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•ENGINEERS FILE NOTE NO .7 .

Published by the BrickDevelopment Association

CISfB' I I

I (21.9) I FG2 I

January 1988

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A REINFORCED

BRICKWORK FREESTANDING

BOUNDARY WALL

By G.D. JohnsonB Eng(Tech) phD MICeram MIStruetE C. Eng.Consulting Engineer, Hertford.

This File Note describes the design andconstruction of a "simple" garden wall- a project which cost less than £4,000.Nonethelesss, it is interesting for at leasttwo reasons. Firstly freestanding walls,

in perhaps too many instances, are notdesigned by structural engineers.Secondly the File Note describes howtight cost constraints were fulfilled byadopting a reinforced solution.

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Fig. 1.

INTRODUCTION

B R I E F

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-- -- -""/-- 1.... ----.. -..... __ -- -.l'-_

-- l.-... -... -...-.....

A

Brickboundary walls are a commonfeature of the built environment in Britain.They not only enhance a property butoften provide an element of both securityand privacy. While much time willundoubtedly be spent on the planning,design and detailing of the dwelling,freestanding walls are seldom subjected toan engineering appraisal but are left to thebricklayers to build.

However, attitudes are changing andmany more walls are now being designedon sound engineering prindples.The BDADesign Guide No 12"The Design ofFreestanding Walls" is a useful publicationin this regard.

As with any structure, economy ofdesign is of importance and this File Notedescribes the design and construction ofone particular boundary wall, whereeconomics, aesthetics and engineeringconsiderations led to the adoption of afully reinforced brickwork solution.

The brief was to design and construct aboundary wall between a public footpathand a private garden to enhance theproperty and to ensure privacy. Awallheight of at least 2m above footpath levelwas therefore required. Cost was animportant consideration since the wallwas to be 25m long. The side of the walladjacent to the footpath had to present asmooth, uninterrupted face free from piersand obstructions. In addition, since part ofthe garden was higher than the path, thewall was required to act as a retaining wallin places. Afurther consideration was thatdisruption to the footpath should beminimal during the construction processas should any damage to the footpath 'ssurface if consequent expensivereinstatement was to be avoided. Finally,as a planning consideration, the brick tobe used for the wall was to blend andcomplement the existing brickwork of theproperty.

STRUCTURAL DESIGN CONSIDERATIONS

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Thebrick chosen to complementtheexisting brickwork was in the upper halfofthe pricespectrum. It was obviouslyimportant to keep costs to a minimumandit was dedded to construct the wall, as faras possible. as a singleleafwall,supportedat intervals by piers.The piers required tobe on the garden elevation so as to leave asmooth sideadjacent to the footpath.Further economy ofmaterialscouldbemade by adopting a reinforced brickworksection for the piers.Thusfor thefreestanding wall allbrickwork couldbeconstructed as a singleskin ofstretcherbond with intermediate 327.5mmsquarehollow piers. At sections where the wall iscalledupon to retain soil, a reinforced

CONCRETE BASE

A142 MESH

Ag. 2.

AA

grouted cavitysectionwas used.Typicalsections are shown in Fig.2.

Somelightbed jointreinforcement wasto be includedin the panels to enhancethe integrity ofthe wall under acddentalloads.

Theconcrete base was designed toresist the applied moments from the piersand to provide stability to the wall as awhole.

Becausethere was to be no significantintrusion into the footpath - thus requiringminimal reinstatment afterwards - thefoundations to the reinforced brick pierswereextended to the rear ofthe wall toprovide the necessary stability.

CONCRETECAPI'lNG_ ,r-- ..,

STAINLESS STEEL '--1:0.1..1MESH IN BED JOINTS -L..-4­TOP. BOTTOM ANDMIDDlE OFWo\ll

A142MESH IN BASE 2112IN WALL FOOTING

B B

MATERIAL CONSIDERATIONS

The brick chosenwas a WaingrovesMuirfield Mixture Rustic - a denseengineering brick suppliedby Butterley Brick.Limited.Thenature ofthis brick is suchthat both sides ofthe wall could beconstructedto a fair-face finish.Thebrickstrength and water absorptionconformedto the BS 3921 requirement fora Class AEngineering Brick.

Consequently no OPC was necessaryatthe base ofthe wall sincea Class Aor Bbrick provides a suitableOPC forfree ­standingwalls. Structurally this isimportant With the momentfixityavailable from a brickwork OPC the base ofthe wall panelcan be regardedas acontinuousedge. This reducesthe designmomentin the centreofthe wall, henceleadingto economy.Themortarused was designation (i) -a 1:3 cement: sand mix. Thedensest andmost durableofthe normal buildingmortars,it isappropriateforthis form ofconstruction wherea thin,halfbrick wallis exposedto the rainfrom both sides andhencemaybe subject to frostwhilesaturated. To lessen the possibility oflimestaining(the leeching offree lime from the

Ag.3.

mortarwithin unprotectedbrickworkduringwet conditions) the mortarpurposely contained nolime. Apropercoping placedon a OPC was considered tobe essentialat the head ofthe wall toprotect the brickwork from waterpercolating downwards ifa longmainten­ance-free serviceable life is to be achieved.

The bed jointreinforcement wasspedfied as stainlesssteel sincetheauthor was not convinced that galvanizedmild steelwould remain free ofcorrosionina situationwherethe coveris only12mm. Furthermore, it is normal practiceto find that the reinforcement is laiddryon top ofa courseand then mortar placedon top ofit - irrespective ofwhat thespedfication calls for. This leads to thesteel not beingtotallysurrounded andembedded inthe mortarand this leaves itmoreproneto corrosion.The use ofstainlesssteel in this situation is nowrequired to conform withthe requirementsofBS 5628 Part 2 and Part 3.The majorreinforcement containedin the pierswasconsidered well protectedand ordinaryhighyield steel reinforcement wasspedfied here.

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MOVEMENT JOINT DESIGN

•Normally, inwalling, movement joints arepositioned at 10 to 12m centres. Since thebrickwork isexposedon both sides andthe wall thicknessisone halfbrick(102.Smm) it was felt that the thermalchangesmay be particularly significant.Although the use ofbed joint reinforcementis consideredto restrain the expansivelongterm moisture movement ofbrick-

worksomewhat, itwas nonethelessdecided to position the movementjointsat approximately 8mcentreswhichalsosuited the planofthe wall as influenced bystructural considerations. The maximummovement which wouldreasonably beexpectedto occurat the jointwouldbe ofthe order ofS/6mm, thus a 12mm jointwidthwas spedfied.

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STRUCTURAL DESIGN

COSTINGS

CONSTRUCTION

Thebreakdownofcosts applicable to theboundary wall was as follows:

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When spedal situations prevail, the use ofreinforced brickwork in boundary wallscan effect worthwhile economies. Halfbrick thickwalls can be made to spanconsiderable distances ifsupported on 3edges;the horizontal distance betweenvertical supporting piersor buttressesmay be increasedfurther by the use ofbedjoint reinforcement. (See BS 5628 Parts 1and 2 forthe design ofsuch panels).

As in this case some nominal bed jointreinforcement is thought desirableinwallsoverabout 1.2m highto enhance theoverall integrityofconstructionand tocopewith 'accidentalevents'.

However as withalldesign it isimportant to ensure that the materialspedfication and detailsare also carefullyconsidered. While the strengthrequirementswerean area ofobviousimportancein the design, the nature oftheprojectwas such that other materialproperties- Le, frost resistanceofthe brickand the mortar - were ofequal importanceif a sound longterm solutionto the briefwereto be achieved.

Finally, it is the author's view thatunreinforced masonry freestandingwallswill continue to be used as a popular,straight-forward solutionto the majority ofscreenwalls.Thef1exiblity ofmasonrybonding, however,and its abilitytoaccomodatereinforcement can be used togreat advantage to provide a soundeconomicsolutionwhen particularconstraints are present in the brief.

CONCLUSIONS

reinforcement wouldbe avoidedand aslightreduction in the sizeofexcavationsbe effected, such savingswouldnotoutweigh the extra cost oflabour andmaterials. Using the Small Works Sectionofthe Building PriceBook, comparative"base" pricesforthe reinforced - and asuitable unreinforced - solutionshow theformer to be 27%cheaper than the latter.Of course, both cost figures generated bythe Price Book, exceedthe final cost ofthewall as constructed but it is believed thatthe cost savingratio wouldhave beensimilar.Thereinforced solution, therefore,can be seen to be both technically soundand economically viableforthe designbriefgiven.

£1588.00£1223.00£704.00£211.00

£3726.00(Ex VAT)

Design Data:Brick: Crushing Strength 80 N/mm2

WaterAbs. 2-5%1:3 sand:cement mixGrade 25 to CP110Basic windspeed 40m/sec.

Mortar:Concrete:Windload:

Thelightbed jointreinforcement was'Bricktor' hightensilestainless steel, ­75mmwide.

An unreinforced wall to achievethe sameperformance wouldbe expected to useabout twicethe quantity ofbricks used inthis design. Even though the cost of

No particulardifficulties werefoundwiththe construction. However, a few minorpoints are worthy ofnote.(i) Stainless steel bedjointreinforcement,although available, is not widely enoughused to be a stockitemat many Builders'Merchants. Arrangements to supply thiswere made well in advance.(ii) Forthiswall a spedal copingwas madewhichfitted overthe thickness ofthe wallto ensure stabilitywith a correctoverhangand drip. Itwas placedovera dpc.

Thebricklaying was done by a normal3 man gang.Although the use ofreinforcement in brickwork in this waywas newto them they wereable toconstruct the wall without difficultyincluding handlingand fixing steel andgroutingthe steel in position.Since free­standing walls relyon goodmortar bondto achieve structural stabilityit wasconsideredessential to ensure that nounapprovedadditiveswereallowed in themortar and that allbed jointsand perpendjoints werefully filled.

LabourBricksOtherMaterialEquipment Hire

• 1, Deslgn Load.

p. - 0.5 kN/m' x 2 x 1.2 x 1.2

- 1.4 kN/m

Po = 1/ 2 x O.33 x 20 x 11' x 1.6- 6.4kN/m

CALCULATIONS

A - (2 x 2) + 0.33 x 2 - 4.66m'@ 0.5 kN/m' Therefore: LOAD - 2.33 kNTherefore:

M d - (2.33 x (2 x 2/3 + 0.6) x 1.2)- 5.4 kN.m/m

SecTiON

SecTiON THRO' IlETAlNING WALL

<t.

330Say lever arm - 0.9 x 2 - 149mm

I. I I_ ,I330/2 330/2

SECTION THRO'COlUMN THUS:

,----...,."

t1!t1zM _ 5.~~ 10' _ 36.3 kN

Therefore: 36 3 10' 115A,REQD= . x x .

460

Use 1NO 112 - 113m m'xx• 3. Wind Panel.

Use A98 MESH (A, - 98mm' /m.)

Say lever arm = 0.95 x 150= 143mm

- 0.17 N/mm'

= 1.77 x 1()6 (mm' /m)- 1/61000 x 103'

0.31 x 1()61.77 x 1()6

Z

Mz

"I, - 1.2 ; "Im - 3.5W. - 0.5 kN/m'~ - 0.35h/L _ 0.50 Therefore : IX = 0.032

M - ex YrWk .l2- 0.032 x 1.2 x 0.5 x 4'

- 0.31 kN.m/m

AS TYPE D:BS 5623 ParI 1.

FOR GROUTEDCAVITY WAll THUS:

~l~...m

l00mm ~150-.l

Mu = p. (2/2 + 1.1) + Po H/3- 5.2 kN.m/m

2.~_

:h~ef~e~ 36 x 10' x 1.15'" eq 460

- 9Omm2/m

•SHADED AREA = LOADTOREINFORCED BRICKWORK

COlUMN. • f.. 41EQD = 0.17 x 3.5 - 0.61 N/mm'

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II

I I330mm

FOR BRICK OF WATER ABS < 7'1>and In 1:1/4:3 MORTAR

I.. - 2.0 N/mm' > 0,6 N/mm'Therefore: OK

Include so me bed Joint reinforcement to assistIntegrity. (Accldenlal Damage and ResistThermal!Moisture Expansion).

Concrete bases to suit ground condirlons andfor no tension under moment condlrlons.

• It will be noled from Table 3 of BS 5628 Part 1that ANYbrick would have been sarlsfactory. Thehigh strength In this case Is coincidental followingthe aestherlc choice of the brick.

The Association would be interested to hear. from Engineers or Architects. of projects which theyconsider worthy of inclusion in The BDA Engineers' FileNote Series . All initial submissions should

contain reference to the particular area of the design which, it is considered. would be of interest tothe design profession as a whole . All enquiries should be addressed to The Technical Editor.

JMorton B5cPhDCEng MICE MICeramMlnstM.

The views expressed in this FileNote are those of the Authors. Readers are expressly advised that theydo not necessarily reflect the views of the Association.

To demonstrate the initia l engineering decisions taken. scheme calculations have been included in theNote. They are NOT intended to be full and detailed calculations and they should NOT be read as such.

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C TheBriCk DNelopiltill Assodll bon

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