design of brick fin walls in tall single-storey buildings 0690

8/8/2019 Design of Brick Fin Walls in Tall Single-storey Buildings 0690

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W .G. CURTIN ME ng FlC E FIStructE MConsEG. SHAW CEng FISt ru ctE MCo nsEJ.K. BECK C En g MIStructEW.A. BRAYB Eng CE ng MICE MIStructE

: (21) : F

June 198 0

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DESIGN or BRICK FINWALLS IN TALL SINGLE-STOREY BUILDINGS


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Ed ited by Robert G. D. Bro wn MA CE ng M ICE

Design ofb .. ick fin wallsinlallsingle-slo .. eybuildingsW.G . CURTIN MEng FICE FIStructE MConsEG. SHAW CEng FIStructE MConsE1.K. BECK CEng MIStructEW.A. BRAY BEng CEng MICE MIStructE

The Brick Development Association1


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CON TENTS

SECT ION I : INTROD UCTIO N AN DG ENERAL D ESI G NIntrodu ctionCo m pa r ison o f fin walls a nd di aphr agm wa lls

General designStructural principle sArchitectura l t reatmentR oo fOpenings in wa llsJ oint sT hermal insulationSound in su lationDa mp proo f cou rse sFoundati onsTemporary propp ingStructur al de sign meth odEx perience and performan ce of fin wall sFurther a pplicationsFuture progre ss

SECT I ON 2: D ESI G N PR I N CIP L ESC ritical d esign c onditionInter act io n be tween l eave sEffecti ve s ectionDesign considerat ionsUpliftCr itical s tressesDifferential stiffnessDesign bendin g moment sD irec tion s of bendingT rial a nd er ror d esignSpacing of fin s

Design o f brick fin walls ill roll sill/de -storey buildings

SE C TION 3 : D ESI GN EXAM PLE5 Sy mbols5 Design problem6 Design a pproach

6 Int r oduction6 C harac teristic l oads7 Desig n loads9 Design ca ses

II Deflec tion of roo f wind girder12 Effec tive flange w idth12 S pacing of fi ns12 Tr ial section12 Proppe d canti lever acti on12 Stabil ity moment o f re sistance12 All owable flexural co mpressive stresses12 Co mparison o f m oments13 Bending m oment dia grams13 Stre sses at le vel o f m aximum wall m oment13 De sign flexural s trength

Fin s with deflected roo f pr op16 S uggested d esign procedure1616 SECT ION 4 : R EFERENCES161617

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171717

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INTRODUCTION ANDGENERAL DESIGN

INTROD UCTION. Th e fin wa ll, in whic h d eep pie rs o r fins a re used

to suppo rt conv entional cavi ty wa lls h as beenshown b y so me co nsiderab le ex perience t o beparticularly s uited , as are di a phra gm walls, fortall single -storey wi de spa n b uild ings s uc h a sasse mbly a nd spor ts h alls , gy mnasiums, swi m mingpools, i ndust rial buildi ngs, e tc.

Fi n wall buildin gs a re a not he r t yp e of ma son rystr uc ture th at obvia te t he nee d for s tee l o rreinfo rced concr ete c olumns , ex ternal cl addingan d int ernal lini ng.

I

co ntrac to r is ne ede d t o for m a du rable ,maintenance free, attractive a nd economica l wall.

Th e fin wa ll was developed from t he di a ph r agmwall (l ) when an a rchitect wishe d to ma intain acavi ty a nd have g reater sco pe for a rchitecturalexpres sion . The wid th o f th e cav ity may beselected w ithin the lim itati o ns o f the co de.BS 5628 ' ' ' , to a llo w f or th e use o f cavi ty insulatio n.

2norm al w alltiesi n

acco rdance w ith B 55628 - - -~ ~Th e bri ckw or k T section f o rm ed ( Fig 2) by t hefin a nd o uter leaf of th e cavity wall p rovide s themain s truc tura l member , an d t he i nne r leaf is theinternal lining. Th u s, on ly o ne ma teria l used byone tr ade und er th e d irec t con t ro l of the ge ne ra l

3

-

-flnw.1I

/

/.....

r o o f _

--lephregm ". llounda


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And , if necessary, the thickness of the o uter leafmay be increased to allow wider spacing of fins.

(6) They have gr eater p otential for use in multistorey struct ures.

Compar ison of fin wall s and diaphragm wall sAdvantag es of fi n wall s o ver diaphra gm walls:(I) Less roof area is required (see Fig 3)(2) Less foundation area is required (see Fig 3)(3) There is generally less cutting of bricks forbonding

(4) There is obvious scope for architectural effect(5) They are easier to post-tension when required

typical simp'" building pl anopening

Di sadvantages o f fi n wall s compared K'ithdiaphra gm wall s:(I ) A larger site area is req uired (see Fig 3) , sothey may not b e viabl e for restr icted sites(2) Th e non-symmetrical section does not have asimilar resistance to ben ding in bo th d irections

an d a slightly greater cross-sectional a rea ofbrickwork is sometimes required(3) The cavity is not wide enough to accommoda teservices(4) More vertical plumbing lines (ie. corners offins and fin wall connec tions) a re re quired, so thelabour costs of the wall ten d to be higher.

GENE RAL DESIG NStructural principlesIn the current co des of pra ctice, pi e rsa rerecognise d as a means of increasing the verticalload-carrying ca pacity o f plane wa lls. Th e effect ofpiers is presente d as a n ap parent increase i n th eeffective thickness of the pla ne wall. T his, ofcourse , will give a lowe r slenderness r at io an dthus the wa ll ca n be a llowed to car r y a largervertical load. In C P I l l ' " no specific guidance

~ - - A W Phape canvary (hollow fin)' I

pacing can vary

I"IT j ! - - / " - lproportionscan vary

5

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 Z I 7 7 7 ?

mkiftn .... lI .

r6 -

I

'D-t - O

c -O D0

Dtapered fln s tapped fln bevelled fin PIIr11ne' fln porthole"n pertonted

tapered fin

6


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7be velled fins

roof le vel

ele vation ons tepped linwa ll a rches

roo '

bric: karch

elevationexample of treatment at v. .

co merf in ma y benecessary toresistarch thrust _ _ J HCtlon

was given for the design of walls with piers underlateral load . as 5628 gives some guidance.

In the concept of the fin wall given in this guide ,the whole fin plus the wall is u sed in determiningthe slenderness rati o although , as with di aphragmwalls , for tall single- storey buildings, verticalloading is unlikely to be criti cal. The criticalloading ca se for de signing such w alls willgenerally be that of wind load s and for thi s aT section is con sidered.

Architectural treatmentA typical simple plan layout for a rectangularbuilding is shown in Fig. 4.

Examples of the variation s which can be made to

thi s plan are that the sizes and spacing of the fin s,the details at the corne rs and even the ba sic wallprofile can be varied in many diff erent ways .Fig 5 show s some example s. The variations canhave structural implications, and the selectedprofile mu st be checked to suit the structuralneed s. .

On elevation , the fin can be t apered, bevelled,profiled , etc . and some t ypical shape s a re shownin Fig 6 .

The t reatment at the eaves of the roof , thevariet y and mi xtures of brick s, and the type of fingive s the construction many po ssible ae stheticeffect s (see Fig 7). Thu s, the de signer ha s greatscope for architectural treatment.

Some typical example s of fin w all con structionare shown in the illu strations to this section .

When using a variety of brick s (for example , adifferent facing on the fins and o uter l eaf fromtho se used in the inner leafof the cavity ), caremust be taken to see th at the bricks are

compatible , particularl y with reg ard to th ermaland moisture movement to prevent crit icaldifferential movement occurring which ma ydamage or affect the structural behaviou r of thewall. The de sign calculations too , under suchDesign o f brick fin wa lls in tall s ingle-storey buildings

Ahop e Two ex a mple s of fi ns s toppe d of f be low the r oo f lin e ,

7


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...----.!----8

Below , left lind right Fi ns flush with th e roo/ line. No tea ccess to d ownpipe located inside thefin .Centu left S u ppedfins s topped of f be low th e roof lin t'.Centu right Ra kedfins s toppe d of f be low th e roof line .Foot of page,lelt Raked fins c arried a bove t he r oof li ne.Foot o/page , right R oo/ brought to o utside a/fi ns .


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re action res istedonend wall

wind suction- - - - - = = ~ _

reactlc n resistedon e nd wall -..II

8

wind pr essure= ~ tt

wind direction

wind pressure

~ wind d irectionin

I too ies r o o ,~ roo pr opsth is s ide this side

-- roof be am I--lloorslab

lin

plan

wind suctio n - - . .

section A-A

circum stances, must take into account anydifferenc e in the strength of the brick s.

9 plan on roof girderSmainroofbea ms

hori zonta l bracingto g irders

RoofCareful considerati on o f th e st ructural beh avi ou rof th e ro of o f the bu ilding is m ost imp ortant forthe full econ omy to be achieved in the overallbu ilding cost. As with the di aphragm, to o btaingre atest economy, th e roo f of th e fin wall bu ildingshould be u sed as a hor izontal plate to p rop andtie the top of the wall s, and to tr ansfer th e

resulting horizontal reactions to t he gables orother tr ansverse walls o f th e buildin g (see Fig 8).Assuming s uch tran sverse wall s to be spacedwithin a reasonable s pa n f or th e roof plat e.

In Fig 8 , th e plan and section indicate th estructural action of the ro of plate in re sisting a ndtransferring wind force s.

Where the architectural de sign requires that thetop s of walls are not pr otected from f alling rai n,part icular car e mu st be given t o the ch oice o f asu itable brick an d mort ar. C P 121 ' 41 a nd BDA

literature give guid ance o n the selection ofsuit able brick s a nd m ortar . To en sure dur ability,where or dinary qu ality bric ks a re to be used inexp osed s ituations, the m anuf acturers' ad viceshould be sought.Design of brick fin walls in tall single -storey buildings

In some ca ses the ro of deckin g (so metimes actin gin con junction with a co ncrete ri ng beam ) is allthat i s requir ed to give t he nece ssary pl ate actio n

but , in other c ases, horizontal roo f br acing isnece ssary (see Fig 9 ).

In ca ses wher e only th e ring beam and d eckingform the plate, it i s nece ssary to make sure that

9


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10

' - - - - roof beam

---- - - - pa dstone

fin - , . . _ ,/ l r--- +- tt- + - - anchol rod filling

through pecstcoe& into roofbeam

~ ! k =...... +* -+ -- anchorrod wrthend ptate 01 n ut

MCt lon through fin

th e deck and its fixing are stiff eno ugh and st ro ngenough to control th e movement and f orcesinvolved. When a c oncrete rin g beam is used, itsho uld be de signed to help tr ansfer the wind f orces10

A h o. ~ Fi n wall co nstruction of fers a wid e choice o f roofstructures, mater ials and me thod s .In co njunction with a suitable cappin g beam , th e r oof isused as a },or ;::O'l101 plat e mem be r to pr op a nd t ie th e tops ofthe w alls , an d 1 0 tran sfer th e horizon tal r eac tions dill'10 wind 10 th e g able or transve r se wa lls .

from the to ps of the w alls o n which th e wind i sac ting t o th e tra nsve rse w alls o f th e building.

If no cap p ing be am is used, a nd the ro of de adload is s mall , th e main roo f be am o ften require sto be strapped d own and thi s can be done u singrods from the pad stone taken down t o a s uitablelevel to e nsure su ffi cient de ad load to r esist uplift(see F ig 10).

The ro of deckin g can be c onstr ucted from ava riety o f materi als and s upported in many w ays .

The fin al choice depending on t he criteria re latedto the particular proper tie s requi red f or thebuilding and the economy of con struct ion.

When c onsidering the co st of the d ecking , it is


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nec essary to tak e a ccount of its a bility to a ct a s aro of plate. All nec essary br acing s hould beincluded t o give total ro of costs , in o rder th at .theovera ll economy o f th e b uildin g can be examined,rather than individual item s.

Care mu st be tak en to see th at a ll c onnectionstr ansferring wind forces, ie, deck to b ea m , beamto wall, deck to wall, beam to brac e, wall tobra ce, etc a re adequat ely de signed for th e forcesinvolved. In cases wh ere large uplift f orces f romthe ro of require a s ubstant ial an chorage , aconcrete capping beam might be pr ovided towhi ch the main ro of beam s can be fi xed. T hecappin g beam can b e used as a main boom forthe h orizontal br acing which m ay be re quired fo rthe roof 's pl ate ac tion.

In cas es where a concrete cap ping beam i s to be

used , it is usually better co nstructed in pr ecastsections of full ba y length s joi nted with a jo intdetail ca pable of tran sferring th e forces in t hislocation. Th e use o f pre cast concre te capp ingbeam s avoids the pr oblems relating to supportingsh utte ring at a high level and pr eventing gr outruns over facing brickw ork, parti cularly wherethe beam is wider than the c avity wall (see Fig 11 )

Op enings in wallsLarge o penings required in th e main walls cansometimes crea te high local loa ding co nditionsfrom wind a nd vertica l loads, particularly aro undbeam bearings. In such l ocations a beam o r li ntelis normally used to span ov er the opening, a nd anadj ustment to e ither the fin spacing o r fin cr osssection c an usua lly be made to ca rry th eincre ased loads i nvolved (see F ig 12).

~ =~ ~~" , ~" , ~t t i ? ?

, , , ~" ,~? I ? ityp lCIIIdoor opening Ioc.tlona

highloa ds Involved spa cing adju sted

high loads involved fin siz e ad justed

no ch ange needed brick s trength adeq uate

12

L eft F loor 10 ce iling opening , 5 ' Ala ry's Colle ge, Cros by .Archi tec ts : Weightm an & Bu llen. S tructural engi neers:W. G. C urtin & Partners .

Abov e Double fi n mo vement j oint .11

roof bea m

capping beam

.J4- -- cavitywallf in - - - - 1~

11"'0 ' - +j== = =:T F ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

Design of brick fill walls ill loll singl e-storey buildings


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14local extension 01 tin

IO U n d a l io n b e l o w l i n~ r-;::-V11---....... ~ I ~ r strip fooling

cav01y wa ll - , ~ /

FoundationsIn m ost normal g ro und co nditions, t hefoundati ons ca n consis t o f strip foo ting s underth e ma in perimeter wall s with local exte nsionund er th e fin location s (see Fig 14) .

JointsMo vement join ts for s hrinkage a nd exp ansion a rerequir ed a t th e a ppropriate ce ntres r elated to th etype of bric ks a nd m or t ar bein g used, th ediff erential tempe ratur es ex pected, an d inacco rda nce wi t h th e c urrent r ec om m endations forbrickwork in C P 121.

Th e jo ints ca n eas ily be acc om m odated by adouble fi n. o ne o n eac h side of the jo int (s ee F ig

13). I t is imp ort ant to eo nsider ea refully the t ypeof joi nt filler t o be used , an d to prov ide adequatesupervision d uring cons truction t o ens ure th atther e are n o restrict ions t o m ovement.

2 ? 2 2 I

13

co n" .. joinl - - .~ ~~r : I,n/1 /

The rmal in sulationM ost buildin gs constructed to dat e o n th e fin w allprinciple h ave not b een subject t o stat uto ryrequirements for th erm al insulati on. S ince Jun e1979, howe ver. simila r bui ld ings a re req uired to b ein acco rda nce with P ar t FF o f th e Buildin gRe gulat ion s. T he necess ar y insulation will b e th esa me a s a ny o ther c avity wa ll, a nd c an eas ily b ea chieved b y filling th e cavity o r partially fillin gthe cavit y with a s uitable in sulan t. Th e finalch oice o f in sulant will d epend o n th e s ituationa nd th e d esign of th e buil ding . See BDA ' EnergyC onservation , Therm al In sulation o f Bri ckBuild ings com plying w ith Par t FF of th e Build ingRegul ations '!", a nd IJDA D esign Not e No 2' ' ' .

So und in sulationThere a re n o s tatutory requirements. a t pr esent .for so u nd in sulation in th e ty pes o f buildin g wh erefin wall co nstruction h as been o r is lik ely to beused. H owever, th e fin wall itself will substantiallyreduce th e tr ansmission of noise from within o rwith out th e build ing a nd will p er f or m at least a swell a s a bri ckwork par t y wall in h ousing.

Damp pr oof co ursesIn a ddit ion t o th e norm al imper vious requ irementsfor the h orizontal dpc, it is imp or t an t to se lect amaterial which h as th e necessary re sist anc e tosliding a nd squeezing o ut und er horizontal an dvertical l oadin gs respectively . Sh ould it beconsidered desirable to tr ansfer flexu ral tensionto th e found ation , o r should pre stressingtechnique s be employe e, consideration s hould b eg iven t o th e use of a n engineering bri ck o r slatedp c to BS 743 (7). It shou ld b e noted, however,th at it is not recommended that the fl exural

ten sile re sistance a t dp c level be exploited in th eca se o f th e pr o pped fin unle ss a mu ch m or edetai led a nalysis of the defl ection of the pr op isconsidered (se e Secti on 2, Design bendingmom en t s) .12

Thi s ty pe of footing is usually a deq ua te for mostlocations but , o f cour se, th e fo un dation s for eachindi vidual building mu st be d etermined fromco nsideration of th e parti cular site an d gro undco ndition s.

Te mporary proppingLik e m ost o ther wall s, th e fi n wa ll is in a c riticalstate during er ection a nd p rior to t he r o of be ingco nstructed an d fixed. During t hi s period,th eref or e, the contracto r must take the no rmaltemp orar y pr ecautions s uc h a s propping the wa llswith th e b ricklayers' s caffo ld ing o r ot her mean sto en sure th at the w a lls remain unda maged .

St ructural d esign methodTh e main c alculations involved in the de sig n offin w alls ar e for the critica l con dit ion s o fcom b ined d ead a nd wind l oa d ing. T he se tak eint o a ccount the m aximum uplift an d m aximumbending flexura l stresses.

Th e flexur al compre ssive s tresses invo lved wh encombined dead , superi m posed, an d w ind loa dinga re ap p lied can be cri tica l, pa r ticularly when th efin i s bending about it s weake r axi s a nd th estresses a t the e xtreme end of th e fin ar econsidered. Th e ch oice o f br ick an d the mort armu st, ther efore, tak e into acco unt th e ten sile a ndcompr essive s trengt h required, an d th e du rabilityneeded for the in dividual b uilding . T he w a lls a rea ssumed to act a s ' proppe d ' ca ntilever s, wi t h th ero of a cting a s the pro p a nd t ra n sferring thepropp ing forces to the t ran sverse walls ( see Fig 8),an d ' fixed ' at the b ase by virtue of the irse lf-weight. Th e plat e action of the r oof wi ll a llowso me sma ll m ovemen t at th e p rop locat ion , an dth e stiffne ss of th e wa ll wi ll va ry du e to th e effectso f th e g ravitational loads a nd t he loss of flexu ra lten sile re sistance at dpc level.

Within the height o f t he wa ll, there ar e twolocations of c ritical bend ing moments , the se occurat dp c level, locati o n A , an d pa r t way betweendp c a nd roo f level a t location B (see Fig 15) . Du eto the un symmetr ical nat ure of the fi n, it is

imp or t an t to con sider bo th d irections o f wi ndloading in or de r to de termine the critical stress .

Th e calculati ons a re ca rried o ut on a trial an derro r ba sis by adoptin g a tria l section a nd then


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15 ~ p " " ,D...... 8MindV locat,on'Bressure

4

/... dpc" - - - 8M @ ope

iever ioc auc n 'A"

wind load in g be nding mom en t

- ~ I ie Jind 8 Msuction \ loca tion '8- r.doc

""" '- S M @ dPCiev euoc auon "A"

w ind lo ading bendi ng mo ment

checking the stress cond iti on s . Fo r detai leddi scussion an d a de sign ex ample see S ections2 an d 3.

EXPE R IENCE AN D P ER F OR MANCE OFF IN WALLSAt lea st a d ozen fin wall buildin gs hav e beencom p leted an d m ore a re und er construction o r inth e design s tage. Th e buildin gs a lready c ompletedare , o n th e wh ole , s ituated o n ex posed sites in theNort h We st o f England .

Th ose con structed be f or e 1976 h a ve a lreadysurvived :(a) the w orst gal es on rec or d in J an u ar y 1976(b) the hotte st summer o n rec or d(c) t he worst dr ou ght

(d) the wette st a utumn

Th ese, an d th e m ore recent example s, ha ve al sosurvived one of th e most severe winter s thi scentury .

Dvstvn a/brick Jill wulls in tall single-storey buildil1gs

Th e building s hav e a ll per for m ed s uccessfully,a nd n o problem s have devel op ed as a result ofthe method of construction. In p articular th eyhave , both int ernally a nd exte rna lly , with stoodthe hard usage as soc iated with spo rt s hallswith out requirin g maintenanc e.

Further applicationsIn addit ion t o the u se o f fin w alls for newb uildings , the y ha ve a lso b een fo und ve ry u sefulfor s trengthening exis ting bu ilding s. In o neparticular case the re ar wall o f a g randstand,which w as sh owing sign s o f b ecoming unstable ,wa s s trengthened by bondin g int o it , a tpr edetermined centres, a serie s of bri ck fin sd esigned to re sist th e exce ssiv e loading likely tobe applied .

A further appl i cati on wa s the u se o f pos tten sioned fin s to strengthen a retaining wall to a nexisting ba sem ent, wh ere a c hange o f u se res ultedin increa sed lateral loading whi ch mad e it bu lge

an d crack an d become un stable. Th e postten sioned brick fin s proved ea sy to construct a ndeconomical wh en co mpared w ith altern ativefor m s o f construction.

F uture pr ogressA lth ough the fin wall w as developed ma inly foruse in tall, single-storey wid e-sp an build ings, itha s becom e a pparent t o th e a utho rs th at it has amu ch wider a pplicat ion. Fo r example, thepost-tensioned fi n wall used a s a retaining wa ll isattract ive both economically a nd vi sually a nd ha sg rea t p otential for the future.

Th e use o f fin w alls in c onjun ction with spinewalls ca n result in multistorey build ings o funre stricted floor area s for o ffi ce building s,ho spital ward bl ocks an d o ther buildin g for m swhi ch cann ot tolerate th e restricti on s of cr osswallor cel lular c onstruction.

Below a nd to p l ef t overleaf Rudheath County S econdarySchoo l. Ar chite cts : w ilson & Wom ersley , St ructuralenginee rs : W . G . C urti" & Part ner s .


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DESIGN PRINCIPLES

C RIT ICAL DESIGN CONDITIO NThe ca lculations whi ch follow are base d o nreasonable ass umptions, so me of which a re as yetun supported by research. However, s tructure s

wh ich have been designed-in acco rdance wit hth ese the orie s a nd ass umptions h ave per f ormed ,and are p erforming, s uccessfully.

For tall s ingle-storey bu ilding s, the c ritical d esigncondition is rarely gove rned b y ax ial co mpres sivestresse s but b y th e wa ll's re sis tance to lateralforce s from wind press ures . Th e flexural ten silestres ses ge nerally go vern the d esign a nd it is,ther efore, ben eficial to e ither r educe th e flexuraltensile s tresses b y re ducing th e maximum a ppliedbending m oment , o r b y increasing th e sect ionmodulus an d /or increase the co mpre ssive stresse s.

Thi s can be achie ved by:(a) u sing th e roof as a plat e ( see F ig 8) to prop th ewall , thu s reducin g th e bendi ng moment wh encompared w ith a free standing canti lever, a nd ;(b) using a T section - fin wall ;(c) using po st-tensioning to increase thecompressive s tresses a nd t o decrease sect io n sizes.

The de sign o f p ost-tensioned fin w alls will b einclud ed in a future publ ication and i s notco vered in thi s design g uide.

Inter action between leavesA s sh own in Fi g 16, th e fins ar e b onded to o ne o ft he leave s of a cavit y wa ll and c onsidered as aT sect ion combining th e bonded leaf with thefin . The o ther leaf i s con sidere d a s a secondarymember , the cavit y tie s being assumed to beunable to tran smit s ignificant vertical shearforces but capab le of transmitting horizontalforce s acro ss th e cavi ty wid th . The type of tiea ssume d for thi s condition is the gal vanisedve rtica l twist t ie to BS 1243 (9' , and under mostco nditions this is adequate. How ever, thed esigner sho u ld sa tisfy him self that the ties a resuitable for the exp os ure cond iti ons in whichthey a re emplo yed, and th at they can tr an sfer th edesign forc es a dequately.

16

16

Io ~ I

1" ' JB

J1~ ne s a ssumed 10a pphed loadi ng - - + , / . hav e zer o s hear~ . re sistance--02 01

Effecth 'e sect ionBecause of the unsymme trical shape o f th emember , th e g eometrical properties of theeffective sections, when co mbined ben ding an ddirect f orces are con sidere d , can va ry greatlyunder ch anges i n loading partic ularly i f a' cracked sec tion' i s being ana lysed . I t is, th erefore,imp ortant when consi dering the sta b ility m omento f re sistance to a lso con sider ca refully th eeffective s ection bein g s tresse d an d th e effec ts

of a ny cr acked portion o n the ge neralpe rform ance o f the wall. The flexural stres sesmu st be kept within tho se recom mended in th eCode of Practice bu t, a t dpc level, th e majority ofdamp pro o f co urses m ust be cons idered to haveno r esistance to flexura l stre ss, a nd a t th is l evel a'cracked section' is often as sumed. T he m omentof resistance at th is level becomes t he gravitationalmoment o f re sistance fo r the worst load ingcombin ation, which is generally t ha t o f d ead plu swind loading.

DESI G N C ON S IDERATI ON STh e variou s loading comb inations a nd the ir e ffecto n the stress cond itions m ust b e ca refullyconsidered, there fore, one of the first ca lculationsis that o f a ssessing the loads:

i1I '

t


18/34

17

p / unitarea

H

~a w ro l C~ H

bu t varie s

lo ading denec teds hape

s tability m omentof reals tance

free mo mentdu e to wi nd

propped cantil ever8M d ue to w ind

8M d iagram f orcon stant s tlftness

s tabilityresistancemom ent

. 1

a s sumed 8M d iagramfo r de sign

proppedcantilever

max positi ve 8M notnecessarily @ ~ H

load ingdi agram

~ p H 2128

free m omenl pH "8

. 1~8

I.

18

Ha ving o btained th e loading condition, it isimp ort ant befor e progres sing with th e d esign tomak e sure th at th e a ss umed beha viou r o f thestru cture i s und er stood . In th e ca se o f the fin w all

be ing used o n tall s ingle- sto rey bu ilding s. it isa ssumed th at th e wa ll a ct s as a proppedcantilev er (see Fig 8 & Fig 17) wh ere the 'fi xedend m om ent' is that due t o th e vertical l oad s, a ndis kn own as the stability m om ent.

(I) calculate th e positive a nd ne gative windpre ssur e

(2) calcul ate the de ad, imp osed an d wind l oadingo n the roo f and w alls.

UpliftIt is most im por t an t to take ac count of roo fupl ift fo rce s when co nside r ing th e wo rst de signco nditi o n . It is a lso imp or t an t to note tha t th ec ritical sec tio n a t o r n ear th e ba se o f th e wall i susu ally a t th e location o f th e dpc wh ere littl e o rno ten sion is permi ssible, d epending o n t hechosen m embrane.

load will n ot decrease but sligh t cra cking a ndrot ation o f the b a se o f the w all will oc cur a ndproduce incr ea sed bendin g at the upp er loca t ion .It is, th erefore , m ore rea list ic in the d esign to fir stcalcul ate th e sta bility m oment at the ba se d pclevel t aking acco unt of the appr opriate partialsa fe ty factor for loa d s

Dete rmine m aximum c ritical st ressesIt is nece ssary i n th e ca lculatio n s to determine th em aximum crit ical force s, m oment s a nd st ress es inthe wall , which, f or a norm al p ropped c anti lever,occur a t o r ne ar th e ba se o f th e wa ll a nd at apoint appr oximately ~ H fr om t he t op o f the wall.H owever, th e pr opp ed fi n wall will va ry fr om thi sa s explained below.

D ifferential s tiffness w ithin height of wallFor a unif ormly d istrib uted l oad o n a proppedcant ilever of co nstant st iffn ess with a rigid prop ,th e bendin g m oment wou ld be a s shown in Fi g 18 .

Th e design load fre e bendin g m oment c an the n besuperimpo sed up on the stability m oment diagram(see F ig 17). Th e positi on and m agni tude o f themaximum p ositi ve bend ing can then be d etermined a nd the se stress c ondi tion s checked .

C heek b oth dir ections o f bendingI t is imp ortant , wh en considering the bend ingm oments o n th e fin, t o check for the b endingm oment s in each d irection at each lev el, since thecr itical s t ress conditions will not nece ssari lyresult fr om the same direction of ap p lied ben dingm oment (see Fig 19).

However , for th e brick fin w all s hown, so medefl ection will occ ur a t the prop loca tio n, and thewall s trength will vary within th e height o f thewall du e to th e va riat io n at e ach leve l in th ea xial load .

T RIAL AN D ERRO R D ESIGNIt is apparent from the sugges ted proce dure tha tthe d esign mu st co mm ence o n a t rial and e rrorba sis, fir st ch oo sin g a rea sonable s ection an d thenchecking the stres s condi tion s which exi st.

It would, therefore, be merely coincidence if thestability m oment at th e bas e wa s exactl y equ al to

p ~2

whi ch is the c ond ition for the straightforward

pr opped cantilever .

Design bending moment sA s th e applied b ending mom en t is incre ased, thestability m oment a t the b ase for t he sa me a xia lDesign of brick fin walls ill tall single-storey buildings

S pacing of fin sT he choice of a suitable section m ust take intoacc ou nt the c avity wa ll's a bility to act suitab lywith the fin to both tran sfer wind forces to theo verall section a nd to preven t bu ckling of theflan ge of the T sect io n. T hi s invol ves ch oosi ng asuitab le spacing for the fin to contro l bot h th esecond iti on s a nd to t ake in to ac co unt economic

17


19/34

1

1 .1 IC - ~

effectiv e flang e

w ind loa ding

! !

I

II...

dellect ad Shape ! : 7

"n ---tUca.~wa "J u-. - Ion

20

22

critical 8M lor bendingte nsile s tress a t

extreme edge off in

fin

Lcrit ical 8M lor bend ingcom pressions tress at

ext reme edgeof fIn

~~ ~: ;r ~ l effectivelength01he wall is :

teeter x Lorfactor )( Hthe lactortakesintoaccountthe restraintcondition

H

/ / /1

l Ion21 root

-

19

spacing of th e roof beams. T he spacing of the finis, ther efore, gove rned b y the followingcon ditions:(a) Th e cav ity wall ac ting as a co ntinuoushorizontal s lab subjected t o wind l oad , spa nningbetw een t he fins ( see Fig 20).(b) The cavity wall' s a bility t o supp or t vert icalload with out bu ckling. Thi s is gove rned b y theslendernes s ra tio of th e wa ll, BS 5628, cla use 28,(see Fig 21).(c) The a bility of th e cross sec t ion t o resi st th ea pplied l oadin g w ith the leaf and fin ac tingtogether to form a T beam.The eff ective flange of th e T bea m (see Fig 22 ) islimited t o t he least of:(i) th e distance be tween t he ce ntres of th e fins(ii) t he br eadth of th e fin p lus twelve t imes th eeffective th ickness o f the b onded leaf(iii) one -third of the effective spa n of the fin .

It shou ld b e noted th at cla use 36.4.3 of BS5628embraces tw o o f th ese con ditions with ref erence topiered wall s but, since it is felt th at t hedistribut ion o f stre ss into th e flange is a lso rel atedto th e span of th e fin (in a s imilar m anner to are inforced co ncrete T b eam ), a s pan r elated limitis a lso n ecessary .

(d) Th e vert ical shear f orces between the fin and

the b onded lea f resulting from the a pplied bendingmoment o n the T se ction (see Fi g 22a).

(e) The economic spacin g o f th e main roofsupports.

T ypical fin sizes are I -2 m deep at spacings o f3 to 5 m and I ! bricks (327 mm) or 2 brick s (440mm ) wide . Some typical sections and theirpropertie s are shown in Table I. The length andthicknes s of the fin is governed by the tendencyo f the outer edge to buckle under compre ssivebending stress.

The roof plate action and the stresses in thetr ansverse walls which provide the reaction s tothe plate mu st be checked.

22a

shear fa i lu re - j ! - " j ' - - - - - - - f

I t should be n oted that whil st item ( c) restrict sthe flange l ength for the de sign o f the fin , theactu al di stance between th e fin s can be greater.18


20/34


21/34

DESI GN PROBLEM

23

1 0m

roo' plate

278m

rOO'beams @3 .8 m centres

A wa rehouse m easuring 27 m x 46 m o n plan , and 10m high , is s hown in Fi g 23. Th e building i s tobe de signed in bri ckwork, using fi n wa ll co nstruction for it s main ve rtical structure. Th e fin s are t opr oject o n th e ex ternal fac e, a nd th e wall pa nels be tween the fins are to be o f 25 5 mm bri ck cav ityconstruction. Th ere a re no internal walls wi thin the bu ilding. Th e buildin g is part of a maj orde velopment wher e extensive tes ting of materials an d strict sup ervis io n of workman ship will beempl oyed .

Th e a rchitect h as se lected p art icular facing b rick s which are s hown t o ha ve a co mpressive strength of30 N /mm ' and a water a bso rption of 10 % . The facing br icks wi ll be used both inside a nd o utside th ebuilding .

D ESIGN APPROACHIntroductionBS 562 8 o ffers thr ee o ptions f or th e design of laterally loaded walls :

(a) Cl ause 36.4 .3 in which the de sign moment of resis tance of wall panel s is g iven a s f"Z' 1m

and(b) Clau se 36.8 wh ich offe rs tw o fur ther o ptions :(i) de sign lateral s trength equated t o e ffective eccentricity du e to lateral loa ds,

o r(ii) t reating the p anel as a n arch .

Th e last o ption ca n seldom b e appli ed to s ingle -storey bu ildings, due t o inadequate a rch thru stre sistance. Th e remaining tw o o ptions t ake no acco unt of flexural compressive s tresses which , in the finwall de sign concept, cert ainly require careful co nsideration.

For thi s reason, it h as been considered n ecessary , to p roperly ex plain t he mechani sms involve d , todiverge from th e BS 5628 concept of equatin g d esign loads to de sign s trengths. The ana lysis con sider sstresses due t o de sign load s and re lates these t o a llowable flexural stresses in both c omp ression an dtension.

(I) C harac teristic l oads

(a) JJ'ind)'orcesThe b asic wind pr essure o n a buildin g is calculated fr om a numb er o f variab les which incl ude:(i) location o f buildin g, nati onally(ii) top ography o f th e immediat e s urrounding are a(iii) hei ght abo ve gr ound to the t op o f the buildin g(iv) building geometr y.

Fo r th e appr opri ate conditions, th e basic pressure and l ocal pre ssure inte nsities a re given i n C P 3,Chapter Y, Par t II (9) .

20


22/34

In thi s example , th ese values are ass umed to ha ve been computed as :Dynamic wind pre ssure, q = 0.74 kN /m 'c'" o n windw ard face = 0.8c'" o n leeward face = - 0.55cp; on wa lls ei ther = ' + 0.2 o r - 0.3Gr oss win d upl ift = c'" + cp; = 0.53Th erefore c haracteristic wind l oads a re:Pressure o n win dward w all = W" = (c", - c p,) q = (0 .8 + 0.3) 0.74

= 0.81 4 k Im 'Su ction o n leeward wall = Wk2 = (c", - cp;) q = (0 .55 + 0. 2) 0.74= 0.56 k Im'G ross roo f uplift = W" = (c ",, + c p ;) q = 0.53 x O.74

= 0.39 kN /m'

= G ,= 0. 18 kN /m'= 0.27 kN/m '= 0. 15kN /m '

( b) Dea d a nd superim posed load s(i) C haracteristic superimposed l oad

= Ok = 0.75 kN /m'(As suming no a ccess t o roo f, o ther th a n fo r cleani ng o r repai r, in accor dance with C P3, Chapter V,Part 1.)(ii) C haracteristic de ad loadAssum e : metal decking

felt a nd chippingso. w. roo f beam s

T otal o, = 0. 60 kN /m '(2) D esign loadsThe critical l oading conditi on to be c onsidered fo r such a w all is usuall y wind + dead o nly, a lthoughthe loadin g co ndition of dead + super imposed + wind should be check ed .De sign dead load = 0.9 G , or 1.4 G ,De sign wind l oad = 1.4W k or 0 .015 G ,

whiche ver is the larger .

= 1.4 x 0.814 = 1.14 k N/m '= 1.4 x 0.56 = 0.78 kN/m '= 1.4 x 0 .39 = 0.54 kN /m '= 0.54 - 0 .54 = zero

Th erefore, by inspection, the mos t cri tical co mbination of loading w ill be given b y :Design de ad load = 0.9 x 0.6 = 0.54 kN /m 'Design w ind l oads:Pressure, from W"Su ction , from W k2Uplift, fro m W "Design dead - uplift

24 case(1) case (2)

} In ,I 1I II I1 \1 1

~ I 1

I \I

w ind pressu re

~~ I o a dIJtF=

ten sionoccu rson fin face

P'OO _\ deflected

I~SMp e fI

i ~ II I ~tensionoccurson wa llface

ro tation abo utface o f wa ll

rotation abou t e nd o f nn

wind SUCfIOO

Pi9 n load

O.7 8 kN /m 2

/

-11 1\1II \\E II

I II I

Z II /1~ / I~~ II Ici I I

II I I

~I-

n al ele vallon

plan

ne

secno

(3) Design casesInner le af offers minim al resistance a nd is ignored in calculations apart f rom as sisting st iffness offlan ge in bending.No te Vertical loading fr om 011' 11 weight o f effec tive sec tion only a s uplift ex actly cancels 0111 ro of dead loads.Des';:11 oj brick tin walls il l 10 11 single-storey buildines 2 1


23/34

(4) Deflection of r oof wind girderThe wall is designed as a propped cantilever and utilises the fins bonded to the outer lea f to act asvertical T beams resisting the flexure.

The prop to the cantilever is provided by a wind girder within the roof decking system (the design ofthis wind girder is not covered by this guide ). The reaction s from the roof wind girder are transferredinto the lateral gab le shear walls at each end of the building . Horizontal deflection of the roof windgirder, reaching a maximum at midspan, has the effect of producing additional rotation at base level(see Fig 25) and this res ults in a less critical stress condition . However, the critical s tress conditions are

generally experie nced in t he end fins where the roof wind girder d e fl ection is a m inimum.

w ind

i

===;:::;r! ,IIIIII

propmovesdue to roofwind girderdeflection

fin 1 lin 2

section BB

(5) Effective flang e width for T profil eThe dimensional limits for the effective length of the wall permitted to act as the flange of t he T profilea re given in BS 5628, Clause 36.4.3 (b), as 6 x thickness of wa ll for ming th e fl ange, measured as aprojection from each face of the fin, when the flange is continuous. In this design example, as w ill bethe ge neral case in practice, t he wa ll for ming the fl ange is the o uter l ea f o f a cav ity wa ll, as d efined i nBS 5628, Cla use 29. 1.1. It is, t herefore, reaso nable to take adva ntage of th e sti ffening effect o f t he inn erlea f in resis ting b uckling of th e o uter l eaf, when acting as t he fl ange of the T p ro fil e. Th e effectivefl ange length, measured fro m ea ch face of the fin, is t herefore ca lculated as 6 X e ffec tive w all th ickness .

Th us:

effective wall thickness = f(102.5 + 102.5)= 137mm

effective flange width = (6 X 137) + 327 + (6 x 137)= 1971 mm .

(6) Spacing of fin sT he spacing of fins has bee n di scussed o n page 7 - b ut o ne aspect o nly. Th e cap acity of th e wall panelto span bet ween th e fins is co nsidered h ere.

Th ere is no doub t th at th e support p rovided f or th e wall panel a t foundation level will a ssist i n resistingth e flexure du e to wi nd f orces. However , this assistance wi ll d iminish a t th e higher levels o f the w allpanel, a nd the wall s hould b e designed to spa n purely h or izontally between th e fins.

Th e wall panels are taken as continuous spans and the maximum ben ding m oments a re shown in Fig26.22


24/34

26WI< L'internal span ' 2 f -

Wk.L' @edge Wk.L @ t. fin.....,-r off in 12

ct. of fin

The m aximum moment is W ; ~ L' a t th e ed ge of th e fins , f or a n ass umed fln width of ~

W L ' I 14 x L'De sign moment = _ k_'- = . = 0 0814 L '14 14 .

From BS 5 628, Cl ause 3 6.4 .3 :

Ym= fhZ

= 1.1 0 N/mm '

De sign m oment o f re sistance

(i) fh : for wa te r a bsorption 7 % to 12 %set in a d esignation ( iii) m ortar

( ..) Z r 2 1 2 x 0.1 025' x 1.0 __0.0035 m'II : ror e aves = 6

(iii) Ym: from BS 5628, table 4 , specialcat egories of m anuf acturing andconstruction cont rol a re a pplicable = 2.5

h fore. rlesi f . 1.10 x 0.0035 xT ere . ore , es rgn m oment 0 resistance = 2.5 x 10'= 1.54 kN m

From thi s check ma ximum s pan of wall pan el.De sign mo ment = de sign mom ent o f resistance0.0814 L' = 1.54

L J :540.0814L = 4.35 m = maximum fin spacing

Th erefore, 3.80 m fin spacing i s a cceptable.

10'

Theref ore:

(7) T r ial sectio nA t rial section c an be reasonab ly o btained b y providing a sec t ion which h a s a s tability moment of

resistance M R " a t th e level o f M B, equa l t o W k18LH

' und er w ind pre ssure load ing W kl ie wh en rotat ion

a t th e ba se of the wall is a bout th e face o f the flange . For th e purp ose o f th e tri al section a ssessment ,th e stability m oment o f resistance c an b e si mplified t o OH in whi ch :o = tr ial sec t ion c oefficient from T a ble IH = height o f fin wall

Wk , LH '8

1.14 x 3.8 x 10' = 0 108

Ther efore 0 = 5.415 kNm /m height o f wallFrom T able I , select fin w all pr ofile 'K ' .

Note I t is imp ortant that thi s trial sect ion coefficie nt is used o nly f or the selection o f the tri al sect ion.A thorough s tructural an alysis mu st al ways be carried out

27250

(12 )( 137) + 440- 2084

~ ~012 -J+L 1115effective flange i 3800 cJc . 1. I I

p roftle o f trlII l MCtIon

Design of brick jill walls ill rail single-s torey buildings 23


25/34

~ T a ble 1

Fi n refer ence letter A B C D E F G H J K L M N P Q R

F in size( mm) 665 665 7 88 788 890 890 1003 1003 \11 5 \ 115 1227 1227 \ 339 1339 \45 1 145 1x x x x x x x x x x x x x x x x

327 440 327 440 327 440 327 440 327 44 0 327 440 327 440 327 440

E ffect ive width of flange ( m) 1.971 2 .084 1.97 \ 2.084 1.97 \ 2.08 4 1.97 \ 2.084 1.971 2 .084 1.97 1 2.08 4 1.971 2.084 1. 971 2 .084

Ne utral axi s Y I (m) 0.455 0.435 0.524 0 .500 0 .589 0.563 0 .654 0 .626 0 .7 \8 0 .687 0 .780 0.7 47 0 .84\ 0 .807 0.902 0.866

N eutral axis Y . (m) 0.210 0.230 0.254 0.278 0 .30 \ 0 .327 0 .349 0.377 0.397 0.4 28 0.4 47 0.480 0 .498 0 .532 0 .549 0.5 85

Effect ive a rea (m t ) 0.386 0.4611 0.4262 0.5 \52 0 .4595 0 .560\ 0.4965 0 .6098 0 .533\ 0 .659 \ 0.5697 0 .708 4 0.6064 0.7577 0.6430 0.807

o .w. of effective a reaper m height W IkN) 7.720 9 .222 8.458 1 0.216 9 . 190 1 1.202 9.930 12.196 \0 .662 13.182 1 1.394 14 . 168 \2 . \28 1 5. \ 54 \2 .860 16 .140

I NA (m') 0 .0\567 0.01939 0.02454 0.0303 0 .03 59 0 .04426 0 .0502 \ 0.06 187 0. 06746 0.083 12 0 .088 0 .10848 0 .1 \ 208 0 .13826 0. 13992 0. 17277

Z, (m ') 0.0344 \ 0 .0445 0 .04684 0 .06059 0.06096 0 .07862 0 .07677 0 .09883 0 .09395 0 . \2099 0. 11282 0. \4522 0.13327 0 . 17132 0. 15513 0. 1995

Z, (m ') 0.07462 0 .0843 0 .09663 0 .10898 0 . 1\928 0 .13536 0 .14387 0 .\6410 0 .16992 0 . \942\ 0 .19687 0.22 6 0.22506 0.26039 0.25487 0 .2953

Tr ial section coefficientn (k Nrnjrn) 1.6212 2.1210 2.1483 2.840 2.76 62 3.6631 3.4656 4 .5978 4.2328 5 .6419 5.0931 6.8006 6.0397 8.06 19 7.060 \ 9.44 19

ftange lace

Z - ~ Z _ I N A1 - YI ' - y . ~ "rial sec tion coe ffici ent Q = W Y, - - A

Y,


26/34

(8) Co nsider pro pped ca ntilever ac tionW ith 3.80 m fin centre s, design wind lo ads on fin s are :Case (i), suction (see Fig 28)W k2L = 0.78 x 3.8 = 2.964 kN /m of heightCase (ii) , pre ssure (see Fi g 28)Wk,L = I.l4 x 3.8 = 4.332 kN /m of h eight

- ., ........ ( Q - ca M (II) preuu re

H - 10.0m~ - 3 . 7 5

/M f

' \

\ ,< ,

.... (lIIBU.Iog . . m

309 H - 10.0m~ H - 3 .75m

E tie 7r'5 M

~ \ ,~~ I'--..'"

I. MB . 1ca . . (I) BM dlegram

10'

4.332 x 10'8

9 x 4.332 x 10'128

9 x 2.964 x128

= 20.84 kNm2.964 x 10'

8= 37.05 kNm

9W kl LH '128

= 30.46 kNmW k , LH '

8= 54.15 kN m

W LH 'Base moment , MB = k28

Base mo ment, MB

Assuming MR , is g reater than Wk'8LH

' a nd zero d eflection o f the roo f pr op , the following BM

diagrams can be dr awn :Case (i )

9W LH 'Wall m oment , M w = ~ ~ 8

Case (ii)

Wall m oment, M w

The bend ing moment d iagram s s hown in F igs 29 a nd 30 ar e applicable o nly if it can be shown that the

stability m oment o f resistance of th e ' cracked sect io n' MR . at dpc le vel exceed s W k8LH ' . Thi s should

be the fir st check t o be carried o ut, and if MR . is less th an W k; H' the b ase moment is limited to MR .

and the BM di agram mu st be redra wn plotti ng th e free moment d iagram o nto the fi xed e nd momentdia gram wh ich is produc ed by MR , (see F ig 38) .

(9) Stabilit y moment of resistanceIn vari ably, a s is th e case with th is design examp le, t here w ill be a d amp pr oof cour se a t or near to thebase of the w a ll. Few d pcs are ca pable of tran sm itt ing m uch fl exural ten sile s tre ss ac ros s the bed jo int,and in thi s exa mple th e an alysi s considers the 'c racked section' .

res istance 10 ax ialload applied et

centrotd 01rectangularultimatestre ss block

ov erturningrnomenI

the min imum wldth0 1 wall is fullystressed 10 producethe maximumleV8fann fof the axia lloadin the tin 10 generatethe stabi lity momentof resistance MA --10 . . . .

31

Ap p end ix B of OS 5628 di scus ses the appl ication o f a rectangul ar s tress bl ock und er ultim atecondit ions, an d t he sta bility moment o f resistance MR . a t the l evel of MB ca n be ass umed t o bepro vided by th e a xial l oad in the fin section acting a t a lever arm about the centroid o f th e rectangularstre ss block a s show n in Fig 3 1.

D esign o f brick fin walls in fall single -storey buil dings 2S


27/34

( 10) Allowable fl exural compressive str esses, P.",(tak ing into acco unt slenderness ( ~ ) a nd m aterial (Ym)Before th e stabilit y moment of re sistance M R , ca n be comp ared with th e as sume d b ase m om e nt ( M B)

f W k L H ', iderat i be . he cri ff . h II bl fl I .o 8 const eranon mu st given to t e en te n a a ectmg t e a ow a e ex ura com p ressive

stresses, P. be' as thi s val ue dictate s th e s tability moment o f re sistance. T his is demonst rated in Fig 31,in which the mechanism pr oducing th e stability m oment o f re sistance M R. is shown.

Th i s flexural compres sive stre ss can bec ome signi fi cant an d mu st be c hecked taking into acco u nt t hetendency of th e fl ange or fin to bu ckle at t he poi nt of ap p lication of the st ress.

Th ere is limited g uida nce given i n BS 5628 o n th e effec t of slenderness o n th e flexu r al compr essivestrength o f m ason r y. Th i s is be cause th e fl exur al s tre ngth o f m ason r y is a ssum ed t o be limit ed by th eflexural tensile s tresses - whi ch is, perhap s, t rue of pane l wall s a nd the like, bu t not of the a na lysis o fm ore co mp lex geome tric for ms suc h as the fin wa ll.

The approac h to the consideration of slen derness and flexura l comp res sive stresses w hic h f ollows isbelieve d to provi de a sa fe an d practical design. It is expected that curr en t re search wi ll a llow mo reaccurate ana lysi s to be d eveloped

Id entification o f p rob l em:

Ca se ( i) sucti on, s howing zone s o f maximum value s of fl exural c ompression ( Fig 32)

32cue {I}suctton

flexural compr ession her e

~ = = p o i n Ic A.......

8 1 1 _1f t n _

varies ~ H"exu ral compre ssion here and grea lef

lie ...... 1- , , -- - ....

3 3 cu e (II) pressu reflexural compression here

r1exu ral compression herepmp

point of cont ralklllure

varies ~ Hand greater

OM d illgram

Ca se ( ii) pre ssure. s howing z ones o f max i ll111m values offlex ural compression ( Fig 33)C onside ring th e wind s uc tion l oading ca se (i ) , flexural co mpression is ap p lied to the fl ange of the Tprofile at th e level o f M . Th e buckling stability o f th e flange is prov ided by th e projecting fin an d ,therefore. the effective length of th e flange . for slenderne ss considerations. ca n be ta ken as t wice th eoutstan ding length of th e flange from the face o f th e fin. Fu rtherm or e , if th e flange is properl y t ied t oth e in ne r l ea f of the cavi ty wa ll. th e effective thickness o f th e fl ange , fo r s lende rness c onsi de r at ion s, ca nbe take n as 5 th e sum of th e t hicknes ses of the two lea ves of the cav ity w all.

F lex ura l co mpressio n is a lso a pp lied t o th e en d of th e projec ting lin at t he leve l o f M B . For thi s designexamp le, t he found a tion is a ss um ed to co m prise a re inf o rced co nc re te raft sla b a s show n in Fig 3 4 .Th e llexur al compr ess io n a pp lica ble a t th is level is n ot inlluenced by s lenderness co ns ide rat ions a s th eraft fo unda tion ca n be a ss umed to p rovide f u ll lat er al s tab ility.

Slende rness at t h is l evel would req uire caref ul co nsideration if th e fin fo undation was a t a gr eater dept hbe low gro und level.26


28/34

34fin

ILdpc

I + - - r c raftfo undation

Co nsidering th e wind p res su r e load ing cas e (ii), flexura l compr ess ion is appli ed to th e e nd of theproj ecting fin at th e level o f M w-

Th e bu ck lin g s tability o f the fi n ca nno t b e co nsidered to be full y pr ovided by th e flan ge o f the Te eprofi le, as th e flan ge is not o f co mparable la te ra l st iffn ess to the fin a nd wo u ld t end to ro ta t e ina tte mpting to prev en t th e fin bu ckling . Rather , it is cons idered th at th e s lende rness of th e fin s houldre lat e partl y to its height an d , as th e fu ll height o f th e fin w ould be ove r-cautio us, it i s p ro p osed th atthe hei ght between p oin ts o f co n traflex ure w o uld pro vide a dequ ate s afe ty . Th e e ffective t h ickness o fth e fin for slende rness co nside rations i s tak en a s th e ac t ua l t hicknes s.

Pu b e

Th e design flexural co mpre ss ive stress Pub< can ther efor e be ex pre ssed as := r~f k

' m

wh ere Pub

design of brick fin walls in tall single-storey buildings 0690

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