KRISHNA KUMAR S1BM08CCT07

GUIDED BY

Smt MANGALA KESHAVAAssistant Professor,

Dept of Civil Engineering,BMSCE B’lore

EVALUATION OF STRESS REDUCTION EVALUATION OF STRESS REDUCTION FACTORS BASED ON TESTS ON AXIALLY AND FACTORS BASED ON TESTS ON AXIALLY AND

ECCENTRICALLY LOADED WALLSECCENTRICALLY LOADED WALLS

BY

INTRODUCTION LITERATURE REVIEWINITIAL TESTSEXPERIMENTAL STUDIES ON

WALLSANALYTICAL STUDIES COMPARISON OF RESULTS DISCUSSION AND CONCLUSIONSCOPE FOR FURTHER STUDIESREFERENCES

The International Building Code (IBC 2000) defines Masonry as "a built-up construction or combination of building units or materials of clay, Shale, concrete, glass, gypsum, stone or other approved units bonded together with or without mortar or grout or other accepted methods of joining”

Masonry practically considered as the art of shaping and uniting masonry units made of either Natural i.e., Adobe, Granite e.t.c Artificial i.e., Clay blocks , Bricks ,Concrete masonry units, Hollow blocks with aid of cement or iron cramps and lead. It therefore includes cutting ,facing, placing of masonry units into particular forms required to perform operations which needs practical dexterity with some skill in geometry and mechanics

Began as low walls of stones or caked mud Sun-dried bricks and with the availability of fire became

burnt bricks The first sun-dried bricks were made in Mesopotamia

(what is now Iraq), in the ancient city in about 4000 BC. The description of the building skills of early Romans can

be found in the four books of Vitruvius, the famous mason who lived in the first century B.C.

The earliest evidence of masonry construction is the arches found in the excavations at Ur in the Middle East. These ruins have been dated at 4000 B.C. Arch structures dating la 3000 B.C. have been found in Egypt.

The pyramid of Khufu in Egypt built about 2700 B.C remains one of the largest single stone masonry structure built by humans even though its original height of 147 m (482 ft.) is now reduced to 137m.

20th Century Developments Steel Reinforced Masonry

High Strength Mortars

High Strength Masonry Units

Pre-stressed Masonry

GREAT WALL OF CHINA

Bonds in Masonry• Bond is the interlacement of bricks, formed when they lay

those immediately below or above them.• It is the method of arranging the bricks in courses so that

individual units are tied together and the vertical joints of successive courses do not lie in the same vertical line.

Mortar • In masonry construction, mortars constitute only a small

proportion (approximately 7%) of the total wall area, but its influence on the performance of the wall is significant.

• The primary purpose of mortar in masonry is to bind masonry units into an assemblage that acts as an integral element having desired functional characteristics.

Bond masonry units together into an integral structural assembly

Seals joints against penetration by air and water Accommodates small movements within a wall Bonds to joint reinforcement to assist in resisting shrinkage

and tension

Functions

The basic advantages of masonry constn. is that the same element can perform a variety of functions such as sub-division of space, thermal and acoustic insulation, fire and weather protection, energy efficient

In the first half of the present century, masonry construction for multi-storied buildings was very largely replaced by steel and R.C .structure due to excessively thick walls wasteful in terms of space and material

Hence code of practice came into existence through research and experience providing sufficient basis for design of masonry structures.

Development of load bearing masonry

A wall can be defined as an upright member, the width of which exceeds four times its thickness. If this ratio is less than four, the wall is considered as column.

Mainly classified into 2 types

Load bearing type Non load bearing type

Definition Classification

Walls

Cavity wallSolid wall

Panel wall

Types of loading in WallsPrimarily walls are subjected to compression.

But however when walls are loaded eccentrically, they will be subjected to flexure in addition to compression.

Objective of Present study• To study the behaviour of full scale masonry

wall under axial and eccentric loading with S.R=6.0 and (e/t=0.25)

• To compute Stress reduction factor by an Analytical approach. Secant formula has been used for computing stress reduction factors for varying slenderness ratio and eccentricities

• To compare the stress reduction factor obtained from experimental investigation and Secant formula with BS code (BS 5628), Euro code (ENV-1995-1-1-1996) Indian code (IS-1905-1987)

Factors affecting Compressive strength of masonry

Mortar strength Unit strength Relative values of unit and mortar strength Ratio of the units (ratio of height to least

horizontal dimension) Orientation of the units in relation to the direction

of the applied load Bed-joint thickness Workmanship (Hendry,1998) Type of bond

Literature Review

Compressive strength of bricks

***Gumaste(2004)

Earlier studies on Full scale Masonry walls in IndiaRaghunath et al 2003,carried out tests on

un- reinforced and reinforced walls for Axial and eccentric load Eccentric load on 1-brick un-reinforced masonry walls (3 Specimens)

Eccentric load test on 1-brick masonry walls with containment reinforcement (3 Specimens)

It was not possible to sustain the applied load after the observation of the first crack in the entire un-reinforced specimen. These un-reinforced specimen started to rotate as soon as the cracks formed, leading to the failure of wall, which broke into two parts.

Gumaste 2004,had carried out compression tests on 3, storey

height walls. The walls were of following dimensions;

Wall No 1. 720 x 105 x 2770mm(TMB)-1:0:6 mix Wall No 2. 970 x 230 x 2770mm(TMB), 1:0:6 mix Wall No 3. 750 x 115 x 2770mm(WCB),1:1:6 mixThe half brick thick stretcher walls failed due to

material crushing where as the failure of one brick thick English bonded wall was due to a combination of splitting of bricks, bond failure and diagonal shear failure

Stress reduction factors from IS code( 0.54,0.67) were on conservative side as compared to experimental values(0.91,0.83)

Jolad 2008,had carried out compression tests on 2 walls.

The walls were of following dimensions; Wall No1. 1050 x 230 x 2430mm(TMB)-1:0:6

mix axial loaded, S.R=10.57Wall No 2. 1050 x 230 x 2430mm(TMB )1:0:6

mix, eccentric loading (e/t=(1/6)), S.R=10.57

The axially loaded wall exhibited typical compression type failure i.e. crushing of bricks, spalling, vertical cracks. Whereas wall with ecc. loading collapsed and typical flexure failure was noticed

Stress reduction factors from IS code were ( 0.88,0.83-(e)) whereas from experiments it was (0.925,0.370)

Details of tests carried on full scale walls

Behaviour of Masonry under Compression

Masonry loaded in uniform compression will either fail by the development of tensile cracks parallel to the axis of loading or by kind of failure along the lines of weakness

A number of studies have been carried out on full-scale masonry walls however there is little data which is available for masonry using moderate strength bricks in India & in particular, south Indian bricks

In this present experimental investigation an attempt has been made to determine the strength, elasticity and stress reduction factor(Ks) of a masonry wall for SR=6.0 for axial and eccentric and comparing the results obtained with codal provisions of different countries.

Design of Masonry Wall for Vertical Load (IS 1905)

Factors:

Slenderness Ratio

Stress reduction factors

Effective length

Effective thickness

Effective height Eccentricity of load

Thickness of wall

Eccentricity ratio

Masonry Walls:SlendernessandEccentricity

Stocky wall, central load

Slender wall, central load

100% strength

Reduced strength

Masonry Walls:SlendernessandEccentricity

Stocky wall, eccentric load

Slender wall, eccentric load

Reduced strength

Reduced strength

e

e

Effect of slenderness and eccentricity

Masonry Walls: Slenderness and Eccentricity IS 1905 1987

Slenderness Ratio= Effective Height/Effective Thickness (or)= Effective Length/Effective Thickness *** whichever is least

Masonry Walls: Slenderness and Eccentricity IS 1905 - 1987

Effective Height= Actual Height * a number from 0.75 to 1.5, depending on the end fixity

***In the present study depending on boundary condition provided it was considered 0.85 H as per IS 1905 - 1987

Masonry Walls: Slenderness and Eccentricity IS 1905 – 1987Effective Length= Actual Length * a number from 0.80 to 2.0 depending on the end fixity

Eccentricity depends on various factors !

• Extent of bearing• Magnitude of loads• Unequal span lengths of the slabs• Degree of fixity at the support• Moment at floor/roof – wall junction• Pitched roofs• Walls of varying thickness

• Eccentricity of vertical loading at a particular junction in a masonry wall shall depends on factors, such as extent of bearing, magnitude of loads, stiffness of slab or beam, fixity at the support and constructional details at junctions.

• No exact calculations are possible to make accurate assessment of eccentricity. Extent of eccentricity under any particular circumstances has, therefore, to be decided according to the best judgment of the designer. Some guidelines for assessment of eccentricity are given in Appendix A. of IS 1905 - 1987

• Arches, vaults and pillars generally experience eccentric force

VAULT PITHCED ROOF

EXPERIMENTAL PROGRAMME

Basic Tests on bricks

Tests on Mortar

Tests on Masonry prisms

Full Scale Tests on Masonry walls

Tests on Bricks

Avg. Dry density C.O.V Range1.75 g/cc 1.70% 1.6-1.95g/cc ****

Water Absorption(%)(Avg)

C.O.V Range

13.90 2.06% ≤20%**

I.R.A kg/m2/min

C.O.V Rangekg/m2/min

2.815 8.55% 1.35-3.53**

***Sarangpani 1998

Compr . Strength (MPa)

C.O.V Range(MPa)

5.32 2.56% 3-11***

***Sarangpani1998, Raghunath 2003 for T.M.B of Bangalore

Compressive Strength = Ultimate Load MPa. Area of Loading

E = 752.6 MPa from the Graph

TESTS ON MORTAR

Compressive strength of mortar Tests were carried out as per IS 2250-1981 1:6 ratio was chosen and hence mortar cubes were cast

measuring 70.6 mm*70.6mm*70.6mm

Compr strength (MPa)

C.O.V Range(MPa)

10.03 3.87% min3.0**

** As per IS-1905-1987

Flow is defined as the resulting increase in the base diameter of mortar mass expressed as a percentage of original base diameter

This test is useful in determining the optimum water cement ratio for a particular mortar mix

As per IS-2250-1981, Flow value shall be b/w 100-115%

Mortar cone before and after applying jolts

Conical mould

Rigid frame

Brass plate-circular

Tests on Masonry PrismsObjective

• To evaluate basic properties such as compressive strength, elastic modulus, stress-strain relationship

• Presently behaviour of stack bonded prism and 225 mm thick masonry prisms have been studied

• Prisms have been cast using 1:6 mix cement sand mortar 1.2 water cement ratio and table moulded bricks

• An average thickness of 10-12mm was maintained for mortar joints and the prisms were cured for 14 days and tested under compression in Universal testing machine

225 mm masonry prism and stack bonded prism before testing

Stress strain curve for prisms

Modes of failureFailure of brick-mortar bond was often noticed.

crushing of the brick was also seen in prisms

Tensile splitting of the brick was also noticed in the prism

To investigate stress reduction factor of walls,

for slenderness ratio=6.0 for axial loading and eccentric loading(e/t=0.25)

Need for present investigation

EXPERIMENTAL PROGRAMIn the present project, full scale brick wall have

been constructed for both axial and eccentric loading(e/t=0.25) for a slenderness ratio =6

Number of test specimens For axial loading – 2 Nos For eccentric loading -- 2 Nos

In this experiment an attempt has been made to compare the values obtained by experiments, with actual stress reduction factor in Table9, IS-1905-1987.

4 legged loading frame-reaction type Courtesy-RVCE

1.55m

0.95m

0.225m

Arrangement for Ecc.loading

0.5m

1.3m

1.0m

DIAL GAUGEPOSITION

Plan of Eccentric loading arrangement-1

Plan of Eccentric loading arrangement-2

Test Set-up details

WALL1- Front face and Rear face after testing

Crushing of brickvertical splitting

WALL2- Front face and Rear face after testing

Spalling

Vertical cracks

Comments on failure of axial loaded walls

Vertical splitting crack observed in the wall specimen after failure.

Crushing of bricks also observed as seen in the figure

De-lamination of brick due to crushing also observed

The vertical cracks were distributed from top of the specimen to the bottom course

Diagonal shear cracks were also observed in the wall specimen

Eccentric loading before and after failure

CRACK PATTERN-ECCENTRICALLY LOADED

Separation of 2 leaves of wall

CommentsGlobal failure of wall specimen was

observed i.e. all courses from top to the bottom participated in sustaining load

Mainly, horizontal cracks induced due to flexure were observed in this specimen

Vertical cracks along the side face of the specimen were also observed i.e., separation of two leaves of wall

Wall1

Wall2

Wall3

wall4

WALL 4(ECC2)

SUMMARY OF WALL TESTED

Evaluation of stress reduction factors It is virtually impossible to apply an axial compressive load to a

wall or column since this would require a perfect unit with no fabrication errors

The vertical load will, in general, be eccentric to the central axis and this will produce a bending moment in the member

The additional moment can be allowed in 2 ways The stresses due to the equivalent axial loads and bending moments can be

added using the formula belowTotal stress=P/A±M/Z

(or) Reducing the axial load-carrying capacity, of the wall, by a suitable factor known as “Stress reduction factor” in IS-1905-1987

The problem of column buckling has been approached in a different way, by observing that load P applied to a column is never perfectly centric

Due to eccentricity, moment is induced in column which induces bending

Formulation of Secant formula

Whereσmax from experiment on walls‘E’ VALUE from prism experiment(e/t)= eccentricity ratio(l/t)= slenderness ratio

(P/A) values

Stress reduction factor obtained

Comparison with different codes of masonry In present study, In the present study, stress reduction factors

computed from various codes have been compared with the present stress reduction factor obtained from Secant formula for slenderness ratio = 6.0 and eccentricity ratio = 0.25

As per ENV 1996-1-1-1995(code of practice for masonry in Europe), Stress reduction factor is denoted by “øm”. In the present study, the stress reduction factor for slenderness ratio = 6.0 and eccentricity ratio =0.25, is equal to 0.48.

Euro-code (ENV 1996-1-1-1995)

Graph showing stress reduction factors for varying eccentricites and slenderness ratio as per

Euro-code (ENV 1996-1-1-1995)

0.48

• As per BS 5628 (code of practice for masonry in Britain), Stress reduction factor is denoted by “β”. In the present study, the stress reduction factor for slenderness ratio = 6.0 and eccentricity ratio =0.25, is equal to 0.55

British code (BS 5628)

0.55

Conclusion The analytical approach using secant formula was adopted to

evolve the stress reduction factor which gave a value of 0.40 for slenderness ratio of 6.0 and eccentricity ratio of 0.25 (1/4)

IS: 1905-1987 gives a value stress reduction factor of 1.0 for slenderness ratio of 6.0 for all eccentricity ratios (0 to 1/3). However the experiment has shown that there is a reduction in stress reduction factor value

The stress reduction factor value given in Eurocode(ENV-1996) and British code (BS 5628) also reveals the reduction in stress reduction factor for varying (e/t) ratios (eccentricities) against slenderness ratio of 6.0. Hence, the IS code value which is on higher side needs to investigated

Scope for Further Study• In the present case, experiments on full scale

walls were carried out for slenderness ratio of 6.0 and eccentricity ratio of 0.25. This can be extended for varying eccentricities such as (1/24), (1/12), (1/3), ETC AND FOR VARYING ECCENTRICITIES to find stress reduction factors as compared to axially loaded

References AVINASH A.C. (2006) “A Comprehensive study on Masonry units”, M.Tech Thesis,

Department of civil Engineering, BMSCE, Bangalore, India. BEER JOHNSTON DE WOLF, “Mechanics of Materials”,2004, Mc Graw Hill

Publishers New York BS 5628-1:1992,” Code of Practice for Structural use of Unreinforced Masonry” DAYARATHNAM .P (1988),”Brick and Reinforced masonry Structures”, Oxford IBH Publishing Company New Delhi EUROCODE 6 ,“Design of masonry Structures”,(ENV 1996-1-1: 1995) GUMASTE K.S. (2004), “Studies on the Strength and Elasticity of Brick masonry

walls ”, Ph.D Thesis, Dept of Civil Engg, Indian Institute of Science, Bangalore HENDRY A W, “Design of Structural Masonry”, 1998, Mc Millan Publishers London HENDRY A W, B.P.SINHA ,S.R. DAVIES,” Design of Masonry Structures”, 2003,

Chapman and Hall Publishers London IS: 1905-1987, “Code of Practice for structural use of Unreinforced Masonry”, Bureau

of Indian standards, New Delhi IS: 2250-1981, “Code of practice for preparation and use of Masonry mortars”, (first

revision),

JAGADISH .K.S, “Alternative Building Materials”, Indian Institute of Science, 2005, New Age International Publishers, Bangalore

MAC KENZIE ,”Design of Structural Masonry”, 2001,Palgrave Publishers, New York

MAURENBRECHER,1985,”Axial Compression Tests on Masonry Walls and Prisms”, National Research Institute of Canada

NARENDRA TALY, “Design of Reinforced Masonry structures”, 2002 Mc Graw Hill Publishers New York

NIKHIL JOLAD (2008),”Evaluation of Stress Reduction Factors through Experiments on Full Scale Brick Masonry Walls”, M.Tech Thesis. Department of Civil Engineering, BMSCE, Bangalore, India

RAGHUNATH (2003),”Static and Dynamic Behaviour of Brick masonry with Containment Reinforcement” Ph.D Thesis submitted to Department of Civil Engineering, Indian Institute of Science, Bangalore

SAHLIN SVEN, “Structural Masonry”, 1971, Prentice hall Publishers London SARANGAPANI .G (1998), “Studies on the Strength of Brick Masonry” Ph.D thesis

submittedto Dept. of Civil Engineering, Indian Institute of Science, Bangalore SHWETHA( 2009), ”Stress Reduction Factors for Masonry an Analytical Approach”,

M.TechThesis. Department of Civil Engineering, BMSCE, Bangalore, India• SP-20 (1991), “ Handbook on Masonry Design and Construction”, Bureau of

Standards, New Delhi

THANK YOU THANK YOU