cicind model code for chimney

CICIND

Model Code for

Steel Chimneys

Revision 1 – 1999

Amendment A – March 2002

Copyright CICIND 1999

ISBN 1-902998-09-X

Office of The Secretary, 14 The Chestnuts, Beechwood Park, Hemel Hempstead, Herts., HP3 0DZ, UK

Tel: +44 (0)1442 211204 Fax: +44 (0)1442 256155 e-mail: [email protected]

Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

10 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

0.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

0.2 Appendices and Commentaries . . . . . . . . . . . . . . . . . . 3

0.3 Philosophy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

11 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

12 Field of Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

13 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

14 Notations, Units and Definitions . . . . . . . . . . . . . . . . . . . . . 4

4.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

4.2 Subscripts-Superscripts . . . . . . . . . . . . . . . . . . . . . . . . 4

4.3 Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

4.4 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

15 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

16 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

6.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

6.2 Structural steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

6.3 Stainless and alloy steels . . . . . . . . . . . . . . . . . . . . . . . 6

17 Actions (External and Internal) . . . . . . . . . . . . . . . . . . . . . 6

7.1 Permanent Load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

7.1.1 Dust load (temporary load) . . . . . . . . . . . . . . . 6

7.2 Wind . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

7.2.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

7.2.2 Wind Speed . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

7.2.2.1 Basic wind speed . . . . . . . . . . . . . . . . 6

7.2.2.2 Design wind speed . . . . . . . . . . . . . . . 7

7.2.2.3 The influence of topography . . . . . . . . 7

7.2.3 Wind load in direction of the wind . . . . . . . . . . 8

7.2.3.1 Wind load on isolated chimneys . . . . . 8

7.2.3.2 Mean hourly wind load . . . . . . . . . . . . 8

7.2.3.3 Effect of fluctuating part of

the wind-speed . . . . . . . . . . . . . . . . . . 8

7.2.4 Vortex shedding . . . . . . . . . . . . . . . . . . . . . . . . 8

7.2.4.1 General principles . . . . . . . . . . . . . . . 8

7.2.4.2 Estimation of top amplitudes . . . . . . . 9

7.2.4.3 Bending Moments due to

vortex shedding . . . . . . . . . . . . . . . . . 9

7.2.5 Ovalling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

7.2.5.1 Static effects . . . . . . . . . . . . . . . . . . . . 9

7.2.5.2 Dynamic effects . . . . . . . . . . . . . . . . 10

7.2.6 The increase of wind effects by

nearby structures . . . . . . . . . . . . . . . . . . . . . . 10

7.2.6.1 Increase in along-wind load . . . . . . . 10

7.2.6.2 Increase in cross-wind response . . . . 10

7.2.7 Damping ratio . . . . . . . . . . . . . . . . . . . . . . . . 11

7.2.8 The first and second natural frequencies . . . . . 11

7.2.9 Passive dynamic control . . . . . . . . . . . . . . . . . 11

7.2.9.1 Aerodynamic stabilisers . . . . . . . . . . 11

7.2.9.2 Damping devices . . . . . . . . . . . . . . . 11

7.2.9.3 Special chimney designs

for damping . . . . . . . . . . . . . . . . . . . 12

7.3 Earthquake loading . . . . . . . . . . . . . . . . . . . . . . . . . . 12

7.4 Thermal effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

7.5 Explosions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

7.5.1 External explosions . . . . . . . . . . . . . . . . . . . . 12

7.5.2 Internal explosions . . . . . . . . . . . . . . . . . . . . . 12

7.6 Internal effects governing the chimney design . . . . . . 12

7.6.1 High temperature flue gases . . . . . . . . . . . . . . 12

7.6.2 Fire . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

7.6.3 Chemical effects . . . . . . . . . . . . . . . . . . . . . . 12

18 Design of Structural Shell . . . . . . . . . . . . . . . . . . . . . . . . . 13

8.1 Minimum thickness . . . . . . . . . . . . . . . . . . . . . . . . . . 13

8.2 Required checks . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

8.3 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 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 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

CICIND Model Code

8.3.1 Load factors and load combinations . . . . . . . . 13

8.3.2 Second order effects . . . . . . . . . . . . . . . . . . . 13

8.3.3 Biaxial stresses . . . . . . . . . . . . . . . . . . . . . . . 13

8.3.4 Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

8.4 Serviceability of shell . . . . . . . . . . . . . . . . . . . . . . . . 14

8.5 Fatigue check . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

8.5.1 Basic principles . . . . . . . . . . . . . . . . . . . . . . . 14

8.5.2 Fatigue strength . . . . . . . . . . . . . . . . . . . . . . . 14

8.5.3 Influence of high temperature . . . . . . . . . . . . 14

8.6 Allowance for corrosion . . . . . . . . . . . . . . . . . . . . . . 14

8.6.1 External corrosion allowance . . . . . . . . . . . . . 19

8.6.2 Internal corrosion allowance . . . . . . . . . . . . . 19

19 Design Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

9.1 Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

9.1.1 General provisions . . . . . . . . . . . . . . . . . . . . . 19

9.1.2 Bolted connections . . . . . . . . . . . . . . . . . . . . 19

9.1.2.1 Shear . . . . . . . . . . . . . . . . . . . . . . . . 19

9.1.2.2 Bearing on connected surfaces . . . . . 19

9.1.2.3 Tension . . . . . . . . . . . . . . . . . . . . . . . 19

9.1.2.4 Combined loading . . . . . . . . . . . . . . 20

9.1.2.5 Deduction for holes . . . . . . . . . . . . . 20

9.1.3 Welded connections . . . . . . . . . . . . . . . . . . . . 20

9.1.3.1 Full penetration welds . . . . . . . . . . . 20

9.1.3.2 Fillet welds . . . . . . . . . . . . . . . . . . . . 20

9.1.3.3 Weld testing . . . . . . . . . . . . . . . . . . . 21

9.2 Flanged connections . . . . . . . . . . . . . . . . . . . . . . . . . 21

9.3 The support at the base . . . . . . . . . . . . . . . . . . . . . . . 21

9.3.1 Anchor bolts . . . . . . . . . . . . . . . . . . . . . . . . . 21

9.3.2 Grouting . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

9.3.3 Temperature effects . . . . . . . . . . . . . . . . . . . . 21

10 Steel liners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

11 Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

11.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

11.2 Structural shell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

11.3 Structural flanges and opening reinforcement . . . . . . . 22

11.4 Stiffening rings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

11.5 Base plate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

11.6 Straightness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

11.7 Erection tolerance . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

12 Surface Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

13 Openings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

14 Guyed and Stayed Chimneys . . . . . . . . . . . . . . . . . . . . . . 22

14.1 Stayed chimneys . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

14.2 Guyed chimneys . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

15 Protection Against Lightning . . . . . . . . . . . . . . . . . . . . . . 22

16 Access Ladders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

17 Aircraft Warning Lights . . . . . . . . . . . . . . . . . . . . . . . . . . 22

CICIND Model Code page 3

FOREWORD

When it was formed in 1973, the “Comité International desCheminées 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 Denmark G. 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 (flaring off) of industrial waste gases.

This Model Code contains guide-lines which reflect 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 appropriatelyqualified and experienced engineers. The construction and erectionshould be carried out by firms competent in this class of work. At alltimes the work should be under the direction of appropriatelyqualified 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 justified.The commentaries have the following objectives:

a) Justification of the regulations of the model code.

b) Simplification of the use of the model code.

c) Understanding of the meaning of the regulations of themodel code.

d) Documentation of the areas in the model code where the presentknowledge 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) Definition 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 simplifies 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 simple’are 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 of steelwork design and construction only when this was required by thephilosophy outlined above or by specific chimney requirements.

1 SCOPE

This model code relates to the structural design and construction of steel chimneys of circular cross-section, 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 chimneyswith a height less than 15m and a slenderness ratio more than 16. Themodel code does not deal with architectural or thermal aspects of steel chimneys nor with their foundations, except insofar as theyaffect the chimney’s 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 self supporting chimneys taller than 15m. For other chimneyssimplification 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

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 for Dual 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 Stack and a new dynamic vibration absorber” — CICIND REPORTVol. 12, No. 2, 1996

[16] Bunz, G., Diepenberg, H. and Rendie, A.:“Influence of fuel oilcharacteristics 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 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 mit Manteloffnungen”, 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, Steel Liners”, 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 factor

Material properties

f – strength (MPa)

E – modulus of elasticity (GPa)

– stress (MPa)

Loadings

T – temperature in centigrade

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 (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 defined 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 flanges, ultimate limit state may takeaccount of plastic stress distribution

Safety 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 defined below. The choiceof reliability category shall be decided by the chimney owner andrelevant statutory authorities. Most chimneys will, however, beregarded as of Normal reliability.

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 1.1

Load 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 classifications 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 200°C and 1.5 fortemperatures between 200°C and 400°C.

To avoid alarming personnel, the maximum permitted amplitude of oscillations 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 specifiedand 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 of structural 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. f k f y / 1.1

The 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<16 16<t<40 40<t<63 63<t<80 80<t<100 Joules °C

(4)

Fe360 A — 235 225 215 205 195 — —

B FU/FN 28 20

C FN 28 0

D FN 28 20

Fe430 A — 275 265 255 245 235 — —

B FN 28 20

C FN 28 0

D FN 28 20

Fe510 B FN 355 345 335 325 315 28 20

C FN 28 0

D FN 28 20

DD FN 40 20

Table 6.1 – Typical mechanical properties guaranteed at delivery

(1) The values given in the table for the tension test and the bend testare valid for longitudinal specimens, except for strips, plates andwide flats whose width exceeds or is equal to 600mm, fromwhich transverse specimens shall be taken.

(2) FU un-normalised steel, FN normalised steel.

(4) A limit thickness of 36mm applies to sections and all productsusing Fe 510 steel.

(7) Valid for normal sized V-notch ISO specimens for products up to63mm thick; for thicker products, the values shall be agreed uponwhen ordering. For products less than 10 mm thick, specimens of a reduced width (but 5mm) shall be taken, the otherdimensions remaining unchanged; the minimum notch toughnessshall be reduced in proportion to the area of the cross-section (i.e.to the width of the specimen).

6.2.4. At ambient temperatures, calculations shall be based onfollowing properties of carbon steel:

density s: 8.0 Mg/m3

(wide flats and plates) 7.85 Mg/m3

(other steel products)Modulus of elasticity E (tension, compression, bending)

210 GPaShear modulus: G 81 GPaPoisson ratio: 0.3Coefficient of thermal expansion : 1.2·105 / oC

6.2.5 At temperatures T between 200°C and 400°C the properties of steel shall be varied as follows:

Yield stress — see Table 6.2

Youngs modulus:(see Table 6.3)ETE {1 (15.9·105)T (34.5·107)T2

(11.8 · 109)T3 (17.2 · 1012)T4}

Thermal expansion: L / L1.2·105 ·Tm/m(100°CT800°C)

CICIND Model Code

Steel Grade Steel Grade Steel Grade

Fe 360 Fe 430 Fe 510

°C fyT /fy fyT fyT /1.1 fyT fyT /1.1 fyT fyT /1.1

20 1.000 235 214 275 250 355 323

200 0.880 207 188 242 220 312 284

250 0.832 196 178 229 208 295 269

300 0.778 183 166 214 195 276 251

350 0.717 169 153 197 179 255 231

400 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 f y shall bereduced according to Table 6.1.

2) For temperatures higher than 350°C 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 20 200 250 300 350 400

ET in GPa 210 202 198 192 185 174

Table 6.3 – Young’s Modulus of structural steel at

high temperatures

6.3. Stainless and alloy steels

When metal temperatures are expected to exceed 400°C, 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 65°C 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 flue gases.

(Note: the conditions downstream of a flue 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 fluegases 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 flue 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 withflue 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, fittings, linings, flues, 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 first 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 influenced by some or all of the following:

a) local topography

b) the level of turbulence

c) the presence of nearby structures, including chimneys

d) the air density

e) the value of the drag coefficient (shape factor)

f) the values of the natural frequencies of oscillation

g) the amount of structural damping and mass present

h) the configuration of the first few mode shapes

i) 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 defined as follows: It is the mean hourlyspeed, measured 10m above ground level in open flat 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 flat and open. However, in some veryhilly areas, where flat ground is rare, Vb is sometimes measured at thechimney 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 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)·k t · k i {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.

k t topographical factor (see 7.2.2.3)

k i 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 k tto account for the increase of mean windspeed over hills andescarpments in otherwise relatively flat 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 k t is given in the following.

k t 1 0.6 .s for 0.3

k t 1 2 . s . for 0.05 0.3

k t 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, defined 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 of

design and actions on structures — wind actions”

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 7.2.3.2.3

d(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 aira 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 · 1 05Re 7·105

CD 0 7 if Re 7 ·105

for chimneys with helical vanes CD 1.4 (see figure 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 of ladder members have been taken into account.

Figure 7.3

7.2.3.3 Effect of fluctuating part of the wind-speed

The influence of the fluctuating 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·loge t)

T3600f 1

1 i 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 factor

S {1 5.78· ( f 1 / Vb )1.14 · h0.98}0.88

the structural plus aerodynamic damping expressed as afraction of critical damping (see 7.2.7)

f 1 sthe natural frequency in sl of the chimney oscillatingin its first 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 fig. 7.6

123·(f 1 / Vb) · h0.21

{1 (330·f 1 / Vb)2 · h0.42}0.83

B

SE

0.577

(2·loge t)


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 f 1 . d / St or Vcr2 f 2 . d / St ... (7.7)

Normally only the first mode structural frequency (f 1) is relevant.However, for slender chimneys with very low first critical wind-speed, the response to second mode vibration (at frequency f 2) shouldalso be studied.

No significant 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” defined 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 first 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 chimney’s response andresulting stresses, ensuring these stresses remain within the limits of fatigue 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 chimney’s own movement, making the behaviour nonlinear

The approximate maximum value of y can be expressed in terms of two 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 105Re 5 .105

1.0 when Re 5.105

Ca .02 when Re 105

(0.07 0.01 . log10Re) when 105Re 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 Categories

least 5km fetch upwind of water, or

smooth flat country without obstacles

Vcr 10m/s 10m/s 7m/s 7m/s

I 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 · · f n)2 · m(z) · y(z) ... (7.10)

Where: m(z) mass per unit length at height zy(z) maximum amplitude at height zf n 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


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:

f 1 (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 third

d 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 fig 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 significantly increase the chimney’s 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 k i as defined below:-

a) Where the height of the interfering structure is less than half thechimney height, k i 1.0

b) Where the height of the interfering structure is half chimneyheight and it is approximately cylindrical in shape, k i isdetermined from the following expression for values of a/dbetween 1 and 30 (see fig. 7.5):

k i 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 chimney’s 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 magnification factor k c,applied to response amplitude, calculated per equation (7.12), may beestimated as follows:-

For a/d 15 :- k c 1.0For a/d 10 :- k c 1.5

k c 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 chimney’s 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


Figure 7.6 – The reduction of Strouhal Number caused by

aerodynamic interference

When the interfering structure or chimney is less than 2 diametersaway, “Interference Galloping” can cause even greater increases inthe chimney’s response. Probably the best solutions in this casewould be either to fit 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 0.002

Unlined, externally insulated 0.003

Lined with refractory concrete 0.005

Lined with brickwork 0.015

chimneys with steel liners*:-

26 0.006

28 0.002

Coupled group 0.004

Chimney with tuned mass damper (0.02min) see Appendix 2

Table 7.4

Notes: If rotation of foundation decreases the first 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 / ( f 1 . 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 loading

f 1 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 first natural frequency should preferably be calculated with acomputer program. Care must be taken to include for the effects of any supporting structure. Assuming a chimney is on a rigid support,its first natural frequency may be calculated by dividing it into asuitable number of sections using the formula (for the first mode):

f 1 (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 deflection 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 of a finite 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:-

f 2 3.5. (E.I / m . L4) ... (7.17)

Where EYoung’s Modulus

IMoment of inertia of cross section

mmass 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 fitted 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 significantly 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 profile and small size, the associated increase inwind drag is minimised. The use of damping devices, therefore, hasbeen proved to be beneficial in the design of steel chimneys and theycan be safely retro-fitted without incurring significant 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 of nearby chimneys or structures.

Other chimney damping devices such as hanging chains have alsobeen successfully used.

CICIND Model Code

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 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 finally byfield 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 modified Mercalliscale 10 without serious damage. However, in cases where a heavymass (e.g. a water tank or a heavy lining) is fitted 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 flue gases from two or more sourcesat significantly 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 flue gas overthe range of flue gas velocities between 5m/s and 15m/s. For flue 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 fire should be assessed. Chimney fires 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 fires, 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 reflective surface will quickly buckleduring a fire. In such cases, if the risk of internal fire is significant, arefractory concrete internal liner should be installed to provide adegree of fire protection. Typically, a castable refractory liningfollowing the requirements of Appendix 3 will provide sufficient fireprotection 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 flue gas does not contain significant concentrations of halogens (see notes (4) & (5) below) the degree of chemical load isdefined in Table 7.5.

Degree of Operating hours per year when

chemical load 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 7.5

Degree 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 flue 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 flue 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• 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 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 HCl30mg/m3 or of HF 5mg/m3 and if the 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 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 and1 bar pressure)

Hydrogen chloride: 0.1% by weight (1300 mg/m3 at 20°C and 1bar pressure)

6) Saturated or condensing flue gas conditions downstream of a fluegas 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 of carbon 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 fitted with an effectivedynamic 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, flexural stresses being added vectorially toovalling stresses. For unstiffened chimneys (i.e. chimneys withoutstiffening rings or substantial flanged joints) having L/R 50, shelltheory or finite element modelling should be used, consideringflexural 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*) f k ... (8.1)where:

i* stresses multiplied by load factorsf k 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/EI)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:-

M11M1 (12 / 8)

Where M1 is the wind moment at any particular level.

This simplified 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 of the 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

{*x 2 *y2 (*x ·*y) 3 *2} f k ... (8.3)

Note – The ovalling stresses are both negative and positive and themaximum value of expression (8.3) occurs when *

x and *y are of

opposite 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*Bk / m ... (8.4)

where:

*N, *B normal and bending compressive stress atultimate limit state

m material factor 1.10

k critical buckling stress

(1.0 0.412 1.2) f y when 2 ... (8.5a)

0.75 f y / 2 when 2 ... (8.5b)

f y yield strength of steel at design temperature

f y / ( cr) ... (8.6)

cr critical elastic buckling stress 0.605 E t/r ... (8.7)

E Young’s modulus of steel at design temperature

t corroded plate thickness

r radius of the structural shell of the chimney atsection considered

... (8.8)N*NB*B

*N*B

tensile stress per shell theory

tensile stress per beam theory


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 shell’s 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 deflection 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 deflection.

So as not to alarm bystanders, the amplitude of deflection 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 of cracks in the material, especially near welds, thus resulting finally inthe failure of a weakened section. The fatigue of the material dependsessentially on:

– the number of stress cycles N

– the stress range (maxmin)

– the constructional details

The influence of the grade of steel as well as that of the min / max

ratio are negligible.

8.5.2. Fatigue strength

The number of load cycles in the cross-wind direction can becalculated from:-

N 1.26 107T f A eA2

where:-

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 of the 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)k

Where:-

max The maximum stress range due to vortex shedding

wn The fatigue strength after N cycles (see figs. 8.2 & 8.3)

k the (positive) exponent of the fatigue curves. for steel, k 3

Determines the load vs. cycles relationship (Vcr / 8)1.2

Modelling safety factor 1.4(for temperatures up to 200°C)

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 material

with respect to the fatigue categories defined in Figure 8.3

8.5.3. Influence of high temperatures

The few results available show that at 200°C fatigue growth ratesmay be higher than at room temperature, but at 400°C 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 400°C.

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 specified limits of stress and deflection. Internal flangesshall have corrosion allowance Cl and external flanges 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 of 3mm 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


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 acheivedand 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 welds

CICIND Model Code


face of the outer shell requires no corrosion allowance, provided aweather-tight cover is fitted over the air space(s) between the liner(s)and the outer shell.

8.6.1. External corrosion 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 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 Chemical

of metal in contact load per Internal corrosion allowance

with flue gas table 7.5

65°C low not applicable (chem. load always “high”)*

medium not applicable (chem. load always “high”)*

high corrosion a llowance inappropriate, use other

material*

65°C – 345°C low 2mm**

medium 4mm


material

345°C low 1mm

medium 2mm


material

Table 8.2 internal corrosion allowance (CI) — for carbon steel

only (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 /1.1

strength of bolts

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 ( 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:

*ll,u / 1.1 1.45u ... (9.2)

The design bearing stress *l relates to the area obtained by

multiplying 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 360 575 525

Fe 430 690 625

Fe 510 815 740

Table 9.2 – Limit bearing stress l ,u in MPa

9.1.2.3. Tension

The limit state is described:

*tt,u / 1.1 0.73u,B ... (9.3)

for t,u see table 9.3

CICIND Model Code

Minimum value of Limit tensile stress

tensile strength of preloaded bolts

bolt grade u,B t,u t,u /1.1

4.6 400 not recommended



8.8 800 640 580

10.9 1000 800 730

Table 9.3

Limit 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 f y

– in the nett section, the stress shall not exceed 80% of the tensilestrength 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 fillet 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 fillet 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 f y

where uE is the guarantied minimum value of the tensile strength of the weld metal and f y the yield stress of the parent material.

Throat section Contact section

grade w,u w,u /1.1 s,u s,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 w,u and s,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 Fe510.

w,u values given in table 9.4 are valid for electrodes with propertiesof steel Fe 510.


9.1.3.3 Weld Testing

While a minimum, taken at random, of 10% of butt welds and filletwelds shall be tested, the weld testing procedures and quality levels shallbe agreed by the client and the builder. The recommendations of levels‘C’of 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 fig. 8.3 assume welds are madeto 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 of the bolt. However, a distance of 5db is recommended as largerspacings result in excessively thick flanges. 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*bZ*a /w 0.73u,b An ... (9.6)

In the case of cross-vibration (fatigue):

Zb,f Zf a / wR An / 1.1 ... (9.7)

where:

R is the fatigue strength for category 35 MPa

An is the stress section of the bolt

If 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 fig.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 profiled contact areas may reduce the damping ratio used inestimating along and across-wind response. The fitting of gaskets tothe flanges 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 designer’s discretion when no responseto 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 baseplate and concrete foundation must be filled 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 deflect 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 flanges welded in place, shall be fabricatedwithin a tolerance of 3mm on circumference and diagonal. If possible, these measurements shall be made while the shell’s 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 cylinder’s 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.

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 flat and normal to the chimney axis. Before bolting, themaximum gap width on the line of the shell, between matching pairs of flanges, shall be 1mm. Before bolting, the gap at the outer edges of theflanges shall not exceed 1.5mm per 100mm width of flange.

Note: These tolerances may be ignored if the flanges 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 flue 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 within0.5°.

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. Specifications 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 specified 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 satisfied 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 hole’s 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 chimney’s stiffness should be takeninto account when determining the chimney’s natural frequencies.

14 GUYED AND STAYED CHIMNEYS

A stayed chimney is defined 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, fixed 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 of differing 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 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 areaif the chimney is to be built within an aerodrome safe guarding areaas local conditions and restrictions may apply.

cicind model code for chimney

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