Warren
Great Britain — Chairman until Jan. 1998
Great Britain — Chairman after Jan. 1998
Great Britain
Great Britain
Germany
The Netherlands
Italy
Denmark
Great Britain
USA
Expert advice was received from:
B.J. Vickery (Canada)
H. van Koten (The Netherlands) — Chairman from April 2005
The 2010 revision was initiated and directed by:
H. van Koten
G.K. Verboom
The Netherlands
The Netherlands
0. INTRODUCTION
0.1 General
Chimneys are required to carry vertically and discharge to the atmosphere, gaseous products of
combustion, chemical waste gases, or exhaust air or for the combustion (flaring off) of industrial waste
gases.
This Model Code contains guide-lines which reflect the current state of art in the design and construction of
steel chimneys. Nevertheless, the design, fabrication and erection of steel chimneys require a thorough
knowledge of these structures, the properties of the materials used, the actions occurring upon the structure
and the recognized rules of the relevant technologies. The design of steel chimneys should therefore only
be entrusted to appropriately qualified and experienced engineers. The construction and erection should be
carried out by firms competent in this class of work. At all times the work should be under the direction of
appropriately qualified supervisors.
CIC1ND will continue to try to improve the understanding of the behavior of chimneys. Further revisions
of this Model Code will therefore be published from time to time.
0.2 Appendices and Commentaries
This Model Code is accompanied by extensive appendices and commentaries. The appendices provide
information which the committee believes will be of use to a steel chimney designer, even though its
inclusion in a chimney design code could not be justified. The commentaries have the following objectives:
a)
Justification of the regulations of the Model Code.
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CICIND Steel Chimney Model Code
b) Simplification of the use of the Model Code.
c)
Understanding of the meaning of the regulations of the Model Code.
d) Documentation of the areas in the Model Code where the present knowledge is sparse so that
the regulations are possibly or probably not optimal.
The following items are not objectives of the CICIND commentaries:
e)
Change of the meaning of certain regulations of the Model Code where these are falsely expressed
or obviously wrong.
f)
Definition of the meaning of certain regulations of the Model Code which are so badly formulated
that they could easily be misinterpreted even by experts.
Certain information from the Model Code is repeated in the commentaries when this simplifies the
presentation of the ideas.
0.3 Philosophy
One of the main objectives of any code governing construction is the creation of a model which resembles
as far as possible, the real situation. The model should be sufficiently “safe, simple and true”. It is very
rarely that simplicity and truth are compatible, so a model must be used which provides an optimum
compromise between truth, simplicity, safety and economy.
While the judgments of ‘sufficiently true’ and ‘sufficiently simple’ are subjective, ‘sufficiently safe’ is
capable of rational judgment. This code interprets ‘sufficiently safe’ in terms of the social and economic
consequences of failure. It does this by comparing the probabilities of failure for given safety factors during
its design life with the failure probabilities required to satisfy accepted social and economic criteria. This
leads to the development of safety factors which ensure that a chimney will have a probability of failure
during its design lifetime between 10−3 and 10−4 , depending upon its reliability category.
CICIND has departed from generally accepted principles of steelwork design and construction only when
this was required by the philosophy outlined above or by specific chimney requirements.
1. SCOPE
This Model Code relates to the structural design and construction of steel chimneys of circular crosssection, with a minimum height of 15m, with or without linings, and to the design and application of linings
to such chimneys where required. It also relates to chimneys with a height less than 15m and a slenderness
ratio more than 16. The Model Code does not deal with architectural or thermal aspects of steel chimneys
nor with their foundations, except insofar as they affect the chimney’s structural design. The Model Code
does not deal with those aspects of the design and construction of steelwork, refractories and insulation
which are not peculiar to chimneys.
2. FIELD OF APPLICATION
The Model Code is valid for all steel chimneys of circular cross- section. However, the design rules have
been formulated for self supporting chimneys taller than 15m. For other chimneys simplification may be
acceptable. Additional information is given in the Appendices and Commentaries.
3. REFERENCES
[1]
“CICIND model code for concrete chimneys — Fart A, The Shell “, August 1998 CICIND,
Zurich, Switzerland.
[2]
“Eurocode 3.2: Design of Steel Chimneys” ENV 1993-3-2: 1997
[3]
Thom, H.C.S.: “Distribution of extreme winds over oceans” Journal of the Waterways, Harbors
and Coastal Engineering Division. Proc. of the American Society of Civil Engineers, February
1973.
[4]
Vickery, B.J: “Wind loads and design for chimneys “, CICIND REPORT, Vol. 14, No. 2, 1998
CICIND Steel Chimney Model Code
page 3
[5]
Eurocode 1 — Basis of Design and actions on structures — Fart 2—4: Actions on structures —
Wind Actions ENV 1991-2-4: 1995
[6]
Van Koten, H: “A calculation method for the fatigue life of steel chimneys subject to cross-wind
oscillations “, CICIND REPORT, Vol. 14, No. 2, 1998
[7]
Ruscheweyh, H.: “Experience with Vortex Excited Oscillations of Steel Chimneys “, CICIND
REPORT, Vol.11, No. 2, 1995
[8]
Ole Hansen, S: “Vortex — induced vibrations of line-like structures “, CICIND REPORT , Vol.
14, No. 2, 1998
[9]
Van Koten, H: ‘Structural damping”, HERON report no.4, 1977, Delft. The Netherlands
[10]
Berger, G : “Measured damping decrements of steel chimneys and their estimation taking account
of their type “, CICIND REPORT, Vol. 15, No. 1, 1999
[11]
Turner J.G.: “Wind load stresses in steel chimneys “, CICIND REPORT, Vol. 12, No. 2, 1996
[12]
Hirsch, G.& Jozsa, M.: “Optimum control of chimney vibration”, CICII’JD REPORT, Vol. 10,
No. 1,
[13]
Bierrum,N.R.: “Mis-tuned Mass Dampers”, CICII’JD REPORT, Vol. 10, No.2, 1994
[14]
Warren, R.M. & Reid, S.L. “Shell to Flue Impact Damping for Dual Wall and Multi-Flue
Chimneys” CICIND REPORT Vol. 10,N0. 1,1994
[15]
Ruscheweyh, H., Kammel, C. & Verwiebe, C. “Vibration Control by Passive Dampers CICND
REPORT Vol. 12, No. 2, 1996
[16]
Bunz, G., Diepenberg, H. and Rendie, A.: “Influence offuel oil characteristics and combustion
conditions of flue gas properties in W T boilers” Journal of the Institute of Fuel, Sept.1967
[17]
Lech and Lewandowski: ‘Prevention of cold end corrosion in industrial boilers” Corrosion,
March 1979, Atlanta, U.S.A.
[18]
Henseler, F.: “Desulphurisation Systems and their Effect on Operational Conditions in Chimneys
“, CICIND REPORT, Vol. 3, No. 2, 1987.
[19]
“CICIND chimney protective coatings manual”, CICIND, Zurich, Switzerland
[20]
Schulz, U.: “Die Stabilität axialer Zylinderschalen mit Mantelöffnungen”, Bauingenieur 5 1,1976.
[21]
‘European Recommendations for Steel Construction: Buckling of Cylinders” ECCS 1984
[22]
Bouwman, E.P.: “Bolted connections dynamically loaded in tension “. Proceedings ASCE,
Journal of the Structural Division, 59,1982.
[23]
“CICIND Model Code for Concrete Chimneys — Part C, Steel Liners “, December 1995
CIC1ND, Switzerland
4. NOTATIONS, UNITS AND DEFINITIONS
4.1 General
The following list shows only the principles by which the notations and their meanings are related. The
actual notations are mostly explained in the text.
Local factors
γ
load factor
Material properties
f
yield strength (MPa)
E
modulus of elasticity (GPa)
σ
stress (MPa)
Loadings
T
temperature in centigrade
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CICIND Steel Chimney Model Code
V
wind-speed (m/s)
w
wind-force (N/m)
Cross-sectional forces
M bending moment (Nm)
e eccentricity (m)
Dimensions
h
height (m)
z
height above ground level (m)
d
diameter (m)
t
wall thickness (m)
4.2 Subscripts-Superscripts
y
yield limit
k
characteristic value
*
stress multiplied by load factor
cr
critical
4.3 Units
Generally, the units of the SI system are used.
Examples:
•
m (meter) and mm (millimeter) for dimensions
•
MN (Mega Newton) and N (Newton) for forces
•
MPa for stresses
In those cases where other units are used, the relevant references are given.
4.4 Definitions
The common names of parts of a steel chimney are explained in commentary 1.
5. BASIS OF DESIGN AND SAFETY FACTORS
5.1 General
The design of sections subject to permanent load and wind loads in the wind direction is based upon
ultimate limit state conditions, the safety of the chimney being ensured by partial safety factors for loads
and material. The ultimate limit state considered is reached when any part of the section is at the limit
stress. The limit stress is defined as either yield stress or critical buckling stress (whichever is least),
divided by the material safety factor. The calculation of the stress distribution and the strength of the
sections shall therefore be made in accordance with the theory of elasticity.
The use of this procedure, combined with the partial safety factors listed below will ensure that low cycle
fatigue will not contribute to failure of the chimney. In the design of details such as flanges, ultimate limit
state may take account of plastic stress distribution
Safety in the case of response to vortex shedding is ensured by the use in the fatigue calculations of a
suitable Miner Number, a material factor and a modeling factor.
5.2 Reliability differentiation
Different levels of reliability shall be adopted for chimneys, depending on the possible economic and social
consequences of their failure.
CICIND Steel Chimney Model Code
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Two classes of reliability related to the consequences of structural failure are used — Normal and Critical,
as defined below. The choice of reliability category shall be decided by the chimney owner and relevant
statutory authorities. Most chimneys will, however, be regarded as of Normal reliability.
Critical chimneys: Chimneys erected in strategic locations, such as nuclear power plants or in densely
populated urban locations. Major chimneys in industrial sites where the economic and/or social
consequences of their failure would be very high.
Normal Chimneys: All normal chimneys at industrial sites or other locations. (Typically chimneys in
industrial sites, power plants or chimneys less than 100m tall in urban locations, where any domestic
dwelling is outside the falling radius of the chimney).
5.3 Partial Safety Factors
Material safety factor for steel
Load factors for:
1.1
Normal Chimneys
Permanent load
Guy rope pretension
Wind load in wind direction (temperate zones)
Wind load in wind direction (tropical storm zones)*
1.1
1.2
1.4
1.5
Critical Chimneys
Permanent load
Guy rope pretension
Wind load in wind direction (temperate zone)
Wind load in wind direction (tropical storm zones)*
1.1
1.2
1.5
1.6
* See literature (e.g. ref.(3)).
5.4 Cross-wind Effects
Chimneys shall be designed to avoid movements across the wind direction sufficient to cause failure or
fatigue damage or to alarm bystanders.
The code contains means of estimating the amplitude of movement and consequent stress range due to
crosswind loading. Limiting stress ranges are given for various weld classifications and design lives. In
addition to a material safety factor 1.1, applied to fatigue category, a modeling factor of 1.4 shall be applied
to the Miner Number derived in fatigue calculations for temperatures up to 200°C and 1.5 for temperatures
between 200°C and 400°C.
To avoid alarming personnel, the maximum permitted amplitude of oscillations due to cross-wind effects or
aerodynamic interference shall be agreed between the owner and designer. This limit will be governed by
the prominence and visibility of the chimney and the anticipated frequency of recurrence of excessive
movements. Guidance is given in Commentary 3.
6. MATERIALS
6.1 General
The materials generally used for steel chimneys are described in the
CICIND METALLIC MATERIALS MANUAL.
Table 6.1 shown below is a copy of Table 8.1 of this manual.
Special steels can be used provided they are precisely specified and their characteristics, such as yield
stress, tensile strength, ductility and weldability, enable the Model Code to be put into application.
6.2 Structural Steels
The mechanical properties and the chemical composition of structural steels shall comply with local
national standards.
The limit stresses of steel are equal to the yield stress of the steel used, divided by the material factor 1.1:
page 6
fk =
CICIND Steel Chimney Model Code
fy
(6.1)
1.1
Characteristic values of the yield stress of structural and high alloy steels at ambient and higher
temperatures are given in Table 6.1.
At ambient temperatures, calculations shall be based on following properties of carbon steel:
= 8,000 kg/m3
= 7,850 kg/m3
Density: γ s
(wide flats and plates)
(other steel products)
Modulus of elasticity E (tension, compression, bending) = 210,000 N/mm2
Shear modulus: G
= 81,000 N/mm2
Poisson ratio: ν
= 0.3
Coefficient of thermal expansion: α
= 1.2 ⋅10−5 / o C .
Characteristic values of the modulus of elasticity at temperatures up to 600°C are given in Tables 6.2,
and characteristic values of the thermal expansion coefficients are given in Table 6.3.
Values at intermediate temperatures can be obtained by linear interpolation.
Type of steel
Temperature of the material up to oC
EN 10025-2
EN 10025-5
EN 10028
EN 10088
EN 10095
a
20
150
250
350
450
500
2.1
2.05
2.0
1.92
2.1
2.05
2.0
1.92
1.84
1.8
1.7
2.0a
1.64
1.94a
1.56
1.86a
1.49
1.79a
1.42
1.72a
1.385
1.685a
550
600
1.35
1.65a
1.315
1.615a
for calculating compression stresses
Table 6.2 — Characteristic values of E-Modulus (105 N/mm2) as a function of temperature.
Type of steel
Temperature of the material up to oC
100
12.0
200
12.1
300
12.9
400
13.5
500
EN 10025-2
12.0
11.1
16.0
12.1
12.1
16.5
17.0
12.9
12.9
17.0
13.5
13.5
17.5
18.0
13.9
13.9
18.0
EN 10025-5
EN 10028
EN 10088
EN 10095
600
18.5
700
800
19.0
Table 6.3 — Characteristic values of thermal expansion coefficient ( 10−6 K −1 ) as a function of
temperature.
CICIND Steel Chimney Model Code
Table 6.1 — Characteristic values of yield stress fy (N/mm2) as a function of temperature.
page 7
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CICIND Steel Chimney Model Code
6.3 Stainless and Alloy Steels
When metal temperatures are expected to exceed 400°C, stainless or alloy steels should be used.
Ordinary stainless steels (including high molybdenum stainless steel) have poor corrosion resistance in the
presence of condensing sulphuric or other acids in the range of concentrations and temperatures normally
found within chimneys. These materials are therefore not recommended in chimneys burning fuels
containing sulphur under conditions of “medium” or “high” chemical load, see paragraph 7.6.3.
When metal temperatures and condensate sulphuric acid concentrations are expected to be less than 65°C
and 5% respectively, the corrosion rates of high molybdenum stainless steels, such as ASTM Type 316L,
are acceptable. Such conditions can be expected at the top (over a height of about 3 diameters) of any
chimney handling high sulphur flue gases.
Note: The conditions downstream of a flue gas scrubber or the presence of chlorides in the condensate will
radically increase the corrosion rate, possibly rendering these stainless steels unsuitable for these
applications.
Ordinary stainless steels are not suitable for use in contact with flue gases containing alkalis.
In cases where it is not possible to avoid high chemical load on the internal face of the structural shell, see
paragraph 7.6.3, the use of a protective coating may be considered (see ref. [19]). Alternatively, a steel liner
or liners, possibly of titanium or high nickel alloy, is a possible solution. See section 10 on Steel Liners.
Low copper alloy steels have good resistance to atmospheric corrosion, except in a marine environment or
other environment where chlorides are present. These steels also show some corrosion improvement over
carbon steel when in contact with flue gases where acid condensation of SO2/SO3 (not of HCL
condensation) is intermittent only (e.g. during shutdowns of a stack in intermittent service, its metal
temperature being normally above acid dew point).
When the metal temperature is below acid dew point for prolonged periods, the performance of low copper
alloy steels in contact with flue gases is similar to that of carbon steel.
Where stainless or alloy steel components are connected to carbon steel, bolted connections are preferred.
In order to avoid accelerated corrosion due to galvanic action, such connections should include insulating
gaskets. Welded connections are permitted, provided specialist metallurgical control is exercised with
regard to weld procedures, electrode selection, etc. Care should be taken to use the correct coefficient of
expansion for the grade and temperature of the steel being considered.
7. ACTIONS (EXTERNAL AND INTERNAL)
7.1 Permanent Load
The permanent load shall include the weight of all permanent constructions, fittings, linings, flues,
insulation, present and future loads including corrosion allowance.
7.1.1 Dust load (temporary load)
On some process plants there can be a carryover of ash or dust burden. This may adhere to the interior
surface of the structural shell or liner and cause an additional dead load. Such cases should be investigated
at the design stage; the calculated load shall be added to the permanent load calculated in 7.1 above.
7. 2 Wind
7.2.1 General
The wind load on a chimney depends in the first instance upon the magnitude of the wind speeds in the area
in which the chimney is to be erected and their variation with height. Apart from that the wind loads, in the
direction of the wind or perpendicular to that direction, will be influenced by some or all of the following:
a)
local topography
b) level of turbulence
c) presence of nearby structures, including chimneys
d) air density
CICIND Steel Chimney Model Code
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e) value of the drag coefficient (shape factor)
f)
values of the natural frequencies of oscillation
g) amount of structural damping and mass present
h) configuration of the first few mode shapes
i)
effect of ladders, platforms, pipes etc.
7.2.2 Wind speed
7.2.2.1 Basic wind speed
The determination of the effective wind pressure is based on the basic wind speed.
The basic wind speed Vb ,appropriate to the location where the chimney is to be erected, is defined as
follows: It is the 10-minute mean wind speed, measured 10m above ground level in open flat country,
without obstructions, at the chimney location, which occurs once on average every 50 years.
The value of the basic wind speed must be taken from meteorological measurements. An indication of
values of the basic wind speeds for various countries may be obtained from the Commentary No. 3.
Where the terrain of the location of the chimney is hilly or built-up, measurements for the determination of
Vb should be taken as near as possible at a place which is flat and open. However, in some very hilly areas,
where flat ground is rare, Vb is sometimes measured at the chimney location and includes the
“Topographical factor“.
7.2.2.2 Design wind speed
The basis for the determination of the wind loads is the design wind speed which equals the basic wind
speed corrected by two factors taking into consideration the height of the chimney and the topography of its
surroundings. These two factors are: the height factor k(z) and the topographical factor kt.
The design wind speed is determined by the following expression:
V(z) = k(z) ⋅ k t ⋅ Vb
(7.1)
where:
V(z) = 10-minute mean design wind speed at elevation z (m/s)
z
= height above ground level (m)
k(z)
= height factor defined in expression (7.2)
kt
= topographical factor, see 7.2.2.3
 z 
k(z) = a ⋅  
 10 
= k(z min )
b
for z ≥ z min
(7.2)
for z < z min
z min = height below which the wind speed (and turbulence intensity) is assumed to remain the same,
see Table 7.1
Vb
= basic 10-minute mean wind speed (m/s)
The height factor depends on the terrain roughness of the area in which the chimney is to be erected. The
Model Code distinguishes three terrain categories given in Table 7.1.
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CICIND Steel Chimney Model Code
Terrain category
z0 (m)
zmin (m)
I
Sea, lakes or flat and horizontal area with negligible vegetation and without
obstacles
0.01
2
II
Area with low vegetation such as grass and isolated obstacles (trees,
buildings) with separations of at least 20 obstacle heights
0.05
4
III
Area with regular cover of vegetation or buildings or with isolated obstacles
with separations of maximum 20 obstacles heights (industrial area, suburban
terrain, permanent forest)
0.3
8
Table 7.1 ─ Terrain categories.
Notes:
1) No separate terrain category is defined for town areas.
Contrary to buildings most chimneys are located near the town border for which (in most cases)
the smoother terrain category should be used. If the chimney is located far inside a town, typically
10km or more, specialist advice is recommended.
2) The maximum application height of expression (7.1) is 300m.
3) If the suitability of a different formulation of the height factor can be proved (together with an
appropriate formulation of the turbulence intensity, expression (7.3)), it may be used (see
Commentary C3.1.3).
The exposure coefficients a, b and c, used in expressions (7.2) and (7.3) are given in Table 7.2.
Terrain
I
II
III
a
1.18
1.00
0.77
b
0.12
0.16
0.22
c
0.14
0.19
0.28
Table 7.2 ─ Exposure coefficients.
The transition between terrain categories has to be considered when selecting the value of z0 . If the
upstream distance to a terrain with lower roughness is smaller than the larger of 5km or 100h, where h is
the height of the chimney, then the lower value of the terrain roughness should be used.
For terrains with a wind direction dependent roughness the (mean) terrain roughness can be determined by
considering the terrain roughness in angular sectors of say 30o multiplied by an appropriate direction factor
derived from the local directional wind statistics. When there is a choice between two or more terrain
categories in a given sector, then the lowest terrain category should be used.
The turbulence intensity, used in the formulation of the gust factor, see 7.2.3.3, and in the formulation of
cross-wind load, see 7.2.4, is given by:
−b
 z 
Iν (z) = c ⋅  
 10 
= Iν (z min )
for z ≥ z min
for z < z min
,
(7.3)
CICIND Steel Chimney Model Code
page 11
where the coefficients b and c are defined in Table 7.2 and z min in Table 7.1.
7.2.2.3 Influence of topography
Clause 7.2.2.2 requires the determination of a topographical factor kt to account for the increase of mean
wind speed over hills and escarpments in otherwise relatively flat terrain (i.e. it is not for use in
mountainous regions). It should be considered for locations closer than half of the length of the hill slope
from the crest or 1.5 times the height of the cliff.
For certain topographical situations, a method for the determination of kt is given in the following
expression:
k t = 1 + 0.6 ⋅ s
= 1+ 2 ⋅ s ⋅ Φ
=1
for Φ > 0.3
for 0.05 ≤ Φ ≤ 0.3
for Φ < 0.05
(7.4)
where:
Φ
= upwind slope H/L in the wind direction, see Figures 7.1 & 7.2
s
= factor obtained from Figures 7.1 & 7.2
H
= height of hill or escarpment
x
= distance of chimney from crest
z
= height of considered position in chimney
Le = effective length of the upwind slope, defined in Table 7.3
Lu = actual length of upwind slope in the wind direction
Ld = actual length of downwind slope in wind direction
Shallow slope (0.05 ≤ Φ ≤ 0.3)
Steep slope (Φ > 0.3)
Le = Lu
Le = H / 0.3
Table 7.3 ─ Values of Le.
page 12
CICIND Steel Chimney Model Code
Figure 7.1 ─ Factor "s" for cliffs and escarpments.
Figure 7.2 ─ Factor "s" for hills and ridges.
CICIND Steel Chimney Model Code
page 13
7.2.3 Wind load in the direction of the wind
7.2.3.1 Wind load on isolated chimneys
For group interference effects, see 7.2.6.
The design wind load w(z) per unit height is determined by the following expression:
w(z) = w m (z) + w g (z)
(7.5)
where:
wm(z) = 10-minute mean wind load per unit height, see 7.2.3.2.1
w g (z) = static equivalent of the wind load per unit height due to gusts, see 7.2.3.3.1
7.2.3.2 Mean wind load
7.2.3.2.1 Main formula
The 10-minute mean wind load per unit height (N/m) is given by:
w m (z) = 0.5 ⋅ρa ⋅ V 2 (z) ⋅ CD ⋅ d(z) for z > z min
= w m (z min )
(7.6)
for z ≤ z min
where:
ρa
= density of air, see 7.2.3.2.2 (kg/m3)
V(z) = design wind speed at height z, see 7.2.2.2 (m/s)
CD
= shape factor, see 7.2.3.2.3
d(z)
= outside diameter of the chimney at height z (m)
7.2.3.2.2 Air density
At sea level in temperate climates, the density of air ρa is to be taken as:
ρa = 1.25 (kg/m3)
(7.7)
Momentary variations in the density due to atmospheric changes need not be taken into account.
The air density relevant to a chimney site at an altitude h1 (m) can be found from the expression:
ρa = 1.25 −
h1
(kg/m3)
8000
(7.8)
7.2.3.2.3 Shape factor
The shape factor CD depends on the Reynolds number of the chimney and two correction factors taking into
consideration the interference of nearby structures and the end-effect for chimneys with a small aspect
ratio:
CD = k i ⋅ k a ⋅ CD,0 ,
(7.9)
in which:
ki
= interference factor accounting for the influence of nearby structures, see 7.2.6.1;
ka
= end-effect factor for chimneys with an aspect ratio below 20, see expression (7.12);
C D,0 = basic shape factor as a function of the Reynolds number and reflecting the influence of
turbulence and the surface roughness of the stack, see expression (7.11) and Figure 7.3.
The basic shape factor is given by, see Figure 7.3:
page 14
CICIND Steel Chimney Model Code
for Re < 2.5 ⋅105
CD,0 = 1.2
= 1.2 − 3.42 ⋅ {log(Re) − 5.40}
for 2.5 ⋅105 ≤ Re ≤ 3.5 ⋅105 .
= 0.7
for Re > 3.5 ⋅105
(7.10)
Reynolds number Re is defined by:
Re = 6.9 l04 V d,
(7.11)
in which V = V(z) is the design wind speed at the top of the chimney in (m/s) and d is the diameter in (m).
For chimneys with helical vanes CD = 1.4. CD is applied to the outer diameter of the chimney in the vaned
portion and not the outer dimension of the vanes.
For attachments, including ladders, etc., the area presented to the wind for each member must use a force
coefficient of 1.2 for circular members and 2.0 for structural shapes. Typical lengths and widths of ladder
members have been taken into account.
Figure 7.3 ─ Basic shape factor as a function of Reynolds number.
The (basic) shape factor is measured in wind tunnel experiments. Most experiments are so-called 2D, i.e.
the cylinder is extending over the full width or height of the wind tunnel. In this arrangement the influence
of the flow near the free end of a cantilevered cylinder on the shape factor is absent. The resulting mean
shape factor (averaged over the height) of a cantilevered cylinder is smaller than its 2D-value. This
reduction decreases for larger aspect ratios. This reduction, called the end effect factor is given by
expression (7.12) and Figure 7.4:
h
k a = 0.60 + 0.129 ⋅ log  
d
for 1 ≤
h
≤6
d
 h

= 0.70 + 0.574 ⋅ log   − 0.778
d
  

for 6 ≤
=1
for
h
≤ 20
d
h
> 20
d
(7.12)
CICIND Steel Chimney Model Code
page 15
Figure 7.4 ─ End-effect factor as a function of the aspect ratio.
7.2.3.3 Static equivalent of the wind load due to gusts
7.2.3.3.1 Main formula
The static equivalent of the wind load due to gusts is assumed to vary linearly with the height. This causes
an increase of the bending moment above the base compared with the normal gust-loading method.
The wind load due to gusts can be determined by:
h
w g (z) =
3 ⋅ (G − 1) z
⋅ ⋅ ∫ w m (z) ⋅ z ⋅ dz
h 0
h2
(7.13)
where:
G
= gust factor, see 7.2.3.3.2
h
= height of the chimney above ground level
z
= height above ground level
w m (z) = 10-minute mean wind load per unit height at height z, see 7.2.3.2.1
7.2.3.3.2 Gust factor
The gust factor G is given by:
G = 1 + 2 ⋅ g ⋅ Iν (zs ) ⋅ B2 + R 2
(7.14)
where:
2 ⋅ log e (νT) +
g = peak factor =
νT = 600 ⋅ f1 ⋅
R2
2
B + R2
0.577
2 ⋅ log e (νT)
; νT ≥ 48
(7.15)
(7.16)
Iν (zs ) = turbulence intensity at zs = 0.6 ⋅ h , see 7.2.2.2
B2 = background factor =
1
 b+h 
1 + 0.9 ⋅ 

 L ( zs ) 
0.63
(7.17)
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CICIND Steel Chimney Model Code
R 2 = resonance response factor =
SL = power spectral density =
π
⋅ SL ⋅ R h ⋅ R d
4⋅ζ
(7.18)
6.8 ⋅ f L
(7.19)
(1 + 10.2 ⋅ fL )5/3
f ⋅ L(zs )
fL = 1
V(z s )
(7.20)
β
 z 
L ( z ) = turbulent length scale = 300 ⋅ 

 300 
for z min ≤ z ≤ 300m
(7.21)
β
z 
L ( z ) = 300 ⋅  min 
 300 
for z < z min
The exponent β depends on the terrain category and is given in Table 7.4.
Terrain
I
II
III
β
0.13
0.26
0.37
Table 7.4 ─ Exponent β .
Rx =
1
1
−
⋅ {1 − exp ( −2 ⋅ ηx )} ;
ηx 2 ⋅ η2x
Rx = 1
for ηx = 0
(7.22)
with
ηx =
4.6 ⋅ f L
⋅x
L(zs )
for x = d or x = h
(7.23)
ζ = structural plus aerodynamic damping (plus the damping due to an external damping device if
applied) expressed as a fraction of critical damping, see 7.2.7
f1 = natural frequency of the chimney oscillating in its first mode (s-1)
h = height of chimney (m)
7.2.4 Vortex shedding
7.2.4.1 General principals
Vortex-shedding occurs when vortices are shed alternately from opposite sides of the chimney. This gives
rise to a fluctuating load perpendicular to the wind direction. The frequency fs at which vortices are shed is
related to the diameter d and the 10-minute mean design wind speed at the top of the chimney V(h) by the
expression:
St =
fs ⋅ d
,
V(h)
(7.24)
where St = Strouhal number = 0.2
Large structural vibrations may occur if the frequency of vortex-shedding is close to one of the natural
frequencies (fn , n=1,2,…) of the chimney. The wind speed at which the shedding frequency is equal to a
natural frequency is called the critical wind speed ( Vcr ) which is given by the expression:
Vcr,n =
fn ⋅ d
St
where the index n refers to the first, second or higher natural frequency (mode).
(7.25)
CICIND Steel Chimney Model Code
page 17
In most cases the structural movements are dominated by the first natural frequency, but the second and
sometimes even the third natural frequency can be important.
The structural movements can substantially be increased by nearby chimneys in a group or row
arrangement. The influence is discussed in Section 7.2.6.
The cross-wind movements depend strongly on the mass and the damping of the chimney and on the
motion induced aerodynamic forces. The latter may generate a negative aerodynamic damping which
opposes the positive structural damping. Excessively large cross-wind movements occur when the total
damping is close to zero. The cross-wind movements are to a large extent determined by the ratio of two
dimensionless parameters, i.e. the Scruton number Sc and the aerodynamic damping parameter K a . Both
parameters depend on the mode shapes associated with the natural frequencies.
The cross-wind movements are small and (mainly) determined by the classical lift force when
Sc n
4 ⋅ π ⋅ K a,n
1
(7.26)
where Sc n and K a,n are given by, respectively, expression (7.34) and (7.36).
They are large and determined by the motion induced aerodynamic forces when this ratio is smaller than
one.
Cross-wind vibrations may cause fatigue. Fatigue is determined by a combination of the number of load
cycles during the operational life time of the chimney and the stress range occurring; see Section 8.5 for a
detailed discussion.
The effect of vortex shedding need not be investigated when:
Vcr,n > 1.25 ⋅ V(h)
(7.27)
where V(h) is the design wind speed at the top of the chimney.
7.2.4.2 Estimation of top amplitudes
The method described in this section for estimating amplitudes depends upon parameters such as structural
damping, atmospheric turbulence and aerodynamic damping due to the movements of the chimney, the
values of which are not known with certainty. The results of the computation should therefore be treated
with care. A sensitivity analysis of these parameters on the results is suggested to gain insight into which
parameters are critical for the design.
The method to determine the cross-wind response of the chimney closely resembles the model formulation
developed by Vickery and Basu (see ref. [4]). The expressions given in this section apply to single free
standing cylindrical chimneys, including those with minor changes of diameter in the top third of their
height. They can be used to estimate the maximum cross-wind deflection and stress at the critical wind
speed. For chimneys with a more complex geometry the full expressions in Commentary 3 should be used.
For a detailed fatigue analysis the response of the chimney as a function of the wind speed is required and
the expressions given in Commentaries 3 and 4 should be used.
The maximum single amplitude (0 to max) of a point at height z, of vibrations at the critical wind speed in
the nth mode is given by:
y n (z) = k p,n ⋅ σ y,n ⋅ u n (z)
(7.28)
where
2
σ y,n = d1 ⋅ c1,n + c1,n
+c2,n = nth mode standard deviation of deflection
(7.29)
4


 Scn
 


  = nth mode peak factor
k p,n = 2 ⋅ 1 + 1.2 ⋅ arctan 0.75 ⋅ 
 4 ⋅ π ⋅ K   


a,n

 


(7.30)
u n (z) = mode shape of nth natural frequency, with u n (z) = 1 at the point of maximum deflection.
For the first natural frequency a parabolic mode shape may be used, see Remark below.
page 18
CICIND Steel Chimney Model Code


Scn
2
c1,n = 0.5 ⋅ a L,n
⋅ 1
 4⋅π⋅K 
a,n 

c 2,n =
(7.31)
a 2L,n ⋅ Ca,n
(7.32)
K a,n
Ca,n is defined in expression (7.39)
a L,n =
Sc n =
0.4 ⋅ f1
= limiting deflection amplitude as a fraction of d1
fn
4 ⋅ π ⋅ m o,n ⋅ ζ s
ρa ⋅ d12
= Scruton number of mode n
(7.33)
(7.34)
ρa = air density , see 7.2.3.2.2
d1 = outside diameter (averaged over the top third)
h
∫ m(z) ⋅ u n (z) ⋅ dz
2
m o,n = 0
= equivalent mass per unit length of mode n
h
∫
(7.35)
u 2n (z) ⋅ dz
0
m(z) = mass per unit length
c
= structural damping ratio, see Table 7.4
ccr
ζs =
K a,n = K a,max,n ⋅ 1 − 3 ⋅ Iν 
for 0 ≤ Iν ≤ 0.25
= K a,max,n ⋅ 0.25
for Iν > 0.25
(7.36)
= aerodynamic damping parameter of mode n
with
K a,max,n = 2.8
for Ren ≤ 2 ⋅105
= 0.9
for Re n > 5 ⋅105
(7.37)
Values at an intermediate Reynolds number can be obtained by (linear-logarithmic) interpolation.
Iν = turbulence intensity at the top of the stack, see 7.2.2.2.
k t = topographic factor, see 7.2.2.3
The turbulence intensity given by expression (7.3) is for equilibrium meteorological conditions. Under
certain meteorological conditions, giving very cold and stable air, the turbulence intensity can be
suppressed to a much lower value. If this situation can occur it is recommended investigating the
sensitivity of the cross-wind deflection for zero turbulence intensity. Typically, this is relevant for
critical wind speeds of up to 10 m/s.
Re n = 6.7 ⋅104 ⋅ Vcr,n ⋅ d1 = Reynolds number of mode n
Ca,n =
1.42 ⋅10−4 ⋅ρa ⋅ λ ⋅ d13 ⋅ C 2L,n
h
St ⋅ m o,n ⋅ B ⋅ ∫
4
u 2n (z) ⋅ dz
0
λ = load correlation length in diameters d1 ; the recommended value is 1.0
(7.38)
(7.39)
CICIND Steel Chimney Model Code
page 19
B = 0.1 + I ν , but ≤ 0.35 = width of the lift spectrum
(7.40)
CL,n = RMS lift coefficient
= 0.7
for Ren ≤ 2 ⋅105
= 0.2
5
(7.41)
for Ren ≥ 5 ⋅10
Values at an intermediate Reynolds number can be obtained by (linear - logarithmic) interpolation.
Notes:
1) The natural frequencies and the associated mode shapes should preferably be determined by a FE
or similar computer program, taking into account the interaction of the chimney and its foundation
and/or support structure. This influence can be large for elastic foundation structures. A parabolic
mode shape for the first natural frequency is only valid for a stiff foundation; for a sufficiently
flexible foundation or support, the chimney shaft’s mode shape can reduce to a straight line.
2) The formulations given in this section to determine the cross-wind amplitudes are approximations
of more complex expressions given in Commentary C3.3. The approximations are accurate except
for a small range where expression (7.31) changes sign, i.e. for
Sc ≈ 4 ⋅ π ⋅ K a
(7.42)
The difference between the approximations and the full expressions can increase the deflections
by up to 25% or more. Though the deflection itself might be small.
3) It is recommended to apply the full expressions of Commentary C3.3 if expression (7.42) applies.
7.2.4.3 Bending moments due to vortex shedding
In deriving the bending moment associated with the maximum response amplitude of a chimney due to
vortex shedding, the associated inertial force per unit length ( Fn (z) ) may be used:
2
Fn (z) = ( 2 ⋅ π ⋅ f n ) ⋅ m(z) ⋅ y n (z)
(7.43)
7.2.4.4 Chimneys with variable diameter
The expressions given in Section 7.2.4.2 apply to chimneys with a nearly constant diameter. Vortex
shedding occurs in a specific range of wind speeds just below and above the critical wind speed of the
respective mode of vibration.
The critical wind speeds are proportional to the diameter of the chimney; so chimneys with a variable
diameter may show vortex shedding in a specific mode at different levels and wind speeds.
To analyze the cross-wind motion due to the vortex excitations at these levels and ranges of wind velocity
one can make various assumptions such as the extent over which vortex shedding occurs in relation to the
external diameter and apply the expressions of Section 7.2.4.2 to these sections of the stack.
Instead of making these assumptions which are difficult to validate it is recommended to apply the full
expressions given in Commentary 3 and determine the response of the chimney for each vibration mode
over a range of wind velocities given by:
V = between 0.75 ⋅ Vcr,n and 2.0 ⋅ Vcr,n .
The standard deviation of the deflection, expression (7.25), as a function of the wind velocity is used to
compute the partial and the total Palmgren-Miner sum in a fatigue analysis.
page 20
CICIND Steel Chimney Model Code
7.2.5 Ovalling
In most cases, a suitably sized stiffening ring at the top of a chimney will eliminate problems associated
with ovalling.
7.2.5.1 Static effect
The uneven wind pressure distribution around the circumference of a circular cylinder causes bending
moments acting on vertical cross- sections of the shaft. The bending moments have a maximum value of:
M(z) = 0.08 ⋅ w 5sec (z) ⋅ d 2 (z) (Nm/m)
(7.44)
where w 5sec is the wind pressure at height (z) averaged over 5 seconds (N/m).
Note: The assumption that a 5 sec gust wind speed at height z = 1.4·V(z) is safe at all heights.
7.2.5.2 Dynamic effect
Due to vortex excitation ovalling vibration of the shell can occur. These vibrations can be expected if the
frequency of the vortices:
f=
2 ⋅ V ⋅ St
d
(7.45)
coincides with an ovalling frequency of the shell.
The fundamental ovalling frequency of un-stiffened shells is determined by:
f o,1 =
0.5 ⋅ t
d
2
⋅
E
ρs
(7.46)
where:
E
= Young’s Modulus of the steel shell (Pa)
t
= average shell thickness over the top third (m)
d
= shell diameter (m)
ρs
= density of shell material (kg/m3)
Substituting typical values of E and ρs, the associated critical wind speed is then:
Vr = 6500 ⋅
t
(m/s)
d
(7.47)
These vibrations can be reduced sufficiently by stiffening rings. The distance between stiffeners L shall not
exceed the smaller of the following two expressions:
L
d
< 0.56
d
t
and
L 10.9
<
d z 0.06
(7.48)
The associated moment of inertia of the stiffening ring section (together with the participating length of
shell) about its centroid, see Figure 7.5, must be larger than:
I r ≥ 0.060 ⋅
p0 ⋅ d 4 ⋅ ( d + 6 ⋅ b r )
E⋅t
(m4)
(7.49)
where:
p0 = wind pressure = 0.5 ⋅ρa ⋅ V 2
b r = width of the stiffening ring
The participating length of the shell = d ⋅ t but its area must not exceed that of the stiffener ring, see
Figure 7.5.
CICIND Steel Chimney Model Code
page 21
Figure 7.5 ─ Stiffener.
7.2.6 Increase of wind effects by nearby structures
Interference effects, caused by the presence of a nearby structure upwind of a chimney, can increase the
chimney’s quasi static wind load in the wind direction, described in 7.2.3 and its response, normal to the
wind direction, described in 7.2.4. If the interfering structure is itself a chimney, its own response when
downwind of the new chimney should be checked.
7.2.6.1 Increase in along-wind load
For chimneys in a row arrangement the shape factor is smaller than for a single chimney for all spacing
distances. For a staggered configuration the shape factor of the down wind chimney can become negative
for small staggering and a distance below about 3d. The shape factor of the upwind chimney can be
substantially larger than for a single cylinder at these small distances. For the most unfavourable wind
direction the shape factor of the upwind chimney approaches or slightly exceeds the value of a single
chimney. For a side-by-side configuration the shape factor is always smaller than for a single chimney
except for a distance well below 2d.
For distances above 4d the recommended value of the interference factor in the Model Code for chimneys
in row arrangement is one, i.e.
ki = 1 .
When the distance is smaller than 4d the shape factor of the upwind chimney can be substantially larger
than for a single chimney. For these small distances expert advice is required.
7.2.6.2 Increase in cross-wind response
When an approximately cylindrical structure (e.g. another chimney) is upwind and within 15 diameters of a
chimney of similar or smaller height, aerodynamic “Wake Interference” effects can considerably increase
the downwind chimney’s cross-wind response (the diameter concerned being that of the interfering
structure). The increase is not yet fully understood, but is thought to be due to increases in both lift
coefficient and negative aerodynamic damping. Note that aerodynamic stabilizers (e.g. helical spoilers) are
ineffective in controlling response in cases of wake interference. For a spacing ratio (a/d) greater than 10,
the magnification factor kc, applied to the response amplitude, calculated per expression (7.24), may be
estimated as follows:
k c = 1.0
= 1.5
a
≥ 15
d
a
for = 10
d
for
for intermediate values linear interpolation is permitted.
(7.50)
page 22
CICIND Steel Chimney Model Code
For a spacing ratio (a/d) less than 10 there is a risk of very large increases in amplitude. In these
circumstances the chimney’s structural damping should be increased (e.g. by the use of a tuned mass
damper) to ensure that the Scruton Number exceeds 25. At this value of Scruton Number, the amplitude of
response is expected to be minimal.
The associated critical wind speed and value of “c2” in expression (7.28) increase with decreasing values of
a/d due to a reduction in the value of the Strouhal Number. This can be important in the design of a tuned
mass damper. Figure 7.6 shows the relationship between Strouhal Number and a/d.
Figure 7.6 ─ The reduction of the Strouhal Number
caused by aerodynamic interference.
When the interfering structure or chimney is less than 2 diameters away, “Interference Galloping” can
cause even greater increases in the chimney’s response. Probably the best solutions in this case would be
either to fit tuned mass dampers, or to connect structurally, the chimney to the interfering structure, using
an energy absorbing connection system.
7.2.7 Damping ratio
The structural damping ratio (ζs = c / ccr ) without aerodynamic damping is given in Table 7.5.
Type of chimney
Damping Ratio
Unlined, un-insulated
0.002
Unlined, externally insulated
0.003
Lined with refractory concrete
0.005
Lined with brickwork
0.015
Chimneys with steel liners3):
λ < 26
0.006
λ > 28
0.002
Coupled group
0.004
Chimney with tuned mass damper
(0.02min) see Appendix 2
Table 7.5 ─ Structural damping ratio.
CICIND Steel Chimney Model Code
page 23
Notes:
1) If rotation of foundation decreases the first natural frequency more than about 10% the foundation
is considered to be soft and the damping ratio may be increased by 0.0005.
2) λ = liner length / liner diameter
3) In order to ensure impact damping the gap between the liner and its restraint should not be greater
than 50mm.
The damping for wind loading in wind direction can be increased by the aerodynamic damping:
ζ a = 2.7 ⋅10−6 ⋅
V
f1 ⋅ t
(7.51)
in which:
V
= design wind speed V(z) at the top of the chimney for wind loading in wind direction, see 7.2.2.2
V
= 0 for cross-wind loading
f1
= fundamental natural frequency, see 7.2.8
t
= thickness of the wall in the top third. Where chimneys are lined, t = total mass per square meter
over the top third (kg/m2) divided by 7850 kg/m3
7.2.8 First and second natural frequencies
Accurate estimation of the natural frequencies requires the use of a finite element structural program with a
dynamic capability or other advanced computer program. Care must be taken to include for the effects of
any supporting structure and additional masses of ladders, platforms etc.. Assuming a straight chimney on a
rigid support and no additional masses, its first and second natural frequency can be calculated from:
f1,2 =
λ1,2
h
2
⋅
E ⋅ I -1
(s )
µ
(7.52)
in which:
µ
= mass of the shell per unit length (kg / m)
E
= Young’s Modulus (Pa)
I
= moment of inertia of cross section (m4)
λ1,2 = 1.875 and 4.694 for the first and second natural frequency
7.2.9 Passive dynamic control
Steel chimneys must be designed to suppress excessive cross-wind movement. Several options are
available to the designer.
7.2.9.1 Aerodynamic stabilizers
When a chimney stands alone, its cross-wind vibrations can usually be reduced by aerodynamic stabilizers.
The useful effect of three continuous helical vanes has been proved on many steel chimneys. The radial
width of the vanes must be 10% of the diameter. The pitch of the vanes should be 5D. The vanes must be
fitted over at least the upper 1/3 of the height. The extra wind drag due to the vanes must be considered (see
7.2.3.2.3). Aerodynamic stabilizers will not reduce the wind interference effects of nearby chimneys or
structures.
7.2.9.2 Damping devices
Damping devices are attached to a chimney to increase its structural damping, thereby significantly
reducing the cross-wind and along- wind vibrations, including the effects of aerodynamic interference by
other nearby towers or chimneys. Damping devices should be designed to avoid the need for their frequent
routine maintenance.
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CICIND Steel Chimney Model Code
Most such dampers are mounted near the top of the chimney. Because of their profile and small size, the
associated increase in wind drag is minimized. The use of damping devices, therefore, has been proved to
be beneficial in the design of steel chimneys and they can be safely retro-fitted without incurring significant
increase in wind drag loads.
Tuned mass dampers provide an extra mass, coupled to the chimney by an energy absorbing medium,
which absorbs the wind induced energy. Tuned mass dampers have proven effective in reducing selfgenerated along wind and cross-wind vibrations and also the effect of nearby chimneys or structures.
Other chimney damping devices such as hanging chains have also been successfully used.
7.2.10 Special chimney designs for damping
Wind tunnel tests, confirmed by analytical means and field experience, have allowed dual-wall and
multiflue chimneys to be designed using shell-to-shell impact damping, which otherwise would require
aerodynamic stabilizers or mass dampers (see ref. [14] & [15]).
Future special chimney designs and damping devices may prove effective in preventing excessive wind
induced vibrations. These should have been proven initially by wind tunnel tests and finally by field
experience before being universally adopted.
7.3 Earthquake loading
The stress due to wind loading on a steel chimney is usually more than the earthquake stress and,
consequently, normal steel chimneys can resist earthquakes with an intensity of up to modified Mercalli
scale 10 without serious damage. However, in cases where a heavy mass (e.g. a water tank or a heavy
lining) is fitted to the upper portion of the chimney, a special investigation must be made (tanks are outside
the scope of the Model Code). Guyed chimneys must also be subject to special investigation.
7.4 Thermal Effects
When a chimney is restrained from adopting a deformed shape in response to differential expansion,
bending stresses will be introduced in the shell. These deformations can be large when a single unlined
chimney carries flue gases from two or more sources at significantly different temperatures or if a single
side entry source introduces gases at very high temperatures. In addition, the resulting differential metal
temperature will introduce secondary thermal stresses. Typical cases of such restraint are to be found in
stayed and guyed chimneys. More information on the derivation of those stresses may be obtained from the
CICIND Model Code for Concrete Chimneys - Part C: Steel Liners.
7.5 Explosions
7.5.1 External explosions
The resistance of steel chimneys to external explosions is very high. If such explosions can occur in the
direct vicinity such that strengthening for this reason is required, it is outside the scope of this Model Code.
7.5.2 Internal explosions
Internal explosions can occur due to the ignition of soot or explosive gases in the chimney. They are not
normally a cause for concern in the design of a steel chimney. The
CICIND Model Code for Concrete Chimneys — Part B, Brickwork Linings
provides a reference for the likely magnitude of explosion overpressures.
7.6 Internal Effects governing the Chimney Design
7.6.1 High temperature flue gases
In the case of bare steel chimneys, having neither an internal liner nor external insulation, the metal
temperature can be assumed to be about midway between ambient air temperature and that of the flue gas
over the range of flue gas velocities between 5m/s and l5m/s. For flue gas velocities faster than 15m/s or
for steel stacks equipped with either a liner or external insulation, heat transfer calculations shall be made
to determine the maximum metal temperature of the structural shell. These calculations shall assume still
air and highest anticipated air temperature.
CICIND Steel Chimney Model Code
page 25
Consideration must be given to the effects of oxidation when the material being used is close to its
temperature limit. This is especially so with gas turbine exhausts, where levels of excess air can be greater
than those normally experienced. This problem may not be solved solely by an increase in corrosion
allowance as the environment may be polluted by the corrosion product. Expert advice should be sought on
the choice of suitable material.
7.6.2 Fire
The risk of a chimney fire should be assessed. Chimney fires can be caused by ignition of:
•
Unburned fuel carried over from the associated boiler or furnace.
•
Where the associated furnace is in petrochemical service, unburned hydrocarbon carryover
following a furnace tube rupture.
•
Soot, sulphur and other deposits.
During chimney fires, the radiant heat loss to atmosphere from a bare steel chimney is often sufficient to
maintain its temperature at a reasonable level. By contrast an externally insulated steel chimney or a bare
steel chimney close to a reflective surface will quickly buckle during a fire. In such cases, if the risk of
internal fire is significant, a refractory concrete internal liner should be installed to provide a degree of fire
protection. Typically, a castable refractory lining following the requirements of Appendix 3 will provide
sufficient fire protection for most situations.
7.6.3 Chemical effects
Limited exposure to acid corrosion conditions can be permitted in chimneys which, for most of the time,
are safe from chemical attack. Providing the flue gas does not contain significant concentrations of
halogens (see notes (4) & (5) below) the degree of chemical load is defined in Table 7.6.
Degree of
chemical load
Operating hours per year when temperature of the surface in contact with flue
gases is below estimated acid dew point +10°C
Low
< 25
Medium
25 - 100
High
> 100
Table 7.6 ─ Degree of chemical load for gases containing sulphur oxides.
Notes:
1) The operating hours in Table 7.5 are valid for an SO3 content of 15 ppm. For different values of
SO3 content, the hours given vary inversely with SO3 content. When the SO3 content is not known,
chimney design should be based upon a minimum SO3 content amounting to 2% of the SO2
content in the flue gas.
2) In assessing the number of hours during which a chimney is subject to chemical load, account
should be taken of start-up and shut-down periods when the flue gas temperature is below its acid
dew point.
3)
While a steel chimney may generally be at a temperature above acid dew point, care should be
taken to prevent small areas being subject to local cooling and therefore being at risk of localized
acid corrosion. Local cooling may be due to:
• air leaks
• fin cooling of flanges, spoilers or other attachments
• cooling through support points
• downdraft effects at top of the chimney
4) The presence of chlorides or fluorides in the flue gas condensate can radically increase corrosion
rates. Estimation of the corrosion rate in these circumstances depends upon a number of complex
factors and would require the advice of a corrosion expert in each individual case. However, in the
page 26
CICIND Steel Chimney Model Code
absence of such advice, provided the concentrations of HC1 < 30mg/m3 or of HF < 5mg/m3 and
if the operating time below acid dew point does not exceed 25 hours per year, the degree of
chemical load may be regarded as “low”.
5) Regardless of temperatures, chemical load shall be considered “high” if halogen concentrations
exceed the following limits:
• hydrogen fluoride: 0.025% by weight (300 mg/m3 at 20°C and 1bar pressure)
• elementary chlorine: 0.1% by weight (1300 mg/m3 at 20°C and 1 bar pressure)
• hydrogen chloride: 0.1% by weight (1300 mg/m3 at 20°C and 1 bar pressure)
6) Saturated flue gas conditions downstream a desulphurization system: “High” chemical load”.
8. DESIGN OF STRUCTURAL SHELL
8.1 Minimum Thickness
At the time of construction the minimum thickness of the shell of carbon steel chimneys shall be 5mm,
including the corrosion allowance.
8.2 Required Checks
The steel shell of a chimney shall be checked for:
•
carrying capacity
•
serviceability
•
fatigue (unless the chimney is fitted with an effective dynamic control)
The carrying capacity check shall prove that the forces resulting from the working loads multiplied by the
load factors do not exceed the resistance of the shell. The check should comprise both the strength and
stability proof. The calculations shall be carried out for the corroded thickness of the steel (without
corrosion allowance). The serviceability shall be checked under working loads without load factors
A fatigue check shall be carried out if movement due to shedding is expected (see 7.2.4).
For unstiffened chimneys with a ratio of L/R> 50 (where L height of chimney and R radius), stresses may
be safely calculated assuming beam theory, flexural stresses being added vectorially ovalling stresses. For
unstiffened chimneys (i.e. chimneys without stiffening rings or substantial flanged joints) having L/R < 50,
shell theory or finite element modeling should be used, considering flexural and ovalling stresses
simultaneously. This will lead to reduction in compression stress at the chimney base or immediately above
changes in chimney diameter, but will increase compression stresses elsewhere. Similarly, this will lead to
increases in tensile stresses at the base and immediately above a change in chimney diameter, which will be
important in deriving bolt tensions.
The increase in tensile stress in these regions may be approximated by the expression:-
tensile stress per shell theory
6 ⋅ R3
= 1 + 2
tensile stress per beam theory
L ⋅t
(8.1)
8.3 Carrying Capacity of Shell
8.3.1 Load factors and load combinations
The chimney shell shall be designed to resist stresses resulting from the weight of the chimney and the
effect of wind multiplied by the load factors γ:
γ i ⋅ σi = σ*i < f k
where:
σ*i
= stresses multiplied by load factors
fk
= limit stress of steel, see expression (6.1)
(8.2)
CICIND Steel Chimney Model Code
page 27
8.3.2 Second order effects
The effect of the displacement of the load application points due to deformations (second order effect) shall
be taken into consideration if the parameter β > 0.6, where:
β = h⋅
N
E⋅I
(8.3)
and
h
= height of the chimney (m)
N
= total axial load at the base of the shell (without load factor) (N)
E·I = stiffness of the cross section at the base of the chimney (Nm2)
The second order moment M1I is approximately determined from:
 β2 
M1I = M I ⋅ 1 + 

8 

(8.4)
where MI is the wind moment at any particular level.
This simplified approximation may only be used when β <0.8 and Nh / N < 0.1, where Nh is the design
value of the total vertical load at the top of the shell.
Note:
Expression (8.4) is not applicable to guyed chimneys.
8.3.3 Biaxial stresses
In areas subjected to biaxial stresses e.g. due to bending moments and ovalling, the carrying capacity check
shall be based on:
*2
*
*
*2
σ*2
≤ fk
x + σy − σx ⋅ σy + 3 ⋅ τ
Note:
(8.5)
The ovalling stresses are both negative and positive and the maximum value of expression (8.5)
occurs when σ*x and σ*y are of opposite signs.
8.3.4 Stability
The proof of stability of the shell is given if the critical buckling stress divided by 1.1 is greater than the
sum of longitudinal stresses due to bending and compression:
σ*N + σ*B ≤
σk
γm
(8.6)
where:
σ*N , σ*B = normal and bending compressive stress at ultimate limit state
γm
= material factor = 1.10
σk
= critical buckling stress
(
)
= 1 − 0.412 ⋅ λ1.2 ⋅ f y
= 0.75 ⋅
fy
λ2
for λ ≤ 2
(8.7)
for λ > 2
fy
= yield strength of steel at design temperature, see Table 6.2
λ
=
fy
α ⋅ σcr
(8.8)
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CICIND Steel Chimney Model Code
E⋅t
r
σcr
= critical elastic buckling stress = 0.605 ⋅
E
= Young’s modulus of steel at design temperature, see Table 6.3
t
= corroded plate thickness
r
= radius of the structural shell of the chimney at section considered
α
=
α N ⋅ σ*N + α B ⋅ σ*B
(8.9)
(8.10)
σ*N + σ*B
Figure 8.1 ─ Normal and bending stresses.
When imperfections w are smaller than 0.01 ⋅ l , see Figure 8.2:
αN =
0.83
1+
=
r
100 ⋅ t
0.7
r
0.1 +
100 ⋅ t
α B = 0.189 + 0.811 ⋅ α N
for
r
≤ 212
t
for
r
> 212
t
(8.11)
If the imperfections (w) are between 0.01⋅ l and 0.02 ⋅ l , see Figure 8.2, the above expressions may be
used if αl is substituted for α:
w 

α1 = α ⋅ 1.5 −

0.02 ⋅ l 

(8.12)
Imperfections (w) greater than 0.02 ⋅ l shall not be permitted. Stiffeners may be used to increase the shell’s
resistance to buckling. Guidance on the design of such stiffeners is given in:
CICIND Mode/ Code for Concrete Chimneys — Part C — Steel Liners.
Figure 8.2 ─ Shell imperfections.
CICIND Steel Chimney Model Code
page 29
8.4 Serviceability of Shell
The downwind deflection from the centerline of the structural shell under maximum design wind load must
be calculated and reported. As long as the carrying capacity stresses in the structural shell, or any liners, is
not exceeded, no limit is placed on downwind deflection.
So as not to alarm bystanders, the amplitude of deflection from the chimney centerline caused by vortex
shedding shall not be greater than the limit agreed per Section 5.4 of this Model Code.
8.5 Fatigue check
8.5.1 Basic principles
The fatigue check shall ascertain that the movement due to vortex shedding does not result in the initiation
and gradual propagation of cracks in the shell material, especially near welds, thus resulting finally in the
failure of a weakened section. The fatigue of the material depends essentially on:
•
the number of load cycles ( N n ) during the design life time of the chimney
•
the stress range ∆σ = (σmax − σmin )
•
the structural details
The influence of the grade of steel as well as that of the σ min / σ max ratio is neglected.
To prevent fatigue occurring the stress must be smaller than the cut-off limit in the S/N diagram, see Figure
8.3. Generally, this requires a large Scruton number, typically 15 or larger, see expression (7.22). This may
be the case in a multiflue chimney or a chimney fitted with a refractory liner. However, in the case of an
unlined chimney, without a damping device to significantly reduce the cross-wind vibrations, this may
require a large wall thickness making the design uneconomic.
It is recommended to attach a damping device to chimneys with a Scruton number less than 15 and to make
sure the stress range is smaller than the cut-off limit in the S/N diagram.
For existing chimneys the Palmgren-Miner sum and the expected lifetime of the chimney can be estimated
using the expressions given in the next section.
8.5.2 Fatigue strength
The number of load cycles in the cross-wind direction can be calculated from:
{
} ≥ 200 ⋅ T
N n = 3.15 ⋅107 ⋅ T ⋅ f n ⋅ exp(- V12 / Vo2 ) - exp(- V22 / Vo2 )
(8.13)
where:
T = design life time of the chimney in years
Vo = 0.2 ⋅ V(h) = standard deviation of the wind distribution function
(8.14)
V(h) = design wind speed at the top of the chimney
V1 and V2 are the lower and upper limit of the range of wind speeds in which cross-wind vibrations
occur.
This range is determined by the Scruton number, the aerodynamic damping parameter and the
turbulence intensity. A first order estimate is found from Figure C3.12 by the crossings of the
horizontal line given by K a,0 and the line for the respective turbulence intensity, where K a,0 is
given by:
K a.0 =
Sc n
4 ⋅ π ⋅ K a,n
with Sc given by (7.30) and K a,n given by expression (7.32).
The Palmgren-Miner sum is used for the fatigue check, i.e. the chimney is expected to develop a crack,
which ultimately will result in a failure of the weakened section if the factored Palmgren-Miner sum, the
Palmgren-Miner sum multiplied by a partial safety factor, is equal to or larger than 1.
page 30
CICIND Steel Chimney Model Code
The Palmgren-Miner sum is given by the maximum value of the sum of the partial Palmgren-Miner sums
for each active cross-wind vibration mode. The partial Palmgren-Miner sum of mode n is given by:
k
f y ,T 


1  a 
a
a2
 da

M n (z) = N n ⋅ ∫ 
⋅
⋅ 2
⋅ exp  
a
 σ (z)
 2 ⋅ σ 2 (z)  
N
a,n
a,n
a min  W,0  W,0 


where:
(8.15)
a
= stress due to cross-wind oscillations
k
= slope of the fatigue curves in the S/N diagram, Figure 8.3
σa,n (z)
= standard deviation of the cross-wind stress of mode n at height z, see expression (8.16)
a min
= stress cut-off limit = 0.5 x {stress range cut-off limit in the S/N diagram}
f y,T
= yield strength of the shaft’s steel at operation temperature T, see expression (6.2)
N W,o = 5 ⋅ 106 load cycles = reference point in S/N diagram
a W,o
= 0.5 x {stress range of specified detail category at 5.106 load cycles}
The standard deviation of the cross-wind stress σ a,n is given by:
h
2
d(z) ⋅ ∫ m(s) ⋅ ( 2 ⋅ π ⋅ f n ) ⋅ u n (s) ⋅ (s - z) ⋅ ds
z
σa,n (z) =
2 ⋅ I(z)
⋅ σ y,n
(8.16)
where I(z) is the moment of inertia at height z.
Notes:
1) Due to the approximations applied to derive the expressions in Section 7.2.4.2, σ y,n is not a
function of the wind velocity. As a result the integration of the wind and stress distribution
functions are separated, see Commentary 4 for details.
2) For a detailed fatigue analysis the full response curve of the chimney as a function of the wind
velocity is needed, see Commentaries 3 and 4 for details.
The factored Palmgren-Miner sum is given by:
n
M(z) = γ ⋅ ∑ M i (z)
(8.17)
i =1
with:
γ = partial safety factor for fatigue; the recommended value is 10.
If the maximum value of the factored Palmgren-Miner sum is less than one no crack is expected to occur
during the design life time. Nevertheless, occasionally the deflection amplitude may be sufficient to cause
alarm. In such cases the amplitude limitation of Section 5.4 may govern.
CICIND Steel Chimney Model Code
page 31
Figure 8.3 ─ Fatigue strength of the base material
With respect to the fatigue categories defined in Figure 8.4
8.5.3 Influence of high temperatures
The few results available show that at 200°C fatigue growth rates may be higher than at room temperature,
but at 400°C growth rates are lower than at room temperature. Unless more detailed results become
available the modeling safety factor shall be increased to 1.50 in the range of metal temperatures between
200 to 400°C.
8.6 Allowance for Corrosion
Allowance for corrosion shall be the sum of the external (CE) and internal (CI) allowances given in Tables
8.1 and 8.2. This total allowance shall be added to the thickness of the shell required to satisfy the specified
limits of stress and deflection. Internal flanges shall have corrosion allowance CI and external flanges
corrosion allowance CE on all exposed surfaces. The allowances listed in Tables 8.1 and 8.2 are for a 20
year lifetime of the chimney. For longer planned lifetimes, the corrosion allowances should be increased
proportionally. For temporary chimneys, expected to be in service for less than one year, values of CE and
CI = 0 are permissible, except in conditions of high chemical load, when a corrosion allowance of 3mm is
required
For a free-standing chimney with steel liner(s), the internal corrosion allowance only applies to the internal
face of the liner(s). The internal face of the outer shell requires no corrosion allowance, provided a weathertight cover is fitted over the air space(s) between the liner(s) and the outer shell.
page 32
CICIND Steel Chimney Model Code
Figure 8.4 ─ Fatigue resistance of typical details.
Notes to figure 8.4:
1)
Butt welds, when high quality has to be achieved and
verified:
• developed root, cap pass counter welding
• evenly machined surface in stress direction
2)
Butt weld: developed root, cap pass counter welding
3)
Butt weld:
• welded one side only
• through-welding of seam root and plane surfaces
• secured on opposite side by auxiliary welding aid
e.g. weld-pool backing ceramics or copper rail
4)
Butt weld: welded one side only
5)
T ─ joint by double-bevel butt
weld
6)
T ─ joint by double Y –butt weld
with broad root face
7)
T ─joint with special quality
double fillet weld
8)
T ─ joint double fillet weld
CICIND Steel Chimney Model Code
Figure 8.4 ─ Fatigue resistance of typical details (continued).
page 33
page 34
CICIND Steel Chimney Model Code
Figure 8.4 ─ Fatigue resistance of typical details (continued).
CICIND Steel Chimney Model Code
Figure 8.4 ─ Fatigue resistance of typical details (continued).
page 35
page 36
CICIND Steel Chimney Model Code
8.6.1 External corrosion allowance
Type of steel
Allowance
Painted carbon steel
0mm
Painted carbon steel under insulation/cladding
1mm
Unprotected carbon steel
3mm
Unprotected “corten” or similar steel
1mm
Unprotected stainless steel
0mm
Table 8.1. External corrosion allowance (CE).
Note: The external corrosion allowances quoted in Table 8.1 are suitable for a normal environment. When
a chimney is sited in an aggressive environment, caused by industrial pollution, nearby chimneys or
close proximity to the sea, consideration should be given to increasing these allowances.
8.6.2. Internal corrosion allowance
Usual temperature of metal in
contact with flue gas
Chemical load per Table 7.4
Internal corrosion allowance
<65°C
low
not applicable
(chemical load always “high”) 1)
medium
not applicable
(chemical load always “high”) 1)
high
corrosion allowance inappropriate,
use other material 1)
low
2mm 2)
medium
4mm 3)
high
corrosion allowance inappropriate,
use other material 3)
low
1mm
medium
2mm
high
corrosion allowance inappropriate,
use other material
65°C - 345°C
> 345°C
Table 8.2 ─ Internal corrosion allowance (CI) for carbon steel only.
For chimneys handling flue gases.
CICIND Steel Chimney Model Code
page 37
Notes:
1) Provided acid concentration in the condensate is less than 5% and chloride concentration does not
exceed 30mg/m3, high molybdenum stainless steel (such as ASTM Type 316L) is suitable within
this temperature limit, using a corrosion allowance of 3mm for a 20 year life. These conditions
are, however, unlikely to be met in a chimney downstream of a FGD system, generating
condensing gases. In these circumstances great care is required in the protection of the gas face of
the chimney or its liner, e.g. by cladding with a suitable high nickel alloy or titanium or by the
application of a suitable organic coating. For further guidance, see:
CICIND Chimney Coatings Manual.
2) In conditions of low chemical load, “Corten” steel shows some improvement of corrosion
resistance over carbon steel, especially when contact with condensing SO2/SO3 is intermittent or
of short duration (e.g. during repeated shut-downs).
3) In these circumstances, ordinary stainless steels (including high molybdenum stainless steel) have
little better corrosion resistance than carbon steel and are, therefore not recommended. If carbon
steel is used in chimneys subject to high chemical load, it will require protection by an appropriate
coating. For further guidance, see:
CICIND Chimney Coatings Manual.
9. DESIGN DETAILS
9.1 Connections
9.1.1 General provisions
Connections shall be calculated on the basis of forces at least as great as the design forces of the parts they
connect e.g. the carrying capacity check shall be carried out with the same load factors and load
combinations as described under 8.3.1.
9.1.2 Bolted connections
The carrying capacity of bolted connections shall be checked with regard to tension and shear or bearing.
9.1.2.1 Shear
The shear stresses multiplied by the load factors shall not exceed the limit shear stress divided by resistance
factor = 1.1:
τ* <
τu
1.1
(9.1)
The values of limit shear stress are given in Table 9.1.
Bolt grade
Minimum value of the
tensile strength of bolts
τu
τu / l.l
4.6
400
200
182
5.6
500
250
227
6.8
600
300
278
8.8
800
400
364
10.9
1000
500
455
Table 9.1 ─ Limit shear stress (τu) in MPa.
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CICIND Steel Chimney Model Code
The design shear stress τ* relates to the gross area or to the nett area, depending on whether the shear plane
is in the unthreaded or threaded part of the bolt.
9.1.2.2 Bearing on connected surfaces
The design stress on connecting parts shall not exceed the minimum value of the tensile strength of the
connected parts multiplied by 1.45:
σ1* <
σ1,u
1.1
= 1.45 ⋅ σu
(9.2)
The design bearing stress σ1* relates to the area obtained by multiplying the diameter d of the shank by the
thickness of the connected part. Regardless of any preload, the limit stress σ1,u is valid for edge distances
greater or equal 2d in the direction of stress.
Grade
σl,u
σl,u / 1,1
Fe 360
575
525
Fe 430
690
625
Fe 510
815
740
Table 9.2 ─ Limit bearing stress σ1,u in MPa.
9.1.2.3 Tension
The limit state is described:
σ*t <
σ t,u
1.1
= 0.73 ⋅ σ u,B
(9.3)
for σt,u see Table 9.3
Minimum value
of tensile strength
Limit tensile stress of
preloaded bolts
σt,u / 1.1
Bolt grade
σu,B
σt,u
4.6
400
not recommended
5.6
500
not recommended
6.8
600
not recommended
8.8
800
640
580
10.9
1000
800
730
Table 9.3 ─ Limit tensile stress σt,u in MPa.
Note: The stresses given in Tables 9.2 and 9.3 are for ambient temperatures. For stresses at elevated
temperatures refer to the factors in column 2 of Table 6.2.
The tensile stress σt shall be calculated on the net section.
Owing to their considerable susceptibility to fatigue, connections that use bolts in tension shall be made
with pretensioned high strength bolts.
CICIND Steel Chimney Model Code
page 39
9.1.2.4 Combined loading
If the external loading results in a combination of tensile stress σ*t and shear stress τ* in the bolt, the
carrying capacity shall be checked for the condition:
2
2
 τ*   σ*t 
 < 1.0
  + 
 τu   σ t,u 
(9.4)
This check is not necessary if:
τ* < 0.2 ⋅ τu
σ*t < 0.2 ⋅ σ t,u
or
(9.5)
9.1.2.5 Deduction for holes
For parts subjected to tension, the following two conditions shall be checked:
1) in the gross section, the stress shall not exceed the yield stress f y
2) in the net section, the stress shall not exceed 80% of the tensile strength σu
9.1.3 Welded connections
The welding standard considered appropriate for steel chimneys is higher than the minimum standard
allowed for other welded products. An acceptable standard is discussed in 9.1.3.3 below.
9.1.3.1 Full penetration welds
If the quality of the weld is at least equal to that of the parent metal, frill penetration welds have the same
resistance as the connected parts. In this case, no particular checks are necessary. Partial penetration welds
shall be taken as fillet welds and calculated as such. Full penetration welds connecting plates of different
thicknesses have a resistance equal at least to that of the thinnest plate. Partial penetration of butt welds
shall not be permitted.
9.1.3.2 Fillet welds
Regardless of the direction of stress, the two design stresses σ*w and σ*s for fillet welds shall be checked:
• in the throat section a-a: σ*w <
•
in the contact section s-s: σ*s <
σ w,u
1.1
σs,u
1.1
= 0.455 ⋅ σ u,E
= 0.636 ⋅ f y
where σ u,E is the guaranteed minimum value of the tensile strength of the weld metal and fy the yield stress
of the parent material.
Throat section
Contact section
Grade
σw,u
σw,u /l.l
σs,u
σs,u./1.1
Fe 360
255
230
165
150
Fe 430
255
230
180
165
Fe 510
255
230
250
230
Table 9.4. ─ Limit stresses σw,u and σs,u for fillet welds in MPa
The yield stress, tensile strength, strain at failure and notch toughness of the weld metal shall exceed
minimum values for parent material, and, failing a specific agreement, shall be at least equal to those of
Fe 510. σw,u values given in Table 9.4 are valid for electrodes with properties of steel Fe 510.
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CICIND Steel Chimney Model Code
9.1.3.3 Weld Testing
While a minimum, taken at random, of 10% of butt welds and fillet welds shall be tested, the weld testing
procedures and quality levels shall be agreed by the client and the builder The recommendations of levels
‘C’ of ISO 5817 “Arc-welded joints in steel guidance on quality levels and imperfections” should be used,
but subject to agreement between the client and builder, local codes may be substituted.
Note: The fatigue categories listed in Figure 8.4 assume welds are made to ISO 5817 level ‘C’ quality
standards. If local codes are used, the weld categories may require appropriate adjustment.
9.2 Flanged Connections
The use of high strength bolts is recommended. The centers between the bolts should be between 4db and
10db, where db is the diameter of the bolt. However, a distance of 5db is recommended as larger spacing
result in excessively thick flanges. The minimum bolt diameter should be db = 16mm. The stress in the
bolts shall be calculated taking consideration of the eccentricity of the loading transmitted by the shell.
Figure 9.1 ─ Normal flange.
Fig. 9.2 ─ Pre-stressed flange, suitable
for vibrating conditions.
CICIND Steel Chimney Model Code
page 41
In the case of along wind:
Z*b = Z* ⋅
a
≤ 0.73 ⋅ σ u,b ⋅ A n
w
(9.6)
In the case of cross-vibration (fatigue):
Zb,f = Zf ⋅
a σR ⋅ A n
≤
w
1.1
(9.7)
where:
σR = fatigue strength for category 35 MPa
An = stress section of the bolt
If the fatigue load Zf is greater than the fatigue strength divided by 1.10, a joint with contact areas shall be
used, see ref. [22] and Figure 9.2. The pretension of the bolts should provide a sufficient force ZA to
prevent the fatigue in the bolt material:
ZA = 0.73 ⋅ σ u,b ⋅ A n ⋅
w
≥ Zf
a
(9.8)
It should be noted that the change of the type of connection to one with profiled contact areas may reduce
the damping ratio used in estimating along and across-wind response. The fitting of gaskets to the flanges
of structural shells is not permitted.
9.3 The support at the Base
Self-supporting steel chimneys are normally based on a reinforced concrete foundation or a steel structure.
The foundation or structure is loaded by an overturning moment, normal force and shear force through the
base plate and anchor bolts.
9.3.1 Anchor bolts
When fatigue due to vortex shedding is anticipated anchor bolts should be prestressed. Measures must be
taken to ensure that the pre-stressing is not lost during the lifetime of the chimney. An anchorage device
shall be attached to the bottom end of the bolt. The maximum bolt stress should not exceed 73% of the
tensile strength of the material of anchor bolt. Alternative satisfactory methods may be used at the
designer’s discretion when no response to vortex shedding is anticipated.
9.3.2 Grouting
After the chimney has been erected and plumbed (with the use of steel shims which remain in position) the
space between the base plate and concrete foundation must be filled with no shrink grout. The compressive
strength of the grout must be equal to or greater than the compressive strength of the concrete.
9.3.3 Temperature effects
Consideration must be given to the effect that radiant or conducted heat will have upon a concrete
foundation. This is particularly relevant to chimneys serving gas turbines or other high temperature exhaust
systems. There is the possibility of the foundation being damaged if an adequate heat barrier is not
installed. In the majority of situations insulation to contain or deflect radiant heat will suffice.
10. STEEL LINERS
Steel liners inside steel chimneys shall be designed to satisfy the requirements of :
CICIND Model Code for Concrete Chimneys — Part C — Steel Liners.
Advice on the design of steel liners in steel chimneys is given in Appendix 3 to this Model Code.
11. CONSTRUCTION
11.1 General
The following will be observed during shop and site construction as appropriate.
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CICIND Steel Chimney Model Code
11.2 Structural Shell
The tolerances in the fabrication of the shell shall be as follows:
Flat plate prior to rolling shall be laid out and squared to within ±1mm in length, width and on each
diagonal.
A chimney section, with flanges welded in place, shall be fabricated within a tolerance of ±3mm on
circumference and diagonal. If possible, these measurements shall be made while the shell’s axis is vertical.
If this is not possible, the shell shall be adequately braced.
Peaking of a cylinder from a true circle at weld seams shall not exceed 3mm, as measured by a 450mm
long template, centered at the weld and cut to the cylinder’s design radius. Other imperfections shall be
within the limits stated in section 8.3.4 of this model code and assumed by the designer.
Vertical butt weld seams shall be staggered a minimum of 200mm from each other.
Misalignment between plates shall not exceed 1mm.
11.3 Structural Flanges and Opening Reinforcement
These shall be fully welded to the structural shell. Intermittent welding shall not be allowed.
Flanges shall be fiat and normal to the chimney axis. Before bolting, the maximum gap width on the line of
the shell, between matching pairs of flanges, shall be 1 mm. Before bolting, the gap at the outer edges of
the flanges shall not exceed 1.5mm per 100mm width of flange.
Note: These tolerances may be ignored if the flanges are bolted together before they are welded to their
respective shell sections. Their orientation shall be marked prior to their being dismantled after
welding.
11.4 Stiffening Rings
If the design permits the use of intermittent welding, crevices exposed to weather or flue gases shall be
sealed.
11.5 Base Plate
The base plate and all base reinforcement shall be fully welded to the structural shell and to each
component. The base plate shall be perpendicular to the shell plate within ±0.5°.
11.6 Straightness
Adjoining cylinder sections shall be welded together straight in the longitudinal direction to a tolerance of
+/-12mm per 10 m of shell length.
Flanges shall be welded to the structural shell within a perpendicular tolerance of ±0.5°.
11.7 Erection Tolerance
The departure of the chimney from the vertical on erection shall not exceed 25 mm or 1/600 of the height,
whichever is the greater at any point.
12. SURFACE PROTECTION
The exterior and interior surfaces of a steel chimney may be protected from attack by weather and corrosive
gases by various methods. Specifications for different types of protection are given in Appendix 3, see also:
CICIND Chimney Protection Coatings Manual.
13. OPENINGS
The width of a single opening shall not exceed two-thirds of the diameter of the structural shell of the
chimney.
Where large apertures are cut in the shell plates, as for gas inlets or inspection panels, a structural analysis
of the stresses shall be made and compensating material provided, as required, to ensure that the stresses
specified in this Model Code are not exceeded. As a result, it may be necessary to incorporate stiffeners
around the opening. When longitudinal stiffeners are used, their design shall include the effects of
CICIND Steel Chimney Model Code
page 43
circumferential bending stresses in the shell, above and below the opening. Also they shall be long enough
to distribute stresses into the main area of the shell without overstress. (Note: this may generally be deemed
to be satisfied if the stiffeners project above and below the opening a distance at least 0.5 times the spacing
of the stiffeners.). The ends of the longitudinal stiffeners should be tapered in a radial direction (see cases
16.1-3 in Figure 8.4).
Additional horizontal stiffeners may be used to absorb the circumferential bending stresses. These
stiffeners may be attached between the longitudinal stiffeners, at the hole’s edge and at the end of the
longitudinal stiffeners.
A suggestion for stiffeners is given in the Commentaries for this Model Code.
Smaller apertures in the shell plates, not equipped with stiffeners, shall have the corners radiused to a
minimum of 10 t, where t is the thickness of the plate.
The effect of openings upon the chimney’s stiffness should be taken into account when determining the
chimney’s natural frequencies.
14. GUYED AND STAYED CHIMNEYS
A stayed chimney is defined as one which derives lateral (but not vertical) support from another structure.
A guyed chimney derives lateral support from guy ropes.
The foregoing structural design rules are valid for self-supported chimneys, acting as cantilevers, fixed at
their bases, with or without liners. Some of the rules (e.g. those related to thermal and chemical load) are
relevant also to chimneys that are guyed or stayed. Rules governing the structural design, related to wind or
earthquake loading do not, however, apply to these chimneys.
14.1 Stayed Chimneys
Stayed chimneys are supported laterally at one or more elevations above their bases. The number of lateral
supports will be governed by buckling considerations per section 8.3.4 above and by the need to avoid
oscillations due to vortex shedding, but shall be kept to the minimum possible. To avoid vibrations due to
vortex shedding, the natural frequencies should ensure that Vcr (assuming St = 0.2) > 1.25 • design wind
speed at the relevant elevation. The prime concern of the design should be to ensure that vertical expansion
is not restricted.
In designing the shell and lateral supports, the forces induced by the restraint of differential thermal
expansion shall be considered. Differential expansion can be expected if two or more gas streams of
differing temperatures enter the chimney at different points. Guidance on the determination of these forces
may be found in:
CICIND Model Code for Concrete Chimneys, Part C ─ Steel Liners.
The design of the supporting structure is outside the scope of this Model Code.
14.2 Guyed Chimneys
Design rules for guyed chimneys are given in Appendix 4 to this Model Code
15. PROTECTION AGAINST LIGHTNING
A steel chimney can be considered as a continuous metal structure and thus be used as its own lightning
protection system. Consequently it requires no air termination or down conductor. It is sufficient to ensure
that the conduction path is electrically continuous and that it is adequately earthed.
16. ACCESS LADDERS
A specification for access ladders and hooks is given in Appendix 5.
17. AIRCRAFT WARNING LIGHTS
It is advisable to contact the local aeronautical authority for the area if the chimney is to be built within an
aerodrome safe guarding area as local conditions and restrictions may apply.