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