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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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FORCE DISPLACEMENT RELATIONSHIP
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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FORCE DISPLACEMENT RELATIONSHIP
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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BEHAVIOUR OF WALLS BOX ACTION AND BANDS
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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BEHAVIOUR OF WALLS BOX ACTION AND BANDS
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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DESIGNING OF MASONRY INFILLED FRAME
Lateral restraint details to a free infill panel
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DESIGNING OF MASONRY INFILLED FRAME
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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IMPROVEMENT OF SEISMIC BEHAVIOUR
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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IMPROVEMENT OF SEISMIC BEHAVIOUR
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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SEISMIC DESIGN OF MASONRY BUILDINGS
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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SEISMIC DESIGN OF MASONRY BUILDINGS
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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SEISMIC DESIGN OF MASONRY BUILDINGS
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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SEISMIC DESIGN OF MASONRY BUILDINGS
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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SEISMIC DESIGN OF MASONRY BUILDINGS
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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SEISMIC DESIGN OF MASONRY BUILDINGS
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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RESTORATION & STRENGTHNING
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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RESTORATION & STRENGTHNING
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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