km c554e-20150630104853 · consisted basically of two universal beams. 457 mm x 152 mm. of 900 mm...
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548
SHEAR STRENGTH OF MASONRY WALLS
J.R. RIDDINGTON, B.Sc . , Ph . D. , C.Eng., MICE University of Sussex, Brighton BN1 9QT, England
M.Z . GHAZALI, M.Sc, Ph.D . , P.Eng, MIEM Universiti Teknologi Malaysia,
54100 Kuala Lumpur
ABSTRACT
This pape r presents some experimental resul ts from a research programme which is attempting to develop a fundamental theory for shear failure in masonry . The results from a series of brick triplet tests with precompression stress leveIs up to 2 N/mm' and from a series with precompression stress leveIs up to 7 N/ mm' were reported earlier. This test work resulted in a general hypothesis for shear failure being proposed in which it was suggested that the basic mo de of failure changes from joint slip to mortar tensile failure as precompression increases. As the triplet has only two mortar layers and is always loaded symmetrically, the failure pattern can always be predicted. Consequently, there was considered to be a need to investigate the behaviour of larger masonry structures. In this paper experimental results from small wall samples are reported, and these are shown to support the previously presented hypothesis. The average shear stresses in the walls at failure were found to be Iower than the stresses in the corresponding triplet samples, but this was to be expected since the walls were subjected to bending as well as shear Ioading.
INTRODUCTION .
In order to understand better the mechanics of shear wall behaviour, while
at the same time avoiding expensive full scale tests, many investigations
[1 - 7] have been carried out into the strength of masonry bedjoints. The
forms of test sample used were the triplet prism formed from three bricks,
the couplet prism formed from two bricks, the small square shear panel ar
small discs cut out from a larger brickwork prism . Results from triplet
and couplet tests have always been presented as average stresses acting on
the joints. Experimental results from triplet samples subjected to
549
precompression, shear and bending have led to a hypothesis being proposed
[8,9] that the failure of masonry in shear is caused by either shear slip
at the brick-mortar interface ar by diagonal tensile failure in the mortar
layer, and that such fa1lure 1s initiated when local failure cri teria are
violated. As the triplet sample has only two mortar layers and is always
loaded symmetrically, the failure pattern can always be predicted . This is
not the case, however, wi th larger masonry structures, which have many
layers of mortar, both in the vertical and the horizontal directions .
There are more possibilities, and local failure can be propagated in any
direction along the brick-mortar interface.
Consequently, there was considered to be a need to carry out tests on
larger masonry structures. Three types of wall sample were tested, 5-brick
high walls using mortars whose water-cement ratio were 0.9 (WH series) and
1.1 (WL series) and 9-brick high walls using mortar whose water-cement
ratio was 0.9 (TWH series). These are shown in Figure 1. AlI the walls
were formed from solid-solid bricks with mortar of designation ii (BS
5628) . For the WH and TWH walls, the consistency of the mortar was
nominally 10 mm as determined by the dropping ball test whilst for the WL
walls, it was 18 mm.
~-L __ r-~ __ .-~L--. __ ~ __ ~ _SHEAR LOAD
UNIFORM PRECOMPRESSION
la) S - BRICK HIGH I WH 11 WL)
REACTION R -I-~--l---~--+--,---+--,.----!
UNIFORM PRECOMPRESSION
I b) 8 -BRICK HI8H I TWH)
Figure 1. Wall samples.
550
As experimental testing of a wall as a cantilever is difficult . due to
the fact that the base has to be prevented from sliding and overturning .
the walls were tested as deep beams using three point loading. together
wi th uniform normal precompression . The most important aspect of the
loading arrangement was to ensure that both shearing and bending were
produced in the wall samples. with the shearing being dominant .
WALL TEST PROCEDURE
The wall samples were tested in the apparatus shown in Figure 2. This
consisted basically of two universal beams. 457 mm x 152 mm. of 900 mm
length. a hydraulic jack with a 500 KN load cell . a 1000 KN load cell
attached to the vertical hydraulic ram of a Mitchell testing machine and
two 3.5 metre lengths of threaded rod o The two universal beams acted as
bearing plates. exerting approximately unlform normal compressive force on
the wall . whilst the 1000 KN load cell and the vertical hydraul1c ram
provided the shear force . The hor izontal force necessary to produce the
normal compression was obtained by the self-reacting mechanism of the
threaded steel rods and the hydraulic ramo Shear force and normal
compressive fo r ce at failure were measured by the two load cells and were
recorded using a Solatron Orion data logger .
LOAD FROM MITCHELL r---'
6mm PLYWOOD ::s::z:
04 x I05x 30 PLATE
I- f- I-
12mm WEB f- f- f- r- 500kN LOAD CELL
STIFFENER "- I- f- I- \ .J . " .. ~ " ;,,, , , .. '" , " I I , •• I I I • , , •• " • 1 1 ."" I
f- I- !I I I LlI UB 457 x 152x52 I- l- r- f.! HYDRAULlC RAM
o g 32
L f- IsR lc.t .~ LJ-40mm 0 high ylelc threaded steel rod
I- f- I--
ERS --º- U n p O O 12 mm END PLA"
STEEL SUPPORTS
Figure 2 . Apparatus for testing wall samples
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The walls were tested with the bed joints vertical. They were
supported on two steel supports , such that the loading was symmetrical . In
principIe the tests were similar to the triplet tests. Precompression was
applied to the walls by the hand operated hydraulic ram until the required
stress leveI was obtained. Subsequent1y, shear load was applied to the
walls through the vertical hydraulic ram which was operated by the Mitchell
testing machine. The failure pattern of the wall was noted.
RESULTS ANO DISCUSSION
The results from the shear tests on the three types of wall WH, WL and TWH
are shown in Figures 3, 4 and 5 . From Figure 3, it can be seen that the
relationship between the average failure shear stress TU and normal
precompression stress 0c can be expressed by two linear relationships.
(a) TU = 0.25 + 0 . 70oc N/mm 2
(for 0c < 1.5 N/mm 2 approximately)
where the coefficient of correlation r
(b) TU = 0.96 + 0.250c N/mm 2
(for 0c > 1.5 N/mm 2)
where r 1.0.
0.95, and
The same resul ts could, however, also be expressed by just one linear
relationship, as:
TU = 0.40 + 0.52oc N/mm 2
where r = 0.96 .
The apparent reduction in slope at high precompression supports the
hypothesis that diagonal tensile cracking in the mortar plays some role in
the ultimate shear failure of masonry structures. At much higher
precompression stress leveIs, it would be expected that the mo de of failure
would be completely due to tensile mortar cracking.
A similar trend although not conclusive can also be detected from the
results from the WL walls, as shown in Figure 4. The relationship between
the shear strength and the average normal precompression can be expressed
as:
TU 0 . 58 + 0 . 50oc N/ mm 2
(r 0.93)
N S S ..... z
552
Wall WH Serl .. exper ImentaI
~ 2
:z: .... I!)
z UI a: .... (1)
a:
"' UI :z: fi)
ar/ d
/ /
/1:.
.V • V·
<;f '
I:. /~ /
/ ,AI
v · Via I I WH 1:.-- Wall WH , , , ,
'2
AVERAGE PRECOMPRESSION IN N/MM2
Figure 3 . Results of wall tests (WH Series)
N :i :i ..... z
3TI'-rT-r~-r~-rTõrr'-õT'-~-.,,-r,,-rTõ~ Wall WL Serl .. experImentaI
~ 2 :z: .... I!) z UI a: ti a:
"' UI :z: (1)
.............. 6 \0-- ...........
"' ......
..... "'6
.......... I:. ............... 6
..........
o ~ t. - - Wall WI. Õ tI I ' , I '~ , , , I J
AVERA6E PRECOMPRESSION IN N/ MM2
Figure 4. Results of wall tests (WL Series)
553
The results of tests on TWH walls are shown in figure 5. The first
crack load, which was observed from the wall tested with a precompression
leveI of 0.83 N/mm' (nominal) is also shown in the figure:
slightly lower than the ultimate failure load.
3
N 2 2
...... z
~ 2 :J: ~ C!)
z UI a: ~ fi)
a:
"' UI :J: fi)
n
Wall TWH Serias experimentai
V ·A
I
V A
Ultlmate Fallur. 1 .t. era ek
2
AVERAGE PRECOMPRESSION IN N IMM2
Figure 5. Results of wall tests (TWH Series)
this is
The results of the wall tests are shown together in Figure 6. From the
figure, no significant difference seems to be noticeable between the
results of WH walls and WL walls. This suggests that mortar consistency
(and hence water-cement ratio) is not a significant factor in determining
the shear strength at lower precompression leveIs of walls formed wi th
these solid bricks. One reason for this is that the solid bricks used have
low water absorption. Hence, there was no rapid absorption of water from
the mortar to the brick, which for the drier mortar mix might have made the
mortar locally stiff and the bond weaker. If the· test had gone up to
higher prestress leveIs where tensi le cracking would be more important,
slightly lower strengths from the WL walls would be expected. The results
of TWH walls are slightly lower than those of WH walls, especially for the
resul t when 0c = 0.83 N/mm'. Because of their shape the TWH walls were
554
subjeeted to relatively greater bending than the WH and WL walls and would
therefore be expeeted to fail at lower shear loads.
3rl~rr.-rr"rT'-"'-"-'''-'~'''-ro.-,,~
t\I ::E ::E ..... z 2 :J: ~ (!)
z
Experimental R •• u Ita
Wall WL, WH a TWH
A ... W IY. ~
... A+ ... (f)
a: ct W :J: (f)
'{ o_
I
t:. ... ...
t:. ...
+
"
+ TWH ... WH A WL
2
AVERAGE PRECOMPRESSION IN N/ MM2
Figure 6. Results of wall tests
The results of the WH wall tests were eompared with the results from
the triplet tests reported earlier [8-9]. For the WL walls a set of
triplets were prepared using the same mortar, and triplet tests were
earried out. The eomparisons are shown in Figure 7 for the WH walls and
Figure 8 for the WL walls.
The results for WL triplets shown in Figure 8 may be expressed by 2
linear relationships:
(a) TU 0.48 + 0.82oe N/mm 2
(for 0e < 2.0 N/mm 2)
where r 0.93
and
(b) TU = 1.7 + 0.320e N/mm 2
(for 0e > 2.0 N/mm 2)
where r 0.90 . ..
N 2 2 -.... z 2 :z: ... CP Z UI Q: ... (/)
Q: cf UI :z: (/)
A
O O
555
EKperlmental R .. ulfs Wall WH
A
A A
A
A A
A WH results
2
AVERAGE PRECOMPRESSION IN N/ MM2
Figure 7. Comparison of wall WH and triplet test results.
N 2 2
-.... z
~
:z: ... CD Z UI Q: ... (/)
Q: cf UI :z: (/)
V
Wall WL Serl .. 8lperlmental
+ __ Trlplet hlgh SX v ... Trlple' low SX &--WaIlWL
2 3
AVERAGE PRECOMPRESSION IN N/ MM2
Figure 8. Comparison of wall WL and triplet test results.
4
556
From the results shown in Figures 7 and 8, it can be seen clearly that
the wall resul ts are lower than the triplet results . This reflects the
limi tation of the average stress method of analysis. The shear stress
dlstribution within larger masonry structures is more complicated than
within a triplet sample with the structures normally being subjected to a
significant degree of bending in addition to shear loading. Furthermore,
the triplet has only one symmetrical possibly line of failure which is
along the brick-mortar interface of the mortar joint. Larger masonry
structures, like walls, have a number of vertical and horizontal mortar
joints along which failure may take place . Failure will develop along the
mortar joints, where shear stress conditions (or diagonal tensile stress)
are worst. The typical failure pattern for the WH (similar to WL) wall is
shown in Plate 1 and for the TWH wall in Plate 2. The failure line follows
basically the imaginary line joining the applied shear load to the
supports; but affectlng only the brick-mortar interfaces.
Plate 1 Failure pattern for WH wall ..
557
Plate 2 Failure pattern for TWH wall
CONCLUSIONS
The relationship between average shear strength and average normal
precompression for walls may be expressed by a Coulomb type of expression.
At higher precompression leveIs, the slope of the linear expressions seems
to be reducing, indicating the possibility of a change in the failure mode.
The shear strength of the walls tested were lower than the strength of
the corresponding triplet samples . Therefore the avp.rage shear strength of
walls cannot normally be accurately predicted by the triplet equation. The
triplet equation may however be used as a guide to the upperbound value of
wall strength.
558
The failure patterns for the walls tested suggest that the worst shear
conditions exist along the brick-mortar interfaces nearest to the imaginary
line connecting the supports to the concentrated shear load.
1. Johnson, F . B. Procedures to Assemblages , Products , ed.
REFERENCES
and Thompson, J .N. , Development of Diametral Testing Provi de a Measure of Strength Characteristics of Masonry Designing, Engineering and Constructing with Masonry F.B. Johnson, Houston , Texas , Gulf Publishing, 1969.
2 . Stafford Smith , B. and Carter, C., Hypothesis of Shear Failure of Brickwork , Jnl . of the Structural Div . , ASCE, Vol. 97, No . ST4, Proc. paper 8029, April 1971 .
3 . Hamid, A. A. and Drysdale, R.G., The Shear Behaviour of Brickwork Bed Joints , Proc . of the British Ceramic Soc., no . 30 , Sept . 1982 .
4 . Hamid, A.A . and Drysdale , R. G. , Behaviour of Brick Masonry under Combined Shear and Compression Loading, Second Canadian Masonry Symposium , Ottawa , Canada, June, 1980 .
5. Jain, A.R. Tests on Brick Couplets, Proc. Instn. Civ. Engrs, Part 2, 1978, 65 , Dec., pp.909-915 .
6. Hofmann , P . and Stockl, S . , Tests on the Shear-Bond in the Bed Joints of Masonry , Proc . Sixth International Brick Masonry Conference , Rome, 1982.
7. Polyakov, S.V., 'Masonry in Framed Buildings', Moscow, 1956. Translated from Russian by G.L. Cairns (1963). Publ i shed by National Lending Library, Boston Spa, U.R.
8 . Ghazali, M. Z. and Riddington, J.R . Shear Strength of Brickwork, Proc . First East Asian Conference on Structural Eng., Asian Inst . Tech . , Bangkok, Vol.1, 1986 .
9. Riddington , J .R. and Ghazali , M. Z. , Shear Strength of Masonry Joints at High Normal Stress LeveIs , 2nd Int . Seminar on Structural Masonry for Developing Countries, Ruala Lumpur, Malaysia, March 1987 .
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