The Design of Laterally Loaded Walls

BS 5&28:Th. Structural U••ofMa.onry Part 1:Unr. lnforcadMa.onry

February 1986

The design

of laterally loaded

walls

+

--Preparedby

BS 5628:The Structural Useof Masonry Part t:Unreinforced Masonry

The design of

laterally loaded

walls

Prepared

by

J. Morton BSc PhD CEng MICE MlnstM

Thispublication is concernedwith the subjectof laterallyloaded walls,with particularreferencetouniform lateral pressures.Itis based onvisual presentations originally givenduring a seriesof !!1

~ seminarsonBS 5628: TheStructural Use ofMasonry:Part 1 in ~ late1978 and subsequently.

~

S

~

cope

• The contentscoverboth thebackground totheCode provisions

3!

aswell astheprovisionsthemselves.Inorder to give thereader

~

an understanding of the Code·.recommendations and the

t

reasoning behind them,the subject isdealt withinitswidest

C1.l

se

nse

.

I: Particular attention is paid toClause36 ofSection 4ofthe

Code whichgivesdetaileddesignrecommendationsfor laterally loadedwalls.

Amendm ents2747(October 1978) and 3445(September 1980) havebeen takenintoaccount. as hasthe latestamendments 4800,March1985.

Acknowledgements

The kind support and guidance of many pecple during the preparation of both the seminars and this publication is gratefully acknowledged. Particular thanks are due to

B

A

Haseltine, currentchairmanof the Techni calDraftingComm it-tee,and toN.J.Tutt who checkedthe design exam ples.

FOREWORD

_

_...

1 ScOpe _ _ _... 1

Acknowledgements

_

_

_

1

1. MATERIAL PROPERTIES

2

Inlroduction

2

Weak and strong direction of bending

3

Wallette test programme

.._

_

_

_

3

Clay bricks 3

Calcium silicate bricks _.. 4

Concrete bricks 4

Concrete blocks 4

Orthogonal ratio

_

_._.

.._._

5

Statistical qui

rks

.._._ __

5

Docking or wetting of bricks

__._

5

Review

._

_

_....

5

2.

DESIGN METHOD

5

Introduction

_

_...

5

Cladding panels _ _ _ _ _ 5

Pa nels whicharch _ __ _ _ _._.. 5

Cladding panels

_

6

Wall tests _ _ _._.__ _ _.._ ..__ 6

Edge supports.. _ _ 6

Modesof failure .._ _ _ _ __. _.. 7

Designmoment ._. . . _ _._... 7

Designmomentofresistance . _... 7

Bending mom ent coeffic ients _... .. 8

Part ialloadfactorfor w ind _._._ _ 8

Check ingthedesign _._ _._ _._ _ _... 8

Shear.._ _ __._._._._ _ _._._._ _._ _ ._._ _ _... 8 Partial safetyfactor for materials __ __._._ _._.__.__ 8

Partial safetyfactorfor shear __. __. 9

Limits to panelsize _ . 9

Edge restra ints__. . _ 9

Metal tiesto columns_ . ._ _.___ 9

Bonded topiers ._._._.._ _._ _ __._._._ __ _._. 9 Bonded retum walls.. ._._._ _ _._ _._ _._._ _._.._ _ 10 Unbonded retum wall usingmetal ties _. ._.__ _ _ _ 10 Freetop edge_. .__._._. .__. . ._._. ._ 10 Wallbuilt up to thestruct u re withsimple anchorages 10

lnsitufloor slabcastontowall 10

Effectof damp proof courses 10

Experimentalvalidation 10

Arching walls

11

Introduction 11

Freestandingwall s .____ 12

Cavitywalls . . . 12

Wa ll panel edge supports . . . ._. .. 12

Walls with openings

13

3.

REFERENCES

_

_

13

4. DESIGN EXAMPLES

_

_

14

Example

1 _ 14

Exam pie 2

_

_

_

_

__

14

Exam pie

3 _ _ _... 15

Example4

15

Example

5

15

Example6

15

Example

7 16

Example

8 16

Example9

17

Example

10 19

Example

11 .. 19

1

IlATERIALPROFDt IllES

The majontyofmasonrycladding panelstend to bend in boththe horizontal and vertical direction s simultaneously; they are essent ially two-way bendingplates(figure1).11ismoredifficu ltto study the materialpropertiesin both directions sim ultaneous ly thanifthepanelissplitintoitshorizontal and vertical modesand each of them studied separately. This facilitates any ex-perimental investigation(frgure2).

IIei di

'!l

perpet

daABr

to

bed

joints

IIei di

'!l

parallel

to

bed

joints

3

ii

~

Ideally, walls like those shown above should be built and ~

tested inbendi ng tofailure.The failuremode willbe aflexu ral Q, crack that will occuralong thebed joint,inthe case of simple

~

vertical bending (left) at or near the position of maximum ~ moment.When the wallissubject tosimple horizontal bending,

2"

the flexural failu recrack will develop through the perpendicu lar ~ joints (perpends) and the bricks,as shown on theright. > Whilefullscale model testsare perhaps ideal. they are also

;jf

Introduction

expensive. It is morecost effective tobuild andtest smallwalls. knownaswallettes.

Two wallettesanda testrig can beseenabove.The wallette. built toaparticular format. in the rig in thebackground is being tested in simp levertical bending.Thewalletteinthe foreground, built to adifferent format. isto be tested insim ple horizontal

bending andwillbe tested inadifferentrig.

Weak and strong direction of bending

The wallette in the test rig in figure 4 will fail, at or near mid-height. when the interface between the bricks and the mortar

bed

is broken. Since thebrickworkhasrelativelyweak tensile properties in both direct and bending tension, it is reasonable to predict that this direction of bending (simple

vertical) will also be relativelyweak. On the other hand, the walletteintheforeground ,whenloaded tofailure intheother rig, will fail whena verticalcrack develops across twobricks and two perpends. Since a brick is much stronger in flexure than a mortar/br ick interface, it is reasonable to predict that this direction ofbending willberelativelystrong.

Experimental evidenceconfirmsthatsimple verticalbending isthe weakdirection,andthatsimple horizontalbending isthe strong direction. For ease of termin ology, the direction of horizontal bending andthe directionofvertical bend ingwillnow be referred to asthe strong andthe weakdirectionsof bending respectively.

Wallette test programme

The wallettetestingprogrammewas carriedout by theBritish Ceramic Research AssociationI, A wide variety of bricks,

commonlyavailablein Britain, was tested in thefourBritish

Standard mortar designations.Theobjective wasto ascertain thefiexural strength,inboth theweak andstrong directions of bending, for the range of modern brickwork designed and constructedinthis country.

Claybricks

For eachclay brick/mo rtarcombi nation,a setofwalletteswas built inboththehorizontalandverticalformats, eachconsisting of five or ten wallettes. This enabled the mean and standard

~

deviationof eachsetof result stobe found.Withthese valuesfor

~

themeanfailure stressandthestandarddeviation,thec

harac-~ teristicflexural stresscouldbe calculated forthe strongand the

~ weakdirectionofbending.But to whatparameterscouldthese

~ resultsbe related?

.. Perhapsthebest known property of bricks istheir comp ressive

~

strength. Doestheflexuralstrength of brickworkrelate tothe

'

!r

com pressivestrengthofthe bricks?Therelationship is shownin

~

figure5.Fromthe limited results available, it wasnot feltto be a t-.; relevant approach,

It isreasonabletopredictthat some relationship wouldexist between the strength of the brick in flexure and the flexural strength of thebrickwork- particularlyinthestrong direction,if

notinthe weak.Figure6showsthatthere is some correlation between these parameters.

After examination of all the possible relationships, it was decidedthattherelationship betweenthe flexuralstrengthofthe brickworkand thewaterabsorption of thebricks' fromwhichit

was constructed wouldbe used " Thegraph aboverelatesthe flexural strengthof the brickworkto the waterabsorpt ion of the brickfor wallettes tested in theweak direction and builtwith designation(i) mortar.Eachpointon the graph represents the mean offiveor tenwallette testswith aparticular brick.The

num berof dots indicates thewide range of bricksthat were tested in order to represent the wide variety of clay bricks available in Britain to-day.The widescatteris notexceptional;it istobe expected whentesting masonry.Itreflects the fact that brickworkhas awide statistic al 'quality' curve,

To com plywith limitstate philosophy, it was necessaryto establish the 95% confidencelimit from allthetestresults.A statisticallybased approachwas used to establish the position of the 95% confidence limitfromthe mean andstanda rddeviations associatedwitheach point.Thislimitis show ninfigure7 asthe curved line,under whichlie5% ofthe results.Itcanbe seen that the flexural strength tendsto decreaseWithanincreaseinthe

waterabsorptionofthe bricks.Because complex curves make

designless straightforward, itwas decided to simplifythe curve

Looking at another simi largraph, fig ure8, this time in the stronq directi on - but sti ll with mortar design ation (i) - the backgroundto anothersectionofTab le 3 ofthe Code (figu re13) can be seen.The approachadoptedis identical to that describec above and thevalueson this graphcorrelateWith those inTable

3.

The relationship between block bulk density and the flexural $lrength was also examined to see

rt

bulk density might be a useful indicator.While certain trends are apparent in figure 12, they werenotfeltto be as relevant as therelationsh ip between flexural strengt hand the compressivestrenqthof the blockitself. Forthis reason, the flexural strengthofblockworkin figure13is

ii'

givenin termsolthecrushingst rengtholthe bloc k.

:<

Considering the strong directio n of bending, it must be

~

remembered that thefailure ot the wallelteinvolves the failu reof Q. both the perpendjointsandthe blocks.Iti~reasonab le to expect ~ an increase in flexural stresswithan increasein blockstrength.

i

This is because the stronger the block in compression, the

i

denserit will be.The denser the block. the greaterthe flexural

1l:

stfrtehnglbhl ofkthte

hblock 'tfseij'IAndIbetthhe grfelaterthe flexural $lrengthf

~

o e ocx. e grea er WI e m uenoe on the strength0 " formats,using a range of calcium silicate bricks representative of modern production in Britain.They were constructeo using four different mortar designations. The results of the large testing programme indicated that only one set of values for the characteristic flexural stress was needed to cover all calcium silicate bnckwork. These values,0.3,0.2,0.9and 0.6Nlmm'for the respective mortar designations. are given in flgure 13. Calciumsilicate bricks available in Bntain do not. of course, have a waterabsorptionvalue associatedWiththeirStanoard

"

,

Aswith calcium silicateand concretebricks,concreteblocks

do

not use the parameter of waterabsorption in their Standard'. While the wallelte testprogramme indicated that there was some relationship between water absorption and flexural $lrength (figure 11), water absorption was not considered to be the most relevantindicatorof the flexuralstrenqthof blockwork.

Conc rete bricks

Concrete bricks were tested in an identical manner to the other brickwalleltetests. The results, see f,gure 13, indicate similar values tothose associated withcalciumsilicatebrickwor k.

Concr eteblo cks

Concreteblocksarecovered inTable 3 (tigure13) in a somewhat different manner to clay. calcium silicate and concrete bricks. While the characteristic flexural stress was ascertained from wallelte tests, a different wallelte format was necessary to account for the d,fference in

urut

size between bricks and blocks. The

wanette

formats for blocks are given in AppendIX

A3of the

Code.

..

waterabsorption.0.

:~

...

l

UI

~

-I

_,

-Flexural strength Nmm'

The relation ship between the cha racteristicflexuralstresses of brickwork- in the weakandstrongdirections- andthe water absorptionofthe clay bricksfromwhich itwas constructedwas established forall the morta rdesignatio ns.Fig ures 9&10 show the graphsfor adesignat ion (iii)(1:1:6)mort ar.Asbefore, fig ure 13contains the values derivedfromthis work.

Calciumsil icatebrick s

The approach for calcium silicate bricks was essentially identical to that usedfor clay bricks. Walleltesets were buiij, in both

Flexural strength water absorption

10

1:1:6 in strong direction

,_ _ • undocked

l . ; : docked

wrthtwo steps at values of 7% and 12% water absorption.This convenientapproach of hav,ng three values of flexural strenqth for three ranges of water absorption, facilitates the material propertiesofbrickwork being descnbec in tabular form .

ij is from this research background that the charactenstic flexural $lressesgiven inTableSot the Code were derived.Thisrs reproduced as figure 13 on page 5.From both the graph and the table, these values are 0.7 Nlmm'

0NA

less than 7%), 0.5 Nlmm'

0NA

greater than 7% and less than 12%), and 0.4 Nlmm'

0NA

greater than 12%) when considenng the weak direction of bending in mortar designation(i).

the wallelteswhen tested inthestrong direct ion.Thistrendwas

found tobe signilica nt in thestrong direction.Itisreflected in

figure 13 which bases flexural strength on the compressive

strengthoftheblock.The characteristic flexural stressvalues range between 0.9 to 0.4 Nlmm', and 0.7 to 0.4 Nlmm'

dependingonthe mortardesignalionfor100 mm blockwork.

In theweakdirection,failure occursbybreakingthetensile bondat the interfacebetweenthe blocksandthemortar bed. Theexperimentalevidencesuggests thatthereisno significant

difference between blockworkconstructedwithweakorstrong

blocks. Consequenttv,for100mm block work there areonly two valuesof 0.25

N

l

m

m

'

and 0.2Nlmm' whicharedependenton themortar designation butindependentofthestrengthof the block Itself.

Since the Code was first published, further tests have

indicatedthattheflexural strengthofthicker walleltesislower

thanthatpredictedbythe wallelteresultsfor100 mmblockwork. The Standard has thereforebeenamendedto take accountof

this.

Forintermediate values ofwall thickness, between 100and

250 mm, the value of characteristic flexural stress can be

obtainedbyinterpolation.

Orthogonal ratio

Theorthogonal ratio is the ratio of the strength inthe weak direction tothe strengthinthe strong direction.For clay,calcium

silicate and concretebricks,the ratiois approximately 1:3 or 0.33

(seefigure 13).For design purposes, a value of 0.35can be universally adoptedfor allthesebricks.

Unfortun ately, with concrete blocks,the ratio variessince the strong directionstresses vary whilsttheweakdirection stresses

remainconstantfor differentstrength blocks.There is,there -fore,nouniquevalue fortheorthogonalratio ofblockwork;itis

necessaryto establisha valueforeach blockwork strength.

Statistical quirks

Therearecertainimplications inherent inastatistical approach.

The graphshow n infigure10,whilstbeingstatisticallycorrect, doesnonetheless misinterpretcertainfacts.Both thecurvedline and the stepped approximation indicate that the strongest brickworkresults when bricks with a waterabsorption of less

than 7% are used,Acloserexaminationof the graph,however, revealsthat brickworkconstructedwith bricks of between 7%

and12% waterabsorptioncanexhibit highervalues of flexural strength thancan be achievedwithmany ofthebricks withless

than 7%waterabsorption . Indeed, thelargestvalueof flexural

strength isachieved witha brick in thewater absorption range 7%-12%.

It isbecauseofsuchstatisticalquirks, that the Codepermits

wallelte teststobecarriedouton particularbricksin accordance

with thetestingprocedure set outin AppendixA3.In this way,a

manufacturer'sclaimof highercharacteristicflexural strengths thanthose intheStanda rdcanbeobject ively assessed bythe designerif,athisdiscretion ,hedecidestoaskforwallelteresults

toBS5628:Part1fromthe materialproducer,

Docking or wetting of bricks

The graphs shown in figures 7,8,9<I. 10make'reference to

'docked'and 'undocked' bricks,Thisisduetotheneedtowet

('dock') certain types of brick if theirwater absorption would

resultinwaterbeing removed fromthemortarattoogreatarate.

Research has indicated thatthishighspeed removal ofwater from the mortar can adversely affect both the compressive strength of the brickwork and the bond at the brick/mortar

interface,

Therateat whichwaterisrem oved frommortaris calledthe Initial Suction Rate of thebrick(ISR).TheISR of abrickcan be

adjusted by partially soaking it. This can be done eithe r by

immersingitinwaterfor ashorttime- typically,a halftothree minutes.Itcan alsobe partlyachievedby sprinklingthestack with water from ahose.Although this isoftendoneonsome

sites, it is not the recommended method. When writing the

specification, itisnormalgoodpractice to suggestthat'before

orders for bricksare placed, the contractor shall satisfythe engineereitherthat the suctionrateof the brick" .doesnot

exceed1.5kg/m'/m in or that he is abletoadjust itsoasnotto exceedthisvalue",

'Docking'orwelting of bricksdoesnotmean saturatingthem, The general rule 'never allow bricks.or brickwork to become

saturated' shouldalwaysbeobeyed,

Summary

The foregoingmaterial constitutesthe background tothesection of the Codewhich dealswith materialproperties. Inparticular, it explains the backgrou nd to Table 3 (figu re 13), and provides

more detailed informationon the large testing programmeon

which itwas based.Not allthegraphshavebeen shownsince theywouldgive nomore informationthaniscontainedin Table

3

.

DESIGN METHOD

Introduction

Turning from the consideranon of material propertiesto the

designofpanels usingthese characteristic flexural stresses,two typesofpanelare particularly important.

Cladd ing panels

Thefirst typeisthe panelwhichtails when thetensilestress in

the extreme fibres equals the ultimate stress. It could be a

vertically spanningwallas shownon the leftof figure14,or a horizontallyspanningpanel as shownon the right.This typeof

wall is very common in Britain,andthevast majority ofbrickwork panels usedtoclad framedbuildingsfallintothis category,

Panels whicharch

The other type of panel, which behaves differently from a

claddingpanel,isthepanelthat'arches'.Such a wall(figure 15),

generates compressive forceswithintheplaneof thewallasit

deflects.Thesecom pressivestresses,generated bythedeflec -tion, aresuperimposed on the tensilestressesasthey develop andpartly orwholly cancelthemout.Consequently,this typeof panelismuch stronger than a cladding panel.

Aparticular typeofwall panel inwhichin-plane forces are presentisaloadbearingwallina structuralmasonrybuilding.In this case,the in·planecom pressivestress presentisthestressin

5

the wall derivedfrom the loadrtis supporting.

II is important to distinguish which type of wall is being designed. and whether it is a wall in which compressive forces can develop.This willoo«be consideredin greaterdepth.

Horizontal

arcIli

Ig

Vertical arc:tiIlg

t5

Cladding panels

Thefirst type of wall to consideristhe cladding panel that falls when the tensile stress developed at the extreme fibres in bending reaches theultima te flexural stress for the material. Having already established the material properties that can adequatelypredict the simple vertica land horizontal bending strengths (figu re 16). it isnow necessary to combine them in two-way bending (figure 11). This essentially relates the

two

know n materialpropertiesdescribedinTable3 oftheCodetothe behaviourof three andfou r -sided plates.

6

Itisa complex analysis and involves thesummation ofthetwo stresses generated from two-cirectional bending (figure 18).

Walltests

To investigate this. a series offull scale wall tests were carried out at the British Ceramic Research Association' (figure 19). Wallettesawaiting testing canalsobe seen inthe foreground of thepicture.

The objective was to findthe relationship between the strength in two-way bending to the two known strengths in the simple vertical andhorizontal directions.

Full scale wall panels were builtin test rigs (figu re20) and an air bag was placed between thewall and a reaction frame behind

it (figure 21).The reaction frame was tied back to thetest rig. Panels of different shapes were then tested using various brick/mortarcombinatio ns. The majority of thepanelsbuilt were standa rdstorey height (figure21)butsome tallerand sho rter :; panels were also tested, suchasthe1'h sto rey heightpanel in ::. figu re 20. The lengt h of thepanels was alsoto be varied - note ~ the two shorter panelsawaiting testing in the foregro und of ~

~ffi21 . ;

Edgesupports

J

Actual failure. when the panel cracks. does not necessarily ~

result in total collapse. The wall in f,gure22 actually crackedata

il:

much lower deflection. In order to demonstrate the failure ~

26

27

7

o

or

a

OorlAa

o

or

a

M

d

=

f

kxZ

Om

f

kx

=

characteristic flexural

strength about vertical axis

Z

=section modulus

om

=

partial safetyfactor

on materials

M=a~,wkl!

where

M : bending moment-about

vertical axis

a :

bending moment

co-efflclent

~,

=

partial safety factor forloads

W

_k

=

characteristic wind load

L

=

panel length

2

Inthedesign ofanybending member,the appliedmomentis norm allyWL2 divided by a factor, for example 8 for simply supportedone way bending. The design momentforpanelsis basedon the same equationbut thedivisor isexpressed as an a coefficient(eg,O. 125for the case above).For limitstate designV" the partialsafety factorfor loads, is necessaryto convert the applied momentinto an (applied)design moment. Notethat the formu la isexpressed intermsof L,the length, andnoth,the height.

Designmomentof resistance

direct ions,andalso forvariouspossibleedgeconditions(figure 25),a usefuldesign techniq ue wouldresult.Thethreepossible edge conditionswouldbe:

1. A free or unsupport ed edge.

2. A supported but pinned edge where no moment would develop.

3. Asupported fullyfixed edge wherea momentequal to the panelmid-spanmoment woulddevelop .

Working on this basis,andtaking intoaccount the walltest result s,the follow ingdesignprocedure was suggested":

Designmoment

The formulato calculatethemomentof resistance isthe normal f x Z term. Because the applied moment uses L (ie, it is associatedwithhorizontal spanning),the moment ofresistance mustuse f" in thestrong direction for compatibility. To be a designmoment ofresistance,thefx Zterm mustbe divided by Failur emodes

wall was grossly deflected (figure 23). This also serves to illustrate the residual strengthof failedpanels;although the wall hasstructu rallyfailedandis grossly deflected,itis stable.Visible in figure23 are theholesthat werecut in the channels formin g the two verticaledges ofthetestrig.Ties wereinsertedinto these holesandwereusedto connect thewalltotheframe, as isdone inpractice- ie, the panels weresupportedin the testframein a similar way to panels in a real building. It was of course recogn isedthat this edge supportsystemwas morelikelyto approximate tosimple supports rather than to fully fixed edge conditions.

i

~

!

The failuremodes of the panels (figure 24)gradually emergedas ~ thetestprogramme progressed.They were found to be fairly 3; similar to those associated with concrete slabs although, of ~ course, brickwork is a brittle material (unlessreinforced) while

~ reinforcedconcreteexhi bitsplastic yield ing. •

~ Itwasrecogn ised that, ifaccountcouldbe takenofthe two o!f different bending momentsatthe centreof the panel in both

31

32

or

BerocAIQ

Forthe majority ofpanelsmetin practice.failure inbendingwill bethe design criterion.It isneverthelessnecessaryto check the

shear. Theaboveformu la givesthisreadily. Partial safetyfactor materials,Vm

These arenormally thetwomainwaysinwhichthedesignis carriedout. Shear

ii

~

~ a,

..

~

->'

The value ofVmcanvarybetween four values dependingon the ~ category

lit

constructioncontrol(workman shipandsupervision)

a

andthe categoryofmanufacturingcontrol(thelikelihoodof weak , units being included inthe consignmentofstructuralunits).This

~

0·040 0·035

0·050 0-<>65 0

·098

0·083 0-081 0

·144

0·50·065

1

·00·091

1·50

·125

Bending momentcoefficientsareset out in Table 9 ofthe Code. Theyare essentiallythe coefficientsgenerated from 'yield line' theory'··which.although notfellto be strictlyapplicable to bnttle materials suchas masonry,nevertheless appearto givereason

-ablecorrelationwith experimental results - at least for solid panelswithout windows ordoors",Theacoefficientdependson theaspect ratio of thepanel(hIL). on the orthogonal ratio~.and the nature of the panel's edge support - whether three or four -side supported. and whether 'simplysupported'or 'fully fixed'.(Note thata setafvalues forahasbeen givenineachof

the tablesfor ~

=

0.35. This value for ~ has been purposely included foralltypesof brickwork}

Partial safety tectorforload(wind), VI Vm'the partial safety factorfor materials.

Normally, VI for general deSign istaken as 1.4_However, a specialcaseismade ofcladding panels which do not affectthe stabilityof themainstructu re- normally aframe.In this case,VI canbereducedto1.2.IImust be clearly understood thatthis is anexception.basednot on designphilosophybut on the needto acknowledge that Without reducing VI to 1.2 many panels currently knownto workwell inpractice could not be justifiedin thedesign process.

Ofcou

rse.shouldthe brickwork panelform part of the structure, and should the removal of the panel endangerthestability of the remaining structure,VI mustbe maintainedat1.4.

By equating thedesignmoment of resistanceto the design moment. Itis nowpossible to obiectivelydesignlaterallyloaded panels.ButItisfirstnecessarytolook in detail atcertainof the terms used.

Bendingmomentcoefficient;

a

Checkingthedesign

The design momentofresistance mustbe equalto orgreater thanthe designmomentand,byre-arrangingtheformulae:

either,

Z isassumedand thef...of the materialis checkedto see if Itis adequate.

or,

the f...of thematenalto be usedISfed into the deSign and the Z

8

valueofthesectionto be usedis clhecked.

approachessentially recogni sesthat therearebonusesforthe designerIfhecan be morecertainabout thestructural units,the mortarandthewaythey are puttogether onsite. Thesedesign

bonuses canbeexploitedif:

1. Thereisasmallerprobability ofthe structural units(bricks! blocks)failing belowtheirspecified strength.

2.Thereis a small er. probab ility of the mortar beingbelowthe strengthspecified.

3.There isasma llerprobabilitythatpoor workmanshipwill be usedin putting thebricks andmortar together onsite.

Condition1is coveredbythemanufactu ringprocess,while 2 and 3 are both factors influenced by site operations and supervision.

Asisthecasewithcompression andshear loadedwalls, itis

possible to specify bricksto an acceptancelim it to BS 3921· where not more than 5% of the bnc kswill have a crushi ng strength below the acceptance limit specified. This is the recuirementforspecial categoryofmanufacturing control.and

the majorityof BOA membercompanies willreadilybe ableto meet thisformof productionqualitycontrol.

To achieve specialcategoryof construction control. mortar tesling should becarriedouttosatisfycondition 2. above.and

the work on site should be supervised bya suitably qualified

personto satisfycondrtion3.

In mostsituations. the designer willnormally basehisdesign onYm =3.50r3. 1since. atthe designstage. the likelycontractor and thelevelofsitesupervisionwill not necessarily be known.

However. lower values of Ymare available, should they be required.

Y

mv

forshear

34

~:2·5

Y~for shearis takenas2.5

Limitsto panel size

Twolimitationstopanelsizesare giveninthe Code:

~ 1. Anoveralllimitonthe areaof thepanel. Detailedrestrictions

<:

are given.depending on the natureofthepanelsupports. ~ 2. Forthreeandfour-sided panels.anoveralllimiton height or

~

lengtn'which mustnot exceed

5Ot.

1

(the effeclivethicknessof the ~ wall).Since

let

foranormalcavity wall isapproximate ly137rnrn, ;;; the practical limit for normal cavity walls spanning in both

~ directionsisabout 6.5 min length andheight.

(I.,.

the effect ive

~

thickness ofa cavitywallis

n

ormally~ (t

l

+

t,)wheret,and t.are

!!.

the actualthicknessofthe

two

leaves.)

Itispossible tobuild panelswhichexceed eitherorboth1and 2 above, but they shouldbeeffectively sub-dividedsothat. while

they maynot appear to be two separate panels. they flex in

bending as though they are. Normally. this sub-div ision is

achieved usingintermediatesupports.Inthis way,conddions1 and2 can be effectivelymet.

Edgerestraint

As With any st ructural member. the question of whether the ends are fully fixed or pinned is not necessanly easy to

determine. The criterion used in

t

he

Code, however, is not difficultto grasp.If the edge of a panel is11l1l%fixed. such that. If loaded. thepaneledge Will fall In flexure before rotating .the panel is assu med tobe 'fullyfixed'.Anythinglessthanthis and

thepaneledgeisassumed to be 'simplysuppo rted'.even If in realityitmay bepartially fixed.

Lookingatdetailedcases:

Meial tiesto columns

Thenon-conti nuousedge. suitab lylied toacolumn. doesnot

give IIl1l%fixity against rotation. It istherefore assumed to be sim plysupported.

Thefullycontinuousedge,achi evedbytakingthewallpastthe colum n. is11l1l%fixed.Itwill break alongthisverticaledge before

it willrotate. Bondedto piers

The above argu men t will alsoapply topiers. While thefIXed support. wherethewallis continuous,isreadityunderstood.the

needfor asim plesupportattheendof the panel isduetothe

abilityofthe pierto rotateintorsion.Iftorsionrestraintwere

present, the endcould be considered fullyfixed.

Bonded relum waifs

Th isfu llfixity IS assumed If thepier becomes a bonded long

retu rnwall. Now both edges are fully fixed. and the wall must crackat theverticalsupports before rotationcanoccur(figu re

38).

Unbonded relum waif usingmelallies

Ifthewall isnot bondedbut merelytied tothelongreturn,some rotation willbe possible.This vertica l edge shouldbeconsidered

tobea sim plesupport.

free lop edge

Irrespectiveofwhat happens at the vertica ledges, the horizontal

edges can also be considered as eithe rfully fixed or simply

supported .In theabove example, thetop edge is actuall yfree

-no support has been provided . This will be designed as a three-sidedpanel.

Waif buill up

10

Iheslruclurewilhsuilable anchorages

Itismore usualto providesome form of support to thetop edge.

In theabove example,the wall has been effectivelypinnedtothe floor above. However, the topedge will stillbe ableto rotate

under lateral load and, therefore, it must be assumed to be simplysupported.

In-situ (loorslab casl on

10

waif

When an in-Situ floorslabIScastontoa wall. theCode suggests

10

that Itcan be considered fully fIXed.

Effecl ofdamp proofcourses

Irrespective of the details at the top of the wall, it must be remembered that most panels havetheir shearand moment resistancelim ited atthei rbasesbythe needto int rod uce dpc

materials.As a result, it is normal to expectthe base of most

panels to be simplysuppo rted.

Experimental validation

Whilst the above app roximations, which give assumed edge

conditio ns,are usefultothebusy design engi neer inpractice,

theyare nonetheless over-simplifications.

In mostofthe casesconside redwhere simplesupportshave

beengiven, a degree of partial rest raint is actuallypresent. In

many ofthe examples,

30%

,

50%or70%fixitymay be present. Besidesthe obviousadvantage of keeping designsimple, this

approachagreeswiththe publishedexperimen talevidence".

Figure44comparesthe predicted failureload, basedonthe designmethod outl ined previously, butusingan estimation of ~ the degree of edge restraint, with the experimental results of

il

walls testedinthe laboratory.Whilstthe correlatio n isreason-

if

ableforfailure loadsof approximately5kNlm2and less, athigher

-g

'

loads, the design method using partial restraints is found toQ.

predict higherst rengths than are fou nd in the laboratory. The

~

strongesttest resultshowninfigure44isfor a wallthat failed

j

between9-10kNlm2, yelthepredicted streng thwas between 14 ~ - 15 kNlm2• It was for this reason that the design method

il.

adopteduses eithe rfullfixityor simplesuppo rtcondit ions. ~

1

1

Ot

particular interest is the caseof the arching action of wind-loaded external walls in a loadbearing structure. In this case,a modified approachtothe archingguidance given in the Code issuggested. In loadbearingwalls,the archingforceis generatedfrom theprecompressivestressalreadypresentdue totheverticalload the wall iscarrying.

Even thoughaloadbearingwallcracksand forms three hinges at A, Band C,itdoes notnecessarilyfail.Failure willonlyoccur whenthelateralloadreaches a levelwhich will movethe point C to the left ofthe lineAB.In otherwords,when6=

t.

The basicequationq,.,=

~

can be establishedby analysing thefree body diagram of one half of the wall (figure 49).This equation is based on stability consideratio ns and not on the flexuralstrengthofthewall.

Theequation was checked againstthe ultimate lateral load of wall panels inafull size5-storey test buildingat the Universityof Edinburgh' (figure 50).The walls were removed byjacking and the failure loadsascertained. Further wall tests,conducted in laboratorymachines undera constant precompression (figu re 51 overleaf),confirmed the earlierconclusionthatthis ultimate load eouat.on gives good correlation with the experimental results,andis a goodfailure predictionequation".

It

is felt that the relatively small scatter associated with the body of test evidenceis dueto the stability natureoftheequation.

The equation is for vertically spanning walls. Three and four-sided loadbearingwallshave evengreater lateral strength. To establish the lateral strength of a three or four-sided loadbearingwall,thestrengthisfirst calculatedfrom the basic equation assumingthe wall spans only vertically.The strength so obtained isthen multipliedby an enhancement factor k, given in Table10of theCode (figure 52overleaf).

The strength enhancement can be signIficant , 3 to 4 for square or nearly squareaspect ratios.Withlam eraspect ratios. as largeconcrete columnsorsubstantial edge beams,itis not abletoextend as itdeflects.Instead, forces aregeneratedwithi n the plane of the wall, and theyeffectively induce restoring moments which stabilise the wall. Walls with arching action present are inherently much stronger than those- such as cladding panels- where archingaction cannot takeplace.The Code givesdesignguidance onthe strengthof this

type

ofwall.

HorizontaIarchi

Ig

This approach" ,unlike partial restraint either estimates the strength realistically or underestimates the strength of the panels,as canbe seeninfigure45, whereallthe pointslie above line A- the45' line. LineErepresentsthedesignmethodwhen using the partial safety factors for both the loads and the materials.

Allowingforprecompression

In many cases, justifying a panel using the recommended designmethod isquite straightforward.In somecases, how-ever,the panel's strength based on this method may be just inadequate forthe load it is being designed to carry.In this situation, the characteristicflexuralstressin the weak direction canbe increased to allowforthe dead weight of the top haf of thepanel.This enhances the strengthofthe panel by:

1, Increasing f", in the weakdirection.

2. Modifying theacoefficientfor3 &4-sided panels by changing

the valueofu.

The strengthenhancement is given bythe formulainfigure 46, below.

->-~ Thedesign method outlinedabovehas dealtwith panels only. ~ However,insituationswherearchingaction cantakeplace,this

~ underestimates the strengthofwalls.

~ To accommodate the deflection induced by loading a wall

-1:

panel,thepanel must increasein length- eitheronplan orin

•

L- -.J 53

DES'GNOlI'BEE

IIDNDIlVGWALLS

pecui"BB

s

l

Xi

eep.

ale

IlIo8d

an

215

nm ....

ecPi*1l1oed an 175nmWllll

Cavitywalls

4-03-0 1·5 1·2

,

1-6 1-5 1·1 1-0

Factor k

52

Number Value of k

ofreturn s Lh 0·75 '·0 2·0 3·0

•

Thusfar. onlythe designof solidsing le-leaf wallshas been discussed . Inthe caseof load bearingcavity walls,it isusually onlythe inner leaf whichis analyse d in arching. sincethisleaf normally carries the sig nificant precompression. In cladding paneldesign.however.both leaves carrysignifica ntlateralload, andth is mustbetaken intoaccount.

Testresults" indicatethatthelateral st rengthof a cavity wallis equal to the sumofthe st reng thofbothindividual leaves.Thisis afort uitous finding and permits an easy and understan dable designapproach .

Wall paneledgesupports

12

when the panel isthree tim esaslong asitishigh .the stiffening effectof the verticaledgesupport dim inishesto virt ually zero and k approac hes1.Effectively. thisrecogn iseswhatcouldrea son-ably be predicted . namely.whenLismuch greater thanh the panel tendsto span purelyvertically .

Freestand ing walls

TheCode givesguidance onthedesign of freestand ing walls.

Thenormal approachforbrickworkfreestanding wallsisbased on flexuralstrengthusingthe modified flexural stress(for weak direct ion bend ing) mentionedearlier(page11).flo<is increased . as before .bytheYmgdterm.Inthecaseof a freestand ingwall. where the flexura lfailure isat the base, gd is based on the dead weightof thefullheightofthe wall.

A fullertreatmentof all aspectsofthe design of freesfanding walls,including details,materials specification, standardsec -tions, strengthdesign and thestabilityof footin gsisgiven in

BDA

Design Guide12".

3' o

~

Whilst the methodusedto design laterally loaded wallsagainst !!.

s-flexu ralfailure hasbeencovered,it is alsoimportan tto ensure [ that thepanelisadequately tiedbackto the structu re.This is ~ morecritical with cladding panels thanwithload bearing walls. ~ Cladding panels are usually tied back to their supporti ng

:l.

structureusing meta l ties. and itis useful to have valuesforthe ~ strength of wall ties used as panel suppo rts . Characteristic •.

valuesforcompression.shearandtensionaregivenin Table 8of

the Code.Whencalculatingdesign strengths,

Y

mis 3

for ties.

Walls with openings

Thedesignmethods coveredsofarhave dealtwithsinq le-leafor

cavity walls of solid construction with no door or window openings.Theintrod uctionofopenings intopanelsreducestheir

ultimate lateral strength. Although some research has been

conducted (figure56)on perforatewalls, nofinal conclusions

have been reached.Consequently, no firm recommendations

have beenincorporated in the Code.Instead.in AppendixD,the

Coderecommendsthat either;

(a) aform of plate analysisis carriedout: or,

(b) a superimpositionmethod isused(figure57).

t. H.W.H. West. H.R Hodgkinson & BA Haseltine. The

resistance of brickwork to lateral loading. Part 1: Ex

-perimental methods and results of tests on small

speci-mens and full sized walls. The Structural Engineer,Vol55,No

10,October1977.

2. BS 3921: Clay bricks and blocks. British Standards

Institution,1974

3. BS 187:Specification for calcium silicate(sandlime&

flintlime) bricks.British StandardsInstitution, 1978

4. BS 6073: Precast concrete masonry units.British St

an-dards Institution,1981.

5. Model specification for clay & calcium silicate

struc-tural brickwork. Special Publication 56, British Ceramic

!2ResearchAssociation,1980(available fromBOA).

•

~ 6. BAHaseltine,H.W.H.West&J.N.Tutt.The resistance of

" brickwork to lateral loading. Part 2:Design of walls to

~

resist lateral loads.TheStructuralEngineer,Vol55,No10,

~

October 1977.

%

7. K.W.Johansen.Yield line formula for slabs.Cement &

'0 Concrete Association.

c

:l'

8.L.L. Jones & RH. Wood. Yield line analysis of slabs.

~ Thames&Hudson, Chatto & Windus,London,1967.

~

Afull treatmentofpanels withopeningsis outsidethescopeof

thispublication.Thereaderisreferredto References12& 13.

The lateral loaddesignof diaphragm andfin wallstructures is

notcoveredin the Code.Indeed, thissubjectisunder reviewby

the CodeTechnical DraftingCommittee.Whilethetheoretical

basis of such designs follows normal sound engineering

principles, asoutlinedabove, the reader is referred to BDA

DesignGuides:8 'Designofbrick fin wallsin tall sinqle-storey

buildings'and 11, 'Designof brick diaphragm walls',formore

detailedguidance141~

REFERENCES

9. A.W. Hendry, B.P. Sinha & A.H.P. Maurenbrecher. Full

scale tests on the lateral strength of brick cavity walls

with precompression. Proc. BritishCeramicSociety21,1974.

10. H.W.H.West. H.RHodgkinson & W.F.Webb.The

resist-ance of brick walls to lateral loading.Proc.British Ceramic

Society 21,1974.

11. J.OA Korff. Design of free standing walls. Design

Guide No12, BrickDevelopmentAssoc iation,February1984.

12. BA Haseltine & J.N. Tutt. Exlernal walls: Design for

wind loads. DesignGuide No4,Brick DevelopmentAssoc

ia-tion,1979.

13.BA Haseltine & J.FA Moore. Handbook to BS 5628:

Structural use of masonry:Part 1: Unreinforced masonry.

BrickDevelopment Association,May1981.

14.W.G.Curtin,G.Shaw,J.K.Beck & WA Bray.Design of

brick fin walls in tall single-storey buildings. DesignGuide

8,Brick DevelopmentAssociation,June1980.

15.W.G.Curtin,G.Shaw, J.K.Beck & WA Bray.Design of

brick diaphragm walls.DesignGuide11,BrickDevelopment

Association,March1982.

16. J.Morton.Accidental damage, robustness

&

stability.

BrickDevelopmentAssociat ion,May1985.

4

DESIGN EXAMPLES

Examp

le

1

A cladding panel2.6mhigh istobe designed incavitybnckworl<. The wallhasnoreturns, and because the verticaledgesare unsupported.It spans vertically.The edgeconditions at the base and the topof the wall are assumed to providesimple supports.The bricksto be used in both leaveshave a water absorption of !f-9'h% and the mortaris1:1:6.Workmanshipand materialsare assumed to be normalcategoryand the hmitingdimensionsare notexceeded.

Estimate the design wind pressure.W. ,whichcan be resistedbythispanel.

The

two

basic equationsare: Designmoment

=

ay, WhL2

Design moment of resistance =

f:;;;!'

Forth ispanel:

a 0.125 for a simplysupportedcase.

Y, 1.2 for a cladding panel whose failure doesnotaffectthestability oftheremaining st ructure.

L 2.6m spanis taken asthe height. h.not the length;since verticalspanning. use flu in the weak directionof bending.

flu

0

.

4N1mm'

for water absorption of 7-12%in1:1:6mortar.

Ym = 3.5 for normalcategoryconstructioncontrol and manufactu ring control of units.

The

strengthof a caVltywall is equalto thesumof thestrengthsof the

two

individual leaves.

Forone leaf: 102 5' Z HXXlx- 6-'- = 1.75x 10"mm3_/mrun. 0.4x 1.75 x 10" Md = 3.5 Nmmlm run=0.2kNmlm run. M = 0.125x 1.2XWkX2.6' . Equating M andM_dgives: _ 0.2 ,_ , Wk - 0.125x 1.2x2.6' kNim -0.2kNlm For cavity wall : W. = 0.2 +0.2=0.4kNlm'

This isalow value.W. isunhkelyto be assmall asthis in practice .and the panelcouldnot bejustified inmanyareasinBritain.

1.3 x 2.0kNlmrun 0.9x 1.3 x 2.0kNim run 2.34

x

10' , = 102.5x

H)3

Nlmm

O

.023N1mm'

= 0.4+(3.5x 0.02)

Nlmm

'

=

0

.

47N1mm

'

!l.1

Ym 0.47 x 1.75 3.5 = O.23 kN1m' Basicequationay,W"L2

Mod ifiedflexuralst ress

0.125x 1.2 x W. x 2.6'

W

.

Example 2

With the above design example try to maximise Wkbytaking these~·weightof the panel intoaccount. KeepYm = 3.5and usethe

samematerials.

Characteristic dead loadG.(assume conservative. ie.

low.

values): Outerleaf(102.5mm) = 2.0 kNlm'

Innerleaf(102.5mm)

plasteredone side = 2.25kNlm'

Theflexural strengthin the weak directioncan bemodrnedto: flu+ Ymgd.

whereg"isthe stressduetothe designverticalload.Thefailurecrack andtherefore thecnticalsection,Ie.thepos.tionofmaximum moment.isassumedto beat rmd-heiq ht.Cons.deronlythe deadweightstressduetothetophalfof the wall.

Outer/ea t Verticalload Design vertica l load St ressdueto design verticalload

Basic equationaYI W. L'

Inner leaf:

Stress due to design vertical load

0. 125x 1.2x W. x 2.6' W. Forthecavltywall.W.

14

0.9x 1.3x 2.25xla' NI ' = 102.5 x

H)3

mm

'i

=

0

.026

Nl

mm

2 _:;.

~

~

~

a

[0.4+(3.5x 30S026)J x 1.75

j

0.24 kN1m' ~ = 0.23+ 0.24kNlm' ~ = 0.47kNlm'

O.4 kNlm'

[0.7+ (3.5x 0.026))x 1.75 3.5

1.75 ,

2x 3.5 : W.=O.99kNlm

conditions.(Notethat the answercould have beenachieved muchquicker.Since f", was increasedbyapproximately20%when modified,W,wouldincreasealsobyapproximafely2O%becauseW, =K f•• is a linear relationship.)

Example

3

A panel identicalto that in Example 1 isto be constructedand1ested in a laboratory.Anestimate is needed, for rig design purposes, of the likelyultimatefailure load.Calculate thisvalue.

At present,themaximumdesignvalue.WI<

=

0.47kNlm2.Thisvalue isderivedusingdesignvalues ofVm.Ytandcharacteristic

valuesfor flexuralstress.Forultimatedesign

Y

m

=VI= 1.0and the mean flexural stressvaluesshould

be

used.

Assuming

no

accuratevaluesforthe mean flexural stressare available,these canbe estimatedfromtigure 9.For bricksof7-12% waterabsorption, the meanflexural stresscanrange betweenapproximately1.0 Nlmm'andthe 0.4Nlmm'f.. value.

Assumef~an= 1.0

Nlmm

'

(strongestbrick/mortarcase).

Modifying thisforselt-weiqht(forsimplicity,takeG.= 2.5kNlm'forbothleaves.2.5kNlm'is conservative,ie,high,forrig design): _ [1 1

x

1

.3x2.5 x

Hi']

-

+

102.5

x

10

3

=1.03 N1mm' Far oneleaf: O 25.1

x

1.0

x

WrnaxX 26. ' = 1.03x11.75 W~. = 2.13kN1m' Forcavilywall: W~. = 4.26kN1m'

This isstillnot a truemaximumvaluesinceitisbased ona prediction of the maximummeanflexural stress.Whilethemean of,say. 10wall tests should beapproximateIy4.26 kNlm'atlateralfailure,theresults will bescatteredaround this mean.

The example could be recalculated if thestandarddeviationassociated With wallette test flexural strengths were known.However,

this is outside thescopeof thispublication. Instead, the rig shouldbe designedusing 4.26 kNlm'together withanadequate factor of

safely.

Example4

.

A panel identical tothat inExamples1&2istobe designedusingdifferentbricksand mortar.Thematerial specificationisnow:

Outer leaf:waterabsorption

<

7%; mortar 1:%:3

Innerleaf: waterabsorption > 12%;mortar1:

Y,,3

Calculate themaximum designwindpressure,W" taking account of theself-weiqht(assumegd= 2.25kNlm'forouterleaf,and 2.50

forinnerleaf).

f", values:outerleafO.7 Nlmm' ;innerleaf0.4

N

/

mm

' .

Outerleaf: 0.125x 1.2x W,x 2.6' W, Innerleaf: , = [0.4 +(3.5 x0.029)) x 1.75 0.125 x 1.2x W, x 2.6 3.5 W, O.25kN1m' For cavilywall: W, 0.4 +0.25 = O.65kNlm'

Hence,bychanging thematerialspecification.thedesign wind pressureWI< canbe increased from 0.47

t

o

0.65kNlm2

•Indeed.rf theinnerleaf were constructedusinga brick of water absorptionoflessthan 7%,W, couldbefurtherincreasedtoapproximately 0.8

kNlm'.Thisisadequateformany situationsin

B

ntain

.

ExampleS

Compare the design windpressureWk=0.65kNlm' ,from the previousexample,withthe designwindpressurefora wall, usingthe

same brickandmortar materials,which is 2.6mlongandpurelyhorizontally spanning(ie,assumethe base ofthe wallisa free edge).

f", values:outerleaf= 2.0Nlmm'; innerleat = 1.1 Nlmm'.

Outer leaf: 0.125x 1.2x W.x 2.6' Innerleaf: , 1.75 , 0.125x 1.2 x W.x2.6 = 1.1x 3.5 : W.=0.54kNim For cavily wall: Wk = 0.99 +0.54 kN1m' l.53kN1m'

This is signrticantlystronger thanthe0.65 kNlm'derived in Example4and demonstratesthe advantageofarrangingpanelsupport

conditionsin a way whichencourages panelstobendintheir stronghorizontaldirection.TheIncreaseinstrengthis solelyduetothis.

A designwindpressureof 1.53kNlm' ismorethan adequate for virtually all conditionsin Britain.(Note:

no

deadweightwas used to

enhancef ",;thisisonlyappropriateforweakdirection bending.)

Example 6

~

RepeatExample5for the practical case where the wall is supportedat itsbase. Assumethe baseof the wallissimply supported,

;: since It Will besittingondpcmaterial.The heightofthe panelis1.3m.No loadsaretakenon thepanel otherthan the windloadingItS

j

surface.

t

Length = 2.6m,height = 1.3m .

"

h!L=

0.5;~= 0.35

~ a= 0.064,Table9 (A),BS5628

~ Outer leaf: , 1.75

~ 0.064x 1.2 xW,x2.6 2x 3.5 ' W""= 1.93

Innerleaf:

0.064

x

1.2

x

W...

x

2.62 ₌ 1.75

1.1 x 3.5 ' Wk,

=

1.06

2.99kNlm'

This isadequate foranywherein Bntain.

Example

7

Acladding panel IS subjectto a designwind pressure,Wk, of 0.8kNlm'(suction).Itis a corner panel,

see

below, freeatthe top edge butsittingon dpc material.~the wall is to be designed USing 102.5mmbnckwork In both leaves, arethere any urmtationson the specificationof the bncksorthe mortar?

FigureA

The assumededgecondmons areshown below.

Thecontmuityof the outer leaf over the columnandaroundthe corner is asumed toprovidefiXitytothe verticaledges.

FigureB

Limiting dimenslons(BS

5628,

CI.36.3): '"= 213(102.5

+

102.5) 137mm Maximum area:}

1

5OOt..'

28m'

AcIualarea = 11.2 m'

Maximum dimension :}50'" 6.85m AcIuallargestdimension 4 m

columns

wall sitsondpc

:

4m /

2·8m

Since neither the maximum area nor the maximum dimension is exceeded, the panel satisfies the limiting dimension requirements.

Basic equationfor oneleaf: oy, WkL'perleaf = !J,d

Y

m

Y

,

= 1.2;Wk=O.4kNlm'(W"" =W,

+

W" but W,=W,):

Y

m

=3.5:

Z

= 1.75x 10" mm3/m. fl.. h 2.8 ocoe icient:

T

=

"4

=O. 7:~= 0.35.

o

= 0.039forhIL=0.5 }

= 0.045for hIL=0.75

BS5628

,

Table9(C)

therefore,o = 0.044for hIL=O. 7, byinterpolation.

M =oy,WkL' = 0.044 x 1.2xO.4x4' kNmlm run = 0.338kNmlm run

= 0.388x 10"Nmmlm run.

1

.7

5 x

1

0"

M,= f"" 35 Nmmlmrun.

Equating

M

and M,gives: f""

~

J;

x 10"= 0.338 x 10".

f""reqd.

=

0.68Nlmm' (strongdirection). Claybricks

Theweakestf""valuein Table3is 0.8Nlmm'.Sincethisexceeds therequired value, any claybricklmortarcombinationcanbeused. Calcium silicate bricks

The

two

valuesof f"" given in Table3,in the strongdirection, are 0.9and 0.6Nlmm'.Only the0.9 Nlmm' is acceptable (ie, mortar designations (i), (ii) & (iii».It is normal,when using calcium silicate brickwork, foramortarno strongerthan designation(iii) to be used.Hence, specificationwouldprobably be 'anycalcium silicatebrickusing a1:1:6 mortar(or equivalent)'.

ExampleS

Because the panel just designed inExample 7 is in a shelteredposition, the architect would liketo use a calcium silicate brick in a 1:2:9mortar (designabon(iv».Can this bejuslifted?

Thewaysin whichthepanelcanbestrengthenedand possiblyjustified are: (i) modifytheflexural strength usingthe panel'sself·weight;

or,

(ii) detailthetop edgeolthepaneltogive a simplesupport:or,

ii

i<

(iii)usehorizontal bedjointreinforcement inoneor bothleaves. "

Since option (iii) is outside thescopeof thispublication, option(i)willbe explored- onlya smallreduction(approximately10%) is ; neededin thef"" required. lithepanel cannot bejustitied,option(ii)will beconsidered. " ~

Option(i) ~

Assume:

Deadload ofboth leaves =2.0kNlm'_{(lowvalueof Gkfor conservative reason)}

Designverticalload

=

O.gx 2.8

=

2.5

k

N

/

m

Stressdueto design 2 5 x 103

verticalload

102.5

x 103 0.025

Nlmm

'

Assumeacalciu msilicate brickusing

1

:2:9mortar (designat ion (iv»).Modifying

I

...

(thisisdone in theweakdirect ion ): I (st rong)

0

.

6

N1

m m

'

;

I (weak)

=

0.2

+

3.5x 0.025

= 0

.

288N1mm

'

Forasingle leaf:

M = ay, W, L'

acoefficient,from Table 9(C).u

=

0.5: a 0.035;

h!L

=

0.5

a 0.043;

h!L

=

0.75

a 0.041;

h!L

=

0.70.byinterpolation .

M 0.041x 1.2 x W,X4' O.79 W, kNmlm run

!od

0.6 x 1.75 x 106 Md _{= Ym = 3.5 x 106} 0.3 kNmlm run EquatingMandMd 0.3 , W, =

0

.79=0.38k

N/m

.

For cavitywall:

W, = 2 x 0.38=0.76

k

N

/m

' .

Thisislessthan W,

=

0.8kNl m' . Option(ii)

Ifcalciumsilicatebrickworkwere tobeusedinconjunction witha 1:2;9 (designation(iv) mortar.it wouldbeadvisable to support the top

0

1

the wall insome way.Normally,th is isdoneusing eitherspecial(sliding)lixingsinconjunctionwithspecial wallties.orbyusing

sectionsofang leorchannelto restrictthelateral movement

0

1

thetop edge.IfthisweredoneWithsuitabledetailsthe panelwouldbe fou r-sided.

Checking thepanel. but nottakingaccountof sell-weight,andrepeatingthe earlier section foroneleaf:

Md = fkx1.75 xl 03.5 6Nmmlmrun. F

t

'

qure

C

M = ay, W,L' .

acoefficient: 2·8m

h!L

=

O

.

7

;

~=

0

.35;

interpolation lrom Table 9 (G) givesa

=

0.031. ~

M = 0.031x 1.2 x0.4x4' =0.238kNmlm run.

EquatingMandMdgivesf...reqd.

=

O

.48

N/

mm

'

.

4m

Thisis less than thef... value

01

0.6

Nl

mm

'

in Table 3.

So

,

thepanel workssatisfactorilyas a tour-sided panel in calciumsilicate

bric kworkina1;2;9 orequivalentdesignation(iv)mortar.

~~__overheadbeam providing

-... -...-- support to topof panel (notshown)

-!;~

~§§§§§§~~

=-

+

-

+-

-

-I-

dPC

t

ray (

not

s

ho

wn

)

Exa

mple

9

A similarpaneltoExample 7 isto be designedlortwodifferentparts ofthe st ructure.The W, valuefor both positions (generalwind pressu re)is O.68kNlm'.Itis proposed to use al00 mm 3.5

Nl

m

m

'

block andaclay brick(bothin1:1:6mortar)f ortheinnerandouter

leavesrespectively. 11thetwo arrangementsforthetwopositions areasshownbelow,commentonanyspecification requirements

fortheclaybricks. columns

....-glazing unit

3m

......o-- - -g ro und floor panel

(1stfloor panelnot shown)

dpc

position1

overheadbeamproviding support tothetop of the panel

(notshown)

17

position2

specialcolumninentrancehall offset fromgridposition&not

supporting edge ofwall ground floor panel (1stfloor panel not shown)

Position1

Assumed edge support conditions: Side 1simplesupport,dpc tray above

Side 2 simplesupport,panel stops - but provide support ties Side3 simplesupport,sits on dpc.

Side 4 fullyfixed.front leatcontinuespast column.

;

®

Inner leaf.'

ay,WkL2

Note:Clause 36.2permits both leavestobeassumedcontinuous,evenIf only one leafiscontinuous, If(r)cavity wall ties areusedin accordance withTable6,(ii)the discontinuous leafisnottbicke:than the one thatiscontmuous.Hence. the assumededgeconditions shownabove applyto both leaves.

tnnerteet

:

100mm3.5N1m m2blockin 1:1:6 mortar.

fluweak = 0.025 Nlmm2; flustrong= 0.45Nlmm2. ~= O. 02Ml.045 = 0.55;hIL =

31

4

= 0.75. FromTable 9(F):

100>

a=0.034; Z = 1000 x 6 = 1.67 x 100mm'/mrun. Equatin9M andMd,ay, WkL2 -

~

-

Y

m

0.45 x 1.67 x 10" 0.034x 1.2x Wk x42 3.5x 10' Wk = 0.33kNlm2.

Since the caVitywall must carryO.68 kNlm2,the outerleaf must carry O.68 - 0.33= 0.35kNlm2.

Outer leaf:

Assume the weakest case in 1:1:6mortarand check the W kfound against theO.35 kNlm2required to be carried:

a = 0.041(~= 0.35;hIL = 0.75;Table9

(F»

flu = 0.9Nlmm2.

Equat_{ingM andMd,}ay, WkL2 -

_-

~

_Y

_m

0.9x 1.75 x 10" 0.041x 1.2 x Wk x 42 3.5x

Hj6

Wk = 0.57kN1m2.

This compareslavourably w rth the O.35 kNlm2the outer leal will be required to carry ilthe inner leaf waslu lly wo r1<ed.

There are no specificationrequirementsforthe clay bricks of the panel in Posrtion1. Any clay brick in 1:1:6 mortarwill be sufficient. (Designload = 0.68kN1m2, Design resistance = 0.9kNlm 2.)

Position 2 "

Assumed edge supportconditions; Same as above (Positio n 1)

exceplthatedge 2 isfree.

®

Repeating the above calculations (in large steps)but basing theacoefficientsonTable 9 (K).

-~

-

Y

m

0.057 x 1.2 x W_{k x}42 = 0.45 3X51.67 Wk = 0.20kNlm 2. Outerleaf: 2 0.9xl .75 0.075x 1.2x Wk x4 = 3.5 Wk = 0.31kNlm2. Cavitywall:

Designwind resistance = 0.20

+

0.31= 0.51kNlm2.

This isnot adequate, anda differentspecificationwill be required.

Either,

recalcu late using an llu of 1.5Nlmm2(to give Wk = 0.52kNlm"l or,

onthe basis thatWkvaries Iineartywith lIu,

0.31x 1.5 2

I

2

Wk 0.9 O.52kN1m when Iu= 1.5N1mm .

ii

Thus.designwindresistance isnow 0.20

+

0.52 = 0.72kN/m2.Thisexceedsthe designwind pressu re of 0.68kNlm2.Therefore, in

:<

posit ion2,the claybricksneedtobe 01lessthan 7% waterabsorptio nina1;1;60r equivalentdesignation(iii)mortar. ~

(Note:It wouldbeQuite possible to investigatethe strengthofthe outer lealusing

a

brickof

1

2%

water absorptionorgreater in

a

g,

strongermortar.Itisunusual, however, todetail a cavity wall with differentmortars ineach leaf;itwouldrequire carefulsupervisionin ~

prsctice.) ..

rtit is planned to use bricks with a wa.t.erabsorption higher than 7%, the strength althe inner leaf will require to be enhanoed.This

i

could be achieved by:

i!

(i) Increasing the block strength specitied. •

•

(iii)Reinf orcingthe Olockwork.

Reinforcingthe outerorinner leaf (orboth) may bethe onlysolutionif a brickofmore than12% waterabsorptionisto be used.

The above optionsassumethat the simplestsolution- namely.providing edge supporttothe assumedfreeedge- cannot be

achieved.This solutionsho uld.of course.be exploredfirst.

Example 10

2·5m

A loadbearingcaVity wall istobechecked for lateralstrength . It iswellsu pported on its

two

vertical edges.but isassumedto besim ply

supported top andbottom.Bothleavescarry theroof equally.W, forthisexposedsite is1.2kNlm'suction.

0.03N1mm' O.02 N1mm' Stress due to designvertical load

Roof /cads:

Lookingfor conservative(unfavourable)conditions.andusingdead

+

wind (noimposed) based on 0.9G, and 1.4 W, .the mini mum

design vertica l load is 6.2kNl mof wall.ie, 3.1kNlmoneach leaf.Both leaveshavebricks of morethan 12% waterabsorprtio n in

1:V,, 3mortar.f,,(weak)=0.4 Nlmm':f,,(strong) =1.1 Nlmm';(assum eu=0.35).

3.1x 1o" 102.5 x 106

0.9 x 1.25 x 2.0x

H

i

'

102.5 x

H

i'

St ress due todesign self·weightof wall

Totalstressfordesignloads

f,, (weak) = 0.4

+

3.5x 0.05= 0.58Nlmm';~=0.5811.1= 0.52.

Rrst checkwhether itwillworkascladding (ie,no allowa nce for verticalload): hIL= 0.45; a= 0.022(Table9(G):~= 0.35).

W L'

h..1

ay

,

k

Y

m

, 1.1x 1.75 ,

0.022x 1.4xW, x5.6 3.5 ;W,= 0.57 kNlm .

Cavity walldesignstrength resistance= 1.14kNlm' ;designload.W,= 1.2kNlm'.

Accounting fordesignverticalstress:

a= 0.018;~= 0.52;hIL = 0.45

W

'

1.1x1.75

0.018x 1.4x ,x 5.6 = 3.5

W, = 0.70 kNlm'

Cavity wall design strength resistance= 1.40kNlm' .

Thisisadequate far W, = 1.2 kNlm'.

Example 11

(To beread inconjunction withBOApubl icat ion 'Accidenta l Dam age,Rabustness& Stab ility"")

A215m m bric kwork wallisto be chec kedto ascertainwhetheritcanWithstand the 'accidental force'equivalent load of34kNlm' (the equivalent gas explosio nlateral pressure).Thedesignload on the wall is140 kNlm run(derived from using partial load factors

appropriate foraccidentaldama ge analysis- Clause 22(d».

I

.J

1 ..

1+102·5mm 215mm ~

t

",

"

s

,!l

accidentaldamage analysisusingthisequationa partialfactor of safetyfor materials represented by

Y

m

=

1.05 canbeused (Clause 37.1.1).

ax

0.215

x

140

ql81

=

1.05

x

2.52 = 36.7kNlm2•

Sincethis is in excess of 34kNlm2,thewallisjudged toremainafter an'accidental event'andis regarded as a protectedmember (Clause37.1.1).

Repeating thedesign,but usingt

=

170mm,

axO.HOx

140

qlat = 1.05

x

2.52

= 2QkNlm2

(ct.

34kNlm2j.

ijthe wall wasconstructedusing the 170mmCalculonbrick, it wouldbejudgedto berell10lledbyan accidentalevent.

(Notethat,"there were oneor two returnson the verticaledge(s),the values of36.7and29

wwrr?

above,shouldbeenhancedbythe appropriate/< factorfrom Table10.)

Assuming that Itwas decidednotto usea 215mm,one brickthick, inner loadbearingleaf,the buildIngwould

need

:

either,

(i) to bechecked to makesurea partialcollapseof unacceptableproportionsdid not ensue; or,

(ii)tobefully horizontallyand verticallytied.

See

Clause37.1and Table12.

Fora fuller explanabon ofthe subjectof accidentalanalysis,see'AccidentalDamage,Robustness<I. Stability'16.

~

t

_!t

..

~

~

I

~

I

- -

-i

)

~

~

..

'0" Readersare expressly advised that whilstthecontentsofthis publication arebelievedtobeaccurate.correct and complete.norelianceshouldbeplacedupon its contents

e asbeing applicable to any particular drcumstances. Any advice. opinion or information containedispublishedonlyonthe footingthatthe Brick Devefopment

f

Assodation.its servants or agents andallcontributorstothispublication shallbeundernoliabilitywhatsoeverin,respectofits contents. C1> Designedandproducedforthe8IidI0eveI0pmentAssocatoo.INoodsideHouse.WInkfield. WIndsor,Berkshire51.42OX.