Masonry Construction in Earthquake Prone Areas

7/29/2019 Masonry Construction in Earthquake Prone Areas

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MASONRY BUILDINGS

Student : Bishanjit Singh Grewal

Guide : Prof PN Rao

BITS Pilani, Hyderabad Campus


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INTRODUCTION

Till 20th century most buildings were masonry constructions.

Gradually reinforced concrete and steel constructions have

become popular.

But masonry still preferred because of good insulation, good

finishing, economical and easy to procure.

Used for infill panels, partitions.

Materials used are bricks, stones, blocks etc joined with lime

mortar and cement mortar.

Used with or without reinforcement.

Structure with reinforcement better suited to withstand

earthquake.



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INTRODUCTION

Reason for poor performance of masonry building in earthquake

The material itself is brittle and its strength degradation due to

load repetition is severe.

Masonry has great weight because of thick walls.

Large stiffness of the material , which leads to large response

to earthquake waves of short natural period.

Quality of construction is not consistent because of quality of

the locally manufactured masonry unit sand unskilled labour

etc that leads to large variability in strength.



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BEHAVIOUR OF UNREINFORCED MASONRY WALLS

Vulnerable to strong earthquake shaking.

Topple easily if pushed horizontally at the top in direction

perpendicular to its plane. This is called Out of Plane Failure.

Out of Plane Failure



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BEHAVIOUR OF UNREINFORCED MASONRY WALLS

A wall offers much greater resistance if pushed along its length.

This is called In Plane Resistance. Such a wall is called a ShearWall.

Seismic capacity based on stability and energy considerations.

Elastic or ultimate strength analysis produce over conservative

results.

In Plane Resistance



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FORCE DISPLACEMENT RELATIONSHIP

Wall subjected to lateral loading

P

F

hW

bDisplacement

Force

A

B

C

Xb Xc

FA

FB

P

F

hW

b

P

P = Pb/2



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Wall behaves elastically upto point A where the base cracks and

force drops from FA to FB.

FB h = Pb + Wb

2 2FB = (P + W)b

2hStabilising force = Pb + Wb

2 2

Unstabilising force = Wx

2Hence

Fh = (Pb - 2Px + Wb Wx)

2



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Solving for xx = Pb + Wb 2Fh

2P + W When F = 0

xc

= Pb + Wb

2P + W

At point A the incremental stiffness of wall becomes negative

so that for a steadily applied force FA, collapse will occur unless

the force FA is transferred by an alternative load path to other

stiffer structural elements.



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BEHAVIOUR OF REINFORCED MASONRY WALLS

Designed for lateral out of plane loads and axial loads.

Lateral loads are transferred to roof, floor or foundation.

Axial loads are transferred directly to the foundation except for

eccentric loading that may cause tension in the wall.

Failure in reinforced masonry wall occurs in

Flexure

Shear



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BEHAVIOUR OF REINFORCED MASONRY WALLS

Failure in Flexure : When ratio of height to length of wall is large

and vertical reinforcement is small.

Failure in Shear: When ratio of height to length of wall is small.

Pattern of cracks in masonry

wall without openings

Pattern of cracks in masonry

wall with openings



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BEHAVIOUR OF WALLS BOX ACTION AND BANDS

Box type construction consists of walls along both axes of building

as shown in diagram below.

For the loading shown, walls A act as shear walls and walls B

topple over but walls A offer resistance to this.



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To provide more stability to the structure, flexural members

known as band or bond beams are incorporated at roof, linteland plinth level.

They provide horizontal reinforcement by taking care of bending

tension in horizontal plane and also distribute the vertical

concentrated loads placed on the walls. During earthquake shaking, a masonry wall gats grouped into

three sub units

Spandrel masonry

Wall pier masonry

Sill masonry

Inertia forces cause the masonry wall piers to disconnect from

masonry above and below.



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Diagonal cracks are likely to develop.

These cracks are checked by providing vertical reinforcement

anchored to the foundation.

Spandrel Masonry

Wall Pier Masonry

Sill Masonry

Plinth Level

Sill Level

Lintel Level

Foundation

Earthquake induced inertia force



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BEHAVIOUR OF INFILL WALLS

In framed structures, the frames are infilled with stiff construction

such as brick or concrete block masonry to create an enclosureand to provide safety to users. Such masonry walls are called

Infill Walls.

Strength and energy dissipation capacity of an infilled wall is

much higher than a bare frame. The major drawback is that it causes stress concentration in

particular members and also torsional deformation of the plane.

The shear distribution throughout the structure is also altered.



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BEHAVIOUR OF INFILL WALLS

Interaction between aframe and infill masonry

Interaction between a frame andhorizontally sheared infill masonry

Interaction between a frame

and partial infill masonry



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DESIGNING OF MASONRY INFILLED FRAME

There are two approaches for the design of a masonry infilled

frame

Qualitative Design Approach : Leads to heavy reinforcement in

both the frame and the masonry which provides an advantage in

case of a major earthquake by providing additional stiffness andabsorbing of greater amount of energy.

Free Infill Panel Approach : Provides a full separation joint

between the masonry and the frame at the ends and the top. Outof plane failure dealt with either the reinforced masonry to act as

vertical cantilever or by providing basket reinforcement.



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Lateral restraint details to a free infill panel



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Basketing Reinforcement (the arrows indicating the direction of restraint provided at

the frame wall junction



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IMPROVEMENT OF SEISMIC BEHAVIOUR

The building should not have re-entrant corners.

The building should not be slender in plan.

The bricks must be stronger than mortar with compressive

strength greater than 35 N/mm2 and very less porosity.

Good interlocking of masonry courses should be ensured at the

junctions.

For a single storey construction, the wall thickness should not be

less than one brick and not less than one and a half bricks for

buildings upto three storeys.

Horizontal reinforcement should be provided in walls to

strengthen them against horizontal in plane bending.

Horizontal bands should be provided at various levels.



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Steel dowel bars may be used at corners and T junctions to

integrate the box action of the walls.

Vertical reinforcing bars should be provided at the corners and

the junctions of the walls to counter the tension produced in

these locations. The amount of steel required will depend upon

number of storeys, storeys height, effective seismic coefficient,importance of the building and soil type.

The size of the openings should be kept small so that the

resistance offered is not reduced and the openings should not be

eccentrically located in order to reduce the torsional moment. Shear reinforcement should be provided in walls to ensure their

ductile behaviour.

Stiff, strong and continuous footing should be used for the

foundations. BITS Pilani, Hyderabad Campus


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The staircase should be completely separated from the building,

otherwise it will act as a cross brace between different floors,thus transferring large horizontal forces at the roof and lower

levels.

Damage in a building with a rigidly

built in staircase

Building with separated staircase



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LOAD COMBINATIONS

The adequacy of the masonry structure and its members is

investigated for the following load combinations : DL + IL

DL + IL + WL

DL + WL

0.9 DL + EL

where

DL Dead load

IL Imposed load

EL Earthquake load

WL Wind load

Permissible stresses may be increased by one third when wind or

earthquake forces are considered along with normal loads.



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SEISMIC DESIGN REQUIREMENTS

Seismic design provisions contained in IS 4326 : 1993.

Small sized buildings of upto three storeys designed as perrequirements of IS 4326 : 1993.

Other important buildings and those located in seismic zone IV

and V should be designed for forces listed in IS 1893 (Part I) and

provisions of IS 1905.



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TYPE OF SHEAR WALLS

Ordinary unreinforced masonry shear wall: They have poor elastic

response and used only in low seismic regions (zone II)and for

buildings of minor importance. Response reduction factor of 1.5

used.

Detailed unreinforced masonry shear wall : These walls are

designed as unreinforced masonry but contain minimum

reinforcement in horizontal and vertical direction. Used for low tomoderate seismic risk zones (zone II and III).

Ordinary reinforced masonry shear wall : These walls follow the

same steel requirements as that of a detailed unreinforced masonry

shear wall. Recommended in zones IV and V with a responsereduction factor of 3.0.

Special reinforced masonry shear wall : These walls are meant to

meet the seismic demands of zones IV and V with a response

reduction factor of 4.0.



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SEISMIC DESIGN OF MASONRY BUILDINGS

Lateral loads are determined.

Base shear is calculated and distributed vertically to differentfloor levels.

For rigid diaphragms, the storey shear is distributed to the vertical

resisting elements in direct proportion to their relative rigidities.

For flexible diaphragm, the exterior vertical resisting elementsshare half the shear of that shared by the interior ones.

The masonry shear walls are assumed to behave as a cantilever.



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Deflection of the wall pier

c = P 4h3 + 3h

Emt d3 d

Rigidity of cantilever pier

Rc = 1/

cwhere

P is the lateral force on the pier wall

Em is the modulus of elasticity of masonry in compression

h is the height of the pierdis the width of the pier panel

tis the thickness of the pier



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For a wall or pier fixed at the top and bottom

Deflectionf = P h

3 + 3h

Emt d3 d

Rigidity

Rf = 1/f

If masonry shear wall segments are combined horizontally, the

combined rigidity is

Rc = Rc1 + Rc2 + Rc3 + ....

For combining the rigidities of segments vertically, the combined

rigidity is

1 1 1 1 .

Rc

Rc1

Rc2

Rc3

+ +=



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For calculating the rigidity of walls with openings

The deflection of the solid wall as a cantilever is calculated as,say, so.

An opening having a height equal to that of the largest

opening is selected .

The deflection of this strip of the wall is calculated as, say, st.

Deflection of piers numbered 2, 3, 4, 5, 6 and 7 is calculated.



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Total deflection of the shear wall is

= so + pst

Rigidity

R = 1 /

The direct shear force in the wall, say I, is given as R i Px/y, where

Px/y is the lateral force applied at the top of the pier and Ri is the

relative stiffness of wall given by



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The torsional shears are given by

where

and are the torsional shears due to seismic forces

along the y and x axis of the building

Ry and Rx are the relative rigidity of each wall along y and x

axis

ey and ex are the respective eccentricities between the

centre of mass and centre of rigidity

J is the relative rotational stiffness of all the walls in the

storey under considerationBITS Pilani, Hyderabad Campus


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Horizontal bands of reinforcement are provided at critical levels

to strengthen the building. These bands can be made ofreinforced brick work in cement mortar not leaner than 1:3.

Lateral forces from winds or earthquakes should be studied

carefully.

Shear walls have to be checked for in plane bending and thetransverse or flexural walls are checked for out of plane forces

along with gravity loads.



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RESTORATION & STRENGTHNING

Some methods adopted for the restoration and strengthening of

masonry structures are discussed below

Grouting : For cracks of width less than 6 mm, the original tensile

strength of the cracked element may be restored by pressure

injection of epoxy or cement mortar, known as grouting.

Guniting : It is a building material consisting of a mixture of

cement, sand, and water that is sprayed onto a mould. The gunite

is placed pneumatically on the surface of masonry in the form of

a slab and may be an expansive cement mortar, quick settingcement mortar or gypsum cement mortar. Also applicable for

cracks wider than 6 mm. where necessary, additional shear

reinforcement may be provided in the gunite slab and covered

with mortar.



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Prestressing : This is a technique by which internal stresses of

suitable magnitude and distribution are introduced so that thestresses resulting from external loads are counter acted to a

desired degree. This method increases the shear strength of the

walls and connections of orthogonal walls.

Strengthening of walls by prestressing



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External binding : Opposite parallel walls can be held to internal

cross walls by pre-stressing bars.Anchoring is done against

horizontal steel channels

instead of steel plates

which run from one crosswall to the other. These

steel channels hold the

walls together and improve

the integral box like action

of the walls.



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THANK YOU

Documents

Masonry Construction in Earthquake Prone Areas