)

Shear Transmission in Reinforced Grouted Cavity Brickwork Beams

Y Osman and Professor A W Hendry

University of Edinburgh

Scotland

ABSTRACT

The paper describes a series of tests carried out with the object of assessing the contributions of compression zone transmission , dowel effect and aggregate interlock to the shear resistance of grouted cavity brickwork beams . Me thods originally devised for reinforced concrete are applied and it i8 shown that compression zone t ransmission predominates wi th dowel effect playing an almost equally important part as the ultimate load ia approached.

Department of Civil Engineering and Building Scicnce , University of Edinburgh, King's Bui ldings, Edinburgh.

817

INTRODUCTION

In recent years, reinforced brick'NOrk has received renewed attention as a structural system. The basie ways of introducing relnforcement are by placing ateeI bara within the mortar joints or in grouted cavity between brickwork leaves. Previous research workers have indicated that thc lules of relnforced concrete can be applied to t.he design ·of reinforced brickwork beaIns provided that em tabl e adjustments are made for differences in materials properties . This paper gives an account of an investigation in which methods originally developed for calculating the relative contributions of variouo shear transmisslon mechanisms in reinforced concrete from experimental results are applied to relnforced brickwork of grouted cavity construction. In such elements shear is always accompanied by bending mament and interaction between these effects is complexo For this rea80n no general theory has been found and previous research has attempted to fomulate design rules by varying the main parameters inDuencing shear resistance. In the present work tests have been carried out to assess the contribution to ahear resistance in a typical section of compression zone transmission, aggregate interlock and dowel erfeet.

MATERIALS FOR TEST BEAMS

Tests were carried Qut on grouted cavity brickwork beams having an overall erosa eectioo 450 x 295mm and span of 5. 34m. as sho",n in figo (1). Two types of brick were uaed for the beams, as shown in Table 1.

Testa on prisma (21] x 100 x 44ümm) gave the c~pressive stre~th Df the masonry,stressed as in the test beams, e.s 12.24 N/rrun and 22.6) N/mm for the two brick types. Youngls modulus f02 the two types 05 brickwork compressed

in this direetion were 11 .97 kN/mm and 18.:;0 kN/mm respectively.

The reinforcing steel was hot rolled,high yield proof stress and ultimate s t resses shown in Table 1. used for the beams testa narnely 0.90% and 1.1+2'1~ .

defonoeu bars having the 2% Two steel ratios were

The oricks were laid in 1:~:4 cement:lline:sand mortar which had a 100mm cube etrength of 21.4 N/mm2.

The grout mix was 1:1/10:3:2 cemeot:lime:pca gravel (101P.m maximum diameter) with 0.75 water. cement ratio.

Tensile stref'..gth tests were carried Qut OH the hrick8 and grout using a splitting method with the resul ts showTI in Table 2.

CONSTRUCTION OF BEWS:

The beams were built in the labol'atory on top of leveI, wooden plank.s covered wi th polythene sheets. The reinforcing bars were placed in 008i tion over the planks and then the brickwork leaves ..... ere built up to six courses. Galvanized fish tai l steel wall ties, were placed in the brick .... .rork as i ndicated in figo 1. After one day, the grout was poured in to th e clean cavity. Beam3 and control specimens were covered with polythene sheets for seven days. Prior to testing, Doe face of each beam was white-washed to facilitate detectiol1 of cracks.

TEST ARRANGEMENTS

In all tests, the beams were supported on rollers on a span of S.]4m. Loads were symmetricalIy placed about mid span 760mra apart which gave a shear apao to depth ratio of 6:1 .

818

Strains were measured by Demec gauge, the points for which were disposed (a) to measuxe longitudinal strains on a 150mm gauge 1cneth and (b) as rosettes to de termine movements across cracks usi ng 50mm gauge l cn&th . For the first purpose t he Demec points were a ttached at different depths from the top of the beam in order that the neutral axis depth could be found and the compression zone shear estimated. The estimation of aggregate interlock and dowel effect required the measurement of vertical . and horizontal displacements across cracks. The application of a small load sufficient to initiate tensilc cracking made it poasible to anticipate the position of the subsequcnt crack pattern, and thua t o locat e the position of the Demec rosettes necessary for these measurements. Fig. 2. shOW9 the disposition of the gauge poi nts in a typical testo

Loada were applied by two hydraulic jacks eúch of which had an electrical resi stance type load cell between the ram and thn top of the beams.

DOWEL TESTS

A number of tests were carried out on beams of the 1m-ler strength masonry in which an artificial crack had been bUiltias indicated in figo 3, as a means Df measuring shear transmission due to dowe effect . Tne central part of the beam waB built first and the crack ~~rface was coat~d with P. T.F.E. material in order t o reduce friction across i t .

The load was appIied centrally to t he I ower part of thc beam through a steel plate passing t hrough the brickwork.

TEST RESULTS

A Bummary of the test beam r esults i8 given in Table 3 . Fig. 4 shOW8 typical compressive zone at a section 2. 0m froffi one of t he oupports . Corresponding shear etresses were calculated from these stro..ins according to the following formula (5):

where

Vcy •

Vc •

Vcy Eb V =

Y

Eb J~.~d O õM bX Y

dn dn

V. B.EbJ J"õb dy . dy O O õ M

shear stress at depth y from the top of the beam Young l s modulus of brickwork Total shear force across section

Vc Shear force t ransmitted through compression zone B Breadth of the section E. x Longi tudinal strains at depth y from the top of the beam M = Bendi ng moment at the secti on dn Depth of neutral axis

A compute r programme was writ ten to eval uate thesc expressions.

Shear transmission by aggregate interlock has been assumed to take place only in the grouted section of the beam as the relati vely smooth brick-mortar interface would be ineffective in shear transfer by this mechanism. The shear force transmitted in this way have been esti rna ted from m€:'asurementn of vertical displacements across she~r cracks and from crack width using the following formula (8):

819

Vg k S v bg Id - do J ;

Ccr .... here

&v vertical displacement acr oss the crack ccr crack \oiid th

k constant, taken as 1.2 bg width 01' grout core d effective depth dn depth Df neutral axis Vg = total shear force carricd by interlock af !,'Tout core

In the tests on dcwel effect , the shear force ( Vd) to cause f ailure by cracking parallel to the reinforcing bar \.ias calc..:ula ted from the following expression:

""here

Vd

Vd/

Vd Va' Ftg K

bb b • g ::;:1'1

lc

6

;

;

O·, ° Ftg [ K bb • b~ - :;;: ~ J lc

' .50 6°·30 Vd

total shear carr ied by dO',.lel actioll applied 3hear force carried by dowel ~ction tensile strength of the grout i~ N/mm a constant, depend on the ratio Ftb/Ftg, where Ftb i9 the tensile strength af the brickwork and taken asO.f7S in the present te:::;ts

""idth af brickwork antl grout core re~pectively bar diameters in one layer _ (rnm) ~omenâ of intria af the COIr,posi w sc-:tion indicated in fig (5a) ~n em vertical displacement c·f dowel f orce in (r.un)

A comparison between this rclati\:n::;hin Dnd exuerimental rr:sults is shown in (fig 5b) .•

On the baeis of the above , estimate!J have bec· n made of the contributiono to shear transfer of compression zone transmission , do ... ,e l effect and aggregate inter­lock at a section in the beams test~d . Some typical resulta are shown in fig 6.

Observed crack patterr.s in typical beams art: nhawn in figo 7.

D1SCUSSION OF RESULTS

Nane of test beams failed in fl exure, thi~ mo!lc having becn prcceded by shear failure.

In alI cases , failure was suuden after progres:3ion of a major acros8 the shear span spreading upwards t o the neutral axis level. took place by splitting along the 1ine of the reinforcement.

diagonal crack Final failure

As will be observed from fjg 6, total shear force calculated from the component valuee falIe short of the actual shear . This was found in alI the beam teste and ie p08Bibly due to an underestimate of aggregate intcrlock which omita any contribution from the masonry. In fact, there may be some ahear t ransmisslon across cracks in the masonry as the bricks tended to rotate and to become wedged acrOSB the cracke as the deformation oi the beamu increased. On conaideration of all the teat resulta, it would appear that compression zone transmisaion accounted for about 4~ of the shear resistance with dowel effect and aggregate interlock each accounting for rather lower proportions.

82U

CONCLUSION

No significant difference was faund in the shear resistance of the beams t ested as between those built of strong and of weak brickwork.

The shear strength of the beams i8 accounted for by t he three main factors of compression zone transmission , dowel effect and aggregate interlock. The contribution of each of these factors, which varied as the load on the beam was increased, has been assessed experimentally by me t hods originally devised for reinforced concrete. The results give an approximate indication of the reI ative magnitude af these mechanisms, and suggest that compression zone t ransfer provided the largest single contribution to shear resistance.

ACKNO\IEDGEMENT

The authors wish to express their appreciation to the technical starf of Civil Engineering and Building Science Department, University of Edinburgh where the experimental work was carried out . Financ iaI support was provided by the Brick Development Associa t ion and the British Ceramic Research Association.

82 1

REFERENCES

1. SUTER, G. T. and BENDRY A.W.

2. SUTER, G.T. and HENDRY A. W.

3. SUTER, G. T. and KELER H.

4. BAUMAN T .

5. TAYLOR, H.P.

6. SINHA, B.P., and HENIlRY A. W.

1. TlIOMAS, K. and O'LEARY D. C.

8. HAMADI, LD. and REGAN P.E.

9. OSMAR, Y. and HENIlRY A.W.

10. B.S. 5628 Part 1 - 1918

11 . HENDRY, A.W.

822

"Shear strength of Reinforced Brick Beé'Jns" Struct. Eng . Val o 53, Na . 6, 1915.

"Limi t State Des ign of Reinforced Br ick Beams" Prac. British Ceramic Soc., No . 24, 1915, pp. 191 - 196.

"Shear strength of Grouted Reinforced Masonry Beams" Prac. 4th l nt. Brick Masonry Conf., Brugges, 1976.

I1Versuche zum studium der Verdubelungswirk­ung der Beigezugkenehrung-eins Stahbeton­balker ll Munch Technischen Hochschule.

I1Further Tests to determine Shear stresses in reinforced concrete beams" C & C A Technical Report 438 - Feb 1910.

"The rrensile strength of brickwork specimens ll

The British Ceramic Research Association.

"Tensile s t rength Tests on Two Ty:pes of Brick" Prac . 2nd lnt . Brick Masonry Conf. England 1910 .

"Behaviour in Shear of Beams wi th Flexural Cracksl! Magazine of Concrete Research VaI. 32, No. 111, June 1980.

UAn Investigation of Reinforced Grout ed Cavi t y Brick Beams 1ll1der Bending and Shear" Bri tish Ceramic Association , Seminar on the Theory of Masonry St ructures, London, July 1980.

Co de of practice of Structural use of masonry.

"S truc tural Brickwork" The Macmillan Press Ltd., U. K. - 1981.

TAllLE 2

Brick Type

A

B

TAllLE 1 BRICK ANO STEEJ, S'rRENG'!'llS

(a) Br ick compr~ ssive strengths

(i) 23 . 86 N/mm2 (ii) 88 .28 N/mm2

(b) Steel properties Diameter mm

Young' s modulus N/mm2

~ proaf stress N/mm2

Ultimate st ress N/mm2

16mm

206 .0

562 .0

575.0

20mm

213·3 572.0

636.7

25mm

215.5

580.0

649.3

SPLITTING TENSlLE STRENGTHS OF BRICKS ANO GROUT

average average Beam Series Splitting tensile Splitting tensile

strength of grout Ftg strength of brick Ftb N/m 2 N/mm2

1 • 1 1. 990 0.274

2.1 79 0·383

1.2 2· 31,1 0·383 2.637 0.574

2. 1 2.799 0.967

2. 351 0.981

2. 2 2.164 0.774 1.890 1.184

A Common clay bricks manufactured from London clay

B Perforated (three 25mm dia. ho I cs) bricks

Tensile strength of brickwork = 2 X SPLITTING FORCE

íTxHxL

where H and 1 height and length of brick units respectively

823

00 N .,.

TAllI.E )

Beam Series

1 . 1

1. 2

2.1

2. 2

SllMMARY OF BEAM TEST RESULTS

Beam No . P76 Mortar Strength

N/mm 2

1. 1. 1 0.905 21. 40

1.1.2 0 . 905 21 .40

1. 2. 1 1. 47 22 · 90

1. 2. 2 1 ·47 22 . 90

2. 1.1 0.905 19.20

2. 1. 2 0 .905 22.20

2. 2.1 1.47 19. 50

2. 2. 2 1· 47 21·50

Grout Brick Ult. Shear Strength Pier V (kN)

N/mm 2 Strength

N/1llJll 2

17.)6 10.55 62 .508

18 . 27 11.91 62.984

18 · 54 14.26 7).855

19.1) 14·26 78 . 815

21 . 12 22.44 60.072

18 .94 22 . 50 63.392

18 .50 24 .)0 66.)92

16.90 22 · 54 62 . 718

o 10 '" '" ~

1'= 0·9%

750

Load CoU

-':::c -o"- . _ .-1 _ - ,-- - , - I - -:rr

' . ;

5340 mm

.... 2Y16 .1. 2Y20 •• 2Y20 •• 2Y25

11001951100 1l=1 .42% 1001s5 1100.

295 295

SECTION A-A

Positions of Ties

FIG. L BEAM CROSS SECTION AND BRICK BONDING ARRANGEMENT

r.:. ~.

AI Bl C1 01 I I ! I

; ,~ct;::

D2 C2 B2 A2 I I I I '

, ~::,_;~~-~;_:I .-.I L~_

RG. 2_ DISPOSITION DF OEMEC POINTS IN TYPICAL TEST

,

825

00 N

'" LOAD lB STEEL PL ATE

r I L I ' ~ ;--, t t ~~.

--::~, ::l, ~, ::::r~=C:=:C~,=. ::r--':T~':-::-=-'~ --------'------r- l....:...-'_ ..

T·~_T~-~~~[ L i~. ~L~.cL T . --.. . -] ;~l-· : -_ ~- ~ J~_~=__ ~~. r~~l"'

.Js I 2 llt 5

~ , "':5 '1 \

DISPt..A(E.MU"" CF 1'l(. .",I'tl f!:t l .-.cr ./

2Y 2 0 2Y25

L

1!.98 I ......-.,...

8ARS ,..., r ASlI RE O A C.RQSS

'1iif:Sf: P'OIwrs

; 579 -I-__ ~ _ 1 4.98..-----1 ~ _______ 4_~ 7 _~ m.m _ _ . . ___ _ . __ . _ ___ _

FIG 3- BEAMS FOR TESTS ON DOWEL EFFECT

2Y16 2Y20

.e co

---I --- .. ,

p~1.45% ?~O.9%

SECTION B- B

00 N ...,

5TRA!M'" I,-S ,-. S"'fFtA ,,,, 1 \~

~ 'ri'" "" 1\ ~, , , ~.w:l I

i", / . 'r:" '00

;-lo., //

IJ) J ,I

"_, '-''''- ...

I li: ;-

/ 1(.0 .I

c', N / ....... l. " ,. l/

'''' r , e.l o ..

A_BEAMS SER IES 2 .1 & 2 .2 8-BEA; .. ~S SERrES 1.2 .. _----

" 08 ",, / ... _ t

-, 5"T~.A ! N), lI)

:") ,,,, ,, / \

I \ ". J / \

'"

"" " o .. 0' o e $ MUiA ST'REC;S 1'J / .. ..,1

c - BEA", S SER IES I . ' ----- --- _ .. _--

FIG. 4 TYPICAL STRAIN ~ SH EAR STRESS IN COMPRESSION ZONE AT LAST LOAD STAGE

FOR SECTlON AT 2000 FROM SUPPORT

828

Vd Va '.0

0.8

0.6

0·4

02

Fig .

t

/

I. •

t

•

CENTRE DF GRAVITY OF COMPOSITE CROSS SECTION CON~snNG OF LONGITUDINAL REINFORCEMENT AND HATCHED GROlJT AAEA WHERE le IS DETERMINED

Flg. 5A-DETERI-IINATION OF Ic

, •

• BEAI-I ,., , BEAI-I'·2

• 8EAI-I 2.'

• BEAI-I2 2

tJ.mm .

58_ COI-IPARISION BETWEEN EXPEf< I I~ENTAL Ri'SULTS ANO

THE PR()-~O~º~E QU~lJ9j;=~J_~'=5'Õ_'p~ljd ---

80

60

: 40 z " ~

z 2 o i= u UJ U'I

U'I U'I O

~ ~ ~ UJ !D

z o w u cc

<l

~ 60 cc « w I U'I

40

20

o E E <I

20

COMP, ZONE

rRANS.

40 o o q 8 o o

60

Vu

V KN.

SEcrlON Ar c, _ BEAM ' .'·2

20

'" N o Ó

40 ;},O o ::: 66

50 V KN

SEcrlON Ar 02 BEA,' 2.'.'

81>-

50-

40

20

50

40

20

o

20 40 50

'" $ ~a5c~~ <5 o qoo-:.-;-""';" Ó o 06000

SECTION AT Q2 - BEAM '·2·'

20 40 50

'" ... '" <D "' ... l"< '" '" :g "'o o 00 0_ o óà ó dó

SECTION Ar c 2 -_BEAM 2.2 .2

FIG .5_SHEAR TRANSFER BY VARIOUS MEC HAN ISM

Vu

80 V KN.

80 V KN

829

,

00 w O

END 1

ENO' Al B c o"

r :L -L,-L L ~--a-1 -~Í--~ , ----r-~.. ,'" "" _:I f-i:;l' T r -' o .. , "Cr·o L . , T-I,~ ( :~I ~: ~ [ I ~--r --"T ..'I~ ... " 10 S

J

"

BEAt-1 , . ' .'

1 o Cf B

~"-~u.~ '"""""l'::::"o " f

T-lO ':""rr-~1'"

BEAt-1 2.'.'

A<­I

FIG.7_0BSERVED CRACK PATERNS IN TYP ICA L BEAt-1S

'I .0-'"" .

L,""]

END 2

ENO 2 ,

""6