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261

A LABORATORY AND IN SITU STUDY OF THE SHEAR STRENGTH OF MASONRY BED JOINTS

R.H. ATKINSON G.R . KINGSLEY

Atk inson-Noland & Assoc iates, Inc. Boulder, Colorado USA

S. SAEB B. AMADEI S. STURE

University of Colorado Boulder, Colorado USA

ABSTRACT

For many exis ting low rise masonry s tructures in seismically ac tive zones, a cr iticaI strength parameter is the shear resistance of the hori­zontal bed joints. This paper presents r esults of a laboratory and field study of the strength and deformability of such joints under the action of both single monotonic and repeated reversed shear loadings.

The laboratory study was conducted us ing a direc t shear tes t device in \"hich both normal and shear forces were applied by servo-controlled loading activators. This system permitted a high degree of loading con­trol which when combined wi t h displacement transducers data provided a good understanding of the shear resistance of masonry bed joint undergoing cycl ic shear loading. The variables in the program included brick unit type, mortar type, joint thickness , normal stress values, and the magni­tude and number of load reversals.

The in-situ phase of the program was conducted by utilizing the hori­zontal shove test in which two fla tjacks are embedded in the wall above and below the t es t unit. The flatjacks permitted the normal stress across the shear failure surface to be varied to determine the effec t of normal stress on the shear resistance.

Typical results from the laboratory and the in-situ tests are pre­sented in thi s paper. The results provide insi gh t as to the shear fa il­ure of horizontal bed joints including peak and residual effec ts, and the effects of cyclic load reversal and degradation.

262

INTRODUCTION

Shear is the dominant mode of failure observed in many masonry build­i n gs subjected t o l ateral loading from earthquakes or other causes. Lateral loading can produce both diagonal cracking failure and horizontal bed joint shear failure modes. The bed joint resistance is of particular concern in the analysis of load bearing stone or brick unreinforced mason­ry structures common in older buildings in the U.S. Unreinforced masonry structures are also found in many areas of the world subject to seismic ac t iv i ty .

The state-of- knowledge concerning shear strength and shear load­displacement behavior of masonry is far less advanced than that concerning compressive behavior even though shear is the dominant mode of failure observed in many masonry buildings subject to seismic loadings. This lack of understanding is reflec t ed by the low values of allowable shear resistance permitted by present U.S. build i ng codes . Particularly lack­ing in the present knowledge of shear behavior of masonry is information on post-peak behavior under cyclic loading and on the deformations associated with pre - peak and post-peak responses. Knowledge of such behavior is essential if adequa t e analytical models are to be developed to describe in- plane behavior of masonry walls.

This paper presents the results of laboratory and field studies on the behavior of masonry bed joints under monotonic and cyclic shear loading . A direc t shear apparatus was used for the laboratory tests and horizontal shove tests were conducted to obtain the mechanical proper t ies of bed joints in the fi e ld.

LABORATORY STUDY OF BED JOINT SHEAR STRENGTH

Direc t Shear Apparatus

Direct shear experiments were conducted using the servo-controlled shear test apparatus available in the rock mechanics laboratory of the Department of Civil Engineering at the University of Colorado at Boulder. This e quipment has been used in t he past for the dyn amic testing of rock joints. Figure 1 shows a side view illustration of t he apparatus. It consists of t\-JO independent normal and shear loading MTS actuators (MTS Systems Corporation), reaction frames, loading fixtures and shear box specimen holders .

The original apparatus specimen holders h1ere modified to accommo­date r unning bond masonry specimens up to 15·2 mm h1ide and 440 mm long, as shoh1n in Figure 2. Bricks are first glued to the th10 specimen-holder platen s using a strong structural epoxy (Sikadur 31). The top and bottom parts of the specimen are then assemble d h1ith a mortar ted joint that h1ill be the test surface. After 24 hours of curing the platens are bolted together to prevent any disturbance of the test specimen during its transport and installation in the shear apparatus. The platens are in turn bolted to the top and bottom support pIates and the actuators of the apparatus . Prior to testing, the bolts keeping the specimen- holder platens together are removed.

~

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263

Vertical and shear displacements are monitored by means of linear variable differential transformers (LVDTs) mounted within the loading actuators. Because of the flexibility of the system, three additional LVDTs are used to measure the vertical displacement of the top support plate. Two LVDTs and their feeler rods are also epoxied directly on the masonry specimens on each side of the bed joint being tested as sho\vn in Figure 2. Thi s provides for accurate measurement of the rela­tive shear displacement along t he bed joint. The normal and shear loads are measured using the load cells of the two actuators. The normal load actuator has a capacity of 736 kN and the shear load actuator has a capacity of 156 kN. Each actuator can be operated independently in force or displacement control modes using a digital function generator. The shear and normal loads and the shear and vertical displacements are recorded with a data acquisition system controlled by an IBM-PC desk top computer.

Experimental Program

The work reported in this paper is par t of a general research project to investigate the mechanical behavior of clay unit masonry bed joints under normal and shear loading. A total of 44 direct shear tests were conducted. The results of these tests wp.re obtained under the following conditions:

i) The normal stress levels ranged between 0.25 and 4.0 MN/m2 •

ii) The mor tar had a volumetric ratio of cement, lime, sand equal to 1-2-9 and a cement water ratio by weight equal to .506.

iii) The bricks were obtained from an old building constructed toward the end of the last century in Boulder, Colorado.

iv) The cycles of shear reversal wer e conduc t ed in displacement control with a frequency of .01 Hz.

v) The bed joints had thicknesses of 7 and 13 mm .

In addition to the direct shear tests, conventional tests were aIs o conducted on cylindrical samples of mortar and brick units in order to determine their ultimate strength and elastic properties (Modulus of Elastici t y and Poisson's Ratio). The results of these tests are reported in Table 1.

TABLE 1 Results of Tests on Mortar and Brick Units

Compressive strength (MN/m2)

Mor tar 4.4

Brick 33.0

Modulus of Elastici t y (MN/m2)

3300

8800

Poisson 's Ratio

0.20

0.16

Normal Load Actuator

Load Cell _________

Top Support Plate

Specimen Reaction Frame

:: li ;! ci trr

264

Normal Load Reaction Frame

( Specimen

) .:i<C ': \ tdl \/' []

::

~

Load Actuator

Horizontal Load Reaction Frame

Bottom Roller Structural Floor Support System

Figure 1. Direct Shear apparatus. Side view schematic illustration.

LVDT Aluminum Rod

~

II II II f'" II ~':K@j" II

I ~ LVDT co;e""" (a)

~

L 289.6

(b)

Figure 2. Modified specimen holder with running bond masonry specimen. a) Side view with LVDT feeler rods. b) Front view.

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265

The results of confine d triaxial compression t e sts on cy l i ndr ical s pecimens of mortar for differen t leve Is of confine ment are pres e n t ed in Fi gure 3.

Hoek cell tests

11 ,-------------------------------------------------~

10

Test

0. 00 2 0.004 0 .0 0 6 0 .008

Axial Stra in

Co nf ; ni ng pressure 1. 7 MPa

0 . 85 flPa

o 4 flPa

0 . 21 MPa

0 .01 0 .012 0.014

Fi gure 3. Re sul t s of Confined Triaxial Compressive Tes t s on Cylindrical Specimens of Mor t ar for Confining Pressures Rangi ng between

0.21 and 1. 7 ~rn /m2 .

Discussion of Tes t Resul t s

Fi gur e 4 shows a typical response curve for a four cycle shear test conduc ted on a masonry specimen at a constant normal load of 15 kN (nominal normal stress equal to .036 MN/m2) . The shear load-relative shea r displacement curve shm-ls a rapid rise in the shear load to a peak fol lowed by a decrease in the shear r es i s tance in the post-peak region to a residua l value. Af t er the first cycle of load reversal, the response curve does no t show a secondary peak and the s hear resistance seems to assume a residual value which is l ess than the value for the firs t cyc le. The value of the residual s hear stre ngth is not affected by the number of cycles. Fi gure 4 shows that a very small amount of de formation is required to r each the first peak load . The slope of the pre-peak part of the horizontal load - horizontal displacement cur ve (the shear stiffness) is not constant. It decreases as the shear load increases indicating a sof tening of the bed joint UpOu shear loading.

The firs t cycle peak and residual nominal shear s trengths are plo tted in Figures 5a and 5b for di ffe r ent leveIs of nominal normal stress and for bed joint thicknesses of 13 and 17 mm, r espec tively. Fo r both joint t hicknesses, increas ing the normal s tress raises the shear strength l eveI. The masonry bed joint shear strength was assumed to be described by a Mohr- Coulomb crite rion T = c + Dn tan ~ where c i s the cohesive s trength and C' n tan ~ the fr ictional strength with tan ~ being the coe ffici ent. This crite rion seems to model quite well the observed

266

Test t39 Test t ype 1, normal load 15. kN

18

16

14

Z 12

.-'< 10 ~ -

~

"O (1j O ...,

..., (1j (a) .w c: O N

-2

." >< -4

O :r: -6

-8

-10

-12

I

1 I I

J ~= -14 -10 -6 -2 10 14

Relative horizontal displacemen t (mm)

Test t39 Tes t type 1, normal load 15 kN

18

17 í/--16

15

14

Z 13 ..-.:

12

"O 11

(1j 10 O

( ..., ..., (b) (1j .w c: O N ." >< O :r:

2 I 0.02 0.04 0.06 0,08 0.1 0.12 0.14 0. 16 0 .1 8 0 .2

Relative horizontal displacement (mm)

Figure 4. Shear Load- Rela tive Shear Displacement Response Curve. Constant Normal Load Equa l to 15 kN. a) Global cyclic loading. b) Prepeak loading r esponse .

-..,

-

~

C\l

3 3.5

CIJ CIJ <li H ...,

2.5 u:J

H C\l <li

,.c: u:J

1.5 ,..., C\l J::

..-! !Oi o Z

0.5

~

C\l 3.5

~ ~

CIJ CIJ <li 2.5 H ...,

u:J

H C\l <li

,.c: 1.5 u:J

,..., C\l J:: . ...; !Oi o 0.5 Z

267

Shear stress versus normal stress Test type 1 , joint thickness 13. mm

o + EF1 2:J 1 tan ~r

tan <PpV 1

peak .. @ ~residual

+

o

2

Nominal Normal Stress (MPa) (a)

Shear stress versus normal stress Test t ype 1 , joint thickness 7. mm

l_-J 'J tan ~ r

Nominal Normal Stress (MPa) (b)

Figure 5. Peak and Residual Shear Strength Envelopes for Bed Joint Thick­nesses of (a) 13 mm and (b) 7 mm.

268

increase in shear strength with the normal stress leveI as shown in Figures 5a and 5b. Table 2 gives the peak and residual values of c and tan ~ and the values of the coefficient of determination R2 for the two bed joint thicknesses.

TABLE 2 Results of the Linear Regression Analysis for Bed Joint Thicknesses

of 7 and 13 mm.

Bed Joint Thicknes s

7 mm

13 mm

Peak Value I Residual Value

Tan ~p cf> (MN/m2 ) ~2 Tan ~ r Cr (MN/m2 ) Rr 2

0.640 0.213 0.995 0.693 0.038 0.996

0.695 0 . 127 0.994 0.678 0.023 0.995 , _________________________ -1

Figure 5a shows that over the investiga ted normal stress range, the difference between the peak and residual shear strengths for a joint thickness of 13 mm is approximately constant. On the other hand, Figure 5b shows that for a thickness of 7 mm, the peak shear strength becomes equal to the residual shear strength at high normal stress l eveIs.

For alI the shear tests conducted in this research program it was found that the prepeak part of the shear stress- shear displacement curve could be modeled using a hyperbolic formulation. This provides a means for assess ing the variation of masonry bed joint shear stiffness with applied normal and shear stresses. The hyperbolic formulation will be presented in another paper.

IN-SITU HEASUREMENT OF BED JOINT SHEAR STRENGTH

The direct shear apparatus used in the laboratory applies a shear stress across the joint under a known magnitude of normal stress. A modification of the in-place shear t es t (1) allows for similar control in the in- situ determination of bed joint shear strength in existing masonry buildings . This permits the measurement of shear strength without dis­turbing the joint as would result from transporting specimens to the laboratory.

The in- situ test is conducted at a location where an in- situ de ­formability test has been conducted using two flatjacks (Ref. 2). A brick unit located mid- distance between the two flatjacks is selected as the test unit. Two units, one on either side of the test unit are carefully removed using a hand held drill to remove the surrounding mor­tar. The test unit is instrumented with a LVDT or other electronic displacement measuring sensor to measure horizontal displacement. A small hydraulic jack is inserted in the space of one of the removed units. The jack is shimmed to provide a properly centered horizontal force on the unit and then is ex tended until it fits snugly in place (Figure 6).

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269

OL~I 1/ 11 II I 19J~ II

JI II

Ilc~L: DI II

~~"""q DI 1/ II II II ! II II II I t: Flati aci<

DI II 11 11 11

Figure 6. In-place shear test se t-up.

~

Ul

E

(!l

Ul {l)

'--'-' Cf)

'-ru {l)

.r: Cf)

In - Place Shear Test - Wall #5 Disp l ac e me n t (mm )

-6 3 -~ 2 -2 1 o O 2 1 4 2 200

120

Flatjac Pressur s As sho1n I \L 1\ í., r 200 psig

~ d~o psg 50 ps 9

~ psig I

~o

-40

-120

I ! ~ --- Op~ J

I ;-J ~ psig I 100 psig

\ I SO Is'lo

V

10 I

10 I

D I

10 I

10

6 3 1.~

0 .8

0.3

-0.3

-0.8

~ I \ 200 r sig

East J -200 -0.25 -O. 17 -0 . 08 O . 00 o . 08

Disp lacement (in . )

Figure 7 . Tupical in-place shea r t es t results.

LVe ~west 0.17

-1.~ 0.25

Cf) ::J co

"' -,

Cf) <T -, co (Jl (Jl

~ -o ~

270

With the flatjack pressures at zero, the pressure in the horizontal jack is slowly increased until the brick unit displaces without further increase in the horizontal load . This corresponds to peak shear strength under a normal force equal to zero. Nex t the pressures in the two fla t­jacks are increased to the firs t increment of normal load pressure and the shove test is repeated. This sequence is repeated several times until a plot of shear strength versus normal pressure is obtained. The horizontal jack can then be transferred to the opposite cavity and the test sequence repeated in the opposite direction to investigate the effect of shear force rever sal on in-situ shear strength.

The results from an in-situ test on materiaIs similar to tho se used in the laboratory s tudy are shown in Figure 7 in the form of shear stress versus shear displacement for both forward and reverse shear. The aver­age shear stress is obtained by dividing the horizontal force by the areas of mortar bed joints above and below the test unit. The vertical "colla~' joint at the inside surface of the test unit is usually neglect­ed since little bond strength is provided by most collar joints due to poor mortar placement. Because of the units which are removed on each side of the test unit, the normal stress on the test unit is greater than the hydraulic pressure in the flatjack. A three-dimensional finite element analysis of the loading geometry showed that the average normal stress on the test unit was 1.71 times the applied hydraulic pressure.

The results from three shove tests conducted at separa te locations on the same tes t wall are shown in Figure 8 to illustrate the repeat­ability of test data. The shove test when combined with the use of flatjacks to provide variations in the normal stress al lows one to directly measure an important material property required to assess structural safety against horizontal loads arising from wind or earth­quake.

Normal Stress (KPa)

200 O 275 ~1 8V 1103 1379

1379

o 160 1103

.... O UJ ::r

Ol a O (1)

CJ - 120 827 -,

UJ Ol n-Ol -, Ql (1)

c- lJl ..., "' UJ

80 c-

551 7<

\O " III r:: ~ UJ

40 275

O O 40 80 120 160 200

0

Normal Stress (psi)

Figure 8. Mohr-Coulomb failure surface for in-situ masonry.

~

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271

SUMMARY

Laboratory studies on the bed joint shear behavior of masonry in­cluding the effects of normal load magnitude and shear load history are presented. Similar studies on the in-situ behavior of joints in existing structures are also presented. Both laboratory and field studies shO\v that peak and residual shear strengths are well represented by the Mohr-Coulomb failure criterion.

ACKNOWLEDGEHENTS

The laboratory studies discussed in this paper are supported by the National Science Foundation under Grant No. NSF ECE-8515318. The field studies are also supported by NSF under Grant No. ECE-8315924.

REFERENCES

1. , Earthquake Hazard Reduction in Existing Buildings, Los Angeles, City Building Code, Division 68, 1981.

2. Noland, J. L., Kingsley, G. R., and Atkinson, R. H., liA Methodology for Incorporating Nondestructive Techniques into the Evaluation of Masonry Structures," Proc. 8th Int. Brick/Block Masonry Conference, Dublin, 1988.