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

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)