THE EFFECT OF HORIZONTAL REINFORCEMENT ON THE STRENGTH AND DUCTILITY OF MASO NRY WALLS AT SHEAR FAILURE

~1 TOMAzEVIc R zARNlé

Senior Research Engineer Research Engineer

Institute for Testing and Research in Materials and Structures, Dimiceva 12, Ljubljana, Yugoslavia

ABSTRACT The effect of horizontal reinforcement on the strength and ductility of masonry walls subjected to cyclic lateral loading has been studied on 16 wall specimens, varying the mortar strength and the amount of horizontal reinforcement. At shear failure the horizontal reinforce­ment significantly improved the ductility of walls, but had no effect on their lateral resistance. It has been observed that the amount of reinforcement, equivalent to the lateral shear resistance of unrein­forced walls, is sufficient for improving the ductility. Larger amount of reinforcement was not fully effective because of unadequate bond and anchorage conditions at shear failure.

1. INTRODUCTION When failing in shear, the unreinforced masonry wall behaves basically as a bri­ttle structural elemento It has already been proposed that the horizontal rein­forcement should be placed in mortar joints in order to increase the strength and ductility of masonry wall, but not many experimental data exist which deal with the effect of such kind of reinforcement. The investigations, carried out so far, have indicated two possible conclusions: in some casee the increase of ductility has been observed, in the others, however, the increase of both ducti­lity and strength has been reported. This paper discusses the results obtained on the series of tests, recently carr­ied out at the Institute for Testing and Research in Materials and Structures in Ljubljana, Yudoslavia, within the first part of a two-year research project. The general conclusions, however, based on the first series of tests only, are still doubtful as far as the behaviour of different types of masonry walls, reinforced by means of horizontally placed reinforcement, is concerned. It is expected, that the formulation of general conclusions will become possible after the completion of the research project, i.e. after the testing of other types of horizontally reinforced masonry walls.

2. DESCRIPTION OF TESTS If masonry walls are reinforced by means of steel reinforcement, the use of lime­cement mortars is not recommended because of possible corrosion of reinforcement. High strength cement mortars are also preferable from the point of view of bond and anchorage of reinforcement, although they are not allowed to be used in sei­smic areas according to the existing Yugoslav seismic regulations. Since the expected lateral resistance of the specimens constructed of high stre­ngth cement mortars in the prototype size would exceed the capacity of the exi­sting testing facilies at the Institute, the reduced size test specimens have been prepared. Alltogether 16 specimens, having dimensions 400x607x63 mm have been constructed, varying the mortar strength and the amount of horizontal rein­forcement, as shown in Table 1. The walls were constructed on reinforced-concrete foundation blocks having screws to fi x them into the test set-up. Hollow parts of concrete blocks have been grouted with mortar, and the horizontal reinforcing bars, having rectangular hooks at both ends, have been placed into the horizontal mortar joints. The choosen dimensions of test specimens, i.e. their height to length ratio, as well as the amount of vertical load should assure the shear fai­lure of walls when subjected to lateral load.

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Table 1: Descripti on of Test Specimens

,.----- '------' Wal 1

Des i gnat ion Hor t a r Grade (I<\Pa )

, Re i ~forcernent I Re inf or ce-Dl ame ter ment rat lo

( "", ) ( \)

A 0/10-1 10 ] A 0/10- 2 I,

1-:-:-~;-:~-~,-I--::-+:-8 -0 -3'-' - : " '" I

, A 10/10-1 I I ----f---- ' i , A 10/15-' '5 I ' I ' 8 0 3.' ' 0. '8

A 20/15-2 I 1-'-3-0/-'°--,- 1---'0-11--8-0 -,.-1 - ~--o-.JI-I'

A 30/10- 2 .

A JO/l5-1

A )0/1~-2 15 i 8 0 ' .1 I 0.31

:

A 40/10-1 10 I 8 04 .2 + I, 0.]7 A 40/1 0-2 4 0 ) . 1 1------4---~'---_- 1

A 40/1 5-1 15 8 e 4 . 2 • 1---"-J-7-, 0 J. '

. ~ ~0/l5-1 _ L ___ --'--____ ~I __ _

Fig.l Shape and Dimen s i ons of Concrete Blocks

AO /lO -1 .2

A 0/ 15 - 1.2

A3 0 / IO-I,2

A 30/ 15 - 1, 2

kOO mm

CF1

l"'1 1 "--i

r~"~ k-~~ B Qk 2 mm

h~~ L~

kl _, H

.63

A20/IO-I,2

A20/ 15 - 1,2

A~O/10 - 1.2

AkO/ 15- 1, 2

kOOmm

Fig.2 Configurati on of Test Specimens

1:3 reduced size hollow concrete blocks (grade B10), as shown in Fig.1, and two different types of cement mortars (grades M10 and M15) have been used for the con­struction of test specimens. Mild steel smooth bars of 3.1 and/or 4.2mm diameter, having 90 degrees hooks at the ends were used for horizontal reinforcement. The shape and the position of reinforcement of the test specimens is shown in Fig.2. The mechanical properties of the constituent materials (concrete, blocks, mortar and reinforcing steel),as given in Table 2, were obtained by means of standard quality control tests. The mechanical properties of walls at compression, howe­ver, were obtained by means of masonry prism tests (Fig.3), especially designed for this occasion.

Tabl e 2: Mechani cal Properties of Materials

Concrete Blocks C~ressl'1e St rength : 12. I "'.

Hortar C~ressive St renQth Bendin g

Cube ("". Pnsm { I"t'a ("'. ) I: 4. 5 - MIO 8.6 12.4 3.2

I: 3. 5 - MI5 16 .8 17 .8 4.1

Yield Elasti c I Yi el d Rei nforcelM:'n t limit Modul us Strain ("'. ) ("" ) ( xI0-3)

o 3 . I 31 4 209 700 1. 50

o 4.2 302 210 500 1. 45

COtTllre ss; ve Elasti c Shear W.ll Strength Modu 1 us Modul us

(MP.) ("'. ) ("'. ) MI O 5.43 3857 41 3

MI 5 6.15 530 1 465

[~PaJ

7.0

6 .0 --

S.O

4.0

3.0

20

1.0

o ~ - .----o 1.0 2.0 3.0 4.0 S.O 6.0 7.0 8 .0 ([lO " J

Fig. 3 Stress-Stra in Diagra ms as Obta ined by Means of Masonry Prism Compress ive Tests

A special testset-up, as shown in Fig.4 has been prepared for the tests, which keeps constant vertical load acting on the wall during the lateral load reversals. Simultaneously, the lower and the upper support planes remain parallel during the

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test, hence clearly defining the boudary conditions. The test specimen was fi­xed onto the rail-guided and roller-supported support beam. During the testing procedure, the horizontally moveable support beam was moved by means of two al­ternatively acting hydraulic actuators. On the upper side the specimen was fi­xed onto the vertically moveable rail-guided and roller-supported upper beam. Reaction forces and bending moments transferred from the specimen into the upper beam were measured by means of four flexural dynamometers, fixed onto the beam. The vertical steel guide-rail plates were kept parallel by means of special screws.

The weight of the upper beam was not sufficient to cause the desired amount of vertical load on the test specimen. Therefore, a hydraulic actuator was fixed onto the testing frame, acting on the upper beam. The actuator was connected to the gas accumulator in order to keep constant the Gil pressure in the actuator during the testing procedure r2gardless to the vertical movements of the upper beam.

The tests were displacement-controlled by means of an x-y plotter, plotting the force-deformation relationship during the testing procedure. After the first cracks developed in the walls, the lateral loading of the same amplitude of de­formations was repeated three times in order to obtain an insight into the stre­ngth and stiffness deterioration at the repeated lateral load reversals. During the tests the propagation of cracks was observed and photographed. For this purpose, one surface of test specimens was painted with gypsum. The follo­wing quantities have been measured (Fig.5):

- forces in hydraulic actuators, - forces in flexural dynamometers of the upper support beam, - horizontal displacements of the upper and lower support beam of the wall, - vertical displacements of the upper support beam, - strain of the wall in two diagonal directions, - strain of reinforcement at four different poillts.

L\lDT-s have been used for- measuring forces in hydraulic actuators and displace­ments, and strain-gages have been used for measuring forces in flexural dynamo­meters and strain of reinforcement. Specially designed electric resistance dila­tometers were used for measuring strain of the wall. The recorded data were com­puter processed for further analysis of test results.

~M tIl

3. TEST RESULTS

Fig.4 Test Set-Up Fig.5 Instrumentation of Test

Specimens

Typical lateral load - horizontal d2formations relationships are presented in Figs.6 and 7 for an unreinforced and reinforced wall, respectively. Typical rela­tionships between the lateral load and strain of the wall are presented in Fig.8, and typical relationships between the lateral load and strain of the wall rein­forcement are presented in Fig.9. Test results are summarized in Table 3, where

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25 .

2 0 .

1=10/15 -1

o (",,,, )

Fig.6 Typical Lateral Load - Deformation of Wall Relationship for Unreinforced Hall

1=140/10-1

E1 ([ - J)

Fig.S Typical Lateral Load - Strain of Wa11 Relationship

25.

20.

15.

,10

5.0

- '5.0

-10.

- 15.

- 20.

1=140/10-1

Fig.? Typical of Wa 11 Wall

1=140/10-t

o ( ",,,,1

Lateral Load - Deformation Relationship for Reinforced

- 2~ .60- .... --:-- "". 3nO ~-!;-~~. 3\-oO-~~--f;;;-~-.{g,0 ~--;-,-'c~ 2'---"-7, "'. 5:--~-.'1.' 52 «(- 1 1

Fig.9 Typical Lateral Load - Strain of Wall Reinforcement Relationship

the following quantities are given:

Ocr deformation of wall at diagonal cracking, H horizontal force at diagonal cracking, o~r deformation of wall at maximum horizontal force, H max maximum horizontal force, omax maximum deformation of wall, H~ax horizontal force at maximum deformation of wall. uma x

Table 3: Test Results

A 0/11..1-' 1.2d Il.!)S -1.10 -b,05 14.20 1.52 -18.72 -2.40 \.52 14.20 - 2.40 -18.72

A 0/10-2 0.74 13.38 -0.81 -14.:.0 ló .72 1.17 -18.35 -'.23 117 16.72 - 1.b9 -17.76

A Oll!;-' 1.48 16.15 -1.30 -19.20 17.84 1. 84 -22 .07 -1.94 1.84 17.84 - 1.94 -2 2.07

A 0/15-i 1.08 15.50 -\.05 -19.74 20.43 2.54 - 22.43 -1.76 2 . S4 20 .43 - 3.30 -16.64

A 20110-1 1.21 13.67 -1.35 -13.99 17.73 4 .02 -19.35 -4.13 6 . :='0 14 . YÓ - 6.ts8 -15.d~ _____ _ ___ _ o .

A 20/1S-1 1.33 16.50 -1.27 -~6.73 21.13 3.55 -21 . 07 - 4. 51 615 11.54 - 6.07 - lO.l i

A lO/l 5-2 1. 02 14.42 - 1.04 -1 7.73 21.42 6.07 - 21.26 -4.~5 7.6;: 19. 58 - 7. 43 -13.0i

A 30110- r 1.25 11 .08 -1.38 -17.50 16 .17 5.19 -13.09 - 3. 97 10. 14 11.3 7 - 10 .43 -19.81 ~----t---t---1---~---1,--t----~--4----+---+---1----+------

A 3011'-1 1.04 10.38 -1.04 -17.11 19 . 93 4.5 7 -1 1.67 -6.08 7.86 9.49 - 7.61 -19.35 ~----t---t---1---~---1~--+ ---+---~---+---1--~~---~----

A 30115- 2 1.01 l'.9b -1.:>2 -16 .70 .lu . 10 4.61 -20 . 12 - 4 .56 6 . 25 17.25 - 6 . 2~ - ltLbl r-'-~r-___l--+---t- - -t--- - -t--- ----- -

A 40/10-1 1.54 15.05 -1.54 -1 7.96 19.59 4.56 -22 71 -458 9.08 12.62 - 9 .06 -13 .60 I----~-~--t---+-___ll__-t_--_t__-_t_-- --- ---- - -----

A 40/10-2 0.99 13.84 -1.03 -16.72 21.10 3 . 5t1 -21. 16 -3 . 56 6.22 14.]5 - 6.07 -1~.5S

~--~I--___l----~--~---t-----------.--~--t---t---t---1------A 40/15 -1 1.01 14.83 - 1. 07 -17. 8 7 1 1 78 4.59 -11.J3 -6.13 10 . 05 11.46 -10.60 -13 . • '

A 4011, -2 1.59 1 •. 67 -1.46 -15.87 21.48 3. 61 -19.74 -3 . 53 4.84 13 .88 - 6.07 -1 •. 09

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Shear failure was observed in the case of all tests, as expected. First, the ho­rizontal tension cracks occurred at the joints between the reinforced-concrete support beams and the wall or in the mortar joints between the first two rows of bloc ks. Then, diagonal cracks developed in the mid-height portion of the wall.

In the case of unreinforced walls one single diagonal crack developed, passing partly through the mortar joints and partly through the masonry units. The diago­nal crack opened until the instantaneous failure of wall took place, in the case of which the upper portion of wall slipped over the lower portion in the diago­nal cracking plane.

In the case of reinforced walls, however, many diagonally oriented cracks deve­loped, uniformly distributed in the middle part of the wall surface. At the in­creased l ateral deformations the cracks opened, and simultaneously the crushing of concrete blocks in the compressed zones of the wall took place. In some ca­ses the hooks of reinforcing bars also sta~ted to bend outwards. At failure, either single parts of the wall already separated by diagonal cracks fall apart, or the wal l settled down because of splitting or crushing of concrete blocks took place. Crack patterns and failure modes of typical walls are presented in Fi gs.10 and 11.

Fig.l0 Typ ical Crack Pattern s - Unreinforced Wall

Fi g.ll Typical Crack Pa tterns - Rei nforced Wall

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4. ANALYSIS DF TEST RESULTS 4.1 The Effect of Horizontal Reinforcement on the Lateral Resistance of Walls

Studying the relationships between the horizontal load, horizontal deformations and strain of the wall and wall reinforcement (Figs.12, 13, and 14), the parame­ters which define the lateral resistance of walls as well as the mechanism of reinforcement action can be defined.

.H[kN] A401lS·1 .HIII.NJ A .c.O/1S - I

5 --

0-+---+----+--· 0"2 o"' o·e O·S 1'0 t t 10"']

- 5 --- - ----I

_ H..

Fig . 12 Typical Lateral Load - Fig.13 Typical Lateral Load - St rain Fig.14 Typical Hor izontal Deformation -Strain of Wall Reinforcement Hystel '~ ~ i 5 Envelopes

Strain of ~Jall Hysteresi s Envelope

of Wa ll Reinforcement Hysteresis Envelope

Previous investigations have shown, that the shear resistance of wall as a stru­ctural element is governed by the maximum principal tensile stresses, occurring in the wall at the given loading conditions. Assuming that the masonry wall is elastic, homogeneous and isotropic structural element, the principal tensile stresses can be calculated by means of the well known equation (Ref .1 and 2):

where:

T

b

C1T = ~ (C1/2)2 + (bT)2 - C1012, (1)

maximum principal tensile stress in the wall as elastic, homo­geneous and isotropic element, average normal stress in the horizontal cross-sectional area of the wall due to vertical load (C1 = 1.66MPa), average shear stress in horizontal 2ross-sectional area of the wall due to lateral load, shear stresses distribution coefficient (b=1.5).

In Table 4 the values of maximum principal tensile stresses at diagonal cracking C1T ,calculated by means of eq.(1) are compared for unreinforced and reinfor­cé~c0alls. In the case of reinforced walls, these values can be considered as equivalent values C1e ,i.e.the values which take into account the effect of reinforcement . An att~~pt has been made, however, to estimate the values of maxi­mum principal tensile sresses for the basic, unreinforced walls, by subtracting the measured contribution of horizontal reinforcement H from the lateral re-sisting force at diagonal cracking Hcr. a,cr

In Table 5 the values of maximum principal tensile stresses C1~ at the maximum lateral load H ,i.e. at the attained lateral resistance of wMll, are compared to the cor~esp~Aaing values of maximum principal tensile stresses at diagonal cracking C1T . ,cr Whereas unique conclusion can be drawn when analyzing the results as given in Table 5, two possible conclusions can be made on the basis of the results pre­sented in Table 4.

The following conclusions can be drawn when analyzing the results as given in

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Table 5: Maximum Principal Tens i 1 e Stresses at Maximum Lateral Laad (Referent ia 1

Tabl e 4: Maximum Pri nc i pa 1 Tensile Stres ses at Diagana 1 Cracking

Tensile Strength) Campa red ta the Ma xi mum Principal Tens i 1 e Stresses at Diagana 1 Cracking _.

Morta,. Reinforcement • . ~,.ade Ra t io °T ,cr "T,e ,. ClT ,cr ( MP,, ) 1'1 1"" 1 1"" 1

O'r ,cr

Horta,. Re in f orcemen t e I , , Grade Rat l0 ClT .u °T , cl'"

ar .u I MP, I 1\ 1 .----

I "" ) 1'"" 1 'T .er 0 . 0 0.34 0.34 1.00

"' 0 0 . 18 0.30 Q.3 4 1.1 3 0.0 0. 48 Q. J4 1. 4 1

0.32 0.2 5 0.38 1.52 "' 0 Q.18 0.56 Q.3 4 1.65

0 . 37 0.26 0 . 4 ) 1.65 0.32 0.6 1 0.38 1 . 61

P.- 0.29 0.38

0.04 0.05

0.0 0.51 0.5 1 1.00

0.37 0. 68 0. 4) \ .58

~ 0 .58 0.38

0.09 0.05

" '5 0.18 0.36 0. 45 1.25 0.0 0. 66 0 .51 1.28

0 . 32 0.28 0.35 1. 25 . '5 0.18 0.69 0. 45 1.53

Q.31 0.33 0. 48 1. 45 0.32 0.65 0.35 1.86

~ 0.37 0. 45

O. la 0.07

0.37 0.68 0. 48 1. 42

~ 0.67 Q. 45

0.03 0.07

Table 4: - the maximum pricipal tensile stresses at diagonal cracking of wall are increa­sing with the increased mortar strength and with the increased reinforcement ratio - the conclusion is based on the comparison between the 0Te /oT

t ' ,cr ,cr ra lOS;

- the maximum principal tensile stresses at diagonal cracking of wall are increa­sing with the increased mortar strength, but are independent on the reinforce­ment ratios - the conclusion is based on the comparison between the values as calculated for the unre~nforced walls 'OT as well as on the comparison between the equivalent values 0T . ,cr ,cr 80th conclusions are possible. However, based on the statistical analysis, the last one is more probable. When analyzing the results in Table 5, the following conclusions can be drawn : - the maximum tensile stresses at the attained latera l resistance of wall incre ­ases with the increased mortar strength;

- the maximum tensile stresses at maximum lateral load, i .e, the lateral resis ­tance of the wall is independent on the reinforcement ratio. The conclusion is confirmed by the relatively small scattering of test results. The possible answer to the above conclusions can be found when analyzing the mechanism of reinforcement action: horizontal reinforcement is carrying the lateral load, acting on the wall, by means of tension. In order to ensure the adequate resistance and ductility of horizontally reinforced masonry wall, two conditions must be fullfilled, theoretically: the condition of the sufficient amount of horizontal reinforcement and the condition of adequate bond and an ­choring of the reinforcement. The amount of horizontal reinforcement shou ld at l east correspond to the lateral resistance of the bas i c, unreinforced wall: when the diagonal cracks develop in the wall, the reinforcement must take over the full lateral load, acting on the wall. Of course this is only possible if the transfer of fo rces is ensured by means of adequate bond between the reinforcement and mortar and by means of adequate ancorage of reinforcement. In the reported series of tests, an attempt has been made to fullfill both conditions.

As far as the load bearing capacity is concerned, the reinforcement could carry the following horizontal shear forces:

~ = 0.18% H = 18.96 kN (wall s A 20), ~ = 0.32% Ha,y = 33.42 kN (wall s A 30), ~ = 0.37 % Ha,y = 42 . 95 kN (wallsA40), a,y

where ~ = 0.18% corresponds to the minimum reinforcement ratio, defined with the

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shear resistance of the basic, unreinforced wall. Adequate anchorage of reinforcement has been provided by means of rectangular hooks at the ends of the reinforcing bars, as well as by selection of high strength cement mortars for the construction of test specimens. Bnsed Oil the analysis of hysteresis envelopes, which show the changes of strain of the wall and reinforcing bars in dependence on the lateral load, acting on the wall, as well as in dependence on the lateral deformations of the wall (Figs.12, 13, and 14), and based on the observed crack patterns (Figs.10, and 11), the mechanism of the reinforcement action can be defined, as shown in Fig.15:

1.0 r---,---_._--..,

o :I:

ó I dmax

a)

1.0

1. o .----y---...,-----,

o :x:

dI dmax

b)

1.0

,. O r---'---'--'-~'"

o :x:

õ I J'max

c)

1.0

Fig.15 Mechanism of Reinforcement Action

When analyzing the reinforcement action, The following characteristic possibili­ties can be distinguished:

- until the diagonal cracking takes place the contribution of reinforcement to the lateral resisting force is very small;

- after the diagonal cracking takes place, the contribution of reinforcement to the lateral resisting force of wall increases with horizontal deformations of wall until the yielding of reinforcement (Fig.15a);

- after the diagonal cracking takes place, the contribution of reinforcement in­creases with deformations of wall until slipping of reinforcement occurs (Fig.15b). After slipping, the contribution of reinforcement might increase again because of the hooks at the ends of reinforcing bars (Fig.15c). Typical diagrams, which show the contribution of reinforcement to the lateral resisting force of wall in dependence on horizontal deformations, are shown in Figs.16 and 17.

Fig.16 Typical Contribution of Reinforcement to the Lateral Resisting Force

lO

1\ lOI1S.1 O. , , o., ,

o .•

Fig.l? Typical Force in Rein­forcement / La tera 1 Load Ra t i o - Deforma t i on of Wall Relationship

Table 6: Bond and Effectiveness of Reinforcement

I H Morta' " i< e i nforcement . a,lN x

Grade Ratio . . H ("",I ('I (""'I (""'I ' .1

0.18 0 .99 1. 03 O.U

.'0 0 .12 0 . 85 0.95 0.30

0 .17 0.71 0.79 0.17

r,- 0. 82

~ 0.18 1.77 2.01 0. 84

.,5 0 .12 1.17 1.82 0.58

0.17 1. 17 0 .19

~ ~ 0.18

In Table 6 the values of the evaluated bond stresses at slipping of reinfor~ement T, the equivalent bond stresses at the maximum force in the reinforcement T and the effectiveness of reinforcement H IH are compared for different types a,max a,y

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of walls. It can be seen that the effectiveness of reinforcement strongly depends on the conditions of bond and anchorage of the reinforcement: it increases with mortar strength, but is inversely proportional to the reinforcement ratio. The reinforcement was fully activated in the case of walls with minimum reinforcement only.

4.2 The Effect of Horizontal Reinforcement on the Ductility of Walls

The deformability properties of walls are defined by means of their stiffness and ductility. In order to evaluate both quantities, the experimentally obtained hysteresis envelopes have been idealized as simple bi-linear elasto-plastic en­velopes, defined by means of elastic stiffness K , lateral resistance of wall H , and ultimate ductility factor d , as shown iN Fig.18. u u The elastic, or effective stiffness of the wall K is defined as the ratio be­tween the lateral force and the corresponding defbrmation Df the wall at diago­nal cracking. Energy conservation principle was used for the determination Df the idealized lateral resistance Df wall H : the area under the experimentally obtai­ned hysteresis envelope must be equal ~o the area under the idealized hysteresis envelope. The ultimate ductility factor d is defined as the ratio between the deformation at the intersection Df the de~cending branch Df the experimental hys­teresis envelope with the idealized hysteresis envelope and the deformation at the idealized elastic limito

Based on the above definitions, capacity reduction factors H IH ,ductility factors at maximum lateral load d as well as ultimate du~ti~~ty factors d have been evaluated, as given in ~~Bfe 7. For this purpose, the experimentall~ obtained hysteresis envelopes have been normalized, as shown in Fig.19.

H"'OI ,--------:;,==--- --, H.

Fig.18 Ideal iza t ion of t he Hysteresis Envelope

04 04

C· I 1.0

:;:::~ j Fig.19 Typical Normalized

Hysteresis Envelope

Tabl e

Morta r Grade ("", I

. '0

. ' 5

7 : Capacity Reduction and Ductil ity Factors

Re i nforce- "u ment rati o ,",." du

( ~ ) ",."

0.0 0. 99 1.13 1. 37

0.18 0 .9' 2 . 52 3.10

0 .32 0.94 2.80 3. 40

0.3 7 0. 95 2. 50 3.13

0.0 0. 97 1. 49 1.59

0 . 18 0. 94 3.25 4 . 15

0. 32 0 .9 3 3.56 ' . 26

0. 37 0.96 3.05 4 .28

The values in Table 7 clearly indicate, that the unreinforced masonry walls be­haves as a brittle structural elemento Adding the horizontal reinforcement, however, the ductility can be essentially improved. Ultimate ductility factors are practically independent on the amount of horizontal reinforcement. A slight increase in ductility only was observed with the increase Df mortar strength, due to the improved bond and anchorage conditions.

4.3 Strength Deterioration and Equivalent Viscous Damping

Tests have shown, that the strength deterioration under repeated load reversals at the same amplitude Df deformations is rathe~ small before the attainment Df maximum load. In such a case, it does not exceed the value Df 5%. However, the strength deterioration becomes significant after the attainment Df maximum load, reaching the value of 30% Df load attained in the first cycle Df loading with the same amplitude of deformation. The process Df strength deterioration tends to stabilize after the third cycle Df load reversals (see Table 8).

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Table 8: Strength Deteri ora t i on at Repea ted Lateral Load reversa l s

"'11 , "z ") , "z H)

~ Z H)

Des;gnat ion ' .. , H) H) 'ma, H) H) ' ma, H) H)

• ZO/l O- l 0.27 0.99 O . • 7 0.97 0.60 0.95 0.9 1

A 10/1 5- 1 0.18 0.96 0. 41 1.00 0.99 0.74 0 . 99

A 10/1 5-1 0.2 4 0. 98 0.48 0. 98 0.81 0 . 91 O. B1

A 30/10 - 1 Q.19 1.00 Q.49 0.98 1.00 0. 87 0.76

A 30/15- 1 0.27 0.98 0. '6 0 . 98 0.97 0.80 0 . 96 0.94

A 30/15 -2 0 . 24 0.97 Q. 40 1. 00 0.98 0 .7 3 0.9' 0.91

A 40/ 10-1 0.15 1. 00 0.39 1.00 0.98 0.83 0.92 0. 86

A 40/1 0-2 0.15 0.98 0.41 0.98 0.95 1.00 0.90 i 0 .00

A 40 /15-1 0 .24 0.97 0.96 0 . 44 0 .97 0. 96 0 .95 0.89 Q.78

A ' 0/15-1 0.2 4 0.98 0 . 40 0.99 0. 95 0 . 86 0.85

Knowing its hysteretic behaviour.the energy absorbtion capacity of a structural element can be estimated. However, when performing the dynamic analysis of the structure, the energy absorbtion capacity is usually expressed in terms of the equivalent damping ratio . For the tested series of walls, the typical changes of the equivalent viscous damping ratio with the increased deformations of walls are presented in Fig.20. The test results have shown that there is practically no effect of the amount of horizontal reinforcement on the value of the equivalent viscous damping ra­tio. The value seems to increase with the increased deformations of wall only.

A40/10 - 1 15

..... 10~

0.5 1.0 1.5 R Co!.)

6 8 la dlmm]

A40115 -1

0 .5 1.0 1.5 R [0/oJ

r -l-,- f ,I~ 4 6 8 10d[mml

5. CONCLUSIONS

5 0 .5 1.0 1.5 R [o!.)

6 8 la cl'lmm]

__ m.a.u,"

0.5 1.0 1.5 R [01oJ

L-~,,-+-,~''---6 8 la &Imm]

Fig.20 Typical Change of Equivalent Viscous Damping Ratio

A series of 16 horizontally reinforced masonry walls has been tested by subjec­ting them to constant vertical load and cyclic lateral load reversals. 1:3 re­duced size hollow concrete blocks (grade 810) and two different types of cement mortar (grades M10 and M15) have been used for the construction of test speci­mens. Mild steel smooth bars of 3.1 and/or 4.2mm diameter. having 90 degrees hooks at the ends. were used as horizontal reinforcement. The reinforcement ratio varied and was equal to 0.18%. 0.32%, and 0.37% with respect to the verti­cal cross-sectional area of the wall.

The following conclusions can be drawn when analyzing the test results:

- when failing in shear. the unreinforced masonry wall. constructed with concre­te blocks and cement mortar, behaves as a brittle structural elemento having minimum ductility;

- the ductility of wall can be essentially improved by means of sufficient amo­unt of horizontal reinforcement. placed into the mortar joints;

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- the shear resistance of walls does not depend on the amount of ho ri zontal re­i nforcement; - the effectiveness of reinforcement strongly depends on the mortar strength: it increases with the increased mortar strength because of the improved bond and anchorage conditions; - the effectiveness of reinforcement is inversely proportional to the reinforce­ment ratio; - for improving the ductility of unreinforced masonry wall, the minimum reinfor­cement is sufficient, i.e. the amount of reinforcemnt corresponding to the late­ral resistance of the basic, unreinforced wall. Larger amount of reinforcement is not economical because of unadequate bond and anchorage conditions at shear failure.

Hence, the following recommendations should be observed for the verification of the earthquake resistance of masonry buildings: the unreinforced masonry buil­dings, constructed with concrete blocks and cement mortar, should be designed for strength, whereas in the of horizontally reinforced masonry buildings of the same kind, the ductility of the walls can be taken into account.

The first series of tests has indicated, that by means of horizontal reinforcement similar effects on the strength and ductility of masonry walls can be obtained as in the case of confined masonry (i.e. the masonry confined by means of reinforced­concrete tie-beams and tie-columns). However, in the case of horizontally reinfor­ced masonry, the cracks are more uniformly distributed, the energy absorbtion capacity is improved and there are no thermal insulation problems.

It has been found, that the same equation can be used for the calculation of the shear resistance of horizontally reinforced masonry wall Hu as in the case of unreinforced wall:

where:

CR A fW t.w

capacity reduction factor, horizontal cross - sectional area of the wall, referential tensile strength of the wall.

__ (2)

The referential tensile strength of the wall ft is defined as the parameter which governs the lateral resistance of masonry'~all at shear failure. Hence, the value of the referential tensile strength of the wall is equal to the value of the maximum principal tensile stress in the wall as a elastic, homogeneous and iso ­tropic structural element at the attained maximum lateral load:

f = a t,w T,u ___ (3)

Based on the analysis of the normalized hysteresis envelopes, the following value of the capacity reduction factors can be used in the calculation:

CR = Hu/Hmax = 0.93.

The following characteristic values of ultimate ductility factors of walls can be taken into account when verifying the earthquake resistance of masonry buildings by means of limit states methods: - for unreinforced walls:

- for reinforced walls: d = 1.20,

u du = 3.20.

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The conclusions, however, are valid for the tested series of walls only. The ge­neral conclusions, already indicated within the first series of tests, will be reported after finishing the research in the near future.

6. ACKNOWLEDGEMENTS The research reported in this paper has been finacially supported by the Research Community of Slovenia, by the National Bureau of Standards, USA, through the US -Yugoslav Joint Board on Scientific and Technological Cooperation, and by the construction company SGD Beton Zagorje.

7. REFERENCES (1) TURNsEK,V., cACOVlc,F. "Some Experimental Results on the Strength of Brick­

Masonry Walls" , Proceedings, 2-IBMaC, Stoke-on-Trent, 1971,

(2) TURNsEK,V.,SHEPPARD,P. "The Shear and Flexural Resistance of Masonry Walls", Proceedings, International Research Conference on Earthquake Engineering, Skopje, 1980,

(3) PRIESTLEY,~1.J.N.,BRIDGEMAN, 0.0. "Seismic Resistance of Brick Masonry Walls" , Bulletin of the New Zealand Socíety for Earthquake Engineering, Vol.7,No.4, 1974,

(4) CANTU,E.,ZANON,P. "Combined Cyclic Testing Procedures in Diagonal Compression on Hollow Clay Block Reinforced Masonry", Proceedings, 6-IBt~aC, Rome, 1982.

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