design of steel free stack

CICINDModel Code forSteel Chimneys

(Revision 1 – December 1999)

Amendment A – March 2002

Commentaries and Appendices(December 2000)

Copyright CICIND 2000ISBN 1-902998-11-1

Office of The Secretary, 14 The Chestnuts, Beechwood Park, Hemel Hempstead, Herts., HP3 0DZ, UKTel: +44 (0)1442 211204 Fax: +44 (0)1442 256155 e-mail: [email protected]

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

Commentary 1 – Glossary of commonly used words . . . . . . . . .3

Commentary 2 – Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6

Commentary 3 – Wind Load . . . . . . . . . . . . . . . . . . . . . . . . . . . .8

C3.1. Wind Speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8

C3.1.1. BasicWind Speed . . . . . . . . . . . . . . . . . . . . . .8

C3.1.2. Wind Maps . . . . . . . . . . . . . . . . . . . . . . . . . . .8

C3.1.3. The Influence of Height . . . . . . . . . . . . . . . . .8

C3.2 The Gust Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15

C3.3 Vortex Shedding . . . . . . . . . . . . . . . . . . . . . . . . . . . .15

C3.4 Movements in the second mode . . . . . . . . . . . . . . . . .16

C3.5 Ovalling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18

C3.5.1 Static effects . . . . . . . . . . . . . . . . . . . . . . . . .18

C3.5.2 Dynamic effects . . . . . . . . . . . . . . . . . . . . . .20

C3.6 Interference effects . . . . . . . . . . . . . . . . . . . . . . . . . .21

Commentary 4 – Fatigue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22

Commentary 5 – Openings . . . . . . . . . . . . . . . . . . . . . . . . . . . .24

Commentary 6 – Chemical Effects and Internal Corrosion . .26

C6.1. Chemical Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . .26

C6.1.1. Attack Due to Sulphur Oxides . . . . . . . . . . .26

C6.1.2 Effects of Flue Gas Desuphurisation . . . . . . .26

C6.1.3. Attack Due to Chlorine, Chlorides

and Fluorides . . . . . . . . . . . . . . . . . . . . . . . .26

C6.2. Internal Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . .26

C6.3 Selection of materials . . . . . . . . . . . . . . . . . . . . . . . .26

Appendix 1 – Base Plate Design . . . . . . . . . . . . . . . . . . . . . . . .28

A1.1 Simple base plates . . . . . . . . . . . . . . . . . . . . . . . . . . .28

A1.2 Base plates with gussets . . . . . . . . . . . . . . . . . . . . . .28

A1.3 Baseplates with gussets and compression rings . . . . .28

A1.4 Grouting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .29

Appendix 2 – Insulation, Linings and Protective Coatings . . .30

A2.1. Insulation

A2.1.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . .30

A2.1.2. Insulation Design . . . . . . . . . . . . . . . . . . . . .30

A2.1.3. Aluminium Cladding . . . . . . . . . . . . . . . . . .30

A2.1.4. Mineral Wool or Foam Insulation . . . . . . . . .31

A2.1.5. Lined and Multiflue Chimneys . . . . . . . . . . .31

A2.2. Linings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31

A2.2.1. General . . . . . . . . . . . . . . . . . . . . . . . . . . . .31

A2.2.2. Design of Separate Liners . . . . . . . . . . . . . . .31

A2.2.3. Design of Linings Attached

Continuously to the Shell . . . . . . . . . . . . . . .32

A2.3. Recommended Start-up Procedures . . . . . . . . . . . . . .32

A2.4. Protective and Decorative Treatments . . . . . . . . . . . .32

Appendix 3 – Guyed Chimneys . . . . . . . . . . . . . . . . . . . . . . . . .33

A3.1. Guyed Chimney expansion . . . . . . . . . . . . . . . . . . . .33

A3.2. Guyed Chimney calculations . . . . . . . . . . . . . . . . . . .33

A3.3 Guy Ropes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .33

Appendix 4 – Access Ladders . . . . . . . . . . . . . . . . . . . . . . . . . .34

A4.1. General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34

A4.2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34

A4.3. Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34

A4.4. Finish . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34

A4.5. Stringers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34

A4.6. Rungs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34

A4.7. Safety Hoops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34

A4.8. Rest Platforms and Landings . . . . . . . . . . . . . . . . . . .35

A4.9. Attachment to Chimney . . . . . . . . . . . . . . . . . . . . . . .35

A4.10.Access Hooks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35

CICINDModel Code for Steel Chimneys

REVISION 1 – DECEMBER 1999

COMMENTARIES AND APPENDICES

TABLE OF CONTENTS

DISCLAIMER

This CICIND document is presented to the best of the knowledge of its members as a guide only. CICIND is not, nor are any of itsmembers, to be held responsible for any failure alleged or proved to be due to adherence to recommendations or acceptance of information

published by the association in a Model Code or in any other way.

CICIND, Talacker 50, CH-8001, Zurich, Switzerland

Copyright by CICIND, Zurich

CICIND Model Code – Commentaries and Appendices page 3

FOREWORDIn December 1999 the Second Edition of the Model Code for SteelChimneys was published. This is now expanded by the publication ofthe Commentaries and Appendixes to this Model Code.

The Intention of this volume is to explain the reasons behind theprinciples set out in the Model Code. It is divided into two parts. TheCommentaries cover the theoretical derivation of the formulae andthe principles used in the Model Code. The Appendices relate tomore practical considerations.

COMMENTARY No. 1

GLOSSARY OF COMMONLY USED TERMS

The numbers in brackets are given in figures C.1.1 and C.1.2.,showing typical chimney designs.

Access door(2.01) – A door for the entry of personnel or other meansof inspection.

Aerodynamic stabilizer (2.03) – A device fitted to the structural shellto reduce wind excited oscillations by modifying vortex shedding

Anchor bolts – See Holding down bolts

Base cone(2.04) – A truncated cone incorporated immediately abovethe baseplate of a chimney.

Baseplate(2.05) – A horizontal plate fixed to the base of a chimney.Also called a bearing plate.

Base stool(2.07) – A construction comprising two vertical plates,welded to the chimney shell and to the baseplate, supporting acompression ring (2.14) through which a holding down bolt passes.

Blanking off plate (2.08) – An imperforate plate fitted immediatelybeneath the inlet of a chimney to prevent the waste gases reaching thelower portion of the chimney. Also known as a false bottom.

Boiler mounted chimney– A chimney supported by a boiler and itsfoundation.

Bracket (2.10) – A construction providing resistance to lateraldisplacement of the chimney and/or supporting part or all of theweight of the chimney.

Bracketed chimney (2.11) – A chimney in which not all externalapplied loads (e.g. wind) are carried exclusively by the structuralshell and for which brackets, attached to an adjacent structure, areprovided to ensure stability. Also known as a braced chimney.

Breeching – see inlet (2.28)

Cap plate (2.12) – A sloping or convex plate fitted to the top of thestructural shell, covering the area between it and the liners andincorporating cravats through which the liners protrude.

Cleaning door (2.13) – A door, normally at the base of the chimney,to permit the remova! of flue dust.

Compression ring (2.14) – A steel plate welded to the shell whichtransfers the forces acting upon the chimney to the holding downbolts. Also known as a base ring.

Cope band (2.15) – A steel section attached to the top of the chimneyaround its perimeter to give added strength and corrosion resistanceat this level.

Cope hood(2.16) – A hood fitted externally to the top of a liner,covering the upstand of the cap plate, to prevent the ingress ofrain water.

Corrosion test piece(2.17) – A fixed or removable steel plate insert,generally of lesser thickness than the shell of the chimney, in contactwith the waste gases and fitted at strategic points where maximumcorrosion is expected to occur.

Cowl (2.18) – A conical or dished cap fitted to the top of the chimneyto reduce the ingress of rain water. Also known as a rain cap.

Cravat (2.19) – An upstand fixed to the roof, roofplate or cap plateto prevent the ingress of rain water (see cope hood). Also known ascounter flashing.

Cross-section– The section of the load bearing steel shell includingthe corrosion allowance.

Damping device(2.20) – A device fitted to the structural shell toincrease its structural damping.

Doubling plate (2.21) – A plate fixed to the shell to reinforce itwhere increased stresses occur.

Double skin chimney (2.22) – A chimney consisting of an outerload-bearing steel shell and an inner liner which carries the fluegases. Also known as a dual wall chimney.

Drag coefficient– see wind force coefficient

Drain pipe (2.23) – A pipe which connects a tundish to a pointoutside the structural shell and used to remove condensate.

Flue – see liner

Guy (2.24) – A wire rope attached at one end to a chimney andanchored at the other so as to provide tensile resistance to the lateraldisplacement of the chimney

Guy band (2.25) – A steel section fitted around the outside of achimney with provision for the attachment of guys.

Guyed chimney (2.26) – A chimney in which not all externallyapplied loads (e.g. wind) are carried exclusively by the structuralshell and for which guys are provided to ensure stability.

Holding down bolts (2.27) – Bolts built into a concrete foundation,brick base or supporting framework to provide anchorage at the baseof the chimney.

Hoops– Horizontal rings forming a cage around ladders.

Inlet (2.28) – A short duct fixed to the shell or baseplate of a chimneyfor the entry of flue gases.

Intermediate cone (2.29) – A truncated cone incorporated in thechimney shell at an intermediate level.

Jointing flange(2.30) – A steel section fitted to the end of a chimneysection to enable sections to be connected together.

Ladder boss– Aboss welded to the chimney shell into which an accesshook or eye can be screwed to provide fixing for temporary ladders.

Lateral supports (2.31) – Supports positioned at appropriate levelswithin the structural shell to locate the liners, allowing independentexpansion of the shell.

Lightning protection system – System to provide electricalcontinuity between the chimney and earth.

Liners (2.32) – Flue gas ducts contained within the structural shell.

Liner base (2.33) – A suitable support positioned at a convenientheight above the baseplate of the structural steel shell to carry theweight of the liners.

Lining (2.34) (see appendix No 2) – A material applied to the internalface of the chimney to prevent the flue gases contacting the innersurface of the steel shell.

Multiflue chimney (2.35) – A group of two or more chimneys withina structural framework or a chimney comprising a group of two ormore liners within a structural shell.

Nett section – The section of the load bearing steel shell withoutcorrosion allowance.

Reinforcement – Structural shapes or plates at or near to shellaperatures to strengthen the shell.

Roofplate (2.36) – A plate which follows the contour of the roofround the chimney where it passes through the roof of a building.Also known as flashing.

Rungs– Horizontal bars in ladders.

CICIND Model Code – Commentaries and Appendices

Safety system– Proprietary fall arrest system fixed to ladder rungsor beside the ladder to give a safe fixing for attachment of operatives’safety harnesses.

Self supporting chimney (2.37) – A chimney in which externallyapplied loads (e g. wind) are carried exclusively by the structuralshell and which, together with the foundation, will remain stableunder all design conditions without additional support.

Splitter plate (2.38) – A vertical plate welded to the interior of theshell between two horizontally opposed inlets to divert the flow of theflue gases into a vertical direction and to inhibit the passage of fluegases from one inlet into the other.

Stay (2.39) – A rigid member providing both tensile and compressiveresistance to the lateral displacement of the chimney. Also known asa lateral brace.

Stayed chimney(2.40) – A chimney in which not all externallyapplied loads (e.g. wind) are carried exclusively by the structuralshell and for which stays, connected to another structure, areprovided to ensure stability.

Stiffening ring – Horizontal members to prevent ovalling and tomaintain the chimney shell circular during fabrication andtransportation.

Strakes– see aerodynamic stabilisers

Stringer – Vertical member of a ladder to which the rungs are attached.

2.03

2.08

Typical general arrangement of three types of self supporting steel chimney.The numbers are related to the text

Figure C1.1


2.11

Typical general arrangement of guyed, stayed and bracketed chimneys.The numbers are related to the text

Figure C1.2

Structural shell (2.41) – The main external structure of the chimney,excluding any reinforcing or flanges.

Top cone(2.42) – A truncated cone or other device fitted at the topof a chimney to increase the gas exit velocity.

Tundish (2.43) – A conical or sloping blanking off plate providedwith facilities for drainage. Also known as a false bottom.

Tuned mass damper– A form of damping device which employs apendulum, tuned to the chimney’s natural frequency. The movingpart of the pendulum is connected to the chimney by an energyabsorbing device.

Vanes– See Aerodynamic stabilizers

Venturi. – See Top cone

Weatherhood(2.44) – A hood designed to shed rain water clear ofthe cravat and prevent its entry into the building. Also known ascounter flashing.

Wind force coefficient– The ratio between the wind pressure on thechimney and the equivalent pressure on the same area normal to thewind direction.


COMMENTARY No. 2

SAFETY

The safety of a chimney is ensured by the use of partial safety factorsat the ultimate limit state. These partial safety factors are listed inparagraph 5.3 of the code. A chimney is thus deemed safe if themaximum stress due to the characteristic load, increased by theappropriate partial load factor, is less than the allowable stress,divided by the partial material safety factor. The level of wind loadfactor chosen ensures that premature failure due to low cycle fatigue,caused by wind gusts in the wind direction, can not occur.

Derivation Of The Partial Load Factor In The WindDirection (Temperate Zones)

The partial load factor for wind load in the wind direction is derivedas follows by considering the social and economic consequences offailure or damage requiring the chimney’s repair or replacement. Thisinvolves deriving the acceptable probability of failure (P) during thechimney’s lifetime, using the following expression given in CIRIA(U.K.) Report No. 63, entitled “Rationalisation of Safety andServiceability Factors in Structural Codes”[1] :

P� 10�4 � Ks� nd / nr ... (C2.1)

Where

nr � average number of people near the structure during theperiod of risk

nd � design life of structure (assumed to be 20 years for asteel chimney)

Ks� a social criterion factor, given in table C2.1

Table C2.1 - Social Criterion Factor

Nature of structure Ks

Places of public assembly, Dams 0.005

Domestic, Office or Trade and Industry 0.05

Bridges 0.5

Towers, Masts, Offshore Structures 5

In order to use equation C2.1 it is necessary to estimate the value ofnr. It is suggested [1, 2] that allowance be made for the number ofpeople likely to be close to the structure at the time that maximumloading can be expected. Since maximum loading is most likely tooccur under extreme wind conditions, it can be assumed that no-onewill be climbing the chimney and no-one will be nearby, exceptthrough necessity.

If we assume nd � 20 years and Ks as 0.05 for “normal” chimneysand 0.005 for critical chimneys, acceptable probabilities can beestimated as summarised in table C2.2:

Table C2.2 Typical failure probabilities forenvironmental economic risk

Environment nr Ks P

Chimney industrial area (“normal” chimney) 0.1 .05 10�3

Chimney in urban area or hospital (“Critical chimney”) 1 .05 10�4

Chimney serving critical plant (“Critical chimney”) 0.1 0.005 10�4

It follows that safety factors should be chosen to give probabilities offailure of 10�3 for a “Normal” chimney and 10�4 for a “Critical”chimney.

The probability of failure depends upon the statistical distributions ofresistance and loading.

The resistance of a steel chimney may be taken as normallydisributed with a coefficient of variation (ratio of standard deviationto mean value) approximately 10%.

The principal load is due to wind. The moment is proportional to thewind pressure, the extreme values of which follow a Fisher-TippettType 1 (FT1) distribution as described in reference 3.

This distribution has a Cumulative Distribution Function (CDF)given by P(q)� exp(�exp(��(q� u)))

in which the constants are the mode u and the dispersion 1/�. Intemperate climates the product u.� � 5; other values may obtainelsewhere (see ref.2)

Now, the characteristic wind is defined as having annual probabilityof being exceeded � 0.02

It follows that the characteristic pressure qk � q�1� �This is converted to standard measure by substituting q� x. u

then Ps1(x) � exp(�exp(��u(x�1)))

The probability distribution function (pdf)

� Ps1(x) � � uexp (� � u(x�1)) Ps1(x)

The 50-year wind pressure is xs50� 1�

The resistance is assumed normally distributed with mean xr andstandard deviation �r

The characteristic value is xr5%� xr � 1.645�r

The load factor F� �

� xr � F�1� �� 1.645�r

the pdf of the resistance is pr (x) � exp�� 2

�The CDF for the wind pressure in period T years is PsT(q)� (Ps1(q))T

The effect of altering the period of exposure from 1 to T years is to

shift the mode from 1 to 1� without altering the shape of

the distribution.

Hence the CDF is PsT(x) � exp(�exp(��u(x�1)�In(T)))

The probability of failure is given by PFT �0��(1�PsT(x))·pr(x)dx

Now the factor F� �w ·�m where �w is the wind load factor and �mthe material factor.

Assuming �m � 1.1, thenif �w � 1.4 PF20� 8·10�4

if �w � 1.5 PF20� 3·10�4

When failure is ductile, additional safety against collapse is derivedfrom the chimney’s residual strength, after mobilisation of itsallowable (yield) strength at one point of its periphery (i.e., at theultimate limit state).

When failure is by buckling, additional safety is implicit in therelationship used between the allowable (critical buckling) strengthand the yield strength of the material. This relationship includes anadditional partial safety factor to ensure that the critical bucklingstress is sufficiently below the lower bound of experimental curvesused as a basis for the design (see ref. 5 ). For normal steel chimneys,this additional partial safety factor lies between 1.2 and 1.33,depending upon the diameter/ thickness ratio.

It is, therefore, proved that wind load factors of 1.4 and 1.5, willensure failure (collapse) probabilities of 10�3 and 10�4, required by“Normal” and “Critical” chimneys, respectively.

In(T)�u

x�xr

�r

12

1�r��2

In(50)� u

xr � 1.645�r

1�In(50)

� u

xr5%xs50

In(50)� u

ddx

In(50)� u


References

(1) Report 63 “Rationalisation of safety and serviceability factorsin structural codes”— CIRIA (U.K.), 1977

(2) BS 8100 Part 2, British Standards Institution, 1996

(3) Bierrum, N.R. — Letter to the Editor,CICIND REPORT Vol. 5, No. 1, 1989

(4) ENV 1991-2-4, CEN, 1995

(5) ‘European Recommendations for steel construction’—European Convention for Construction Steelwork (ECCS), 1978.


COMMENTARY 3

WIND LOAD

At the time of publication of the revised CICIND Model Code forSteel Chimneys (1999), the wind load model currently used in ENV1991-2-4 (eventually intended to form the basis of Eurocode 1, Part2–4: Actions on Structures — Wind Actions) has been shown bycalibration studies by CICIND and others to be unacceptable. In viewof the time expected to elapse before an acceptable model forEurocode 1 is agreed by all parties, CICIND have decided for the timebeing to retain the wind load model described in the 1988 version ofthis Model Code. A recent paper[1] has shown that this model givessafe and reasonably accurate estimates of the wind load on chimneys.

C3.1 Wind-speed

As the basis for the wind-load, the hourly mean windspeed has beenretained. The wind-load is calculated after estimating a turbulenceintensity, by a “gust factor” method[2].

C3.1.1. Basic wind-speed

The basic wind-speed used in deriving wind-loads is the wind-speedaveraged over one hour and measured at 10m above open ground atthe chimney location, which has a probability of exceedence of oncein 50 years.

The value of the basic wind-speed for a given location should beobtained from data collected by meteorological stations.

When wind speeds have been measured over periods less than 50years, the value of the basic windspeed must be extrapolated usingthe Fisher-Tippett Type 1 expression for the statistical distribution ofextreme values, as follows:

P(V)� exp {-exp [��(V � u)]}

Where:

P(V) � probability of excedence of velocity V duringthe relevant period

� slope of curve in Fig. C3.1

u � intercept on vertical axis of curve in Fig. C3.1

For a probability of exceedence, once in 50 years, P(V) = 0.02

In some cases lower values for u and are found (see lit. [3] ).

The relationship between the wind-speed and the return period isgiven in figure C3.1

If the averaging time of the measurement is shorter than one hour, thehourly mean at 10m height may be determined using figure C3.2. Inthis figure the ratio between the hourly mean and shorter averagingperiods of the wind-speed is given for various types of terrain. TableC3.1 gives a quick reference for “Open country” terrain situations.

Fig. C3.1 – Relationship between wind-speedand its return period

Fig. C3.2 – Relationship between windspeedand its averaging time

Table C3.1 – Relationship between commonly quotedwindspeeds at 10m height above grade for

“open ground” situations

Hourly 10-minute 5-second 3-secondmean mean gust gust

Hourly mean 1.0 1.05 1.45 1.5

10-minute mean 0.95 1.0 1.4 1.45

5-second gust 0.7 0.75 1.0 1.05

3-second gust 0.65 0.7 0.95 1.0

Note:- To convert “Fastest mile” windspeed to the above time-averaged windspeeds, use the relationship (velocity� distance /time) to determine the time taken to traverse one mile. This timeshould then be entered in fig. C3.2.

C3.1.2 Wind Maps

When no results of wind-speed measurements are available anindication of the basic wind-speed is given in the figures C3.3, C3.4,C3.5, C3.6, C3.7 and C3.8 for Europe, USA, Asia, Australia, Africaand Brazil.

Some countries have not published wind velocity maps, chosinginstead to specify wind pressure maps or wind velocities at specificlocations. In such cases the customer should specify the windvelocity (vb) to be used in the design. The map showing isopleths forAfrica is unofficial and should be used with caution.

C3.1.3. The influence of the height

The increase of the wind-speed with height is in accordance with thepower law:

Vz � Vb · kp,z0 · (z / 10)�

Vb is the basic windspeed (i.e. measured at 10m above open, levelterrain, without obstructions). The scale factor “kp,z0” and exponent“�” depend on the terrain roughness around the chimney. The valueskp,z0 � 1 and � � 0.14 have been chosen in the Model Code. This isassumed to cover the most common case when the chimney is not inthe centre of cities and not at the sea shore, but somewhere inbetween and clear above the surrounding buildings.

When structures such as buildings are being designed, it is normal toassume different values of � and kp,zo, relevant to the terrainconsidered. This, for instance, would give lower wind velocities intown centres than in open country. When tall structures, such aschimneys, are concerned, however, the wind velocity gradientcontinues to be influenced by the terrain over which it previouslytravelled. In some cases, the previous terrain continues to be ofinfluence after the wind has travelled by as much as 5km overrougher terrain. In addition, the gust factor is a function of theturbulence, so that in town centres, even though the wind velocitymay be less than in open country, the gust factor could beconsiderably higher, partially cancelling out the reduction in dynamicpressure. As a result of these considerations, it was decided to keepthe Model Code simple and use just one terrain category.

30 1

t (secs)

1�

1�


Fig. C3.3 Wind speeds in m/s for Europe (10 min. mean)(note – to convert to Vb – hourly mean, divide by 1.05)

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90(40)

100(40)

110(49)

120(54)

130(58)

140(63)

130(58)

140(63)

150(67)

140(63)150(67)140(63)

130(58)120(54)110(49)

100(45)

90(40)

90(40)

85(38)

90(40)

Fig C3.4 – Wind Speeds in USA

Notes:1. Values are nominal design 3-second gust wind speeds in miles per hour

(m/s) at 33ft (10m) above ground. To derive Vb divide by 1.5.2. Linear interpolation between wind contours is permitted.3. Island and coastal areas outside the last contour shall use the last wind

speed contour of the coastal area.4. Mountainous terrain, gorges, ocean promontories, and special wind

regions shall be examined for unusal wind conditions.

Location V mph (m/s)

Hawaii 105 (47)Puerto Rico 145 (65)Guam 170 (76)Virgin Islands 145 (65)American Samoa 125 (56)

Special Wind Region


Fig C3.5 – Basic windspeeds in m/s for Asia (hourly mean)

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Regions m/s

A 25

B 29

C 34

D 41

Fig 3.6 – Basic Windspeeds for Australia


Fig C3.7 – Basic wind speed Vb in m/s for Africa. Isopleths shown dotted should be used with caution. For final designs localregulations should be used in all cases.


70° 65° 60° 55°50°

45°40°

35°

0°

5°

10°

15°

20°

25°

30°

35°

35

45

40

35

Fig. C3.8 – Windspeeds in m/s for Brazil (3-second gusts)Note – To convert to basic windspeed (hourly mean), divide by 1.5


C3.2 The gust factor

The proposed method for the calculation of the bending moments inthe chimney is based on the gust factor method (see lit. [4])

This conventional approach is:

wg (z)� G · wm (z)

where:

wg (z)� the load at level z

G� the gust factor — a function of wind turbulence and thechimney’s natural frequency, damping and height

wm (z)� the load due to the mean wind velocity

An extension of this method has been proposed by B.J. Vickery (seelit. [5]) to account for the inertial response of a chimney and givemore accurate values of the bending moments at levels above thebase. This method has been adopted in the CICIND Model Code forConcrete Chimneys, Part (a) for the design of concrete shells, wheresteel reinforcement as well as shell thickness, varies often over thechimney height. In the case of steel chimneys however, which arelighter and shorter than concrete chimneys (giving a smaller inertialresponse) and for which there is less scope for changes of thicknesswith height, it was decided to use the simpler conventional method.

C3.3 Vortex shedding

Large vortex-induced vibrations perpendicular to the wind directionmay occur when the vortex shedding frequency coincides with anatural frequency f of the chimney. This occurs at a mean windvelocity “V” equal to the critical wind velocity “Vcr” determined by:

V � Vcr � f ·d / St ... (C3.3.1)

in which d is the predominant chimney diameter over the top thirdand St is Strouhal number.

Vortex-induced vibrations depend strongly on mass and damping ofthe chimney. The risk of large vibrations is judged by the Scrutonnumber Sc defined as:

Sc� ... (C3.3.2)

in which is the structural damping ratio, mo is the effective massper unit height of the chimney as defined in the model code and � isthe density of air.

The risk of large vortex-induced vibrations depends on a combinationof Scruton number and large-scale turbulence intensity of theincoming wind field. High intensity of large-scale turbulence or highScruton numbers reduces the risk of large vortex-induced vibrations.A structure with a given Scruton number may be stable in the kind ofturbulence flow normally encountered but become unstable in rarecases with low turbulence occurring under stable atmosphericstratification.

C3.3.1 Structural Amplitudes

The standard deviation “�y” of the top structural deflection is givenby, see ref. [6]:

� · · ·

... (C3.3.3)

in which Ca, Ka and �L are aerodynamic parameters. Theaerodynamic parameter Ca is found from the generalised vortex-induced wind load on structures without any significant additional

response induced by the chimney’s own motion. The aerodynamicparameters Ka and �L incorporate the effects of the motion-inducedresponse by means of aerodynamic damping:

– The first term {Ka ·� ·d2 / mo} introduces negativeaerodynamic damping

– The second term {1� [�y / (�L ·d)]2} gives the positiveaerodynamic damping — important for large amplitudes andensuring that the response is self-limiting.

For small amplitudes of up to approx. 5% of the diameter, theaerodynamic damping is described sufficiently accurately by the firstterm only.

It can be seen that, when the structural damping is much greaterthan the negative aerodynamic damping, �y is quite small. As the twovalues converge, however, the increase in �y becomes dramatic, untilthe self limiting amplitude is approached and increases becomesmaller (see Fig. C3.9).

The maximum value “y” of the top deflection amplitude is calculatedby multiplying the standard deviation �y with a peak factor kp,i.e. y� kp ·�y. For small amplitudes below approx. 1–2% of thediameter, the peak-factor is approx. 4, corresponding to a stochastictype of vibration. For large amplitudes, the peak-factor is equal toabout 1.5, corresponding to sinusoidal vibrations with constantamplitude. For intermediate amplitudes, the peak-factor increasesgradually with decreasing amplitude. However, for the sake ofsimplicity, the Model Code assumes a sudden change at a value of�y � 4% of diameter.

Fig. C3.9 – Relationship between �y and Structural Damping (�)for given values of Ka, mo and d

Solving equation (C3.3.3) for the standard deviation shows that themaximum value y of the top deflection amplitude (i.e. zero tomaximum) can be expressed by (see Model Code equation 7.9):

y/d � kp · {c1 � (c12 � c2)0.5}0.5 ... (C3.3.4)

where the constants c1 and c2 are equal to:

c1 � 0.5 ·�L2 · {1 � [ · mo / (Ka·� · d2)]} ... (C3.3.5)

or c1 � 0.5 ·�L2 · {1 � [Sc / (4 · · Ka)]}

c2 � ... (C3.3.6)

In smooth flow conditions, �L � approximately 0.4 (see table 1),which gives the following expressions for c1 and c2 (see Model Code,7.2.4.2):

c1 � 0.08 · {1� [� · mo / (Ka ·� · d2)]}

c2 �0.16 ·� ·d3 · Ca

2

Ka · mo · St4 · h

�L2 ·� ·d2 · Ca2 ·d

Ka· mo · St4 · h

�y

dh

�d2

mo

Ca

� {K a·� ·d2 / mo} · {1 � [�y / (�L ·d)]2}1

St2

�y

d

4 · · · mo

� ·d2


For most non-heavily damped chimneys with Scruton numbers lessthan 4··Ka, the influence of the constant c2 is negligible and theamplitude of the structural deflection (0 - max.) can be found from:

y / d� kp · (2 · c1)0.5� 0.4 · kp · {1 � [Sc / (4 · · Ka)]} 0.5 ... (C3.3.7)

In the present simplified and approximate approach, the aerodynamicdamping parameter Ka is estimated for smooth flow cases as afunction of Reynolds number (Re) only. A function of longitudinalturbulence intensity, “I” gives the reduction in turbulent flow, i.e.:

Ka(Re, I) � Ka,max(Re) · K�(I) ... (C3.3.8)

The aerodynamic damping parameter, Ka,max for smooth flow atvarious values of Re is given in Table 1.

The function K� may approximately be determined by:-

K�(I) � 1-31 for 0 I 0.25 and

K�(I) � 0.25 for I � 0.25.

For terrain category 1 (i.e. within 5km of open sea), the minimumturbulence intensity, Imin can be assumed to be 0% for wind velocitiesless than or equal 10m/s and 10% for wind velocities larger than10m/s. For all other terrain categories the minimum turbulenceintensity, Imin can be assumed to be 0% for wind velocities less thanor equal to 7 m/s and 10% for wind velocities larger than 7m/s.Further studies are needed to clanfy the influence of turbulence moreaccurately.

Table C3.2. Aerodynamic parameters in smooth flow. ForReynolds numbers between the limits given, the aerodynamic

parameters are determined by linear interpolation usingln(Re) as argument

Aerodynamic parameter Re < 105 Re = 5 · 105 Re > 106

Ca,max 0.02 interpolation 0.01

Ka,max 1.5 1.0 1.0

�L 0.4 0.4 0.4

Figure C3.10 shows the vortex-induced vibrations as a function ofturbulence intensity for Reynolds numbers equal to 105

and 106, respectively.

C3.3.2 Bending Moments

The bending moments in the chimney can be calculated from theinertial load per unit length (Fw) corresponding to the relevant modeshape (�i), where:

Fw � m · (2ni)2 ·�i · ymax ... (C3.3.9)

where ni � relevant natural frequency

ymax� maximum amplitude at the relevant natural frequency

or from the bending moment due to a force at 1/6 of the chimneyheight from the top, causing the same deflection ymax.

Figure C3.10. Vortex-induced vibrations as function ofturbulence intensity and Reynolds number. It is assumed thatm0 / �d2 � 50 and h/d � 30, which influence the low amplitude

part of the curves shown.

The amplitude should be limited to ensure that stresses are withinpermissible limits, both from the point of view of failure and fatiguelife. In addition, the amplitude should not be large enough to alarmbystanders. This limit is difficult to define in general terms asbystanders’ alarm is subjective, depending upon how often theresponse occurs, its frequency, the visibility of the chimney and thebystanders’ perception of the risk. Definition of the limitingamplitude for this aspect is, therefore left to the owner and thedesigner to agree for each individual case. Some guidance for highlyvisible chimneys with low values of Vcr (� 10m/s within 5km of seaor lake-shore, � 7m/s in inland locations) is given below:

Critical Chimneys – Top double amplitude (peak to peak)should be not more than 10% top diameter

Normal Chimneys – Top double amplitude (peak to peak)should be not more than 25% top diameter

These limits may be increased for less noticeable chimneys and/orthose with higher values of Vcr (i.e. those which rarely see largeamplitude response).

C3.4 Movements in the second mode

Just as in the case of cross-wind response in the fundamental mode,a response to excitation in the second mode, giving a top amplitudeexceeding about 4% of the top diameter, triggers an increasedresponse, initiated by the chimney’s own movement.


In the case of fundamental mode movements, response is onlyimportant to vortices shed over a length near to the chimney top, equalto about 5 top diameters, as demonstrated by Fig. C.3.11.

Vcr � 5.9 m/s, Scr � 4.8, f � 40 hz

Vcr � 4.2 m/s, Scr � 18.7, f � 29.5 hz

Fig. C3.11 – Auto-Spectra of the anemometer signal(velocity signal), measured at Vcr in the wake of the model,

measured over top half

The maximum ampltude in the second mode will occur at the top (seefig. C.3.12). The amplitude reduces to zero over a length of H / 4.This steep reduction means that the length over which vortexshedding is important will be much smaller in the case of secondmode response.

Fig. C3.12 – Mode shapes, first and second mode

In the second mode, the energy due to fluctuating wind pressures willbe applied at the middle part of the chimney. The top amplitude of achimney responding in the second mode will never be as great as thatreached by the same chimney responding in the primary mode. Thisis because much more wind-induced energy would be required in thesecond mode. This is illustrated in Fig. C.3.13, which shows thebending moment causing the same amplitude in the second mode asin the first mode would require about 50 times more energy. On theother hand, the energy required to cause the same base stress in thesecond mode is almost the same as that in the first mode, even thoughtop deflection in 2nd mode is much smaller.

The proposed calculation method is based upon the assumption thatmore or less the same energy is applied in bending, whether thechimney is in the first or the second mode. It therefore follows thatthe top amplitude in the second mode would only be about 1/6 of the

top amplitude in the first mode. The stresses, however, will be aboutthe same in each case.

Fig. C3.13 – Stresses and energy levels in first and second mode

This is partly demonstrated by measured values in a full-scalechimney — see fig. C.3.14. The measured values in this trace are ofstresses at the base and it can be seen that many of the stress cyclesin that part of the response in the second mode are much the same asthose in the first mode. The second mode amplitudes were, however,only about 15% of the first mode amplitudes.

Fig. C3.14

The proposal for determining the top amplitudes in the second mode isgiven in fig. C.3.15. The stresses in both the first and second modesshould be taken into account when dealing with the effects of fatigue.

Fig. C3.15 – Relationship between Scruton Numberand top amplitude

Second Mode

First Mode

Bas

e S

tres

s

Time (secs)

ƒ1 = 0.7 Hz ƒ2 = 2.6 Hz

[M/S r.m.s.]2

Hz

0.0

10.000 100.00HZ

REAL

[M/S r.m.s.]2

Hz

2.0000

0.0

10.000 100.00HZ

REAL


C3.5 Ovalling

The static as well as the dynamically fluctuating pressure causes avarying pressure over the circumference of the chimney. The varyingwind pressure around a circular cylinder causes a “static” ovallingdeformation of the cicle. The dynamics in the wind, including vortexshedding can cause a vibration of the circular shape, the lowest ordermode and most likely to occur being that of ovalling.

C3.5.1 Static ovalling load

The distribution of the wind pressure around the circumference of theshell can be written as:

p� p0 · {�0.823� 0.448cos� � 1.115cos2� � 0.400cos3�

� 0.113cos4� � 0.027cos5�} ... (C.3.5.1)

where: p0 � the wind pressure � 0.5 ·� · v2

� � Angle between wind direction and point oncircumference under consideration

The first term (0.823 · p0) is an overall suction and causes a smalluniform tensile force on vertical cross sections of the shell.

The second term (0.448 · p0 ·cos�) is the pressure in the winddirection (fig. C.3.16) and provides the derivation of the forcecoefficient (shape factor) of 0.7, to give a total load. It causes nodeparture from a circular cross-section.

Fig. C.3.16 – Wind pressure and deflectedshape due to p0cos� term

The third term (1.115 · p0 ·cos� — fig. C.3.17) causes ovalling.

Fig. C.3.17 – Wind pressure and deflectedshape due to p0cos2� term

The remaining terms have little influence.

C3.5.1.1 Unstiffened shells

C3.5.1.1.1 – Effect on vertical moments(stresses on horizontal sections)

An analysis of the deformation and stresses in an unstiffened shell(assuming a rigidly fixed circular base) due to the ovalling load hasbeen given elsewhere in the literature[8]. This considered stresses on

horizontal sections of an unstiffened shell due to the total winddistribution, involving mainly the cos� and cos2� terms (fig. C.3.18)

A major part of the stresses on horizontal sections is due to thetransition from a circular shape at the base to an oval shape.

Fig. C.3.18 – Circumferential wind pressure and deflected shape

Derivation of the increase in tensile stress is fairly straight-forward,as the maximum tensile stresses due to both beam flexure andrestraint of ovalling deformation occur at the base at 180° to the winddirection (i.e. on the up-wind side). Clause 8.2 of the Model Codegives the expression:-

{tensile shell stress � tensile beam sress � (1� {6 / [(l/r) 2 ·(t/r)]}.

Therefore, for t/r� 0.008 and l/r� 50, the increase in tensilestress�30%. This is probably unimportant in the design of chimneyshells, which are usually governed by compressive stresses, but it isimportant in designing the base joint and holding-down bolts. The ModelCode, therefore, calls for shell theory (or the above approximation) to beused for unstiffened chimneys with aspect ratio � 25.

The position regarding compressive stresses is not so simple. Ref. (8)limited itself to consideration of stresses at the base, at 0° to the winddirection. Here, the compression due to beam flexure is reduced oreven reversed by the shell stresses induced locally by restraint ofovalling deformation. However, increases in compressive stress arepossible elsewhere. Increases in compressive stress are due to eitherof two effects:

1) At the base and between values of � about 60° and 120° to thewind, the reduced compression stress due to beam flexure (functionof �) has to be added to the compressive shell stress due to restraintof ovalling (function of 2�) — see fig. C3.19. Significant increasesin total compressive stress only occurr at relatively small values oft/r for l/r ratios less than 30 — see table C3.5.1

2) For relatively thick shells at low l/r ratios, increases ofcompression stress occurr on the down-wind side at 0° to the winddirection, at heights about 6 diameters above the base — see tableC3.5.2. This is due to contraflexure effects, associated withrestraint of ovalling, causing compressive stresses at this height.


Fig. C3.19 – Stresses at chimney base

Therefore, combining both tables it can be seen that consideration ofshell stresses leads to significant increases in compressive stresses,either at the base or at a height about 6 diameters above the base for l/rratios � 30. Guidance regarding these increases is given by fig. C3.20

Fig. C3.20 – Increases in compressive stress over lower 6diameters of an unstiffened chimney, due to shell effects

C.3.5.1.1.2 – Effect on horizontal moments (stresses onvertical sections)

The distribution of ovalling pressure� 1.115 · p0 ·cos 2�

Where p0 is the wind pressure, averaged over 5 seconds.

Away from the ends of a long, unstiffened shell, the consequentbending moment at position � is m0, where:

m0 � · R2 · p0 ·cos 2� ... (C3.5.2)

and m0 (max)� 0.07 · p0 · d2(Nm/m)

1.1154

3.0

2.0

1.0

.004 .006 .008 .01 .011

�CT

�CB

t / R

= 20lR

= 30lR

Flexure

Oval

Circle

Ovalling

Wind

Total Tension

Upwind

NetCompression Max Compression

0° 90° 180°DownWind

t/r l/r beam stress � beam stress shell stress total stress ratiomax. at � at �MPa degrees MPa MPa MPa

0.004 20 2.3 90 0.0 7.3 7.3 3.18

30 6.0 70 2.0 6.0 8.0 1.35

40 11.5 70 4.0 6.0 10.0 0.87

0.005 20 1.9 90 0.0 4.8 4.8 2.63

30 4.8 70 1.7 3.7 5.4 1.13

40 9.2 70 3.2 3.7 6.9 0.75

0.006 20 1.6 90 0.0 3.3 3.3 2.13

30 4.0 70 1.4 2.5 3.9 0.98

t/r l/r max. comp. height (z) beam stress total stress ratioshell stress at z at z

MPa (x dia.) MPa MPa

0.011 20 0.9 6.2 1.3 2.2 1.64

30 0.9 6.2 8.8 9.7 1.11

0.010 20 0.9 6.2 1.5 2.3 1.57

30 0.9 6.2 8.8 9.7 1.10

40 0.9 6.2 23.1 24.0 1.04

0.008 20 0.8 6.2 1.8 2.7 1.43

30 0.8 6.2 11.0 11.8 1.03

0.006 20 0.4 7.4 1.2 1.6 1.32

30 0.4 7.8 11.0 11.4 1.03

Table C3.5.1 – Max. Compression Stresses atBase of Unstiffened Chimney

Table C3.5.2 – Increases in compressive stress at 0o to wind (downwind side),about 6 diameters above base of an unstiffened chimney.


(Note: 0.07 increased to 0.08 in Model Code (equation 7.11), to allowfor effect of initial curvature)

The associated deflection of an unstiffened shell at point � is w0,where:

w0 � ·cos 2� ... (C3.5.2)

and w0 (max)� 0.06 · p0 · d4 / (E · t3)

C3.5.1.2 Stiffened shells

The addition of correctly sized circumferential stiffeners at the top andat the correct spacing will reduce shell stresses due to ovalling tonegligible values. In considering the effect of stiffeners the followingapproach is used:

Based upon the theory of shells[9], the deformation (w) at a distance(height) x from the stiffener is (with a small approximation) given bythe following function:

w � w0 · {1 � e��x/2 ·[cos(��x/2)�sin(��x/2)]} ... (C3.5.4)

where: �� / 2� ·(t/R)0.5

Substituting ��/2 � 1.52 · (t)0.5 / (R)1.5, the deformation of thestiffened shell becomes close to that of an unstiffened shell at adistance 1.58 · R ·(R/t)0.5, or 0.56 · d ·(d/t)0.5 The deformation of theshell above and below the stiffener is shown in fig. C3.21.

Fig. C3.21 – Ovalling deformation of a cylinderwith a stiff ring at x � 0

It can be seen that the ovalling deformations and, therefore stresses,remain low (about 0.03w0) if the distance between stiffeners ofinfinitely high stiffness is smaller than 0.56 · d ·(d/t)0.5.

The maximum bending moment in the stiffener at this spacing isobtained after integration of the shear forces in the shell:-

M � 0.028 · p0 · d3 ·(d/t)0.5 (Nm) ... (C3.5.5)

In order to be effective, the deformation of the stiffener under thismoment must be much smaller than w0 — this requirement beingmore important than its strength.

The deformation of the ring (with spacing� L) is obtained byintegration of the bending moment M. The result is:

When L�1.58 · R ·(R/t)0.5:

w � ·cos 2� ... (C3.5.6)

This must be much less than w0, say 1/5.

Therefore, Ir must be, say, greater than 5 times (0.06 · d1.5 · t2.5). Thiswill ensure ovalling stresses in the shell are reduced to about 20% ofthose in an unstiffened shell.

i.e The spacing (L) of stiffening rings should be 0.56 · d ·(d/t)0.5

and the moment of inertia (Ir) of the stiffening ring (includingparticipating shell (see Model Code Fig. 7.4) should be:

Ir � 0.3 · d1.5· t2.5 when L� 0.56 · d ·(d/t)0.5 ... (C.3.5.7)

Ir � 0.3 · d1.5· t2.5· L / 0.56 · d ·(d/t)0.5 ... (C.3.5.8)when L� 0.56 · d ·(d/t)0.5

C3.5.2 Dynamic component of ovalling

C.3.5.2.1 - Unstiffened shells

The resonance frequency of the fundamental (ovalling) vibrations foran unstiffened cylinder is given by:

f1 � ·�� 0.49 · ·�� ... (C3.5.9)

where E � Young’s modulus of the shell� � Density of the shellA � Cross-section area of shell (� t m2/m)I � Moment of inertia of shell about its vertical axis

(� m4/m)

R, d and t� Radius, diameter and thickness of shell

In the case of steel:

f � 2560 · t / d2 ... (C3.5.11)

The frequency of vortex shedding relevant to ovalling� 2 · St · V / d

Therefore large scale resonant movemements can occur if:

2560 · t/d2 � 2 · St · V / d

For St� 0.2, therefore, Vcr� 6500 · t / d ... (C3.5.12)

To ensure that ovalling vibrations do not occur, it is necessary toincrease the moment of inertia of the shell to give a value of Vcrsufficiently high to avoid a build up of periodic excitation. Assumingthat Vcr� 30 m/s is high enough to achieve this, the required valueof I is then given by:

f � 2 · St · Vcr / d� ��Giving:

I � · ... (C3.5.13)

For Vcr � 30m/s, St� 0.2, � � 7850 kg/m3 andE� 210 · 109 N/m2, thereforeI � 7.4 · 10�6 · A · R2 � 1.8 · 10�6 · d2 · t (m4/m height)

For an unstiffened shell, this means t3 / 12� 1.85 · 10�6 · d2 · t... (C3.5.14)

i.e. t/d must be � 0.004, otherwise stiffening rings will be requiredto avoid the risk of ovalling vibrations.

C.3.5.2.2 – Stiffened shells

Assuming the top of the chimney is stiffened by a ring satisfyingequation (C3.5.8), ovalling vibrations can still occur at lower levelsif the t/d ratio is � 0.004. These vibrations are defined by:

� · A · R4

7.2 · E42 St2 Vcr2

R2

7.2 · E · I� · A · R4

12

t3

12

E�

td2

7.2 · E · I� · A · R4

12

0.19 · p0 · R5.5

E · Ir ·(t)0.5

��1·52x

Rt

R

Deformation with rings

at distances x = 1.32R��Rt

Ring Saffener (Deformation Zero)

wwo

2(3)0.25· R

12 · R4 ·1.115 · p016 · E · t3


w � � 4w � �

� � · t · � 2

w � 0 ... (C3.5.15)

Where w� deformationx � coordinate along the shell (i.e. vertical direction)y � coordinate along the circumferenceT � Time

The solution is approximated by:

w � w0 ·cos�t ·cos (2y/R) · cos ( · x / L) ... (C3.5.16)

Where L � distance between stiffening ringsw0 � deformation of unstiffened shell� � 2 · · ff � frequency

Substituting in equation (C3.5.12) gives:

�2 � · ... (C3.5.17)

An approximation is:

�� (E /�)0.5· {1 / [R � (4 · L2) / (2 ·R)]} ... (C3.5.18)

Therefore L2 � ( /2)2 · {[(R / 2 · f) · (E /�)0.5] � R2}

Assuming that Vcr� 30m/s is high enough to avoid oscillations andf � 0.2·Vcr/R and substituting E� 210·109 N/m2 and � � 7850Kg/m3:

L � 18 · R, or 9 · d ... (C3.5.19)

From equation (C3.5.14), we have seen that the minimum value of Iper unit height to avoid oscillations is:

I � 1.85 · 10�6 · d2 · t (m4/m height)

Assuming the stiffener to provide the equivalent I of a length ofshell� 9 ·d, Ir of stiffener (including participating shell — see ModelCode, Fig. 7.4) ) must be:

Ir > 1.75 · 10�5 · d3 · t ... (C3.5.20)

C3.6 Interference Effects

In considering the effect of aerodynamic interference by an upstreamcylindrical structure on the cross-wind response of a chimney, it isgenerally accepted that the value of lift coefficient increases with thelocalised small-scale turbulence associated with wake buffetting[1].In Reference [1], however, Vickery acknowledges in paragraph 5.2that this does not explain the full increase in cross-wind response. Hestates that: “Across-wind response of the downstream structure isenhanced but the mechanism is not completely clear”. He assumesthat a second contribution comes from reinforcement of themovement by buffeting at a similar frequency to that of vortexshedding by the downwind chimney. Presumably this reinforcementcan be expressed by an increase in negative aerodynamic damping.

Unfortunately little research data is yet available to define the way inwhich the increase in negative aerodynamic damping is affected byspacing, Scruton Number, or large-scale atmospheric turbulence.Therefore, for spacings between chimney and interfering structureless than 10 diameters, the Model Code merely recommends additionof structural damping to increase the chimney’s Scruton Number tomore than 25. At this point it is unlikely that excessive response willbe experienced. When research data is available, more definite designguidance can be given.

{( /L)2 � (2/R)2} 4 � { 4 / (R2 · L4)}{( /L)2 � (2/R)2} 2

E · t2

12 ·�

�2

�y2

�2

�x2

�2

�T2

E · t�4wR2 �x4

�2

�y2

�2

�x2

Et3

12(1��2)

Literature

[1] B.J. Vickery — “Wind loads and Design for Chimneys”—CICIND REPORT, Vol. 14, No. 2, 1998

[2] A.G. Davenport — “Wind structure and wind climate”—Seminar on Safety of Structures, Trondheim, 1977.

[3] P.J. Rijkoort and J. Wieringa — “Extreme wind-speeds bycompound Weibull analysis of exposure-corrected data”.Journal of Wind Engineering, no. 13, 1983.

[4] A.G. Davenport — “Gust loading factors” — Proc. ASCEJournal Struct. Div., Vol. 93, No, ST 5, June, 1967.

[5] B.J. Vickery — “Wind-induced loads on reinforced concretechimneys” — Nat. Seminar on Tall Reinforced ConcreteChimneys, New Delhi, 1985.

[6] S. O. Hansen — “Vortex Induced Vibrations of Line-LikeStructures”— CICIND REPORT, Vol. 15, No. 1, March 1999

[7] Shoei-Sheng Chen — “Flow-induced vibration of circularcylindrical structures”. Hemisphere Publishing Corporation 1987.

[8] H. van Koten — “The Stress Distribution in Chimneys due toWind Pressure”— CICIND REPORT Vol. 11, No. 2, 1995

[9] H.van Koten — “Structural analysis of shells”— TechnicalUniversity of Delft.


COMMENTARY No. 4 – FATIGUE

When we consider the long term history of movement of a chimneysubject to cross-wind movement in response to vortex excitation, wemust take into account the following phenomena:

(1) Movement is subject to a “start-up” and a “wind-down” phase atthe beginning and end of each response excursion (see Fig. C4.1)

(2) The stress at a point on the chimney tends to vary, reducing as thewind direction changes and its speed departs from its criticalvalue, all due to atmospheric turbulence. The degree of reductiondepends upon the level of turbulence.

Fig. C4.1 Typical trace of cross-wind oscillations

Further, in inland locations and at relatively high critical windspeeds,atmospheric turbulence is high enough to ensure that the maximumamplitude rarely occurs. This was demonstrated by a series of longterm measurements (varying between 93 days and 322 days) of theresponse of four steel chimneys in Germany[1] — see fig. C4.2. It canbe seen from these histograms that amplitudes exceeding 90% ofmaximum occurred only rarely, varying from about 10 cycles during93 days at Aachen to about 100 cycles during 264 days at Cologne.

The method in the Model Code takes these facts into account anddevelops a spectrum of response, using the Miner Rule to determinefatigue life. The Miner sum is:

M � (�max/ �wn)k ·(logen)�k� ... (4.1)

Where �max � the maximum stress, per section 7.2.4 of theModel Code

�wn � the stress causing cracks after n cycles(per Wohler curve)

k � 3 in the case of fatigue in steel

� � a function (dependent upon Vcr) defining theshape of the load/cycle collective curve(Fig. C4.3) as follows:-

� � �max· {1 � (log n / logn1)} � ... (4.2)

� � (Vcr / 8)1.2 ... (4.3)

n � Number of load cycles due to cross-wind excitationduring the lifetime T

Fig. C4.2 – Histograms of long term response offour full-scale chimneys

Fig. C4.3 Load/cycle collectives for various values of �

To determine the number of load cycles(n), it is first necessary toknow the number of occasions the wind will blow at its criticalvelocity (Vcr). This is determined from considerations of theprobability of their occurrence — P(Vcr):

P(Vcr)� 2 · · e�(Vcr / V0)2

... (4.4)

Where Vo � wind velocity averaged over one year� approx. Vb(h) / 4

Vb(h) � hourly mean velocity at chimney top, withexceedance probability of once in 50 years.

It is assumed that the chimney responds at wind velocities between1.1Vcr and 0.9Vcr.

VcrV0

2


Also a reduction has to be introduced to account for changes in thewind direction, so that the point of maximum stress is moved awayfrom the point under consideration. The stress at a given point isproportional to cos2� and the total effect is roughly:-

(1 / 2) ·0

�2

cos2� d� � 0.5

As a result,

n� 3.15 · 107 · T · f · 4 · 2 · 0.5 · 0.1 · A · e�A2

n� 1.26 · 107 · T · f · A · e�A2 ... (5) (see Model Code 8.5.2)

Where A� 4 · Vcr /Vb(h)f � Resonance frequency

The load/cycle collective predictions over 20 years, calculated byequations (3) & (5) are shown by the dotted lines in Fig. C4.2.

Because the spectrum was derived from long term measurements onrelatively few chimneys, a modelling safety factor � 1.4 isintroduced in the expression for the Miner Number.

Literature

[1] W. Langer, H. Ruscheweyh & C. Verwiebe — “Untersuchungendes Querschnittverhalten von Original Stahlschornstein” —Forschungsbericht P. 230

[2] H. van Koten — “A Calculation Method for the Cross-WindVibrations of Chimneys” — CICIND REPORT Vol. 14, No. 1,June 1998


COMMENTARY No. 5 – OPENINGSOpenings have to be strengthened to prevent local reduction of:

StrengthResistance against — fatigue

— instability

The strength of the cross-section with openings is the same as thestrength of an undisturbed section if the section modulus is the same.This equality of section moduli is sufficient to fullfill the firstcondition of strength.

The moment of inertia of a circle with an opening subtended by theangle 2� is:

I � d3 � t / 8� { � � � sin�cos� � [(2sin2�) / ( � �)]}

Derivation formulae for cross section properties of chimneys (bothunreinforced and reinforced) and of chimneys with more than oneopening at the same elevation are given in Table C5.1

If � is small then the value of I is close to that of the complete circle(0.125� d3 � t). As � increases, however, the value of I dropsrapidly (see Fig. C.5.1). The same holds for section modulus. Toreplace the lost material, reinforcing stiffeners are welded verticallyto the chimney on each side of the opening. To be effective, the cross-section area (A) of each of the reinforcing stiffeners should be at leastequal to A� 1.25� R� t � (sin�)0.5.

A cross section with an opening is sensitive to the effects of buckling.This is due to the stiffness of the weakened cross-section beingreduced by the possibility of the shell bending in or out at the edgesof the opening. To prevent this the reinforcement stiffeners have to be

placed normal to the shell {see Figures C5.2 & C5.3) andconcentrated along the edge of the opening.

However, sudden ending of of the reinforcement above and below theopening can cause stress concentrations. These can treble stresseslocally and lead to fatigue damage such as local cracks. To avoid this,in the case of openings with width greater than 40% of the chimneydiameter locally, the vertical stiffeners should connect at each endwith a horizontal stiffener extending around the full circumference(see fig. C5.2).

Fig C5.1 – Reduction of inertia at openings

When the width of opening is less than 40% of the chimney’s diameterlocally, it is not necessary to provide a horizontal stiffener extendingaround the full circumference and a more local arrangement may beused (see fig.. C5.3). Vertical reinforcement should be continuedabove and below the opening to a point where the added stress isunimportant. The code deems that continuing the reinforcementbeyond horizontal stiffeners above and below the opening a distanceat least 0.5 times the width of the opening will suffice.

2

1

�°

2

2

1

�°

2

W1

R2t

2

1

�°

2

�°�°

Rt

MW2

�2 =

�1 =W2

R2tI

R3t

MW1

Fig. C5.1 – Derivation formulae for section properties of chimneys with openings (a � reinforcement area)

G1 G1

G

G

�

G

G

G1 G1

a a

aa

G1 G1

G

G

e

2 1G1 G1

G

G

e

2 1a

a

0

0

� � � �

A � 2tr ( � 2�)

IGG � 2tr3 (/2��sin�cos�)ZGG � IGG / rcos�

IG1G1 � 2tr3 (/2��sin�cos�)ZG1G1 � IG1G1/r

IG1G1 � 2tr3 (/2��sin�cos�)� 4ar2sin2�

ZG1G1 � IG1G1/r

IG1G1 � tr3 (��sin�cos�)ZG1G1 � IG1G1/r

IG1G1 � tr3 (��sin�cos�)�2ar2sin2�

ZG1G1 � IG1G1/r

IGG � 2tr3 (/2��sin�cos�)� 4ar2cos2�

ZGG � IGG / rcos�

IGG � tr3 {��sin�cos�

�[2sin2�/(��)]}Z1

GG � IGG / (e�rcos�)Z2

GG � IGG / (r�e)

IGG � I00 � Ae2

Z1GG � IGG / (e�rcos�)

Z2GG � IGG / (r�e)

A � 2tr ( � 2�) � 4a A � 2tr ( � �)e � rsin� / (��)

A � 2tr ( � �) � 2a

e �

I00 � tr3 ( � �� sin� cos�)

� 2ar2cos2�

tr2 sin� � arcos�

tr (��) � a


Fig. C5.2 – Suggested detail of reinforcement forwide openings (> 0.4D)

If the vertical height of the opening is more than twice its horizontalwidth, a stability check is needed. Guidance on such checks is givenin the chapter on bending of plates under lateral loads in “Plates andshells”, by Timoshenko.

When the duty of the chimney requires flue gas inlets whose widthexceeds two-thirds of the structural shell’s diameter, a possiblesolution would be to provide a large number of small circularopenings, giving a total area equivalent to that required.Reinforcement could then be threaded between the small holes andaround the whole group, as required.

Fig. C5.3 – Suggested detail of reinforcement fornarrow openings (< 0.4D)

Even though it is reinforced to ensure the section complies withstrength requirements, the presence of an opening can reduce locallythe stiffness of the chimney and affect its natural frequencies. Thisreduced stiffness should therefore be taken into account whenderiving the chimney’s dynamic response. This is done by takingaccount of the reduced local stiffness at the opening when calculating“x” for each section in equation 7.16 of the Model Code.


COMMENTARY 6 –CHEMICAL EFFECTS AND INTERNALCORROSION

C6.1. Chemical effects

C6.1.1. Attack due to sulphur oxides

The most common form of internal chemical attack is due to acidsformed by the condensation of sulphur oxides in the flue gas. Sulphuris found in all solid and liquid fuels to varying degrees and can alsobe found in gaseous fuels. During the combustion process, nearly allsulphur in the fuel is oxidised to sulphur dioxide (SO2) which can beabsorbed by condensing water vapour to form sulphurous acid.

A small quantity of sulphur dioxide (SO2) is further converted tosulphur trioxide (S03). The quantity depends in a complex mannerupon the sulphur content of the fuel, the amount of excess airavailable during combustion, temperature in the combustion chamberand the presence of catalysts such as iron oxides. This smallconcentration of S03 (usually measured in PPM), gives rise to mostof the acid corrosion problems encountered in chimneys. This isbecause on condensation, the S03 ions combine with water vapour toform sulphuric acid whose concentration can be as high as 85%.

Condensation of these acids takes place when the temperature of theflue gas falls below their respective acid dew point temperatures(ADP), or when the flue gas comes into contact with a surface, at orbelow the relevant acid dew point temperature.

The acid dew point temperature of sulphuric acid depends upon theconcentration of S03 in the flue gas (see Fig C6.1). Provided thetemperature of the flue gas and the surfaces with which it can comeinto contact are maintained 10°C above the acid dew point estirnatedfrom Fig. C6.1, there is no danger of acid corrosion due to this cause.

Alternatively, suitable acid resisting coatings can be applied toprotect the steel. Guidance on suitable coatings and theirperformance is given in “CICIND Manual for Chimney ProtectiveCoatings”.

The acid dew point of sulphurous acid is about 65°C, a little abovethe water dew point. If the fuel is contaminated, other acids, such ashydrochloric and nitric acid can be expected to condense in the sametemperature range. Thus, even if fuel and combustion processes arechosen to minimise production of S03, or if flue gases are scrubbedto remove most of the S03 and SO2, severe corrosion can be expectedif the temperatures of the flue gas or the surfaces with which it cancome into contact fall below 65°C, or the acid dew point temperaturerelevant to the reduced S03 concentration, if this is higher. Again, asafety margin is recommended of 10°C above the acid dew pointtemperature estimated from figure C6.1.

C.6.1.2 Effects of Flue Gas Desulphurisation (FGD)

Despite the removal of most of the sulphur oxides during FGD, asevere corrosion risk remains. This is because, downstream of ascrubber, the flue gas is usually very wet and its temperature is oftenvery low — low enough to be below the (low) value of acid dewpoint temperature (ADP) associated with the reduced sulphur oxidecontent. Fig. C6.2 shows the relationship between temperature andacid concentration to be expected and demonstrates that flue gascondensing at temperatures as low as 80°C can end up as quiteconcentrated acid. Also the flue gas often contains chlorides, carriedover from the scrubbing materials.

All steels except the very expensive high nickel alloys and titaniumwould deteriorate very quickly in this environment. To minimise theexpense, methods have been developed to apply very thin sheets ofalloy or titanium to the inner face of carbon steel or other vulnerableliners. Some organic coating materials have also been developed forthis duty.

C6.1.3. Attack due to chlorine, chlorides and fluorides

Chlorides are found in most solid fuels, including refuse and in manyliquid fuels. It is also sometimes found as a pollutant in some FGDprocesses. Upon combustion chlorides are transformed into freechloride ions which, on contact with water vapour are transformedinto hydrochloric acid. The highest condensation temperature atwhich hydrochloric acid has been found is 60°C. Thus, when any fluesurface falls below this acid dew point, very serious corrosion willoccur. This dew point is close to the water and sulphurous acid dewpoint. Even very small amounts of chlorides in combination withother condensed acids can cause serious corrosion problems.

Hydrogen chloride, hydrogen fluoride and free chlorine in flue gasesalso become corrosive in their vapour stage. Stainless steels areattacked at temperatures above 320°C. Fluoride vapours arecorrosive to stainless steels at temperature above 250°C.

C6.2. Internal Corrosion

The internal corrosion allowances in table 8.2 of the Model Code arebased upon limited exposure to condensing sulphuric acid per FigC6.1. They are derived from the relationship between “Peakcorrosion rate” and “S03 concentration” shown in figure C6.3. This,in turn, was derived from the upper bound of a family of curveswhich show the same relationship observed in practical situations.See lit. [2] and [3]. A safety factor of 4 has been used in arriving atthe corrosion allowances.

Fig. C.6.2 – Phase diagram: sulphuric acid – water vapour

C6.3 Guideline to choice of liner metallic materials

Guidelines on the suitability of various metals and alloys for therange of chemical risks to be found in chimneys will be given inCICIND’s “Metallic Materials Manual” (to be published in 2001).

Literature

[1] “Desulphurisation Systems and their Effect on OperationalConditions in Chimneys”, Henseler, F., CICIND REPORT,Vol. 3, No. 2, 1987

[2] “Influence of fuel oil characteristics and combustionconditions on the gas properties in water tube boilers”BunzG., Diepenberg H, and Rundle A. — Jnl of the Institute ofFuel Sept 1967

[3] “Prevention of cold end corrosion in industrial boilers”. Lechand Landowski — “Corrision” — March 1979


Fig. C6.1 – Relationship between ADP and SO3 concentration

peak corrosionrates(micron/1000 hours)

*) ppm = part per million (10–6)

SO1 concentration (ppm by vol) *)

Fig. C6-3 – Relationship between peak internal corrosion rates and SO3 concentration

CICIND Model Code – Commentaries and Appendices Amendment A – March 2002

APPENDIX 1 –DESIGN OF CHIMNEY BASE PLATESThis appendix is intended to give guidance on rationalising baseplatedetails. In the following calculations, base plate bearing stress (�*c)and maximum bolt tension (Pb*) are calculated for factored load andoverturning moment. In the case of bases with a compression ringand/or gussets the values of �*c and Pb* are calculated using elasticanalysis as a reinforced concrete ring assuming the modular ratio of12 [1],[2]. The area of steel bolts is taken as the thread root crosssection area of the bolts. In chimneys requiring an increase in designtensile stress at the base on account of clause 8.2 of the Model Code,the value of Pb* should be factored accordingly.

A.1.1 Simple baseplates, with no gussets or compressionrings (Fig. A.1.1)

Fig. A1.1 – Simple Baseplate

On the compression side, the vertical shell force is distributed over astrip of width (2.l3 � ts), where l3 is chosen to limit the pressure onthe grout (�*c) to no greater than fkg / 1.5.

The maximum baseplate stress (�*)is then given by:

�* � 3 .�*c . (l3 / tb)2 fk / 1.1 ... (A1.1)

where fk � characteristic strength of the bottom plate steel

�* c � pressure on the grout

ts � thickness of shell

fkg � characteristic compressive strength of the grout

On the tension side, the values of l1 and l4 should be adjusted to givevertical and rotational equilibrium. The active circumferential lengthof the baseplate may be taken as 3 ·l2 or the bolt spacing, whicheveris the lesser.

The bolt tension (Pb*) then � p* · (l1 � l2) / l1 ... (A1.2)

Where p* is the vertical tensile force in the shell per bolt.Assuming a distribution of baseplate stress over a length of 3 ·l2:

�* � 2 · p* / tb2 � fk / 1.1 ... (A1.3)

Both equations A1.1 and A1.3 have to be satisfied.

A1.2 Baseplates with Gussets (Fig. A1.2)

Fig. A1.2 – Baseplate with gussets

The maximum baseplate stress (�*) is given by the followingexpression:

�* � �1 . �* c . l2 / tb2 � fk / 1.1 ... (A1.4)

where �1 is given by:

l / b �10 3.000.2 2.680.3 2.300.4 1.850.6 1.250.8 0.831.0 0.511.25 0.301.5 0.22

and l � the outstand of the basplate from the chimney shellb � distance between gussets

The baseplate stresses (�*) on the tension side may be calculated usingthe method described in lit. [1]. For the particular case of l � 4 · D:

�* � �2 · Pb* / t b2 fk / 1.1 ... (A1.5)

Where �2 is obtained as follows:

l / b �20.2 2.380.3 2.280.4 2.070.5 1.870.6 1.650.8 1.331.0 1.061.25 0.811.5 0.62

Both equations A1.4 and A1.5 must be satisfied.

The height of the gussets (h) should be sufficient to maintainacceptable shell stresses. The stress in the shell (�*s) is given by thefollowing expression:

�* s� w* . [( � / ts) � (�3 . Rs/ ts2)] fk / 1.1 ... (A1.6)

Where: � and �3 are given by:

No. of gussets 3(equally spaced)

6 1.00 0.5312 1.93 0.2616 2.50 0.2020 3.20 0.1624 3.83 0.1328 4.47 0.1132 5.10 0.09840 6.37 0.07960 9.55 0.05280 12.74 0.039

100 15.92 0.031

and Rs� shell radius

w* � the radial force on the shell per unit height of gussetat the top of the gussets, given by the followingexpression:

w* � 3 . M* / h2

Where M* is the bending moment at the base of each gussetplate due to out of balance forces under the baseplate.

M* � P* . 2D per gusset on the tension side

� �*c . 6 . D2 .b per gusset on the compression side

CICIND Model Code – Commentaries and Appendices Amendment A – March 2002 page 29

Allowance should be made for stress concentrations that may occurat the top of the gussets.

A1.3 Baseplate with gussets and compression ring(See Fig. A1.3)

Fig. A1.3 – Baseplate with gussets & compression ring

The baseplate stresses are calculated in the same way as in sectionA1.2 above using equation A1.4.

The compression ring bending stresses (�*) are calculated in thesame way as in section A1.2 above, using equation A1.5, substitutingtc (thickness of compression ring) for tb (baseplate thickness). Addedto this is a direct circumferential stress arising from the out of balancemoment caused by the eccentricity of the bolts, giving a total stress:

�* � �2 · Pb* / t c2 � Pb* · N / (30 · · D · tc) � fK / 1.1

where N� number of bolts

A gusset plate thickness of 0.25D will suffice if it is of a steel whoseyield strength at least equals that of the bolts.

Notes regarding the derivation of 1 and 2

Stress coefficients �1 and �2 were obtained as follows:

�1 is the coefficient applicable to the compression side and is derivedfrom Timoshenko’s work on a rectangular plate fixed on three sidesand free on the fourth. This is a reasonable assumption becausepressure under the base inside the shell will produce fixity. At thegussets there is fixity by virtue of the continuity of the basplate.

�2 is the coefficient applicable to the tension side. In the literature [1]this is taken from a model comprising a rectangular plate simplysupported on all sides, with a patch load at the centre representing thebearing of the nut. This is not a true reflection of the boundaryconditions which are more truly fixed on two opposite sides (at thegusstes), one side being pinned (at the shell) and the fourth side free.Neither is the effect of the holding down bolt hole considered. In thisAppendix, therefore, the values of �2 have been derived from plateelement FE analysis, using the more realistic above boundaryconditions and allowing for the bolt hole in the plate.

A1.4 Grouting

Note – If the chimney is intially levelled using a nut placed on theholding down bolt under the baseplate, this nut should be loosenedafter packers are introduced.

Fig. A1.4 provides guidance on the grouting procedure to beused under chimney baseplates.

References:

[1] Brownell & Young — “Process Equipment Design”,Chapter 10

[2] Pinfold, G.M. — “Reinforced Concrete Chimneys and Towers”


APPENDIX 2 – INSULATION AND PROTECTIVELININGS AND COATINGS

A.2.1 Insulation

A2.1.1 General

In order to minimise loss of heat from a chimney and to maintain thetemperature of the shell or liner(s) above flue gas acid dewpointlevel, insulation may be fitted. But it should be appreciated that,however effective the insulation, acid will condense if the flue gastemperature entering the chimney is at or below its acid dewpointtemperature.

Even if metal in contact with flue gas is generally at temperaturesabove its acid dewpoint, rapid local corrosion can occur at cold spots.In order to eliminate cold spots careful attention should be given tothe following details:

– Potential air leaks should be eliminated by properly sealingflanged joints, inspection/cleaning doors, expansion joints andinstrumentation apertures. The long-term effectiveness of sealingmaterials at the relevant service temperatures should bedemonstrated.

– Direct metal/metal contact between steel liners and the structuralshell should be avoided. Liner support should incorporate athermal isolation device.

– Attachments such as guy ropes, aerodynamic stabilizers, ladders,platforms and pipes can act as cooling fins. Their attachment tometal in contact with flue gas should incorporate a thermalisolation device.

A2.1.2. Insulation design

Insulation should be designed to maintain the surface in contact withthe flue gas above acid dew point temperature everywhere, when theflue gas is at normal operating condition and at abnorrnal conditionsif they can last for more than 25 hours per year (see table 7.1 of theModel Code). For design purposes, the following parameters shouldbe used:

– Theoretical acid dewpoint, calculated taking account of sulphurcontent and excess combustion air should be increased by asafety margin of 10°C. If data is not available to permitcalculation of the flue gas acid dew point temperature, thefollowing values should be used for minimum metal temperaturein contact with flue gas:

• When fuel is oil and/or gas, containing more than 0.5% byweight of sulphur, 175°C

• When fuel is coal containing more than 0.5% by weight ofsulphur, 135°C

• When fuel contains less than 0.5% by weight of sulphur, 100°C

– Ambient air temperature should be the minimum winter airtemperature at the chimney location, obtained by averaging themean temperature each night over a period of one month.

– Wind velocity should be assumed to be 5m/s.

The temperature of the metal in contact with flue gas should bechecked for the condition of highest anticipated flue gas temperature.For this check the following design parameters should be assumed:

– Ambient air temperature should be maximum anticipated airtemperature at the chimney location.

– Zero wind velocity.

The design of insulation thickness to satisfy the requirements of thisclause should be based upon the conductivity value of the insulationmaterial, provided by the insulation manufacturer. If such data is notavailable, typical values listed in table A3.1 may be used.

overall average Utype of insulation thickness values W / (m2 K)

aluminium 6mm air gap 4.5

aluminium 18mm air gap 4.0

mineral wool 25mm 2.3*




expanded mineral 50mm 1.15*




* These values apply for a mean insulation temperature of 40°C. They should beincreased by 5% for each 50°C increase in mean insulation temperature.

Table A2.1 Typical insulation conductivities

Mineral wool or foam insulation exposed to weather should beprotected by weather proofed cladding. Design of this cladding andits fixings should ensure its integrity under the action of wind at avelocity of 1.5� basic wind-speed at the relevant height (perparagraph 7.2.2.of the Model Code). The design should take accountof the variation of wind pressure around the surface of the chimneyat a given elevation.

A2.1.3 Aluminium cladding

Aluminium cladding enclosing a narrow airspace is an effective formof insulation, due to its high thermal reflectivity. (Note — Sheet steelor other forms of cladding may be suitable in certain cases.)

The exterior of the steel shell beneath the cladding should be coatedwith heat resisting paint.

The cladding should consist of aluminium sheet not less than 1.0mmthick with symmetrical flange covers made in halves frorn aluminiumsheet which also shall not be less than 1.0mm thick.

The cladding should be made in strakes, using a number of equalplates per strake. All seams should be connected by aluminium alloyrivets at not more than 100mrn centres. Vertical seams of each strakeshould be set at the midpoint of the strake beneath.

The cladding should be fitted with its internal face the required distanceaway from the external face of the chimney shell, this distance beingmaintained by continuous circumferential spacers of the requiredthickness low conductivity tape coincident with the horizontal joints ofthe aluminium. The tape should be cemented into position by means ofsodium silicate or other suitable adhesive. The ends of the horizontalrivets in the aluminium sheets serve to retain the tape in position aftererection. The circumferential spacers divide the airspace between thesteel shell and the aluminium cladding into sections not more than1.5m high, thus reducing convection heat losses.

When the length of the prefabricated sections of shell betweenflanges is not a whole multiple of the strake width, only one make-upstrake per section of chimney should be used.

All projections should be clad. Cleaning doors and other pointswhere access is required should be “boxed in” with removablealuminium panels.

The airspace at the top of the chimney should be completely sealed toprevent ingress of moisture between the steel shell and the cladding.

Each upper strake of aluminium should lap over the lower strake bya minimum of 25mm, The vertical seams similarly should have aminimum lap of 25mm.

To permit examination of the steel shell of the chimney withoutremoving the cladding, 150 mm square openings, located at carefullyseiected points and covered by removable panels approximately230mm square, may be provided. Suitable positions are:


– diametrically opposite any inlet

– approximately 1,25m from the top of the chimney

Great care should be taken to ensure that dissimilar metals do notcome into contact with each other. If it is essential in the design thattwo dissimilar metals have to be connected, a suitable non-conductiveand impervious film or agent should be placed between them.

A2.1.4. Mineral wool or foam Insulation

Wrapping the steel shell with a suitable grade insulation material ofsufficient thickness provides more effective insulation thanaluminium cladding with the usual 6mm air gap.

Thicknesses of over 50mm are applied in two separate layers, theouter layer being fitted so that the vertical and horizontal joints arestaggered from the joints of the inner layer. If a stiffener or flange ofthe chimney section projects past the outer face of the insulation, itshould be wrapped with an additional layer of the same thickness forat least 75mm on each side of the flange or stiffener. Insulation has tobe protected from the weather, a convenient way of doing this is tocover it with metal cladding, designed as descibed above.

The insulation should be fixed to the steel shell by wrapping it aroundso that the ends butt. It can be secured in place by steel strapping. Atleast two bands of strapping should be used for each strake ofinsulation. Insulation tends to compact and slip down the surface ofthe steel during transportation and erection thus leaving bare patchesof steel which are potential “cold spots”. The slipping of theinsulation may be prevented by welding steel pins to the shell. Onlow chemical load chimneys the pins can project through theinsulation and have spring retaining washers fitted.

On medium chemical load chimneys it is advisable to use short pinswhich only project half the thickness of the insulation so as to prevent“cold spots” forming.

Usually an interval of 600mm is used between the pins.

A2.1.5. Lined and multiflue chimneys

The sapce between the outer shell and the liner of a double skinchimney can be filled with mineral wool, expanded mineral, or othersuitable insulator.

When expanded mineral is used as insulation, the design andfabrication of the chimney must ensure that there are no voids oropenings out of which the expanded mineral can leak. A suitabledrain off position must be provided at the lowest point of theexpanded mineral area to ensure that the expanded mineral can bedrawn off if access to the interior of the chimney shell is required.

Notices should be fitted to the exterior of the chimney warning thatthe chimney has been filled with expanded mineral.

After 6 to 12 months, expanded mineral insulation compacts by about10% thus leaving areas of the liner exposed. It is essential that thisvoid is “topped up” with more expanded mineral and that adequateprovision is left in the cap plate for topping up to take place.Sometimes a second “topping-up” is necessary after a further 12month period.

A2.2 Protective linings

A2.2.1 General

Linings may be required in steel chimneys for one or more of thefollowing purposes:

– To maximise the strength of the structural shell by keeping it cool

– As fire protection

– To protect an externally insulated structural shell fromexcessively hot flue gases. These could be generated by anoperational upset or occur when an energy conservation systemis by-passed.

– Corrosion protection

– To act as insulation to maintain the flue gas temperature above itsacid dew point.

– Reduce potential for aerodynamic instability.

Chimney linings may be:

a) Separate liners, with a space between the liners and the outerstructural shell. More than one liner may be accomodated withinthe structural shell, to form a “multi-flue” chimney.

b) Attached continuously to the inner face of the structural shell.Such linings may be either cast against the structural shell, or beapplied by spray, trowel or brush. Such linings may be:

– castable refractory

– solid grade diatomaceous concrete

– chemical resistant coatings

– fibreglass reinforced plastic (FRP)

A2.2.2 Design of separate liners

A2.2.2.1. General considerations

For information on the design of separate liners see the “CICINDModel Code for Concrete Chimneys, Part C - Steel Liners”.

Lateral support should be provided between the liner and thestructural shell as near as possible to the top of the chimney.

Additional lateral supports may be required at intermediate elecationsbetween the top of the liner and its base, depending upon considerationsof stability and dynamic response, but their number should beminimised as far as possible. The lateral restraints should be designedto permit the linings to expand freely both vertically and radially.

A gap between the liner and its lateral restraint(s) of between 3mmand 6mm (the larger gap being appropriate for larger diameter liners)will ensure that impact damping enhances the structural dampingsufficiently to avoid problems of cross-wind oscillation in most cases.

The liner should be designed to resist stresses due to loads imposedby the lateral restraints, as the structural shell moves under the effectof wind or earthquake.

The presence of horizontal restraints between the liner and structuralshell may prevent the liner from adopting a distorted shape in responseto differential expansion. As a result, bending stresses may beintroduced in both the liner and the structural shell, These stresses canbe very high when a single liner carries flue gases from two or moresources with different temperatures. In addition, the resultingdifferential liner temperature will introduce secondary thermal stresses.

A cover should be provided at the top of the structural shell to giveweather protection to the airspace between liner and shell. The designof this cover should permit free expansion of the liner. Sufficientradial clearance should be incorporated to permit any relativemovement, between liner and shell, that may be allowed by thelateral restraint system. In the design of this cover, special attentionshould be paid to the integrity of its fastenings, bearing in mind therisk of acid corrosion, stress corrosion and fatigue cracking whichmay be caused by aerodynamic “flutter”.

A2.2.2.2 Steel liners

Unprotected steel liners should not be used in conditions of highchemical load (see table 7.1 of Model Code). In conditions of low ormedium chemical load, internal corrosion allowances listed in table 8.2of the model code may be used. In conditions of high chemical load(such as downstream of FGD), unprotected steel can be replaced by (orprotected by “Wallpapered” coatings of) high nickel alloys, titanium orother metals. Guidance on choice of these materials is contained inCICIND’s “Metallic Materials Manual”, to be published in 2001.

Liner supports and lateral restraints should incorporate thermalinsulation so as to avoid formation of localised cold spots on thelining surfaces due to conduction of heat to the structural shell.


Consideration should be given to the risk of fire and/or hightemperature excursions described in paragraphs 7.6.1 and 7.6.2 of themodel code. If the risk is significant, consideration should be givento the provision of fire protection.

A2.2.2.3. Plastic liners

Plastic and FRP liners are suitable for conditions of “high chemicalload” (see table 7.1 of the Model Code), combined with lowtemperatures. In order to prevent material degrading, the temperatureof these linings should not be allowed to exceed 100°C. Short termexcursions to 150°C can be tolerated if the right type of plastic ischosen, but the life is reduced.

In order to ensure liner temperature is maintained below 100°C, anautomatically controlled quenching system may be installedupstream of the chimney, which is activated when the flue gastemperature exceeds 100°C.

A2.2.3 Design of linings attached continuously to the shell

A2.2.3.1 General

Lining or coating selection criteria and quality standards to be usedduring surface preparation and lining installation are detailed in theCICIND “Chimney Protective Coatings Manual”.

A2.2.3.2 Castable refractory linings (includingdiatomaceous concrete linings)

Castable refractory should be insulating type with a minimum bulkdensity, after drying, of 1000kg/m3. Diatomaceous concrete shouldbe of the “solid” grade. They should be single layer construction,installed without vapour stops. They may be cast against the innerface of the steel shell or they may be applied by a gunning process.Mixing procedures and water quantities shall follow themanufacturers’ recommendations.

The minimum thickness of lining shall be 50mm. Linings 50mm to65mm thick shall be reinforced by electric welded wire mesh. Themesh should be 50� 50mm with wire of minimum diameter 2mm, orit may be 100� 100mm with minimum wire diameter 3mm.

The mesh should be positioned 20mm from the surface of the steel shelland should be anchored to it by steel studs, welded at 450mm spacing.

Linings thicker than 65mm shall be reinforced by arc welded “V”studs, randomly orientated and at a minimum spacing of 16 persquare metre.

A corrosion resistant metal cap should be provided at the top of therefractory to protect its horizontal surface from the weather.

Providing its surface in contact with flue gas is above acid dew point,this type of lining provides corrosion protection to the steel chimneyor liner to which it is applied. Application of such a lining wouldconvert a steel chimney, classed as being under “High chemical load”when unprotected, to a “Low chemical load” classification.

A2.2.3.3 Fibreglass reinforced plastic (FRP) linings

The use of plastic and FRP for linings applied to steel chimneys isseverly restricted by their tendency to separate from the steel, due todifferential expansion. To minimize this problem, lining temperaturesshould not exceed the following values:

– epoxy resins, 80°C – polyesters, 60°C

It is essential that the FRP linings adhere firmly to the inside face ofthe chimney shell so that the surface does not crack or spall. If theacid flue gas penetrates the FRP it will attack the steel shell.

A2.2.3.4 Chemical resistant coatings

Guidance on the selecion and application of chemical reisistantcoatings is given in the CICIND Chimney Protective Coatings Manual.

In the selection of a coating for internal use, consideration should begiven to the maximum temperature to which it will be subjected, both

when wet and when dry. Only coatings should be used that have beenproved capable of retaining their protective properties in theseconditions throughout the life of the chimney. Also, the chosencoating material should have expansion characteristics compatiblewith those of the shell throughout the relevant temperature range.

A2.3 Recommended start-up procedures for newcastable refractory in steel chimneys or liners.

The start-up procedures should follow the refractory manufacturer’sinstructions. If none are available, the following procedures may beused:

– Hold gas temperature in the range of 70°C– 90°C for at least3 hours.

– Control subsequent increases in temperature and gas flow so thatno part of the liner is exposed to a gas temperature increaseexceeding 50°C/hr. All parts of the lining should be exposed to gastemperature at least 75% of design temperature for at least 6 hours.

These requirements also apply to old refractory linings which havebeen left exposed to weather and have become soaked with water.

A2.4 Protective and decorative treatments

Treatment selection criteria and quality standards to be used duringsurface preparation and coating application are detailed in theCICIND “Chimney Protective Coatings Manual”.

Stainless steel is normally supplied in its mill finish condition, which isa matt, light grey. Polishing to achieve a shiny finish involves extra cost.

Weathering steel, unless grit blasted, may not oxidise evenly.


APPENDIX No. 3 – GUYED CHIMNEYS

A3.1. Thermal expansion effects

Steel chimneys are subject to thermal expansion when the shell isheated by the flue gases and, to a small extent by strong sunlight andby large variations in ambient temperature. The vertical expansioncan be considerable on tall chimneys with reasonably high flue gastemperatures, especially if they are externally insulated.

For example, the vertical expansion of a steel chimney with a guyband 80m above ground level and with a shell temperature of 250°C,would be 280mm.

This vertical expension expansion can greatly affect the tension in theguy wires and the consequent compressive load on the chimney shell.

The stresses in guy ropes and shell should be checked under both“hot” and “cold” conditions. For instance, if the guy wires arecorrectly tensioned when the chimney is “cold”, the verticalexpansion when the chimney goes on load will increase the tensionin the guy ropes, it will also increase the vertical component in theshell plate, when it could in extreme cases produce buckling.However, if the guy wires are tensioned when the chimney is “hot”,when it goes off load the chimney will reduce in height and the guywires will lose part of their tension. This could cause more movementunder wind load than is desirable. In order to avoid these problems,a compromise initial guy rope tension under cold conditions may benecessary i.e. a tension that allows some lateral deflection of thechimney under design wind and “cold” conditions, while increasingthe vertical load in the chimney by a significant but safe margin under“hot” conditions.

Alternatively, if a chimney is used on a constant load 24 hours a dayfor long periods and maintenance resources permit, the guys caninitially be correctly tensioned when the chimney is cold. When thechimney starts up and is heated to its operating temperature, the guyscan be readjusted to the correct tension after the chimney hasexpanded. As soon as the heat load is reduced and the chimneyresumes its “cold” height, however, the guys must be retensioned.

A3.2. Calculations

A3.2.1 Normal conditions

The guyed chimneys shall be calculated taking into the considerationsecond order effects. The decisive winddirections which should betaken into account are given in figure A3.1

Fig. A3.1 – Wind directions for guyed chimneys

The stability of the structure and foundation as a whole or any part ofit should be investigated.

Weight of anchorage should be provided such that:

M � 1.4 Mw� 1.35 Mm� 0.9 Me� 0.9 Ma

in which:

M � combined moment

Mw � overturning moment produced by the design wind andimposed loads

Mm � overturning moment produced by dead-weight or otherpermanent loads which may act to increase combinedmoment

Me � overturning moment produced by permanent loads whichact at all times to reduce combined moment

Ma � restoring moment produced by the foundation (includingguy rope anchorages) without exceeding allowable materialstresses or the foundation allowable bearing pressure.

In determining the support provided by the windward guy ropes, therelative stiffnesses of the chimney (acting as a cantilever) and the guyropes, including their non-linear behaviour, should be taken intoaccount. Many modern structural computer programs have routinesfor analysing guyed structures, which do this automatically. Ifcalculations are made by hand, however, guy rope tensions shouldfirst be calculated, assuming the chimney is pinned at its base.Horizontal deflections at the rope attachment points should then bedetermined. The stack shell should then be analysed as a cantilever,propped by springs at the rope attachment points. The stiffness ofthese springs is determined by the deflections and horizontalcomponents of tension in the ropes, previously calculated. Secondorder effects should be considered.

A3.2.2 Abnormal conditions

The stability of the chimney should be checked at 0.1 � DesignWindspeed, assuming one of the guy ropes to be broken.

A3.3 Guy ropes

Guy ropes should be provided in at least 3 vertical planes. T he anglebetween any two planes should not exceed 130°. Guy ropes shouldnot slope more than 60° to the horizontal.

Guy ropes shall be of galvanized steel wire, with steel cores,complying with ISO/R346. The wires should have a minimum tensilestrength of 1450 N/mm2, A completed rope should be evenly laid andfree from loose wires, disturbed strands or other irregularities andshould remain in this condition when properly unwound from the reelor coil. Fittings should be of galvanized steel. Prior to erection,completed guy ropes should be greased and subjected to a tensileforce amounting to 20% of their minimum breaking load for a periodof 30 minutes.

Guy ropes and fittings should be designed so that their minimumbreaking strength exceeds 3 � maximum calcuiated load, due to thesum of pretension, design wind and chimney expansion.

After erection and while the chimney is cold, the guy ropes should bepretensioned so as to minimise top deflection of the chimney. Thepretension may be measured by the use of a suitable instrument andshould be not less than 15% nor more than 30% of the calculatedmaximum tension due to design wind under the hot condition.

Attachments of the guy ropes should be positioned sufficiently farbelow the chimney top to avoid corrosive effects of the flue gases.A minimum distance of 3m is recommended.


APPENDIX No. 4 – ACCESS LADDERS

A4.1. General

This section specifies the requirements for steel ladders, permanentlyfixed to steel chimneys, to provide means of access. They are to befixed to the chimney in a continuous vertical length interspersed withlandings and/or rest platforms as required.

There may be relevant local requirements or standards which aremore stringent than those detailed below and, in these cases, theymust be followed.

An alternative to the caged ladder system is an open ladder with aproprietary safety system, either running beside the ladder orcentrally between the stringers.

Rest platforms as described in A5.8 should still be incorporated at therelevant levels.

A4.2. Definitions

For the purpose of this appendix the following definitions shall apply:

1) Stringers. The side members of the ladder to which the rungsare fitted.

2) Safety hoop. A bar fixed to the stringers to enclose the path ofpersons climbing the ladder, to prevent them falling outwards.

3) Rest platform. A platform provided to enable the personclimbing the ladder to rest.

4) Landing. A platform provided to enable access to part of or thewhole of the circumference of the chimney.

A4.3. Materials

The materials used for the construction of ladders, hoops, platformsand rest platforms shall be of carbon steel and conform to Euronorm28–32, except those components within 3 diameters of the chimneytop which, in the case of chimneys carrying flue gas with high SO2/SO3content, should be of high molybdenum stainless steel (ASTM 316L orsimilar) or should be protected by an acid-resistant coating.

A4.4. Finish

All burrs, weld-flash, sharp edges and other imperfections likely tocause injury to the hands of a person using the ladder, shall beremoved and made smooth before the finishing treatment.

Depending on the situation and atmospheric conditions in which theladders are to be used, they shall be given a suitable protective finish.

Hot dip galvanizing is not recommended for ladder components orconnections manufactured by a cold forming process. Galvanizingmay only take place after drilling, bending, sawing, etc.

A4.5. Stringers

Stringers shall be of flat bar of minimum dimensions 65� 10mm.The stringers shall be parallel and straight throughout the rungportion and the distance between the stringers measured from theinside faces shall not be less than 300mm and not more than 450mm.

The stringers shall extend upwards, to a height of not less than1075mm above the upper platform and shall be securely fastened attheir extremities. Such extension of the stringers shall not encroachon the clear width of the platform passageway.

Where, in order to step from the ladder into a landing platform, it isnecessary to pass between the extended portion of the stringers, theseshall be opened out from platform level to provide a clear width of600–675mm between them at handrail level.

Where access to an upper platform is from the side or front of aladder, the ladder itself shail be extended above the platform level fora distance of not less than 1075mm or equivalent handholds shallbe provided.

Stringers should, if possible, be in a continuous length, but where theyare in more than one length they shall be joined by fishplates on theinsides of the stringers, either welded or bolted. If bolts are used theyshall be countersunk on the stringer and not less than 12mm in diameter.There shall be not less than two bolts on each side of the joint.

A4.6. Rungs

Rungs shall be of round bar not less than 20mm diameter. If the baris reduced in diameter at the ends for welding, the reduced diametershall be 6 mm less than the diameter of the bar and there shall be a1.5mm radius at the root of the shoulder.

The rungs in a ladder or flight of ladders, shall be uniformly spacedthroughout at centres of 225mm minimum to 300mm maximum. Thetop rung shall be on the same level as the platform which shall beextended, if necessary, to limit, to not more than 75mm, the gapbetween the rung and platform. Alternatively the platform may beextended to replace the top rung.

Rungs shall be fitted into holes drilled in the stringers and secured bywelding. Rungs shall be welded to the stringers with or withoutshouldering. Holes in the stringers shall be drilled to give a 1mmclearance and where shouldered rungs are used, holes shall becountersunk 1.5mm to clear the root radius (see figure A4.1).

Fig. A4.1 – Attachment of ladder rungs to stringers

A4.7. Safety hoops

If safety hoops are fitted to the ladder, the following provisionsshall apply.

All ladders rising 2300mm or more from a lower platform or groundlevel to the top rung shall be fitted with safety hoops, the spacing ofwhich shall be uniform and at intervals not exceeding 1000mmmeasured along the stringer. The lowermost hoop shall be fitted to thestringers at a height of 2300� 0� 75mm from a lower platform orground in order to give sufficient overhead clearance when getting onto the ladder. The uppermost hoop shall be fixed in line with anyguard rail to the upper platform but in any case shall be at a height ofnot less than 1075mm above the level of this platform.

A4.7.1. Size of hoops

Circular pattern. The width across the hoop shall be 690 to 760mm.The distance from the centre line of stringers to the inside of the backof the hoop, measured at right angles to the stringers, shall be 760 to850mm (see figure A4.2).

Rectangular pattern. The width across the hoop shall be 690 to760mm. The distance from the centre line of stringers to the inside ofthe back of the hoop, measured at right angles to the stringers, shallbe 690 to 760mm. The radius of the corners shall be not less than150mm (see figure A4.2).

The minimum dimensions of the hoop and strap material shall be50� 8mm. At least three vertical straps shall be fitted internally tobrace the hoops; one of these straps shall be at the centre back of thehoop, and the others spaced evenly between the centre back of thehoop and the ladder stringers.


Fig. A4.2 – Ladder hoops

Hoops and straps shall be fixed by bolting or welding. If bolts areused they shall be countersunk, inserted from the inside of the strapor hoop and shall he not less than 12mm diameter. The assembly ofhoops and straps shall be suitably braced unless secured to thestringers by double bolting, or welding.

A4.8. Rest platforms and landings

When required, rest platforms shall be provided at intervals of notgreater than 20m. Landing places, other than working platforms,which are provided specifically at rest platforms shall be at least825mm square and shall have a guardrail at a height of 1075mmabove the platform level with an intermediate rail and toeboards.

When required, landings shall be provided at suitable levels to provideaccess to sampling points etc. These landings are to be adequatelysupported from the chimney shell and shall have a minimum width of825mm. They are to be fitted with a guardrail at 1075mm above theplatform level, with an intermediate rail and toeboards.

A4.9. Attachment to chimney

The ladder shall be vertical except where it follows the slope of acone section.

Stringers shall be attached to the chimney by suitable connectionswhich shall be firmly attached to the stringers and the chimney andbe sufficiently close together to make the ladder rigid throughout itslength. The connections shall be of sufficient length to give aclearance of not less than 200mm behind the rungs. Suitableprovision shall be made at fixing points for any differential expansion(except at platforms and landings) .

A4.10. Access hooks

A4.10.1. General

This section specifies requirements for hooks which are intended toprovide means of access for inspection and maintenance only bysteeplejacks and members of similar trades who normally fit theirown ladders.

The hooks may be of two types:

a) Those welded permanently to the steel shell

b) Those which are screwed into sockets welded to the. shell of thesteel chimney

A4.10.2. Use of access hooks

The hooks shall be in a vertical line on the exterior of the structure.The use of access hooks inside chimneys exposed to corrosive gasesis not recommended. The first hook should be 1.2m� 50� 0mmabove access level.

The hooks should be spaced at multiples of 1.5m vertical centres witha local tolerance of �50mm which will accommodate the majority ofthe various lengths of ladders used by steeplejacks.

The hooks are to be used for the temporary attachment of laddersonly except as noted below.

A pulley is sometimes rigged from the top of a steeplejacks’ ladderfor the purpose of lifting small loads for maintenance of the chimney.It is important that such loads shall be kept as light as possible and inno circumstance should any single load exceed 50kg. If a hook isused directly for lifting purposes, the weights of the lifting devicesuspended from it and of the load to be lifted should together notexceed 50kg.

A4.10.3. Materials

Hooks shall be made from steel complying with the requirements ofEuronorm 25–72. In a normalised condition the steel shall have aminimum tensile strength of 430N/mm2 and a maximum tensilestrength of 500N/mm2. The sockets shall be made from round steelbar complying with the requirements of Euronorm 25–72.

A4.10.4. Design

The design shall be as shown in figure A4.3 for the welded hooks.

The design shall be as shown in figure A4.4 for the screwed hooksand sockets.

It is recommended that the screwed type of hook be used on insulatedchimneys i.e., those with mineral wool or aluminium cladding as thehook does not project through the insulation. This projection couldcause “cold spots” on the chimney shell.

An insulating spacer should be attached to the face of the socket tominimise heat conduction between the face of the socket and thesurface of the aluminium cladding.

Fig. A4.3 – Welded ladder hooks

Fig. A4.4 – Screwed ladder hooks and bosses


A4.10.5. Construction

The hooks shall be hot forged by hand out of solid bar. The hooks shallpass visual examination to ensure freedom from surface defects andshall be cleanly forged in such a manner that the microscopic flowlines follow the body outline of the hook. The whole of the shank shallbe forged in one piece, integral with the hook. The hooks shall benormalised after the completion of all forging operations by heatingthem uniformly in a furnace until the whole of the metal has attaineda temperature between 880°C and 910°C and then cooled in still air.

A4.10.6. Method of fixing

The welded type hook shall be fixed to the chimney by means of afillet weld of 6mm leg size on each side of the shank and returnedacross the top and bottom. After welding to the structure, a test shall

be carried out by suspending from the hook a mass of 200kg when nofracture, crack or visible deformation shall occur. The socket of thescrewed-type hook shall be fixed to the chimney by means of a filletweld of 6mm leg size for the whole of the perephery of the socket.

For new chimneys the welding should be carried out in thefabrication shop.

It is normal practice for the steeplejack firm to supply the screwedtype hooks for their own use when they ladder the chimney.

design of steel free stack

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