cicind part 1

CICINDModel Code forSteel Chimneys

(Revision 1 - December 1999)

Amendment A - March 2002

Commentaries and Appendices

(December 2000)

Copyright CICIND 2000, 2002ISBN 1-902998-17-0

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

.~_":.'"

,(

CICINDModel Code for Steel Chimneys

REVISION 1 - DECEMBER 1999

COMMENTARIES ANO APPENDICES

TABLE OF CONTENTS

Foreword , 3

Commentary l -Glossary of commonly used words 3

Commentary 2 - Safety 6

Commentary 3 - Wind Load 8

C3.l. Willd Speed 8

C3.1.I. BasicWind Speed 8

C3.1.2. Wind Maps 8

C3.l.3. The Influence of Heighl 8

C3.2 The GUSl Faetor 15

C3.3 Vortex Shedding 15

C3A Movements in tbe 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 EtTects and InternaI Corrosion . .26

C6.1. Chemical Effects 26

C6.l. 1. Attack Due to SlIlphur Oxides 26

C6.l.2 Effects or Plue Gas DeslIphurisalion 26

C6.1.3. Anack Due to Chlorìne, Chlorides

and Pluorides 26

C6.2. Intemal Corrosion 26

C6.3 Selection of materials 26

Appendix 1- Base PIate Design 28

A 1.1 Simple base plates 28

A1.2 Base plates wìtb gussets 28

A 1.3 Baseplates with gussets and compression rings 28

AIA Groutìng 29

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

AZ.l. Insulation

A2.l.! GeneraI .30

A2.1.2. InslilatìonDesign .. , , .. '" 30

A2.I.3. AluminÌum Cladding 30

A2.l.4. MineraI Wool or Foam Inslilatìon .31

A2.l.5. Lined and Multiflue Chìmneys " .31

A2.2. Linings 31

A2.2.l. GeneraI .31

A2.2.2. Design of Separate Liners 3 I

A2.2.3. Design of Linings Attached

Continuously to tbe Shell " 32

AZ.3. Recommended Start-up Procedures 32

A2A. Protective and Decorative Treatments .32

Appendix 3 - Guyed Chimneys 33

A3.l. Guyed Chimney expansion 33

A3.2. Guyed Chimney calculations 33

A3.3 Guy Ropes , .33

Appendix 4 - Access Ladders 34

A4.1. GeneraI .34

M.2. Defì.nitions .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 PIatforms and Landings .35

A4.9. Attachment to Chimney 35

A4.1 O. Access Hooks 35

DISCLAIMER

This CICIND document is presemed to tbe best of the knowledge of its members as a guide only. CIClND is not, oor are any of its

members, to be held responsible for any failure aIIeged or proved to be due to adherence to recommendations or acceptance of informatioo

published by tbe associatiou in a Model Code or in any otber way.

CICIND, Talacker 50, CH-800l, Zurich, Switzerland

Copyright by CICIND, Zurich

CICINO Model Code - Commentaries and Appendices

FOREWORDIn December 1999 the Second Edition of the Model Code for Steel

Chimneys was published. This is now expanded by the pubIication 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 and

the principles used in the Model Code. The Appendices relate tomore practicalconsiderations.

COMMENTARY No.1

GLOSSARY OF COMMONLY USED TERMS

The numbers in brackets are given in figures C.1.l 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 51001 (2.07) - A construction comprising two vertieal 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 afalse bottom.

Boiler mounted cbirnney - A chimney supported by a boiler and itsfoundation.

Bracket (2.10) - A construction providing resistance to lateraldisplacement of the chimney andlor supporting part or alI of theweight of the chimney.

Bracketed chimney (2.11) - A chimney in which not alI extemalapplied loads (e.g. wind) are carried exclusively by the structuralsheIl 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 shelI, covering the area between it and the liners andincorporating cravats through which the liners protrude.

Cleaning door (2.13) - A door, normaIly at the base of the chimney,to permit the remova! of fiue dust.

Compression ring (2.14) - A steel plate welded to the sheIl 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 externaIly 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 strategie points where maximurncorrosion 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.

page3

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 structuraldamping.

Donbling 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 fIuegases. Also known as a dual wall chìmney.

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.

FIue - 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 ali extemaIlyapplied loads (e.g. wind) are carried excJusively 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 previde 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 fIue gases.

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

Jointing f1ange (2.30) - A steel section fitted to the end of a chimneysection to enable sections to be connected together.

Ladder boss - A boss 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 Iiners, allowing independentexpansion of the shell.

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

Liners (2.32) - FIue 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) -Amaterial applied to the internaiface of the chimney to prevent the fIue gases contacting the innersurface of the steel shell.

Multiflue cbirnney (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 sbell withoutcorrosion allowance.

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

Roofplate (2.36) - A plate whieh follows the contour of the roofround the chimney where it passes through the roof of a building.Also known as jlashing.

Rungs - Horizontal bars in ladders.

page4 CICINO Model Code -Commentaries and Appendices

Safety system - Proprietary fali arrest system fixed to ladder rungsor beside the ladder to gìve a safe fixing for attachment of operatives'safety harnesses.

Self supporting chimney (2.37) - A chimney in which externallyapplied Ioads (e g. wind) are carried exclusively by the structuralsheli and which, together with the foundation, will remain stableunder alI design conditions without additional supporto

Splitter plate (2.38) - A vertical plate welded to the interior of theshell between two horizonta!Iy opposed inlets to divert the fiow oftheftue gases into a vertical direction and to inhibit the passage of ftuegases from one inletinto the other.

Stay (2.39) - A rigid member providing both tensile and compressiveresistance to the lateral displacernent of the chimney. ALso known asa Lateral brace.

Stayed chimney (2.40) - A chimney in which not alI externallyapplied loads (e.g. wind) are carried excIusively by the structuralshell and far 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.

Typic:al gérteralarrangement of three types of self supporting steelchimney.The numbers are related to the texi

IIIIIiII

I i 1--STRUCTlIRAL{.",_~.:."_."""..l SHELl 24 f

l.INEIl 232

•••.•...•..OAHPINGOEVICE 220

IIII

I IL,,,,"~••,,,,,,••~

TOP CONE 2.42

--HELICALAEROOYNAMIC

SI ABlllZERS

2.03

---INTERMEDIA TECONE 219

JOINTINGFl ANGE 2.3D

L1NERS 2.32

MINER A L WOOl

INSULA 110N

--STRunURAL

SHELL 241

1. .~..; .•.~.

c OPE f1QOO 2 16

INI.ETS 2.28

JOINTING'FLANGE Z3D

ACCESS

HOOKS 202 ---....

--STRUCTURAl

SHrtl141

'BASEPLA TElQS

HOlOING OOWNBOllS 2.n

SPLITTER

PlA TE 2.38

INl ns 228

/ACCESS

. __ OOOR 201

lINER

BASE 2.33

BASE CONE 204

A(CESS

. ........-OOOR 2.01

: IT.....[/COHPRESSION RING 2.14__ BASE STOOl 2.01

'-...HOlOING DDWN

BOl.TS z.n

/'TUNOISH 143

BASE OR Bf ARINGf'lAH zos

DRAIN

PIPE 2 23 -.--.

BASESTOOL 207.

;;ELF SlJPPORTING

I1ULl.!..FLUjJ;HIMNE Y

235

SELF SUPPORTING

CHIMNEY

237

DOUBLE SKIN

CHIMNEY

Z2Z

Figure C1.1

CICINO Model Code - Commentaries and Appendices page5

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

Top cone (2.42) - A truncated cane 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 far drainage. Also known as afa/se bottom.

Thned mass damper - A forro 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 stabiIizers

Venturi. - See Top cone

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

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

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

'INLET226

2.11

JOINfiNGFLANGES

2.30

SUPPORfiNGFRAME

BRACKE1S -~2.10

CLEANINGDOORS213

/COPE6AND .2.15 (OPE SAND

, 215CORROSIONTEST P1EC[--I-O211

= -=-

-•••••

••••

•••••

-•.•.

-

••••

i='"

'-'O

O

O

v

(OPE BANO215

NLE!228

DOUBLINGPLATES221

~A(CESSDOOR2.01

STAY

.......-- 239

~JOINTINGFl.ANGES230

ELEVA TiON'

BASEPLA TE

205 ~'D

GUY \oIIRE

2.2ì.

-lOP SECflON

INlET

228

(oPE 6AND215 '

/COWL 216

iI

.J

I,!;y,-II

'I--INTERMEDIA TEI S[(fI0N

I

~' JOINTING

I fLANGC230

: /ClEANING, /OOOR213

/BLANKINGOH PLATE208

ROOFPLA TE

236 "

(RAVAT2.19

ACCESS

DOOR

201 ----

--BASESEOION

/BASEPLATE/ 205

o&f-CHIMNEYFLUE

::.---STAY 2.39

_5UPPORTINGCOLUHN

m=------SUPPORfiNG

FRAME

STArs239

BRACKET 210

PLAN PLAN PLM!

GUYED CHIMNEY

226

STAYEO CH/MNEY

WITH THREE FLUES

(CENTRAL COLUMN)

240

STAYEO CH/MNEY

WITH FOUR FLUES

(EXTERNAL FRAME)

240

[2RACKETED SINGLE

FLUE CHIMNEY

211

Figure C1.2

page 6

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 Iisted 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 partialload factor for wind load in the wind direction is derivedas folIows by considering the soci al and economie consequences offailure or damage requiring the chimney's repair or replacement. Thisinvolves deriving the acceptable probability of failure (P) duriI1Jgthechimney's lifetime, using the following expression given in CIRIA(U.K.) Report No. 63, entitled "Rationalisation of Safety andServiceability Factors in Structural Codes"[l]:

.h (C2.l)

Where

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

Dd = design life of structure (assumed to be 20 years for asteel chimney)

Ks = a sodal criterion factor, given in table C2.!

Table C2.1 - Social Criterion Factor

Nature 01 structure Ks

Placesof publicassembly,Dams 0.005

Domestic,OfficeorTradeand Industry 0.05

Bridges 0.5

Towers,Masts,OffshoreStructures 5

In order to use equation C2.! it is necessary to estimate the value ofnr. It is suggested [l, 2] that allowance be made for the number ofpeople Iikely to be dose to the structure at the time that maximumloading can be expected. Since maximum loading is most Iikely 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 far criticaI chimneys, acceptable probabilities can beestimated as summarised in table C2.2:

Table C2.2 Typical failure probabilities forenvironmental economie risk

Environrnent nrKsP

Chimneyindustriaiarea ("normal"chimney)

0.1.0510-3

Chimneyin urbanarea or hospital("Criticaichimney")

1.0510-4

Chimneyservingcriticaiplant("Criticalchimney")

0.10.00510-4

It follows that safety factors should be chosen to give probabilities offailure of 10-3 for a "Normal" chimney and 10-4 far a "CriticaI"chimney.

Thc 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%.

CICIND Model Code - Commentaries and Appendices

The principalload is due to wind. The moment is proportional to thewind pressure, the extreme values of which foIlow a Fisher- TippettType I (Fr!) distribution as described in reference 3.

This distribution has a Cumulative Distribution Function (CDF)given by P(q) = exp( -exp( -a.(q - u»)

in which the constants are the mode u and the dispersion 1Ia.. Intemperate climates the product U.a. = 5; other values may obtaineIsewhere (see ref.Z)

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

. . ( In(50) )It foIlows that the charactenstic pressure qk = q 1 + ~

This is converted to standard measure by substituting q = x. u

then Ps1(x) = exp( -exp( -au(x-l»)

The probabiIity distribution function (pdi)

d

= dx PBl(x)=auexp(-au(x-l)Ps1(x)

Th 50 . d . l In(50)e -year Wlll pressure IS Xs50 = +-au

The resistance is assumed norma1ly distributed with mean Xr andstandard deviation <Tr

The characteristic value is Xr5% = Xr - 1.645<Tr

xr5% xr-1.645<Tr

The load factor F = xs50 = l In(50)+-au

( In(50) ):. xr = F 1+ -- + 1.64S<Tr. au

( O)?)

l 1 x-xr-

the pdf of the resistance is Pr(x) = <Tr~ exp ( -"2 -;;:;-

The CDF far the wind pressure in period T years is PsT(q) = (P'I(q»T

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

. 1 l In(T)'th l' th h fshltt the mode from to + -- Wl out a tenng e s ape oa.u

the distribution.

Hence the CDF is PsT(x) = exp( -exp( -au(x- I) +In (T)))

The probability of failure is given by PFT = I(1-PsT(x»)'Pr(x)dxo

Now the factor F = 'Yw' 'Ym where 'Yw is the wind load factor and 'Ymthe material factor.

Assuming 'Ym= 1.1, thenif 'Yw= lA PF2o= 8.10-4if 'Yw= 1.5 PF20 = 3.10-4

When faiIure is ductile, additional safety against collapse is derivedfrom the chimney's residual strength, atter mobilisation of itsallowable (yield) strength at one point of its periphery (Le.. at theultimate Iimit state).

When failure is by buckling, additionaI safety is implicit in thereiationship used between the allowable (critical buckling) strengthand the yield strength of the material. This relationship inc1udes anadditional partial safety faetor to ensure that the criticaI bueklingstress 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 diameterl thickness fatio.

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 "CriticaI" chimneys, respectively.


(' 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,CICINn REPORT VoI.5, No.1, 1989

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

(5) 'European Recommendations for steel construction' European Convention for Construction Steelwork <ECCS), 1978.

page7

page 8 CICINO Model Code - Commentaries and Appendices

COMMENTARY 3 2.11

built up terroin

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

~

././r-

./ y---or.-./-..-r

./'".-...- L--~--- - -

WINDLOAD

At tbe time of publication of tbe revised CICIND Model Code farSteel Chimneys (1999), tbe wind load mode1 currently used in ENV1991-2-4 (eventually intended to forro tbe basis of Eurocode 1, Part2-4: Actions OD StructureS - Wìnd Actions) has been ShOWDbycalibration studies by CICIND and otbers to be unacceptable. In viewof tbe time expected to elapse before an acceptable modeI forEurocode 1 is agreed by all parties, CICIND have decided for tbe timebeing to retain tbe wind Ioad modeI described in tbe 1988 version ofthis Model Code. A recent paper[l] has shown tbat this model givessafe and reasonably accurate estimates of tbe wind load on chimneys.

V·lVb

l.S1.~1.3

1.2

1.11.8

3800 600 300 60 30

--+- t (secs)

?pen counlry

sea cOil$l

P(V) = exp {-exp [ -:-Q(V - u)))

C3.1 Wind-speed

As tbe basis for tbe wind-Ioad, tbe hourly mean windspeed has beenretained. Tbe wind-Ioad is caJculated after estimating a turbulenceintensity, by a "gust factor" metbod[2J.

C3.1.1. Basic wind-speed

Tbe basic wind-speed used in deriving wind-Ioads is tbe wind-speedaveraged aver one hour and measured at 10m above open ground atthe chimney Iocation, which has a probability of exceedence of oncein 50 years.

Tbe 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, tbe value of tbe basic windspeed must be extrapolated usingtbe Fisher-Tippett Type l expression for tbe statistical distribution ofextreme values, as follows:

Where:

P(V)

u

probabiIity of excedence of velocity V duringthe relevant period

sIope of curve in Fig. C3.1

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

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

"open ground" situations

Hourly1O-minute5-second3-secondmean

meangustgust

Hourly mean

1.01.051.451.5

1O-minute mean

0.951.01.41.45

5-second gust

0.70.751.01.05

3-second gust

0.650.70.951.0

Note:- To convert "Fastest mile" windspeed to the above timeaveraged windspeeds, use tbe relationship (velocity = distance /time) to deterrnine 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 tbe 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 BraziI.

Some countries have not published wind velocity maps, chosinginstead to specify wind pressure maps or wind ve10cities at specificlocations. In such cases the customer should specify tbe windvelocity (Vb) to be used in the designo The map showing isopleths forAfrica is unofficial and should be used with cantiano

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

1In some cases lower values for u and - are found (see lit. [3] ).

Q

C3.1.3. The influence of the height

Tbe increase of tbe wind-speed with height is in accordance with tbepowerlaw:

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

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

If the averaging time of tbe measurement is shorter than one hour, thehourly mean at 10m height may be determined using figure C3.2. Inthis figure tbe ratio between tbe hour1y mean and shorter averagingperiods of the wind-speedis given for various types of terrain. TableC3.l gives a quick reference for "Open country" terrai n situations.

Vb is tbe basic windspeed (Le. measured at 10m above open, levelterrain, without obstructions). The scale factor "kp,zo" and exponent"Q" depend on tbe terrain roughness around tbe chimney. Tbe values

kp,zO= 1 and Cl = 0.14 have been chosen in tbe Model Code. This isassumed to cover the most common case when the chimney is not inthe centre of cities and not at tbe sea share, but somewhere inbetween and c1ear above tbe surrounding buildings.

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

hurrJc.ne lOnas

nOrmal wind condition(no hurrieaneJ

SOD lODOSO 100

_iinveartlO

-,/./

l

~......"..-:: f-'"-

;

•

--

0.1

I..

CICIND Model Code - Commentaries and Appendices page9

6ì

o 0-500 ••••M.S.L

O SOOm-1500m·H.Sl..

~ ma",· JlXlOm. M.S.L

JISW ••3DlXl",. H.S.l.

Mean ~ 5P"d io> mlS cl DmQbcp". open t.nan.R.tun p..-cxI 50~.

10 min. mean

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

\\.J

...l,

!Ilo'6c::

8-c.«"Oc:l'Il

!Ilor:l'Il'EQ)

EEo

ClIQ)

"8GG5"Oo::::!:

OzC3

C3

~

90(40)

100(45)110(49)

t:.Jw-~.

90(40) ~

LocationVmph(mls)

100(45) 130(58)

Hawail105(47)

110{49} 120{54}

Puerto Rico145(65)Guam

170(76)Virgin Islands

145(65)American Samoa

125(56)

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

(mls) 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.

90{4O)

100(40)

Special Wind Region

o..-CI>

Cll'IlC. Fig C3.4 - Wind Speeds in USA

i:ìj

CIC/ND ModelCode- CommentariesandAppendices page11

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

REGION C

Tropical Cyclones

Insels trom f 50 kmsmoothed coastline t 100 km

~'6cQ)a.~ucca

III

.~cQ)

EEoOI

Q)UoOQiuo~ClZ(3(3

REGION D

Severe Tropical

Cyclones

'2.00

Regions

A

B

C

D

Basic windspeed Vb(hourly mean)

mls

25

29

34

41

REGION ANormal

0.

'\)

Maryoorough250

300

C\I,-Gl

g>a. Fig 3.6 - Basic Windspeeds for Australia

QICIND Model Code - Commentaries and Appendices page 13

- ---- -

\'®

Tropicalstorm zone

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

page 14 crCIND Model Code - Commentaries and Appendices

70°

____--l

----

50"

45'

I

40'

35"

IO·

5°

10'

15"

20°

25"

Fig. C3.S - Windspeeds in mls far Brazil (3-second gusts)Note - To convert to basic windspeed (hourly mean), divide by 1.5

CICINO Model Code - Commentaries and Appendices page 15

where:

Wm(Z) = the load due to the mean wind velocity

Wg(Z) = G o wrn(z)

(J'y

response induced by the chimney's own motion. The aerodynarnicparameters K" and aL incorporate the effects of the motion-inducedresponse by means of aerodynamic damping:

- The first term {Ka op' tfl/1llo} introduces negativeaerodynamic damping

- The second term {l - [(Jy/(aL .d)J2} 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 accurateIy by the firstterm onlyo

It can be seen that, when the structural damping t is much greaterthan the negative aerodynarnic damping, (Jy is quite small. As the two

vaIues converge, however, the increase in (Jy becomes dramatic, untilthe self limiting amplitude is approached and increases becomesmaller (see Fig. C309).

The maximum value "y" of the top deftection amplitude is calculatedby multiplying the standard deviation (Jy with a peak factor kp'Leo y = kp olTyoFor small amplitudes below approxo 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 amplitudeo However, for the sake ofsimplicity, the Model Code assumes a sudden change at a value of

lTy= 4% of diametero

..o(C303ol)V = Vcr = f· d / St

in which d is the predominant chimney diameter over the top thirdand St is StrouhaI numbero

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

G = the gust factor - a function of wind turbulence and thechimney's natural frequency, damping and height

Wg (z) = the load at level Z

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 chimneyo This occurs at a mean windvelocity "V" equal to the criticaI wind velocity "V cr" determined by:

An extension of this method has been proposed by Eol Vickery (see]it. [5]) to account for the inertiaI response of a chimney and givemore accurate values of the bending moments at levels above thebaseo 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 heighto In the case of steel chimneys however, which arelighter and shorter than concrete chimneys (giving a smaller inerti alresponse) and for which there is less scope for changes of thicknesswith height, it was decided to use the simpler conventional methodo

,P=- C3.2 The gust factor(

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

This conventional approach is:

4°1To{01lloSc = jJpoa-

..o(C3.302) Fig. C3.9 - Relationship between O'y and Structural Damping (9for given values of Ka, mo and d

where the constants Cl and Cz are equal to:

Solving equation (C3.3.3) for the standard deviation shows that themaximum value y of the top deflection amplitude (Leo zero tomaximum) can be expressed by (see Model Code equation 709):

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

The risk of large vortex-induced vibrations depends on a combinationof Scruton number and large-scale turbulence intensity of theincorning 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 f10w normally encountered but become unstable in rarecases with low turbulence occurring under stable atmosphericstratificationo

"o (C303.4)

..o(C303.5)

In smooth f10w conditions, aL = approximate!y 0.4 (see table l),which gives the foIlowing expressions far Cl and C2 (see Mode! Code,7.2.402):

C3.3.1 Structural Amplitudes

The standard deviation "(Jy" of the top structural deftection is givenby, see ref. [6]:

~=_l_. Ca .ptfl.!i

d StZ {-{Kaopotfl/mo}o{l-[lTy/(aL'd)]2} mo h

cz =QLZ o p o tflo CaZ o d

K. o mo . St4 . h"o (C3.3.6)

... (C3.3.3)

in which Ca, Ka and aL are aerodynamic parameterso Theaerodynamic parameter Ca is found from the generaIised vortexinduced wind load on structures without any significant additional

Cl = O.OS·{l- [~·mo/(Ka· p'd2)]}

0.16· p.tf3, C.2C -

2 - Ka 'm,. ·8(4. h

page 16 CICIND Model Code - Commentaries and Appendices

For most non-heavily damped chimneys with Scruton numbers lessthan 4·'lT·K", the influence of the constant ~ ìs negligìble and theamplitude of the structural deflection (O - max.) can be found from:

y/d = kp.(2'C1P5 = OA·kp· {I- [Sc/(4·'lT·K,,)]}O.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 longìtudinalturbulence intensity, "f' gives the reduction in turbulent flow, Le.:

... (C3.3.8)

0.30

0.15

0.00

sRe = lO

The aerodynarnic damping parameter, Ka,max for smoothflow atvarious values of Re is given in Table 1.

The function Kv may approximately be determined by:-

KJl) = ]-3] for O,,;;],,;;0.25 and

KJl) = 0.25 for l> 0.25.

For terrain category 1 (Le. within 5km of open sea), the minimumturbulence intensity, lmin can be assumed to be 0% for wind velocitiesless than or equa! lOm/s and 10% for wind velocities larger than10m/s. Far alI 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.

0.30

0.15

0.00

O

o

5 lO 15 20Scruton number, Se.

6Re,;= IO

5 IO 15 20Scruton number. Se.

25

25

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

parameters are determined by Iinear interpolation usingIn(Re) as argument

Aerodynamic parameter Re < 1()5Re = 5 .105Re> 1lJ6

Ca,max

0.02interpolation0.01

Ka,max

1.51.01.0

aL

0.40.40.4

Figure C3.10 shows the vortex-induced vibrations as a function ofturbulence intensity for Reynolds numbers equal to 105and 106, respectively.

C3.3.2 Bending Moments

The bending moments in the chimney can be calculated from theinertialload per unit length (Fw) corresponding to the relevant modeshape (~i)'where:

Figure C3.10. Vortex-induced vibrations as function ofturbulence intensity and Reynolds number. It is assumed thatmol pd2 = 50 and h/d = 30, which influence thelow amplitude

part of the curves shown.

The amplitude should be limited to ensure that stresses are withinpermissible lirnits, 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 generai terms asbystanders' a!arm 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 individuai case. Some guidance for highlyvisible chimneys with low values of Vcr « 1Om/s within 5km of seaor lalee-shore, < 7m/s in inland locations) is given below:

Criticai Chimneys - Top double amplitude (peale to peale)should be not more than 10% top diameter

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

... (C3.3.9)These limits may be increased for less noticeable chimneys andJorthose with higher values of Vcr (Le. those which rarely see largeamplitude response).

where ni = relevant natura! 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 deftection Ymax'

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.

CICINO Model Code - Commentaries and Appendices page 17

In the case of fundarnental mode movements, response is onlyimportant to vortices shed over a length near to the chirnney top, equaIto abour 5 top diameters, as demonstrated by Fig. C.3.11.

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

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

measured over top half

stress 2" u stress l"

FirsIMo(J. /"

4'---llme (secs)~1, = 0.7 Hz 12 = 2.6 Hz

----

l' mode= 2' mode

B.1SE:+aa 1.61E'+61 z."6E:+atJ 3.~at+011 1.1et+6I'

J7top deflection 21t=O.15xl'"

\Venerqy 1",. enerqy 2-

top-deflection 2~""1·

-. H

-1.0

-.6

-. <

t .•I .•~! ~'f00di -B.D~CQ -.2

La

.H

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

This is partly demonstrated by measured values in a fulI-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 cyc1esin 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.

)1IO

r

100.00° top

o top100.00

HZ

HZ

// 1/

IO..87t5•2r ~iam~tlrr

Vcr = 5;9 m/s, Scr = 4.8, f = 40 hz

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

10.000

10.000

0.0

0.0

[MIS r.m.s.p REALH<

2.0000

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 iinportant will be much smalIer in the case of secondmode response.

Fig. C3.14

The proPOSaIfor 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 deaIing with the effects of fatigue.

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

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

In the second mode, the energy due to f1uctuating 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 iIlustrated 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 smaIler.

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

, 0'1.-- ....---- ·-""~---""---··-l

Re < 3.105

IO 15Sc- 2.0

paga 18

C3.5 Ovalling

The static as well as the dynamicaIly fiuctuating pressure causes avarying pressure over the circumference of the chimney. The varyingwind pressure around a circular cylinder causes a "static" ovallingdefonnation of the cic1e. The dynarnics 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.


horizontal seetions of an unstiffened shell due to the total wind

distribution, involving mainly the cos<\>and cos2<\>terms (fig. C.3.18)

A major part of the stresses on horizontal seetions is due to thetransition from a cìrcular shape at the base to an ovai shape.

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

C3.5.1 Static ovalling load

The distribution ofthe wind pressure around the circurnference oftheshell can be written as:

p = Po' {-0.823 + 0.448cos</J + 1.l15cos2</J +OAOOcos3</J

- 0.113cos4</J - 0.027cos5</J} ... (C.3.5.1)

where: Po = the wind pressure = 0.5 . p . v2

</J = Ang1e between wind direction and point oncìrcumference under consideration

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

The seeond term (0.448· Po . eos</J) is the pressure in the winddirection (fig. C.3.16) and provides the derivation of the foreecoefficìent (shape factor) of 0.7, to give a total load. It causes nodeparture from a eireular cross-section.

Wmd- Direction

Derivation of the increase in tensile stress is fairly straight-forward,as the maximum tensile stresses due to both beam flexure and

restraint of ovalling deformation oeeur at the base at 180° to the winddireetion (Le. on the up-wind side). Clause 8.2 of the Model Codegives the expression:-

{tensile sheli stress = tensile beam sress x (1 + {6 / [(l/r)2. (t/r)]).

Fig. C.3.16 - Wind pressure and deflectedshape due to Pocosepterm

The third terrn (1.115 . Po . cos</J- fig. C.3 .17) causes ovallìng.

Fig. C.3.17 - Wind pressure and deflectedshape due to Pocos2epterm

The remaining terrns have little infiuence.

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 sheli(assurning a rigidly fixed cireular base) due to the ovalling load hasbeen given elsewhere in the literature[8J. This considered stresses on

Therefore, for t/r = 0.008 and IIr = 50, the increase in tensilestress = 30%. This is probably unimportant in the design of ehimneyshells, 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)Iimited itself to consideration of stresses at the base, at 0° to the winddirection. Bere, the compression due to beam flexure is reduced oreven reversed by the shell stresses induced loeally by restraint ofovalling deformation. However, increases in compressive stress arepossible elsewhere. Inereases in compressive stress are due to eitherof two effects:

l) At the base and between values of <\>about 60° and 120° to thewind, the redueed compression stress due to bea.rn flexure (functionof <\»has to be added to the compressive shell stress due to restraintof ovalling (function of2<\» - see fig. C3.l9. Significant increasesin total compressive stress only occurr at relatively small values oftJr for l/r ratios less than 30 - see table C3.5.1

2) For relatively thìck 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 contrafiexure effeets, associated withrestraint of ovalling, causing compressive stresses at this height.

CICINO. Model Code - Commentaries and Appendices page 19

r'" UrI/rbeam stresscjlbeam stressshell stress

Imax. atcjl, MPadegreesMPaMPa

0.004

202.3900.07.3

30

6.0702.06.0

40

11.5704.06.0

0.005

201.9900.04.8

30

4.8701.73.7

40

9.2703.23.7

0.006

201.6900.03.3

30

4.0701.42.5

Table C3.5.1 - Max. Compression Stresses at

Base of Unstiffened Chimney

total stress

MPa

7.3

8.0

10.0

4.8

5.4

6.9

3.3

3.9

rafio

at cjl

3.18

1.35

0.87

2.63

1.13

0.75

2.13

0.98

Ur IIrmax.comp.height (z)beam stresstotal stressratiashell stress

atzat zMPa

(x dia.)MPaMPa

0.011

200.96.21.32.21.64

30

0.96.28.89.71.11

0.Q10

200.96.21.52.31.57

30

0.96.28.89.71.10

40

0.96.223.124.01.04

0.008

200.86.21.82.71.43

30

0.86.211.011.81.03

0.006

200.47.41.21.61.32

30

0.47.811.011.41.03

Table C3.5.2 - Increases in compressive stress at 00 to wind (downwind side),

about 6 diameters above base of an unstiffened chimney.

Ovai

Circle

~Wind <TCT

<TCB

3.0

2.0

1.0

1-=20R

~ ~ -=30R

Therefore, combining both tables it can be seen that consideration of

shell stresses leads to significant increases in compressi ve stresses,

either at the base or at a height about 6 diameters above the base for 1/r

ratios < 30. Guidance regarding these increases is given by fig. C3.20

Fig. C3.19 - Stresses at chimney base

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

.01 .011.ooa

ti R

.006.004

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

The distribution of ovalling pressure = 1.115· PO' cos2<jJ

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

Away from the ends of a long, unstiffened shell, the consequentbending moment at position <jJis mo, where:

1.115

ma = -4-' R2. PO' cos 2e!> h. (C3.5.2)

and mo (max) = 0.07· Po' d2(NmJm)

-l TOlal Ten";on

Upwind

OvallingFlexure

0<DownWind

lNel _

Compression t

page20 CICINO Model Code - Commentaries and Appendices

(Note: 0.07 increased toO.08 in Model Code (equation 7.11), to allowfor effect of initial curvature)

The associated deftection of an unstiffened shell at point <I>is wo,where:

w = wo' {I - e-kyxJ2 . [cos(À'yxl2)+sin(À:yxl2)]} ... (C3.5.4)

2

where: À:y12 = (3)0.25. R . (tIR)0.5

Substituting À"{12= 1.52· (t)0.51 (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)O.5The deformation of theshell above and below the stiffener is shown in fig. C3.21.

... (C3.5.9)

... (C3.5.8)

... (C3.5.11)

... (C3.5.I2)

2560· tld2 = 2· St· V/ d

f = 2560 . t I d2

Ir > 0.3· dl.5 . t2.5. L/ 0.56· d . (dft)O.5

when L <0.56· d . (dft)O.5

For St = 0.2, therefore, Vcr = 6500· ti d

The frequency of vortex shedding relevant to ovalling = 2 . St· V I d

Therefore large scale resonant movemements can occur if:

To ensure that ovalIing 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 rnfs is high enough to achieve this, the required valueof I is then given by:

1 fi!lJ.2.E.If=2·St·Vcr/d=-27l' p·A·R4

C3.5.2 Dynamic component of ovalling

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

t3(= - m4/m)12

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

In the case of steel:

C.3.5.2.1 - Unstiffened shells

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

l J 7.2·E·I t rp:-f1 = 27l" P . A . R4 = 0.49· d2 . ..; ~

This must be much less than Wo,say IIS.

Therefore, Ir must be, say, greater than 5 times (0.06' dl.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)O.5and the moment of inertia CIr)of the stiffening ring (includingparticìpating shell (see Model Code Fig. 7.4) should be:

Ir> 0.3· d1.5. t2.5 when L = 0.56· d· (df1)°·5 ... (C.3.5.7)

... (C3.5.2)

2

Defonnation with rings

at dlstances x = 1.32R{~"

Ring Saffener (Deformation Zero)

Fig. C3.21 - Ovalling deformation of a cylinderwith a stiff ring at x = O

o

....

1.2S

Li:1.0o.n;:j 0.25

12·R4.1.1I5·poWo= 16. E. t3 . cos 2<1>

and wo (max) = 0.06· Po' d41 (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:

It can be seen that the ovalIing deformations and, therefore stresses,remain low (about 0.03wo) if the distance between stiffeners ofinfinitely high stiffness is smaller than 0.56· d· (dlt)O.5.

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

For Vcr > 30rnfs, St = 0.2, P = 7850 kg/m3 andE = 210 . 109 N/m2, therefore

I> 7.4.10-6. A· R2 = 1.8.10-6. d2. t (m4/m height)M = 0.028· Po' d3 . (d/t)0.5 (Nm) ... (C3.5.5)

Giving:

p·A·R47.2·E ... (C3.5.13)

In order to be effective, the deformation of the stiffener under thismoment must be much smaller than Wo - 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:

For an unstiffened sheIl, this means t3/12 > 1.85.10-6• d2. t

... (C3.5.14)

i.e. tld must be > 0.004, otherwise stiffening rings will be requiredto avoid the risk of ovalling vibrations.

When L = 1.58 . R . (R1t)O.5:

0.19· Po' R5.5

w= E.Ir.(t)O.5 ·cos2<1>... (C3.5.6)

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 t1d ratio is < 0.004. These vibrations are defined by:

CICIND Model Code - Commentaries and Appendices page 21

The solution is approximated by:

w = Wo' cos wt· cos (2y/R)· COS (1T' x/L)

Where w = defonnation

x = coordinate along the shell (Le. vertical direction)y = coordinate alongthe circurnferenceT=Time

Where L = distance between stiffening ringsWo = deformation of unstiffened shellw =2·1T·f

f = frequency

Substituting in equation (C3.5.l2) gives:

E·t2 {('ll'/L)2+ (2/R)2}4+ {'ll'4/(R2.l4)}wZ = 12· p . {('ll'/L?+ (2/R?F

An approximation is:

00= (E/ p)05. {li [R + (4· U)/(1TZ, R)]}

Therefore L2 = (1T/2)z. {[(R/ 21T' f). (E/ p)O.5] - RZ)

Assurning that V cr > 30mls is high enough to avoid oscillations andf=O.2,Vc/R and substituting E=2IO·109 N/m2 and p=7850Kglm3:

Literature

[Il B.J. Vickery - "Wind loads and Design Jor Chimneys" CICIND REPORT, VoI. 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 oJ exposure-corrected data".Journal ofWind Engineering, no. 13,1983.

[4] A.G. Davenport - "Gust loading Jactors" - Proc. ASCEJournal Struct. Div., VoI. 93, No, ST 5, June, 1967.

[5] B.J. Vickery - "Wind-induced loads on reinforced concretechimneys" - Nat. Seminar on Tal1 Reinforced ConcreteChimneys,New Delhi, 1985.

S. O.Hansen - "Vortex lnduced Vibrations oJ Line-LikeStructures" - CICIND REPORT, VoI. 15, No.1, March 1999

Shoei-Sheng Chen - "Flow-induced vibration of circularcylindrical structures". Hernisphere Publishing Corporation 1987.

H. van Koten - "The Stress Distribution in Chimneys due toWind Pressure" - CICIND REPORT VoI. 11, No.2, 1995

H.van Koten :- "Structural analysis oJ shells" - TechnicalUniversity of Deift.

... (C3.5.l5)

w = __E_t3_ [_iJ_2+_iJ_2]4 w + _E_· t_il_4w_ +l2(1-v2) ilx2 ily2 R2 ilx4

a2 [il2 a2 ]2+p·t·- -+- w=O

. iI'f2 ilx2 ily2

L< 18·R, or9·d ... (C3.5.l9)

From equation (C3.5.I4), we have seen that the rninimum value oflper unit height to avoid oscillations is:

(m4/m height)

Assurning the stiffener to provide the equivalent I of a Iength 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[l].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 the

movement 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 aerodynarnic damping is affected byspacing, Scruton Number, or large-scale atmospheric turbulence.Therefore, for spacings between chimney and interfering structureless than lO diameters, the Model Code merely recommends additionof structural damping to increase the chirnney'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.

page 22 CICINO Model Code - Commentaries and Appendices

COMMENTARY No.4 - FATIGUE

When we consider the long terro history of movement ofa chimney

subject to cross-wind movement in response to vortex excitation, wemust take iuto 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. J)

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

Aachen

C7, 'a 102.7 N/mm*

B = 28 m

Ve< B m/s93 days

"1-1H '" 35 m

Ve: 2.5 m/s264 days

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

IO' lO' lO' la' IO' 101 10'

H = 38 m

Ve. 3.5 m/s152 days

t- t. R(!Ckiinghausena, = 17.1 N/mml

1.0 f-=~.~:S:~:::::::):::::::T:.~:::r::::::~

::rt~t~!l~'~~lI .O. 2 •.•.... ~•....•.. ~..•..•. " ~ ...•.•. ~.. : •.•.• \

0.0 .....• :...•... : _..~.....• ~ ~ ;•.... ,'"I

Pirna

01 - 11,6 N/mml

B = 60

Ve>: 8 m/s

322 days

TlM( rUNCTICtl -CHRN l

nAxlvALuc]-B.914E-sz VD!=ITf\-poltàs'" 489G

'::i..i-.: 1-.2 i-.' j

-.61-.' 1

-1.0 j -----~------c-----~ ~----~ , ~e B.\5E+es 1.61(.01 2.<t6( .•.~u 3.Z8t.~al ';.t3(-tOI S,

Further, in inland locations ami at relatively high criticaI windspeeds,atmospheric turbulence is high enough to ensure that the maximul11amplitude rarely occurs. This was demonstrated by a series of longterm measllrements (varying between 93 days and 322 days) of theresponse· of four steel chimneys in Germanyll! - see fig. C4.2. It canbe seen from these histograms that amplitudes exceedìng 90% ofmaximum occurred only rarely, varying from about la cycles during93 days at Aachen to about 100 cycles during 264 days at Cologne.

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

... (4.1)

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

~og nWhere CT max the maximum stress. per sectioll 7.2.4 of the

Model Code Fig. C4.3 Loadlcycle collectives far various values of 11.

CTwn

k

the stress causing cracks after n cycles(per WohJer curve)

3 in the case of fatiglle in steeI

To deterrnine the number of load cycles(n), it 1s first necessary toknow thc number of occasions the wind will blow at its criticaI

velocity (Vcr). This is detennined l'rom cOllsiderations of theprobability of their occurrence - P(V cr):

a function (dependent lIpon Ver) dellning theshape of the load/cycle collective curve(Fig. C4.3) as follows:-

Vcr 'P(Vcr) = 2· --, . e'-(Vcr/vot

Vo-À

U = umax' (l - (log n flogo j) l).. ... (4.2)

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

'" (4.4)

À = (Ver /8)1.2 ... (4.3)hourly mean velocity at chimney top, withexceedance probability of once in 50 years.

n Number ol' load cycles due to cross-wind excitationduring tbe lifetirne T

It is assumed that the chimney responds at wind velocities betweenUVcr and O.9Vcr.


Also a reduction has to be introduced to account for changes in thewind direction, so that the point of maximum stress is moved awayfram the point under consideration. The stress at a given point isproportional to cos2<1> and the tota] effect is raughly:-

(1 /2'IT)' rCOS2<l>d<l> = 0.5()

As a result,

il = 3.15.107 ·T·f·4· 2 ·0.5·0.1 ·A·e-A2

= 1.26.107• T· f·A . e-A' ... (5) (see Model Code 8.5.2)

Where A = 4· Vcr Nb(h)f = Resonance frequency

page23

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

[l] W. Langer, H. Ruscheweyh & C. Verwiebe - "Untersuchungendes Querschnittverhaiten von OriginaI Stahischornstein" Forschungsbericht P. 230

[2] H. van Koten - "A Calculation Method for the Cross- WindVibrations of Chimneys" - CICIND REPORT VoI. 14, No. l,lune 1998

page 24 CICIND Model Code - Commentaries and Appendioes

Fig C5.1 - Reduotion of inertia at openings

When tbe width of opening is less than 40% of tbe chimney's diameterlocally, it is not necessary to provide a horizontaI stiffener extendingaround the full circurnference and a more 10caI arrangement may beused (see fig.. CS.3). VerticaI 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 O.S times the width of the opening will suffice.

placed normaI to the shell {see Figures CS.2 & CS.3) andconcentrated aIong tbe edge of the opening.

However, sudden ending of of tbe reinforcement above and below tbeopening can cause stress concentrations. These can treble stresseslocally and lead to fatigue damage such as 10caI cracks. To avoid this,in tbe case of openings with width greater than 40% of the chimneydiameter locally, tbe verticaI stiffeners should connect at each endwith a horizontal stiffener extending around the full circumference(see fig. CS.2).

COMMENTARY No.5 - OPENINGS

Openings 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 equaIity of section moduli is sufficient to fullfill tbe firstcondition of strength.

The moment of inertia of a circIe with an opening subtended by tbeangle 2<f>is:

1= d3 X t/8 X {'!T- <P - sin<f>cos<f>- [(2sin2<f»/('!T- <f»]}

Derivation forrnulae for cross section properties of chimneys (bothunreinforced and reinforced) and of chimneys with more tban oneopening at the same elevation are given in Table CS.I

lf 13 is small then tbe value of I is cIose to that of the complete circle(0.12S X '!Td3X t). As 13 increases, however, tbe value of I dropsrapidly (see Fig. C.S.l). The same holds for section modulus. Toreplace the lost material, reinforcing stiffeners are welded verticaIlyto the chimney on each side of the opening. To be effective, tbe crosssection area (A) of each of the reinforcing stiffeners should be at leasteqUaI to A = 1.25 X R X t X (sinl3)O.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 tbis tbe reinforcement stiffeners have to be

'!TL7T~

t 2 t 2

1 !!!J.R"t 1 R't 1

.:E. 'TI' ~1T'2 2

~lìtr ---JIo- W

(!:):~'/= :,

~' /• t

7T~.R

l 2 ,--,"0' =.M..R2t 1 W1

""2 'lT-~'

G

A = 2tr ('TI"- 2f3)

IGG= 2tr3 (7T/2-f3-sinl3cosf3)ZGG= IGG/ rcosf3

IG'G'= 2tr3 (7T/2-f3+sinf3cosf3)

Ze'G1 == IG1G1/r

A = 2tr ('TI"- 2f3)+ 4a

IGG= 2tr3 ('TI"/2-f3-sinf3oosf3)+ 4ar2cos2f3

~G = IGG/ rcosl3

IG1G'= 2tr3 ('!T/2-f3+sìnf3cosl3)+ 4ar2sin213

Z"PG' == IG1G,!r

A = 2tr ('TI"- f3)e = rsinf3/ ('TI"-f3)

IGG= tr3 {7T-I3-sinl3cosf3-[2sin213/('TI"-I3)]}

Z1GG = IGG/ (e+rcosl3)Z2GG = IGG/ (r-e)

IG'G1= tr3 ('TI"-I3-sinf3cosl3)

ZG1Gl= IG1G'/r

A = 2tr ('TI"- f3)+ 2a

tr2 sin[3 - arcos[3e=--~---tr ('!T-l') + a

100 == tr3 ('TI"- f3- sinf3 COSI3)

+ 2ar2cos2f3

IGG = 100 - Ae2

Z1GG = IGG/ (e+rcosf3)Z2GG= IGG/ (r-e)

IG'G1= tr3 (7T-I3-sinl3cOSI3)+2ar2sin2f3

ZG'G1= IG'G,/r

Fig. C5.1 - Oerivation formulae for section properties of chimneys with openings (a = reinforcement area)

CICINO Model Code - Commentaries and Appendices page25

Fig. C5.2 - Suggested detail of reinforcement farwide openings (> 0.40)

If the vertical height of the opening is morethan twice its horizontalwidth, a stability check is needed. Guidance on such checks is givenin the chapter on bending of plates under lateralloads in "Plates andshells", by Timoshenko.

When the duty of-the chimney requires fiue 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.

SectionAA

L = 0.5 openingwidth

Fig. C5.3 - Suggested detail of reinforcement farnarrow openings « 0.40)

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'sdynamic response. This isdoneby takingaccount of the reduced local stiffness at the opening when calculating"x" for each section-in equation 7.16 of the Model Code.

Section AA


C6.2. Internai Corrosion

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

The intemal cOITosion allowances in table 8.2 of the Model Code are

based upon limited exposure to condensing slllphurie acicl per FigC6.1. They are derived l'rom the relationship be.tween "Peakcorrosio!1 rate" and "S03 concentration" shown in figure C6.3. This,in turn, was clerived l'rom the upper bound 01' a family 01' curveswhich show the same relationship observecl in practical situations.See Iìt. [2] and [3]. A safety factor 01' 4 has been used in arriving atthe corrosion allowances.

10080

Condensate

40 60H2 804: Gew %

20

Mixture 01gas and condensate

200

oO

40

160()°!::I

F! 120ClQ.E~

80

Gas240

C6.1.3. Attack due to chlorine, chlorides and fluorides ('Chlorides are found in most solicl fuels, inclucling refllse and in manyliquid fuels. lt is also sometimes faund as a pollutant in some FGDprocesses. Upon combustion chlorides are transformed into freeehloride ions which, on eontaet with water vapollr are transformedinto hydrochloric acido 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 cIose to the water and sulphurous acid dewpoint. Even very small amounts 01' chlorides in combination withother condensed acids can cause seI10us corrosion problems.

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

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 aGid can be expected to con dense in the sametemperature range. Thus, even il' fuel and combustion processes arechosen to minimi se production of S03' or if ftue gases are scrubbedto remove most of the SOo and S02, severe cOITosion can be expectedif the temperatures of tlle tlue gas or the surfal~es with which it cancome into contact fall below 65°C, or the acid dew point temperaturerelevant lO the reduced S03 concentralion, if this ìs higher. Again. asafety margin is recommended of IOeC above the acid dew pointtemperature estimated from figure C6.1.

C6.1. Chemical effects

C6.1.1. Attack due to sulphur oxides

The most cammon form of internaI chemical attack is due to acids

formed by the condensation of sulphur oxides in the Hue gas. Sulphuris found in al! solid and liqllid fuels to varying degrees ancl can alsobe founcl in gaseous l'uels. Dwing the combustion process, nearly alIsulphur in the l'Ùel is oxidised tosulphur diox,ide (S02) which can beabsorbed by condensing water vapour to fonn sulphurous acido

Asmall quantity (jl' sulphur clioxide (S02) is further convetted tosulphur trioxide (SO)). The quantity depends in a complex mannerupon the sulphur content of the fuel, the amount 01' exeess airavanable cluring combustion, temperature in the combustion chamberand the presenee of catalysts sueh as iron oxicles. This smalleoneentration of S03 Cusual1y measurecl in PPM). gives rise to mostof the acid eorrosion problems encountered in ehimneys. This isbeeause on eondensation, the S03 ions combine with water vapour toform sulphuric acid whose concentration can be as high as 85%.

Conclensation of these aeids takes pIace when the temperature of thefIue gas fa11s below their respective acid dew point temperatures(ADP). or when the ftue gas comes into contaet with a surface, at orbelow the relevant acid dew point temperature.

The acid dew pointtemperature of sulphuric acid depends upon theconcentration of S03 in the ftue gas (see Fig C6.1). Provided thetemperature 01' the tlue gas and the surfaces with which it can comeinto contact are maintained loec above the acid dew point estimatedfrom Fig. C6.I, 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 far Chimney ProtectiveCoatings".

COMMENTARY 6 -CHEMICAL EFFECTS ANO INTERNAL

CORROSION

C.6.1.2 Effects of Flue Gas Desulphurisation (FGD)

Despite the removal of most of the sulphur oxides dwing FGD, asevere corrosion risk remai ns. This is because, downstream 01' ascrubber, the ftue gas is uSllally 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 guiteconcentrated acido Also the Uue gas often contains chlorides, carriedover l'rom the serubbing materials.

C6.3 Guideline to choice of Iiner metallic materials

Guidelines on the stùtability 01' various metals and alloys l'or therange of chemical risks to be found in chimneys will be gìven inCJCIND's "Metallic Materials Manual" (to be published in 200 I).

Literature

[l] "Desulphurisation S:vstems and their Effèct 011 OperationalConditions in Chimneys". Henseler, E, CICIND REPORT,VoI. 3, No.2, 1987

AlI steels except the very expensive high nickel alloys and tÌtanÌumwOllld deteliorate very quickly in this environment. To minimise theexpense, methods have been developed to apply very thin sheets al'al10y or titanillm to the inner faee of carbon steel or other vllinerabieIiners. Some organic coating materials have aiso been developed l'orthis duty.

[2] "lnjluence offuel oil characleristics Clnd combusliollconditions 011 the gas properties in.water tube boilers" BunzG., Diepenberg H, and Rundle A. - Jnl 01' the lnstitute 01'FuelSept 1967

[3J "Prevention olcold end corrosioll in industriai boilers". Lechami Landowski - "Conision" - March 1979

CICIND Model Code - Commentaries and Appendices page27

.• 1

Ec.c.:;'".S

100 :EK

'S

.2:;li>

E r.o'..

i1l>1.000

IO

"·.1 1_1 ~••

·1

a. -lO'U20 J ~

by valume -

_.,:'1 1-_==1== I ~• o5

10·1, .. r I J I f I '0.1IO 90 IUO 110 110 130 140 ISO

O.wJloill.1. ·C·U! UrTll LTU in 11TTTITTJIIT-IrUlJ IO

160 110 100 190150140130

'~'I:t;l:I'l:l;I:f:-t'~" l'li'".,L •....... 1

'r·.~ ."~. I ::.:. A.,;+I.t l-~l '.:

-·d:..

lO"120

10-1

Dcvvpoirl1, "c

Fig. C6.1 - Relationship between ADP and 503 concentration

peak corrosionrates(micranl

1000 hours)

".

" ..so, concentration (ppm by voi) ,

1 ppm = part per miflion (1O~

Fig. C6-3 - Relationship between peak internai corrosion rates and 503 concentration

page28 CICIND ModelCode - Commentaries and Appendices Amendment A - March 2002

APPENDIX 1-

DESIGN OF CHIMNEY BASE PLATES

This appendi x is intended to give guidance on rationalising baseplatedetails. In thc following calculations, base plate bearing stress (0"*c)and maximum boll teasion (Pb *) are calculated for factored load andovertuming momento In thc case of bases with a compression ringandlor gussets tbc values of 0"*c and Pb* are calculated using elasticana1ysis as a reiaforced concrete ring assuming the modular ratio of12 11l.l21.Thc· area of stecl bolts istak.enas the thread root cross

section area of the bolts. In chimneys requiring an increase in designtensiJe. stress at the base 011account 01'c1ause 8.2 of thc Mode! Code,the value or Pb* should be factored according1y.

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

The maximum baseplate stress (0"*) is givcn by the followingexpression:

0"*=131'0"*c,P/tb2<fk/U ... (A1A)

where 131is given by:

l/b 131

O 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

Fig. A1.1 - Simple Baseplate

... (AJ.5)

Where 132is obtained as follows;

l/b 132

0.2 2.380.3 2.28OA 2.070.5 1.870.6 1.650.8 1.33l.0 1.061.25 0.811.5 0.62

anel 1 = the outstand of the basplate from the chimney sheJlb = distance between gussets

The baseplate stresses (cr*) on the tension side muy be calculated usingthc method described in !it [1]. For the particular case of 1= 4· O:

... (ALl)

Tension side

p* perboll

+ ------- ---- ------tb

er*e

On the cOl11pression side, the vertica1 shell foree is distributed over astrip of width (2.13 + t,), where 13 is chosen to limit the pressure onthe grOtit (O"*c)to no greater than fkg I ].5.

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

Both equations AlA and A1.5 must be satistìed.

The height of the gussets (h) should bc sufficient to maintainacceptab1e shell stTesses. Thc stress in the sheJl (O"*s)is givcn by thefollowing expression:

where ti,

fkg

characteristic strength of the botto m plate steel

pressure on the grout

thìckness of shell

eharaetel;stic compressive strength of the grautrr*, = w*. [(exI t,) + (133' Rslt}»):s fk Il. l '" (AJ.6)

On the tension side, the values al' Il and 14 should be adjusted to givevertical and rotational equilibrium. Thc active circumferentiallengthof the buseplate may be taken as :I ./2 or the bolt spacing, whicheveris the lessero

Thc bolt tcnsion (Pb*) then = p*. (lI + 12)/1, ... (A1.2)

Where p* is the verticaltensile force in the sheJl per boll.Assuming a distribution al' baseplate stress over a length al' 3 .12:

0-* = 2· p* IlbZ < fk/l.l ... (A1.3)

Both equations A 1.1 und A 1.3 have to be satisfied.

0.530.260.200.160.130.11

0.0980.0790.0520.0390.03.1

1.001.93

2.503.203.834.47

S.IO

6.379.55

12.7415.92

No. of gussets(equally spaced)

6J2

1620

242832406080

100

Where: ex and 133 are given by:

and R, = shelJ radius

w* = the radiai foree 011the sheIJ per unit height of gussetat the top of the gussets, given by the followingexpression:

w* = 3. M*lh2

Where M* is the bending moment al the base of each gussetplate due to out of balanee forces under the basepJate.

M* = p* , 20 per gusset on the tension sidc

= O"*c. 6 . 1)2. b per gll~set on thc compression side

2D

D

Rodius Rs

ts

erOe

Fig. A1.2 - Baseplate with gussets

CompressionSide

Gussel spocing b.equolly spoced

cbout bolts.

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

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

Allowancc should be made for stress concentrations that may occurat the top of thegussets.

A 1.4 Grouting

Tbe baseplate stresses are calculated in the same way as in sectionAl.2 above using equation AIA.

The compression ring bending stresses (a*) are calculated in tbcsame way as in sectiol1 A 1.• 2 above, using equation A 1.5, substitutingtç (tbickness of compression rillg) for tb (baseplate thickness). AcldedlO this is a direct cìrcumferential stress arising from the out ofbalanccl1lomenl caused by Ùle eccentlicity 01' the bolts, giving a total stress:

NON SHRINK GROUTOF' THE SAME STRENGTHAS THE CONCRETE BASE.

BASEPLATE

COMMENTARY 7

PACKER PLACED NEXT TO BOLT

Note - If the chimney is intially levelled using a nut placed on theholding down belt under the basep late , this nut should be loosenedafter packers are introduced.

2D

TensionSlde

2D

------------------

ts

a'c

CompressionSide

Gusset spocing05 above

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

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

where N = number of bolts Fig. A1.4 provides guidaoce 00 the grouting procedure to beused under chimney baseplates.

A gussel plate thickness of O.25D wil! suffice il' it is of a steel whoseyie.lelstrength at least equals that of the bolts.

Notes regardillg tbe derivation of ~1 and ~2

Stress coetTicients 131 and [32 were obtained as follows:

~1 is tbe coefficient applicable to the compression side anel is derivedl'rom Timoshenko's work on a rectangular plate fìxed 011three sidesand l'ree on the fOllrth. This is a reasonable assumption becausepressure under the base inside the shell will produce fixity. At thegussets there is fixity by virtue of thc continuity of tbe basplate.

132 is the coefficient applicable lo the tension side. In tl1e literature [l]this is taken from a model comprising a rectangular plate simplysupporled on ali sides, with a patch load at the centre representing thebearing of the nu!. This is not a (fue reflection of the boundaryconditions which are more truIy fixed on two opposi te sides (at tbegusstes), one side being pinned (at tbe shell) and the fourtb side free.Neither is t!le effect of tbe holding down bolt hole consielered. ln thisAppendix, therefore, tbe values of 132 have been derived from plaleelement FE analysis. using tbe more realistic above boundaryconditions anel allowing l'or the bolt hole in tbe plate.

References:

[I] Brownell & Young - "Process bquipment Design",Chapter lO

[2] Pinfold, C.M. - "ReÙ~forced Conaete Chimneys and Towers"


A2.1.1 Generai

A2.1.2. Insulation design

APPENOIX 2 - INSULATION ANO PROTECTIVE

LlNINGS AND COATINGS

Direet metal/metal contaet between steelliners and the stmctural

shell shollld be avoided. Liner support should incorporate atbermal isolation devic:e.

Attachments such as guy ropes, aerodynamic stabilizers, ladders,platforms and pipes can aet as cooling fins. Their attaehment tometal in contact with fiue gas shouId incorporate a tbermalisolation device.

overall average U('"type of insulationthicknessvalues W I (m2 K)

aluminium

6mm airgap4.5

aluminium

1Bmm air gap4.0

mineral wool

25mm2.3*

minerai woo/

50mm1.15*

minerai wool

75mm0.7*

minerai wool

100mm0.5*

expanded minerai

50mm1.15*

expanded minerai

75mm0.7*

expanded minerai

100mm0.5*

expanded minerai

150mm0.35*

Table A2.1 Typical insulation conductivities

A2.1.3 A1uminium cladding

Minerai wool or foa111insulation exposed to weatber ShOllId beprotected by weather proofed c1adding. Design of this cladding andits fixings should ensure its integrity under tbe actioll of wind at avelocity of 1.5 X basie wind-speed at the relevanl height (perparagraph 7.2.2.of the Model Code). The design should take accountof the variation of wind pressure around the surface 01' the chimneyat a given elevation.

Aluminium c1adding enclosing a narro w airspace is an effective formof insulation, due to its high tbermal reflectivity. (Note - Sheet steelor otber fonl1S of cladding may be sLlitable in certain cases.)

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

The cladding should consist of aluminium sheet not less than LOmmthick with symmetrical fiange eovers made in haives from aluminiumsheet which also shal1not be less than l.Omm thick.

The cJadding should be made in strakes, using il number of equalplates per strake. AH seams shollld be connected by aluminium alloyrivets at not more than 100mm centres. Vertical seams of each stTakeshould be se! at tbe midpoint of thc. strake beneath.

The eladding should be fitted with its il1temal face the required distanceaway from the extemal face of the chimney shell, this distance beingmaintained by continuous circumfercntial spacers of tbe reqtùredthiclmess low conductivity tape coiucident with thc horizontal joints ofthe aluminium. The tape should he eemented into position by means ofsodium silicate or otber suitable adhesive. The ends of the horizontal

rivets in the alunùnium sheets serve lo retain tbe tape in position aftererection. The circumferential spacers divide the airspace between tbestecl shell and tbe aluminium cladding into sections not more than.I.5m high, tbus reducing convection heat losses.

When the Iength of the prefabricated sections of shell betweentlanges is not a whole multiple of tbe strake widtb, only one make-upstrake per section of chimney should be used.

AH projections shouId be elad. Cleaning doors ami other pointswhere access is required shollld be "boxed in" with removabiealuminium panels.

The airspace at the top of tbc chimney should be completely sealed toprevcnt ingress of moisture between tbc stce] shell and the cladding.

Each upper strake or aluminium should lap over the lower strake bya minil1lum of 25mm, The vertical seams similarly shouId have aminimum lap of 25mm.

To permit examination of the steel shell of the chimney withoutremoving the cladding, 150 mm square openings, [ocuted at careful1yseiected points and covered by removable panels approximately230mm square, may be provided. Suitable positions are:

* These vaiues apply for il mean insulation temperature of 40oG. They ShOllld be

increased by 5% for each 500e increase in mean insulation temperature.

1nsulation

lnsulatìon should be designed to maiutain the surface in contad withthe tlue gas above aGid dew point temperature everywhere, when tbefiue gas is at normal operating conditioo and at abnOlmal conditlonsif they can last for more than 25 hours per year (see table 7.1 of theMode! Code). Far design purposes, the following parameters shollidbe used:

Theoretical acid dewpoint, calculated taking aecount of sulphurcontent and excess comhustion air should be increased by asafety margio of IO"C. lf data is not available to permitcalculation of the t1ue gas acid dew point temperature, thefollowiog values ShOllldbe used for minimum metal temperaturein contaet witb fiue gas:

• When l'uel is oil andlor gas, containing more than 0.5% byweight of sulphur, 175"C

• When fuel is coal containing more tbau 0.5% by weight ofsulpllllr, 135"C

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

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

Wind velocity shouId be assumed to be 5m/s.

The temperature of the metai in contact witb fiue gas shouId bechecked for the condition of highest anticipated fiue gas temperature.Far this eheck the following design parametcrs should he assumed:

Ambient air tempermure should be maximum anticipatcd airtemperature at the chimney location.

Zero wind velocity.

The design or insulation thickness lO satisfy the requìrements 01' thisclause should be based upon the eonductivity value of the insulationmateria l, provided by the insulation manufacturer. ]f such data is notavailable, typical values listed in table A3.1 may be used.

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

Even if metal in contact with fiue gas is generally at temperature,above its aeid dewpoint, rapid local corrosion can oceur at cold spots.In arder to eliminate cold spots careful attendon should be given tothe following details:

Potenti al air leaks should be eliminated by properly sealingflanged joints, inspeetion/cleaning doors, expansion joints andinstrumentation apertures. The long-term effeetiveness of sealingmaterials at the relevant service temperatures should bedemonstrated ..

A.2.1

C/CIND Model Code - Commentaries andAppendices

- diametrically opposite any inlet

- approximalely l,25m from the top of tbe chimney

Greal care should be taken lo ensure tl1at dissimilar metals do nolcome into contact with each olher. lf it is essential in the design thattwo dissimilar metals have to be connected, a suitable non-conductive

and impervious film or agent should be placed between tbem.

A2.1.4. Minerai wool or foam Insulation

Wrapping thc· sleel shell with a suitable grade insulation material 01'

sufficient thickness provides more effcclive insulation tbanaluminium cladding with thc usual 6mm air gap.

Thicknesses of over 50mm are applied in two separate layers, theouter layer being filted so that the vertical and horizontal joints arestaggered from the joints of the inner layer. Ifa stiffener or f1ange ofthe chimneysectiol1 projects past thè outer face of the insulation, itshould be wrapped with an additionallayer of thc same thickness forat least 75mm on each side ofthe f1ange or stiffener. Insulation has tobe protected from the weather, a convcnient way of doing this is tocover it with metal cJadding. designed as descibed above.

The insulation should be fixed to the steel shell by wrapping it aroundso that the ends butt. It can be seenred in pIace by steel strapping. Atleast two bands of strapping should be used for each strake ofinsulation. lnsulation tends to compact and slip down the surface ofthe steel during ti-ansportation and erection thus leaving bare patchesof steel which are potential "cold spots". The slipping of theinsulation may be prevented by wclding steel pins to thc 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 nsed between the pins.

A2,1,5. Lined and multiflue chimneys

The sapce between the outer shell and the lincr of a double skinchimney can be iì.lled with mineraI wool, expanded mineraI, or othersuitable insulator.

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

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

After 6 to 12 months, expanded mineraI insulation compacts by about10% thus leaving areas of the liner exposed. It is essential that thisvoid is "topped up" with more expanded mineraI and that adequateprovision is 1eft in the cap plate for topping up to take piace.Sometimes a seeond "topping-up" is necessary after a further 12month periodo

A2.2 Protective linings

A2.2.1 Generai

Linings may be require.d in steel ehimneys for one or more of thefollowing purposes:

To maximise the stre.ngth ofthe structural shell by keeping it cool

As fire proteetion

- To proteet an externally insulated struetural shell fra mexcessively hot fiue gases. These could be generated by anoperational upset or occur when an energy conservation systemis by-passed.

- Corrosion protection

page 31

- To aet as insulation to maintain the fine gas temperature above itsacid dew point.

- Reduce potenti al for aerodynamic instability.

Chimney Iinings may be:

a) Separate !iners, with a space between the liners and tbe outerstructllral shell. More than one liner may be accomodated withinthe structural shell, to form a "mlllti-flue" chimney.

b) Attached continuously to the inner face of the structural shell.Snch Iinings may be either cast againsl thestructural shell, or beapplied by spray, trowel or brush. Sueh Iinings may be:

- castable refractory

- soJid grade diatomaceous concrete

- chemical resistant coatings

- libregJass reinforced pJastic (FRP)

A2.2.2 Design of separate Iiners

A2.2.2.1. GeneraI considerations

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

Latera] support should be provided between the Iiner and thestructural shell as near as possible to the top of the chimney.

Additional lateral supports may be require.d at inteonediate elecationsbetween the top of the liner and its base, depending npon cOllsiderationsof stability and dynamic response, but their nllmber should beminimised as far as possible. The lateral restraints should be. designedto pem1it the linings to expand freely both vertically and radialJy.

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

The liner shollid be designed to resist stresses due 10 loads imposedby the latera] restraints, as thc structural shell moves under the effectof wind or earthquake.

The presence of horizontal restraints between the liner and structuralshell may prevent tbc liner from adopting a distorled shape in responselO differenti al expansion. As a result, bending stresses may beintroduced in both the liner and the structural shell, These stTesses canbe very high when a single Iiner carries flue gases fTom two or moresources with different temperatures. In addition, the resnltingdifferentialliner temperature will introduce secolldary 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 pennit free expansion of the linee Sufficientradiai clearance should be incorporated to pelmit any relativemovement, between liner and shell, that may be aIJowed by thelatera! restraint system. In the design of this cover, speciaJ attentionshonld re paid to the integrity of its fastenings, bearÌng in mind therisk of acid cOlTosion, stress corrosion and fatigne cracking whichmay be caused byaerodynarnic "fiutler".

A2.2.2.2 Steelliners

Unprotected steeJ liners should not be used in eonditions of highchemicalload (see table 7.1 of Model Code). In eonditions of low ormedium chernicalload. internai eorrosion allowances listed in table 8.2of the model code may be lIsed. In conditions of high chemical load(such as downstream 01' 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 in

CICIND's "Metallie Materials Mannal", to be publishe.d 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 shelI.

page 32

Consideration should be given to the risk 01' fire andlor hightemperature excursions described in paragraphs 7.6.1 and 7.6.2 ofthemodel code. 11' the risk is significant, consideration should be givento the provision 01' fire protection.

A2.2.2.3. Plastic liners

Plastic and FRP Iiners are suitable for conditions 01' "high chemicalload" (see tabJe 7.1 of the Model Code), combined with low

temperatures. In order to prevent material degrading, the temperature01' these linings should not be allowed to exceed 100°C. Short termexcursions to 150°C can be tolerated if the right type 01' piastic ischosen, but the life is reduced.

In order to ensure liner temperature is maintained below 100°C, anautomatically conlrolled quenching system may be installedupslream 01' the chimney, which is aetivated when the fiue gastemperature exceeds IOQoC.

A2.2.3 Design of linings attached continuously to the shell

A2.2.3.1 Generai

Llning or coating selection criteriu and qualilY standards to be useddllring slIrface preparation and Iining installation are detailed in thecrCIND "Chimney Protective Coatings Manual".

A2.2.3.2 Castable refractory linings (includingdiatomaceous concrete linings)

Castable refractory should be inslIlating type with a minimum bulkdensity, after drying, 01' lOOOkg/m3• Diatomaceous concrete shouldbe of the "solid" grade. They shouId be single layer cOl1stTUction,installed without vapour stops. They may be cast against thc innerface of the steel shel1 or they may be applied by a gunning processoMìxing procedures and water quantities sha11 fo11ow the.manufacturers' recommendations.

The minimum thickness 01' lining sha11 be SOmmo Linings 50mm to65mm thick shall be reinforced by eIectric welded wire· mesh. Themesh should be 50 X 50mm with wire of minimum diameter 2mm, oril may be 100 X IOOmm with minimum wire diameter 3mm.

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

Linings thicker than 65mm shall be reinforced by arc welded "V"studs, randomly orientated and at a minimum spacing of 1.6 persquare metre.

A corrosion resistant meta! cap should be provided at the top 01' therefractory to protect its horizontal sUlt"ace from the weather.

Providing its surface in contaet with flue gas is above acid dew point.this type of lining provides corrosion protection to the steel chimneyor Iiner to which il is applied. Application of such a lining wouldconvert a steel chimney, c!assed as being under "High chemicalIoad"when unprotected, to a "Low chemical load" classification.

A2.2.3.3 Fibreglass reinforced plastic (FRP) linings

The use of plastic and FRP for Iinings 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 vailles:

- epoxy resins, SODC - polyesters, 60°C

Il is essential that the FRP linings aclbere lìrmly to tbe inside face ofthe chimney shell so that tbe surface does not crack or spalI. lf tbcacìd fiue gas penetrates the FRP il wiU attack the steel shel!.

A2.2.3.4 Chemical resistant coatings

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

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


when wet and when dry. Only coatings should be used that have beenproved capabie of retaining their protective properties in theseconditions throughout the life of the chimney. AIso, 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 Iiners.

The start-up procedures ShOllld follow the refractory manufacturer'sinstrllctions. If none are available, thefollowing procedures may beused:

- Hold gas temperature in the range of 70°C-90"C for at leasl3 hours.

- ContraI subsequent increases in temperature und gas fiow so that

nopart of the Iiner is exposed to a gas temperature. im:reaseexceeding 50°CIhr. AH parts 01' the lining should be exposed to gastemperature at least 75% of design temperature for at least 6 hours.

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

A2.4 Protective and decorative treatments

Treatmenl selection cliteria and quality standards to be used duringsurface preparation and coating application are detailed in theCIClND "Chimney Protective Coatings ManuaI".

Stainless steel is nornlalIy supplied in its mill finish condition, which isa matt, light grey. Polishing to achieve a shiny tìnish involves extra COS1.

Weathering steeI, unless gril blasted, may 110toxidise evenly.


APPENDIX No.3 - GUYED CHIMNEYS

A3.1. Thermal expansion effects

Steel chimneys are subject to therrnal expansion when tbe shellisheate<! by the flue gases and, to a small extent by strong sunlight andby large variations in ambient temperature. The vertical expansioncan be considerable on tali chimneys with reasonably high tlue gastemperatures, especìal1y if tbey are extemally insulated.

For example, thevertìcal expansion of a steel chimney witb a guyband 80m above ground level and witb a shelt temperature of 250°C,would be 280mm.

This vertical expension expansion can greatly affect tbe tellsion in theguy wires and tbe consequent compressi ve load on tbe chimney sbel1.

Tlle stresses in gUy ropes arld shel1 should be chec;ked under both"hot" and "cold" conditions. For instance, if thé guy wires arecorrectly tensioned when tbe chimney is "cold", tbe vertìcalexpansion when the chimney goes on 10ad will increase the tensionin the guy ropes, it wìl1 also increase the vertical component in theshell plate, when it could in extreme cases produce buckling.However, if tbe guy wires are tensioned when the chimney is "hot",when it goes off load the chimney will reduce in height and tbeguywires williose part of their tension. This could cause more movementunder wind load tban is desirable. In order to avoid tbese problems,a compromise initial guy rape tension under cold conditions may benecessary Le. a tension that allows some lateral deftection of thechimney under design wind and "cold" conditions, while increasingthe verticalload in thc 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, tbe guys caninitially be correctly tensiom:d 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 tbe chimney hasexpanded. As soon as the heat load is reduced and the chimneyresumes its "cold" hcight, however, the guys must be retensioned.

A3.2. Calculations

A3.2.1 Normal conditions

Thc guyed chimneys shall be calcuiated taking into thc considcrationsecond order effects. The decisive winddirections which should be

taken into accollnt are given in figure A3.1

Fig. A3.1 - Wind directians far guyed chimneys

Thc 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= l.4Mw+ 1.35Mm-O.9Me<O.9Ma

in which:

M = cornbined rnomcnt

Mw = overturning moment produced by tbe design wind andirnposed loads

page33

Mm = overtuming momentproduced by dead-weight or otberpermanent loads which may act to increase combinedmornent

Me = overtuming moment produced by permanent loads whichact at all times to reduce combined moment

Ma = restoring moment producedby tbe foundation (includingguy rape anchorages) without exceeding allowable materialstresses or tbe foundation allowable bearing pressure.

In determining tbe support provided by the windward guy ropes, therelative stiffnesses of tbe chimney (acting as a cantilever) and the guyropes, including tbeir non-linearbehaviour, should be taken intoaccount. Many modern structural computer programs have routinesfor analysing guyed structures, which do this automatically. Ifcalcuiations are made by hand, however, guy rape tensions shouldfust be calculate<l,assumingtbe.chimn(lyispjnned at itsbase.Horizontal deftections at tbe rope attachment points sbould tben bedetermined. Tbe staèk shell should tben be analysed as a cantilever,propped by springs at tbe rope attachment points. The stiffness oftbese springs is determined by the deflections and horizontalcomponents of tension in tbe ropes, previously calculated. Secondorder effects should be considered.

A3.2.2 Abnormal conditions

ThestabiIity of tbe chimney should be checked at 0.1 X DesignWindspeed, assuming one of tbe guy ropes to be broken.

A3.3 Guy ropes

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

Guy ropes shall be of gaivanized steel wire, witb steel cores,complying witb IS01R346. The wires should have a minimum tensilestrengtb of 1450 N/mm2, A completed rape should be evenly laid andfree from loose wires, disturbed strands or otber irregularities andshould remain in this condition when properly unwound fromtbe reelor coiI.Fittings sbould be of galvanized steeI. 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 tbeir minimumbreaking strengtb exceeds 3 X maximum calcuiated load, due to tbesum of pretension, design wind and chimney expansion.

After erection and while tbe chimney is cold, tbe guy ropes should bepretensioned sO as to minimise top deflection of tbe chimney. Thepretension may be measured by tbe use of a suitable instrurnent andshould be not less tban 15% nor more than 30% of tbe calculated

maximum tension due to design wind under the hot condition.

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

page 34

APPENDIX No.4 - ACCESSLADDERS

A4.1. Generai

This section specifies the requirements for steelladders, permanentlyfixed to steel chimneys, to provide means of accesso They are to befixed lo the chimney in a continuous verticallength interspersed withIandings an<lJor rest platforms as required.

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

An altemative to the caged ladder system is an open Iadder with aproprietary safety system, either running beside the ladder orcentraily between the stringers.

Rest platforms as described in A5.8 shmùd stili be incorporated at therelevant levels.

A4.2. Definitions

For the purpose 01' this appendix the following definitions shall apply:

I) Stringers. The side members of the Iadder to which the rungsare fitled.

2) Safely hoop. A bar fixed to Ihe slringers to enclose the path ofpersons climbing the ladder, to prevent them falling outwards.

3) ResI platform. A platfonn provided to enable the personclimbing the ladder lo rest.

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

A4.3. Materials

The materiais 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 clùmneytop which, in the case of chimneys carrying flue gas with high S02/S03content, should be of high molybdenum stainless steel (ASTM 316L orsimilar) or should be protected by an acid-resistant coating.

A4.4. Finish

AH burrs, weld-ftash, sharp edges and other imperfections likely tocause injury to the hands of a person using the Iadder, shall beremoved and made smoothbefore the finishing treatment.

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

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

A4.5. Stringers

Stringers shall be of flat bar of minimum elimensions 65 x IOmm.The stringers shaU be parallel anel straight throughout the rungportion anel the distance between the stringers measurcd from theillside faces shall not be less than 300mm and noI more than 450mm.

Thc stringers shal] extend upwards, to a height of not less than1075mm above the upper platform and shall be seclIrely fastened attheir extrcmities. Such extension of the stringers shall noI encroachon thc c1ear width of the platform passageway.

Where, in order to step from the laclder into a landing platfonn, it isnecessary to pass between the extended portion of the stringers. theseshall be opened out from platform leve! to provide a clear width of600-675mm belween them at handraillevel.

Where access to an upper platform ìs from the side or front of a

ladder, the ladder itself shail be extended above thc plalform level fara ctistance of not less than 1075mm or equivalent handholds shallbe provided.


Stringers should, if possibIe, re in a continuous Jength. but where theyare in more than one length they shall be joined by lishplates on theinside.<;of the stringers, either welded or bolted. .lf bolts afe used theyshal! be countersunk 011 the stringer and not Iess than 12rom in diameter.There shail be not less than two bolts on each side of the joint.

A4.6. Rungs

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

The rungs in a ladder or ftight of ladders,shall be uniformly spacedtbrollghout at centres of 225mm minimum to 300mm maximllm. Thetop rung sha11 be on the same levei as the platform which shall beextended. if necessary, 10 limit,.tO not.IDore than 75mm, thegapbetween the rung and platform. Alternatively the platform may beextended to replace the top rung.

Rungs shall be fitted into holes dòlled in the stringers and secured bywe!ding. Rungs shaU be welded to the stringers with or withoutshouldering. Holes in the stringers shall be drilled to give a 1romcIearance and where shouldered rungs are used, holes shail becountersunk 1.5mm to c!ear the root radius (see figure A4.1).

l mm CLEARANCE 1 mm CLEARANCEHOLE HOLE

Fig, A4,1 - Attachment of ladder rungs to stringers

A4.7. Safety hoops

lf safety hoops are fittcd to thc ladder, the following provisionsshall apply.

AUladders òsing 2300mm or more from a lower platform or groundlevel to the top rung sha11be fitted with safcty hoops, the spacing ofwhich shail be uniform and at intervais not exceedillg IOOOmmmeasured along the stringer. The ]owernlost hoop shall be fitted to thestringers at a height of 2300 - O+ 75mm from a Iower platfann orgraund 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 ofnOlless than 1075mm above the level of this platform.

A4. 7.1. Size of hoops

Circular pattem. 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 11gureA4.2).

Rectangular pattem. The width across the hoop shal1 be 690 to760mm. The distance from the centre line of stringers to the insidc ofthe back of the hoop, measured at right angles to the stringers, sha11be 690 lO 760mm. The ractius of the comers shal1 be noI less !han150mm (sce figure A4.2).

The minimum dimensions of the hoop and strap mateIial shall be50 X 8mm. AI Ieasl three vertical straps shall be fitted intemally tobrace the hoops; one of these straps sha11be al the centre back of thehoop, and thc others spaced evenly betweell the centre back of thehoop and the ladder stringers.

CICIND Model Code - Commentaries and Appendices page 35

A4.10. Access hooks

It is recommended that the screwed type of hook be used on insulatedchimneys Le., 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 faee of tbe socket tominimise heat conduction between the face of the socket and the

surface of the aluminium cladding.

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.

A4.10.3. Materials

~-~~---\mm .

Hooks shall be made from steel complying with the requirements ofEuronofl11 25-72. In a normalised condition tbe stecl shall have a

minimum tensile strength of 430N/111I112 and a muximum tensilestrength of 500N/mm2• The sockets shal1 be made from round steelbar complying with the requirements of Euronofl11 25-72.

6 mm FILlE'T WELD

~

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

A pulley is sometirnes rigged from the top of a steeplejacks' ladderfor the purpose of lifting smallloads for maintenance of the clùmney.It is important that such loadsshall be kept as Iight as possible and inno circumstance should any single load exceed 50kg. lf a hook isused directly forlifting purposes, the weights of tbe lifting device

suspended from it and of the load to be Iifted shouId together notexceed 50kg.

690mm MIN.760 mm MAX./

HOOP5 ANO HOOPSTRINGER5 FROMso mm x B mm MINIMUM

300mmM1N.4S0mmMAX,

Fig. A4.2 - Ladder hoops

~I({--!; E -oc",on•... '"

A4.9. Attachment to chimney

The ladder shall be vertical except where it follows tbe slope of acane section.

Stringers shall be attached to lhe chimney by suitable connectionswhich shall be lirmly attached to tbe stringers and the chimney andbe sufficiently close together to make tbe ladder rigid throughollt itslength. The connections shall be of sufficiellt length to gìve ac1earance of not less than 200111111behind the rungs. Suitableprovision shall be made at fixing points for any differenlial expansion(except at platforms and landings) .

A4.8. Rest platforms and landings

When required, l'est platfofl11s shall be provided at intervals òf notgreater than 20m. Landing places, other than working plalt'orms,which are provided specifìcally at l'est plalforms shall re al least825111msquare and shall have a gllardrail at a heìght of 1075mmabove the platform level with an intermediate rail and toeboards.

When reqllired, landings shall be provided al suitable levels to provideaccess to sampling points etc. These landings are to be adeqllatelysupported fTOmthe ehimney shell and shall have a minimum widtb of825mm. They are to be fìtted with a guardrail at1075mm above theplatform level, witb an intermediate rail and toeboards.

Hoops and straps shall be fixed by bolting or welding. If bolts areused they shall be countersunk, inserted fr0111the inside of the strapor hoop and shal1 he not less than 12mm diameter. The assembly ofhoops and straps shall be suitably braced unless secured to thestringers by double bolting, or welding.

Fig_A4.4 - Screwed ladder hooks and bosses

Fig_A4.3 - Welded ladder hooksA4.10.1. Generai

This section specifies requirements for hooks which are intended topro vide means 01' access l'or inspection and maintenance only bysteeplejacks and members of similar trades who normally tit lbeirown 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 verticalline on the exterior of the structure.

The lise of access hooks inside chimneys exposed to corrosive gasesis not recollllllended. Tbe first hook should be 1.2m + 50 - Ommabove access leve!.

The hooks should be spaced at multiples of l.5m vertical eentres witba loeal tolerance of +50mm which will accommodate the majOlity orthe various length.s of ladders used by steeplejacks.

IF ALUMINIUM CLAOOING 15USED AN INSULATING WASHER5HALL BE PLACEO BETWEENFACE OF BOSSANO THEALUMINIUM.

EXTERNAL 80SS INTERNAL BOSS

page 36

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 8uch a maoner that the mit:roscopic flowlines follow tbe body outline of tbe hook. The whole ofthe shank shallbe forged in one piece, integraI with the hooIe. The hooks shall benormalised after tbe completioo of all forging operations by heatingthem uniformly in a furnace until the whole of tbc metal has attaioeda temperature between 880°C and 910°C and tben cooled io still air.

A4.10.6. Method of fixing

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


be carried out by suspending [rom the hook a mass 01' 200kg when nol'racture, crack or visible deformation shall oecur. Tbe socket of the

screwed-type hook shall be lìxed to the chimoey by means of a finetweld of 6mm leg size for the whole of the perephery of tbe socket.

Por new chimneys t.he welding should be carried out in tbefabtication shop.

lt is normaI practice for the steeplejack firm to supply tbe screwedtype hooks far tbeir own use when they ladder tbe chimney.

Date post:	11-Jan-2016
Category:	Documents
Upload:	tommaso
View:	98 times
Download:	16 times

cicind part 1

Documents