effect of central openings on the strength of …...doi : 10.23883/ijrter.2018.4406.ikxwe 51 effect...

DOI : 10.23883/IJRTER.2018.4406.IKXWE 51

Effect Of Central Openings On The Strength Of Confined Masonry

Subjected To Lateral In-Plane Loading (An Experimental Study)

Ahsan Ul Haq1, Zahid Ahmad Chat

2

1M-Tech (Structural Engineering) NIT-Srinagar

2M-Tech (Structural Engineering) NIT-Srinagar

Abstract Confined masonry construction is made up of masonry walls and confining ties, which are built on

all four sides of each wall. This system is a conventional form for constructing houses as well as a

good alternative for post-disaster reconstruction of the seismically damaged and/or collapsed

buildings in many countries. Window and door openings appear in many panels of confined masonry

buildings, but many codes do not consider the effect of these openings in the strength and the

stiffness of confined masonry panels. In this study, the influence of area of openings on the stiffness

and the strength of confined masonry walls is investigated. An experimental program is used to

investigate the strength and stiffness of confined masonry walls; four specimens were used:-confined

masonry wall without opening, confined masonry wall with opening of 20% wall area, confined

masonry wall with opening of 25% wall area, and confined masonry wall with opening of 30% wall

area.

The results show that openings significantly reduce the lateral load resistant properties of the

confined masonry.

I. INTRODUCTION In many developing countries, masonry is used for housing because of its low material cost and

simplicity of construction. Masonry is a sturdy and durable material for wind and vertical loads that

houses must routinely withstand. However, if it is unreinforced, earthquake shaking can easily render

a masonry building a pile of rubble with obvious serious consequences for the inhabitants. For many

years now, reinforced concrete frame construction has also been very popular. In this type of

construction, builders add unreinforced masonry walls after they construct the reinforced concrete

frame. Intuitively, reinforced concrete frame construction might seem a better solution to resist

earthquakes than unreinforced masonry. Unfortunately, these frames require a high degree of skill to

build properly. The interaction of the unreinforced masonry infills with the frames causes brittle

behavior that is only, at best, marginally better than unreinforced masonry construction. Given the

universal popularity of masonry and the widespread availability of cement, reinforcing steel, and

aggregate, confined masonry is a simple solution. By making some inexpensive and easy changes to

traditional construction materials and procedures, the risk of casualties can be significantly reduced.

Very few cases of collapse have been reported in past earthquakes worldwide.

To construct a confined masonry house, a builder begins with the masonry walls much in the same

way as with unreinforced masonry construction. The difference is that the builder leaves vertical

slots in the walls every three or four meters. After finishing the walls at the first floor, the builder

then places steel reinforcing in the vertical slots and along the tops of the wall. Forms (usually

wooden boards) are also placed at the same locations and concrete is cast into the wall slots and at

the top of the wall. The verticals are called “tie columns” and the horizontals are called “bond” or

“tie beams.” Although they look a lot like the more traditional columns and beams of a reinforced

concrete frame, their function during an earthquake is quite different. In essence, they confine the

International Journal of Recent Trends in Engineering & Research (IJRTER)

Volume 04, Issue 11; November - 2018 [ISSN: 2455-1457]

@IJRTER-2018, All Rights Reserved 52

masonry and force it to work together with the concrete ties. The result is dramatically better

performance during earthquakes and a major reduction in the risk of collapse.

The confining members are effective in:

• Enhancing the stability and integrity of masonry walls for in-plane and out-of-plane earthquake

loads (confining members can effectively contain damaged masonry walls),

• Enhancing the strength (resistance) of masonry walls under lateral loads, and

• Reducing the brittleness of masonry walls under earthquake loads and hence improving their

earthquake performance.

It should be noted that the term “confined masonry” is used in a general sense for different forms of

masonry construction reinforced with additional steel, timber, or concrete elements. However, the

focus of this document is on clay brick or concrete block masonry walls “confined” with reinforced

concrete tie-beams and tie-columns.

Confined masonry walls can be constructed using different types of masonry units like clay burnt

bricks, hollow clay tiles, concrete blocks etc

II. OBJECTIVES OF THE STUDY The objective of this study is to find experimentally:

a). The strength and stiffness of confined masonry walls.

b). The effect of central opening on the lateral strength of confined masonry walls.

III. METHODOLOGY

An experimental study is performed to obtain the lateral stiffness of confined masonry walls. Four

confined masonry walls were tested to investigate their characteristics and compare the same. The

four types of walls tested are:

1. CM wall without opening-“CM”.

2. CM wall with opening and area of opening 20% - “CM+O20

”.


”.


”.

Two model specimens were tested for each type of wall. All the model specimen had brick masonry

panel confined by tie columns and tie beams. The walls of height 1.3m, width 1.2m and thickness

0.1m were used. The size of bricks used is 230mm×100mm×67.5mm. The thickness of bond used is

10mm. The size of columns and beams is 100mm×100mm. The size of opening were: 62mmx50mm

(20% opening area), 75mm×52mm (25% opening area) and 75mmx62mm ( 30% opening area).

Guidelines For The Confined Masonry: Following are some guidelines which have been taken into account for the construction of confined

masonry walls:

Guidelines From The Earthquake Resistant Confined Masonry Construction, By Svetlana

Brzev:

• The tie-column reinforcement should consist of four 10 mm diameter deformed bars (4-10 mm

bars) for longitudinal reinforcement, and 6 mm ties at 200 mm spacing (6mm @200 mm). Vertical

bars should be lapped by a minimum of 500 mm (or by at least 40 times the bar diameter).

• The minimum tie-column cross sectional dimensions are 100 mm by 100 mm.

• The tie-beam reinforcement should consist of four 10 mm diameter deformed bars for longitudinal

reinforcement, and 6 mm stirrups at 200 mm spacing. The tie-beam reinforcement needs to be

continuous, with the longitudinal reinforcement bars lapped by at least 500 mm.




IS Code Guidelines For Openings In Bearing Walls: The IS code is silent about confined masonry, but for openings in the bearing walls it recommends

the following guidelines:

● The guidelines on the size and position of opening are given in the clause 8.3.1 of IS 4326:

1993.

Characteristics of Materials To better understand the structural behavior of masonry wall it is important to have some knowledge

of the properties of component materials. Masonry is a multi-component assembly; in the current

study the wall specimens consisted of clay burnt bricks, cement mortar, concrete, and reinforcing

steel.

The main purpose of testing the component materials was to characterize the materials, to facilitate

the comparisons with other published results and design standards and to ensure that the quality of

materials was being maintained.

Material

Strength

Mean value(MPa)

Bricks Compressive strength 8.3

Mortar (1:6) Compressive strength 3.8

Concrete(1:2:4) Compressive strength 16.2

Steel Yield strength 425

Parameters Studied The parameters studied in this experimental program were the effect of central opening and effect of

reinforcement around the opening on the strength and stiffness of confined masonry walls.

Construction The specimens were built upon a concrete floor in the loading frame. The bricks were placed in

stretcher courses. The construction is carried in such a way so that toothed wall is made. After every

two layers of brick course the concrete is poured in form-work of columns, already placed at the

sides of constructing wall, so that the monolithic confined masonry wall is prepared. The tie-beam is

finally laid at the top.

To cure them, the specimens were wetted for seven days. The mortar joints of the concrete masonry

unit specimens were wetted twice a day.

Testing Procedures The testing set up is shown in Figure below. A horizontal load was applied along the axis of the top

beam using hydraulic jack. The walls were vertically confined. The walls were also restrained to

translation. During testing, the development of cracks and damage were registered. The load and

deformation of the walls at top were recorded manually.

Qualitative Behaviors Of The Specimens All the specimens failed in shear. The walls show almost linear behaviour initially followed by the

development and propagation of cracks, thus the later non-linear load deformation zone. The cracks

were mostly diagonal. The bottom of the compression tie-column got crushed and deformed i.e.

formation of plastic hinge. The cracks propagate though the mortar joint due to a low adherence

between the mortar and the masonry units.

International

IV.

Confined Masonry Wall Without Opening

Model 1

S.No Load(tons) Deflection(cm)

1. 0 0

2. 1 0.05

3. 2 0.1

4. 3 0.3

5. 4 0.4

6. 5 1.0

7. 5.5 1.5

8. 6 2.0

9. 5 4.0

10. 3.5 6.0

11. 3 8

12. 3 10

13. 2.5 12


Volume 04, Issue 11; November - 2018

RESULTS AND DISCUSSIONS

Masonry Wall Without Opening-“CM”

Model 1 Model 2

Deflection(cm) S.No Load(tons) Deflection(cm)

1. 0 0

0.05 2. 1 0.05

3. 2 0.1

4. 3 0.3

5. 4 0.4

6. 5 0.7

7. 6 1.0

8. 6.5 1.6

9. 6 1.8

10. 5 2.0

11. 3.5 3.5

12. 3 5

13. 3 8

14. 3 10

15. 2.5 13

Journal of Recent Trends in Engineering & Research (IJRTER)

[ISSN: 2455-1457]

Deflection(cm)

International

Average values:

Peak load = 62.5 KN

Initial stiffness = 10 KN/mm

Deformation capacity = 12.5 cm

Confined masonry wall with opening

Model 1


1 0 0

2 1 0.2

3 1.5 0.3

4 2 0.8

5 2.5 1.2

6 3.2 1.8

7 2 2.5

8 1.5 3.5

9 1.5 5

10 1 6



Confined masonry wall with opening-“CM+O20

”

Model 1 Model 2


1. 0 0

2. 1 0.2

3. 1.5 0.3

4. 2 0.9

5. 2.5 1

6. 3 1.3

7. 3.5 1.6

8. 2 2.2

9. 1.5 3

10. 1.5 6

11. 1 8


[ISSN: 2455-1457]

Deflection(cm)

International

Average values:

Peak load = 33 KN


Deformation capacity = 7 cm


Model 1


1 0 0

2 1 0.3

3 1.5 0.5

4 2 1.0

5 2.5 1.5

6 2 2.0

7 1.5 3.0

8 1.5 4.0

9 1 6.0




”

Model 1 Model 2


1. 0.0 0.0

2. 1.0 0.3

3. 1.5 0.5

4. 2.0 0.8

5. 2.8 1.0

6. 2.5 1.5

7. 2.0 2.0

8. 1.5 3.0

9. 1.5 4.0

10. 1 7.0


[ISSN: 2455-1457]

Deflection(cm)

International

Average values:

Peak load = 26.5 KN




Model 1


1 0 0

2 1 0.6

3 1.5 1

4 1.8 2

5 1.5 4

6 1.5 6

7 1 6.5

Average values:

Peak load = 17.7 KN

Initial stiffness = 1.58 KN/mm





”

Model 1 Model 2


1. 0 0

2. 1 0.6

3. 1.5 0.9

4. 1.7 2.2

5. 1.5 4

6. 1.5 5

7. 1 6


[ISSN: 2455-1457]

Deflection(cm)

International

From the above graph of Load-deflection of all the three types of wall specimens, the maximum load

was carried by confined masonry wall without opening i.e. “CM” and least by the confined masonry

wall with opening 30% i.e. “CM+O

masonry wall without opening i.e. “CM” and minimum in the “CM+O

stiffness was also seen in the “CM” and the least initial stiffness was observed in the “CM+O

From the crack pattern (diagonal cra

In case of “CM” the crack was diagonal and propagated from central portion towards corners. In case

of wall with opening i.e. “CM+O” the cracks were diagonal and concentrated around the open

corners.

COMPARISON OF RESULTS:

1. MAXIMUM LATERAL STRENGTH:a). [CM+O

20] / [CM] = 0.528

b). [CM+O25

] / [CM] = 0.424

c). [CM+O30

] / [CM] = 0.28

2. DEFORMATION CAPACITY: a). [CM+O

20] / [CM] = 0.56

b). [CM+O25

] / [CM] = 0.54

c). [CM+O30

] / [CM] = 0.52

3. INITIAL STIFFNESS: a). [CM+O

20] / [CM] = 0.50

b). [CM+O25

] / [CM] = 0.30

c). [CM+O30

] / [CM] = 0.158



deflection of all the three types of wall specimens, the maximum load


wall with opening 30% i.e. “CM+O30

”. The maximum deflection was also observed in the confined

masonry wall without opening i.e. “CM” and minimum in the “CM+O30

”. The maximum initial

stiffness was also seen in the “CM” and the least initial stiffness was observed in the “CM+O

From the crack pattern (diagonal cracking) it was evident that all the wall specimen failed in shear.


of wall with opening i.e. “CM+O” the cracks were diagonal and concentrated around the open

MAXIMUM LATERAL STRENGTH:

2. DEFORMATION CAPACITY:

V. CONCLUSION


[ISSN: 2455-1457]

deflection of all the three types of wall specimens, the maximum load


was also observed in the confined

”. The maximum initial

stiffness was also seen in the “CM” and the least initial stiffness was observed in the “CM+O30

”.

cking) it was evident that all the wall specimen failed in shear.


of wall with opening i.e. “CM+O” the cracks were diagonal and concentrated around the opening




The research presented in this thesis investigated the structural performance of confined masonry

walls under lateral in-plane loading. The test data consists of results obtained for-confined masonry

wall without opening and confined masonry wall with opening.

Taking into account the findings from this study, the following conclusions can be drawn:

1. The presence of openings in the walls drastically reduces the earthquake resistant properties

of the buildings.

2. The incorporation of opening of area 20% of the wall area causes: decrease in lateral strength

by about 47%, decrease in deformation capacity (or ductility) by 44% and initial stiffness by about

50%.



70%.



86%.

5. The strength of confined masonry walls with 20% opening area is about twice as that of

confined masonry wall with 30% opening area.

6. The stiffness of confined masonry walls with 20% opening area is more than thrice as that of

confined masonry wall with 30% opening area.

7. There is hardly any change in deformation capacity between the wall with 20% opening area

and wall with 30% opening area.

REFERENCES 1. IS 2250: 1981, “Code of practice for preparation and use of masonry mortars”.

2. is 2212: 1991, “Brick works- code of practice”.

3. is 1077: 1992, “Common burnt clay building bricks and specifications”.

4. IS 4326: 1993, “Earthquake resistant design and construction of buildings- code of practice”.

5. Svetlana Brzev, Earthquake-resistant confined masonry construction, National Information Center of

Earthquake Engineering, Indian Institute of Technology Kanpur(India), December 2007

6. Eshghi, S. & Pourazin, K. 2009. In-Plane Behavior of Confined Masonry Walls – with and without

Opening, International Journal of Civil Engineering, 7(1): 49-60.

7. Sassan Eshghi & Behrang Saffari, 2009. Effect of openings on the lateral stiffness and strength of

confined masonry walls, International Journal of Civil Engineering.

8. Masayuki kasoki, kenji Kikuchi & Hideko Nonaka, Experimental study on reinforcing methods for

window openings in confined masonry walls, 35th conference on our world in concrete and structures : 25-27

August 2010, Singapore.

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