CICIND - Model code for steel chimneys_1999.pdf

CICINDModel 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] CICIND Model Code for Steel Chimneys REVISION 1 – 1999 TABLE OF CONTENTS Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 10 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 0.1 0.2 0.3 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Appendices and Commentaries . . . . . . . . . . . . . . . . . . 3 Philosophy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 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 7.4 7.5 Earthquake loading . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Thermal effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 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 8.2 8.3 Minimum thickness . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Required checks . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Carrying capacity of shell . . . . . . . . . . . . . . . . . . . . . 13 11 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 12 Field of Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 13 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 14 Notations, Units and Definitions . . . . . . . . . . . . . . . . . . . . . 4 4.1 4.2 4.3 4.4 5.1 5.2 5.3 5.4 6.1 6.2 6.3 7.1 7.2 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Subscripts-Superscripts . . . . . . . . . . . . . . . . . . . . . . . . 4 Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Reliability differentiation . . . . . . . . . . . . . . . . . . . . . . . 4 Partial Safety Factors . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Cross-wind effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Structural steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Stainless and alloy steels . . . . . . . . . . . . . . . . . . . . . . . 6 Permanent Load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 7.1.1 Dust load (temporary load) . . . . . . . . . . . . . . . 6 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 15 Basis of Design and Safety Factors . . . . . . . . . . . . . . . . . . . 4 16 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 17 Actions (External and Internal) . . . . . . . . . . . . . . . . . . . . . 6 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 . . . . . . . . . . . 19 9. 13 8. . . . . . . . . . . . . . . .2 Guyed chimneys . . . . . . . . . . . . .2 Bolted connections . . . . . . . . . . . . . . . . . 21 11 Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 . . . . . . . . . . . . . . . . . . . 14 8. . . . . 21 9. . . . . . .2. . . . . . . . . . . . . . . . . . . .1 General provisions . . 20 9. . .1. . . . 22 11. . . . . .1 Anchor bolts . . . .2 Fatigue strength . . . . . . . . .2.3. . . . . . . . .3 Temperature effects . . . . . . . . . . .1. .6 Allowance for corrosion . 21 9.1 Basic principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 Full penetration welds . . . . . . . . . . . . . . . . . . . 19 9. 14 8. . .1. . . . . . . . . . . . . . . . . . 20 9. . . . . . . . . . . . . . . . . . . . . . . .1. . . . . . . . . . .1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2 Grouting . . . 21 9. . . . . . . . . . 21 9. . . . . . . . . . . . . . .1. . . .2 Fillet welds . . . . . . . . . . . . . . . . . . . . 22 11. . . 22 16 Access Ladders . . 14 8. . . .4 Combined loading . . . . . . . . .3 Structural flanges and opening reinforcement . . . . . . . . . . .5. . . . . . . . . . . . . . . . . . . . . . . . .1.3 Flanged connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 Connections . . . . . . . . . . . . 22 15 Protection Against Lightning . . . . . . . 22 14. . . . . . . . . . . . . . . . . . 13 8. . . . . . .2. 22 12 Surface Protection . . . . . . . . . . . . . . . . . . . . . . .7 Erection tolerance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 11. . . .3 Tension . . 19 8.5 Deduction for holes .1. . . . . 22 13 Openings . . . . . 20 9. . . . . . . 21 11. . . . .3 Biaxial stresses . . .4 8. . . . . . . . . . . .1. . . . . . . . . . . .5 Serviceability of shell . . . . . . . .2 Structural shell . . . . . . . . 22 11. . . .page 2 CICIND Model Code 8. . .6. . . . . . . . .3. 13 8. . . . . 20 9. . . . .3. . . . . . . 14 8. . . . . . . 19 9. .6. .4 Stability . . . . . . . . . . . . . 19 9. . . . . . . . . . . .3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6 Straightness . . . .3. . . . . . . . . . . . . . . . 19 9. 14 Fatigue check . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 8.1. . . . . . .3. . . 21 10 Steel liners . . . . . .4 Stiffening rings . . . . . . . . 21 The support at the base . . . . . . . . . . .1 Shear . . . . . . . . . . . . . . . .1 External corrosion allowance . . . . . . . . . . . . 22 14 Guyed and Stayed Chimneys . . . . . . . . .3. 14 8.3 Welded connections . . . . . . . . . . . .3. . . . . . . . . . . . . . . . . . .2. . . . . . .2 Bearing on connected surfaces . . .3 Influence of high temperature . . . . . . . . . . . . . 20 9. .2 Second order effects . . . . . . .2 Internal corrosion allowance . . . . . .3. .1 Stayed chimneys . . 19 9. . . . . . . . . . . 19 9. . . . . . . . . .3 Weld testing .1. . . . . . . . . . . . . . . . . . . .2. . . . . .1. . 22 14. . . . . . .5. . . . . . . . . .2 9.3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 19 Design Details . . . . . . . . . . . . 21 11.5 Base plate . . .5. 21 11. . . . . . . . . . . . . . . . . . . .1 Load factors and load combinations . . . . 22 17 Aircraft Warning Lights . . . . The construction and erection should be carried out by firms competent in this class of work.2: Design of Steel Chimneys” ENV 1993-3-2: 1997 Thom. It also relates to chimneys with a height less than 15m and a slenderness ratio more than 16. The following items are not objectives of the CICIND commentaries: [4] [5] [6] . of the American Society of Civil Engineers. The design of steel chimneys should therefore only be entrusted to appropriately qualified and experienced engineers.CICIND Model Code page 3 FOREWORD When it was formed in 1973. 1998 Great Britain – Chairman after Jan. b) Simplification of the use of the model code. 1998 Expert advice was received from: B. The committee comprises: J. This Model Code contains guide-lines which reflect the current state of art in the design and construction of steel chimneys. Since 1988. August 1998 CICIND. with or without linings. internationally. However. 0. The commentaries have the following objectives: a) Justification of the regulations of the model code.N. depending upon its reliability category.2. This code interprets ‘sufficiently safe’ in terms of the social and economic consequences of failure. H: “A calculation method for the fatigue life of steel chimneys subject to cross-wind oscillations”. Pritchard Max Beaumont Michael Beaumont G. d) Documentation of the areas in the model code where the present knowledge is sparse so that the regulations are possibly or probably not optimal. the science and technology of chimneys has advanced and in 1995. c) Understanding of the meaning of the regulations of the model code. chemical waste gases. Roberts B. CICIND REPORT. the real situation. Warren Great Britain – Chairman until Jan. 2. the properties of the materials used. and to the design and application of linings to such chimneys where required. Vol. Bouten R. CICIND appointed a committee to revise the Model Code. “Eurocode 3. recognising current best international practice and knowledge.S. Additional information is given in the Appendices and Commentaries. except insofar as they affect the chimney’s structural design. H. Further revisions of this model code will therefore be published from time to time.S. Switzerland. CICIND REPORT.A. 2. the design.: “Distribution of extreme winds over oceans” Journal of the Waterways. e) Change of the meaning of certain regulations of the model code where these are falsely expressed or obviously wrong. Zurich. CICIND will continue to try to improve the understanding of the behaviour of chimneys. Vickery (Canada) H. a subcommittee was appointed in 1981. The model should be sufficiently “safe. Nevertheless. 0. f) Definition of the meaning of certain regulations of the model code which are so badly formulated that they could easily be misinterpreted even by experts. Berger J. While the judgements of ‘sufficiently true’ and ‘sufficiently simple’ are subjective. with Commentaries being published the following year. with a minimum height of 15m. 14. At all times the work should be under the direction of appropriately qualified supervisors. 14. No. Ole Hansen G. Proc.J. simplicity.REFERENCES [1] [2] [3] “CICIND model code for concrete chimneys — Part A. B. It is very rarely that simplicity and truth are compatible. FIELD OF APPLICATION The model code is valid for all steel chimneys of circular crosssection. The model code does not deal with architectural or thermal aspects of steel chimneys nor with their foundations. the design rules have been formulated for self supporting chimneys taller than 15m. For other chimneys simplification may be acceptable. CICIND has departed from generally accepted principles of steelwork design and construction only when this was required by the philosophy outlined above or by specific chimney requirements. van Koten (The Netherlands) 0. Vol.1 General Chimneys are required to carry vertically and discharge to the atmosphere. so a model must be used which provides an optimum compromise between truth. This leads to the development of safety factors which ensure that a chimney will have a probability of failure during its design lifetime between 10Ϫ3 and 10Ϫ4. It does this by comparing the probabilities of failure for given safety factors during its design life with the failure probabilities required to satisfy accepted social and economic criteria. 2. The Shell”. 1998 Great Britain Great Britain Germany The Netherlands Italy Denmark Great Britain U. Harbors and Coastal Engineering Division.C. As a means to this end. No. safety and economy. February 1973. Appendices and Commentaries This Model Code is accompanied by extensive appendices and commentaries. The model code does not deal with those aspects of the design and construction of steelwork. This document was published in 1988. Vickery. Certain information from the model code is repeated in the commentaries when this simplifies the presentation of the ideas. refractories and insulation which are not peculiar to chimneys.M. gaseous products of combustion.3 Philosophy One of the main objectives of any code governing construction is the creation of a model which resembles as far as possible. the “Comité International des Cheminées Industrielles” (CICIND) adopted as a major goal the harmonisation of national codes for the design of industrial chimneys. ‘sufficiently safe’ is capable of rational judgement. 1998 Eurocode 1 — Basis of Design and actions on structures — Part 2 – 4: Actions on structures — Wind Actions ENV 1991-2-4: 1995 Van Koten. fabrication and erection of steel chimneys require a thorough knowledge of these structures. Ghermandi S. INTRODUCTION 0. charged with drafting a proposal for a model code for steel chimneys which reflected the current “state-of-the-art” and a consensus of views. Pinfold R. or exhaust air or for the combustion (flaring off) of industrial waste gases. the actions occurring upon the structure and the recognised rules of the relevant technologies. 3.J: “Wind loads and design for chimneys”. even though its inclusion in a chimney design code could not be justified. The appendices provide information which the committee believes will be of use to a steel chimney designer. 1 SCOPE This model code relates to the structural design and construction of steel chimneys of circular cross-section. simple and true”. Vol. H: “Structural damping”. 1.1982. Vol. Kammel. 2. CICIND REPORT. 2. The Netherlands 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. 1999 [11] Turner J.: “Influence of fuel oil characteristics and combustion conditions of flue gas properties in W T boilers” Journal of the Institute of Fuel. & Reid. Definitions The common names of parts of a steel chimney are explained in commentary 1. December 1995 CICIND. The ultimate limit state considered is reached when any part of the section is at the limit stress. a material factor and a modelling factor. CICIND REPORT. 1987. the safety of the chimney being ensured by partial safety factors for loads and material.. ST9. No. [10] Berger. UNITS AND DEFINITIONS 4. Vol.4.11. CICIND REPORT. be regarded as of Normal reliability. R. 5. divided by the material safety factor.R. N. G : “Measured damping decrements of steel chimneys and their estimation taking account of their type”. CICIND REPORT. 1994 [13] Bierrum. [19] “CICIND chimney protective coatings manual”. Vol. The choice of reliability category shall be decided by the chimney owner and relevant statutory authorities. In the design of details such as flanges. No. A. E. 1. 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. C. Switzerland 4.M. No.2 Reliability differentiation Different levels of reliability shall be adopted for chimneys. & Verwiebe. ultimate limit state may take account of plastic stress distribution Safety in the case of response to vortex shedding is ensured by the use in the fatigue calculations of a suitable Miner Number. No.: “Die Stabilitat axial belasteter Zylinderschalen mit Manteloffnungen”. S: “Vortex — induced vibrations of line-like structures”. 1994 [14] Warren. “Shell to Flue Impact Damping for Dual Wall and Multi-Flue Chimneys” — CICIND REPORT Vol. however. 15. 1984 [22] Bouwman. C.S. The limit stress is defined as either yield stress or critical buckling stress (whichever is least).G. Zurich. 1996 [16] Bunz. No. U. Bauingenieur 51.: “Wind load stresses in steel chimneys”. Journal of the Structural Division.& Jozsa. 2.. F. 1977. Vol. CICIND.page 4 CICIND Model Code [7] [8] [9] Ruscheweyh. H.2. Vol.P. Steel Liners”. 10. 3. HERON report no. depending on the possible economic and social consequences of their failure. H. [18] Henseler. General The following list shows only the principles by which the notations and their meanings are related.1967 [17] Lech and Lewandowski: “Prevention of cold end corrosion in industrial boilers” Corrosion. The use of this procedure.L.: “Optimum control of chimney vibration”. U. No. H. No. 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) . 2. Most chimneys will. The actual notations are mostly explained in the text. 14. 1995 Ole Hansen.: “Desulphurisation Systems and their Effect on Operational Conditions in Chimneys”. S. No. The calculation of the stress distribution and the strength of the sections shall therefore be made in accordance with the theory of elasticity. and Rendie. M. 5.1.: “Mis-tuned Mass Dampers”. Atlanta. Two classes of reliability related to the consequences of structural failure are used — Normal and Critical. 10. BASIS OF DESIGN AND SAFETY FACTORS 5.: “Experience with Vortex Excited Oscillations of Steel Chimneys”. 10. [23] “CICIND Model Code for Concrete Chimneys — Part C. Units Generally. Vol. G. 12.A. 12. Subscripts-Superscripts y – yield limit k – characteristic value * – stress multiplied by load factor cr – critical 4. combined with the partial safety factors listed below will ensure that low cycle fatigue will not contribute to failure of the chimney.: “Bolted connections dynamically loaded in tension”. 1994 [15] Ruscheweyh. G. 1996 [12] Hirsch.4. the units of the SI system are used. 1. as defined below.3. the relevant references are given. Diepenberg. Sept. CICIND REPORT . March 1979. Switzerland [20] Schulz. CICIND REPORT. Delft. 2. “Vibration Control by Passive Dampers — a Numerical and Experimental Study of the Damping Effect of Inner Tubes Inside a Steel Stack and a new dynamic vibration absorber” — CICIND REPORT Vol.1 General The design of sections subject to permanent load and wind loads in the wind direction is based upon ultimate limit state conditions. NOTATIONS. Proceedings ASCE. [21] “European Recommendations for Steel Construction: Buckling of Cylinders” ECCS/CECM/EKS.1976. CICIND REPORT. N0. 4. 1998 Van Koten. specimens of a reduced width (but Ͼ 5mm) shall be taken.2 · 10Ϫ5 · T m/m (100°C р T р 800°C) .8 · 10Ϫ9)T3 Ϫ (17. The yield stresses at high temperatures are given in Table 6.3 Partial Safety Factors Material safety factor for steel Load factors for: Normal Chimneys – Permanent load – Guy rope pretension – Wind load in wind direction (temperate zones) – Wind load in wind direction (tropical storm zones)* Critical Chimneys – Permanent load – Guy rope pretension – Wind load in wind direction (temperate zone) – Wind load in wind direction (tropical storm zones)* * See literature (e.2. applied to fatigue category.6 Fe430 275 265 255 245 235 Fe510 355 345 335 325 315 Table 6.85 Mg/m3 ϭ (other steel products) Modulus of elasticity E (tension. the other dimensions remaining unchanged.4. 6. such as nuclear power plants or in densely populated urban locations. plates and wide flats whose width exceeds or is equal to 600mm. Limiting stress ranges are given for various weld classifications and design lives.4 1. Fe 430 and Fe 510.2.2. tensile strength. divided by the material factor 1.2.(3)).5 for temperatures between 200°C and 400°C.2 · 10Ϫ12)T4} Thermal expansion: ⌬L / L ϭ 1. Notch Toughness (7) t<16 16<t<40 40<t<63 63<t<80 80<t<100 Joules °C (4) Fe360 A B C D A B C D B C D DD — FU/FN FN FN — FN FN FN FN FN FN FN 235 225 215 205 195 — 28 28 28 — 28 28 28 28 28 28 40 — ϩ20 0 Ϫ20 — ϩ20 0 Ϫ20 ϩ20 0 Ϫ20 Ϫ20 1. Steel grade ASTM A36 has similar properties to Fe 360. 5.2.e. the ductility requirements.1. Fe 360.5 1. 5. At ambient temperatures. except for strips. 6. Guidance is given in Commentary 3. The code contains means of estimating the amplitude of movement and consequent stress range due to crosswind loading. Normal Chimneys — All normal chimneys at industrial sites or other locations.5 At temperatures T between 200°C and 400°C the properties of steel shall be varied as follows: Yield stress — see Table 6. In zones where bearing elements are subjected to tension as a result of external loads or in zones of three-dimensional stress.2 Youngs modulus:(see Table 6. a modelling factor of 1. To avoid alarming personnel.e. In addition to a material safety factor 1.5 1.1 The yield stresses of structural steels at normal ambient temperature are given in table 6.4 Cross-wind Effects (Vortex shedding) Chimneys shall be designed to avoid movements across the wind direction sufficient to cause failure or fatigue damage or to alarm bystanders.3 Coefficient of thermal expansion ␣: ϭ 1.1 1.1 Steel Class De-oxidation Grade procedure (2) Yield stress in MPa for thickness (mm) Min. from which transverse specimens shall be taken. 6. For products less than 10 mm thick. This limit will be governed by the prominence and visibility of the chimney and the frequency with which maximum amplitudes can be expected. calculations shall be based on following properties of carbon steel: ϭ 8. lit.1: i.3. the minimum notch toughness shall be reduced in proportion to the area of the cross-section (i.2. in addition to the minimum strength values. shall be considered. (Typically chimneys in industrial sites. 6.5 · 10Ϫ7)T2 ϩ ϩ (11. bending) ϭ 210 GPa Shear modulus: G ϭ 81 GPa Poisson ratio: ␯ ϭ 0. General The materials generally used for steel chimneys are described below. for thicker products.1 1. (2) FU ϭ un-normalised steel. the maximum permitted amplitude of oscillations due to cross-wind effects or aerodynamic interference shall be agreed between the owner and designer.1 The mechanical properties and the chemical composition of structural steels shall comply with local national standards. power plants or chimneys less than 100m tall in urban locations.1 – Typical mechanical properties guaranteed at delivery (1) The values given in the table for the tension test and the bend test are valid for longitudinal specimens. compression. where any domestic dwelling is outside the falling radius of the chimney).2·10Ϫ5 / oC density ␥s: 6.3) ET ϭ E {1 ϩ (15.2.CICIND Model Code Amendment A – March 2002 page 5 Critical chimneys — Chimneys erected in strategic locations.2 1. (7) Valid for normal sized V-notch ISO specimens for products up to 63mm thick.1.1. FN ϭ normalised steel. For the most commonly used grades of steel. the values shall be agreed upon when ordering.0 Mg/m3 ϭ (wide flats and plates) ϭ 7.1 gives the mechanical properties. to the width of the specimen). 6.4 shall be applied to the Miner Number derived in fatigue calculations for temperatures up to 200°C and 1.g. The limit stresses of steel are equal to the yield stress of the steel used. (4) A limit thickness of 36mm applies to sections and all products using Fe 510 steel. 1. such as yield stress. MATERIALS 6. ductility and weldability.9 · 10Ϫ5)T Ϫ (34. fk ϭ fy / 1. Major chimneys in industrial sites where the economic and/or social consequences of their failure would be very high. Structural Steels 6.2.2 1. enable the Model Code to be put into application. Special steels can be used providing that they are precisely specified and that their characteristics. Table 6. a steel liner or liners. provided specialist metallurgical control is exercised with regard to weld procedures. These steels also show some corrosion improvement over carbon steel when in contact with flue gases where acid condensation of SO2/SO3 (not of HCL condensation) is intermittent only (e.1.717 0. flues. its metal temperature being normally above acid dew point). without obstructions. The value of the basic wind must be taken from meteorological measurement.1.2. platforms.2. Where stainless or alloy steel components are connected to carbon steel.880 0. the use of a protective coating may be considered (see lit[19]). electrode selection. In cases where it is not possible to avoid high chemical load on the internal face of the structural shell. Apart from that the wind loads. except in a marine environment or other environment where chlorides are present. is a possible solution.6. 4) Linear interpolation is acceptable Temperature °C ET in GPa 20 210 200 202 250 198 300 192 350 185 400 174 Table 6. Welded connections are permitted. during shutdowns of a stack in intermittent service. possibly rendering these stainless steels unsuitable for these applications. pipes etc.3. Wind 7. bolted connections are preferred. Dust load (temporary load ) On some process plants there can be a carry over of ash or dust burden. .1 General The wind load on a chimney depends in the first instance upon the magnitude of the wind speeds in the area in which the chimney is to be erected and their variation with height. etc. Stainless and alloy steels When metal temperatures are expected to exceed 400°C. possibly of titanium or high nickel alloy.2.832 0.1. Basic wind speed The determination of the effective wind pressure is based on the basic wind speed. such connections should include insulating gaskets.1 above.3 – Young’s Modulus of structural steel at high temperatures 6.page 6 CICIND Model Code Steel Grade Fe 360 °C 20 200 250 300 350 400 fyT/fy 1.) Ordinary stainless steels are not suitable for use in contact with flue gases containing alkalis. 3) Special attention should be paid to the modulus of elasticity at high temperatures for stainless steel.1 214 188 178 166 153 138 Steel Grade Fe 430 fyT 275 242 229 214 197 178 Steel Grade Fe 510 fyT 355 312 295 276 255 230 fyT/1. ACTIONS (EXTERNAL AND INTERNAL) 7.2.1 250 220 208 195 179 162 Table 6. at the chimney location. This may adhere to the interior surface of the structural shell or liner and cause an additional dead load. 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.1 323 284 269 251 231 209 When the metal temperature is below acid dew point for prolonged periods. 7. are acceptable. the corrosion rates of high molybdenum stainless steels. 7. present and future loads including corrosion allowance. 7. Alternatively.6. Low copper alloy steels have good resistance to atmospheric corrosion.1.000 0.3.3. appropriate to the location where the chimney is to be erected. Such cases should be investigated at the design stage. 7. where flat ground is rare. Vb is sometimes measured at the chimney location and includes the “Topographical factor”. see paragraph 7. the performance of low copper alloy steels in contact with flue gases is similar to that of carbon steel. is defined as follows: It is the mean hourly speed. In order to avoid accelerated corrosion due to galvanic action. which occurs on average once every 50 years. will be influenced by some or all of the following: a) local topography b) the level of turbulence c) the presence of nearby structures. The basic wind speed Vb. 2. An indication of values of the basic wind speeds for various countries may be obtained from the Commentary No. fyT/1.2 – Yield stresses of structural steel in MPa (thicknesses t р 16mm) Note 1) For thicknesses greater than 16mm the yield stress fy shall be reduced according to Table 6. see paragraph 7. See section 10 on Steel Liners. Care should be taken to use the correct coefficient of expansion for the grade and temperature of the steel being considered. 2) For temperatures higher than 350°C alloy steels should be considered. fittings. linings. insulation.2.3. measurements for the determination of Vb should be taken as near as possible at a place which is flat and open. the calculated load shall be added to the permanent load calculated in 7. measured 10m above ground level in open flat country.778 0. Permanent load The permanent load shall include the weight of all permanent constructions. in the direction of the wind or perpendicular to that direction. stainless or alloy steels should be used. Such conditions can be expected on the external surface at the top (over a height of about 3 diameters) of any chimney handling high sulphur flue gases. Wind speed 7. Where the terrain of the location of the chimney is hilly or built-up. However.647 fyT 235 207 196 183 169 152 fyT/1.g. Ordinary stainless steels (including high molybdenum stainless steel) have poor corrosion resistance in the presence of condensing sulphuric or other acids in the range of concentrations and temperatures normally found within chimneys. such as ASTM Type 316L. When metal temperatures and condensate sulphuric acid concentrations are expected to be less than 65°C and 5% respectively. These materials are therefore not recommended in chimneys burning fuels containing sulphur under conditions of “medium” or “high” chemical load. in some very hilly areas. (Note: the conditions downstream of a flue gas scrubber or the presence of chlorides in the condensate will radically increase the corrosion rate.1. This value has been chosen since many chimneys are in open terrain or project well above the surrounding buildings. the topographical factor kt and the interference factor ki.14. the topography of its surroundings and the existence of adjacent objects.2 – Values of Le .2 H = height of hill or escarpment x z = distance of chimney from crest = height of considered position in chimney Le = effective length of the upwind slope.2. The design wind-speed is determined by the following expression: V(z) ϭ Vb · k(z) · kt · ki {m/s) where: V(z) ϭ hourly mean wind speed at elevation z (m/s) z ϭ height above ground level (m) Vb ϭ basic wind speed (m/s) k(z) ϭ Height factor ϭ (z/10)␣ ␣ ϭ 0.3) Le ϭ Lu Steep slope (␾ Ͼ 0.e.2. 7.2.2.3 The influence of topography Figure 7.1 and 7. defined in table 7.2. 7.5 times the height of the cliff. 7.2. For certain topographical situations.1 & 7.1) If the suitability of a different value of [␣] can be proved (together with an appropriate scale factor).1. kt ϭ 1 ϩ 0.CICIND Model Code page 7 7.6.2 – Factor ‘s’ for hills and ridges Shallow slope (0.2..3 kt ϭ 1 for ␾ Ͻ 0.1) Clause 7. Design wind speed The basis for the determination of the wind loads is the design wind speed which equals the basic wind speed corrected by three factors taking into consideration the height of the chimney.2 Lu = actual length of upwind slope in the wind direction Ld = actual length of downwind slope in wind direction Figure 7.3 Figures 7. (7.2 requires the determination of a topographical factor kt to account for the increase of mean windspeed over hills and escarpments in otherwise relatively flat terrain (i. It should be considered for locations closer than half of the length of the hill slope from the crest or 1. a method for the determination of kt is given in the following.2) s = factor obtained from Figs. kt ϭ topographical factor (see 7.1 & 7.3) ki ϭ interference factor (see 7. s .3). ␾ for 0.2.05 Ͻ ␾ Ͻ 0. it is not for use in mountainous regions).1 – Factor ‘s’ for cliffs and escarpments .2.05 Ͻ ␾ Ͻ 0.2. s for ␾ Ͼ 0..2 from ENV 1991-2-4 — “Eurocode 1 – Basis of design and actions on structures — wind actions” Table 7.05 Table 7. it may be used (see Commentary C3.3 kt ϭ 1 ϩ 2 .6 . These three factors are: the height factor k(z).1 – Values of kt Where:␾ = upwind slope H/L in the wind direction (see Figs.3) Le ϭ H / 0. .2.2 and for different distances this number is as shown in fig.2.2..2.4.089 log10 h B ϭ background turbulence ϭ {1 ϩ (h / 265)0.2 CD ϭ 0 7 if Re Ͻ 3 · 105 if Re Ͼ 7 · 105 .3).2. where Re ϭ 6.98}Ϫ0.. including ladders.2 (m/s) CD ϭ shape factor.2. etc.1.4) CD ϭ 1.2.2.3.21 {1 ϩ (330 · f1 / Vb)2 · h0.0 for structural shapes.3.3. see 7. The frequency (f) at which vortices are shed is related to diameter (d) and wind velocity (V) by the expression: St ϭ f d / V where St ϭ Strouhal number .3 G ϭ the gust factor. see 7. The Strouhal number decreases with decreasing distance (A) of nearby chimneys in a row arrangement.36 {log Re Ϫ 5. Wind load in the direction of the wind 7.9 · 104 · V · d. Shape factor The shape factor CD depends on the Reynolds number Re of the chimney (see Fig..3. Main formula The mean wind load per unit height is: wm(z) ϭ 1/2␳a · V(z)2 · CD · d(z) where: ␳a ϭ density of air.2. (7.2.4 Vortex shedding Momentary variations in the density due to atmospheric changes need not be taken into account..2.3) Figure 7.63}Ϫ0. Wind load on isolated chimneys (For group interference effects. G ϭ gust factor ϭ 1 ϩ 2 · g · i · Ί๶๶ {B ϩ ๶๶ (E · ๶ S /๶๶ ␨)} .2.3.3. (7.2) 7.1.2.4 (see figure 7.. see 7.88 ␨ ϭ the structural plus aerodynamic damping expressed as a fraction of critical damping (see 7. see 7.2.2. Air density At sea level in temperate climates.6 .2. CD is applied to the outer diameter of the chimney in the vaned portion and not the outer dimension of the vanes..7) f1 ϭ sthe natural frequency in sϪl of the chimney oscillating in its first mode h ϭ height of chimney in metres 7.page 8 Amendment A – March 2002 CICIND Model Code 7. For attachments.3 7.2 Ϫ 1. see 7.14 · h0.2. wm(z) ϭ wm(10) 7.42}0.3.2.6) The design wind load w(z) per unit height z is determined by the following expression: w(z) ϭ wm(z) · G (N/m) where: wm(z) ϭ mean hourly wind load per unit height.25 Ϫ (h1 / 8000) kg/m3 7. (7. see formula 7.2.3 Effect of fluctuating part of the wind-speed The influence of the fluctuating part can be found by multiplying with the gust factor G.88 E ϭ energy density spectrum ϭ 123 · (f1 / Vb) · h0.25 kg/m3 i ϭ turbulence intensity ϭ 0.311 Ϫ 0..5) V(z) ϭ wind speed at height z. Typical lengths and widths of ladder members have been taken into account.. The air density relevant to a chimney site at an altitude h1 (m) can be found from the expression: ␳a ϭ 1.83 S ϭ size reduction factor S ϭ {1 ϩ 5.2. Mean hourly wind load 7. the area presented to the wind for each member must use a force coefficient of 1.3}. CD ϭ 1. 7.2 for circular members and 2.3.3 d(z) ϭ outside diameter of the chimney at height z (m) Note: For z Ͻ 10m.6) for chimneys with helical vanes CD ϭ 1. (7.2.2 (kg/m3) where: g ϭ peak factor ϭ Ί๶๶ (2 · log ๶๶ e ␷t) ϩ ␷T ϭ 3600 f1 B␨ Ί๶ ΂1ϩ๶ SE ΃ 0.3.2.48) if 3 · 105 Ͻ Re Ͻ 7 · 105 7.78 · ( f1 / Vb )1. in which V ϭ V(z) is the mean wind speed at the top of the chimney in m/s and d is the diameter in m.3..577 ๶๶ Ί๶๶ (2 · log e ␷t) (N/m) . For A/d Ͼ 15 the Strouhal number is 0. 7. the density of air ␳a is to be taken as: ␳a ϭ 1.2.3 .1 General Principles Forces due to vortex shedding cause cross wind response of a chimney.3.. (7. 3. log10Re) ϭ .5 and that movement does not exceed the limits agreed per Section 5.2.08 ϫ w5 sec (z) ϫ d2(z) (Nm/m) . the associated inertial force per unit length [F(z)] should be used...4.2. ␲ .1 7. a suitably sized stiffening ring at the top of a chimney will eliminate problems associated with ovalling.3.1 above) The Strouhal Number St (see 7.2.2.2 Estimation of cross-wind amplitudes due to vortex shedding The method described in this section for estimating amplitudes depends upon parameters such as structural damping and atmospheric turbulence.4.3) The local minimum atmospheric turbulence intensity (I).5 when Ίc1 ϩ Ί๶๶๵ c12 ϩ๶๶ c2 Ͼ 0.9 and 7. The bending moments have a maximum value of: M ϭ 0.5.04 Ka ϭ Kamax .01 .9 and 7.3 Bending Moments due to vortex shedding In deriving the bending moments associated with the maximum response amplitude of a chimney due to vortex shedding. h) ϭ 1. Ίc1 ϩ Ί๶๶๵ (c12 ϩ๶ c2) where d1 c1 c2 Kp ϭ mean diameter over top third of chimney height ϭ 0. see Table 7.2.2 ϫ the design wind-speed at the top of the chimney. log10Re) when 105 Ͻ Re р 5 .04 ϭ 4 when Ίc1 ϩ Ί๶๶๵ c12 ϩ๶๵ c2 Ͻ 0.2. mo) / (Ka .2). or estimating (per 7.07 Ϫ 0. 7. for slender chimneys with very low first critical windspeed.075 Ϫ 0.1 Table 7. However.. The cross-wind movements depend strongly on the mass and damping of the chimney.2.2. mo c / ccr ␳a d12 . a parabolic mode shape may be assumed. The top amplitude (y) of a chimney moving across the wind because of vortex shedding depends upon:The Scruton Number Sc (see 7.4...1 above) The Reynolds Number Re (ϭ 6. A major determining property is the dimensionless “Scruton number” defined as: Sc ϭ where: 4 .CICIND Model Code Amendment A – March 2002 page 9 Significant amplitudes usually only occur when the shedding frequency (which increases as the wind speed rises) co-incides with a structural frequency. ␳a . If the Scruton number is greater than 5.5.715 .0 Ca ϭ . cross-wind oscillations could be violent. making the behaviour non linear The value assumed for minimum local turbulence intensity (I) shall be as listed in Table 7. Ca2 / (Ka . (1 Ϫ 3 . whose values are not known with certainty. or smooth flat country without obstacles Vcr I р 10m/s 0 Ͼ 10m/s 0. therefore be treated with care and should not be assumed to be accurate. 7. the response to second mode vibration (at frequency f2) should also be studied. the designer may choose between providing stabilisers or damping devices (see 7. d / St . The results of the calculation should.7) The approximate maximum value of y can be expressed in terms of two quantities.2.11) ..3 The chimney’s own movement.2.5 0 ͵ m(z) u (z) dz h 2 1 0 ϭ (5. F(z) ϭ (2 · ␲ · fn)2 · m(z) · y(z) Where: m(z) ϭ mass per unit length at height z y(z) ϭ maximum amplitude at height z ϭ natural frequency of nth mode fn In deriving the fundamental mode maximum amplitude at height z from the maximum amplitude at the chimney top (per 7..3 All other terrain Categories р 7m/s 0 Ͼ 7m/s 0.1 Static effect The uneven wind pressure distribution around the circumference of a circular cylinder causes bending moments acting on vertical crosssections of the shaft. This occurs at the critical wind speed (Vcr) which is derived by the following expression: Vcr1 ϭ f1 . The addition of stabilisers or damping devices (see 7.. d1 . ensuring these stresses remain within the limits of fatigue per 8.2. St4 .10).10) .2) the chimney’s response and resulting stresses. I) when Re р 105 . d2)} ϭ 0.4.9 · 104 · V · d) — see 7. (7. No significant movement due to vortex shedding will be found if the critical wind-speed exceeds 1.4.02 ϭ (0.08 {1 Ϫ (␨ .2. (7. d / St or Vcr2 ϭ f2 .25kg/m3 d1 ϭ the diameter (averaged over the top third) u1 (z) ϭ the mode shape of the first resonance frequency c / cr ϭ ␨ ϭ damping ratio (see Table 7.8) Kamax ϭ 1. mo . Ovalling In most cases.9) Normally only the first mode structural frequency (f1) is relevant.. 105 ϭ 1. 105 when Re р 105 when 105 Ͻ Re р 106 when Re Ͼ 106 mo ϭ ͵ u (z) dz h 2 1 ␳a ϭ air density ϭ 1. 7. (7.2.4.01 when Re Ͼ 5 .4) h ϭ the height of the chimney If the Scruton number is less than 5. (7. ␳a .10) is mandatory in this case. c1 and c2 as follows:y ϭ Kp .16 . d3 .2. Chimney Location Open Sea or Lake shore with at least 5km fetch upwind of water..4. (7. Fig.4 The associated critical windspeed and value of “c2” in equation (7.2. ki ϭ 1.2 Dynamic effect Due to vortex excitation ovalling vibration of the shell can occur. (7.5 t / d2) ..2 Ϫ . 7. calculated per equation (7. For a spacing ratio (a/d) greater than 10. The associated moment of inertia of the stiffening ring section (together with the participating length of shell) about its centroid (see fig 7.4).12) 7.4) must be larger than: I ϭ 1. but is thought to be due to increases in both lift coefficient and negative aerodynamic damping.2. t / d m/s .14) For closer spacing this value of I may be reduced proportionately.t Ί๶๵ When an approximately cylindrical structure (e.500 .g. the design windspeed per equation 7.3.5.12).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).6.. the amplitude of response is expected to be minimal.6. The distance between stiffeners shall not exceed 9 · d. 7. The participating length of the shell ϭ Ί๶๶ (d . helical spoilers) are ineffective in controlling response in cases of wake interference. Note – These spacing and minimum “I” requirements should not be confused with those of stiffeners sometimes required as reinforcement to resist the static ovalling effect (7.5 kc ϭ 2.1a/d for a/d between 10 and 15 For a spacing ratio (a/d) less than 10 there is a risk of very large increases in amplitude. The increase of wind effects by nearby structures Interference effects.2 Effect on cross-wind response C L t Centroid of stiffener and participating shell d.5 Ϫ 0.5): ki ϭ 1. may be estimated as follows:For a/d Ͼ 15 :.4 V(z) is safe at all heights.4).75 · 10Ϫ5 · d3 · t m4 . The increase is not yet fully understood..g. Note the assumption that 5 sec gust windspeed (m/s) at height z ϭ 1. (7. another chimney) is upwind and within 15 diameters of a chimney of similar or smaller height.2. but its area must not exceed that of the stiffener ring (see Fig.0 For a/d ϭ 10 :. 7.6 shows the relationship between Strouhal Number and a/d. (7. described in 7.13) .. applied to response amplitude...3 and its response. If the interfering structure is itself a chimney. due to a reduction in the value of the Stouhal Number. The fundamental ovalling frequency of unstiffened shells is determined by: f1 ϭ (0. a ϭ distance of chimney down-wind from the interfering structure (centre to centre) d ϭ diameter of the interference structure Fig.g.4.2. t).5 – Effect of interference on downwind loading 7. Ί๶๶๶ E / ␳s Where E ϭ Youngs Modulus of the steel shell t ϭ the average shell thickness (in m) over the top third d ϭ the shell diameter (in m) ␳s ϭ density of shell material Substituting typical values of E and ␳s. In these circumstances the chimney’s structural damping should be increased (e.2.1 Effect on wind load in the wind direction When interference effects are expected from a nearby structure. 7. either during transport/erection or as a result of the design wind load (8. These vibrations can be expected if the frequency of the vortices (f ϭ 2 · V · St / d) coincides with an ovalling frequency of the shell. aerodynamic “Wake Interference” effects can considerably increase the downwind chimney’s cross-wind response (the diameter concerned being that of the interfering structure). the magnification factor kc.2. This can be important in the design of a tuned mass damper. normal to the wind direction. e d/2 Figure 7. the associated critical windspeed is then Vcr ϭ 6. caused by the presence of a nearby structure upwind of a chimney. Note that aerodynamic stabilisers (e.kc ϭ 1. ki is determined from the following expression for values of a/d between 1 and 30 (see fig.2. .1 used to determine the wind load should be increased by a factor ki as defined below:a) Where the height of the interfering structure is less than half the chimney height.0067a/d These vibrations can be reduced sufficiently by stiffening rings. At this value of Scruton Number.5. can significantly increase the chimney’s quasi static wind load in the wind direction.kc ϭ 1.12) increase with decreasing values of a/d. 7.1) or to prevent local buckling. described in 7. by the use of a tuned mass damper) to ensure that the Scruton Number exceeds 25.0 b) Where the height of the interfering structure is Ͼ half chimney height and it is approximately cylindrical in shape. its own response when downwind of the new chimney should be checked.6. 7. using an energy absorbing connection system.0005.2. “Interference Galloping” can cause even greater increases in the chimney’s response. Damping devices Damping devices are attached to a chimney to increase its structural damping. 7.. The radial width of the vanes must be 10% of the diameter.9. the associated increase in wind drag is minimised. Tuned mass dampers provide an extra mass.3..2.16) is the mass of the section including the lining or covering (in kg) is the deflection of the same section due to the force equal to gravity acting normal to the centre-line at the mass centre (m).8) t is the thickness of the wall in the top third.9 Passive Dynamic Control Steel chimneys must be designed to suppress excessive cross-wind movement. Aerodynamic stabilisers will not reduce the wind interference effects of nearby chimneys or structures. For a chimney with constant diameter and thickness...2.3). or to connect structurally. The pitch of the vanes should be 5 D. I ๶ /m ๶.015 0. coupled to the chimney by an energy absorbing medium.8 The first and second natural frequencies The first natural frequency should preferably be calculated with a computer program. the chimney to the interfering structure. uninsulated Unlined. Several options are available to the designer.5 . however. The extra wind drag due to the vanes must be considered (see 7.1 Aerodynamic stabilizers When a chimney stands alone. its first natural frequency may be calculated by dividing it into a suitable number of sections using the formula (for the first mode): f1 ϭ (1 / 2 . The use of damping devices. (7. The vanes must be fitted over at least the upper 1/3 of the height.02min) see Appendix 2 Accurate estimation of the second natural frequency requires the use of a finite element structural program with a dynamic capability or other advanced computer program. t ϭ total mass per square metre over the top third (kg/m2) divided by 7850 kg/m3 .. 7. ๶ L๶ . Most such dampers are mounted near the top of the chimney.7 Damping ratio The structural damping ratio (␨ ϭ c / ccr) without aerodynamic damping is given in table 7.2. is the value of gravitational acceleration (m/s2) When the interfering structure or chimney is less than 2 diameters away. including the effects of aerodynamic interference by other nearby towers or chimneys. the following expression may be used:4) f2 ϭ 3. ๶๶ ͚ms ๶ · x๶ /͚ ๶๶ ms · ๶๶ x2] (secϪ1) in which: ms x ge Figure 7.003 0. the wind speed V(z) at the top of the chimney (7. its cross-wind vibrations can usually be reduced by aerodynamic stabilizers.6 – The reduction of Strouhal Number caused by aerodynamic interference .2. which absorbs the wind induced energy. Damping devices should he designed to avoid the need for their frequent routine maintenance. Assuming a chimney is on a rigid support. 7.2. Where chimneys are lined.7 .17) Where E ϭ Young’s Modulus I ϭ Moment of inertia of cross section m ϭ mass per unit length 7.2. for wind loading in wind direction.2. Because of their profile and small size.CICIND Model Code Amendment A – March 2002 page 11 7. Ί๶๶ [ge . Probably the best solutions in this case would be either to fit tuned mass dampers. externally insulated Lined with refractory concrete Lined with brickwork chimneys with steel liners*:␭ Ͻ 26 ␭ Ͼ 28 Coupled group Chimney with tuned mass damper Table 7. ␲) . has been proved to be beneficial in the design of steel chimneys and they can be safely retro-fitted without incurring significant increase in wind drag loads.2. thereby significantly reducing the cross-wind and alongwind vibrations. 10Ϫ6 .9. Ί๶๶ (E . ␭ ϭ liner length / liner diameter * – In order to ensure impact damping the gap between the liner and its restraint should not be greater than 50mm. therefore. The damping for wind loading in wind direction can be increased by the aerodynamic damping: c / ccr ϭ 2. The useful effect of three continuous helical vanes has been proved on many steel chimneys.2) V ϭ 0 for cross-wind loading f1 is the fundamental natural frequency (7. Care must be taken to include for the effects of any supporting structure. (7. Tuned mass dampers have proven effective in reducing selfgenerated along wind and cross-wind vibrations and also the effect of nearby chimneys or structures. t) in which: V is.004 (0.. V / (f1 . Notes: If rotation of foundation decreases the first natural frequency more than about 10% the foundation is considered to be soft and the damping ratio may be increased by 0.005 0.15) .002 0. Type of chimney Unlined. Other chimney damping devices such as hanging chains have also been successfully used.4.2.4 Damping Ratio 0.2.006 0.002 0. (7. sulphur and other deposits. Guyed chimneys must also be subject to special investigation. spoilers or other attachments • cooling through support points • downdraft effects at top of the chimney 4) The presence of chlorides or fluorides in the flue gas condensate can radically increase corrosion rates.5. Typical cases of such restraint are to be found in stayed and guyed chimneys. For different values of S03 content. 2) Where the associated furnace is in petrochemical service. 7.5. Chemical effects Limited exposure to acid corrosion conditions can be permitted in chimneys which. Internal effects governing the chimney design 7.page 12 CICIND Model Code 7.1. if the risk of internal fire is significant. More information on the derivation of those stresses may be obtained from the CICIND Model Code for Concrete Chimneys — Part C: Steel Liners. Future special chimney designs and damping devices may prove effective in preventing excessive wind induced vibrations. However. The CICIND Model Code for Concrete Chimneys — Part B.025% by weight (300 mg/m3 at 20°C and 1 bar pressure) . 7. [14] & [15]). where levels of excess air can be greater than those normally experienced. Chimney fires can be caused by ignition of: 1) Unburned fuel carried over from the associated boiler or furnace.10 Special chimney designs for damping Wind tunnel tests. the metal temperature can be assumed to be about midway between ambient air temperature and that of the flue gas over the range of flue gas velocities between 5m/s and 15m/s. However. 7. Estimation of the corrosion rate in these circumstances depends upon a number of complex factors and would require the advice of a corrosion expert in each individual case. In such cases.5.6.2.2. Consideration must be given to the effects of oxidation when the material being used is close to its temperature limit.5. have allowed dual-wall and multiflue chimneys to be designed using shell-to-shell impact damping. Providing the flue gas does not contain significant concentrations of halogens (see notes (4) & (5) below) the degree of chemical load is defined in Table 7. 7. which otherwise would require aerodynamic stabilisers or mass dampers (see ref.6. confirmed by analytical means and field experience.6. Brickwork Linings provides a reference for the likely magnitude of explosion overpressures. 7. having neither an internal liner nor external insulation. heat transfer calculations shall be made to determine the maximum metal temperature of the structural shell. chimney design should be based upon a minimum S03 content amounting to 2% of the SO2 content in the flue gas. account should be taken of start-up and shut-down periods when the flue gas temperature is below its acid dew point. 5) Regardless of temperatures. Expert advice should be sought on the choice of suitable material. Degree of chemical load low medium high Operating hours per year when temperature of the surface in contact with flue gases is below estimated acid dew point ؉10°C Ͻ 25 25 Ϫ 100 Ͼ 100 Table 7. Thermal effects When a chimney is restrained from adopting a deformed shape in response to differential expansion. 2) In assessing the number of hours during which a chimney is subject to chemical load. provided the concentrations of HCl Ͻ 30mg/m3 or of HF Ͻ 5mg/m3 and if the operating time below acid dew point does not exceed 25 hours per year. care should be taken to prevent small areas being subject to local cooling and therefore being at risk of localised acid corrosion. This problem may not be solved solely by an increase in corrosion allowance as the environment may be polluted by the corrosion product. it is outside the scope of this model code. By contrast an externally insulated steel chimney or a bare steel chimney close to a reflective surface will quickly buckle during a fire. are safe from chemical attack. normal steel chimneys can resist earthquakes with an intensity of up to modified Mercalli scale 10 without serious damage.3. When the S03 content is not known. Fire The risk of a chimney fire should be assessed. bending stresses will be introduced in the shell. Earthquake loading The stress due to wind loading on a steel chimney is usually more than the earthquake stress and. These calculations shall assume still air and highest anticipated air temperature. These should have been proven initially by wind tunnel tests and finally by field experience before being universally adopted.6. 7. in the absence of such advice.5 Degree of chemical load for gases containing sulphur oxides Notes: 1) The operating hours in table 7. Local cooling may be due to: • air leaks • fin cooling of flanges. These deformations can be large when a single unlined chimney carries flue gases from two or more sources at significantly different temperatures or if a single side entry source introduces gases at very high temperatures. for most of the time. Internal explosions Internal explosions can occur due to the ignition of soot or explosive gases in the chimney. a castable refractory lining following the requirements of Appendix 3 will provide sufficient fire protection for most situations. chemical load shall be considered “high” if halogen concentrations exceed the following limits: Hydrogen fluoride: 0.g. unburned hydrocarbon carryover following a furnace tube rupture. In addition. the radiant heat loss to atmosphere from a bare steel chimney is often sufficient to maintain its temperature at a reasonable level. External explosions The resistance of steel chimneys to external explosions is very high. in cases where a heavy mass (e. 3) While a steel chimney may generally be at a temperature above acid dew point. If such explosions can occur in the direct vicinity such that strengthening for this reason is required.1. Typically. the hours given vary inversely with S03 content. a refractory concrete internal liner should be installed to provide a degree of fire protection.4. the degree of chemical load may be regarded as “low”. High temperature flue gases In the case of bare steel chimneys. the resulting differential metal temperature will introduce secondary thermal stresses.3 are valid for an S03 content of 15 ppm.2. This is especially so with gas turbine exhausts.3. They are not normally a cause for concern in the design of a steel chimney. a water tank or a heavy lining) is fitted to the upper portion of the chimney. Explosions 7. For flue gas velocities faster than 15m/s or for steel stacks equipped with either a liner or external insulation. a special investigation must be made (tanks are outside the scope of the model code). 3) Soot. During chimney fires. 7. consequently. stresses may be safely calculated assuming beam theory.. Second order effect The effect of the displacement of the load application points due to deformations (second order effect) shall be taken into consideration if the parameter ␤ Ͼ 0.. the carrying capacity check shall be based on 2ϩ␴ Ί๶๶ {␴*x๶๶ ๶๶ *y2 Ϫ ๶( ๶ ␴* ๶๶ ๶๶ 3␶ *2} р fk x · ␴* y) ϩ๶๶๵ .2) fy when ␭ р Ί๶ ϭ 0. Required checks The steel shell of a chimney shall be checked for: – – – carrying capacity serviceability fatigue (unless the chimney is fitted with an effective dynamic control) and: h ϭ height of the chimney (m) N ϭ total axial load at the base of the shell (without load factor) (N) E I ϭ stiffness of the cross section at the base of the chimney (Nm2) The second order moment M11 is approximately determined from:M11 ϭ M1 (1 ϩ ␤2 / 8) Where M1 is the wind moment at any particular level.2.3. (8. considering flexural and ovalling stresses simultaneously. For unstiffened chimneys (i..6) . ␴*B ϭ normal and bending compressive stress at ultimate limit state ␥m ␴k ϭ material factor ϭ 1. (t/R)]} tensile stress per beam theory 8. 8. 8. Biaxial stresses In areas subjected to biaxial stresses e.e.1% by weight (1300 mg/m3 at 20°C and 1 bar pressure) Hydrogen chloride: 0. The check should comprise both the strength and stability proof.3) occurs when ␴*x and ␴*y are of opposite signs.75 fy fy ␭ ␴cr E t r ␣ / ␭2 when ␭ Ͼ Ί๶ 2 ϭ yield strength of steel at design temperature ␣ ␴cr) ϭ Ί๶๶ fy / (๶๶๵ ϭ critical elastic buckling stress ϭ 0. chimneys without stiffening rings or substantial flanged joints) having L/R Ͻ 50.. 8.2) . which will be important in deriving bolt tensions. For unstiffened chimneys with a ratio of L/R Ͼ 50 (where L ϭ height of chimney and R ϭ radius). shell theory or finite element modelling should be used.. A fatigue check shall be carried out if movement due to vortex shedding is expected (see 7..0 Ϫ 0.3) The carrying capacity check shall prove that the forces resulting from the working loads multiplied by the load factors do not exceed the resistance of the shell.1 Minimum thickness At the time of construction the minimum thickness of the shell of carbon steel chimneys shall be 5mm. Load factors and load combinations The chimney shell shall be designed to resist stresses resulting from the weight of the chimney and the effect of wind multiplied by the load factors ␥: (␴i ␥i) ϭ (␴i*) р fk where: ␴i* ϭ stresses multiplied by load factors fk ϭ limit stress of steel 8.1) Note – The ovalling stresses are both negative and positive and the maximum value of expression (8.8) ␴*N ϩ ␴*B . (8. 8. The calculations shall be carried out for the corroded thickness of the steel (without corrosion allowance).. This simplified approximation may only be used when ␤ Ͻ 0. DESIGN OF STRUCTURAL SHELL 8.3...CICIND Model Code Amendment A – March 2002 page 13 Elementary chlorine: 0.1.5 .7) . Stability The proof of stability of the shell is given if the critical buckling stress divided by 1.4.g..3.. Where Nh is the design value of the total vertical load at the top of the shell.605 E t/r ϭ corroded plate thickness ϭ radius of the structural shell of the chimney at section considered ␣N ␴*N ϩ ␣B ␴*B ϭ .412␭ 1.6. Similarly. (8.1% by weight (1300 mg/m3 at 20°C and 1 bar pressure) 6) Saturated or condensing flue gas conditions downstream of a flue gas desulphurisation system shall always be considered as causing “High” chemical load. due to bending moments and ovalling. this will lead to increases in tensile stresses at the base and immediately above a change in chimney diameter.2. Carrying capacity of shell 8.5b) ..10 ϭ critical buckling stress 2 ϭ (1. (8. (8. (8. including the corrosion allowance. (8. (8....3. where:␤ ϭ h (N / E I)0.. This will lead to reduction in compression stress at the chimney base or immediately above changes in chimney diameter.2.3. but will increase compression stresses elsewhere.. It is not applicable to guyed chimneys.4) ϭ Young’s modulus of steel at design temperature . The serviceability shall be checked under working loads without load factors.3.5a) . The increase in tensile stress in these regions may be approximated by the expression:tensile stress per shell theory ϭ 1 ϩ {6 / [(L/R)2 .1.. (8.8 and Nh / N Ͻ 0.4).1 is greater than the sum of longitudinal stresses due to bending and compression: ␴*N ϩ ␴*B Ͻ ␴k / ␥m where: ␴*N. flexural stresses being added vectorially to ovalling stresses. occasionally movement amplitude may be sufficient to cause alarm.189 ϩ 0.5 ϭ 0.1.2 ϭ Modelling safety factor ϭ 1. In such cases the amplitude limitation of Section 5.1) the above formulae may be used if ␣1 is substituted for a: ␣1 ϭ ␣ [1.811 ␣N If the imperfections (w) are between 0. Basic principles The fatigue check shall ascertain that the movement due to vortex shedding does not result in the initiation and gradual propagation of cracks in the material. 8.9b) The amplitude of movement varies. (8. For a free-standing chimney with steel liner(s).4 may govern. especially near welds.2.2 – Fatigue strength of the base material with respect to the fatigue categories defined in Figure 8. 8.5. thus resulting finally in the failure of a weakened section.9a) .4 (for temperatures up to 200°C) ␣B ϭ 0..1): ␣N and: ␣N ϭ 0. The internal The influence of the grade of steel as well as that of the ␴min / ␴max ratio are negligible.5. cycles relationship ϭ (Vcr / 8)1. kϭ3 ϭ Determines the load vs. The allowances listed in tables 8. This total allowance shall be added to the thickness of the shell required to satisfy the specified limits of stress and deflection. the corrosion allowances should be increased proportionally. except in conditions of high chemical load.3 8.1 and 8. So as not to alarm bystanders.5 Ϫ w / 0.9c) If the factored Miner Number (M*) is less than 0. Stiffeners may be used to increase the shell’s resistance to buckling. no limit is placed on downwind deflection. 8. Allowance for corrosion Allowance for corrosion shall be the sum of the external (CE) and internal (Cl) allowances given in tables 8. (8.3) k ␭ ␥ ϭ the (positive) exponent of the fatigue curves. Figure 8. 8.2 are for a 20 year lifetime of the chimney. Influence of high temperatures The few results available show that at 200°C fatigue growth rates may be higher than at room temperature. Fatigue check 8.5.6. (8.02l shall not be permitted. The fatigue of the material depends essentially on: – – – the number of stress cycles N the stress range ⌬␴ ϭ (␴maxϪ ␴min) the constructional details Figure 8. As long as the carrying capacity stresses in the structural shell.2 no cracking will occur during the required lifetime. Unless more detailed results become available the modelling safety factor shall be increased to 1.. Internal flanges shall have corrosion allowance Cl and external flanges corrosion allowance CE on all exposed surfaces.83 / (1 ϩ r / 100t)0.01l and 0. Serviceability of shell The downwind deflection from the centreline of the structural shell under maximum design wind load must be calculated and reported.2 & 8. is not exceeded. The effect of fatigue due to all of the load cycles can be expressed by considering the factored Miner Number M*:Where M* ϭ ␥ .page 14 Amendment A – March 2002 CICIND Model Code When imperfections w are smaller than 0. Guidance on the design of such stiffeners is given in CICIND Model Code for Concrete Chimneys — Part C — Steel Liners.1 ϩ r / 100t)0. but at 400°C growth rates are lower than at room temperature..01l (Fig. M ϭ ␥ (␴max / ␴wn)k · (logeN)Ϫk␭ Where:␴max ϭ The maximum stress range due to vortex shedding ␴wn ϭ The fatigue strength after N cycles (see figs. the internal corrosion allowance only applies to the internal face of the liner(s).02l (see Fig.4 of this model code. For longer planned lifetimes.7 / (0.4. .1 8. for steel.2.5 for r/t р 212 for r/t Ͼ 212 .. the amplitude of deflection from the chimney centreline caused by vortex shedding shall not be greater than the limit agreed per Section 5.5.. expected to be in service for less than one year.3.50 in the range of metal temperatures between 200 to 400°C.26 ϫ 107ϫ Tϫ fϫ Aϫ eϪA2 where:T ϭ The required lifetime of the chimney in years f ϭ The resonance frequency A ϭ 4Vcr / V V ϭ The design wind velocity V(z) at the top of the chimney . or any liners. Fatigue strength The number of load cycles in the cross-wind direction can be calculated from:N ϭ 1.02 l] Imperfections (w) greater than 0. Nevertheless. when a corrosion allowance of 3mm is required. 8. values of CE and CI ϭ 0 are permissible. For temporary chimneys.1 and 8.. 8. with maximum movement only representing a small proportion of the total number of cycles. butt weld: – developed root.g. 2. 8.3 Type of welding: 1. 8. butt welds. 6.2 — Chimneys” Notes to Fig. butt weld: – welded one side only T – joint by double-bevel butt weld T – joint by double Y – butt weld with broad root face T – joint with special quality double fillet weld T – joint double fillet welds . cap pass counter welding butt weld: – welded one side only – through-welding of seam root and plane surfaces – – secured on opposite side by auxiliary welding aid – e. 7. weld-pool backing ceramics or copper rail 4. 3. when high quality has to be acheived and verified: – developed root. cap pass counter welding – evenly machined surface in stress direction.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. 5. page 16 CICIND Model Code . CICIND Model Code page 17 . page 18 CICIND Model Code . caused by industrial pollution. especially when contact with condensing SO2/SO3 is intermittent or of short duration (e.8 8. ordinary stainless steels (including high molybdenum stainless steel) have little better corrosion resistance than carbon steel and are.g.1 ϭ 0. Tension The limit state is described: ␴*t р ␴t. + In these circumstances.u / 1.. load always “high”)* not applicable (chem. 9.1.u in MPa 9.1.1 182 227 278 364 455 Table 8.g. consideration should be given to increasing these allowances. the carrying capacity check shall be carried out with the same load factors and load combinations as described under 8. In these circumstances great care is required in the protection of the gas face of the chimney or its liner.6. These conditions are. (9. Grade Fe 360 Fe 430 Fe 510 ␴l. bolt grade minimum value of the tensile strength of bolts 400 500 600 800 1000 ␶u 200 250 300 400 500 ␶u/1. 8. External corrosion allowance painted carbon steel painted carbon steel under insulation/cladding unprotected carbon steel unprotected “corten” or similar steel unprotected stainless steel 0mm 1mm 3mm 1mm 0mm 9. The design shear stress ␶* relates to the gross area or to the nett area.2.g.45: ␴*l р ␴l.u see table 9.1.1.2. provided a weather-tight cover is fitted over the air space(s) between the liner(s) and the outer shell.3 . use other materialϩ 1mm 2mm corrosion allowance inappropriate. use other material* 2mm** 4mmϩ corrosion allowance inappropriate. 9. 9.1 Limit shear stress (␶U) in MPa. (9.8 10. by cladding with a suitable high nickel alloy or titanium or by the application of a suitable organic coating. General provisions Connections shall be calculated on the basis of forces at least as great as the design forces of the parts they connect e.2. nearby chimneys or close proximity to the sea. (9. see the CICIND Chimney Coatings Manual. it will require protection by an appropriate coating.. therefore not recommended.1.2.2. load always “high”)* corrosion allowance inappropriate.5 low medium high low medium high low medium high .1. Regardless of any preload.u / 1.6 5. “Corten” steel shows some improvement of corrosion resistance over carbon steel.3. Connections 9. high molybdenum stainless steel (such as ASTM Type 316L) is suitable within this temperature limit. depending on whether the shear plane is in the unthreaded or threaded part of the bolt.45 ␴u ␴*l . Bearing on connected surfaces Table 8. 8.2) Notes: * Provided acid concentration in the condensate is less than 5% and chloride concentration does not exceed 30mg/M3.6 6.1.1 The values of limit shear stress are given in Table 9. the limit stress ␴l. The design bearing stress relates to the area obtained by multiplying the diameter d of the shank by the thickness of the connected part.1. Bolted connections The carrying capacity of bolted connections shall be checked with regard to tension and shear or bearing. Table 9.2 internal corrosion allowance (CI) — for carbon steel only (for chimneys handling flue gases) The design stress on connecting parts shall not exceed the minimum value of the tensile strength of the connected parts multiplied by 1. If carbon steel is used in chimneys subject to high chemical load. For further guidance.u 575 690 815 ␴l. Shear The shear stresses multiplied by the load factors shall not exceed the limit shear stress divided by resistance factor ϭ 1. however.1.1: t* р ␶u / 1.CICIND Model Code page 19 face of the outer shell requires no corrosion allowance.1.. Internal corrosion allowance Usual temperature of metal in contact with flue gas Ͻ 65°C Chemical load per table 7. see the CICIND Chimney Coatings Manual..1 are suitable for a normal environment.6.9 65°C – 345°C Ͼ 345°C not applicable (chem.1. e. When a chimney is sited in an aggressive environment. during repeated shut-downs).3) .B for ␴t.2 – Limit bearing stress ␴l. For further guidance.2..3. using a corrosion allowance of 3mm for a 20 year life. generating condensing gases.1 525 625 740 ** In conditions of low chemical load.1) Internal corrosion allowance 4.u / 1. External corrosion allowance (CE) Note: The external corrosion allowances quoted in Table 8. unlikely to be met in a chimney downstream of a FGD system.1 ϭ 1. DESIGN DETAILS 9. use other material Table 9.u is valid for edge distances greater or equal 2d in the direction of stress.73 ␴u..1. For stresses at elevated temperatures refer to the factors in column 2 of Table 6.1.1.u / 1.u not recommended not recommended not recommended 640 800 Limit tensile stress of preloaded bolts ␴t.6 6. 9.1 ϭ 0.4 are valid for electrodes with properties of steel Fe 510. Limit stresses ␴w. strain at failure and notch toughness of the weld metal shall exceed minimum values for parent material.2.u /1. the following two conditions shall be checked: – – in the gross section.u and ␴s. Combined loading If the external loading results in a combination of tensile stress ␴t* and shear stress ␶* in the bolt.2 ␴t. tensile strength. In this case. .1 165 180 250 150 165 230 Note! The stresses given in Tables 9. Deduction for holes For parts subjected to tension. An acceptable standard is discussed in 9.3 below.3.1 ϭ 0.u)2 р 1. no particular checks are necessary. If the quality of the weld is at least equal to that of the parent metal.2 ␶u or ␴t* р 0.u ␴s. the stress shall not exceed the yield stress fy in the nett section. (9 5) . Owing to their considerable susceptibility to fatigue.4) grade Fe 360 Fe 430 Fe 510 9.u / 1.455 ␴uE – in the contact section s-s: ␴s* р ␴s.3 are for ambient temperatures.3 Limit tensile stress ␴t. Full penetration welds 580 730 Table 9.u for fillet welds in MPa The yield stress. the two design stresses ␴w* and ␴s* for fillet welds shall be checked: – in the throat section a-a: ␴w* р ␴w.9 ␴u. the stress shall not exceed 80% of the tensile strength ␴u Table 9.2 and 9.u in MPa.4. Welded connections Minimum value of tensile strength bolt grade 4.8 10.1.2.2.page 20 CICIND Model Code 9.u values given in table 9. The tensile stress ␴t shall be calculated on the nett section. shall be at least equal to those of Fe510.1.0 This check is not necessary if: ␶* р 0. the carrying capacity shall be checked for the condition: (␶* / ␶u)2 ϩ (␴t* / ␴t.5.B 400 500 600 800 1000 ␴t. and. Partial penetration welds shall be taken as fillet welds and calculated as such. 9.u ␴w.1 The welding standard considered appropriate for steel chimneys is higher than the minimum standard allowed for other welded products..1. Throat section ␴w. (9..3. failing a specific agreement. ␴w.6 5.3. full penetration welds have the same resistance as the connected parts. Partial penetration of butt welds shall not be permitted. 9.u / 1. Fillet welds Regardless of the direction of stress.636 fy where ␴uE is the guarantied minimum value of the tensile strength of the weld metal and fy the yield stress of the parent material. connections that use bolts in tension shall be made with pretensioned high strength bolts.1 255 255 255 230 230 230 Contact section ␴s.1.8 8... Full penetration welds connecting plates of different thicknesses have a resistance equal at least to that of the thinnest plate.2.u/1.4.3.u .1. If local codes are used. [22] and fig.6) .73 · ␴u. There is the possibility of the foundation being damaged if an adequate heat barrier is not installed.3. If possible. with flanges welded in place. 9. The fitting of gaskets to the flanges of structural shells is not permitted. (9. taken at random. normal force and shear force through the base plate and anchor bolts. Other imperfections shall be within the limits stated in section 8.1 where: ⌬␴R is the fatigue strength for category 35 MPa An is the stress section of the bolt ..2. 9.7) The tolerances in the fabrication of the shell shall be as follows: Flat plate prior to rolling shall be laid out and squared to within Ϯ1mm in length. Grouting After the chimney has been erected and plumbed (with the use of steel shims which remain in position) the space between the base plate and concrete foundation must be filled with nonshrink grout. local codes may be substituted. An anchorage device shall be attached to the bottom end of the bolt. 8.3 Temperature effects Consideration must be given to the effect that radiant or conducted heat will have upon a concrete foundation.1 General The following will be observed during shop and site construction as appropriate. The maximum bolt stress should not exceed 73% of the tensile strength of the material of anchor bolt. 9.3. The minimum bolt diameter should be db ϭ 16mm.f ϭ Zf a / w р ⌬␴R An / 1. Structural shell If the fatigue load Zf is greater than the fatigue strength divided by 1. The support at the base Self-supporting steel chimneys are normally based on a reinforced concrete foundation or a steel structure. In the majority of situations insulation to contain or deflect radiant heat will suffice.3. (9. where db is the diameter of the bolt.1 Normal flange In the case of along wind: Z*b ϭ Z* a / w р 0. Note The fatigue categories listed in fig.b · An · w/a у Zf .4 of this model code and assumed by the designer. a joint with contact areas shall be used (see lit. 9. Fig. This is particularly relevant to chimneys serving gas turbines or other high temperature exhaust systems.. 9. the shell shall be adequately braced.1. If this is not possible. The pretension of the bolts should provide a sufficient force ZA to prevent the fatigue in the bolt material: ZA ϭ 0.. of 10% of butt welds and fillet welds shall be tested.CICIND Model Code page 21 9. STEEL LINERS Steel liners inside steel chimneys shall be designed to satisfy the requirements of CICIND Model Code for Concrete Chimneys — Part C — Steel Liners.2 Flanged connections The use of high strength bolts is recommended.. 9.2 Prestressed flange. A chimney section.2.2. as measured by a 450mm long template. shall be fabricated within a tolerance of Ϯ3mm on circumference and diagonal. The centres between the bolts should be between 4db and 10db. the weld categories may require appropriate adjustment. The foundation or structure is loaded by an overturning moment. However.10.7) 10.3. the weld testing procedures and quality levels shall be agreed by the client and the builder.. 11. width and on each diagonal. Fig. Advice on the design of steel liners in steel chimneys is given in Appendix 3 to this Model Code. Alternative satisfactory methods may be used at the designer’s discretion when no response to vortex shedding is anticipated. but subject to agreement between the client and builder.2). It should be noted that the change of the type of connection to one with profiled contact areas may reduce the damping ratio used in estimating along and across-wind response.3.b An In the case of cross-vibration (fatigue): Zb. these measurements shall be made while the shell’s axis is vertical. suitable for vibrating conditions Misalignment between plates shall not exceed 1mm.3 assume welds are made to ISO 5817 level ‘C’ quality standards.2. Anchor bolts When fatigue due to vortex shedding is anticipated anchor bolts should be prestressed. Peaking of a cylinder from a true circle at weld seams shall not exceed 3mm. Measures must be taken to ensure that the prestressing is not lost during the lifetime of the chimney. 9. The compressive strength of the grout must be equal to or greater than the compressive strength of the concrete. (9. .. 11. The stress in the bolts shall be calculated taking consideration of the eccentricity of the loading transmitted by the shell. 9.73 ␴u.3 Weld Testing While a minimum.3. Vertical butt weld seams shall be staggered a minimum of 200mm from eachother. centred at the weld and cut to the cylinder’s design radius. The recommendations of levels ‘C’of ISO 5817 “Arc-welded joints in steel — guidance on quality levels and imperfections” should be used. CONSTRUCTION 11. a distance of 5db is recommended as larger spacings result in excessively thick flanges.1.2. 3). however.page 22 CICIND Model Code 11. Before bolting. 17. 14 GUYED AND STAYED CHIMNEYS A stayed chimney is defined as one which derives lateral (but not vertical) support from another structure. The effect of openings upon the chimney’s stiffness should be taken into account when determining the chimney’s natural frequencies.3. The prime concern of the design should be to ensure that vertical expansion is not restricted. The design of the supporting structure is outside the scope of this Model Code. OPENINGS The width of a single opening shall not exceed two-thirds of the diameter of the structural shell of the chimney. 8. the gap at the outer edges of the flanges shall not exceed 1. fixed at their bases.2) Ͼ 1. A suggestion for stiffeners is given in the Commentaries for this Model Code. To avoid vibrations due to vortex shedding. The foregoing structural design rules are valid for self-supported chimneys. ACCESS LADDERS A specification for access ladders and hooks is given in Appendix 5. A guyed chimney derives lateral support from guy ropes. Rules governing the structural design. 12.7 Erection tolerance The departure of the chimney from the vertical on erection shall not exceed 25 mm or 1/600 of the height.1 – 3 in Fig. their design shall include the effects of circumferential bending stresses in the shell. In designing the shell and lateral supports. Their orientation shall be marked prior to their being dismantled after welding. Part C — Steel Liners. but shall be kept to the minimum possible. those related to thermal and chemical load) are relevant also to chimneys that are guyed or stayed.g. the maximum gap width on the line of the shell.5mm per 100mm width of flange. above and below the opening. with or without liners.5 times the spacing of the stiffeners. Intermittent welding shall not be allowed.5°. The number of lateral supports will be governed by buckling considerations per section 8. whichever is the greater at any point. Also they shall be long enough to distribute stresses into the main area of the shell without overstress. as required. acting as cantilevers. Specifications for different types of protection are given in Appendix 3. it may be necessary to incorporate stiffeners around the opening. It is sufficient to ensure that the conduction path is electrically continuous and that it is adequately earthed. 11. Note: These tolerances may be ignored if the flanges are bolted together before they are welded to their respective shell sections.2 ϫ maximum windspeed at the relevant elevation (10 minute mean). AIRCRAFT WARNING LIGHTS It is advisable to contact the local aeronautical authority for the area if the chimney is to be built within an aerodrome safe guarding area as local conditions and restrictions may apply. 16. the natural frequencies should ensure that Vcr (assuming S ϭ 0. When longitudinal stiffeners are used.4 Stiffening Rings If the design permits the use of intermittent welding. shall be 1mm. SURFACE PROTECTION The exterior and interior surfaces of a steel chimney may be protected from attack by weather and corrosive gasses by various methods.3 Structural Flanges and opening reinforcement These shall be fully welded to the structural shell. between matching pairs of flanges. crevices exposed to weather or flue gases shall be sealed.4 above and by the need to avoid oscillations due to vortex shedding. Before bolting.5 Baseplate The baseplate and all base reinforcement shall be fully welded to the structural shell and to each component. See also CICIND Chimney Protection Coatings Manual.5°. Some of the rules (e. .2 Guyed Chimneys Design rules for Guyed chimneys are given in Appendix 4 to this Model Code 15. where t is the thickness of the plate. the forces induced by the restraint of differential thermal expansion shall be considered. Where large apertures are cut in the shell plates. Smaller apertures in the shell plates. (Note: this may generally be deemed to be satisfied if the stiffeners project above and below the opening a distance at least 0. As a result. The ends of the longitudinal stiffeners should be tapered in a radial direction (see cases 16. Flanges shall be flat and normal to the chimney axis. PROTECTION AGAINST LIGHTNING A steel chimney can be considered as a continuous metal structure and thus be used as its own lightning protection system. a structural analysis of the stresses shall be made and compensating material provided. These stiffeners may be attached between the longitudinal stiffeners. to ensure that the stresses specified in this Model Code are not exceeded. The base plate shall be perpendicular to the shell plate within Ϯ0. at the hole’s edge and at the end of the longitudinal stiffeners. Guidance on the determination of these forces may be found in CICIND Model Code for Concrete Chimneys. Additional horizontal stiffeners may be used to absorb the circumferential bending stresses. 11. related to wind or earthquake loading do not. 11.1 Stayed chimneys Stayed chimneys are supported laterally at one or more elevations above their bases. Flanges shall be welded to the structural shell within a perpendicular tolerance of Ϯ0. 14.). shall have the corners radiused to a minimum of 10 t. Differential expansion can be expected if two or more gas streams of differing temperatures enter the chimney at different points. Consequently it requires no air termination or down conductor. 14. apply to these chimneys. 11. not equipped with stiffeners. 13.6 Straightness Adjoining cylinder sections shall be welded together straight in the longitudinal direction to a tolerance of Ϯ12mm per 10m of shell length. as for gas inlets or inspection panels. 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