New UNREINFORCED MASONRY STRUCTURES 2 - DEUkisi.deu.edu.tr/ozgur.ozcelik/Ekonomi/ARCH...
Transcript of New UNREINFORCED MASONRY STRUCTURES 2 - DEUkisi.deu.edu.tr/ozgur.ozcelik/Ekonomi/ARCH...
-
UNREINFORCED MASONRY
STRUCTURES
- PART I -DEFINITIONS AND PROBLEMS UNDER
LATERAL LOADS
-
Some Definitions
• UnReinforced Masonry (URM): Defined as
masonry that contains no reinforcing in it.
• Masonry Unit: Clay brick or natural stone
element used to construct masonry.
• Mortar: Binding element being used to
connect masonry units, typically composed of
lime or cement, or both, and sand.
-
Masonry Walls
• Walls most generally are made of “brick”,
“hollow concrete blocks”, “hollow clay
tiles ??”, “stone” and “adobe”, and are
load bearing.
• Solid clay-brick unit masonry is the most
common type of masonry unit.
-
Types of Masonry Wall Units
-
Masonry Walls
A common type of unreinforced masonry wall in
one- or two-story buildings is approximately a 30-35
cm thick, and uses a pattern of brickwork. In this
pattern, most of the bricks are laid running parallel
with the wall (these are known as stretchers).
Approximately every sixth horizontal row, there will
be a row of bricks with their ends rather than their
sides visible (these are known as headers), as
illustrated in Figure 8.
-
Basic Brickwork Terminology
Bed
Joint
Head
Joint
Course - horizontal layer of brick
-
Course: Continuous layer
Wythe: Continuous vertical section
A Pattern of Brickwork
-
A TYPICAL MASONRY STRUCTURE
MAIN COMPONENTS
• Main components of URM structures:
• Footing and/or foundation wall (concrete,
masonry, or rock)
• Load bearing masonry exterior walls,
• Wood frame floor(s) (in most of the cases),
• Roof system,
• At the interior, there is some type of bearing
wall(s), normally URM or wood.
-
Foundation Walls
• Foundation walls for URM buildings
may be either concrete, masonry, or
rock,
• If the foundation wall is unreinforced
masonry or rock, it can break apart at
the mortar joints when seismic activity
occurs,
• Many times, these walls have been
badly deteriorated from moisture
penetration over the life of the
building,
• The mortar used in many older
buildings contains very little cement
and is normally very soft and weak.
-
Load Bearing Walls
• A bearing wall is defined as a “wall which
supports any vertical load in a
building/structure as well as its own weight”.
The distance
between the wall
units is sometimes
larger to provide
insulation. This type
of wall configuration
is called “cavity
walls”. How to
connect different
wall layers (there is
no mortar)?
-
Floor and Roof Diaphragms
• Three categories of diaphragms can
be identified:
• rigid concrete slab diaphragms,
• flexible wood,
• metal diaphragms,
Intermediate systems such as hollow
concrete planks and brick spanning
between beams also exist.
• In flexible diaphragms, excessive
deflections can lead to “out-of-
plane??” wall damage.
• Hollow brick/concrete systems may
lack adequate interconnections to
function as a continuous load path.
Flexible Wood Diaphragm
Hollow Brick System
-
Floor and Roof Diaphragms
An Intermediate Floor Diaphragm System
-
Wooden Joist System for Floors
-
A Typical Single Story URM Building
System as a Whole
-
A Typical Multi-story Unreinforced
Masonry Building
-
What is Wrong with URM Structures?
• Masonry is one of the oldest building materials and
has been considered the most durable,
• It is behavior under vertical/gravity static loads is
superb (under compressive forces),
• But earthquake (EQ) ground shaking has been found
to be very damaging to URM buildings,
• Previous EQs have shown that masonry structures
are the most vulnerable of all building types to the
lateral (earthquake) forces.
-
Why Earthquake Loads are Damaging
to URM?
• Masonry material performs well under compressive
forces,
• EQ loads generates tensile as well as shear forces in
the material which is intrinsically weak in tension,
• Lack of additional material (e.g., steel reinforcement)
withstanding well to tension forces makes URM
building very vulnerable for EQ loads.
-
Then Why URM Structures are Still
Being Studied?
• In many modern building codes, it is prohibited to
build URM building in high seismic regions,
• But in existing building stocks, URM structures DO
exist, therefore identifying them in building
inventories, assessing their earthquake performance,
and retrofitting them are important and valid tasks.
-
A Recent Inventory Study Conducted in
Balçova (2011)
• Entire Balçova area has been studied to
characterize its building inventory, following
figures were identified,
– Out of 7628 buildings 2660 many of them are URM
buildings (35% of the total),
– Only %23 of them have projects, the remaining do
not have projects (majority),
– It is estimated that 25-30% of Izmir’s entire building
stock is composed of URM buildings.
-
Other URM Components
Non-wall Components
� In URM structures there are other components called
non-wall component. In many cases, especially under
earthquakes, their response becomes important, and
damage to these components may occur before
damage on the walls becomes significant.
� Below, common behavior modes of non-wall
components are discussed.
-
Non-wall URM Components
Parapets
� These short extensions
of walls above the roof
typically occur at the
perimeter of the
buildings and are
primarily present for
aesthetic reasons.
� As originally
constructed, they are
not braced back to the
roof and are thus
susceptible to brittle
flexural out-of-plane
failure.
From Adjacent
Building
Building Itself
-
Parapets(Are they just simple extensions?)
-
Non-wall URM Components
Appendages
• This category includes veneer,
cornices, brackets, statuary, and
any minor masonry feature that is
susceptible to falling,
• Damage may result from excessive
accelerations of appendages and
deformations that cause
connection failures between the
appendage and the structure,
• Delamination of veneer can result
from missing or inadequate ties,
• Pounding against adjacent
buildings can lead to localized
falling hazards.
-
Canopy Failure
Are they just shades and shelters?
-
Non-wall Components
Wall-Diaphragm Ties• Generally limited to low-strength
tension connections in which one
end of a steel bar is embedded one
wythe in from the outer face of the
wall and the other end is
hammered into the side of a wood
joist.
• Wall-diaphragm separation due to
inadequate or missing tension ties
can lead to out-of-plane failures of
walls;
• Missing shear ties can lead to the
diaphragm sliding along the in-
plane walls and then pushing
against the walls perpendicular to
the movement, resulting in corner
damage to the walls.
Missing shear ties!
In-plane walls
Tension Ties
-
URM Components
More Details on Wall Components
• URM wall elements can be subdivided into five component
types based on the mode of inelastic behavior.
• The majority of modes relate to “in-plane damage”, but
“out-of-plane” damage can occur as well in each of the
systems, often in combination with in-plane damage.
• The five component types are described below.
-
“In-Plane” and “Out-of-Plane” (OOP)
Damage in URM building
In-plane Wall
DamageOOP Wall
Damage
-
Wall Components
Solid Cantilever Wall (URM1)
URM1: Such walls are
typically found adjacent to
other buildings or on alleys,
and they act as cantilevers
up from the foundation.
Typical Inelastic/Failure Behavior
-
Overview of In-plane Failure Modes on
a Solid Masonry Wall
-
Wall ComponentsWeak Pier in Perforated Wall
(URM2 and URM4)
URM2: This component is a
weak pier in a perforated wall. In
this system, inelastic
deformation occurs in the piers.
URM4: This component is a
strong spandrel in a weak pier-
strong spandrel mechanism.
Strong spandrels do not
experience damage.
Typical Inelastic/Failure
Behavior
-
Wall ComponentsWeak Spandrel in Perforated Wall
(URM3)
URM3: This component is a
weak spandrel in a perforated
wall.
Inelastic deformation occurs
first in the spandrels then
leading to inelastic
deformation and damage in
the piers.
Typical Inelastic/Failure Behavior
-
Wall ComponentsWeak Joints in Perforated Walls
(URM5)
URM5: Perforated wall with panel
zone weak joints. Inelastic
deformation occurs in the region
where the pier and spandrel intersect.
Such damage is not observed
generally in experimental tests, nor is
it seen in actual earthquakes, except
at outer piers of upper stories.
Typical Inelastic/Failure Behavior
-
Understanding the Response of Structural
Components in URM Buildings under
EQ Loads
-
• The walls are weak in resisting horizontal forces (and they
lack ductility),
• The walls are heavy (they have high mass, leading to high
inertial forces),
• Diaphragms are excessively flexible (insufficient lateral
support for the walls),
• Diaphragm-to-wall connections are either absent or weak,
• Parapets and ornamentation are common (and made of
masonry).
What Makes URM Buildings Weak Under
EQ Forces (Lateral Forces)?
-
• Masonry materials are intrinsically strong when
compressed under the gravity loads but are
weak in resisting earthquake forces, which
make materials flex and also shear,
• When an earthquake shakes an unreinforced
masonry building, it causes the building’s walls
to flex out-of-plane and to shear in-plane,
• Unreinforced masonry is weak in resisting both
of those types of forces.
• Mortar is the “glue” that holds the masonry
units together; however, when it eventually
cracks, it does so in a brittle manner, similar to
the way that the bricks crack.
An Intrinsic Problem
Brittle Material
-
Other Problems
Maintaining Integrity
Furthermore a number of common failure modes of URM
buildings related to maintaining integrity have repeatedly been
observed in earthquakes. These modes can be grouped in the
following categories:
• Lack of anchorage,
• Anchor failure,
• In-plane failures,
• Out-of-plane failure,
• Combined in-plane and out-of-plane effects,
• Diaphragm-related failures.
-
Four Recognized In-plane Failure Modes
-
• Rocking of a wall and its foundation on
the supporting soil has been observed in
the field.
• Though recognized as a potentially
favorable mode of nonlinear response
and a source of damping rather than
significant damage,
• Excessive rocking could theoretically lead
to some instability and nonstructural
damage in the superstructure,
• Several technologies being used to
encourage rocking
Foundation Rocking – Mode 1
-
Wall-Pier Rocking – Mode 1
• In the wall-pier rocking behavior mode,
after flexural cracking develops at the
heel, the wall or pier acts as a rigid body
rotating about the toe.
-
Toe Crushing – Mode 2
Characteristics of toe crushing
• Loss of material at toe of pier
• Vertical load carrying capacity generally maintained
• Not really a failure
-
• In this type of behavior sliding occurs on bed joints. Commonly observed
both in the field and in experimental tests,
• There are two basic forms: sliding on a horizontal plane, and a stair-
stepped diagonal crack where the head joints open and close to due to
movement on the bed joints.
Bed Joint Sliding – Mode 3
-
Bed Joint Sliding – Mode 3
Stair-step Type
-
Bed Joint Sliding – Mode 3
Horizontal TypeCharacteristics of bed joint sliding
• Governed by friction,
• Consumes energy,
• Pseudo ductility source,
• Vertical load carrying capacity generally
maintained
• Not really a failure
Horizontal Sliding
-
Diagonal Tension Failure – Mode 4
Characteristics of diagonal tension failure
• Governed due to tension stresses (shear
forces lead to tension stresses), that is
why also called shear failure,
• No bed joint sliding,
• Observable in both piers and spandrels.
Tension Cracks on a Pier
Tension Cracks on a Pier and Spandrels
-
Diagonal Tension Failure – Mode 4
Spandrel Failure
Pier Failure
-
Out-of-plane (OOP) Failure Modes
There are three types of OOP damage:
• One-way bending between vertical supports,
• Two-way bending, end walls represent either 3
or 4 boundary conditions,
• Corner failure.
-
Out-of-plane Failure
One-way Bending
-
Out-of-plane FailureTwo-way Bending
-
Out-of-plane FailureTwo-way Bending
-
Out-of-plane FailureTwo-way Bending
Spalled
corner
-
Out-of-plane FailureCantilever Action
Complete collapse of a gable
by cantilever action (lack of
anchorage)
Diaphragm is visible
-
Out-of-plane FailureCorner Damage
-
Out-of-plane FailureMixed Mode Failure
(In-plane, out-of-plane and corner effects)
Very common type of damage (heavy roof usually the culprit)
-
UNREINFORCED MASONRY
STRUCTURES
- PART I -DEFINITIONS AND PROBLEMS UNDER
LATERAL LOADS
-
Some Definitions
• UnReinforced Masonry (URM): Defined as
masonry that contains no reinforcing in it.
• Masonry Unit: Clay brick or natural stone
element used to construct masonry.
• Mortar: Binding element being used to
connect masonry units, typically composed of
lime or cement, or both, and sand.
-
Masonry Walls
• Walls most generally are made of “brick”,
“hollow concrete blocks”, “hollow clay
tiles ??”, “stone” and “adobe”, and are
load bearing.
• Solid clay-brick unit masonry is the most
common type of masonry unit.
-
Types of Masonry Wall Units
-
Masonry Walls
A common type of unreinforced masonry wall in
one- or two-story buildings is approximately a 30-35
cm thick, and uses a pattern of brickwork. In this
pattern, most of the bricks are laid running parallel
with the wall (these are known as stretchers).
Approximately every sixth horizontal row, there will
be a row of bricks with their ends rather than their
sides visible (these are known as headers), as
illustrated in Figure 8.
-
Basic Brickwork Terminology
Bed
Joint
Head
Joint
Course - horizontal layer of brick
-
Course: Continuous layer
Wythe: Continuous vertical section
A Pattern of Brickwork
-
A TYPICAL MASONRY STRUCTURE
MAIN COMPONENTS
• Main components of URM structures:
• Footing and/or foundation wall (concrete,
masonry, or rock)
• Load bearing masonry exterior walls,
• Wood frame floor(s) (in most of the cases),
• Roof system,
• At the interior, there is some type of bearing
wall(s), normally URM or wood.
-
Foundation Walls
• Foundation walls for URM buildings
may be either concrete, masonry, or
rock,
• If the foundation wall is unreinforced
masonry or rock, it can break apart at
the mortar joints when seismic activity
occurs,
• Many times, these walls have been
badly deteriorated from moisture
penetration over the life of the
building,
• The mortar used in many older
buildings contains very little cement
and is normally very soft and weak.
-
Load Bearing Walls
• A bearing wall is defined as a “wall which
supports any vertical load in a
building/structure as well as its own weight”.
The distance
between the wall
units is sometimes
larger to provide
insulation. This type
of wall configuration
is called “cavity
walls”. How to
connect different
wall layers (there is
no mortar)?
-
Floor and Roof Diaphragms
• Three categories of diaphragms can
be identified:
• rigid concrete slab diaphragms,
• flexible wood,
• metal diaphragms,
Intermediate systems such as hollow
concrete planks and brick spanning
between beams also exist.
• In flexible diaphragms, excessive
deflections can lead to “out-of-
plane??” wall damage.
• Hollow brick/concrete systems may
lack adequate interconnections to
function as a continuous load path.
Flexible Wood Diaphragm
Hollow Brick System
-
Floor and Roof Diaphragms
An Intermediate Floor Diaphragm System
-
Wooden Joist System for Floors
-
A Typical Single Story URM Building
System as a Whole
-
A Typical Multi-story Unreinforced
Masonry Building
-
What is Wrong with URM Structures?
• Masonry is one of the oldest building materials and
has been considered the most durable,
• It is behavior under vertical/gravity static loads is
superb (under compressive forces),
• But earthquake (EQ) ground shaking has been found
to be very damaging to URM buildings,
• Previous EQs have shown that masonry structures
are the most vulnerable of all building types to the
lateral (earthquake) forces.
-
Why Earthquake Loads are Damaging
to URM?
• Masonry material performs well under compressive
forces,
• EQ loads generates tensile as well as shear forces in
the material which is intrinsically weak in tension,
• Lack of additional material (e.g., steel reinforcement)
withstanding well to tension forces makes URM
building very vulnerable for EQ loads.
-
Then Why URM Structures are Still
Being Studied?
• In many modern building codes, it is prohibited to
build URM building in high seismic regions,
• But in existing building stocks, URM structures DO
exist, therefore identifying them in building
inventories, assessing their earthquake performance,
and retrofitting them are important and valid tasks.
-
A Recent Inventory Study Conducted in
Balçova (2011)
• Entire Balçova area has been studied to
characterize its building inventory, following
figures were identified,
– Out of 7628 buildings 2660 many of them are URM
buildings (35% of the total),
– Only %23 of them have projects, the remaining do
not have projects (majority),
– It is estimated that 25-30% of Izmir’s entire building
stock is composed of URM buildings.
-
Other URM Components
Non-wall Components
� In URM structures there are other components called
non-wall component. In many cases, especially under
earthquakes, their response becomes important, and
damage to these components may occur before
damage on the walls becomes significant.
� Below, common behavior modes of non-wall
components are discussed.
-
Non-wall URM Components
Parapets
� These short extensions
of walls above the roof
typically occur at the
perimeter of the
buildings and are
primarily present for
aesthetic reasons.
� As originally
constructed, they are
not braced back to the
roof and are thus
susceptible to brittle
flexural out-of-plane
failure.
From Adjacent
Building
Building Itself
-
Parapets(Are they just simple extensions?)
-
Non-wall URM Components
Appendages
• This category includes veneer,
cornices, brackets, statuary, and
any minor masonry feature that is
susceptible to falling,
• Damage may result from excessive
accelerations of appendages and
deformations that cause
connection failures between the
appendage and the structure,
• Delamination of veneer can result
from missing or inadequate ties,
• Pounding against adjacent
buildings can lead to localized
falling hazards.
-
Canopy Failure
Are they just shades and shelters?
-
Non-wall Components
Wall-Diaphragm Ties• Generally limited to low-strength
tension connections in which one
end of a steel bar is embedded one
wythe in from the outer face of the
wall and the other end is
hammered into the side of a wood
joist.
• Wall-diaphragm separation due to
inadequate or missing tension ties
can lead to out-of-plane failures of
walls;
• Missing shear ties can lead to the
diaphragm sliding along the in-
plane walls and then pushing
against the walls perpendicular to
the movement, resulting in corner
damage to the walls.
Missing shear ties!
In-plane walls
Tension Ties
-
URM Components
More Details on Wall Components
• URM wall elements can be subdivided into five component
types based on the mode of inelastic behavior.
• The majority of modes relate to “in-plane damage”, but
“out-of-plane” damage can occur as well in each of the
systems, often in combination with in-plane damage.
• The five component types are described below.
-
“In-Plane” and “Out-of-Plane” (OOP)
Damage in URM building
In-plane Wall
DamageOOP Wall
Damage
-
Wall Components
Solid Cantilever Wall (URM1)
URM1: Such walls are
typically found adjacent to
other buildings or on alleys,
and they act as cantilevers
up from the foundation.
Typical Inelastic/Failure Behavior
-
Overview of In-plane Failure Modes on
a Solid Masonry Wall
-
Wall ComponentsWeak Pier in Perforated Wall
(URM2 and URM4)
URM2: This component is a
weak pier in a perforated wall. In
this system, inelastic
deformation occurs in the piers.
URM4: This component is a
strong spandrel in a weak pier-
strong spandrel mechanism.
Strong spandrels do not
experience damage.
Typical Inelastic/Failure
Behavior
-
Wall ComponentsWeak Spandrel in Perforated Wall
(URM3)
URM3: This component is a
weak spandrel in a perforated
wall.
Inelastic deformation occurs
first in the spandrels then
leading to inelastic
deformation and damage in
the piers.
Typical Inelastic/Failure Behavior
-
Wall ComponentsWeak Joints in Perforated Walls
(URM5)
URM5: Perforated wall with panel
zone weak joints. Inelastic
deformation occurs in the region
where the pier and spandrel intersect.
Such damage is not observed
generally in experimental tests, nor is
it seen in actual earthquakes, except
at outer piers of upper stories.
Typical Inelastic/Failure Behavior
-
Understanding the Response of Structural
Components in URM Buildings under
EQ Loads
-
• The walls are weak in resisting horizontal forces (and they
lack ductility),
• The walls are heavy (they have high mass, leading to high
inertial forces),
• Diaphragms are excessively flexible (insufficient lateral
support for the walls),
• Diaphragm-to-wall connections are either absent or weak,
• Parapets and ornamentation are common (and made of
masonry).
What Makes URM Buildings Weak Under
EQ Forces (Lateral Forces)?
-
• Masonry materials are intrinsically strong when
compressed under the gravity loads but are
weak in resisting earthquake forces, which
make materials flex and also shear,
• When an earthquake shakes an unreinforced
masonry building, it causes the building’s walls
to flex out-of-plane and to shear in-plane,
• Unreinforced masonry is weak in resisting both
of those types of forces.
• Mortar is the “glue” that holds the masonry
units together; however, when it eventually
cracks, it does so in a brittle manner, similar to
the way that the bricks crack.
An Intrinsic Problem
Brittle Material
-
Other Problems
Maintaining Integrity
Furthermore a number of common failure modes of URM
buildings related to maintaining integrity have repeatedly been
observed in earthquakes. These modes can be grouped in the
following categories:
• Lack of anchorage,
• Anchor failure,
• In-plane failures,
• Out-of-plane failure,
• Combined in-plane and out-of-plane effects,
• Diaphragm-related failures.
-
Four Recognized In-plane Failure Modes
-
• Rocking of a wall and its foundation on
the supporting soil has been observed in
the field.
• Though recognized as a potentially
favorable mode of nonlinear response
and a source of damping rather than
significant damage,
• Excessive rocking could theoretically lead
to some instability and nonstructural
damage in the superstructure,
• Several technologies being used to
encourage rocking
Foundation Rocking – Mode 1
-
Wall-Pier Rocking – Mode 1
• In the wall-pier rocking behavior mode,
after flexural cracking develops at the
heel, the wall or pier acts as a rigid body
rotating about the toe.
-
Toe Crushing – Mode 2
Characteristics of toe crushing
• Loss of material at toe of pier
• Vertical load carrying capacity generally maintained
• Not really a failure
-
• In this type of behavior sliding occurs on bed joints. Commonly observed
both in the field and in experimental tests,
• There are two basic forms: sliding on a horizontal plane, and a stair-
stepped diagonal crack where the head joints open and close to due to
movement on the bed joints.
Bed Joint Sliding – Mode 3
-
Bed Joint Sliding – Mode 3
Stair-step Type
-
Bed Joint Sliding – Mode 3
Horizontal TypeCharacteristics of bed joint sliding
• Governed by friction,
• Consumes energy,
• Pseudo ductility source,
• Vertical load carrying capacity generally
maintained
• Not really a failure
Horizontal Sliding
-
Diagonal Tension Failure – Mode 4
Characteristics of diagonal tension failure
• Governed due to tension stresses (shear
forces lead to tension stresses), that is
why also called shear failure,
• No bed joint sliding,
• Observable in both piers and spandrels.
Tension Cracks on a Pier
Tension Cracks on a Pier and Spandrels
-
Diagonal Tension Failure – Mode 4
Spandrel Failure
Pier Failure
-
Out-of-plane (OOP) Failure Modes
There are three types of OOP damage:
• One-way bending between vertical supports,
• Two-way bending, end walls represent either 3
or 4 boundary conditions,
• Corner failure.
-
Out-of-plane Failure
One-way Bending
-
Out-of-plane FailureTwo-way Bending
-
Out-of-plane FailureTwo-way Bending
-
Out-of-plane FailureTwo-way Bending
Spalled
corner
-
Out-of-plane FailureCantilever Action
Complete collapse of a gable
by cantilever action (lack of
anchorage)
Diaphragm is visible
-
Out-of-plane FailureCorner Damage
-
Out-of-plane FailureMixed Mode Failure
(In-plane, out-of-plane and corner effects)
Very common type of damage (heavy roof usually the culprit)
-
UNREINFORCED MASONRY
STRUCTURES
- PART I -DEFINITIONS AND PROBLEMS UNDER
LATERAL LOADS
-
Some Definitions
• UnReinforced Masonry (URM): Defined as
masonry that contains no reinforcing in it.
• Masonry Unit: Clay brick or natural stone
element used to construct masonry.
• Mortar: Binding element being used to
connect masonry units, typically composed of
lime or cement, or both, and sand.
-
Masonry Walls
• Walls most generally are made of “brick”,
“hollow concrete blocks”, “hollow clay
tiles ??”, “stone” and “adobe”, and are
load bearing.
• Solid clay-brick unit masonry is the most
common type of masonry unit.
-
Types of Masonry Wall Units
-
Masonry Walls
A common type of unreinforced masonry wall in
one- or two-story buildings is approximately a 30-35
cm thick, and uses a pattern of brickwork. In this
pattern, most of the bricks are laid running parallel
with the wall (these are known as stretchers).
Approximately every sixth horizontal row, there will
be a row of bricks with their ends rather than their
sides visible (these are known as headers), as
illustrated in Figure 8.
-
Basic Brickwork Terminology
Bed
Joint
Head
Joint
Course - horizontal layer of brick
-
Course: Continuous layer
Wythe: Continuous vertical section
A Pattern of Brickwork
-
A TYPICAL MASONRY STRUCTURE
MAIN COMPONENTS
• Main components of URM structures:
• Footing and/or foundation wall (concrete,
masonry, or rock)
• Load bearing masonry exterior walls,
• Wood frame floor(s) (in most of the cases),
• Roof system,
• At the interior, there is some type of bearing
wall(s), normally URM or wood.
-
Foundation Walls
• Foundation walls for URM buildings
may be either concrete, masonry, or
rock,
• If the foundation wall is unreinforced
masonry or rock, it can break apart at
the mortar joints when seismic activity
occurs,
• Many times, these walls have been
badly deteriorated from moisture
penetration over the life of the
building,
• The mortar used in many older
buildings contains very little cement
and is normally very soft and weak.
-
Load Bearing Walls
• A bearing wall is defined as a “wall which
supports any vertical load in a
building/structure as well as its own weight”.
The distance
between the wall
units is sometimes
larger to provide
insulation. This type
of wall configuration
is called “cavity
walls”. How to
connect different
wall layers (there is
no mortar)?
-
Floor and Roof Diaphragms
• Three categories of diaphragms can
be identified:
• rigid concrete slab diaphragms,
• flexible wood,
• metal diaphragms,
Intermediate systems such as hollow
concrete planks and brick spanning
between beams also exist.
• In flexible diaphragms, excessive
deflections can lead to “out-of-
plane??” wall damage.
• Hollow brick/concrete systems may
lack adequate interconnections to
function as a continuous load path.
Flexible Wood Diaphragm
Hollow Brick System
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Floor and Roof Diaphragms
An Intermediate Floor Diaphragm System
-
Wooden Joist System for Floors
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A Typical Single Story URM Building
System as a Whole
-
A Typical Multi-story Unreinforced
Masonry Building
-
What is Wrong with URM Structures?
• Masonry is one of the oldest building materials and
has been considered the most durable,
• It is behavior under vertical/gravity static loads is
superb (under compressive forces),
• But earthquake (EQ) ground shaking has been found
to be very damaging to URM buildings,
• Previous EQs have shown that masonry structures
are the most vulnerable of all building types to the
lateral (earthquake) forces.
-
Why Earthquake Loads are Damaging
to URM?
• Masonry material performs well under compressive
forces,
• EQ loads generates tensile as well as shear forces in
the material which is intrinsically weak in tension,
• Lack of additional material (e.g., steel reinforcement)
withstanding well to tension forces makes URM
building very vulnerable for EQ loads.
-
Then Why URM Structures are Still
Being Studied?
• In many modern building codes, it is prohibited to
build URM building in high seismic regions,
• But in existing building stocks, URM structures DO
exist, therefore identifying them in building
inventories, assessing their earthquake performance,
and retrofitting them are important and valid tasks.
-
A Recent Inventory Study Conducted in
Balçova (2011)
• Entire Balçova area has been studied to
characterize its building inventory, following
figures were identified,
– Out of 7628 buildings 2660 many of them are URM
buildings (35% of the total),
– Only %23 of them have projects, the remaining do
not have projects (majority),
– It is estimated that 25-30% of Izmir’s entire building
stock is composed of URM buildings.
-
Other URM Components
Non-wall Components
� In URM structures there are other components called
non-wall component. In many cases, especially under
earthquakes, their response becomes important, and
damage to these components may occur before
damage on the walls becomes significant.
� Below, common behavior modes of non-wall
components are discussed.
-
Non-wall URM Components
Parapets
� These short extensions
of walls above the roof
typically occur at the
perimeter of the
buildings and are
primarily present for
aesthetic reasons.
� As originally
constructed, they are
not braced back to the
roof and are thus
susceptible to brittle
flexural out-of-plane
failure.
From Adjacent
Building
Building Itself
-
Parapets(Are they just simple extensions?)
-
Non-wall URM Components
Appendages
• This category includes veneer,
cornices, brackets, statuary, and
any minor masonry feature that is
susceptible to falling,
• Damage may result from excessive
accelerations of appendages and
deformations that cause
connection failures between the
appendage and the structure,
• Delamination of veneer can result
from missing or inadequate ties,
• Pounding against adjacent
buildings can lead to localized
falling hazards.
-
Canopy Failure
Are they just shades and shelters?
-
Non-wall Components
Wall-Diaphragm Ties• Generally limited to low-strength
tension connections in which one
end of a steel bar is embedded one
wythe in from the outer face of the
wall and the other end is
hammered into the side of a wood
joist.
• Wall-diaphragm separation due to
inadequate or missing tension ties
can lead to out-of-plane failures of
walls;
• Missing shear ties can lead to the
diaphragm sliding along the in-
plane walls and then pushing
against the walls perpendicular to
the movement, resulting in corner
damage to the walls.
Missing shear ties!
In-plane walls
Tension Ties
-
URM Components
More Details on Wall Components
• URM wall elements can be subdivided into five component
types based on the mode of inelastic behavior.
• The majority of modes relate to “in-plane damage”, but
“out-of-plane” damage can occur as well in each of the
systems, often in combination with in-plane damage.
• The five component types are described below.
-
“In-Plane” and “Out-of-Plane” (OOP)
Damage in URM building
In-plane Wall
DamageOOP Wall
Damage
-
Wall Components
Solid Cantilever Wall (URM1)
URM1: Such walls are
typically found adjacent to
other buildings or on alleys,
and they act as cantilevers
up from the foundation.
Typical Inelastic/Failure Behavior
-
Overview of In-plane Failure Modes on
a Solid Masonry Wall
-
Wall ComponentsWeak Pier in Perforated Wall
(URM2 and URM4)
URM2: This component is a
weak pier in a perforated wall. In
this system, inelastic
deformation occurs in the piers.
URM4: This component is a
strong spandrel in a weak pier-
strong spandrel mechanism.
Strong spandrels do not
experience damage.
Typical Inelastic/Failure
Behavior
-
Wall ComponentsWeak Spandrel in Perforated Wall
(URM3)
URM3: This component is a
weak spandrel in a perforated
wall.
Inelastic deformation occurs
first in the spandrels then
leading to inelastic
deformation and damage in
the piers.
Typical Inelastic/Failure Behavior
-
Wall ComponentsWeak Joints in Perforated Walls
(URM5)
URM5: Perforated wall with panel
zone weak joints. Inelastic
deformation occurs in the region
where the pier and spandrel intersect.
Such damage is not observed
generally in experimental tests, nor is
it seen in actual earthquakes, except
at outer piers of upper stories.
Typical Inelastic/Failure Behavior
-
Understanding the Response of Structural
Components in URM Buildings under
EQ Loads
-
• The walls are weak in resisting horizontal forces (and they
lack ductility),
• The walls are heavy (they have high mass, leading to high
inertial forces),
• Diaphragms are excessively flexible (insufficient lateral
support for the walls),
• Diaphragm-to-wall connections are either absent or weak,
• Parapets and ornamentation are common (and made of
masonry).
What Makes URM Buildings Weak Under
EQ Forces (Lateral Forces)?
-
• Masonry materials are intrinsically strong when
compressed under the gravity loads but are
weak in resisting earthquake forces, which
make materials flex and also shear,
• When an earthquake shakes an unreinforced
masonry building, it causes the building’s walls
to flex out-of-plane and to shear in-plane,
• Unreinforced masonry is weak in resisting both
of those types of forces.
• Mortar is the “glue” that holds the masonry
units together; however, when it eventually
cracks, it does so in a brittle manner, similar to
the way that the bricks crack.
An Intrinsic Problem
Brittle Material
-
Other Problems
Maintaining Integrity
Furthermore a number of common failure modes of URM
buildings related to maintaining integrity have repeatedly been
observed in earthquakes. These modes can be grouped in the
following categories:
• Lack of anchorage,
• Anchor failure,
• In-plane failures,
• Out-of-plane failure,
• Combined in-plane and out-of-plane effects,
• Diaphragm-related failures.
-
Four Recognized In-plane Failure Modes
-
• Rocking of a wall and its foundation on
the supporting soil has been observed in
the field.
• Though recognized as a potentially
favorable mode of nonlinear response
and a source of damping rather than
significant damage,
• Excessive rocking could theoretically lead
to some instability and nonstructural
damage in the superstructure,
• Several technologies being used to
encourage rocking
Foundation Rocking – Mode 1
-
Wall-Pier Rocking – Mode 1
• In the wall-pier rocking behavior mode,
after flexural cracking develops at the
heel, the wall or pier acts as a rigid body
rotating about the toe.
-
Toe Crushing – Mode 2
Characteristics of toe crushing
• Loss of material at toe of pier
• Vertical load carrying capacity generally maintained
• Not really a failure
-
• In this type of behavior sliding occurs on bed joints. Commonly observed
both in the field and in experimental tests,
• There are two basic forms: sliding on a horizontal plane, and a stair-
stepped diagonal crack where the head joints open and close to due to
movement on the bed joints.
Bed Joint Sliding – Mode 3
-
Bed Joint Sliding – Mode 3
Stair-step Type
-
Bed Joint Sliding – Mode 3
Horizontal TypeCharacteristics of bed joint sliding
• Governed by friction,
• Consumes energy,
• Pseudo ductility source,
• Vertical load carrying capacity generally
maintained
• Not really a failure
Horizontal Sliding
-
Diagonal Tension Failure – Mode 4
Characteristics of diagonal tension failure
• Governed due to tension stresses (shear
forces lead to tension stresses), that is
why also called shear failure,
• No bed joint sliding,
• Observable in both piers and spandrels.
Tension Cracks on a Pier
Tension Cracks on a Pier and Spandrels
-
Diagonal Tension Failure – Mode 4
Spandrel Failure
Pier Failure
-
Out-of-plane (OOP) Failure Modes
There are three types of OOP damage:
• One-way bending between vertical supports,
• Two-way bending, end walls represent either 3
or 4 boundary conditions,
• Corner failure.
-
Out-of-plane Failure
One-way Bending
-
Out-of-plane FailureTwo-way Bending
-
Out-of-plane FailureTwo-way Bending
-
Out-of-plane FailureTwo-way Bending
Spalled
corner
-
Out-of-plane FailureCantilever Action
Complete collapse of a gable
by cantilever action (lack of
anchorage)
Diaphragm is visible
-
Out-of-plane FailureCorner Damage
-
Out-of-plane FailureMixed Mode Failure
(In-plane, out-of-plane and corner effects)
Very common type of damage (heavy roof usually the culprit)