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CICINDModel Code forSteel Chimneys
Revision 1 1999Amendment A March 2002
Copyright CICIND 1999ISBN 1-902998-09-X
Office of The Secretary, 14 The Chestnuts, Beechwood Park, Hemel
Hempstead, Herts., HP3 0DZ, UKTel: +44 (0)1442 211204 Fax: +44
(0)1442 256155 e-mail: [email protected]
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Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . 310 Introduction . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . 3
0.1 General . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . 30.2 Appendices and Commentaries . . . . . . .
. . . . . . . . . . . 30.3 Philosophy . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . 3
11 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . 312 Field of Application . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . 313 References . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . 314 Notations, Units and Denitions . . . . . . . . . . . . . .
. . . . . . . 4
4.1 General . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . 44.2 Subscripts-Superscripts . . . . . . . . .
. . . . . . . . . . . . . . . 44.3 Units . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . 44.4 Denitions .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4
15 Basis of Design and Safety Factors . . . . . . . . . . . . .
. . . . . . 45.1 General . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . 45.2 Reliability differentiation . .
. . . . . . . . . . . . . . . . . . . . . 45.3 Partial Safety
Factors . . . . . . . . . . . . . . . . . . . . . . . . . . 55.4
Cross-wind effects . . . . . . . . . . . . . . . . . . . . . . . .
. . . . 5
16 Materials . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . 56.1 General . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . 56.2 Structural steels
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56.3
Stainless and alloy steels . . . . . . . . . . . . . . . . . . . .
. . . 6
17 Actions (External and Internal) . . . . . . . . . . . . . . .
. . . . . . 67.1 Permanent Load . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . 6
7.1.1 Dust load (temporary load) . . . . . . . . . . . . . . .
67.2 Wind . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . 6
7.2.1 General . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . 67.2.2 Wind Speed . . . . . . . . . . . . . . . . . . . .
. . . . . . . 6
7.2.2.1 Basic wind speed . . . . . . . . . . . . . . . .
67.2.2.2 Design wind speed . . . . . . . . . . . . . . . 77.2.2.3
The inuence of topography . . . . . . . . 7
7.2.3 Wind load in direction of the wind . . . . . . . . . .
87.2.3.1 Wind load on isolated chimneys . . . . . 87.2.3.2 Mean
hourly wind load . . . . . . . . . . . . 8
7.2.3.3 Effect of uctuating part ofthe wind-speed . . . . . . .
. . . . . . . . . . . 8
7.2.4 Vortex shedding . . . . . . . . . . . . . . . . . . . . .
. . . 87.2.4.1 General principles . . . . . . . . . . . . . . .
87.2.4.2 Estimation of top amplitudes . . . . . . . 97.2.4.3
Bending Moments due to
vortex shedding . . . . . . . . . . . . . . . . . 97.2.5
Ovalling . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9
7.2.5.1 Static effects . . . . . . . . . . . . . . . . . . . .
97.2.5.2 Dynamic effects . . . . . . . . . . . . . . . . 10
7.2.6 The increase of wind effects bynearby structures . . . . .
. . . . . . . . . . . . . . . . . 107.2.6.1 Increase in along-wind
load . . . . . . . 107.2.6.2 Increase in cross-wind response . . .
. 10
7.2.7 Damping ratio . . . . . . . . . . . . . . . . . . . . . .
. . 117.2.8 The rst and second natural frequencies . . . . .
117.2.9 Passive dynamic control . . . . . . . . . . . . . . . . .
11
7.2.9.1 Aerodynamic stabilisers . . . . . . . . . . 117.2.9.2
Damping devices . . . . . . . . . . . . . . . 117.2.9.3 Special
chimney designs
for damping . . . . . . . . . . . . . . . . . . . 127.3
Earthquake loading . . . . . . . . . . . . . . . . . . . . . . . .
. . 127.4 Thermal effects . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . 127.5 Explosions . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . 12
7.5.1 External explosions . . . . . . . . . . . . . . . . . . .
. 127.5.2 Internal explosions . . . . . . . . . . . . . . . . . . .
. . 12
7.6 Internal effects governing the chimney design . . . . . .
127.6.1 High temperature ue gases . . . . . . . . . . . . . .
127.6.2 Fire . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . 127.6.3 Chemical effects . . . . . . . . . . . . . . . .
. . . . . . 12
18 Design of Structural Shell . . . . . . . . . . . . . . . . .
. . . . . . . . 138.1 Minimum thickness . . . . . . . . . . . . . .
. . . . . . . . . . . . 138.2 Required checks . . . . . . . . . . .
. . . . . . . . . . . . . . . . . 138.3 Carrying capacity of shell
. . . . . . . . . . . . . . . . . . . . . 13
CICINDModel Code for Steel Chimneys
REVISION 1 1999
TABLE OF CONTENTS
DISCLAIMER
This CICIND Model Code is presented to the best of the knowledge
of its members as a guide only. CICIND is not, nor are any of
itsmembers, to be held responsible for any failure alleged or
proved to be due to adherence to recommendations or acceptance of
information
published by the association in a Model Code or in any other
way.
Extracts from standards are reproduced with the permission of
BSI under licence number PD\1999 1591.Complete copies of the
standard can be obtained by post from BSI Customer Services, 389
Chiswick High Road, London W4 4AL, UK
CICIND, Talacker 50, CH-8001, Zurich, Switzerland
Copyright by CICIND, Zurich
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page 2 CICIND Model Code
8.3.1 Load factors and load combinations . . . . . . . . 138.3.2
Second order effects . . . . . . . . . . . . . . . . . . . 138.3.3
Biaxial stresses . . . . . . . . . . . . . . . . . . . . . . .
138.3.4 Stability . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . 13
8.4 Serviceability of shell . . . . . . . . . . . . . . . . . .
. . . . . . 148.5 Fatigue check . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . 14
8.5.1 Basic principles . . . . . . . . . . . . . . . . . . . . .
. . 148.5.2 Fatigue strength . . . . . . . . . . . . . . . . . . .
. . . . 148.5.3 Inuence of high temperature . . . . . . . . . . . .
14
8.6 Allowance for corrosion . . . . . . . . . . . . . . . . . .
. . . . 148.6.1 External corrosion allowance . . . . . . . . . . .
. . 198.6.2 Internal corrosion allowance . . . . . . . . . . . . .
19
19 Design Details . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . 199.1 Connections . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . 19
9.1.1 General provisions . . . . . . . . . . . . . . . . . . . .
. 199.1.2 Bolted connections . . . . . . . . . . . . . . . . . . .
. 19
9.1.2.1 Shear . . . . . . . . . . . . . . . . . . . . . . . .
199.1.2.2 Bearing on connected surfaces . . . . . 199.1.2.3 Tension
. . . . . . . . . . . . . . . . . . . . . . . 199.1.2.4 Combined
loading . . . . . . . . . . . . . . 209.1.2.5 Deduction for holes .
. . . . . . . . . . . . 20
9.1.3 Welded connections . . . . . . . . . . . . . . . . . . . .
209.1.3.1 Full penetration welds . . . . . . . . . . . 209.1.3.2
Fillet welds . . . . . . . . . . . . . . . . . . . . 209.1.3.3 Weld
testing . . . . . . . . . . . . . . . . . . . 21
9.2 Flanged connections . . . . . . . . . . . . . . . . . . . .
. . . . . 219.3 The support at the base . . . . . . . . . . . . . .
. . . . . . . . . 21
9.3.1 Anchor bolts . . . . . . . . . . . . . . . . . . . . . . .
. . 219.3.2 Grouting . . . . . . . . . . . . . . . . . . . . . . .
. . . . . 219.3.3 Temperature effects . . . . . . . . . . . . . . .
. . . . . 21
10 Steel liners . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . 2111 Construction . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . 21
11.1 General . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . 2111.2 Structural shell . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . 2111.3 Structural anges and
opening reinforcement . . . . . . . 2211.4 Stiffening rings . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . 2211.5 Base plate
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2211.6 Straightness . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . 2211.7 Erection tolerance . . . . . . . . . . . . .
. . . . . . . . . . . . . . 22
12 Surface Protection . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . 2213 Openings . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . 2214 Guyed and Stayed
Chimneys . . . . . . . . . . . . . . . . . . . . . . 22
14.1 Stayed chimneys . . . . . . . . . . . . . . . . . . . . . .
. . . . . . 2214.2 Guyed chimneys . . . . . . . . . . . . . . . . .
. . . . . . . . . . . 22
15 Protection Against Lightning . . . . . . . . . . . . . . . .
. . . . . . 2216 Access Ladders . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . 2217 Aircraft Warning Lights . . . .
. . . . . . . . . . . . . . . . . . . . . . 22
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CICIND Model Code page 3
FOREWORDWhen it was formed in 1973, the Comit International
desChemines Industrielles (CICIND) adopted as a major goal
theharmonisation of national codes for the design of
industrialchimneys. As a means to this end, a subcommittee was
appointed in1981, charged with drafting a proposal for a model code
for steelchimneys which reflected the current state-of-the-art and
aconsensus of views, internationally. This document was published
in1988, with Commentaries being published the following year.Since
1988, the science and technology of chimneys has advancedand in
1995, CICIND appointed a committee to revise the ModelCode,
recognising current best international practice and knowledge.The
committee comprises:
J. Roberts Great Britain Chairman until Jan. 1998B.N. Pritchard
Great Britain Chairman after Jan. 1998Max Beaumont Great
BritainMichael Beaumont Great BritainG. Berger GermanyJ. Bouten The
NetherlandsR. Ghermandi ItalyS. Ole Hansen DenmarkG. Pinfold Great
BritainR.M. Warren U.S.A.
Expert advice was received from:B.J. Vickery (Canada)H. van
Koten (The Netherlands)
0. INTRODUCTION
0.1 General
Chimneys are required to carry vertically and discharge to
theatmosphere, gaseous products of combustion, chemical waste
gases, orexhaust air or for the combustion (aring off) of
industrial waste gases.This Model Code contains guide-lines which
reect the current stateof art in the design and construction of
steel chimneys. Nevertheless,the design, fabrication and erection
of steel chimneys require athorough knowledge of these structures,
the properties of thematerials used, the actions occurring upon the
structure and therecognised rules of the relevant technologies. The
design of steelchimneys should therefore only be entrusted to
appropriatelyqualied and experienced engineers. The construction
and erectionshould be carried out by rms competent in this class of
work. At alltimes the work should be under the direction of
appropriatelyqualied supervisors. CICIND will continue to try to
improve the understanding of thebehaviour of chimneys. Further
revisions of this model code willtherefore be published from time
to time.
0.2. Appendices and Commentaries
This Model Code is accompanied by extensive appendices
andcommentaries. The appendices provide information which
thecommittee believes will be of use to a steel chimney designer,
eventhough its inclusion in a chimney design code could not be
justied.The commentaries have the following objectives:a)
Justication of the regulations of the model code.b) Simplication 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
orprobably not optimal.
The following items are not objectives of the
CICINDcommentaries:
e) Change of the meaning of certain regulations of the model
codewhere these are falsely expressed or obviously wrong.
f) Denition of the meaning of certain regulations of the
modelcode which are so badly formulated that they could easily
bemisinterpreted even by experts.
Certain information from the model code is repeated in
thecommentaries when this simplies the presentation of the
ideas.
0.3 Philosophy
One of the main objectives of any code governing construction is
thecreation of a model which resembles as far as possible, the
realsituation. The model should be sufficiently safe, simple and
true. Itis very rarely that simplicity and truth are compatible, so
a modelmust be used which provides an optimum compromise between
truth,simplicity, safety and economy.While the judgements of
sufficiently true and sufficiently simpleare subjective,
sufficiently safe is capable of rational judgement.This code
interprets sufficiently safe in terms of the social andeconomic
consequences of failure. It does this by comparing theprobabilities
of failure for given safety factors during its design lifewith the
failure probabilities required to satisfy accepted social
andeconomic criteria. This leads to the development of safety
factorswhich ensure that a chimney will have a probability of
failure duringits design lifetime between 103 and 104, depending
upon itsreliability category.CICIND has departed from generally
accepted principles ofsteelwork design and construction only when
this was required by thephilosophy outlined above or by specic
chimney requirements.
1 SCOPE
This model code relates to the structural design and
construction ofsteel chimneys of circular cross-section, with a
minimum height of15m, with or without linings, and to the design
and application oflinings to such chimneys where required. It also
relates to chimneyswith a height less than 15m and a slenderness
ratio more than 16. Themodel code does not deal with architectural
or thermal aspects ofsteel chimneys nor with their foundations,
except insofar as theyaffect the chimneys structural design. The
model code does not dealwith 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
selfsupporting chimneys taller than 15m. For other
chimneyssimplication may be acceptable.Additional information is
given in the Appendices andCommentaries.
3.REFERENCES
[1] CICIND model code for concrete chimneys Part 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
EngineeringDivision. Proc. of the American Society of Civil
Engineers,February 1973.
[4] Vickery, B.J: Wind loads and design for chimneys,
CICINDREPORT, Vol. 14, No. 2, 1998
[5] Eurocode 1 Basis of Design and actions on structures Part 2
4: Actions on structures Wind Actions ENV1991-2-4: 1995
[6] Van Koten, H: A calculation method for the fatigue life of
steelchimneys subject to cross-wind oscillations, CICINDREPORT,
Vol. 14, No. 2, 1998
-
page 4 CICIND Model Code
[7] Ruscheweyh, H.: Experience with Vortex Excited
Oscillationsof Steel Chimneys, CICIND REPORT, Vol.11, No. 2,
1995
[8] Ole Hansen, S: Vortex induced vibrations of
line-likestructures, 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
chimneysand their estimation taking account of their type,
CICINDREPORT, Vol. 15, No. 1, 1999
[11] Turner J.G.: Wind load stresses in steel chimneys,
CICINDREPORT, Vol. 12, No. 2, 1996
[12] Hirsch, G.& Jozsa, M.: Optimum control of chimney
vibration,CICIND REPORT, Vol. 10, No. 1, 1994
[13] Bierrum, N.R.: Mis-tuned Mass Dampers, CICIND REPORT,Vol.
10, No. 2, 1994
[14] Warren, R.M. & Reid, S.L. Shell to Flue Impact Damping
forDual Wall and Multi-Flue Chimneys CICIND REPORTVol. 10, N0. 1,
1994
[15] Ruscheweyh, H., Kammel, C. & Verwiebe, C.
VibrationControl by Passive Dampers a Numerical and
ExperimentalStudy of the Damping Effect of Inner Tubes Inside a
Steel Stackand a new dynamic vibration absorber CICIND REPORTVol.
12, No. 2, 1996
[16] Bunz, G., Diepenberg, H. and Rendie, A.: Inuence of fuel
oilcharacteristics and combustion conditions of flue gasproperties
in W T boilers Journal of the Institute of Fuel,Sept.1967
[17] Lech and Lewandowski: Prevention of cold end corrosion
inindustrial boilers Corrosion, March 1979, Atlanta, U.S.A.
[18] Henseler, F.: Desulphurisation Systems and their Effect
onOperational Conditions in Chimneys, CICIND REPORT, Vol.3, No. 2,
1987.
[19] CICIND chimney protective coatings manual, CICIND,Zurich,
Switzerland
[20] Schulz, U.: Die Stabilitat axial belasteter Zylinderschalen
mitManteloffnungen, Bauingenieur 51,1976.
[21] European Recommendations for Steel Construction: Bucklingof
Cylinders ECCS/CECM/EKS, 1984
[22] Bouwman, E.P.: Bolted connections dynamically loaded
intension. Proceedings ASCE, Journal of the StructuralDivision,
ST9,1982.
[23] CICIND Model Code for Concrete Chimneys Part C,
SteelLiners, December 1995 CICIND, Switzerland
4. NOTATIONS, UNITS AND DEFINITIONS
4.1. General
The following list shows only the principles by which the
notationsand their meanings are related. The actual notations are
mostlyexplained in the text.Local factors
load factorMaterial properties
f strength (MPa)E modulus of elasticity (GPa) stress (MPa)
LoadingsT temperature in centigradeV 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 limitk characteristic value
* stress multiplied by load factorcr critical
4.3. Units
Generally, the units of the SI system are used.Examples:
m (metre) and mm (millimetre) for dimensions and MN (Meganewton)
and N (Newton) for forces MPa for stresses
In those cases where other units are used, the relevant
referencesare given.
4.4. Definitions
The common names of parts of a steel chimney are explained
incommentary 1.
5. BASIS OF DESIGN AND SAFETY FACTORS
5.1 General
The design of sections subject to permanent load and wind loads
inthe wind direction is based upon ultimate limit state conditions,
thesafety of the chimney being ensured by partial safety factors
for loadsand material. The ultimate limit state considered is
reached when anypart of the section is at the limit stress. The
limit stress is dened aseither yield stress or critical buckling
stress (whichever is least),divided by the material safety factor.
The calculation of the stressdistribution and the strength of the
sections shall therefore be madein accordance with the theory of
elasticity.The use of this procedure, combined with the partial
safety factorslisted below will ensure that low cycle fatigue will
not contribute tofailure of the chimney.
In the design of details such as anges, ultimate limit state may
takeaccount of plastic stress distributionSafety in the case of
response to vortex shedding is ensured by theuse in the fatigue
calculations of a suitable Miner Number, a materialfactor and a
modelling factor.
5.2 Reliability differentiation
Different levels of reliability shall be adopted for chimneys,
depending onthe possible economic and social consequences of their
failure.Two classes of reliability related to the consequences of
structuralfailure are used Normal and Critical, as dened below. The
choiceof reliability category shall be decided by the chimney owner
andrelevant statutory authorities. Most chimneys will, however,
beregarded as of Normal reliability.
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CICIND Model Code Amendment A March 2002 page 5
Critical chimneys Chimneys erected in strategic locations, such
asnuclear power plants or in densely populated urban locations.
Majorchimneys in industrial sites where the economic and/or
socialconsequences of their failure would be very high.Normal
Chimneys All normal chimneys at industrial sites or otherlocations.
(Typically chimneys in industrial sites, power plants orchimneys
less than 100m tall in urban locations, where any domesticdwelling
is outside the falling radius of the chimney).
5.3 Partial Safety Factors
Material safety factor for steel 1.1Load factors for:Normal
Chimneys
Permanent load 1.1 Guy rope pretension 1.2 Wind load in wind
direction (temperate zones) 1.4 Wind load in wind direction
(tropical storm zones)* 1.5
Critical Chimneys Permanent load 1.1 Guy rope pretension 1.2
Wind load in wind direction (temperate zone) 1.5 Wind load in wind
direction (tropical storm zones)* 1.6
* See literature (e.g. lit.(3)).
5.4 Cross-wind Effects (Vortex shedding)
Chimneys shall be designed to avoid movements across the
winddirection sufficient to cause failure or fatigue damage or to
alarmbystanders.The code contains means of estimating the amplitude
of movementand consequent stress range due to crosswind loading.
Limiting stressranges are given for various weld classications and
design lives. Inaddition to a material safety factor 1.1, applied
to fatigue category, amodelling factor of 1.4 shall be applied to
the Miner Number derivedin fatigue calculations for temperatures up
to 200C and 1.5 fortemperatures between 200C and 400C.To avoid
alarming personnel, the maximum permitted amplitude ofoscillations
due to cross-wind effects or aerodynamic interferenceshall be
agreed between the owner and designer. This limit will begoverned
by the prominence and visibility of the chimney and thefrequency
with which maximum amplitudes can be expected.Guidance is given in
Commentary 3.
6. MATERIALS
6.1. General
The materials generally used for steel chimneys are described
below.Special steels can be used providing that they are precisely
speciedand that their characteristics, such as yield stress,
tensile strength,ductility and weldability, enable the Model Code
to be put intoapplication. In zones where bearing elements are
subjected to tensionas a result of external loads or in zones of
three-dimensional stress,the ductility requirements, in addition to
the minimum strengthvalues, shall be considered.
6.2. Structural Steels
6.2.1 The mechanical properties and the chemical composition
ofstructural steels shall comply with local national
standards.6.2.2. For the most commonly used grades of steel, Fe
360, Fe 430and Fe 510, Table 6.1 gives the mechanical properties.
Steel gradeASTM A36 has similar properties to Fe 360.6.2.3. The
limit stresses of steel are equal to the yield stress of thesteel
used, divided by the material factor 1.1: i.e. fk fy / 1.1The yield
stresses of structural steels at normal ambient temperatureare
given in table 6.1. The yield stresses at high temperatures
aregiven in Table 6.2.
Steel Class De-oxidation Yield stress in MPa for thickness (mm)
Min.Grade procedure Notch
(2) Toughness(7)
t
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page 6 CICIND Model Code
Steel Grade Steel Grade Steel GradeFe 360 Fe 430 Fe 510
C fyT/fy fyT fyT/1.1 fyT fyT/1.1 fyT fyT/1.120 1.000 235 214 275
250 355 323200 0.880 207 188 242 220 312 284250 0.832 196 178 229
208 295 269300 0.778 183 166 214 195 276 251350 0.717 169 153 197
179 255 231400 0.647 152 138 178 162 230 209
Table 6.2 Yield stresses of structural steel in MPa(thicknesses
t 16mm)
Note
1) For thicknesses greater than 16mm the yield stress fy shall
bereduced according to Table 6.1.
2) For temperatures higher than 350C alloy steels should
beconsidered.
3) Special attention should be paid to the modulus of elasticity
athigh temperatures for stainless steel.
4) Linear interpolation is acceptable
Temperature C 20 200 250 300 350 400ET in GPa 210 202 198 192
185 174
Table 6.3 Youngs Modulus of structural steel athigh
temperatures
6.3. Stainless and alloy steels
When metal temperatures are expected to exceed 400C, stainless
oralloy steels should be used.Ordinary stainless steels (including
high molybdenum stainless steel)have poor corrosion resistance in
the presence of condensingsulphuric or other acids in the range of
concentrations andtemperatures normally found within chimneys.
These materials aretherefore not recommended in chimneys burning
fuels containingsulphur under conditions of medium or high chemical
load, seeparagraph 7.6.3.When metal temperatures and condensate
sulphuric acidconcentrations are expected to be less than 65C and
5% respectively,the corrosion rates of high molybdenum stainless
steels, such asASTM Type 316L, are acceptable. Such conditions can
be expectedon the external surface at the top (over a height of
about 3 diameters)of any chimney handling high sulphur ue
gases.(Note: the conditions downstream of a ue gas scrubber or
thepresence of chlorides in the condensate will radically increase
thecorrosion rate, possibly rendering these stainless steels
unsuitable forthese applications.) Ordinary stainless steels are
not suitable for use in contact with uegases containing alkalis.In
cases where it is not possible to avoid high chemical load on
theinternal face of the structural shell, see paragraph 7.6.3, the
use of aprotective coating may be considered (see lit[19]).
Alternatively, asteel liner or liners, possibly of titanium or high
nickel alloy, is apossible solution. See section 10 on Steel
Liners.Low copper alloy steels have good resistance to
atmosphericcorrosion, except in a marine environment or other
environmentwhere chlorides are present. These steels also show some
corrosionimprovement over carbon steel when in contact with ue
gases whereacid condensation of SO2/SO3 (not of HCL condensation)
isintermittent only (e.g. during shutdowns of a stack in
intermittentservice, its metal temperature being normally above
acid dew point).
When the metal temperature is below acid dew point for
prolongedperiods, the performance of low copper alloy steels in
contact withue gases is similar to that of carbon steel.Where
stainless or alloy steel components are connected to carbonsteel,
bolted connections are preferred. In order to avoid
acceleratedcorrosion due to galvanic action, such connections
should includeinsulating gaskets. Welded connections are permitted,
providedspecialist metallurgical control is exercised with regard
to weldprocedures, electrode selection, etc.Care should be taken to
use the correct coefficient of expansion forthe 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
permanentconstructions, ttings, linings, ues, insulation, present
and futureloads including corrosion allowance.
7.1.1. Dust load (temporary load )
On some process plants there can be a carry over of ash or
dustburden. This may adhere to the interior surface of the
structural shellor liner and cause an additional dead load. Such
cases should beinvestigated at the design stage, the calculated
load shall be added tothe permanent load calculated in 7.1
above.
7. 2. Wind
7.2.1 General
The wind load on a chimney depends in the rst instance upon
themagnitude of the wind speeds in the area in which the chimney is
tobe erected and their variation with height. Apart from that the
windloads, in the direction of the wind or perpendicular to that
direction,will be inuenced by some or all of the following:a) local
topographyb) the level of turbulencec) the presence of nearby
structures, including chimneysd) the air densitye) the value of the
drag coefficient (shape factor)f) the values of the natural
frequencies of oscillationg) the amount of structural damping and
mass presenth) the conguration of the rst few mode shapesi) the
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
basicwind speed.The basic wind speed Vb, appropriate to the
location where thechimney is to be erected, is dened as follows: It
is the mean hourlyspeed, measured 10m above ground level in open at
country,without obstructions, at the chimney location, which occurs
onaverage once every 50 years.The value of the basic wind must be
taken from meteorologicalmeasurement. An indication of values of
the basic wind speeds forvarious 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 aspossible at a place which is at and open.
However, in some veryhilly areas, where at ground is rare, Vb is
sometimes measured at thechimney location and includes the
Topographical factor.
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CICIND Model Code page 7
7.2.2.2. Design wind speed
The basis for the determination of the wind loads is the design
windspeed which equals the basic wind speed corrected by three
factorstaking into consideration the height of the chimney, the
topographyof its surroundings and the existence of adjacent
objects. These threefactors are: the height factor k(z), the
topographical factor kt and theinterference factor ki.
The design wind-speed is determined by the following
expression:
V(z)Vb k(z) kt ki {m/s) ... (7.1)where:
V(z) hourly mean wind speed at elevation z (m/s)z height above
ground level (m)Vb basic wind speed (m/s)k(z)Height factor
(z/10)
0.14. This value has been chosen since many chimneys are inopen
terrain or project well above the surrounding buildings.
kt topographical factor (see 7.2.2.3)ki interference factor (see
7.2.6.1)If the suitability of a different value of [] can be proved
(together withan appropriate scale factor), it may be used (see
Commentary C3.1.3).
7.2.2.3 The influence of topography
Clause 7.2.2.2 requires the determination of a topographical
factor ktto account for the increase of mean windspeed over hills
andescarpments in otherwise relatively at terrain (i.e. it is not
for use inmountainous regions). It should be considered for
locations closerthan half of the length of the hill slope from the
crest or 1.5 times theheight of the cliff.
For certain topographical situations, a method for the
determinationof kt is given in the following.
kt 1 0.6 . s for 0.3kt 1 2 . s . for 0.05 0.3kt 1 for 0.05
Table 7.1 Values of kt
Where:-
= upwind slope H/L in the wind direction (see Figs. 7.1 &
7.2)s = factor obtained from Figs. 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, dened in table
7.2
Lu = actual length of upwind slope in the wind direction
Ld = actual length of downwind slope in wind direction
Shallow slope (0.05 0.3) Steep slope ( 0.3)Le Lu Le H / 0.3
Table 7.2 Values of Le
Figure 7.1 Factor s for cliffs and escarpments
Figure 7.2 Factor s for hills and ridges
Figures 7.1 and 7.2 from ENV 1991-2-4 Eurocode 1 Basis ofdesign
and actions on structures wind actions
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page 8 Amendment A March 2002 CICIND Model Code
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 z is
determined by thefollowing expression:
w(z)wm(z) G (N/m) ... (7.2)where:
wm(z)mean hourly wind load per unit height, see formula 7.3
G the gust factor, see 7.2.3.3
7.2.3.2. Mean hourly wind load
7.2.3.2.1. Main formula
The mean wind load per unit height is:
wm(z) 1/2a V(z)2 CD d(z) (N/m) ... (7.3)
where:
a density of air, see 7.2.3.2.2 (kg/m3)V(z) wind speed at height
z, see 7.2.2.2 (m/s)CD shape factor, see 7.2.3.2.3d(z) outside
diameter of the chimney at height z (m)
Note: For z 10m, wm(z)wm(10)
7.2.3.2.2. Air density
At sea level in temperate climates, the density of air a is to
be taken as:
a 1.25 kg/m3
Momentary variations in the density due to atmospheric
changesneed not be taken into account.
The air density relevant to a chimney site at an altitude h1 (m)
can befound from the expression:
a 1.25 (h1 / 8000) kg/m3 ... (7.4)
7.2.3.2.3. Shape factor
The shape factor CD depends on the Reynolds number Re of
thechimney (see Fig. 7.3), where Re 6.9 104 V d, in whichVV(z) is
the mean wind speed at the top of the chimney in m/s andd is the
diameter in m.
CD 1.2 if Re 3 105
CD 1.2 1.36 {log Re 5.48) if 3 105 Re 7 105CD 0 7 if Re 7
105
for chimneys with helical vanes CD 1.4 (see gure 7.3}. CD
isapplied to the outer diameter of the chimney in the vaned portion
andnot the outer dimension of the vanes.
For attachments, including ladders, etc., the area presented to
thewind for each member must use a force coefficient of 1.2 for
circularmembers and 2.0 for structural shapes. Typical lengths and
widths ofladder members have been taken into account.
Figure 7.3
7.2.3.3 Effect of fluctuating part of the wind-speed
The inuence of the uctuating part can be found by multiplying
withthe gust factor G.
G gust factor 1 2 g i {B(E S/ )} ... (7.5)
where:
g peak factor (2 loget)
T3600 f1
1i turbulence intensity 0.311 0.089 log10 h
B background turbulence {1 (h / 265)0.63}0.88
E energy density spectrum
S size reduction factorS {1 5.78 ( f1 / Vb )1.14 h0.98}0.88
the structural plus aerodynamic damping expressed as afraction
of critical damping (see 7.2.7)
f1 sthe natural frequency in sl of the chimney oscillatingin its
rst mode
h height of chimney in metres
7.2.4 Vortex shedding
7.2.4.1 General Principles
Forces due to vortex shedding cause cross wind response of
achimney. The frequency (f) at which vortices are shed is related
todiameter (d) and wind velocity (V) by the expression:
St f d / V ... (7.6)where StStrouhal number
The Strouhal number decreases with decreasing distance (A) of
nearbychimneys in a row arrangement. For A/d15 the Strouhal number
is0.2 and for different distances this number is as shown in g.
7.6
123 (f1 / Vb) h0.21{1 (330 f1 / Vb)2 h0.42}0.83
BSE
0.577(2 loget)
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CICIND Model Code Amendment A March 2002 page 9
Significant amplitudes usually only occur when the
sheddingfrequency (which increases as the wind speed rises)
co-incides witha structural frequency. This occurs at the critical
wind speed (Vcr)which is derived by the following expression:
Vcr1 f1 . d / St or Vcr2 f2 . d / St ... (7.7)
Normally only the rst mode structural frequency (f1) is
relevant.However, for slender chimneys with very low rst critical
wind-speed, the response to second mode vibration (at frequency f2)
shouldalso be studied.
No signicant movement due to vortex shedding will be found if
thecritical wind-speed exceeds 1.2 the design wind-speed at the
topof the chimney.
The cross-wind movements depend strongly on the mass anddamping
of the chimney. A major determining property is thedimensionless
Scruton number dened as:
Sc ... (7.8)
where:
mo
a air density 1.25kg/m3
d1 the diameter (averaged over the top third)
u1 (z) the mode shape of the rst resonance frequency
c / cr damping ratio (see Table 7.4)
h the height of the chimney
If the Scruton number is less than 5, cross-wind oscillations
could beviolent. The addition of stabilisers or damping devices
(see 7.2.9 and7.2.10) is mandatory in this case.If the Scruton
number is greater than 5, the designer may choosebetween providing
stabilisers or damping devices (see 7.2.9 and7.2.10), or estimating
(per 7.2.4.2) the chimneys response andresulting stresses, ensuring
these stresses remain within the limits offatigue per 8.5 and that
movement does not exceed the limits agreedper Section 5.4.
7.2.4.2 Estimation of cross-wind amplitudes due tovortex
shedding
The method described in this section for estimating
amplitudesdepends upon parameters such as structural damping
andatmospheric turbulence, whose values are not known with
certainty.The results of the calculation should, therefore be
treated with careand should not be assumed to be accurate.
The top amplitude (y) of a chimney moving across the wind
becauseof vortex shedding depends upon:-
The Scruton Number Sc (see 7.2.4.1 above)The Strouhal Number St
(see 7.2.4.1 above)The Reynolds Number Re ( 6.9 104 V d) see
7.2.3.2.3)
The local minimum atmospheric turbulence intensity (I), seeTable
7.3
The chimneys own movement, making the behaviour nonlinear
The approximate maximum value of y can be expressed in terms
oftwo quantities, c1 and c2 as follows:-
yKp . d1 .c1 (c12 c2) ... (7.9)
where
d1 mean diameter over top third of chimney height
c1 0.08 {1 ( . mo) / (Ka . a . d2)}
c2 0.16 . a . d3 . Ca2 / (Ka .mo . St4 . h)
Kp 1.5 when c1 c12 c2 0.04
4 when c1 c12 c2 0.04
Ka Kamax . (1 3 . I)
Kamax 1.5 when Re 105
(5.075 0.715 . log10Re) when 105 Re 5 . 105
1.0 when Re 5 . 105
Ca .02 when Re 105
(0.07 0.01 . log10Re) when 105 Re 106
.01 when Re 106
The value assumed for minimum local turbulence intensity (I)
shallbe as listed in Table 7.3.
Chimney Location
Open Sea or Lake shore with at All other terrain Categoriesleast
5km fetch upwind of water, orsmooth flat country without
obstacles
Vcr 10m/s 10m/s 7m/s 7m/sI 0 0.1 0 0.1
Table 7.3
7.2.4.3 Bending Moments due to vortex shedding
In deriving the bending moments associated with the
maximumresponse amplitude of a chimney due to vortex shedding,
theassociated inertial force per unit length [F(z)] should be
used.
F(z) (2 fn)2 m(z) y(z) ... (7.10)
Where: m(z) mass per unit length at height zy(z) maximum
amplitude at height zfn natural frequency of nth mode
In deriving the fundamental mode maximum amplitude at height
zfrom the maximum amplitude at the chimney top (per 7.2.4.2),
aparabolic mode shape may be assumed.
7.2.5. Ovalling
In most cases, a suitably sized stiffening ring at the top of a
chimneywill eliminate problems associated with ovalling.
7.2.5.1 Static effect
The uneven wind pressure distribution around the circumference
of acircular cylinder causes bending moments acting on vertical
cross-sections of the shaft. The bending moments have a maximum
value of:
M 0.08w5 sec (z) d2(z) (Nm/m) ... (7.11)
0
h
m(z) u12 (z) dz
0
h
u12 (z) dz
4 . . mo c / ccr
a d12
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page 10 Amendment A March 2002 CICIND Model Code
Where w5 sec is the wind pressure at height (z) averaged over 5
sec(m/s). Note the assumption that 5 sec gust windspeed (m/s) at
heightz 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) coincides with an ovalling frequency of the
shell.The fundamental ovalling frequency of unstiffened shells
isdetermined by:
f1 (0.5 t / d2) . E / s ... (7.12)Where EYoungs Modulus of the
steel shell
t the average shell thickness (in m) over the top thirdd the
shell diameter (in m)
s density of shell material
Substituting typical values of E and s, the associated
criticalwindspeed is then
Vcr 6,500 . t / d m/s ... (7.13)
These vibrations can be reduced sufficiently by stiffening
rings. Thedistance between stiffeners shall not exceed 9 d. The
associatedmoment of inertia of the stiffening ring section
(together with theparticipating length of shell) about its centroid
(see g 7.4) must belarger than:
I 1.75 105 d3 t m4 ... (7.14)
For closer spacing this value of I may be reduced
proportionately.
Note These spacing and minimum I requirements shouldnot be
confused with those of stiffeners sometimes requiredas
reinforcement to resist the static ovalling effect (7.2.5.1) orto
prevent local buckling, either during transport/erection oras a
result of the design wind load (8.3.4).
The participating length of the shell (d . t), but its area must
notexceed that of the stiffener ring (see Fig. 7.4).
Figure 7.4
7.2.6. The increase of wind effects by nearby structures
Interference effects, caused by the presence of a nearby
structureupwind of a chimney, can signicantly increase the chimneys
quasistatic wind load in the wind direction, described in 7.2.3 and
itsresponse, normal to the wind direction, described in 7.2.4. If
theinterfering structure is itself a chimney, its own response
whendownwind of the new chimney should be checked.
7.2.6.1 Effect on wind load in the wind direction
When interference effects are expected from a nearby structure,
thedesign windspeed per equation 7.1 used to determine the wind
loadshould be increased by a factor ki as dened below:-a) Where the
height of the interfering structure is less than half the
chimney height, ki 1.0b) Where the height of the interfering
structure is half chimney
height and it is approximately cylindrical in shape, ki
isdetermined from the following expression for values of a/dbetween
1 and 30 (see g. 7.5):ki 1.2 .0067a/d
a distance of chimney down-wind from the interferingstructure
(centre to centre)
d diameter of the interference structure
Fig. 7.5 Effect of interference on downwind loading
7.2.6.2 Effect on cross-wind response
When an approximately cylindrical structure (e.g. another
chimney)is upwind and within 15 diameters of a chimney of similar
or smallerheight, aerodynamic Wake Interference effects can
considerablyincrease the downwind chimneys cross-wind response (the
diameterconcerned being that of the interfering structure). The
increase is notyet fully understood, but is thought to be due to
increases in both liftcoefficient and negative aerodynamic damping.
Note thataerodynamic stabilisers (e.g. helical spoilers) are
ineffective incontrolling response in cases of wake
interference.For a spacing ratio (a/d) greater than 10, the
magnication factor kc,applied to response amplitude, calculated per
equation (7.12), may beestimated as follows:-
For a/d 15 :- kc 1.0For a/d 10 :- kc 1.5kc 2.5 0.1a/d for a/d
between 10 and 15
For a spacing ratio (a/d) less than 10 there is a risk of very
largeincreases in amplitude. In these circumstances the chimneys
structuraldamping 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 ScrutonNumber, the amplitude of response is expected to be
minimal.The associated critical windspeed and value of c2 in
equation(7.12) increase with decreasing values of a/d, due to a
reduction inthe value of the Stouhal Number. This can be important
in the designof a tuned mass damper. Fig. 7.6 shows the
relationship betweenStrouhal Number and a/d.
Centroid ofstiffener andparticipating
shell
e
t
d / 2
CL
d . t
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CICIND Model Code Amendment A March 2002 page 11
Figure 7.6 The reduction of Strouhal Number caused byaerodynamic
interference
When the interfering structure or chimney is less than 2
diametersaway, Interference Galloping can cause even greater
increases inthe chimneys response. Probably the best solutions in
this casewould be either to t tuned mass dampers, or to connect
structurally,the chimney to the interfering structure, using an
energy absorbingconnection system.
7.2.7 Damping ratio
The structural damping ratio ( c / ccr) without
aerodynamicdamping is given in table 7.4.
Type of chimney Damping Ratio
Unlined, uninsulated 0.002Unlined, externally insulated
0.003Lined with refractory concrete 0.005Lined with brickwork
0.015chimneys with steel liners*:-
26 0.006 28 0.002
Coupled group 0.004Chimney with tuned mass damper (0.02min) see
Appendix 2
Table 7.4
Notes: If rotation of foundation decreases the rst natural
frequencymore than about 10% the foundation is considered to be
softand the damping ratio may be increased by 0.0005. liner length
/ liner diameter* In order to ensure impact damping the gap between
theliner and its restraint should not be greater than 50mm.
The damping for wind loading in wind direction can be increased
bythe aerodynamic damping:
c / ccr 2.7 . 106 . V / (f1 . t) ... (7.15)
in which:
V is, for wind loading in wind direction, the wind speed V(z) at
thetop of the chimney (7.2.2.2)V 0 for cross-wind loadingf1 is the
fundamental natural frequency (7.2.8)t is the thickness of the wall
in the top third.Where chimneys are lined, t total mass per square
metre over thetop third (kg/m2) divided by 7850 kg/m3
7.2.8 The first and second natural frequencies
The rst natural frequency should preferably be calculated with
acomputer program. Care must be taken to include for the effects
ofany supporting structure. Assuming a chimney is on a rigid
support,its rst natural frequency may be calculated by dividing it
into asuitable number of sections using the formula (for the rst
mode):
f1 (1 / 2 .) . [ge .ms x /ms x2] (sec1) ... (7.16)
in which:
ms is the mass of the section including the lining orcovering
(in kg)
x is the deection of the same section due to the force equal
togravity acting normal to the centre-line at the mass centre
(m).
ge is the value of gravitational acceleration (m/s2)Accurate
estimation of the second natural frequency requires the use ofa
nite element structural program with a dynamic capability or
otheradvanced computer program. For a chimney with constant
diameterand thickness, however, the following expression may be
used:-
f2 3.5 . (E . I/ m .L4) ... (7.17)
Where EYoungs ModulusIMoment of inertia of cross sectionmmass
per unit length
7.2.9 Passive Dynamic Control
Steel chimneys must be designed to suppress excessive
cross-windmovement. Several options are available to the
designer.
7.2.9.1 Aerodynamic stabilizers
When a chimney stands alone, its cross-wind vibrations can
usuallybe reduced by aerodynamic stabilizers. The useful effect of
threecontinuous helical vanes has been proved on many steel
chimneys.The radial width of the vanes must be 10% of the diameter.
The pitchof the vanes should be 5 D. The vanes must be tted over at
least theupper 1/3 of the height. The extra wind drag due to the
vanes must beconsidered (see 7.2.3.2.3).Aerodynamic stabilisers
will not reduce the wind interference effectsof nearby chimneys or
structures.
7.2.9.2. Damping devices
Damping devices are attached to a chimney to increase its
structuraldamping, thereby signicantly reducing the cross-wind and
along-wind vibrations, including the effects of aerodynamic
interference byother nearby towers or chimneys. Damping devices
should hedesigned to avoid the need for their frequent routine
maintenance.
Most such dampers are mounted near the top of the
chimney.Because of their prole and small size, the associated
increase inwind drag is minimised. The use of damping devices,
therefore, hasbeen proved to be benecial in the design of steel
chimneys and theycan be safely retro-tted without incurring
signicant increase inwind drag loads.
Tuned mass dampers provide an extra mass, coupled to the
chimneyby an energy absorbing medium, which absorbs the wind
inducedenergy. Tuned mass dampers have proven effective in reducing
self-generated along wind and cross-wind vibrations and also the
effect ofnearby chimneys or structures.
Other chimney damping devices such as hanging chains have
alsobeen successfully used.
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page 12 CICIND Model Code
7.2.10 Special chimney designs for damping
Wind tunnel tests, conrmed by analytical means and eld
experience,have allowed dual-wall and multiue chimneys to be
designed usingshell-to-shell impact damping, which otherwise would
requireaerodynamic stabilisers or mass dampers (see ref. [14] &
[15]).Future special chimney designs and damping devices may
proveeffective in preventing excessive wind induced vibrations.
Theseshould have been proven initially by wind tunnel tests and
nally byeld experience before being universally adopted.
7.3. Earthquake loading
The stress due to wind loading on a steel chimney is usually
morethan the earthquake stress and, consequently, normal steel
chimneyscan resist earthquakes with an intensity of up to modied
Mercalliscale 10 without serious damage. However, in cases where a
heavymass (e.g. a water tank or a heavy lining) is tted to the
upper portionof the chimney, a special investigation must be made
(tanks areoutside the scope of the model code). Guyed chimneys must
also besubject to special investigation.
7.4. Thermal effects
When a chimney is restrained from adopting a deformed shape
inresponse to differential expansion, bending stresses will
beintroduced in the shell. These deformations can be large when
asingle unlined chimney carries ue gases from two or more sourcesat
signicantly different temperatures or if a single side entry
sourceintroduces gases at very high temperatures. In addition, the
resultingdifferential metal temperature will introduce secondary
thermalstresses. Typical cases of such restraint are to be found in
stayed andguyed chimneys. More information on the derivation of
thosestresses may be obtained from the CICIND Model Code for
ConcreteChimneys 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
thatstrengthening for this reason is required, it is outside the
scope of thismodel code.
7.5.2. Internal explosions
Internal explosions can occur due to the ignition of soot or
explosivegases in the chimney. They are not normally a cause for
concern inthe design of a steel chimney. The CICIND Model Code for
ConcreteChimneys Part B, Brickwork Linings provides a reference for
thelikely 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 norexternal insulation, the metal temperature can be assumed
to be aboutmidway between ambient air temperature and that of the
ue gas overthe range of ue gas velocities between 5m/s and 15m/s.
For ue gasvelocities faster than 15m/s or for steel stacks equipped
with either a lineror external insulation, heat transfer
calculations shall be made todetermine the maximum metal
temperature of the structural shell. Thesecalculations shall assume
still air and highest anticipated air temperature.
Consideration must be given to the effects of oxidation when
thematerial being used is close to its temperature limit. This is
especiallyso with gas turbine exhausts, where levels of excess air
can be greaterthan those normally experienced. This problem may not
be solvedsolely by an increase in corrosion allowance as the
environment maybe polluted by the corrosion product. Expert advice
should be soughton the choice of suitable material.
7.6.2. Fire
The risk of a chimney re should be assessed. Chimney res can
becaused by ignition of:1) Unburned fuel carried over from the
associated boiler or furnace.2) Where the associated furnace is in
petrochemical service,
unburned hydrocarbon carryover following a furnace tube
rupture.3) Soot, sulphur and other deposits.During chimney res, the
radiant heat loss to atmosphere from a baresteel chimney is often
sufficient to maintain its temperature at areasonable level. By
contrast an externally insulated steel chimney ora bare steel
chimney close to a reective surface will quickly buckleduring a re.
In such cases, if the risk of internal re is signicant, arefractory
concrete internal liner should be installed to provide adegree of
re protection. Typically, a castable refractory liningfollowing the
requirements of Appendix 3 will provide sufficient reprotection for
most situations.
7.6.3. Chemical effects
Limited exposure to acid corrosion conditions can be permitted
inchimneys which, for most of the time, are safe from chemical
attack.Providing the ue gas does not contain signicant
concentrations ofhalogens (see notes (4) & (5) below) the
degree of chemical load isdened in Table 7.5.
Degree of Operating hours per year whenchemical load temperature
of the surface in contact with flue
gases is below estimated acid dew point 10C
low 25medium 25 100high 100
Table 7.5Degree of chemical load for gases containing sulphur
oxides
Notes:
1) The operating hours in table 7.3 are valid for an S03 content
of 15ppm. For different values of S03 content, the hours given
varyinversely with S03 content. When the S03 content is not
known,chimney design should be based upon a minimum S03
contentamounting to 2% of the SO2 content in the ue gas.
2) In assessing the number of hours during which a chimney
issubject to chemical load, account should be taken of start-up
andshut-down periods when the ue gas temperature is below itsacid
dew point.
3) While a steel chimney may generally be at a temperature
aboveacid dew point, care should be taken to prevent small areas
beingsubject to local cooling and therefore being at risk of
localisedacid corrosion. Local cooling may be due to: air leaks n
cooling of anges, spoilers or other attachments cooling through
support points downdraft effects at top of the chimney
4) The presence of chlorides or uorides in the ue gas
condensatecan radically increase corrosion rates. Estimation of the
corrosionrate in these circumstances depends upon a number of
complexfactors and would require the advice of a corrosion expert
in eachindividual case. However, in the absence of such advice,
providedthe concentrations of HCl 30mg/m3 or of HF 5mg/m3 and ifthe
operating time below acid dew point does not exceed 25 hoursper
year, the degree of chemical load may be regarded as low.
5) Regardless of temperatures, chemical load shall be
consideredhigh if halogen concentrations exceed the following
limits:Hydrogen uoride: 0.025% by weight (300 mg/m3 at 20C and 1bar
pressure)
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CICIND Model Code Amendment A March 2002 page 13
Elementary chlorine: 0.1% by weight (1300 mg/m3 at 20C and1 bar
pressure)Hydrogen chloride: 0.1% by weight (1300 mg/m3 at 20C and
1bar pressure)
6) Saturated or condensing ue gas conditions downstream of a
uegas desulphurisation system shall always be considered ascausing
High chemical load.
8. DESIGN OF STRUCTURAL SHELL
8.1 Minimum thickness
At the time of construction the minimum thickness of the shell
ofcarbon steel chimneys shall be 5mm, including the
corrosionallowance.
8.2. Required checks
The steel shell of a chimney shall be checked for: carrying
capacity serviceability fatigue (unless the chimney is tted with an
effective
dynamic control)The carrying capacity check shall prove that the
forces resulting fromthe working loads multiplied by the load
factors do not exceed theresistance of the shell. The check should
comprise both the strength andstability proof. The calculations
shall be carried out for the corrodedthickness of the steel
(without corrosion allowance). The serviceabilityshall be checked
under working loads without load factors.A fatigue check shall be
carried out if movement due to vortexshedding is expected (see
7.2.4).For unstiffened chimneys with a ratio of L/R 50 (where L
heightof chimney and R radius), stresses may be safely
calculatedassuming beam theory, exural stresses being added
vectorially toovalling stresses. For unstiffened chimneys (i.e.
chimneys withoutstiffening rings or substantial anged joints)
having L/R 50, shelltheory or nite element modelling should be
used, consideringexural and ovalling stresses simultaneously. This
will lead toreduction in compression stress at the chimney base or
immediatelyabove changes in chimney diameter, but will increase
compressionstresses elsewhere. Similarly, this will lead to
increases in tensilestresses at the base and immediately above a
change in chimneydiameter, which will be important in deriving bolt
tensions.The increase in tensile stress in these regions may be
approximatedby the expression:-
1 {6 / [(L/R)2 . (t/R)]}
8.3. Carrying capacity of shell
8.3.1. Load factors and load combinations
The chimney shell shall be designed to resist stresses resulting
fromthe weight of the chimney and the effect of wind multiplied by
theload factors :
(i i) (i*) fk ... (8.1)where:
i* stresses multiplied by load factorsfk limit stress of
steel
8.3.2. Second order effect
The effect of the displacement of the load application points
due todeformations (second order effect) shall be taken into
considerationif the parameter 0.6, where:-
h (N / E I)0.5 ... (8.2)
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 M11 is approximately
determined from:-
M11 M1 (1 2 / 8)Where M1 is the wind moment at any particular
level.This simplied approximation may only be used when 0.8 andNh /
N 0.1. It is not applicable to guyed chimneys.Where Nh is the
design value of the total vertical load at the top ofthe shell.
8.3.3. Biaxial stresses
In areas subjected to biaxial stresses e.g. due to bending
moments andovalling, the carrying capacity check shall be based
on
{*x2 *y2 (*x*y) 3*2} fk ... (8.3)Note The ovalling stresses are
both negative and positive and themaximum value of expression (8.3)
occurs when *x and *y are ofopposite signs.
8.3.4. Stability
The proof of stability of the shell is given if the critical
bucklingstress divided by 1.1 is greater than the sum of
longitudinal stressesdue to bending and compression:
*N *B k /m ... (8.4)where:*N, *B normal and bending compressive
stress at
ultimate limit statem material factor 1.10k critical buckling
stress
(1.0 0.412 1.2) fy when 2 ... (8.5a) 0.75 fy /2 when 2 ...
(8.5b)
fy yield strength of steel at design temperature fy / ( cr) ...
(8.6)cr critical elastic buckling stress 0.605 E t/r ... (8.7)E
Youngs modulus of steel at design temperaturet corroded plate
thicknessr radius of the structural shell of the chimney at
section considered
... (8.8)N *N B *B*N *B
tensile stress per shell theorytensile stress per beam
theory
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page 14 Amendment A March 2002 CICIND Model Code
When imperfections w are smaller than 0.01l (Fig. 8.1):N 0.83 /
(1 r / 100t)0.5 for r/t 212 ... (8.9a)
and: N 0.7 / (0.1 r / 100t)0.5 for r/t 212 ... (8.9b)B 0.189
0.811 N
If the imperfections (w) are between 0.01l and 0.02l (see Fig.
8.1) theabove formulae may be used if 1 is substituted for a:
1 [1.5w / 0.02 l] ... (8.9c)Imperfections (w) greater than 0.02l
shall not be permitted.Stiffeners may be used to increase the
shells resistance to buckling.Guidance on the design of such
stiffeners is given in CICIND ModelCode for Concrete Chimneys Part
C Steel Liners.
Figure 8.1
8.4. Serviceability of shell
The downwind deection from the centreline of the structural
shellunder maximum design wind load must be calculated and
reported.As long as the carrying capacity stresses in the
structural shell, or anyliners, is not exceeded, no limit is placed
on downwind deection.
So as not to alarm bystanders, the amplitude of deection from
thechimney centreline caused by vortex shedding shall not be
greaterthan 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
vortexshedding does not result in the initiation and gradual
propagation ofcracks in the material, especially near welds, thus
resulting nally inthe failure of a weakened section. The fatigue of
the material dependsessentially on:
the number of stress cycles N
the stress range (max min) the constructional details
The inuence of the grade of steel as well as that of the min
/maxratio are negligible.
8.5.2. Fatigue strength
The number of load cycles in the cross-wind direction can
becalculated from:-
N 1.26 107T fA eA2where:-
T The required lifetime of the chimney in years
f The resonance frequency
A 4Vcr / V
V The design wind velocity V(z) at the top ofthe chimney
The amplitude of movement varies, with maximum movement
onlyrepresenting a small proportion of the total number of cycles.
Theeffect of fatigue due to all of the load cycles can be expressed
byconsidering the factored Miner Number M*:-Where M* .M (max /wn)k
(logeN)kWhere:-max The maximum stress range due to vortex
sheddingwn The fatigue strength after N cycles (see gs. 8.2 &
8.3)k the (positive) exponent of the fatigue curves. for steel, k3
Determines the load vs. cycles relationship (Vcr / 8)1.2 Modelling
safety factor 1.4
(for temperatures up to 200C)If the factored Miner Number (M*)
is less than 0.2 no cracking willoccur during the required
lifetime. Nevertheless, occasionallymovement amplitude may be
sufficient to cause alarm. In such casesthe amplitude limitation of
Section 5.4 may govern.
Figure 8.2 Fatigue strength of the base materialwith respect to
the fatigue categories defined in Figure 8.3
8.5.3. Influence of high temperatures
The few results available show that at 200C fatigue growth
ratesmay be higher than at room temperature, but at 400C growth
ratesare lower than at room temperature. Unless more detailed
resultsbecome available the modelling safety factor shall be
increased to1.50 in the range of metal temperatures between 200 to
400C.
8.6. Allowance for corrosion
Allowance for corrosion shall be the sum of the external (CE)
andinternal (Cl) allowances given in tables 8.1 and 8.2. This
totalallowance shall be added to the thickness of the shell
required tosatisfy the specied limits of stress and deection.
Internal angesshall have corrosion allowance Cl and external anges
corrosionallowance CE on all exposed surfaces. The allowances
listed in tables8.1 and 8.2 are for a 20 year lifetime of the
chimney. For longerplanned lifetimes, the corrosion allowances
should be increasedproportionally. For temporary chimneys, expected
to be in service forless than one year, values of CE and CI 0 are
permissible, exceptin conditions of high chemical load, when a
corrosion allowance of3mm is required.For a free-standing chimney
with steel liner(s), the internal corrosionallowance only applies
to the internal face of the liner(s). The internal
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CICIND Model Code page 15
Figure 8.3 Fatigue resistance of typical details(continued on
pages 16, 17 and 18)
From ENV 1993-3-2 : 1997 Eurocode 3: Design of Steel Structures
Part 3.2 Chimneys
Notes to Fig. 8.3
Type of welding:1. butt welds, when high quality has to be
acheived
and veried: developed root, cap pass counter welding evenly
machined surface in stress direction.
2. butt weld: developed root, cap pass counter welding3. 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 only5. T joint by double-bevel
butt weld6. T joint by double Y butt weld with broad root face7. T
joint with special quality double llet weld8. T joint double llet
welds
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page 16 CICIND Model Code
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page 18 CICIND Model Code
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CICIND Model Code page 19
face of the outer shell requires no corrosion allowance,
provided aweather-tight cover is tted over the air space(s) between
the liner(s)and the outer shell.
8.6.1. External corrosion allowance
painted carbon steel 0mmpainted carbon steel under
insulation/cladding 1mmunprotected carbon steel 3mmunprotected
corten or similar steel 1mmunprotected stainless steel 0mm
Table 8.1. External corrosion allowance (CE)
Note:
The external corrosion allowances quoted in Table 8.1 are
suitable fora normal environment. When a chimney is sited in an
aggressiveenvironment, caused by industrial pollution, nearby
chimneys orclose proximity to the sea, consideration should be
given toincreasing these allowances.
8.6.2. Internal corrosion allowance
Usual temperature Chemicalof metal in contact load per Internal
corrosion allowancewith flue gas table 7.5
65C low not applicable (chem. load always high)*medium not
applicable (chem. load always high)*high corrosion allowance
inappropriate, use other
material*65C 345C low 2mm**
medium 4mm
high corrosion allowance inappropriate, use othermaterial
345C low 1mmmedium 2mmhigh corrosion allowance inappropriate,
use other
material
Table 8.2 internal corrosion allowance (CI) for carbon steelonly
(for chimneys handling flue gases)
Notes:
* Provided acid concentration in the condensate is less than 5%
andchloride concentration does not exceed 30mg/M3, highmolybdenum
stainless steel (such as ASTM Type 316L) issuitable within this
temperature limit, using a corrosion allowanceof 3mm for a 20 year
life. These conditions are, however, unlikelyto be met in a chimney
downstream of a FGD system, generatingcondensing gases. In these
circumstances great care is required inthe protection of the gas
face of the chimney or its liner, e.g. bycladding with a suitable
high nickel alloy or titanium or by theapplication of a suitable
organic coating. For further guidance, seethe CICIND Chimney
Coatings Manual.
** In conditions of low chemical load, Corten steel shows
someimprovement of corrosion resistance over carbon steel,
especiallywhen contact with condensing SO2/SO3 is intermittent or
of shortduration (e.g. during repeated shut-downs).
+ In these circumstances, ordinary stainless steels (including
highmolybdenum stainless steel) have little better
corrosionresistance 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 furtherguidance, see the 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 greatas the design forces of the parts they connect e.g. the
carryingcapacity check shall be carried out with the same load
factors andload combinations as described under 8.3.1.
9.1.2. Bolted connections
The carrying capacity of bolted connections shall be checked
withregard to tension and shear or bearing.
9.1.2.1. Shear
The shear stresses multiplied by the load factors shall not
exceed thelimit shear stress divided by resistance factor 1.1:
t* u / 1.1 ... (9.1)
The values of limit shear stress are given in Table 9.1.
bolt grade minimum value of the tensile u u/1.1strength of
bolts
4.6 400 200 1825.6 500 250 2276.8 600 300 2788.8 800 400 364
10.9 1000 500 455
Table 9.1 Limit shear stress (U) in MPa.
The design shear stress * relates to the gross area or to the
nett area,depending on whether the shear plane is in the unthreaded
orthreaded part of the bolt.
9.1.2.2. Bearing on connected surfaces
The design stress on connecting parts shall not exceed the
minimumvalue of the tensile strength of the connected parts
multiplied by1.45:
*l l,u / 1.1 1.45u ... (9.2)The design bearing stress *l relates
to the area obtained bymultiplying the diameter d of the shank by
the thickness of theconnected part. Regardless of any preload, the
limit stress l,u is validfor edge distances greater or equal 2d in
the direction of stress.
Grade l,u l,u / 1.1
Fe 360 575 525Fe 430 690 625Fe 510 815 740
Table 9.2 Limit bearing stress l,u in MPa
9.1.2.3. Tension
The limit state is described:
*t t,u / 1.1 0.73u,B ... (9.3)for t,u see table 9.3
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page 20 CICIND Model Code
Minimum value of Limit tensile stresstensile strength of
preloaded bolts
bolt grade u,B t,u t,u/1.1
4.6 400 not recommended5.6 500 not recommended6.8 600 not
recommended8.8 800 640 58010.9 1000 800 730
Table 9.3Limit tensile stress t,u in MPa.
Note! The stresses given in Tables 9.2 and 9.3 are for
ambienttemperatures. For stresses at elevated temperatures refer
tothe factors in column 2 of Table 6.2.
The tensile stress t shall be calculated on the nett
section.
Owing to their considerable susceptibility to fatigue,
connections thatuse bolts in tension shall be made with
pretensioned high strength bolts.
9.1.2.4. Combined loading
If the external loading results in a combination of tensile
stress t*and shear stress * in the bolt, the carrying capacity
shall be checkedfor the condition:
(* / u)2 (t* /t,u)2 1.0 ... (9.4)This check is not necessary
if:
* 0.2 u or t* 0.2t,u ... (9 5)
9.1.2.5. Deduction for holes
For parts subjected to tension, the following two conditions
shall bechecked:
in the gross section, the stress shall not exceed the yield
stress fy in the nett section, the stress shall not exceed 80% of
the tensile
strength u
9.1.3. Welded connections
The welding standard considered appropriate for steel chimneys
ishigher than the minimum standard allowed for other
weldedproducts. 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,full penetration welds have the same resistance as the
connectedparts. In this case, no particular checks are necessary.
Partialpenetration welds shall be taken as llet welds and
calculated assuch. Full penetration welds connecting plates of
differentthicknesses have a resistance equal at least to that of
the thinnestplate. 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
w* ands* for llet welds shall be checked:
in the throat section a-a: w* w,u / 1.1 0.455uE in the contact
section s-s: s* s,u / 1.1 0.636 fy
where uE is the guarantied minimum value of the tensile strength
ofthe weld metal and fy the yield stress of the parent
material.
Throat section Contact sectiongrade w,u w,u /1.1 s,u s,u /
1.1
Fe 360 255 230 165 150Fe 430 255 230 180 165Fe 510 255 230 250
230
Table 9.4. Limit stresses w,u and s,u for fillet welds in
MPa
The yield stress, tensile strength, strain at failure and notch
toughness ofthe weld metal shall exceed minimum values for parent
material, and,failing a specic agreement, shall be at least equal
to those of Fe510.w,u values given in table 9.4 are valid for
electrodes with propertiesof steel Fe 510.
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CICIND Model Code page 21
9.1.3.3 Weld Testing
While a minimum, taken at random, of 10% of butt welds and
lletwelds shall be tested, the weld testing procedures and quality
levels shallbe agreed by the client and the builder. The
recommendations of levelsCof ISO 5817 Arc-welded joints in steel
guidance on quality levelsand imperfections should be used, but
subject to agreement between theclient and builder, local codes may
be substituted.Note The fatigue categories listed in g. 8.3 assume
welds are made
to ISO 5817 level C quality standards. If local codes areused,
the weld categories may require appropriate adjustment.
9.2 Flanged connections
The use of high strength bolts is recommended. The centres
betweenthe bolts should be between 4db and 10db, where db is the
diameter ofthe bolt. However, a distance of 5db is recommended as
largerspacings result in excessively thick anges. The minimum
boltdiameter should be db 16mm. The stress in the bolts shall
becalculated taking consideration of the eccentricity of the
loadingtransmitted by the shell.
Fig. 9.2.1 Normal flange
In the case of along wind: Z*b Z* a /w 0.73u,b An ... (9.6)In
the case of cross-vibration (fatigue):Zb,f Zf a / w RAn / 1.1 ...
(9.7)where:R is the fatigue strength for category 35 MPaAn is the
stress section of the boltIf the fatigue load Zf is greater than
the fatigue strength divided by1.10, a joint with contact areas
shall be used (see lit. [22] and g.9.2.2). The pretension of the
bolts should provide a sufficient forceZA to prevent the fatigue in
the bolt material:
ZA 0.73 u,b An w/a Zf ... (9.7)
Fig. 9.2.2 Prestressed flange, suitable for vibrating
conditions
It should be noted that the change of the type of connection to
onewith proled contact areas may reduce the damping ratio used
inestimating along and across-wind response. The tting of gaskets
tothe anges of structural shells is not permitted.
9.3. The support at the base
Self-supporting steel chimneys are normally based on a
reinforcedconcrete foundation or a steel structure. The foundation
or structureis loaded by an overturning moment, normal force and
shear forcethrough the base plate and anchor bolts.
9.3.1. Anchor bolts
When fatigue due to vortex shedding is anticipated anchor
boltsshould be prestressed. Measures must be taken to ensure that
theprestressing is not lost during the lifetime of the chimney.
Ananchorage device shall be attached to the bottom end of the
bolt.The maximum bolt stress should not exceed 73% of the
tensilestrength of the material of anchor bolt. Alternative
satisfactorymethods may be used at the designers discretion when no
responseto vortex shedding is anticipated.
9.3.2. Grouting
After the chimney has been erected and plumbed (with the use
ofsteel shims which remain in position) the space between the
baseplate and concrete foundation must be lled with nonshrink
grout.The compressive strength of the grout must be equal to or
greaterthan the compressive strength of the concrete.
9.3.3 Temperature effects
Consideration must be given to the effect that radiant or
conductedheat will have upon a concrete foundation. This is
particularlyrelevant to chimneys serving gas turbines or other high
temperatureexhaust systems.There is the possibility of the
foundation being damaged if anadequate heat barrier is not
installed. In the majority of situationsinsulation to contain or
deect radiant heat will suffice.
10. STEEL LINERS
Steel liners inside steel chimneys shall be designed to satisfy
therequirements of CICIND Model Code for Concrete Chimneys Part C
Steel Liners. Advice on the design of steel liners in steelchimneys
is given in Appendix 3 to this Model Code.
11. CONSTRUCTION
11.1 General
The following will be observed during shop and site construction
asappropriate.
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 within1mm in length, width and on each diagonal.A chimney
section, with anges welded in place, shall be fabricatedwithin a
tolerance of 3mm on circumference and diagonal. Ifpossible, these
measurements shall be made while the shells axis isvertical. If
this is not possible, the shell shall be adequately braced.Peaking
of a cylinder from a true circle at weld seams shall notexceed 3mm,
as measured by a 450mm long template, centred at theweld and cut to
the cylinders design radius. Other imperfectionsshall be within the
limits stated in section 8.3.4 of this model codeand assumed by the
designer.Vertical butt weld seams shall be staggered a minimum of
200mmfrom eachother.Misalignment between plates shall not exceed
1mm.
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page 22 CICIND Model Code
11.3 Structural Flanges and opening reinforcement
These shall be fully welded to the structural shell.
Intermittentwelding shall not be allowed.
Flanges shall be at and normal to the chimney axis. Before
bolting, themaximum gap width on the line of the shell, between
matching pairs ofanges, shall be 1mm. Before bolting, the gap at
the outer edges of theanges shall not exceed 1.5mm per 100mm width
of ange.Note: These tolerances may be ignored if the anges are
bolted togetherbefore they are welded to their respective shell
sections. Their orientationshall be marked prior to their being
dismantled after welding.
11.4 Stiffening Rings
If the design permits the use of intermittent welding,
crevicesexposed to weather or ue gases shall be sealed.
11.5 Baseplate
The baseplate and all base reinforcement shall be fully welded
to thestructural 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
thelongitudinal direction to a tolerance of 12mm per 10m of
shelllength.
Flanges shall be welded to the structural shell within a
perpendiculartolerance of 0.5.
11.7 Erection tolerance
The departure of the chimney from the vertical on erection shall
notexceed 25 mm or 1/600 of the height, whichever is the greater at
anypoint.
12. SURFACE PROTECTION
The exterior and interior surfaces of a steel chimney may
beprotected from attack by weather and corrosive gasses by
variousmethods. Specications for different types of protection are
given inAppendix 3. See also CICIND Chimney Protection Coatings
Manual.
13. OPENINGS
The width of a single opening shall not exceed two-thirds of
thediameter of the structural shell of the chimney.
Where large apertures are cut in the shell plates, as for gas
inlets orinspection panels, a structural analysis of the stresses
shall be madeand compensating material provided, as required, to
ensure that thestresses specied in this Model Code are not
exceeded. As a result, itmay be necessary to incorporate stiffeners
around the opening. Whenlongitudinal stiffeners are used, their
design shall include the effectsof circumferential bending stresses
in the shell, above and below theopening. Also they shall be long
enough to distribute stresses into themain area of the shell
without overstress. (Note: this may generallybe deemed to be
satised if the stiffeners project above and below theopening a
distance at least 0.5 times the spacing of the stiffeners.).The
ends of the longitudinal stiffeners should be tapered in a
radialdirection (see cases 16.1 3 in Fig. 8.3).Additional
horizontal stiffeners may be used to absorb thecircumferential
bending stresses. These stiffeners may be attachedbetween the
longitudinal stiffeners, at the holes edge and at the endof the
longitudinal stiffeners.
A suggestion for stiffeners is given in the Commentaries for
thisModel 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 thethickness of the plate.
The effect of openings upon the chimneys stiffness should be
takeninto account when determining the chimneys natural
frequencies.
14 GUYED AND STAYED CHIMNEYS
A stayed chimney is dened as one which derives lateral (but
notvertical) support from another structure. A guyed chimney
deriveslateral support from guy ropes.
The foregoing structural design rules are valid for
self-supportedchimneys, acting as cantilevers, xed at their bases,
with or withoutliners. Some of the rules (e.g. those related to
thermal and chemicalload) are relevant also to chimneys that are
guyed or stayed. Rulesgoverning the structural design, related to
wind or earthquake loadingdo not, however, apply to these
chimneys.
14.1 Stayed chimneys
Stayed chimneys are supported laterally at one or more
elevationsabove their bases. The number of lateral supports will be
governed bybuckling considerations per section 8.3.4 above and by
the need toavoid oscillations due to vortex shedding, but shall be
kept to theminimum possible. To avoid vibrations due to vortex
shedding, thenatural frequencies should ensure that Vcr (assumingS
0.2) 1.2maximum windspeed at the relevant elevation (10minute
mean). The prime concern of the design should be to ensurethat
vertical expansion is not restricted.
In designing the shell and lateral supports, the forces induced
by therestraint of differential thermal expansion shall be
considered.Differential expansion can be expected if two or more
gas streams ofdiffering temperatures enter the chimney at different
points. Guidanceon the determination of these forces may be found
in CICIND ModelCode for Concrete Chimneys, Part C Steel Liners.
The design of the supporting structure is outside the scope of
thisModel Code.
14.2 Guyed Chimneys
Design rules for Guyed chimneys are given in Appendix 4 to
thisModel Code
15. PROTECTION AGAINST LIGHTNING
A steel chimney can be considered as a continuous metal
structureand thus be used as its own lightning protection
system.Consequently it requires no air termination or down
conductor. It issufficient to ensure that the conduction path is
electrically continuousand that it is adequately earthed.
16. ACCESS LADDERS
A specication 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 areaif the chimney is to be built within an aerodrome safe
guarding areaas local conditions and restrictions may apply.