[20] - Structural Faults & Repair (Masonry Walls Srg)

7/27/2019 [20] - Structural Faults & Repair (Masonry Walls Srg)

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IN-PLANE SHEAR REINFORCEMENT OF MASONRY PANELS WITH HIGH

STRENGTH STEEL CORDS

Prof Antonio Borri, Giulio Castori & Marco Corradi

University of Perugia

Dept of Civil and Environmental Engineering

Via Duranti, 93 - 06125 PerugiaItaly

[email protected]

KEYWORDS: Masonry panels, Strengthening, Steel Cords

ABSTRACT

In the area of the reinforcing techniques most widely used at present, the use of new materials may

contribute significantly to the solution of different structural problems. The aim of this paper is to analyze

the results of a new series of tests, designed to find the experimental values for the shear strength and

stiffness of masonry panels reinforced with high-strength steel cords embedded in a cementitious matrix(SRG: Steel Reinforced Grout). More than 30 solid brick panels were constructed in the laboratory and

then were strengthened with various types of steel cords according to differing schemes. Diagonal

compression tests were carried out on the panels in order to measure shear strength, stiffness and

ductility. The purpose of the tests was to analyze the effectiveness of the intervention, above all as a

technique of seismic upgrading against in plane mechanisms of collapse. The results of the experiments

carried out show a significant increase in strength and shear stiffness, with interesting implications for the

practical utilization of the technique studied.

INTRODUCTION

A great deal of experimental research has been carried out in the past few years in order to study the shear

behaviour of masonry structures. This research has highlighted the low strength of historic masonrystructures to seismic stress and therefore questions both the shear and tensile strength of masonry walls.

Starting in the 1990s, an extensive experimental investigation of masonry panels obtained from buildings

in Tuscany was carried out by Chiostrini et al. (2000). Other experimental research to evaluate the

mechanical behavior of masonry structures was carried out by Turnsek et al. (1980), Sheppard (1985),

Chiostrini et al. (1994), Anzani et al. (1998) and finally by Corradi et al. (2002) on panels obtained from

buildings hit by the Umbria Marche earthquake. Far fewer have been the experiments carried out to

determine the efficacy of various techniques of shear strength reinforcement of masonry walls, such as

the use of grout injections, ferro cement, deep repointing of mortar joints and, among the latest

investigations, the use of composite materials based on carbon, glass or aramidic fibers. The injectability

of masonry walls was studied by Baronio et al. (1992). Other reinforcing techniques have been analyzedby Modena (1994), Binda et al. (2000) and Borri et al. (2008). Corradi et al. (2003) have instead analyzed

the case of the reinforcing of historic masonry walls through the use of composite strips.

None of the above mentioned shear reinforcing techniques presents such an advantage as to be preferred a

priori to another, however each can be effective for specific masonry work or in the solution of a

particular problem. Among the latest techniques, the use of composite materials FRP (Fiber Reinforced

Polymers) for reinforcement of masonry panels can result in significant increases in shear strength,

without causing excessive increases in shear stiffness. The long term effectiveness of these upgrading

works can, however, be compromised by the environmental aging or low fire resistance of the epoxy

resins used to glue the fibers to the masonry surfaces. A possibly interesting development in the use of

composites regards innovative materials, based on high strength steel wires forming cords that areembedded in either an epoxy (Steel Reinforced Polymer) or cementitious (Steel Reinforced Grout) matrix.

The use of these materials in seismic upgrading works presents some interesting aspects; there are several


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were used, whereas the steel cords used for the experiments were 3X2 type. The laminates were provided

in a low density (with 1.6 cords/cm) strips 50 mm wide and 500 mm long.

a) b)

Figure 1: Pull tests specimens.

All tests were conducted using a close loop load configuration, where no external reaction is required

(Figure 2). A direct tensile force was applied to the reinforcement using a 100 kN manual hydraulic jack.

To transfer the load to the strip, two steel plates were used to provide a reaction against the jack and to

spread the load evenly across the strip width. Both plates, in fact, had a central square hole, through

which the jack was placed, and were bolted together to secure the free end of the sheet. Before placing the

specimens in the testing apparatus a 20 mm thick steel restraining plate and a 5 mm thick piece of ply

wood, both with a small gap to allow the reinforcement to pass through, were placed on top of the

specimen. This restraining plate provides full restraint at the loaded end of the specimen.

STEEL FLAT PLATE

STEEL RESTRAINING PLATE PLY WOOD

CONCRETE BLOCKS

HYDRAULIC JACK

SRG STRIP

SOLID CLAY BRICKS

CONCRETE BLOCKS

STEEL PLATE WITH CENTRAL SQUARE HOLE

Figure 2: Test setup scheme for pull tests.

Two different failure modes were detected, namely: failure in cord matrix interface (a) and failure in

overlay (b).

All specimens GM tested failed with the same failure mode, denoted above as failure mode (a), i.e. they

failed by sliding shear along the cord matrix interface (Figure 3a). Conversely, both specimens GS and

GB failed with two different modes, namely, a group of specimens failed in cord matrix interface, while

the remainder failed in the overlay, i.e. they failed in the masonry substrate matrix interface (Figure 3b).

Closer inspection of the reinforcement after debonding in the brick showed that a thin layer of masonry

(between 1 and 5 mm), near the loaded end, was still attached to its surface, indicating that, in such a

region, failure was in the masonry itself.


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a) b)

Figure 3: Pull tests failure mode: a) failure in cord matrix interface; b) failure in overlay.

As for specimens GM, the mean value of the failure load (196 N/mm) was 81% of steel cord strength, that

indicates a not perfect bonding between cord\s and matrix. For series GS, although a common

cementitious grout rather than fiber reinforced grout was used, the failure load (277 N/mm) increased of

approximately 41% with respect to series GM, denoting a good interface bond between the steel cords

and the matrix. Finally, series GB showed a lower (-14%) mean value of the failure load (239 N/mm)

with respect to series GS. However, even in this case, the interface bond between the steel cords and the

matrix was satisfactory (the mean value of the failure load was 98% of the steel cord strength).

Table 2: Pull test results.

Specimen Series GM

GM.01 GM.02 GM.03 GM.04 GM.05 GM.06 GM.07

Matrix Type Mapefinish

Failure load (N/mm) 194 148 188 189 224 189 233

Failure load/

Tensile strength0.80 0.61 0.78 0.78 0.93 0.78 0.96

Failure mode a a a a a a aSpecimen Series GS

GS.01 GS.02 GS.03 GS.04 GS.05 GS.06 GS.07

Matrix Type 1:1 mix of sand and cement mortar (small diameter aggregates)

Failure load (N/mm) 212 297 284 295 306 260 282

Failure load/


Failure mode b a a a a a a

Specimen Series GB

GB.01 GB.02 GB.03 GB.04 GB.05 GB.06 GB.07

Matrix Type 1:1 mix of sand and cement mortar (big diameter aggregates)

Failure load (N/mm) 244 230 284 201 229 190 294

Failure load/


Failure mode a a a a a b a

EXPERIMENTAL PROGRAM

(1)Test MatrixA total of thirty-one solid brick masonry panels (515510125 mm) were manufactured for this

experimental program. Seventeen walls were built with hydraulic lime mortar and the remaining ten with

cement mortar (Table 3). All the specimens, except the control ones, were strengthened using medium

density, high strength steel cords (type 3SX and 3X2). In order to study the influence of the eccentricityof the strengthening, the strips were applied on both sides or only at one side of the panels. Moreover the

strip widths varied between 25 mm and 50 mm.


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Table 3: Description of the specimens.

Specimen Mortar Type Reinforcement

Type

Reinforced

Sides

Reinforcement width

(mm)

PRN 1 Hydraulic Lime - - -



PRN 4 Hydraulic Lime - - -PRN 5 Hydraulic Lime - - -


PRN 7 Hydraulic Lime 3SX 2 50






PRN 13 Hydraulic Lime 3X2 2 25

PRN 14 Hydraulic Lime 3X2 2 25PRN 15 Hydraulic Lime 3X2 2 50



PRC 1 Cement - - -

PRC 2 Cement - - -

PRC 3 Cement 3X2 2 50





PRC 8 Cement 3X2 1 50PRC 9 Cement 3SX 2 50

PRC 10 Cement 3SX 2 50





The same configuration of the reinforcing system was investigated for all the panels: strips as grid

arrangement (Figure 4). More in detail, the reinforcing system is made up of two strips placed centrally

(one horizontally, the other vertically) together with four other strips (two horizontal and two vertical)positioned 50 mm from the edge of the panel. The main function of the strips positioned along the edges

of the panel was principally to anchor the two central ones, which were more stressed, rather than

contribute to the reinforcement.


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a) b)

Figure 4: Reinforcement layout: a) 50 mm wide strips; b) 25 mm wide strips.

(2)Test SetupThe diagonal compression load is applied on the corners of the walls via a hydraulic actuator and

controlled by a load cell. The experimental setup for the diagonal compression is presented in Figure 5. A

single cycle of monotonically increasing loads at a speed of 0.50.7 kN/sec was applied to test the panels.The force was applied to the wall by steel shoes placed at the top corner, and transmitted to similar shoes

at the bottom corner. The displacements of compressed and stretched diagonals of masonry panels are

measured by two LVDT transducers.

Figure 5: Geometrical configuration and boundary conditions for masonry panels tested in diagonal

compression.

EXPERIMENTAL RESULTS

(1)Panels constructed with hydraulic lime mortarAs expected from the material characterization, all the unreinforced specimens presented brittle failure

due to splitting along the loaded diagonal, with crackings that appear suddenly only in the mortar joints

(Figure 6a). As for the reinforced panels, the failure mechanism involved the crisis of mortar joints and,

only in a limited central area, of bricks in compression (Figure 6b). The diagonal compression tests

highlighted that the cementitious grout used to glue the cords to the masonry surface worked efficiently.

In fact cords did not detach until failure of the panel was reached. This essentially depends on the

particular type of failure, which did not involve the tensile failure of the steel cords. Cords remained

attached to the cementitious matrix, which in turn had detached from the masonry once the masonry was

completely crushed.


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a) b)

Figure 6: Failure modes: a) splitting along the loaded diagonal (unreinforced panels); b) masonry

crushing with consequent detachment of the steel cords (reinforced panels).

The average value of the shear strength capacity of the unreinforced panels, used as reference value for

the comparison with the strengthened specimens results, is equal to 0.094 MPa. Reinforcement using 3SX

and 3X2 cords resulted in a strong increase in shear strength. Of particularly significant interest, with

respect to the maximum shear stress (max), is the category of the panels reinforced with 3SX cords (50

mm wide) placed on both sides of the masonry panel. The greatest increase in shear strength, (+315%)

was measured on these panels (Table 4).

Table 4: Panels constructed with hydraulic lime mortar: experimental results.

Specimen ReinforcementReinforced

sides

Reinforcement width

(mm)

max

(MPa)

max,reinf/

max,unrreinf

G1/3

(MPa)

PRN 1-2-3-4-5-6 - - - 0.094 - 36

PRN 7, PRN 8 3SX 2 50 0.390 4.15 377PRN 9, PRN 10 3SX 2 25 0.340 3.62 92

PRN 11, PRN 12 3SX 1 50 0.209 2.22 344

PRN 13, PRN 14 3X2 2 25 0.225 2.40 39

PRN 15 3X2 2 50 0.278 2.96 397

PRN 16, PRN 17 3X2 1 50 0.250 2.66 337

Figure 7 shows the stress strain curves for the ten panels constructed using hydraulic lime mortar.

0

0,1

0,2

0,3

0,4

0,5

0,000 0,005 0,010 0,015 0,020 0,025 0,030

Angular st rain

Shearstress(MPa)

PRN7

PRN9PRN15

PRN14PRN13

PRN11

PRN8

PRN10

PRN12

PRN16

PRN12

PRN17

Figure 7: Stress strain diagram of panels constructed with hydraulic lime mortar.


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(2)Panels constructed with cement mortarThe unreinforced panels reach a crisis due to the opening of cracks which pass through the thickness of

the wall along the compressed diagonal. These cracks, differently from the panels with a hydraulic lime

mortar, also involve the solid bricks. The reinforced panels present a failure mode similar to that of the

unreinforced panels. As to the steel cords, for the most part these remain adherent to the masonry panel

surfaces, while localized detachments, in some cases extensive, can be observed near the cracks in the

masonry wall along the compressed diagonal. The notable diagonal compression stresses in the testsdetermine, in some cases, limited crushing of the masonry wall, resulting in small expulsions of brick

fragments.

The results of these tests, given in Table 5, highlight different stiffness and shear strength, for both the

unreinforced as well as the reinforced panels, when compared to the tests on specimens with a hydraulic

lime mortar. In particular, the average shear strength of the two unreinforced panels was 0.540 MPa,

while significant increases resulted in shear stiffness. These values were notably higher (up to 5-20 times

as much) than the panels using a hydraulic lime mortar. In fact, the cement mortar is characterized by a

tensile strength which, though immodest, determines a strong increase in the stiffness and shear strength

of the masonry wall. Therefore, the reinforcing action of the steel cords is less significant when compared

to that on the panels using a hydraulic lime mortar. In Table 5 it can be seen that there is little differencein the increase in shear strength between the types of reinforcement experimented. Shear stiffness

increased up to 103%, while average increases of 55% were measured for shear strength.

Table 5: Panels constructed with cement mortar: experimental results.

Specimen ReinforcementReinforced

sides

Reinforcement width

(mm)

max

(MPa)

max,reinf/

max,unrreinf

G1/3

(MPa)

PRC 1, PRC 2 - - - 0.540 - 987

PRC 3, PRC 4, PRC 5 3SX 2 50 0.804 1.49 5750

PRC 6, PRC 7 3SX 2 25 0.917 1.70 6429

PRC 8 3SX 1 50 0.740 1.37 5021

PRC 9, PRC 10, PRC 11 3X2 2 25 0.738 1.37 2874

PRC 12, PRC 13 3X2 2 50 0.865 1.60 1797

PRC 14 3X2 1 50 1.093 2.03 2273

In Figure 8 and Figure 9 is given the stress strain curves, respectively for the panels reinforced with

3X2 and 3SX cords, and a comparison with the unreinforced panels. It can be observed that the reinforced

panels are also characterized by a greater apparent ductility when compared to those unreinforced.

0

0,2

0,4

0,6

0,8

1

1,2

0 0,001 0,002 0,003 0,004 0,005 0,006

Angular str ain

Shearstress

(MPa)

PRC8

PRC3

PRC2

PRC4

PRC5

PRC7

PRC

PRC1

Figure 8: Stress strain diagram of panels constructed with cement mortar and reinforced with 3X2

cords.


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0

0,2

0,4

0,6

0,8

1

1,2

0 0,001 0,002 0,003 0,004 0,005 0,006

Angular st rain

Shearstress(

MPa) PRC12

PRC101

PRC2

PRC14

PRC1PRC11

PRC1

PRC9

Figure 9: Stress strain diagram of panels constructed with cement mortar and reinforced with 3SX

cords.

ANALYSIS OF THE RESULTSAnalytical models available in literature to evaluate the shear strength of unreinforced masonry (URM)

shear walls are here reported. All of them are based on the linear effects superposition, which derive from

the implicit assumption of plastic stress redistribution. Even though the latter assumption is not properly

introduced when dealing with composites, at present no more appropriate approaches are available.

According to this, the maximum strength obtained during the tests was compared with those given by the

formulas for reinforced masonry proposed by Eurocode (Eurocode 6, 1996):

r,h yk vkcr

m s

tff tdV 0.9d

= + (1)

where:

vk vko 0f f 0.4= +

by Tomazevic (Tomazevic et al, 1993):

*

vk0 0cr r,h yk *

vk0

fV 0.9tl 1 0.4A f

b f

= + + (2)

and by Triantafillou (Triantafillou, 1998):

cr vk r,h r r,uV f td 0.9d E r t = + (3)

where:

( ) ( )2

r,u r,e r,h r r,h r r 0.0119 0.0205 E 0.0104 E = = +

The symbols used in the formulations are as follows: Vcr is the critical lateral load; tis the wall thickness,lits length, d = 0.8lis the effective depth; b is the shear stress distribution coefficient (= 1.5 for parabolic

distribution); m and sare coefficients (= 1) from Eurocode 6; 0 is the design compressive stress; r,h isthe horizontal reinforcement ratio computed on the wall section; Ar,h is the area of the horizontal


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reinforcement, fyk is the characteristic tensile stress of the reinforcement, Er is the reinforcement elastic

modulus and r,u is the reinforcement tensile ultimate strain.

Note that the characteristic initial shear strength of masonry (fvk0) should be determined from sliding tests

in accordance with EN 1052-3. In the absence of such data, it has been determined from the values given

in Eurocode. Conversely, the value of the characteristic initial shear strength (f*

vk0) of Eq. (2) has been

obtained by diagonal compressive tests.

Also, the formulation (3) considers a factor of efficiency r, which depends on the failure mode(reinforcement rupture or debonding). The expression of r, given by the Eq. (3), was found by

Triantafillou (Triantafillou, 1998) for concrete members (r,e is the effective reinforcement strain). On thebasis of the present experimental database it would be possible to provide a better calibration of the r

factor for masonry; in fact, by measuring the effective strain for each different reinforcement type it ispossible to newly determine all coefficients of Eq. (3) by polynomial interpolation.

0,3

90

0,8

77

0,4

38

0,7

92

0,3

40

0,5

18

0,2

39

0,5

01

0,2

09

0,5

18

0,2

39

0,5

01

0,2

25

0,5

71

0,2

68

0,4

56

0,2

78

0,9

81

0,4

96

0,7

14

0,2

50

0,5

71

0,2

68

0,4

56

0,0

0,2

0,4

0,6

0,8

1,0

1,2

PRN

7-8

EUROCODE6

TOMAZEVIC

TRIANTAFILLOU

PRN

9-10

EUROCODE6

TOMAZEVIC

TRIANTAFILLOU

PRN

11-12

EUROCODE6

TOMAZEVIC

TRIANTAFILLOU

PRN

13-14

EUROCODE6

TOMAZEVIC

TRIANTAFILLOU

PRN

15

EUROCODE6

TOMAZEVIC

TRIANTAFILLOU

PRN

16-17

EUROCODE6

TOMAZEVIC

TRIANTAFILLOU

Shearstrength(N/mm

2)

a)

0,8

65

1,0

61

0,6

68

0,7

94

0,7

38

0,6

51

0,4

40

0,5

36

1,0

93

0,6

51

0,4

40

0,5

36

0,8

04

0,9

57

0,6

10

0,8

72

0,9

17

0,5

98

0,4

11

0,5

81

0,7

40

0,5

98

0,4

11

0,5

81

0,0

0,2

0,4

0,6

0,8

1,0

1,2

PRC

3-4-5

EUROCODE6

TOMAZEVIC

TRIANTAFILLOU

PRC

6-7

EUROCODE6

TOMAZEVIC

TRIANTAFILLOU

PRC

8

EUROCODE6

TOMAZEVIC

TRIANTAFILLOU

PRC

9-10-11

EUROCODE6

TOMAZEVIC

TRIANTAFILLOU

PRC

12-13

EUROCODE6

TOMAZEVIC

TRIANTAFILLOU

PRC

14

EUROCODE6

TOMAZEVIC

TRIANTAFILLOU

Shearstrength(N/mm

2)

b)

Figure 10: Correlation between experimental and predicted strength: a) PRN tests; b) PRC tests.


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The comparison among the experimental and the predicted values of the shear strength is reported in

Figure 10. As for the PRN tests, from this analysis it is clear that both the formula proposed by Eurocodeand Triantafillou overestimate the strength of the reinforced masonry, particularly excessive for the

contribution of the reinforcement. Conversely, the formula proposed by Tomazevic appears to be more

conservative as it provides lower shear strengths. As for the PRC tests, despite the same mechanicalparameterEwas maintained, with respect to the PRN tests, there is an underestimation of the strength.

Also, despite the same amount of reinforcement was maintained for one side e two sides set up, both inPRN and PRC tests the related shear strength differed, in some cases, of more than 10 20% for the two

configurations. Therefore, for a better calibration of the formulas, also the geometrical reinforcementarrangement should be considered, as it can noticeably affect the composite strengthening effectiveness.

CONCLUSIONS

Reinforcement of masonry panels using steel cords presents numerous positive characteristics and the

results of the experimental research carried out even in the differentiation of the various tests suppliedinteresting results, highlighting practical limits and solutions. The use of a cementitious grout in place of

the epoxy resins normally utilized in the FRP reinforcements presents some positive characteristics,including lower material and installation costs and a higher fire resistance. Although the cementitious

grout is characterized by lower shear and tensile strength compared to the epoxy resins, it is able to carryout its job of tranferring the stresses between the masonry and the steel cords, guaranteeing theeffectiveness of the reinforcement.

There were significant increases in strength and shear stiffness for both 3X2 and 3SX cords and thecollapse of the panels occurred due to either shear failure or a partial detachment between the

cementitious matrix and the masonry support. The technique was more effective if the reinforcement wasapplied on low shear strength masonry (constructed using a hydraulic lime mortar). In all cases, the steel

cords determined significant increases in the shear stiffness and ductility of the masonry panels.

Research must still be carried out on various aspects of the utilization of SRG composites, such as the

role of the size of the reinforcing mesh and the joining between the panel faces.

ACKNOWLEDGEMENTS

The authors would like to acknowledge the Department of Civil Protection Consorzio RELUIS for

supporting this collaborative research.

REFERENCES

Anzani, A., Baronio, G. and Binda, L. (1998), Multiple leaf stone masonry as a composite: the role ofmaterials on its behavior and repair, Incomarech Raphael 97/E/412 PACT 55, 177-212. Baronio, G., Binda, L. and Modena, C. (1992), Criteria and methods for the optimal choice of groutsaccording to the characteristics of masonries, International workshop CNR-GNDT, Effectiveness ofinjection techniques for retrofitting of stone and brick masonry walls in seismic areas, Milan, Italy, 139-157. Binda, L., Penazzi, D., Tedeschi, C. and Baronio, G. (2000), Deep repointing of rubble stonemasonries in seismic areas, Final report of Maintenance of pointing in historic buildings: decay andreplacement EU contract ENV 4-CT98-710. Borri, A., Binda, L., Corradi M. and Tedeschi, C. (2008), Experimental evaluation of shear andcompression strength of masonry wall before and after reinforcement: deep repointing, Construction and

Building Materials, Elsevier, Vol. 22, No. 4, 463-472. Chiostrini, S. and Vignoli, A. (1994), In-situ determination of the strength properties of masonrywalls by destructive shear and compression tests,Masonry International, Vol. 7, No. (3), 87-96.

Chiostrini, S., Galano, L. and Vignoli, A. (2000), On the determination of strength of ancient masonrywalls via experimental tests, Proceedings of the 12th World Conference on Earthquake Engineering,

Auckland, New Zealand, Paper No. 2564.


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Corradi, M., Borri, A. and Vignoli, A. (2002), Strengthening techniques tested on masonry structuresstruck by the Umbrian-Marche earthquake of 1997-1998, Construction and Building Materials, Vol. 16,No. 4 , 229-239.

Corradi, M., Borri, A. and Vignoli, A. (2003), Experimental study on the determination of strength ofmasonry walls, Construction and Building Materials, Vol. 17, 325-337. ENV 1996-1-1 (1996),Eurocode 6: design of masonry structures. Modena, C. (1994), Repair and upgrading techniques of unreinforced masonry structures utilizedafter the Friuli and Campania-Basilicata earthquake,Earthquake Spectra, Vol. 10, No. 1. Sheppard, P.F. (1985),In-situ test of the shear strength and deformability of an 18th century stone andbrick masonry wall, Proceedings of the 7th International Brick/Block Masonry Conference, Melbourne,

Australia, 149-160. Tomazevic, M., Lutman, M. and Petrovic, L. (1993), In plane behavior of reinforced masonry walls

subjected to cyclic lateral loads, Report to the Ministry of Science and Technology of Republic of

Slovenia, parts 1 and 2, Liubljana, Slovenia. Turnsek, V. and Sheppard, P.F. (1980), The shear and flexural resistance of masonry walls,Proceedings of the Research Conference on Earthquake Engineering, Skopje. Triantafillou, TC. (1998), Strengthening of masonry structures using epoxy-bonded FRP laminates,

Journal of Composite Constructions, ASCE, Vol. 2, No. 2, 96-104.

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[20] - Structural Faults & Repair (Masonry Walls Srg)

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