w. G. CURTIN FICE FIStructE MConsE PhD MEngG. SHAW CEng FIStructE MConsEJ. K. BECK CEng MIStructEW . A. BRAYBEng CEng MICE MIStructE

CI/SfB (21) I F I

April 1983

LOADBEARING BRICKWORKCROSSWALLCONSTRUCTION

Front cover MEDICAL RESIDENTS' HOME,TEACHING HOSPITAL, LIVERPOOL

This is one of the tallest halt-brick slender crosswallstructures in Europe.

There were two main restrictions on this project sitearea and construction costs . The cost of the hospitalhad already exceeded the budget, and this block hadto be a no- frills bui ld",g. Loadbeanng brickwork waschosen for the optimum economy and reliability,

The struc tura l design uses ha lf-b rick (102.5 mm)internal loadbearing walls throughout, except whe resoun d o r fire requtatrons demanded thicker walls, eg,around staircases and lill shaf ts. External walls aresimple cavi ty walls with ha lf-b rick thick leaves. Floo rslabs are solid reintorced concrete, 150 mm thi ck,pa rtia lly precasf to minimise shuttering ope rations onSite, and compnse 65 mm thick prestressed plankswilh an 85 mm fhick insrtu topping.

The maximum designed masonry strenqtn in thelowest storeys required bricks with a crushing strengthof 50 N mm 2 set In a designation (ii) (1 :t:4,D mortar.To achievea satisfactory compromise betweeneconomyand unnecessary and counter-productiveconfusion for the contractor, the masonryspecification was reduced at three levels In the heightof the structure. The bottom three storeys werecons tructed in engineering bricks. the top storeys In

commons. and the Intermediate floors in mediumstrength bricks.

Price £4 .00

~ - ---------~

lyptCal upper ltoor plan

For co nstruction purposes, the buildinq was dividedInto two halves. At each level, when the bricklayers hadcompleted one half, they moved imo the other whilethe concrete floors were constructed", the first half. Inth is way. continuity of work was maintained at alltimes for the retatively small work terce.

Architects Wilham Holford & ASSOCIatesStructural eng ineers W . G. Curtin & Partners.

W. G. CURTIN FICE FISlruclE MConsE PhD MEngG. SHAW CEng FISlruclE MConsE]. K. BECK CEng MISlruclEW. A. BRAYBEng CEng MICE MISlruclE

Loadbeal'ingbrickworkcrosswallconslruclion

CONTENTSI. INTRODUCTION 3Crosswall Construction 3Typical applications 4Common factors influencing design co nsidera tions of a ll forms of multi -sto rey struct ures 5Stability 6Accide nta l damage 10External walls 10Co ncrete roof slab /loadbearing wall connection 12Choice of brick and mortar strengths 12Movement joints 12Provision for services 13Vertical alignment of loadbearing walls 13Foundations 14Flexibility 14Elevational treatment of crosswall structures 15Speed of erection 15Podium construction 15Partitions 15References 15

2. DES IG EX,\;\ IPLE I 21Hostel building 9-store)s high 21Building geometry 21Characteristic loads 21Design of typical internal crosswall 21Design of external cavity wall for wind 25Overall stability 28Accidental damage design 29

3. DESI GN EX,\;\ IPLE 2 31Commercial office development 4-storeys high 31Building geometry 31Characteristic loads 32Design of typical internal crosswall 32Design of externa l cavity wall for wind 33Overa ll stabi lity 36Accidenta l damage 36Other applications 36

The Brick Development Association1

ST JOHN RIGBY SCHOOL,ORRELL , LANCSThis was the job (1958) tha t was to herald a new era instructural brickwork design. The original design wasfor a steel frame with 4in breeze block classroomseparating walls. For acoustic reasons, the client thenchanged the br ief from 4in breeze block partitions to9in brickwork. This meant massively heavier loads onbeams and columns. Thus, all the steelwork sectionswould have to be increased in size - and cost.

The first cost-saving solution was to pin each floor liftof brickwork tightly up against the soffit of the steelbeams over, so that, in effect, the 4-storey height wallswould be virtua lly self-supporting and not imposeextra loads on the steel frame. The stress in thebrickwork, due to its self-weight, at the base of thewalls was checked and found to be insignificant.

A check was then made to determine whether the wallscould possibly ca rry the 7.5 m spans of floors and roof .They could - and the structura l steel frame wasredundant.

Engineering bricks were used at the projec ting ends ofthe crosswa lls to prevent damp penetration - avertica l dpc being ruled out because of the danger atthe projecting ends peeling off from the internalcrosswalls.

The fina l surprise to the structu ral designers was todiscover that as compared with reinforced conc reteand steel-f ramed structu res, the brickwork solutionwas not only chea per but considerably faster to build .

ArchitectsWeightman & BullenStructural engineers W,G,Curtin &Partners.

plon

2

8 ~ . fIocw , 8"'''Iioces soan

I I

EJ I ftoor ; ~ EJsoan "'0:r'"

i I Iglazed elevations 6-8m

For office buildings where the room functionsare accurately known in ad vance, the crosswallcentres can be predetermined. Where greate rflexibility is requ ired in some areas, it is oftenpossible to span the floor in the opposite directiononto the corrido r and external walls for thatarea of the layout, and to introduce demountabl epartitions to suit requirements. However, wheremaximum flexibility is required, the crosswallform ofconstr uction is more restrictive than thespine wall form, where the floors span betweenexternal and corridor (or spine) walls throughout.This latter form of construction will be thesubject of a future BOA Design Gu ide.

more than adequate to prov ide longitudinalstability, which is discu ssed in more detail later.

In many cases, the long floor spans are mosteconomically formed in precast prestressedconcrete units, seated about 100 mm onto thewalls. To give some continu ity and resistance tothe negative moments which will occur in practice(even though, in theory, the units are 'simply'supported), it is advisable to use an rc in-situ in-fillwithin the pc floor over the wall support. Thi s willassist in providing a rob ust floor slab, bett erequipped to resist forces due to accidental dam age(see Figure 3). It is necessary to comply with theBuilding Regulation covering progressive collapsefrom accidenta l damage when the building is fiveor more storeys in height.

floOr units

insitu intiU

Crosswall

longitudinallorcement

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3

bedrooms. (It is, perhaps, regrettable that thereis no similar requ irement for hotel bedrooms).A half-brick wall has an average sound reductionof 42 dB and, if plastered both sides, 50 dB.(b) Party walls - Building Regulations require21 5 mm brick, or similar, between adjace ntdomestic units.(c) Fire barriers - in many insta nces, BuildingRegulations require 215 mm th ick brick, or similar,around stai rcases, lift shafts, vertical serviceducts, etc, in additio n to compartmenting firebreaks alo ng the length of the building - 102.5 mmthick clay brickwork provides 2 hours fireresistance.

These functio nal demands dictate the need forwalls which, if checked, are likely to be equallycapable offulfilling the structural function , thuseliminati ng the need for a stru ctural frame.

Typical applicationsOffice blocks and School classroom blocksLayouts for offices and classrooms can varygreatly, but a typical plan shape is shown inFigure 2.

Where wide-span units are used to provide afairfaced soffit, the in-situ in-fill shown in Figure 4should still be provided.

4

The 'alternate spa ns loaded ' condition, and theresulting bending moments and eccentricity ofloading induced into the walls due to deflect ion ofthe floor units and rotation at the supports, arerarely critical. Nevertheless, the effect ofeccentricity on the bea ring stresses should betaken into account. The reinforcement in the in-filltends to reduce the effect of eccentricities anddistribute the uneven stresses. Many schoo lbuildings were erected in the late '50s to ear ly '70susing high alumina cement in the precast floorunits. Subseq uently, all these build ings had to beinvestigated and, so fa r as the authors' experienceand knowledge are concerned, none of the wallsshowed any distress due to eccentric load ing.

The crosswalls usually need to be 215 mm thickto carry the loads. Gable and externa l walls arenormally in 265 mm cavity brickwork. Co rridorwalls should be at least 102.5 mm for aco usticand fire resistance. The external and corridorwalls, together with the stai rcase, are norma lly4

Bedroom block sFigure 5 shows a typical basic floor plan of abedroom block. Many build ings of thi s type arefive to ten sto reys high, and need to be checked foraccidental damage under Building Regulation017. Floors are usually in-situ continuous

plon

2

8 ~ . fIocw , 8"'''Iioces soan

I I

EJ I ftoor ; ~ EJsoan "'0:r'"

i I Iglazed elevations 6-8m

For office buildings where the room functionsare accurately known in ad vance, the crosswallcentres can be predetermined. Where greate rflexibility is requ ired in some areas, it is oftenpossible to span the floor in the opposite directiononto the corrido r and external walls for thatarea of the layout, and to introduce demountabl epartitions to suit requirements. However, wheremaximum flexibility is required, the crosswallform ofconstr uction is more restrictive than thespine wall form, where the floors span betweenexternal and corridor (or spine) walls throughout.This latter form of construction will be thesubject of a future BOA Design Gu ide.

more than adequate to prov ide longitudinalstability, which is discu ssed in more detail later.

In many cases, the long floor spans are mosteconomically formed in precast prestressedconcrete units, seated about 100 mm onto thewalls. To give some continu ity and resistance tothe negative moments which will occur in practice(even though, in theory, the units are 'simply'supported), it is advisable to use an rc in-situ in-fillwithin the pc floor over the wall support. Thi s willassist in providing a rob ust floor slab, bett erequipped to resist forces due to accidental dam age(see Figure 3). It is necessary to comply with theBuilding Regulation covering progressive collapsefrom accidenta l damage when the building is fiveor more storeys in height.

floOr units

insitu intiU

Crosswall

longitudinallorcement

_8-8

I v / /1 B-, I

: •,eon

I V I ~ I, ,

I

d V ~ B'" 1MI

!

I / , I rein

3

bedrooms. (It is, perhaps, regrettable that thereis no similar requ irement for hotel bedrooms).A half-brick wall has an average sound reductionof 42 dB and, if plastered both sides, 50 dB.(b) Party walls - Building Regulations require21 5 mm brick, or similar, between adjace ntdomestic units.(c) Fire barriers - in many insta nces, BuildingRegulations require 215 mm th ick brick, or similar,around stai rcases, lift shafts, vertical serviceducts, etc, in additio n to compartmenting firebreaks alo ng the length of the building - 102.5 mmthick clay brickwork provides 2 hours fireresistance.

These functio nal demands dictate the need forwalls which, if checked, are likely to be equallycapable offulfilling the structural function , thuseliminati ng the need for a stru ctural frame.

Typical applicationsOffice blocks and School classroom blocksLayouts for offices and classrooms can varygreatly, but a typical plan shape is shown inFigure 2.

Where wide-span units are used to provide afairfaced soffit, the in-situ in-fill shown in Figure 4should still be provided.

4

The 'alternate spa ns loaded ' condition, and theresulting bending moments and eccentricity ofloading induced into the walls due to deflect ion ofthe floor units and rotation at the supports, arerarely critical. Nevertheless, the effect ofeccentricity on the bea ring stresses should betaken into account. The reinforcement in the in-filltends to reduce the effect of eccentricities anddistribute the uneven stresses. Many schoo lbuildings were erected in the late '50s to ear ly '70susing high alumina cement in the precast floorunits. Subseq uently, all these build ings had to beinvestigated and, so fa r as the authors' experienceand knowledge are concerned, none of the wallsshowed any distress due to eccentric load ing.

The crosswalls usually need to be 215 mm thickto carry the loads. Gable and externa l walls arenormally in 265 mm cavity brickwork. Co rridorwalls should be at least 102.5 mm for aco usticand fire resistance. The external and corridorwalls, together with the stai rcase, are norma lly4

Bedroom block sFigure 5 shows a typical basic floor plan of abedroom block. Many build ings of thi s type arefive to ten sto reys high, and need to be checked foraccidental damage under Building Regulation017. Floors are usually in-situ continuous

5lighlWeig'"partman

275 mmcavitywall6

IMngroom bedroom

bedroom

bed bedroom room

bathhandining

,102.5mm service andIoadbearing ventilahonductsdivision walls

krtchen

• •1-- - --2 15mm party walls 1seMceduets102,Smmcrosswalls

concrete slabs. Where the external side walls andthe corridor walls are load bearing, the floor slabsmay spa n two ways. Some minor increase inreinforcement is all that is usually necessary tocope with the accidenta l damage provisions.

Crosswalls usually need to be 102.5 mm thick inorde r to carry the loads and to provide soundinsulation . It is not uncomm on to return the endsof the crosswalls, at their j unctions with theexterna l and cor rido r walls, to improve theirsta bility.

Crosswa ll structures can , of course, be built muchhigher than ten sto reys. However, as with allhigh-rise construction, the costs tend to increasefaste r than the increase in height.

Lo... to medium-risefiat stup to six storeys}A typical floor plan is shown in Figu re 6.

The demand for high-rise flats (which were moresuited to cellular masonry construction) haswaned, and there is now more interest in medium­rise blocks. These are a hybrid form of theclassroom and bedroom blocks, d iscussed ear lier,in that they tend to comprise a mixture of215mmand 102.5 mm crosswalls. The party walls, spacedat about 12m centres, need to be 21 5 mm th ickto comply with the sound requirements of theBuilding Regulations, and the intermediatecrosswalls 102.5 mm thick to give good acousticperformance. Co rrido r walls and externa l wallsare generally of mason ry construction and areused to provide longitud inal sta bility. They mayalso be subject to the requirements of the BuildingRegulat ions for flanking sound transmission.

Floo rs are nearly always of in-situ concrete con­struction. Timber floors could be used in low-riseconstru ction , if/ire regulations permit. It should beremembered that the requirements for the strap­ping and tying of timber floors are different fromand greater than those for concrete floors (seeAppendix C, BS 5628', and Structural Masonryl.oacIlJ('ar;n.1t I"·;CJ..H'O,./" crosswall construction

Designers' Manual ' ). Ca re should be taken toensure tha t the floor construction forms anefficient acoustic barrier.

Common factors influencing design considera tionsof all forms of multi-storey structuresA resume of the more common factors which haveto be considered when designing crosswall andother multi-storey structures is given below, andeach item is then considered in greater detail.

1. S tability , A building must be stable underverti cal and horizontal (wind) loads on both itslongitudinal and lateral axes. Co nsideratio n mustbe given to the effect of openings in the walls onthe stiffness of the building and the design of theshear walls.2. Accidental damage. The design should takeaccount ofgood engineering practice, and forbui ldings of 5 or more storeys comply with D 17of the Building Regulations.J . External ...alls. Support and restraint of theouter leafi s necessary , even where the wall isnon-Ioadbearing. Th is should not be confused withdesign agai nst acciden tal damage.4. Concrete roofslab]...all connections. In-situconcrete roo f slabs should not usually be castdirectly onto masonry walls. As the roof expandsand contracts, due to thermal and othermovement s, the wall will tend to crack,pa rticular ly at the connection. A sliding joi ntshould be form ed between the walls and the roofslab.5. Choice of brick and mortar. Whilst it is qu itesimple to design every wall in every storey heightwith a different brick and mortar, thi s increasesthe costs, planning and supervision of thecontract. On the other hand, although the useof only one br ick laid in one class of mortarsimplifies planning and supervision enormously.it may not be the most economical solutio noverall. Thus, before mak ing a choice, the costimpl icat ions should be carefully considered.6. Movement joints, As with other structural

5

mater ials, movement joints must be incorporatedin the structure. Whilst brickwork structures canprovide a certain amount of resistance to damagedue to movement, it is still necessary to installmovement joints.7, Provisionfor services. Early planning of serviceruns is necessary, so that openings in brickworkframes can be built in.8. Vertical alignment ofloadbearing walls, Forsimplicity. speed of construction and costconsidera tions, walls should remain in the samevertical plane from founda tions to roo f. Where,for special reasons, the occasiona l wall cannot belined up, it is not difficult to acco mmodate suchplan changes - though it does tend to increasecosts.9. Foundations. The foundations for load bearingbrick structures are generally simpler than thosefor structural frames. Th e loads are spread alongthe walls, founded on strip foot ings, so thatcontact pressures are low. In framed structures,load s are often concentra ted at the colum n points,so that contact pressures are high.

10. Flexibility. Occasionally, over a period oftime, there is a need to alter a structure to meetchanged funct ional requirements. In manysituations, brickwork structures are more readilyadaptable to alteration than steel or concreteframe s.

StabilityFigure 7 shows the main forces acting on astructure.

Vertical stabilityIt is rare for vertical instability, ie, collapse orcracking of masonry under vertical load s, to be amajor problem - provided, of course, that thecompressive stresses in the brickwork are keptwithin the allowable limits, the necessaryrestrain ts to prevent buckling are provided, andthe walls are founded on adequate foundations.

Horizontal stability ( at right angles to thecrosswalls}The wind acts on the externa l walls or claddingpanels. These tran sfer the wind force to thefl oors and roof which, acting as hor izont alplates, in turn transfer the force to the transversewalls (see Figure 8). The wind force createsracking in the transverse walls (generally termedshear walls), as shown in Figure 9, but such wallsare highly resistant to rack ing stresses. Th eracking stresses are usually either eliminated bythe vertical compressive load on the walls, and/o rresisted by the allowable tensile stresses in themasonry. If the tensile stress should exceed theallowable limits, consideration should be givento reinforcin g or post-tensioning the walls.

The stresses at the base of the wall are due to thecombined effect of the vertical loading and themoment induced by the wind force, and aredetermined using the normal elastic stressdistribution formula (see Figure 10) :

f = W ± MA Z

7 ,----- vertical loadlng(dead and supenmposed)

There is usually little danger, in properl y plannedmulti-storey masonry structures, of walls

9

r---""-,,- - - - - - - - - ---- - - floor actingas horizontal plate(or beam) translening WInd forces

as reactionsto -

~---transverse walls. ( ie:oossweus .shear walls. pemtcos . etc)

eJdemal wanOf"-ngpane!spanning betweenf\oor'plates'

6

overturning, or failing in horizont al shear,a lthough this does depend on the designer 's skillin producing a suitab le layout.

beams. However, th is is very rare ly a di ffi cultpro blem to overcome if sufficient forethought isgiven to the plan form and the str uctura l layou t.

Longitudinal stabilityUnstiffened crosswall structures - ie, crosswallswitho ut stiffness at right angles to the plane ofthe wall - may not be stable under longitudinalloading from wind, and could collapse like ahouse of cards (Figure I I).

To prevent such actio n, longitudinal bracing isnecessary. Th is is usually provided (see Figure 12)by either :(a) corr idor walls(b) longitudinal externa l walls

(c) stiff vertical box sections formed by the wallsto staircase, lifts and service ducts, or(d) crucifor m, T, Y, L-shaped block plans , or otherplan forms which provide longitudinal stiffness orrobu stness.

Designfor windIn most brickwork cros swall struct ures, thestresses due to wind are insignificant comparedto th ose due to dead and imposed loadin g, asthe worked exam ples will show later.In a steel or concrete frame, the beams andcolumns are of relatively similar stiffness, rigidlyconnected , and are of the same material. However,with a brickwork struc ture, it may not act as arigid frame because the walls are relat ively sturdyand the floor slabs comparative ly flimsy. Forinstance, in a cross wall structure with an internalcor rido r, the walls, being stiff, act as separatevertical can tilevers, and the corr ido r floors tend to

r-r

"""I II ;<J-- wind

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~~= P~~ + _M_" ja x d " b x d2

A =b xd 6

CCJ+ z "~'f = ~- ~­

A Z

wind {terce Plever arm h

11

10

Multi-storey masonry structures tend to rely fortheir stability on their own weight in resistinghorizont al forces due to wind. They are notcapable, as can be steel or concrete fra mes, ofbeing conside red as fully rigid frames for designpurposes. In steel or concrete structures, rigid­jointed frames tend to be necessary to resistlateral wind loading. It is not usually poss ible todevelop as much rigidity at the jun ctions of br ickwalls and conc rete floor slabs as there can be, forexample, between in-situ conc rete columns and

(aj corridor walls ..1 (b) external face walls--.Y (e) vertical box sections

crucjorm T plan lplan Yplan

(d) plan forms givingstiffness in two directions

l.oadbearing brick work cross wall cons truc tion 7

13 Ipin-jointed props

, 7 ,- • I - - -/, I I • __1, , I ._ .f,,

I I I, I I, I I I I I

~I I I / /

II I II I f ] II f f I II I f I I II I I I I

corridor

crosswall structurewithout I. c ol I. 1 ,Icorridors; acting as solid,single vertical cantilever

doub le cantilever 8(.100~

is stiffer than wall (b) which is stiffer than wall (c).The gable wall (d), with small widely spacedwindows , may be considered to act similarly towall (a) if the openings are relati vely small.However , if the windo ws were deepened, thewall would approach the condition of wall (c).

- r-e-

=3~ = 3 ~4 4

2

- ~ ~ '--

t,direction 15

16

z

=

corridor wall

Trectangular

n 6m 14

I' ' Iw;1I 1 I wall

y

I I floor slabs

I : corridor

Only rarely do the calcul ations become verycomplex. If they do, however, or if the designeris in any doubt as to the stiffness of the walls orstruct ure, he should either refer to one of themany computer programme s on the market, orcarry out a model test. If a computer is used, thedesigner should satisfy himself that the programmeis suitable and well founded, and that the resultsof the analysis are reasonable.

The stiffness of a wall is relative to its secondtL '

moment of area, I, = 12'In the crosswall structure shown in Figure 14 :wall x ha s an I value proportional to 3 ' = 27,wall y has an I value proportional to 6'=216.Thus, wall y is eight times as stiff as wall x. Sincethe walls are tied together by floor slabs, they arelikely to deflect equally, and wall y can beassumed to car ry eight times the wind force ofwall x.

The distribution of wind forces, particul arly ontall slender crosswall structures, between walls ofdiffering stiffnesses may need consideration . Someof the main points are illustrated below.

In Figure 16, the I and Z sectio ns are stiffer thanthe T section which, in turn, is stiffer than therectangular section.

act as pin-jointed props (see Figure 13). If bot hwalls are of the same length , L, and thickness, t,they share the wind force equally. When they arenot of equal length , they then share the wind forcein proportion to their relat ive stiffness - if theydeflect equally, as they are likely to do, becauseof the floors' actio n in transferring the force.

In Figure 15, the floor plan of a block of flat sshows walls of differing length (and, therefor e,stiffness) and of differing positions in relation tothe wind. The main wind force would be resistedby walls I, assisted by walls 2, with some helpfrom walls 3, and little help fro m walls 4. Anexperienced designer would probably, at first, onlycheck the effect of walls I and 2 in resisting wind,and then , if they were inad equate, consider theassistan ce of walls 3. He would be likely toignore the minimal effect of walls 4 in resistingthe wind forces. The use of walls 1 only, wouldnecessitat e a long span for the plate action of theroof or floors.

Walls ofdiffering sectionWhen externa l or corridor walls are bond ed intocrosswalls, they change the shape of a cross wallfrom a simple rectangular 'plate' section into aT, I or Z section. This can give the crosswallincreased stiffness, and hence increased stability.

Openings in wallsIntuitively, it can be seen that wall (a) in Figure 178

Stability ofshear wallsThe stress condition for the design of shear walls

0 0

---{> ---{> ---{> ---{>0 00 00 0

17

wallwith noopenings

(a)

wall withcoroor

(bl

gablewallwith'slot'window

(el

gable wallwithsmallwindows

(d)

18 p 19~BM

(a)loading (a) loading

(b)stressblock

"' fk [~]~~"'&n flexuraltensile. stress

flexuralcompressivestress

(b)stress block

stressblockunderaxial loading stress blockunder flexural loading

has been briefly d iscussed, and is based on theW M

formula :f= A ±Z

Brickwor k design involving flexural stresses isalmost invariably limited by the flexural tensilestrength of the mason ry. This is not surprisingsince the ratio of compressive strength to tensilestrength is in the order of 20 to 1. Occasionally,the flexural compressive stresses can becomesignificant, and limitin g so far as the strength ofcertain element s are concerned . Such elementsinclude geometr ical sections such as the d iaphragmwall 3 and the fin wall ' , as well as shear wallswhich are discussed in detail here, and fullyanalysed in Str uctural Masonry Designer s'Manual ' , Flexural compressive stresses are notcovered in BS 5628, but the method of analysiswhich follows is believed to provide a safe andreliable design, and is based on sound engineeringprinciples.

Figure 18 shows the stress block across a wallsubject to purely axial loadin g, and with noeccentricity of that load . The maximumcompressive stress allowable in the wall sectionis limited by the masonry's tendency to buckle,hence the inclusion of the capacity reductionfactor, p.

Figure 19 shows the stre ss block across a wallsubject to purely flexural loading conditions.

BS 5628: Part I does not different iate betweenaxial compression and flexural compression.However , it is genera lly accepted that allowableflexural compressive stresses may be higher thanallowable axia l compressive stresses. The flexuraltensile stresses will, as stated earlier, normallybe the limiting factor . However, if the axia lcompressive stresses already in the wall are addedto the flexural compressive stresses, th is mayproduce a more critical design condition.Considera tion must be given to the need forlimiting the flexural compressive stresses , due tothe possibility of the section buckling under theapplication of such stress (see Figure 20).

Th e autho rs consider tha t the following designmeth od provides a safe and practical solution :(a) In the first instance , the wall should bechecked for maximum axial loading only, usingthe design principles given in BS 5628, clause 32.The capacity reduction factor, p, applicable tothis stage of the design, should be derived fromthe maximum slenderness ra tio. The maximumallowable stress under thi s loadin g condition is :Pf,Ym

(b) The additional compressive stress resultin gfrom the bending due to th e lat eral loading is thenconsidered, and the maximum allowable

combined compressive stress is l.l fk X 1. inYm'

LIJllc/he//rillg hrickl1'(Jrf.. crosswall construction 9

IhNr WIU, bending ,bout majornl, 01' wall

The stress blocks for the two stages of the designare shown in Figure 21 . The application of thisproposed design will be demonstrat ed in theworked examples which follow.

Th e design examples which follow will alsobriefly consider the implicat ions ofaccidenta ldamage , but it is worth repeating here thegeneral recommendations of BS 5628, which maybe interpreted as follows:(I) The designer responsible for the overa ll

stability of the structure should ensure that thedesign, details, fixing, etc, of elements or partsof the structure are compatible, whether or notthe design and details were made by him. All toooften, the design of a buildin g comprises a seriesof element designs, carri ed out by the respecti vesuppliers of precast floors, timber trussed rafterroofs , stee l floor beams, etc, and no one isappoin ted to be responsible for the overallsta bility. A situation which has, uponinvestigation by the authors, been respon sible fornumero us disastrous consequences in the past,and one which should not be allowed to prevailin the future.(2) The designer should consider the plan layoutof the structure, return s at the ends of walls,interaction between intersecting walls, slabs,tru sses, etc, to ensure a stable and robu st design.The collapse of any part of a structure should notbe out of prop orti on to the cause of the collapse,as was the case in the Ronan Point disaster of1968. Progressive collapse is far less likely tooccur in properly designed and detailed brickworkstructures than the untied industrialised precastwalling systems of that era .(3) The designer should check that lateral forcesacting on the whole structure are resisted by thewalls in the plane s parallel to those forces, or aretransferred by them by plate action of the floors,roof, etc, or that the forces are resisted by bracingor other means.

External wallsExterna l walls can be solid, cavity, d iaphragm,fin, or have piers . It is quite common for the outerleaf of a cavity wall, or the face of a solid wall,to be in a different type of unit from the innerleaf or face. In cavity wall construction, a veryfrequent example is the use of a clay facing brickexterna lly and an insulating block internally.

The structure must have adequate residualsta bility not to collapse completely, and theCode further advises that the designer shouldsatisfy himself that ' . . . further collap se of anysignificant proportion or the structure is unlikelyto occur' . The structure is not necessarilyrequired to be serviceable after the event , butcollapse should be limited to provid e a mean s ofescape for the occupants and adequate stabilityto faciiltate its demoliti on or rehabilitati on .

<}-- wind load

floors can offer It' wallrestraint to 't'shearwall-- , '

~ -----1------.'~ - - --T- - - - - -

------~------extra tendency of j'wall to buckle atthis endowing tocombination of axial "/(,<"-"v.(/ /and flexural elevalton on ,hear wallcompressive Istresses - - --..·~t...::;~;::::~ :a i -i tef

{,,.,- Tplanon ~h••r WIll

axia l compressive, + [.1~~~r~~~~~,~Sive-flexural compreesrve stressesstresses

aire•• In,he.r WIll

20

which a 10% increase has been applied to theflexural aspect of the stress, in a similar mannerto Appendi x B of BS 5628. The capacity reducti onfactor, p, should be derived fro m the slendernessratio, which incorporates the effective thicknessappropri ate to the direc tion of the bucklin gtendency (ie, perpendicular to the direction ofapplication of the bend ing) as shown in Figure 20.

Accidental damageDesign for accidental dama ge is a subject initself, and to provide a thorough commentary onall aspects of it would require a documen t aslarge again. Readers are again referred toStructural Masonry Designers' Manu al', inwhich a whole chapter has been devoted to th isimportant subject, and to BS 56282

, clause 37.

21

axialloadplusbending movment

(a) load ing (b ) stress block under axial loading (c) stress block undercombined axialand lateral loading

10

Note that , in the case of a solid wall with differentbrick s on the outer and inner faces, the brick sshould have com pat ible movementcharact eristics.

Cavity walls are more popular than solid wallsbecau se they are more resistant to rainpenetration , and have bett er thermal insulationproperties. However, care and attention must begiven to the choice and fixing of the wall ties.Often, the outer leaf only helps to stiffen the innerloadbearin g leaf - but thi s action is only possiblewith sufficient, good and durable ties. BS 5628advises that the external leaf of a cavity wallshould be supported at least every third storey,to reduce the effect of loosening of the wall tiesowing to the differential movement of the extern aland internal leaves. The Code allows an except ionto thi s rule for buildings not more than 4 storeysand 12 m in height , where the outer leaf may beuninterrupted for its full height.

Whilst, to some extent, both leaves carry the windload , in addition to car rying their own weight , theinner leaf usually supports most of the floor load .Where the outer leaf carries its self weight only,the choice of facing brick is not usually restrictedby strength requ irements.

22

'-' '-J

BE dpc

~'O ------- metetcramos

brick ',._"'-----'-_,Bend /or adhesiveslip •

_ concrete slabcompressible .Il[ _sea lanl----O D

DO00......., .......,

23

,~:.1JA----== outer leal of clayfacingbricks .

" L_ - - - - - - inner leaf 01lightweightinsulatingblockworx

. - floorslabspanningontoexternalwall

------ - compressible filler athead of inner leaftopermitdeflection of

floorslab

24

to minimise the effects of cold bridging at thefloor/wall junctions.

_ concreteslab

..... 00000...... ,.....,

LJ LJ

00stainlesssteel ~ ,Qorother durable I ~~";t.;;;t:====-- --- dpcmetal angle~ j!!

compressible == -searanl---~=!.J';:=;-----

Th e externa l walls in Figure 12(b) may be subjectto high lateral loads combined with on ly minim alvertical loads. Such brick walls do not have a highresistance to bending perpendicular to the ir plane.Th e wall panels in the top storey are most at risk,becau se they are likely to be subject to thegreatest wind pressure whilst the onlycompensating precompression is the verticalloadin g from the roof and the wall' s own weight.If a lightweight tim ber roof is used, there could bewind uplift forces, which would have to becounteract ed by strap ping the roof down to thewalls. Th ere would then be no verticalprecompression in the top sto rey walls.

Generally, thi s is not a significant problem withloadbearing brickw ork - but it can be, if thebrickwork is non-loadbearing and is used merely(and wastefully) as a cladding to a steel orconcrete-framed structure. However, whereload bearin g panels do lack sufficientprecompression , the problem can be overcome by

II

When using a non-structural block for the innerleaf, conside ratio n should be given to providing aflexible jo int to prevent load s being transferredinto it.

For mult i-storey structures with large areas offloor spa nning onto externa l walls, the load s onthe walls may be very high. Th eir strength andthermal requirements may appea r to conflict.There are at least two possible solutions to thisproblem :(a) carry the floor load s on a dense inner leaf,using insulat ion in the cavity ;' ,(b) car ry the floor load s on the outer leaf, itsspecification unchanged if appropriate, so makin gthe insulat ion independent of the structure.

With current thermal requirement s, insulationwill usually be within the cavity, Ca re should betaken, when carrying the floor on the out er leaf,LoadlwuriuK brick work crosswull com /ruction

When the floor slab bear s on a half-b rick outerleaf, it is preferable to carry it across to theoutside face. If it is conside red necessary to maskthe slab, this may be achieved with brick slips.Details of types, fixings, etc, can be fou nd inmodern text books on build ing construction. Atypical detail is shown in Figure 22. If the floorslab bea rs on a one brick or thicker wall, thefloor slab can be masked by a course of bricks,see Figure 23.

Engineers tend to prefer bricks rather than slips,and to anch or them (and thu s restrain the outerleaf) to the slab by anchor ties or steel angles, assho wn in Figure 24.

r;ij~=======-_--nJdlensioned 25j.., -1onlUOspanner

QT1post-tenaklnlng pnnclpte In top storey of multHtorey ttrueture

post-tensioning the wall (Figure 25). Therelatively small diameter rods can be anchoredinto the floor slab below, at regular, designedcentres, and extend up the wall panel through thecavity, where they sho uld be provided with someform ofcorrosion protection , preferably aproprieta ry tape. The rod s a re anchored againat the head of th e wall panel, possibly onto aconcrete ring beam, roof slab or pad ston e,through a steel bea ring plate. By mean s of nutson their thread ed end s, th e rod s can be ten sion edto a designed value, using a simple torquewrench, with du e a llowa nces being mad e for thevario us forms of losses. In th is way, thepreviously inadeq uate preco mpression can beincreased to assis t the wall's sta bility.

Concrete roof slab/loadbearing wall connectionWhilst it is good prac tice, a nd structurallyben eficial, to cast floor slabs onto the walls, it isinadv isable to cas t the roof slab directly on thetop of the upper storey wal l. T he roof slab willtend to expand an d contract with temperaturevaria tions, and if it is restrained by the slab/wallconnection, either it or the wall will crack.

In order to reduce this effect, the roof slab shouldbe separated from the supporting wall. Aneffective separation joint can be achieved byinse rting two layers ofdpc (see Figure 26) or aproprieta ry jointing material. It is essen tial thatthe joint is flat , otherwise a slip plane will not beformed.

Choice of brick and mortar strengthsUs ually, the bottom sto rey mason ry will be themost highly stressed. The stress diminishes ateach sto rey height , a nd th e to p sto rey is usuallythe most lightl y st ressed .

Inevitably, within anyone sto rey height , somewall s will be more heavily stressed than others.For example in, say, a six-sto rey hostel block , th ecrosswalls ma y be 102.5 mm thick and the wall ssurrounding the sta ircase may, for fire protectionpu rposes, be 215 mm thick whil st only carryi ngth e same load as the crosswalls. Thus, it followsth at every storey height could be ofa different12

st rengt h of br ickwork, and th at , within an yonesto rey height, va ria t ions in brickwork strengthcould be employed. Howe ver , a ny savi ngs inmaterial cos ts du e to the widespread varia tionwould be swallowed up by th e extra costs oforganising, sort ing, stacking, supervising, etc .

It is generally advisable to use a maximum of onlythree mortar strengt hs: I :1:3 below dpc level a ndin extremely high ly st ressed work; 1:1 :6 (or 1:1:4)for external a nd highly stressed work; I :2:9 forinternal work (ie, BS 5628 morta r designat ion s(i), (iii) an d (iv) respective ly).

It is difficult for administrative or supervisorystaff to check the strength of th e br icks a nd themo rtar mix by sight. Reducing th e cementconte nt of th e mortar only produces a minimalsavi ng in the cost per m ' of the ma son ry. Everyeffort should be mad e to keep the walls of aconstant thick ness throughout their height. Itsho uld be kept in mind that a slende r, highlystressed wall is usually cheaper th an a th ick wallcarrying a low stress. Brickwork strengths sho uldgenerally be un iform througho ut anyone sto rey,and cha nges in st rength sho uld be limited toapproxi ma tely eve ry three sto reys.

No te tha t a top sto rey wall , due to its sma llpre-load, may have excess ive flexural tensilestress resul ting from wind forces, a nd mayreq uire specific br ick an d morta r st rengt hs tocope with this.

M ovement jointsOn lon g crosswall st ructures, it is esse ntia l toinsert movement joints to counter th e effects ofthermal a nd moisture movements. They a re alsoadvisable on str uctures liabl e to und ergo excess ivedifferential settlement and mining subs ide nce.Movement joints sho uld also be used to brea k upLand T plan shapes, an d other simi lar build ing

26-----:.-__ooncrete roof

~sIab

---,--sIip plane(2 1aye<s'" dpc )

27 .....",

out~ leaf

U M W --'h'

oom~~i::(metener _

W;.- doublecrosswalls

Jffl gap throug h» » ;; 22 22 22 ? fIootslab

? 22 22 2? 21· a 22 V d

configurations, when they are sensitive tomovement. A typical meth od of achieving this , incrosswall structures, is shown in Figure 27.

Services, finishes, etc, which have to cros s themovement gap should be provided with flexibleconn ections, as in concrete or steel-framedstructures. The spacing of movement joints mustrelate to their function, and there are no rigidrules applicable to the dete rmination of spacing.For example, to provide for moi sture and thermalmovement of the masonry, 12 m spacing ofcontrol joints is usually adequate for claybrickwork, wherea s much closer spacing, say5 m or 6 m, is necessary for calcium silicatebrickwork. Further advice is given in CP 121.Settlement and mining movement joint cent rescan only be assessed from a consideration of therelevant sub-soil and mining information.

Provision for servicesInevitably , pipes for hot and cold water supply,conduits for electrical cables, a ir-conditioningducts, etc, have to pass through loadbearing brickwalls. The openings or holes for these servicesmust always be pre-plann ed. Services engineersare accustomed to ind iscriminate breaking outoflarge holes and cutting chases in relat ivelyth ick walls of traditional br ickwork construction,when upgrading or changing the services inexisting buildings. They do not always appreciatethat ad hoc alterations cannot be permitted inmodern , slender, highly stressed walls. Holes andchases should not be cut without the priorapprova l of the structural designer.

Pre-formed openings can easily be arranged by

leaving out bricks when building the wall. If theopenings are large , or could cau se over-stressingor undesirable stress concent rati on in thesurro unding masonry, reinforcement can be laidin the bed joints above the openings - andaround, if necessary - to distribute the stress.Detailed drawings of service holes and chasesshould be given to the contractor before thecommencement of building opera tions. Atypical builders-work drawing is shown inFigure 28.

Chases should be sawn out to the depth agreed bythe structural designer, and not be hacked outby hammer and chisel except in special cases.Horizontal or diagonal chases are rarelypermi ssible in highly-stressed zones, since theytend to redu ce the effective cross-sectional areaand increa se the buckling tendency of the wall.Nor are vert ical chases usually permi ssible inhalf-brick thick walls (102.5 mm) without carefuldesign checks, since they can induce verticalsplitt ing in the masonry.

Holes for vertical service runs th rough floor slabsform a very useful site aid in sett ing out andchecking the vertical alignment of walls. Verticalducts can easily be formed by making minoradjustments to the wall layouts (see Figure 29).

Vertical alignment of loadbearing wallsWhilst engineers and architects have longaccepted the need for column grid layouts, andare well aware of the need to line-up columnsfrom foundations to roof (ie, co lumn positionsshould , where possible, remain consta nt), theydo not, at first, readily accept the same discipline

28 1I d 1111I 17'

external 1I 1I 1-cav ity wall 25mm wide x 10 mm deep

1I 11 '/ chase f()( conduit

11 1I :f75 mmT r:: openirlQ for

'~'"--]a~400mm ~ light SWItch

l 00mm 11l00 mm 1200 mm

opening for H 11sevcee T75mmIh'1I A :""1 75mm

1I ~ ~mm

holes inslabfor se rvices- - - ---j

timbercover

?2 ;

serviceL.--- - - duct

« 22 " 22 U~~::Z::=2222 =

" " " " ~~iU~corridor

22 22;; & 'V 2< ) ;

2< « «

29

Loadbcaring brick work crosswall construction 13

tongnge

",Ie

31

oon'nge o'

mason<ybeam

,-- - - - windows

~-- bandsof brickwofX ofdifferentcolourbricks

and mortar Of variationin bonding

crossweus

]-- - - wi"""'"

w .~l/ Ioadbearingwen I

W compression liang

hcomposite

r oo o I shallowrc 100forming tension fla

L.,

external walls alternate bayssteppedback

30

crosswanspm;eetedbeyond face 10'express the structure',

Brickwork structures can accommodate a widerange offunctional requ irements. It is simply aquestion of choosing the form best suited to thefuncti on .

No n-Ioadbearing partitio ns can still be placedpractically anywhere in a loadbearing brickworkframe. Nevertheless, as with steel or concretecolumns. it is desirable that the load bearingwalls are lined up. They can , ofcourse, be movedout of line - but this may mean expensive andcomplex beam and beam-support layouts. Thisfactor, more than any ot her, has tended tomilitate agai nst the use ofl oadbea ring brickwork,especially in situations where the ground floorlayout differs from the upper floors. For example.in a hotel bedroom block, the gro und floor mayrequ ire large open spaces for restau rant, receptio nareas, etc. The conflicting needs of the groundfloor and the upper floors can easily be reconciledby pod ium constructio n (see lat er).

Th e authors' experience has shown that designersqu ickly adapt to the need for planning discipline,and welcome the benefits of repetiti on of floorlayouts, windows, doors and other furniture ,service runs, finishes, etc, with the accompanyingsavings in cost and erection time, and thesimplicity of construction.

FoundationsThe nar rowest strip foot ing that can beconveniently dug by an excavator usually result sin a founda tio n area such that the soil con tac tpressures are low. For example, a nine-sto reyhostel block with 102.5 mm crosswalls, foundedon a 600 mm wide concrete strip footi ng, wouldhave a contac t pressure of only about 325 kN/m' .When the ground bearing capacity is so low thatpiling is necessary, the wall itself ca n be treatedas the compression flange of a compositereinforced concrete/masonry grou nd beam, withattendant savings in founda tio n costs (seeFigure 30). It should be noted, however , that theuse of a wall as a composi te beam may part iallylimit its adaptability should it become necessaryto change the str ucture at a later dat e.

. in brickwork structures - no doubt because theyhave been used to placing non-Ioadbear ing wallsorpartitions anywhere.

typical crosswallbuilding plan showing key activities 32

o. O.aoo: rmrn mrO O·'·'O,Do::1' "·0:"._00·"o 0 " ''oQ) 0,.., oC . ' ~ steel fixe r

ccncretor fixing shurterer brick layercastingslabs reinlorcement erecti~ building walls

I 2 3 tormwo •

Becau se brick walls are pliant, compared tostructural steel or rc frame s, they are particularlyeconomical on sites subject to mining or othersubsidence. Reinforcing the lower and upper plansbed joints at each storey height results in a wall L..- ...J

that is highly resistant to differential settlement,although care must be tak en to ensure full andadequat e cover to the reinforcement.

FlexibilityMan y designers think that brickwork structuresare inflexible - that it is difficult to alter them ,once they are buil t. Th is is not so. Fo r example,14

rc deck

34

~reinforcement

____ flexible joint

===:JI ~ -- ----d-eb: =from wall

33j crosswans

----- f------ I-- f------ r--- f------ I-- f------

concrete deck

:- r- f- f- ~ !-...V ,/ podium

podium with steel, concrete or brickwork columnssuppo rting a concrete deck, as shown in Figure 33.Depending on the load from the crosswalls, thedeck can be of plate or waffle slab con struction ,diagrid or T-beam . Th e deflection of the deckund er the crosswalls sho uld be assessed, eventhough clay brickwork oft en has an inherentflexibility that enables it to adjust to the deflect ionof a concrete beam.

The spate of conversion , alteration andrehabilitation of brickwork structures in the '70sgave masonry designer s the opportunity to provethat it is often easier to alter a brickworkstructure than a steel or concrete structure. It isoften easier to dem olish a brick wall than a steelor concrete column. And it is far simpler to forman opening in a brick wall than in a reinforcedconcrete wall. Generally, it is cheaper to bond in,thicken, brace, or otherwise strengthen a brickwall than a steel column. It is often easier andquicker to repair an overloaded brick wall or archthan the equivalent in steel or concrete.

one of the authors' most interesting change-of-useprojects was the successful conversion of aVictorian ice-cream factory into an old people 'shome .

Elevational treatment of crosswall structuresLong side walls pierced by hole-in-the-wallwindows can be visually dull. There are manyways of overcoming this - for example by usingdecor ative brickwork and/or modelling theelevation (see Figure 31 ).

Although alterations to modern, highly-stre ssed,load bearing brickwork structures require carefulattenti on , it is onl y on rar e occasions, whenwholesale altera tions are required for a radicalchange of use, that br ickwork structures becomeinflexible.

Speed of erectionThe speed of con struction of crosswall buildin gis very impressive, particularly if the plan formand size of the str ucture allow it to be con stru ctedin qu arters, using the sequential meth od wherebythe trade s can follow each other aro und thebuild ing fro m one quarter to the next as theycomplete their section of the work (see Figure 32).

From the stages indicat ed in the diagram, thebricklayers on co mpletion of Bay 4 would moveup to the next floor and sta rt work in Bay I. Theother trades, ie, shutte rers, steel fixers andconcreto rs, would all move on one bay - theconstruction continuing to spira l up the building,keepin g all trades co nsta ntly empl oyed.

Podium constructionA common objection to the use of crosswallco nstruction is that the gro und floor planningrequi rement s often demand more open space thancrosswalls permit. Typical examples are receptionareas and restaurants in hotel s, car parking forflat s, recreation areas and shops in studenthostels. But the floors above, with regular walllayouts, are ideal for crosswall con struction.

PartitionsWhere non-l oad bearing brick partition s are builtparallel to lon g span floor slabs, particularlywhere prestressed concrete floor s are empl oyed,the deflection of the floor may be of suchmagnitude as to cause cracking in the masonry.Thi s is generally caused by the brickworkatte mpting to arch over the deflected floor - whichproblem can be minimi sed by the introdu ction ofa dp c membrane beneath th e par tition , and theaddition of bed joint reinforcement in lowerco urses, as indicated in Figure 34.

References(I) Structural Masonry Designers' Manual,Curtin, Shaw, Beck & Bray, Granada Publi shingTechnical Division , 1982.(2) BS 5628: Part I : 1978: The structural use ofmasonry , British Standards Institution .(3) CP 121: Part I: 1973. Walling: Brick and blockmasonry, British Standards Institution .(4) Design of brick diaphragm walls, Curtin, Shaw,Beck & Bray, Brick Development Associati on ,1979, revised ed. 1982.(5) Design of brick lin walls in tall single-storeybuildings, Curtin, Shaw, Beck & Bray, BrickDevelopment Associati on , 1980.

Frequently, there is no need to frame the wholestructure, merely becau se of the ground floorplanning requirements. A different structuralform can be used for the ground storey, and acommon solution to the problem is to form aL Olll /f" 'lI l' illg In-:j(:kworl: crosswoll construction 15

Above WOMEN'S HALL OF RESIDENCE ,BANGOR UNIVERSITYBy the time this design was started (1960) and followingthe experience at St John Rigby (page 2) andsubsequent loadbearing brickwork schools , thedesigners were sufficiently experienced in structuralbrickwork design to be confident that 9in and 6in(Calculon) walls were unnecessa rily thick, and that4tin thick walls would be adequate.

However, no test data on such slender crosswalls wasavailable. Calculations and design assumptions werechecked and re-checked , and compression testsca rried out by Professor A. W. Hendry on storey-heightpanels at Liverpool University confirmed expectations.Later, after completion of the building , a full -scalemodel of the structure was built and tested byProfessor Hendry at Edinburgh University.

A major problem at the start of the cont ract was inachieving the standa rd of brickwork specified . Neitherthe site agent nor the clerk of works could convincethe operatives of the more exacting standards ofbrickwork required. Finally W. G. Curtin tried withslides of research work calculations, specificationclauses, and a cardboard model which showed howthe structure would work. The message got through.The standard of work shot up, and the turnover otlabour ceased. So good was the standard ofbricklaying, that the client agreed to the erection of apanel of brickwork carrying the initials ot thebricklayers.

Architects Colwyn Foulkes & PartnersStructural engineers W. G. Curtin & Partners

16

Above LINNET LANESHELTERED HOUSING, LIVERPOOLA single block of 32 flats. visually divided by a recesson the fron t elevation to give the appearance of twodomestically-scaled units. Half-brick and one-brickthick crosswatls support precast concrete floors . withlocal insitu areas giving support to cantileveredbalconies. Reinforced concrete footings onvibro -compacted sub-strata.Architects Build ing Design GroupStructural engineers W. G. Curtin & Partners

Right CHRISTOPHER GRANGE BLINDINSTITUTE , WEST DERBY, LIVERPOOLAn early use of podium cons truction. Ground floor isan open plan rc framed area providing reception,meeting . occupational therapy areas. chapel . etc.There was no need to carry the frame through to thebedroom and flat block over - as is usually done.Loadbea ring brick crosswalls built ott the rc podiu mprovided a much more econo mica l solution.Traditional strip and pad foundations.Architects Roy Cro ft & PartnersStructural engineers W. G. Curtin & Partners.

Left ST PETER'S COURT SHELTEREDHOUSING, ROCK FERRYAn extensive three-storey scheme for MerseysideImproved Housing Associa tion . Structure incorporat eshalf-brick and one-brick thick crosswalls, short spanpreca st concrete floors, and rc staircases.Foundations incorporate vibro -compacted ground inassociation with reinforced strip footings.Architects Merseyside Improved Housing ArchitectsStruc tura l engineers W. G. Curtin & Partners

l.oadbraring "rieJ.. work cross wall construction 17

sectton

Above MULBERRY COURT ,OXFORD STREET, LIVERPOOLRequirements were for a development providingpremises for the Midland and National Westminsterbanks and three shop units, premises for the JointService Units, and residential accommodation forstudents in such a form as to be suitable for use byeither undergraduates, postgraduates, marriedstudents and staff .

The site lies between buildings diverse in form andcharacter, and thus posed difficult problems in thecreation of a good visual relationship between the newbuildings and their existing neighbours . To deal withthe problem of scale, the two lower storeys werevisually combined into one thus helping to achieve atransition from the large scale of Mount Pleasant onone side and the smaller one of the precinct on theother .

The structural form is podium construction with aconc rete frame up to first floor level and loadbearingbrickwork above. Allowance was made for differen tialmovement between the grou nd floor framed structureand the brick struc ture by dividing the residentia lblock into units and providing movemen t joints acrossthe building.

Architects Manning Clamp & PartnersStruc tural engin eers W. G. Curtin & Partners

18

J

Lllat/hearing hl'icJ,. l\'o,.J.. crosswoll cvnstrnction

Left and above SUB-DIVISION POLICEHEADQUARTERS, ELLESMERE PORTLoadbearing brick columns (some backed by internalcrosswalts) with rc edge beams spanning between.Half-brick and one-brick thick internal crosswa lls.Accommoda tion incorporates large open area s.A similar sized steel-tramed structu re was startednearby at the same time. By the time the steel trameha d been encased in conc rete, the police were movinginto their load bearing brick structu re.

Architects Paterson Macaulay & OwensStruc tural engineers W. G. Curtin & Partners

Below and left below BAPTIST MEN'SMOVEMENT SHELTEREDACCOMMODATION, PRINCES AVENUE,LIVERPOOLA five- and four-storey block with a mansard roof .built in a conse rvation area and designed to blendwith the adjacent properties. lnsitu concrete floorscarried on load bea ring brick crosswalls. Can tileveredwindow details. Provisions for vertica l expa nsionincorporated in externa l walls. Reinforced footings onvibro-co mpacted fill to old chu rch basement.

Architects David Parry, OUigg in & Gee Assoc iatesStructura l engineers W. G. Curtin & Partners

second floof plan

19

D

Left and above BLAIR COURT,BIRKENHEADA six-storey block of flats at the edge of BirkenheadPark for Merseyside Improved Housing Associa tion.Built on a sloping site, the front entrance is at groundlevel, with a basement opening out to a lower level atthe rear of the building. Loadbear ing half-brick andone-brick thick crosswalls support insitu rc fioors .Common room in the basement area incorporates rcframes . Mansard roof at top level to conform toadjacent bUilding heigh ts.

Architects Paterson Macaulay & OwensStructural engineers W. G. Curtin & Partners

Dground 'Ioor plan

Above and right GAMBIER TERRACE,LIVERPOOLPart of a fine Rege"cy terrace overlooking theAnglican cathed ral. Unfortunately, the building wascrumbling and pract ically beyond repair . It wasdecided to preserve the facade, demolish the rest ofthe building, erect a new structure and pin the

restored facade to it. Scheme comprises a four- andsix-storey block of sheltered housing, with serni-insitufioors spanning onto loadbearing brick crosswatls.Original facade was temporarily propped - finalrestraint being provided by the six-storey element.

Archifects David Parry, Quiggin & Gee AssociatesStructural engineers W. G. Curtin & Partners

20

DESIGN EXAMPLE 1

HOST EL BUILDING 9-STOREYS HIGHBuilding geometry (see Figures 34 & 35)Overall height = 24.30 mOverall length = 32.00 mOverall width = 14.00 mFloor to floor height = 2.70 mSpan of rc floor s = 4.00 m (150 mm thick in-sit u co ncrete).

Assum e : ma sonry density = 19.ookN/m 'concre te density = 24.00 kN /m '

Roof a nd floor slabs are 150 mm thick in-situ reinforced concrete. Exte rnal faci ng br icks selected havea wate r absorptio n of 7%, a nd a compressive strength of 35 N/mm '. Designati on (iii) morta r ( I :1 :6)will be used exte rnally th rou ghou t. Extensive q uality co ntrol and testin g of material s will be ca rriedo ut, a nd supervision of the reputabl e con tracto r will be maintained a t a ll times .

Characteristic loadsRoof: dead load, G•• 150 mm slab = 3.60

1.00screed to falls , say = .:..:..=,.::-..,...,...,.;--:

4.60 kN/m 'imposed loa d, Q. , (no direct acce ss) 0.75 kN /m '

Floors: dead load , G•• 150 mm slab = 3.60partitio ns = 0.90services. etc = 0.20finishes, etc = 1.30

76.7oo~k:-:NC:-/:-m-:'

imposed load , Q. , (bedrooms) 2.00 kN /m '

Wind loading: is ass umed to have been ca lculated on th e basis of CP 3: Chapter V : Part 2 : 1972, togive a maxim um character istic wind pressure on the walls of + 0.90 kN/m ' , an d a maximu m gro sscharacteri stic wind up lift on the flat roof of + 1.25 kN/m ' .

Design of typical internal crosswallLoading

Characteristic Characteristic Imposeddead loads imposed load lessfloors & roof kNfm load reduct ions kNfm

roof (4 x 4.6) 18.4 (4 x 0.75) - 3.0 0% 3.008th 18.4 + (4 x 6.0) 42.4 3 + (4 x 2.0) - 11.0 10% 9.907th 42.4 + 24.0 66.4 II + 8 = 19.0 20% 15.206th 66.4 + 24.0 90.4 19 + 8 - 27.0 30% 18.905th 90.4 + 24.0 114.4 27 + 8 = 35.0 40 % 21.004th 114.4 + 24.0 138.4 35 + 8 - 43.0 40% 25.803rd 138.4 + 24.0 162.4 43 + 8 - 51.0 40% 30.602nd 162.4 + 24.0 186.4 51 + 8 = 59.0 40% 35.401st 186.4 + 24.0 210.4 59 + 8 - 67.0 40% 40.20

From previou s experience it is expected that a 102.5 mm thick wall will be adequate for the full heightLoadbearing brickwork crasswall construction 21

of the intern al crosswalls. Less expe rienced designers a re referred to Structural Ma sonry Designers'Manual' for guidance o n obta ining tr ial wall sections, and for table s of design loads for solid walls.The mas onry strengths required will be ca lculated at the four levels marked with an asteri sk inFigure 35 (ie, lowest, fourth and seventh sto reys).

5.0-r-4.0- r-- 4_0~ 4.0 -r- 4.O.,.. 4.0~ 4.0 -,.3.0

~ ~ i IT±Tl~]6 .2S E6 11-1T11:1_

typical floor ~an 35

32 0 ~

14 .0

root

81h

7th

6th

51h

41h 2.70 24.30

3rd

2nd

lsi 2.70

A' ground 2.70t....:J .. -

typical section

The design vertical strength of the internal crosswalls will be assessed initially and, for this, the loadingcombination of dead plus imposed will be con sidered :hence , y, = 0.9 G, or 1.4 G,and = 1.6 Ok(BS 5628, clause 22).

Design /oads(inc/uding own weight ofmasonry}At level A :dead load , floors & roof = 210.4 x 1.4 = 294.56imposed load = 40.2 x 1.6 = 64.32dead load masonry = 0.1025 x 19 x 24.3 x 1.4 = 66.25

total n; = 425.13 kN/m

At level B:dead load , floor s & roof = 138.4 x 1.4 = 193.76imposed load = 25.8 x 1.6 = 41.28dead load masonry = 0.1025 x 19 x 16.2 x 1.4 = 44.17

tot al nw = 279.2 1 kN /m

At level C :dead load, floor s & roof = 66.4 x 1.4 92.96impo sed load = 15.2 x 1.6 24.32dead load masonry = 0.1025 x 19 x 8.1' x 1.4 22.08

total n; = 139.36 kN /m

Capacity reduction/actorThe in-situ concrete floor slab can be assumed to provide enhanced resistance to lateral movement.Hence, the effective height of the wall may be tak en as 0.75 times the clear height (BS 5628, clau se28.3.1.1).Therefore ,effective height = (2.70 - 0. 15)0.75 = 1.91 mand

. h, 1.91slenderness rauo = t:" = 0.1025 = 18.6

22

The proport ion of dead load to imposed load in this exa mple will en sure th at th e resultant eccent ricity ,of th e loading system will be within 0.05t, an d therefore will no t influence th e capacit y reduction factor.This can be ver ified by a simple calcula tio n of th e th eoreti cal posi tio n of th e res ulta nt load , based on theass umptio ns give n in BS 5628, clau se 31.

The dead an d imposed load s above th e slab level under co nsiderat ion may be taken as axia l, and th ealternate spans loaded situation will be ana lysed . Thus, the load ing a rra nge me nt show n in Figure 36is appropriate to th e maximum possible ecce ntr icity of load.

calculation of eccentricity of load 36

axial /oad from wallsand floors over O,9G,

dead only(O,9G,)

I I I' - 51·' - 51· '

masonry specifications shown on typical sec tion

Check th e condition at level A .R 1 = (1.4 x 6 x 2) + (1.6 x 2 x 2)

= 16.8 + 6.4 = 23.2 kNjrnR 2 = (0.9 x 6 x 2) = 10.80 kN/m .M inimum load in wa ll above I st floor slab

= 0.9 (186.4 -+- 0. 1025 x 19 x 2 1.6)= 205.62 kN /m .

Result ant positi on fro m left ha nd face of wa ll(205.62 x 0.05 1) + (23.2 x 0.017) + (10.8 x 0.085)

205.62 + 23.2 + 10.8= 0.0493 m .

Th erefor e, eccentrici ty from <I.. a t to p of wall= 0.051 - 0.0493 = 0.0017 m

ex = 0.0 171. ie, less tha n 0.051.Check the co ndition at level C.R 1 = (1.4 x 6.0 x 2) + ( 1.6 x 2.0 x 2)

= 16.8 + 6.4 = 23.2 kNjrnR 2 = (0.9 x 6 x 2) = 10.80 kN jrn.Minimu m load in wall above 7th floor slab

= 0.9(66 .4 + 0.1025 x 19 x 5.4)= 69.22 kN rrn.

Resultant position fro m left hand face of wall(69.22 x 0.051) + (23.2 x 0.0 17) + (10.8 x 0.08 5)

69.22 + 23.2 + J0.8= 0.0469 m.

T herefo re, ecce ntricity from <I.. at top of wall= 0.05 1 - 0.0469 = 0.004 1

ex = 0.004 1t, ie, less tha n 0.051.Then , at a ll levels up to level C :with slende rness ra tio = 18.6a nd ex = 0 to 0.05t ,fro m BS 5628, ta ble 7, ~ = 0.749(A bove level C, ass ume an eccentricity of 0.11.)

Partial safety f actorf or material streng thF ro m BS 5628, clau se 27, both manufactu ring and co nst ruction control can be cla ssed as 's pecia l' fo rLoodbearini: bricl: work crosswall construct ion 23

bricks with a compressivestrength or SON/rrm' setin a designation (iii) mortar

bncks with a compressivestrength of27.5 N/mm' setin a designation (ii) mortar

R aJl loadbearing walls

bricks witha compressivestrengthof 20 N/mm' selindesignation(iii) mortar

87654

321

G

extemal lacingbfid<wor1( to havea comp<essoves.engthof35 Nfrrvn2 andewete-

~C~adesignation(iii) mortar

37

aa~anngwaJIs lO have awaler absorption of 7%to 12%

the conditions described at the beginning of th is design example. Thus. from BS5628. Table 4. Ym = 2.5.

Th is value must be assessed by the de signer. based upon the conditio ns prevailing. for each ind ividu al project.

Masonry strengths r~quiudFrom BS 5628. clause 32.2.1 :

design vertica l load resistance = /3tf. .. n;Ym

Therefore. characteristic strength of mas on ry required. f••O. Ym

= Jjt.For narrow brick walls. BS 5628 permits a stress increase factor of 1.15 (see BS 5628. clause 23.1.2).Therefore. characteristic strength of ma sonry required. f••

n.,¥m= 1.15/31"

At level A :characteristic st rength of masonry req uired, f••

425.13 x 2.5 x 10'1.15 x 0.749 x 102.5 x 1000

= 12.04 N/mm '.Use bricks with a compressive strength of 50 N/mm '. set in a designation (ii) mortar.At level B:characteristic strength of ma sonry required. f••

279.21 x 2.5 x 10'= 71'""'.1-::5-'x:":"::0'::.7=47:9;-:'-x=7I0"'2::':.~5 :":x'---;I"'OOO=

= 7.91 N/mm' .Use bricks with a compressive strength of 27.5 N/mm ' . set in a designation (ii) mortar.At level C:characteristic st rength of ma sonry required . f••

139.36 x 2.5 x 10'= 71.'""'1-::5-x:"::":'0'::.7;;47:9;-:'-x=7I0"'2::':."5-'x--;I"'OOO=

= 3.95 N/mm' .Use bricks with a compressive strength of 10 Njmrn", set in a designation (iii) mortar.

It may be noted that the majority of bricks classified as 'commons' have a compressive strength ofatleast 20 Njrnm ", although some ca lcium silicate bricks can be oflower st rength. In practice. therefor e. it maybe considered that the required compressive strength of 10 N/mm' is an unreasonably low specifica tio n.

= 42.4 x 1.4 = 59.36= I I.Ox I.6 = 17.6= 0.1025 x 19 x 5.4 x 1.4 = 14.72

91.68

Check characterist ic wall strength above level C :with slenderness ratio = 18.6and e, = 0.1t.from BS5628. Table 7./3 = 0.682.Design loads at level D :dead load. floors & roofimposed loaddead load ma sonry

Characteristic strength of masonry required. f••91.68 x 2.5 x 10'

= '71'""'.\-:5,.....:.x-'0=-.'=68"'2=--x::.;..;;.10"'2"'.-=5-'x----:I-=OOO=

= 2.85 N/mm'.Therefore. brick and mortar strength specified for level C is adequate. The masonry specification.reducing in the upper storeys. is shown in Figure 37.24

bricks with a compressivestrength or SON/rrm' setin a designation (iii) mortar

bncks with a compressivestrength of27.5 N/mm' setin a designation (ii) mortar

R aJl loadbearing walls

bricks witha compressivestrengthof 20 N/mm' selindesignation(iii) mortar

87654

321

G

extemal lacingbfid<wor1( to havea comp<essoves.engthof35 Nfrrvn2 andewete-

~C~adesignation(iii) mortar

37

aa~anngwaJIs lO have awaler absorption of 7%to 12%

the conditions described at the beginning of th is design example. Thus. from BS5628. Table 4. Ym = 2.5.

Th is value must be assessed by the de signer. based upon the conditio ns prevailing. for each ind ividu al project.

Masonry strengths r~quiudFrom BS 5628. clause 32.2.1 :

design vertica l load resistance = /3tf. .. n;Ym

Therefore. characteristic strength of mas on ry required. f••O. Ym

= Jjt.For narrow brick walls. BS 5628 permits a stress increase factor of 1.15 (see BS 5628. clause 23.1.2).Therefore. characteristic strength of ma sonry required. f••

n.,¥m= 1.15/31"

At level A :characteristic st rength of masonry req uired, f••

425.13 x 2.5 x 10'1.15 x 0.749 x 102.5 x 1000

= 12.04 N/mm '.Use bricks with a compressive strength of 50 N/mm '. set in a designation (ii) mortar.At level B:characteristic strength of ma sonry required. f••

279.21 x 2.5 x 10'= 71'""'.1-::5-'x:":"::0'::.7=47:9;-:'-x=7I0"'2::':.~5 :":x'---;I"'OOO=

= 7.91 N/mm' .Use bricks with a compressive strength of 27.5 N/mm ' . set in a designation (ii) mortar.At level C:characteristic st rength of ma sonry required . f••

139.36 x 2.5 x 10'= 71.'""'1-::5-x:"::":'0'::.7;;47:9;-:'-x=7I0"'2::':."5-'x--;I"'OOO=

= 3.95 N/mm' .Use bricks with a compressive strength of 10 Njmrn", set in a designation (iii) mortar.

It may be noted that the majority of bricks classified as 'commons' have a compressive strength ofatleast 20 Njrnm ", although some ca lcium silicate bricks can be oflower st rength. In practice. therefor e. it maybe considered that the required compressive strength of 10 N/mm' is an unreasonably low specifica tio n.

= 42.4 x 1.4 = 59.36= I I.Ox I.6 = 17.6= 0.1025 x 19 x 5.4 x 1.4 = 14.72

91.68

Check characterist ic wall strength above level C :with slenderness ratio = 18.6and e, = 0.1t.from BS5628. Table 7./3 = 0.682.Design loads at level D :dead load. floors & roofimposed loaddead load ma sonry

Characteristic strength of masonry required. f••91.68 x 2.5 x 10'

= '71'""'.\-:5,.....:.x-'0=-.'=68"'2=--x::.;..;;.10"'2"'.-=5-'x----:I-=OOO=

= 2.85 N/mm'.Therefore. brick and mortar strength specified for level C is adequate. The masonry specification.reducing in the upper storeys. is shown in Figure 37.24

Design of external cavity wall for windThe critical design case will occur on the top storey at the gab le, where the minimum compression onthe wall is further reduced due to wind uplift pressures on the roof slab. Walls subject to high lateralloading, and low compress ive load , are more likely to fail due to flexural tensile stresses, rather thanaxial compressive stress or buckling.

Consider roofupliftminimum roof dead load = 0.9 x 4.6 = 4. 14 kN/m 'maximum wind uplift on roo f = 1.4 x 1.25 = 1.75 kN/m 'nett roof dead load (afte r uplift) = 4.14 x 1.75 = 2.39 kN/m 'nett roof dead load on wall = 2.39 x 2 = 4.78 kN jm

Design methodBS 5628 acknowledges that there is not a precise design method for such walls, but suggests twoapproximate methods:(a) designing as a wall panel supported on a num ber of sides;or,(b) designing as an arch spanning between supports .

Regard ing the second option, for the wall under consideration there is insufficient dead load availableto resist an arch thrust in the vertical plane. In any case, the autho rs consider that the op tion ofdesigning as an arch can be difficult to justify, and should not generall y be used.

Tak ing the first option, BS 5628, clause 36.4.3, gives the flexural strength of vertically loaded pane ls as:

fk • h- Z, were1mfk • is the characteristic flexural strength.1m is the partia l safety factor for materials,Z is the section modu lus.No account is taken of the considerab le ass istance to this resistance moment that is provided by thevertical compressive loads.

The authors consider that the following design method, in which the applied bending moments areassessed from basic pr inciples and the compre ssive loads in the brickwork are exp loited , provides asafe and practical design based on sound and reliable engineering principles.

Stability moment of resistanceClause 36.5.3 of BS5628 gives the design moment of resistance for free-standing walls as :

(fk• + gd ) X Z,

1min which the assistance provided by the axial compressive stress, gd, is exploited. However, this formulais based on elastic analysis, and is limited by the flexural tensile resistance which may, in fact, be zeroat the base of the wall if a felt dpc is present.

The stability moment of resistance concept exploits the gravitati onal mass of the brickwork , plus anynett roof loads, to generate a resistance momen t. Under lateral wind pressure loading, the wall willtend to rota te at dpc level on its leeward face, and 'crack' at the same level on the windward face, asindicated in Figure 38.

38

",ackal dpc

<J-- - - - - - --'--"""I I '- wall rotat es

-{ I f+. about this2 po int

Loatlhearillj! brick work cross wall construction 25

In limit state design, the previous knife-edge concept of the point of rota tion is replaced with arectangular stressed area, in which the minimum width of br ickwork , w. , is st ressed to the ultimate toproduce the maximum lever arm for the ax ial load to generate the maximum stability mom ent ofresistance MR •. The ultimate stress applied to th is minimum width of brickwork is termed the'allowable flexural co mpressive stress', P ub"

I.If. l3 . hichP ubc = ---, In W rc

Ym1.1 is the stress increase factor to take acco unt of the flexural aspect of f.,f. is the cha racteristic comp ressive stres s,Ym is the partial safety factor for materials,13 is the capacity reduction facto r which, owing to the restraint available a t floor level, is taken as

1.0.

For this design example, the axia l load differs in each leaf of the cavity wall, and the total stabilitymoment of resista nce will be equ al to the sum of the stability moments of resistance of the two leaves,provided they are tied in accordance with the provisions of BS5628, clau se 36.4.5. This is con sideredacceptable, since the resistance moment is genera ted by the rotat ion of the leaves and, once it hasreached its full value, does not reduce significantly th rou gh furthe r rotation.

w.

lever a rm

lever a rm

inner leaf

outer leaf, w.

Allowable flexural compressive stresses:

I f 1.1 x 8.5 (35 N{ ' b . k . desi . (...) )outer ea , Pub< = 2.5 10m nc s 10 esignauon III mortar

= 3.74N{mm' ,

. I f 1.1 x 5.8 (20 N{ ' b . k . desi . (iii) )inner ea , P ub' = 2.5 10m ric s 10 esignatron III mortar

= 2.552 N{mm' .Minimum axial load in leaves :outer leaf = 0.9 x 19 x 0.1025 x 2.7

= 4.732 kN{m,= 4.78 + 4.732 (roof dead - roof uplift + ow brickwork)= 9.512 kN{m.

Minimum width s of stress block s:axial load,

P ub<4.732 x 10'3.74 x 1000

= 1.26 1010,102.5 - 1.26

2= 50.621010(see Figure 39),

9.512 x 10'inner leaf, WI = 2.552 x 1000

= 3.72 1010,102.5 - 3.72

= 2= 49.39 10m(see Figure 39).

1, 102 5"I

~.~ ::J2.552 Nlmm'ol3 72mm

49.39 mm -

outerleaf;

102 .5

~.-n ---';74 Nlmm'

rJ l ' 26 mm

SO.62mm

inner leal; 39

MR. a4 .732 x 0.05062 -0.24 kN.m MR. "'9.512 x 0.04939 ..0.47 kN.m

total MA . .. 0 .24 + 0.47 .. 0 .71 kN.m

stability rncMnt.nt or ,..Iatllnce

26

Stabilit y moments of resistance:outer leaf, MR. = 4.732 x 0.05062

= 0.24 kNm,inner leaf, MR. = 9.512 x 0.04939

= 0.47 kNm,total M R. = 0.24 + 0.47

= 0.71 kN m.

This stability moment of resistance is now considered to provide partial fixi ty to the base of the wall span.

0.7 12.55

point of zero shear fromroof prop

shear at roof prop

Design bending momentCalculate position of maximum span moment by locating zero shear position:

y,W. h MR.= - 2- - - h-

1.4 x 0.9 x 2.552

= 1.329 kN/m,1.329

1.4 x 0.9= 1.055 m,= (1.329 x 1.055) - (1.4 x 0.9 x 1.055'J

2maximum wall moment

= 1.402 - 0.70 1M ; = 0.70 1 kNm

It is considered that , if the stability moment of resistance at the base of the wall should exceed

y,~•..1:: - that of a propped cantilever - the applied moment should be limited to this value, and the

. d "I . hi h h . II Id b 9y,W. h'wall designed as a true proppe ca nt, ever In w IC t e maximum wa moment wou e 128

The bending moment diagram for this example is show n in Figure 40.

40 <)- prop force 1.329 kN/m

1.055 m

~----maximumwall

momen10.701 kN.m

~Slat>"ly momenl o,resistance - 0.71 kN.m

des~n bending moment diegr.m

= 0.0646 N/mm' .

= 1.751 x IO 'mm 3•

Check stresses at level ofM w

The stresses at the level of the maximum wall moment, M , ; will be calculated using the formula

C" + gd) Z, given in BS 5628, clause 36.5.3.Ym

Hence, design mome nt of resistance = ( f•• + gd) Z, where :Ym

f.. is the characteristic fl exural strength (tensile),Ym is the partial safety facto r for materials,gd is the design vert ical dead load per unit area,Z is the section modu lus of wall section.For both leaves,Z _ 1000 X 102.5'

- 6For outer leaf,gd = 0.9 x 19 x 1.055 = 0.018 N/mm '.For inner leaf,

O0 84.78

gd = . 1 + 102.5

Loadbearing brick work crosswotl construction 27

As, at the level of Mw , the two leaves deflect by the same amount and their stiffnesses are equal, thebending moment at thi s level will be shared equally between them .Hence , BM per leaf = 0.701 x 0.5

= 0.35kNm,design M R of outer leaf(7% water absorption bricks

in designation (iii) mortar) = (~:; + 0.018) 1.751 x 106

= 0.382kNm.Thi s is satisfactory as it exceeds the applied bending moment of0.35 kNm.

Now calculate the characteristic flexural strength required for the inner leaf brickwork :

0.35 x 10' = (~·.5 + 0.0646 ) 1.751 x 10' ,

(0.35 x 10' )

required f.. = 2.5 1.751 x 10' - 0.0646

= 0.338 N/mm'.

Then, from Table 3 ofBS 5628, the inner leaf bricks are required to have a water absorption of between7% and 12%, and be set in a designation (iii) mortar. This compares with the requirement for 20N/mm 'bricks, set in a designation (iii) mortar, from the compressive strength part of the calculatio ns. The finalchoice ofbricks must balance these requ irements.

41shear walla proyktlng0Yef'II1I a bility

Overall stabili tyIn line diagram form , Figure 41 shows the main walls which will provide overall sta bility to thestructure. The numbered walls provide stiffness to the narrow axis, whilst the lett ered walls providestiffness to the longer axis of the building. The unmarked walls, although capable of offering someresistance to the wind forces, are ignored to simplify the calculations.

Most of the walls, particularly the main crosswalls numbered 3 to 16 inclusive, intersect with otherwalls to form T, L, and Z-shapes on plan - thu s providing extremely stiff sections to act as shear walls.However, and again for simplicity ofcalculation , only the stra ight rectangular sections of these wallswill be considered as effectively resisting the lateral wind moments on the bui lding as a whole. If thisproved to be insufficient, a calculation based on the actual number and geometric shape of the shearwalls would be carried out, shari ng the wind load in accordance with the loaded conditio n and shearwall stiffness.

Relative stiffness ofshear wallsThe total wind moment acting on the building will be shared between the shea r walls resisting thatmoment, in proportion to thei r stiffnesses and the load and span configurat ions. For simplici ty, th is will

L'be considered as equating to ~L '

B,K.N.P,QC.D. S,T.U.VA,F.RtEG.H.J.L, M

Longitudinal walls (see Figure 41) :wall letter length, L,

(m)2.53.54.05.0

28

L'

15.642.964.0

125.0

No ofleaves( x6)(x 12)( x 7)( x 5)

93.6514.8448.0625.0

1681.4

wind moment shareL ' t :!:L'0.00930.02550.03810.0743

Lat itudinal walls (see Figure 41) :wall number length, L,

(m)1, 2,17 5.03 to 16 inclusive 6.0

L '

125.0216.0

No efleans( x 5)( x 14)

625.03024.0

wind moment shareL ' ! l:L '0.0340.059

andZ

Therefore, gd

3649.0

Note that in calculating the relati ve stiffness, the externa l cavity walls (A, B, C, D, E, R, S, T, U, V,I , 17) are each considered to provide two leaves of 102.5 mm brickwork to resist the wind moment.

Design wind moments on buildingLongitudinal direction :1.4 x 0.9 x 14 x 24.3 x 12.15 = 5208 kNm .Latit udinal direct ion:1.4 x 0.9 x 32 x 24.3 x 12.15 = 11 904kNm.

These are the maximum total wind moments acting in each direction , which the shear walls are req uiredto resist within the lowest sto rey height of the building.

Consider typical crosswall ( 7) in ground storeyShare of total wind moment = 0.059 x 11904

= 702.3 kN m.Minimum dead load in wall = (0.9 x 210) + (0.9 x 19 x 0. 1025 x 24.3)

= 231.6 kN/m.231.6 x 10'102.5 x 1000

= 2.259 N/mm ' ,102.5 x 6000'

6= 6.15 x 10' mm'.

Design moment of resista nce = (~: + gd) Z

= (~: ; x 2.259) 6.15 x 10'

= 15123 kNm at base of wall.Which exceeds the applied wind moment of 702.3 kNm, thus ju stifying the simplifica tion of thecalcu lations, mad e earl ier, in assessing the relat ive stiffnesses.

Thi s design momen t of resistance has been calculated on the assumption that full restraint aga instbuckling is provided by the gro und floor slab and, therefore, no stress reduction factors are app licable ­which is a perfectly reasonable prop osition. However, at mid-storey height , the buckling tend ency ofthe length of shear wall und er flexur al compressive stress can have an effect on the allowable stres ses,and the design meth od described ea rlier in 'Stabil ity of shear walls' will be applied as follows :

I fl I· 1.1~fk

allowab e exura compressive stress, Pu be , = -­Ym

1.1 x 0.749 x 12.22.5

= 4.021 Njmm >

231.6 x 10' 702.3 x 10 'design flexural compressive stress,fub" = 102.5 x 6000 + 6.15 x 10'

= 0.38 + 0.114= 0.494 N/mm'.

Ot her shea r walls provi ding overall stability to the building shou ld be checked using the same basicprin ciples but, by inspection , these also should prove to be comfortably withi n the allowable stressesgiven in BS 5628.

Accidental damage designThe performance parameters for designing buildings to withstand accidental damage are set out insectio n 5 of BS 5628, and under Building Regulation D 17. Int erpretation of the wording of both thesedocuments can be complex and confl ict ing. The main objective of the rules for accidenta l damagedesig n is to produce a sufficiently robust st ructure to withstand damage of limited proport ions. Hence,the design techniqu es sho uld be a combinatio n of stress calculat ions and descriptive reasoning todemonstrate, beyond reason able doubt , the inherent robustness of the structure under various damagesituations.Loadbearing brickwork crosswall construction 29

T his bui ld ing. be ing over 5 storeys in height. is classed as a Catego ry 2 buildi ng. Ta ble 12 of 13S 562Hgives three options for designing and detailing such a structure for accidental damage. Option I. inwhich vertical a nd horizontal elemen ts (un less 'protected ') arc to be proved removable. one at a time.without ca usi ng co llapse. is considered most appropriate for this type of building.

Cross walls removedThe normal span of the in-s itu concrete 11001' is perpendicular to the line of the crosswa lls. If a crosswa llis removed. the slab is de signed 10 spa n in the opposi te direction onto the corridor and e1evationalwalls using. if necessary. increased distribution reinforcement designed on the appropriately reducedpartial safety factors. In addition. the slab would tend to hang. in a catenary. between the crosswallseither side of the removed wall.

Ca lculations could be prepared for either of these alternative means of support. and should beaccompanied by a commentary on the as sumptions made for the design method chosen.

Gahle wall removed13S 562Hstates that for walls without vertical lateral suppo rts . the whole length ofexterna l wallsmust be considered removable. whilst for simila r internal walls. only 2.25h need be considered a s theremovable length . The Build ing Regulations do not differentiate between interna l and external walls,hu t limit the removable length to 2.25h for all walls. II seems to the authors particularly ha rsh toconsider. say . in a spine wall stru cture of 30 m or more in length . the po ssibi lity of an incident capableof removing such a disproportionate length of external wall. II is sugg ested. therefore. that in certaincircumsta nces the designer sho uld use his d iscret ion in assessing a realistic but reasonable length fo rrem oval.

Having assessed the rem ovable length of gable wa ll. co nsideratio n ca n now be given to the a lterna tivemea ns of support fo r the st ructure, following its removal. If the length removed is not excessive,co nsideration may be given to co mposite ac tio n of th e masonry over ac ti ng with the 11 0 01' slabimmediately above the removed length of wa ll. T his. togethe r with th e arching effect of the ma sonry tosprea d the load s over 10 the other side of the removed length of wa ll. may be a ll that is necessar y. withthe add itional reinforcement . if any. being added periphera lly in thc in-s itu 11001' slab .

A mo re complex ana lysis might consider two adjacent 11 0 0 1' sla bs acting a s the flange s of deep I-bca rnswith the corridor and eleva tional walls between them acting as the webs of the same beam. Theseco mposite sections may be used to ca nti lever from the last crosswall and could support , at the end of thecan tilever . a similar I-sha ped composite beam uti lising the gable wall as the web . Thus. a framework ofcomposite beams is provided . and reinforced accordingly. to suppo rt the structure over (see Figure 42).

428Ssessment of des6gntoraccklental damage

e1evationalwan

gable wallremoved ----'

wall rotenon under I.tltfalload

composrteI sectlOt'lS.,....------","""","ng lXl<rido<wall,

/' ;:;dfloor slabs cantlievet'out from last c:rosswan

It may well be thai . in th e lower storeys of a multi-s torey load beari ng brick structure, th ere is eno ughco mpress ive load from a bove to ena ble the wa lls to be designed to withstand the lateral force of 34kN/m ' . thus defining that wa ll as a ' protected' member which does no t ha ve to be co nsidered rem ovable.

Th e relat ive simplicity with which the requi remen ts for acc ide nta l damage ca n be met in loadbearingbrickwork design is indicative of the general robustness of thi s for m ofco nstruction. T his ro bustnesswas d ramaticall y demonstrat ed du ring the last war, when nume ro us masonry struct ures, with a la rmingportion s and co rne rs blown out through bomb da mage, rem ained sta ble. Many were simplyst reng thened locally to continue their useful life.

311

DESIGN EXAMPLE 2

COMMERCIAL OFFICE DEVELOPM ENT 4-STOREYS H IGHBuilding geometry (see Figures 43 & 44)Overall height = 13.6 mOverall length = 46.0 mOverall width = 46.0 mF loor to floor height = 3.2 mSpan of precast floors = 7.0 mAssume : masonry density = 19.0 kN fm 3

precast floors = 2.75 kNfm' .

707.070

office office.----> .---->

otnce.---->

office office.----> .---->

43

r r r r-~~"""'Ii----",-r-----''';'rri''l'''''

8.0

I:l-18.0 2.0t-

8 0

IL 1-_~_....._~_~_~ ,

46.0

office

1office

17.0

o"oce

1o"oce

17.0

typical floor plan

office1 7 0

46.0!.-----------,,~---------

31

.20

.20

.20

.20

0.80

Uflatroof A1-

[OJ IT:]] IT:]] [0] office office 3(L

[OJ IT:]] IT:]] [0] office office 3

I ~

[OJ IT:]] [0] [0] off~ office 3

IL

0=0 [OJ [OJ rr::::::Il office office 3G.... .....

44

Ro of and floor slabs are 225 mm th ick prestressed concrete units. External facing bricks selected havea water abso rption of 6.5% and a compressive strength of 50 N/mm'. and are to be set in a designation(ii) mortar. Extensive qu ality control and testing ofma terials will be exercised throughout. and strictsupervision will be permanently employed.

Characteristic loadsRoof dead load, G. , PC units

screed to falls

imposed load , Q•• (offices)

Floors

= 2.75= 1.25

4.00kN/m'imposed load . Q••(with direct access) = 1.50 kN/m'

dead load . G., PC units = 2.75part ition s = 1.85finishes = 1.35services = 0.30

6.25 kN /m'= 2.50kN/m'

Wind loadingIs assumed to have been calculated on the basis ofCP 3: Cha pte r V: Part 2: 1972. to give a maximumcha racteristic wind pressure on the walls of + 0.70 kN /m ' . and a maximum gross characteristic winduplift on the flat roof of + 1.05 kN /m ' .

kNjm10.5025.2036.4044.10

Characteristic Imposedimposed load lessload reductions(7 x 1.5) = 10.5 0 %10.5 + (7 x 2.5) = 28.0 10%28.0 + 17.5 = 45.5 - 20 %45.5 + 17.5 = 63.0 - 30 %

kN jm28.0071.75

= 115.50159.25

(7 x 6.25)+ 43.75+ 43.75

Design of typical internal crosswallLoading

Characteristicdead loadsfloors & roof(7 x 4)28 +71.75

115.5

roofth irdsecondfirst

With a floor to floor height of3.20 m, a half brick thick wall would be approaching the limit ofmaximum slenderness ra tio , and would therefore require a relat ively high strength brick. In addition , abearing width of 102 mm is not adeq uate to receive long-s pan prestressed concrete units fro m eitherside . Hence, 215 mm thick walls will be adopted for the main load bearing walls.

De sign wall in the lowest storey for the loa ding combination ofdead plu s imp osed. Thus, the partialsafety facto rs will be:"(to dead load = 0.9 G. or 1.4 G.and imposed loa d = 1.6 Q. (BS 5628, clause 22).

Design loadsdead = 159.25 x 1.4 = 222.95imposed = 44.10 x 1.6 = 70.56masonry = 0.215 x 19 x 12.8 x 1.4 = 73.20

total n, = 366.71 kN /m .

Capacity reduction factorAs for the previous design example, the concrete floor slabs can be assumed to provide enhancedresistance to lateral moveme nt. Hence :32

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

effective height = 0.75 x clea r heighttherefore, h" = (3.20 - 0.225) 0.75 = 2.23

. h" 2.23 0 4slenderness rat io = t:;" = 0.215 = I . .

The calculation of the eccentricity of the load is carried out in a similar ma nner to the previous designexample, and from Figure 45 the following values can be calculated :

45c.lculMton of eccentrtctty of Io.ct

axial load rrom wallsand floots over 0.90_

.n.I. , I, .1107.5 107.5

R, = 40.425 kNR, = 19.69 kNminimum loa d in wall above first floor = 139.24 kNresultant position from left hand face = 100 mmeccentricity about il. at top ofwall = 7.5 mm,therefore,e, = 0.035t (which is less than 0.05t).Thus, from as5628, Table 7:with slende rness ratio = 10.4

and e, = 0.035tP= 0.962.

Partial safe ty factorf or material strengthSimilar conditions exist to those applicable in the previous design example. Hence, from as 5628,Ta ble 4 :Ym = 2.5.

Ma sonry strength required

Characterist ic strength of masonry required , f. , = niJ; m

366.71 x 2.5 x 10'0.962 x 215 x 1000

= 4.43 N/mm '.Use bricks with a compressive strength of 15 Njmrn ", set in a designation (iv) mortar.

Design of external cavity wall for windThe design principles will be simila r to those used in the previous design example. Considera tion willalso be given to the walls running parallel to the span of the pc units and, for these, a 215 mm thickinner leaf will be checked. In both cases, the wall in the topmost storey will be designed .

Case I , wall supporting roof unitsConsider roofupliftminimum roof dead load = 0.9 x 4.0 = 3.60 kN/m 'ma ximum wind uplift on roo f = 1.4 x 1.05 = 1.47 kN/m 'nett roofdead load (after upl ift) = 3.60 - 1.47 = 2.13 kN/m 'nett roof dead load on wall = 2.13 x 3.5 = 7.455 kN/m 'Loadbearing hricJ.. work crosswall construction 33

= 4.664 Njmm'

= 7.5 mm

= 47.5mm

= 50.5 mm

= 1.936 Njmm'

== 1.5 mm

inner leaf, Pubc

inner leaf, Ws

outer leaf, P ube

Calculate stability moment of resistaneeAssume 102.5 thick brickwork for both leaves.Allowable flexural compressive stresses:

1.1 0 x 10.602.5

1.10 x 4.402.5

Minimum axial loads in leaves:outer leaf = 0.9 x 0.1025 x 19 x 4.0 = 7.011 kN/minner leaf = 7.011 + 7.455 = 14.466 kN /mMinimum width of stress block s:

7.011 x 103

= 4.664 x 1000102.5 - 1.5

214.466 x 10 3

= 1.936 x 1000102.5 - 7.5

= 2lever arm

lever arm

outerleaf, w.

Stability moment ofresistanceouter leaf = 7.0 I I x 0.0505 = 0.354inner leaf = 14.466 x 0.0475 = 0.687

total MR. = 1.041 kNmThe stability moment of resistance is shown diag rammatically in Figure 46.

46outer leaf:

1"'02.5 ' I

~.11J 664N/mm'

lJ~1 5mmSO.5mm

Inner leal :

' I 102.5 _I

~.~=:Il.938N/mm,

CJ ~75mm47.5mm

47

]

prop f0f'C:8 1.243 kN /m

height to top1.268m [01parape t wall ]

""1.268 + 0.8 - 2.068m

-------maxlmum wallmoment M.. - 0.788 kN.m

MR,. 7.011)(0.0505 -0.354 kN.m MR•• , • .466)(O.0475 - O.687kN.m

total MA. - 0.354 +0.687 - 1.041 kN.m

It8blltty moment of ,,"INne.

This stability moment of resistance is less than the design bending moment at the base of the wall whichwould be applicable to a true propped cantilever. Hence, the stabi lity moment of resistance isconsidered as a partially fixed base, and the maximum design bending moment in the height of the wallis calculat ed to coincide with the level of zero shear.

Maximum wall moment

Shear at roof prop

Point of zero shear fromroof prop

y,W. h MR .= - 2 - - - h-

1.4 x 0.7 x 3.2 1.041- - -2 3.2

= 1.243 kNjm.1.243

- 1.4 x 0.7= 1.268 m.

1.268')Maximum wall moment, M; = (1.243 x 1.268) - (1.4 x 0.7 x - 2-

= 1.576 - 0.788= 0.788 kNm.

The design bend ing moment d iagram is shown in Figure 47.34

= 1.5mm= 50.5mm

The two leaves of the cavity wall deflect, under wind load, by the same amount and are of equalstiffness. Therefore, the maximum wall moment will be shared equally between two leaves.Thus, M; per leaf= 0.394 kNm .

Now calculate the characteristic strengths required for the brickwork in the two leaves to withstandth is design bending moment.

From f.. required = (~w - gd). as in the previous design example:

1000 x 102.5'outer leaf,Z 6 = 1.751 x lO' mm '

gd = 0.9 x 19 x 2.068 = 0.035 N/mm'inner leaf, Z = as above = 1.751 x 10' mm '

7.455gd = 0.035 + 102.5 = 0.108 N/mm ' .

Characteristic strengths required

Outer leaf, f•• requ ired = (~:~~~ : :~: - 0.035) 2.5

= 0.475 N/mm' .Which is less than the f•• provided of 0.5 N/mm' for the facing bricks and mort ar specified.

Inner leaf, f•• required = G:~~~ : : ~ : -0.108) 2.5

= 0.292 N/mm '.Therefore, use bricks with a water absorption of between 7% and 12% set in a designation (iv) mort arfor the inner leaf.

Case 2, check the non-loadbearing external wallsS tability moment of re.,i.,tance(215 thick inner leaf )Allowable flexural compressive stresses. as calculated for load bearing external wall .Minimum axia l load in leaves:outer leaf = as for previous wall = 7.011 kN /minner leaf = 0.9 x 0.215 x 19 x 4.0 = 14.706 k /mMinimum width s of stress blocksouter leaf, w. = as for previous walllever arm

inner leaf, w.

lever arm

14.706 X 10'1.936 x 1000215 - 7.6

- 2

= 7.6mm

= 103.7 mm

Stabiiity moments ofresistanceouter leaf = 7.011 x 0.0505 = 0.354inner leaf = 14.706 x 0.1037 = 1.525

total MR. 1.879 kNm

The stability moment of resistance exceeds the applied design bending moment appl icable to a truepropped cantilever, which is calculated as:YrW•h' = 1.4 x 0.7 x 3.2' = 1 254 kN

8 8 . m.

Hence, the wall will be designed as a propped cantile ver, and the condit ion at i h down from theroofwill be examined.

Maximum wall moment s

= 9YrW•h' = 9 x 1.4 x 0.7 x 3.2' = 0 706 kNM ; 128 128 . rn,

The two leaves will deflect equally under wind loadin g, but they are not of equal stiffness, hence themaximum wall moment will be shared between the two leaves in proportion to their stiffnesses.

Thu s:

ou ter leaf, I1000 X 102.5'= - --:-;:-- -

12= 89.7 x lO'mm'

l.oa dbearing brickwork crosswall construction 3S

inner leaf, 11000 X 21 5 '

=12

Therefore. share of M.

out er leaf, M. = 0.706 ( 89.78: ~28. 2)

inner leaf, M. = 0.706 ( 89.78~8~~8 . 2)

= 828.2 x 10' mm'

= 0.069 kNm

= 0.637 kNm

Characteristic strengths requiredFrom :

requ ired f•• = (~. - g.) r;1000 x 102.5'

outer leaf, Z = 6 = 1.751 x 10'mm '

g. = 0.9 x 19 x 2.068 = 0.0354 Nlmm '. 1000 x 215'Innerleaf,Z = 6 = 7.70x IO' mm '

g. = 0.9 x 19 x 2.068 = 0.0354 Nlmm '

Therefore :

outer !caff•• required = (~:~~~ : :~: - 0.0354) 2.5

= 0.010 Nlmm'. f . d ( 0.637 x 10 ' 003 )Inner leaf •• require = 7.70 x 10' - . 54 2.5

= 0.12 Nlmm'

Each of these requ ired values is less than those calculated for the loadbeari ng external walls, hence thesame brickwork specificatio ns will be adequate.

In each of these external wall designs, consideration must also be given to the effect of windowopenings. and for a suggested design method for dealing with such a perforated wall, readers arcreferred to Structural Masonry Designers' Manual.

The requirements for supporting the outer leaf of cavity walls in tall buildings have been discu ssedear lier under External Walls . Whilst this structure is of four storeys only it is 13.2 m high. Thu s, asupport is necessary, and it should preferably be introduced at second floor level.

Overall stabilityByconsidering the plan shape of the whole building, and the disposition of the substantial crosswallsand corridor spine walls, it can be stated that 'by inspection ' the overall stability of the bui lding docsnot require to be ju stified by calculation . The previous design example, on a very much more slenderstructure employing narrower crosswall s, demonstrated that the stre sses in the shear walls resistinglateral wind loading were relatively insignificant.

Accidental damageThe building is four storeys high and, therefore , does not come under the requi rements of D 17of theBuilding Regulations. As such, the building does not require any additional precautions to be takenwith regard to accidental damage, other than those generall y provided for in clause 2 of BS 5628 andsummarised earlier in this publication. Tying of the adjacent spans of precast concrete units. as shownin Figures 3 & 4, can provide an effective and inexpensive means of alternative support in a collapsesituation, and is, perhaps. the least that the designer may consider to be warra nted.

Other applicationsThe building considered has been described as an office building, providing a number of adeq uate lysized roo ms between the widely-spaced crosswalls. The design would require little altera tion to suit thefunctions of a school classroom building , in which the room sizes are compatible, and only a sma llincrease in the imposed load , with possib ly a compen sating reduc tion in the partitions load . being alltha t is necessary.

Similarly, small four or six bed wards in modern hospita l developments would be admira bly suited tothis form of construction, with the application of an almo st identical design process.36

Above NURSES ' HOSTEL, OXFORD STREETMATERNITY UNIT, LIVERPOOLTwin six-storey units with insitu co ncrete floors carriedon half -brick thick crosswans. Provisions for verticalexpan sion incorporated in external walls.

Architects Ormrod & PartnersStructural engineers W. G. Curlin & Partners.

Back cover BLAIR COURT. BIRKENHEADsee page 20.

Rradrrs arl' f'xprl'uly Qd~/J,.d ,hat, whifst th.. ro"t,."u of this pub/if"ul;orl 0" bt-fi,~,.d to ~ aU"Ho lr. correct Gffd complru. "0 ,,.llallet' $llDuld IN placrd Ilf'Olt Itsro..t,.,,/S as bt-ift6 applirablf' 10 afly /NI',ir"1a, rirrUmSlaftCf's. All)' odvier. opi"io" or in/ormolloff ('o" IQ;/" d if ,publishrd o"ly (}If til, /oot'''' ,114' , II,. B';ck.lH",lop"',.", AuOC"iatiQ", /U Sf" "'''' '1 or OKl',ds ,,,, d all u m tribut," s to 'his ".,blit'Qtlon shall '" lIt1lh, ItOliabIlity wha tMW""" i" rrlpHl 01 Its COII,,.,,U.

Dffiped and produced for The 8rick De~dopmenl AMOCialion . WOO<bide Hocse , Winkf~ld . Windsor , Berkshire SU 2DX. Tc:kphone Winkf>eld Row (0J4.4)AA.~5 1 try ROIUI ld Adams Associates. Printed by Rollkecp Ltd .

Loadbea..ing bdckwol'kc..os~llconsll'uclion