4 . c . 4

LATERAL STRENGTH OF REINFORCED BRICK WALLS

DESIGN FOR WIND LOADS

ARr~E _A·JDERT ?esearch Assi.5 ta'1t

.~.N DERS LOSB'::RG ProfessoI'

ChalmeY's Un:- veY'3ity of C'ec'moloPd, GoteDor(J, Sve!"Íge

LATERAL STRENGTH OF REINFORCED BRICK WALLS

DESIGN FOR WIND LOADS

At the Chalmers University of Technology a number of

4~ " br ick panel walls with 0 1' without joint reinfor ce ­

ment aI'e tested with lateral loadi ng, simulating the

wind pressure . Some of the main conclusions o f t he

te s ts are :

1. The bending stiffness decreases considerably

with increased stress , both be f ore and after

crowking .

2. The crack load is not appreciably influenced

by the j oint reinforcement .

3. I n walls ~ith mainly hori zontal and inclined

failure lines , for example obl ong walZs suppor ted

along 4 edges , the j oint r ei nfoI'c ement is in­

effective Qnd cannot be utilised to increase the

ultúna.te load .

On basis of the test resu lts , some preliminary des i gn

rules for wind- loaded reinforced brick panels are

given .

BlEGEFESTIGKEIT VON BEWEHRTEM

ZIEGELMAUERWERK UNTER WINDLAST

An de r Chalmers University of Technology wird gegen ­

wiirtig eine Reihe von 4 1/2 Zo II Ziege ZWandtafe ln

mit und ohne Lagerfugenarmierung unteI' horizontaler

Belastung, die den Winddruck simuliert, gepY'Üft .

Einige de r wichtigsten Schlussfolgerungen der Ver­

suche sind:

1. ) Die Biegesteifigkei t nimmt betr iichtlich ab, wenn

die Spannungen zu nehrren . Dies gilt sowohl jÚI'

den Zv.s tand vor als auch nach Rissbi ldung .

2. ) Die Riss last wi r d durch die Fugenbewehrung nur

unwesentlich beeinflusst .

3 . ) In Wiinden, die hauptsiich lich horizontale und

geneigte Bruch linien aufweisen, zum Beispie l in

vie rseitig gestützten W'ànden. ist die FugenaI'­

mierung unwirksam und kann die Bruch last nicht

steigern .

Aufgrund de r Versuchsergebnisse werden einige Ent ­

wurfsrichtlinien fÜ:t' bewehrte Mauerwer kstafe ln unteI'

Windbelastung angegeben .

4.c.4-0

RESISTANCE LATERALf: DE MURS EN

MACONNEille ARM1:,'E

CALCUL DES EFFETS DU VENT

A l ' Uniw Y's ité dE TeChnologie d.e Chalmer s , d.es essais

cra t é té e ffeetués sur un c(lY'tedn norr/bre d.e pans d.e mur

en briq',ws d.e 10 em d ' épaisseur avec ou sans ar>mature

dans l es j oint s . Le s elois ons ont été sowzises à des

charges latémles súnulant la pression du vent o

Quelques conelusions principales de ees essais s ont

les suivantes :

1) La réS1: s tance à la fle.'Cion dirrinue eonsiéM rrib lemen i;;

avee l' aUgl,'/entat1:on de l a con tr'ainte , tant avant

qu ' acr es la fic9urat ion .

2) La ch arge de fissumtion n' es t pas s ensiblement

i nfbCrlc;ég par le renforCX!ment d.es joints .

3) Dans les murs ou les lignes d.e fissuration sont

pl'i rwipaZel1ent horizoY/tales ou incli nées, par

e:r:u~['le Cks mIAm oblongs soutenus par les 4 bonls,

I 'Cl l"TI1.1fur>e dans les joints n'a pas d ' influence e t

ne ce::'ilie t pas d.e ma,jorer la charge t otale sur ee s

Sur b('1.s e des résultats de ees es sais , quelques r egles

de eal cuZ pré liminaires sont étab lies pour de s murs

en maçonnerie ame s ous l es effets du vent o

LATERALE STERK1'E VAN GEWAPENDE MUREN.

BEREKENING VAN WINDBELASTING .

cp de Chalmers University of Teehnology werd een aan­

tal baksteenpanelen van 4 1/2 li, met en zonder voeg­

versterldn.(J getest op laterale belasting, die wind­

druk simuleerde . Tot de belangrijkste konklusies be­

hor en

1. de buigstijf heid ver>mindert sterk bij gestegen span­

ning, zowe l voor als na het optreden van seheuren .

2. de belasting waarbij seheuren optreden wordt niet

merkbaar beinvIoed door de wapening .

3 . in mlAren met vooral horizontale en gebogen breuk­

lijnen, bijvoorbee ld in de mlAren die aan de vier

kanten steun krijgen, is het wapenen van de voegen

niet effektief en kan niet worden gebruikt om de

breuklast te verhogen .

cp basis van de testresultaten kunnen sommige voorlo­

pige ontwerpl'egels voor baksteenpaneIen onder wind­

Iast worden opgesteld .

1 . INTRooUCTIoN . PRACTICE DF oESIGN

When designing such a large brick panel wall, that this wall will not sustain the design wind load with a sufficiant margin Df safety, it has nowadays become still more usual to reinforca the bed joints in order to increase the load capacity . In lack Df design rules and experimental basis , the engineer must rely on his own judgement and experience. Usually one or two cp 6 -cp 8 mlT, deformed bars in avery third to every sixth bed joint are recommended .

For factory building \-Jalls with large spans , construc­ted as cavity walls , where the usual matal ties are not expacteo to giva sati3factory cooperation betwaen the brick laaves, joint reinforcemant can ba usad, complataj with some type Df horizontal trus3 laddar.

2 . BUl LDING CoDES

The new Swedish Building Co da regulations for masonry [SBN 75 Chapter 24) states requiraments for covering mortar thickness and permissible stresses in brick­work and reinforcement . No design rules at all are given for laterally loaded reinforced masonry .

3 . FoRMER TESTS

only a few tests with laterally loaded reinforced masonry walls are reported in the literature .

Holgate 1931 presents the results from tests in New Zealand on the lateral strength Df two 9 " reinforced brick walls , subjectad to combined static and dynamic load .

Krôuss and Vodges 1932 account for tests in USA with a nurnbar of brick floor 31abs , 9 . 5 cm thick , spanning one way and reinforced with 1 cp 3/8 " in every bed join~ . The brick slabs generally failed in shear; only in a f e"! cases the reinforcement reached the yield stress .

Granholm 1943 reports testa , carried out at the Chalmers University Df Technology , with three 5" brick masonry plates, five courses wilJe and reinforced with 2 cp 10 rnm smooth bars , O yield = 520 N/mm 2

, in every bed joint . Failure occurred by yielding Df the ten­sile reinforcement.

4 . RECENT TESTS AT THE CHALMERS UNIVERSITY DF TECHNoLoGY

The statical behaviou r Df the reinforced brick wall in cracking stage and at failure has up to now not been known. Therefere a basis for a correct design method has been lacking. Usual Swedish practice Df design is to calculate the reinforced wall as a slab spanning one way, usinB the permissible benrling compressive stresses in the rnasonry and tensile stresses in the re­i nforcement , stated in the Swedish Building Code .

The permissible design stresses in the Swedish Build­ing Co de for rainforced masonry assume that the rein­forcernant and the masonry work together, and are based on a few tests with walls and beams , loaded in bending i n their own vertical plane, for example a window lintel .

The real extent Df cocperation in a wind-loaded rein­forced brick wall was up to now not investigated , and an action Df research has therefere been looked upon as important.

4 . c . 4- 1

Within the scope Df the project "Lateral strength Df masonry walls ", sponsored by the Swedish Council for Building Rese arch , a series Df wall tests has recently been performed at th e Division Df Concrete Structures , Chalmers Univers ity Df Technology . The tests ara planned in consu ltation wi th the Swedish Association Df Brick Manufacturers , which also has contributed financially .

The tests included six 4 1/2 " brick panel walls with and without horizon tal reinforcement. The aim was mainly to study the effect Df a varying amount Df joint reinforcement .

For the tests perforated clay bricks with the density 1 . 3 und compressive strength 45 MPa were used . The mortal' consisted Df special masonry cement an d masonry sand O - 4 mrn in proportions 1 : 0 by weight [mortar type B) OI' 1 : 3 : 5 [mortar type A) . The joints were made 15 rnrn thick and completely filled .

The wall s, with 3 . 5 m length an d 2 . 0 height , were simply supported along the four edges and after about 28 days curing subjected to a uniforrnly distributed lateral load by rneans Df an air- bag filled by corn­pressed air and placed between the wall and a re­sisting plate , 5ee Losberg-Johansson 1969 .

The walls were reinforced in some Df the bed joints with two ~ 6 Ks40 deformed bars [o yield = 400 N/mm 2

)

with a horizontal mortar cover Df 30 mm according to the Swedish Building Code .

In connection with the wall tests detail tests were rnade with masonry beams spanning one way, simply supported over a 1 . 6 m s~an and subjected to two line loads, in order to determine the ultimate bending moment in horizonta l and vertical direction respec­tively .

5 . THE FUNCTIoN DF THE JolNT REINFoRCEMENT. ANALYSIS DF TEST RESULTS

5.1 Bend ing stiffness

The reinforcement does not influence , significantly upon the stiffness before cracking [stage I) , see Fig. 2 . Even with reinforcement in every bed joint [beam A62) , the stiffness could not be increased compared with unreinforced brickwork .

After cracking [stage 11) , however , the stiffness i5 highly de~endent on the amount Df joint reinforcement .

Figure 3 shows measured bending stiffness for brick masonry beams with different amounts Df joint rein­forcement .

5.2 Cracl<- load

The crack moment is not appreciably influenced by the joint reinforcement . In Figure 4 , i . e. measured and calculated crack loads for b ric k panel walls , sup­ported along fou r edges, are compa r ed . At calculation the walls ha ve be en assumed t o behave as isotropic plates with Poiss on ' s ratio as 0 . 15 . In realit y , the walls are very anisotropic with moduli i Df elasticity varying wi th the local bending stress , see Figure 2 .

5 . 3 Crack widths

The joint reinforcement distributes t he horizontal strains, which in unreinforced masonry are more con­centrated to the perp end joints . oue to this fact, the width Df the vertical cracks will be restricted. At the oblong reinforced walls, simply supported along four edges. the first horizontal crack, about 0 . 2 mm wide, developed at about 70% Df the ultimate load. The crack width had at about 90% Df the ultimate load

4 . c . 4- 2

increased to about 0. 5 - 1 .0 mm .

5.4 Ul timate load

At successive increase Df the latera l load against the wal l , the f i r st crack will de ve l op in that pe r pe nd ~oi n~ or bed joi nt , where the ultimate e xtensibility 15 flrs t exceeded. After formation Df a fina l envelope- li ke yie ld li ne pat tern t he wall behaves as a mechanism, where th e di f f erent parts are twi s t ed r e ­lat ed to each othe r . The reinforcement i s not abl e t o p r event torsio n i n the yi eld l ines. Th e bond bet ween mortar and brick fa ils , an d the l oad cannot be any more increased. Thus t he ul timate l oad i s reached .

In Figure 4 meas ured an d calculated ultimat e l oads are c~~lPar~d for brick walls, supported a long fo ur edges, vil t;h dl fferent amounts Df rei nforcalTIent but t he same aspect ratio a/b = 1 . 8 . Obviously, the convent i ona l joint rein{o rcement is practically ineffect ive for thsse walls. The variations falI within t he normal st r ength dispersion.

The ultimate loads are ca l c ulated wi th hel p Df yi eld line analogy, see Losber g a nd J oh ans s on 1868 , a t which the uI U .rnate bendi ng Iwme nt s horizonta lly and vert i­cally are taken from detail tests on s imp l y s upport ed strips Df the brick masonry . The same t ype Df r ein ­forcRd detail tests used now, where the j oint rei n­forcement was effect ive upto yielding, a r e obvio usly not auequate for ca Iculation Df t he ultimat e l oad Df the wall. The reason for th i s is discussed bel ow .

The ma xi mum re i nforcement stress i n the walls imme di ­ate ly before failure amounts to on l y 10 - 40 MP a [N/mITI 2 . ) . The bo nd bet we en brick a nd ITIortar i s ob ­viou s ly not s uff icient to gi ve the rei nforcelTIent the i nte nded fu nc tion. I n order to obta in the int e nded cooperat i on betwee n joint reinfo r cement and ma s onry , it is nec essary to impro ve t he bond and s hear capaci ty i n the brick- mortar i nterfa ce .

It may be observed, t ha t the above r eferred i nsuffi­cient cooperõtion between joint reinforcelTIe nt and masonry is only valid for the wall tests support ed on alI four euges , where there is biaxial bend ing a nd twisti ng ITIolTIents in the wall . When testing simp l y supported masonry beams, there are no difficul ties to utili ze conventional joint reinforcernent up to yielding.

From the discussio n above it may be rea l ised , that the reinforcement is more effective at shorter a nd hi gher wa l ls and at walls supported along three euges [as discussed below) e.g . when ther e is a mor e pronounced beam action.

After the test se r ies ment i oned above wi th wal l s s up­ported along 4 edges, some walls with th e s ame di men­sions but supported along 3 edges, have been tested . One wall [86 5:61) was unreinforced , one other [865 :60) was reinforced with 2 ~ 6 Ks40 in every th ird bed j oint. The test resu l t, see Table 1, s hows f a i r l y good agr a3ment be t wee n experiments and th eory . The crack l oads are calc ul a t ed wi t h el ast i c theory (5.2 ) , t he ult i mat e l oa ds wi t h simple yie ld line an a logy without corner spal l ing .

Table 1

Wall No.

86 5:61

865:60

Measured and calculated crack lo d tima t e l oads for 4 1/2" brick pa an SI and 1 . d . e wal a l ln mortar typ e B, supported aI edges Si de ratio a/b = 3.40/1.95 ~n~.

Rein­f orce ment

unreinforced

8 x 2 ~ 6

Crack load kN/m 2

Mea- Calcu­ured lated

6.2

7. 0

5.7

5.7

6.2

10.1

Figure 5 sh ows t he ul t i mat e bending moment in t o the amount Df joint r e inf orce ment for the simp support ed reinforce d br i ck ma s on ry beam detail Full l i ne curve represe nts t he tests, dotted line ~ ul ated w~ t h rea l yi eld st res s and nominal yield l n the re lnfor c ~ment r es pective ly. The inner lever arrn i s t he n estlmat ed 0 . 9 h, where h = effective he i ght = 8.5 cm .

5 . 5 oi scussion Df the analo gy

If yield l i ne analogy may be applied, it is demanded among others , that the reinforcement must be effective up t o yie l ding r ange. If this should be possible in wa ll r egi ons wi t h inclined failure lines, e.g. where there are twisting moments, the reinforcement and the mas onry mus t act t ogether also after cracking.

Figure 6 shows the s train in the reinforcement in re­l a t ion t o the be ndi ng moment f or brick masonry beams. s panning one way , wi t h di fferent amounts Df reinforce­ment. obvi ously , t he reinforced masonry analbgous with reinforced c oncrete has a considerable ductility in t he direct ion Df t he reinforcement (horizontally). Vert i cally, where t he modul us Df rupture is not in­f l uenced by t he j oint reinforcement, the masonry ShoW5 a marked elasto-p l as t ic behavio ur wi th a bending stiff­ness r apidl y decreas ing with i ncreased stress (see Figure 2) .

Wi th regard t o th e facts mentioned abo ve, it is quite reasonab l e that yield line analogy may give a good estimate Df the ultimat e load Df the wall, provided t hat t he s upport condi t ions and a s pect ratio Df the wall a r e such that mas on r y and j oint reinforcement could act t oget her quite up to ultimate stage.

6 . PRELIMINARY oESIGN RU LES

The e f fectiv i ty Df t he joint relnforcement is decided by t he supporting cond i ti ons and si de ra t io Df the wal l . Hori zont al re i nfor cement is most effective in wall parts with ve rtica l failure lines, for example at a free upper edge , or in walls spanning one way between vert ica l support s . In walls with only in­clined or hori zont a l failure lines the conventional joint reinfo rceme nt i s not s o use f ul.

on the basis Df the t e st resu l ts r eferred to above , s ome pre l iminary des ign r ules for wind loaded, re­i nforced brick walls are given in the following. The experiments are hitherto li mited to simply supported ob l ong wall s with a side ratio Df about 1.8. For other support condi ti on s and s i de ratios the results may be appli ed wit h some cau t io usness .

The crack load can approxima t el y be calculated accord­ing t o e lastic theo r y f or i s otropic plate with Poisson's rat i o 0 . 15 - 0 . 20 , whe reby for perforated bricks in mo r tar t ype B the moduli i Df rupture can be taken as 2 . 0 MPa horizontally and 0 . 7 MP a vertically.

The ultima te l oad can be estimated with yield line analogy . The ultimate bending moment in vertical direction is taken from bend i ng tests ar can be esti ­mated by calculation with a modu lus of rupture as above . The horizontal ultimate bend i ng moment is calculated analogous ly as for reinforced concrete slabs , spann i ng one way . The inner leve r arm can approximately be taken as 0 . 9 times the effective he i ght .

Normally a reinforced b ric k wall is under- reinforced , e . g . the failure is caused by yielding i n the tensile re i nforcement . The least amoun t Df tens i le re i nforce ­ment s hould be 1 . 5 cm2 ;m f or e xa mple 2 ~ 8 Ks40 in every fourth bed joint , otherwise the calculated ulti ­mate moment wi ll be less tha n t he crack moment , see Figure 5 .

In walls with mainly ho rizontal and inclined failure lines , for examp le oblong vJalls supported along 4 ed~es, the joint r e i nforcement is i ne f fective and can no t be utilized to increase the ultimate load .

The factors Df safety may be judged with r egard to strength dispersion , wo rk ma nship , the consequences Df some c racking , etc . The des ign wind load is normally of very short durat i on , and eventually developing cracks may partly be c l osed again at un l oading . The safety against cracking may the r efo re ofte n be chosen some'.'Jhat lower than the safety agai nst failure .

7 . REFERENCES

1931 P . Holgate : Brick Wall Tests . Pamphlet iss ued by Amalgômated Brick and Pipe Co _, Wellington , New Zealand 1931 .

19 32 Krauss - Vodges : Results Df Tests on Ten Oemon ­strations Df Re inforced Brick Structures with Summary Coveri ng Tests on Thirteen Structures . Journal Df the American Ce ramic Society 1932 .

1943 Hj . Granholm : Ar me ra de tegelkonstrukt ioner . (Reinforced brick structuresJ . Chalmers Univer­sity Df Technology . Pub l . No . 16 . Gothenburg 1943 .

1969 A. Losbe r g - S . Joha nss on : Sideway pressure on masonry wal ls Df brickwork . Pape r presented at the International Sympos ium on Bearin g Wa lls in Warsaw . June 1969 , arra nged by th e I nt e rnat ional Council for Bu i lding Researc h.

" I

t

i I

, I

I ; L ___ ___ __ __ _ ___ ~

I a=3 .4m _4 - .1

Fig . 1 Outline of test wall

q

kp/ m 2

4 . c . 4- 3

Modulu s of e l a s ticity Bending stiffness

EI MNm' E MPa

15000-,..-------,

---, AS3 "'< 3 x 2 4>-6 1,5

10000 E.l "-,,_

""-''\~62--<:'-' ' 1.0

E" 12 x 2 4>6 '

5000 0,5

O-r~~-r~~-r'-rO S kNm /m

Bend ing moment m

Fig . 2 E1 modulus of elasticity for un­

reinfor ced brick masonry beams (mortar t ype BJ in hori zontal bending, average of 3 tes ts .

modulus of elasticity for similar beams in vertical bend­ing, average o f 2 tests .

Modulus of elasticity and bending stiffness be ­f ore cr acking f or 4 1/2" brickwork in mortar type B. The modulus of elasticity is deter­mined by measured curvature on 60 cm length of beam part with cons t ant bending moment o

Bending s tif fness EI

MNm'

O.I.,------r, --------,

A62 112x24>61 0,3

A51.(4x24>6 1

0,2

0.1

10 15 kNm / m

Bending momen t m

Fig . 3 Bending sti f fness after cracking for 4 1/2 " brick masonry beams

4 . c . 4-4

Lateral load w

kN/ m' 15 ~----------

"~ "~o o" . ""/" /' I I me as. ult. load ./

I ;. 5~ /

51 moinent d e t~rm~ning ;3 1

rmea s. crackload o o

í ca l e . c rack l oad o

5 f-- -- - _. --L ._--.---- --_

O~~~~~~~~~""I~-n~ 0,5 1.0 1,5 2.0 2_5 cm'/m

Amount o f r e inforcemen t A r

Ultimate moment

mult

kNm/m ,----------- - -

A62

O-~-r--r--r-'-'-~-'-~ O L. cm2/m

Amount of t ensil e reinforcement

Fig . 5 Ultimate bending moments i n relation to the amount of joint reinforcement for

Fig . 4 Comparison between measured and calcu­Zated crack loads and ult imate loads

4 1/ 2" simp ly supported reinforced brick masonry beams

f or 4 1/2 " brick panel walls laid in mortar type B, suppor ted along four edges . Side ratio a/b = 3. 40/1 . 90 = 1. 8.

Fig . 6

Bending mome nt m

kNm / m

15

10

A62 112x 2<»6)

A54 14x2<»6)

O 4~

Re info rc ement s train E r

Reinforcement strain in relation to bending moment for 4 1/2" simp ly supported reinfor ced brick masonry beams