deficiencies and vulnerability assessment of buildings and retrofit

8/13/2019 Deficiencies and Vulnerability Assessment of Buildings and Retrofit

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DEFICIENCIES AND VULNERABIL ITYASSESSMENT OF BUILDINGS AND

RETROFIT


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WHAT IS SEISMIC VULNERABILITY?

Vulnerability is defined as:

The degree of loss to a given element at Risk(or set of elements) resulting from a

given level of Hazard.

The Vulnerability of an element is defined as a ratio of the expectedloss to the maximum possible loss on a scale from 0 to 1 or 0% to100%.

Seismic Risk= (Seismic Hazard) x (Vulnerability) x (Value)


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SO

Seismic Vulnerabilities may be associated with

all built infrastructure, natural landscape, soil

and human lives

EARTHQUAKE

is one natural disaster that effects all theelements identified above


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DAMAGESBY

EARTHQUAKES!


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BHUJ EARTHQUAKE-2001


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BHUJ EARTHQUAKE-2001


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Aftermath

KASHMIR EARTHQUAKE-2005


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Aftermath



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The main road linking Islamabad and Muzaffarabad



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An Aerial View of Razed Houses in Muzaffarabad



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We Shall Restrict Ourselves to BuildingVulnerabilities

Now

Buildings are made up of either manmade/manufactured materials

and/or

Natural Occurring Materials


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Classified as:

Rural Buildings/HousesUrban Buildings/Houses

both could be:

EngineeredMarginally EngineeredNon-Engineered


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MATERIAL USED:

Rural Buildings

Urban Buildings

Mostly MUD with thatched,

tinned and/or asbestos sheet

roofing

Stone rubble masonry

Soil stabilized blocks and sand

cement blocks

Mostly Reinforced Concrete

Steel, in Industrial Buildings


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All Material have inherent weakness leading to

deterioration, some of them are:

Brittleness

Non resistance to chemical attacks

Prone to fire

Do not have ability to perform in adverse loading

conditions

Vulnerable to natural disasters


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REINFORCED CONCRETE BUILDINGS

Versatile Material

Most Researched Material

Stood Test of Time

Materials Readily Available

However

possess inherent weaknesses leading to

deterioration and adding vulnerabilities if not

handled properly


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IMPORTANT TERMINOLOGIES

Stability

Serviceability

Lateral Stiffness

Lateral Strength

Ductility

Drift

Deformations


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LOADS ON STRUCTURES

AND ITS ELEMENTS&

THEIR RESPONSE


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Seismic forces on the elements of

shear wall building system

Source: Earthquake Resistant Design of Structures, Pankaj Agarwal and Manish

Shrikhande


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Buckling under compression

Source: Structures in Architecture, The Building of Buildings, Mario Salvadori and

Robert Heller


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Spandrel beam under torsion


Robert Heller


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Reinforced concrete in bending


Robert Heller


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Forces in ductile and elastic system

Source: Concrete Structures in Earthquake Regions: Design and Analysis, Edmund

Booth and Richard Fenwick


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Load-deflection behaviour of a flexural member

Source: Reinforced Concrete Design, R. Park and T. Pauly


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Source: Concrete Structures in Earthquake Regions: Design and Analysis, Edmund

Booth and Richard Fenwick

Favourable and unfavourable arrangements of relative member strengths


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Blade of grass survives, but the tree does not!

Source: Hand Book on Seismic Retrofit of Buildings, Central Public Works Department,

Indian Building Congress, IIT, Madras


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Earthquake MotionsSource: Structures in Architecture, The Building of Buildings, Mario Salvadori and

Robert Heller


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Elastic behaviour of a building (Murty, 2005)




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Conventional Structure

Source: Earthquake Risk Reduction, David Dowrick


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COMMONVULNERABILITIES

&DEFICIENCIES


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Schematic diagram illustrating local geology and soil features


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Source: Seismic Conceptual Design of BuildingsBasic Principles for Engineers,

Architects, Building Owners and Authorities, Hugo Bachmann


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VERTICAL IRREGULARITIES

Vertical Discontinuities in Load Path or Load Transfer

Major contributor to structural damage during strong earthquakes

Provision for adequate strength and toughness of individual elementsin the system should be made

All structural and non structural elements should be adequately tied tothe structural system, and the load path must be complete and

sufficiently strong

The diaphragm must have adequate stiffness to transmit force


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Vertical Discontinuities

Vertical elements such ascolumns or shear wallsdont continue to

foundation

Causes excessive ductilitydemand in local members

below

Different from soft story

Karachi

PhotobyTomTobin


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Simple rules for elevation shapes of aseismic buildings. (Onlywith dynamic analysisand careful detailing should these rules be broken)


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Simple rules for vertical frames in aseismic buildings


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Related to Vertical Discontinuities

Likely Damage

Zone 4 Medium likelihood of gross loss of life

High likelihood of major damage / closure

Zone 3 Medium likelihood of gross loss of lifeHigh likelihood of major damage / closure

Zone 2b Low likelihood of gross loss of life

Medium likelihood of major damage / closure

Zones 2a and 1 Low likelihood of major damage / closure


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Geometry

Changes in plandimensions of lateralforce resisting systemover height

Can causeconcentration of

damage

Different from soft story

Photo by Greg Deierlein


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Simple rules for plan layouts of aseismic buildings. (Onlywith dynamic analysis

and careful detailing should these rules be broken)


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Simple rules for widths of beams and columns in aseismic reinforced concrete

moment-resisting frames



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Load Path

An earthquake excites thebuildings mass,generating inertial forces

The lateral force resistingsystemtakes these forcesto the soil

The route the forces take

is called the load path Simplified load path for a

moment-resisting frame

Beams &

flooring

ColumnFoundation

Earthquake shaking

Inertial

forces

Shears &

moments

Soil


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EXAMPLES OF LOAD PATH

IRREGULARITIES

Discontinuous columns, shear walls, bracing frames

creating a floating box type situation.

Source: Earthquake Resistant Design of Structures, Pankaj Agarwal and Manish Shrikhande


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Most critical region of damage is the connecting element (link betweendiscontinuous column to lower level column) and lower storey column.

Primarily, therefore, is the strength of the beams that support the load of

discontinuous frame.

Spectacular failure witnessed due to discontinuity of vertical elements ofthe lateral load resisting systems e.g. infill walls that are present in

upper floors are discontinued in the lower floor, caused several

collapses in BHUJ-EARTHQUAKE.

Another example of discontinuous shear wall is the Olive View Hospital,which nearly collapsed due to excessive deformation in the first two

stories during 1972 San Fernando earthquake and was subsequentlydemolished.


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Example of Incomplete Load Path

Photo courtesy Melvyn Green

Delhi


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Damage Due to Incomplete Load Path

Photo courtesy Patrick Murphy Corella

2004 Al-Hoceima , Morocco Eq.

Photo courtesy US National Geophysical Data Center

1999 Koaeli, Turkey Eq.


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Related to Load Path

Likely Damage

Zone 4 Depending on load path deficiency, high to medium

likelihood of gross loss of life

Zone 3 Depending on load path deficiency, medium to low

likelihood of gross loss of life; high to medium likelihood ofmajor damage / closure


Depending on load path deficiency, medium to low

likelihood of major damage / closure



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Irregularity in strength and stiffness

A Weak storey is defined as one in which the storeyslateral strength is less than 80% of that in the storey above.

A storey lateral strength is the total strength of all seismicresisting elements sharing the storey shear for the direction

under consideration i.e. the shear capacity of the column or

the shear walls or the horizontal components of the axial

capacity of the diagonal braces.


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The deficiency that usually makes a storey weak is inadequate strength

of the frame columns.

A SoftStoreyis one in which the lateral stiffness in less than 70%

of that in the storey immediately above, or less then 80% of the

combined stiffness of the three stories above. So;

Weak Storeyis related to Strength

and Soft Storeyto Stiffness


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The essential characteristics of a weakor softstorey consistof a discontinuity of strength or stiffness, which occurs at the

second storey connections



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This discontinuity is caused by lesser strength or increased flexibility

deflections in the first storey of the structure, which in turn results inconcentration of forces at the second storey connection, resulting in

concentration of inelastic actions.

Though it may have functional and/or technical advantage, due toreduced spectral acceleration and base shear due to increased

natural period of vibration as in a base isolated structure, however, the

price of this reduction is paid in the form of an increase in structural

displacement and inter-storey drift, thus entailing a significant P-

effect, threatening stability of structure.

Failure of reinforced concrete buildings due to soft storey haveremained the main reason in past earthquakes.


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Mexico city 1985 EQ, soft first stories were majorcontributor to 8% of serious failures

Number of cases of soft storey failure have also beenreported in Algeria earthquake 1980, San Salvador

earthquake 1986, North ridge earthquake 1994, Bhuj

earthquake, 2001


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It is recognized that this type of failure results from the combination

of several other unfavorable factors, such as, torsion, excusive

mass on upper floors, P- effects and lack of ductility in the bottom

storey, leading to local stress concentration accompanied by large

plastic deformations. Soft Stories, therefore, deserve special

consideration in analysis and design, specially the columns of first

storey have to be designed on the basis of adequate capacity

and/or ductility.


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Soft Story Damage

Chi-Chi,

1999Chi-C

hi,

TaiwanE

q.

Photo courtesy Japan-Hong Kong Recon. Team, U. of Kyoto

Before During

After

Deformation and damage

concentrate here


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Soft Story Damage

PhotocourtesyPaci

ficEarthquakeEngineeringRes

earchCenter

1999 Koaeli, Turkey Eq. Ahmedabad, 2001 Bhuj, India Eq.

Photo courtesy Indian Institute of Technology , Kanpur


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Related to Soft Story

Likely Damage

Zone 4 High likelihood of gross loss of life


Zone 2b Medium likelihood of gross loss of life


Zones 2a and 1 Low likelihood of gross loss of life



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Related to Weak Story

Likely Damage



Zone 2b Medium likelihood of gross loss of life


Zones 2a and 1 Low likelihood of gross loss of life



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Mass Irregularities

Mass irregularities are considered to

exist where the effective mass of anystorey is more than 200% of theeffective mass of an adjacent storey.

Effective mass is the real massconsisting of the dead weight of the

floor plus the actual weight of partitionand equipment.

Excess mass can lead to:

(a) Increase in lateral inertial forces

(b) Irregular responses and complexdynamics.

(c) Shifting of centre of gravity of lateralforces above the base if heavy massesin upper floor



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Related to Mass Irregularities

Likely Damage



Zone 3 Low likelihood of gross loss of lifeHigh likelihood of major damage / closure





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Numerous examples of buildings reported to have collapsed due to

mass irregularities, in Bhuj e.g. Mansi complex is believed to have

failed due to massive swimming pool at the upper floor.

Vertical Geometric Irregularity

A vertical setback is a geometric irregularity in a vertical plane.

It is considered, when the horizontal dimension of the lateral forceresisting system in any storey is more then 150% of that in an

adjacent storey.


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A set back can also be visualized as a vertical re-entrant corner.

The general solution of the set back problem is the total seismicseparation in plan, other wise lateral force resisting elements must bechecked by using dynamic analysis.



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Related to Vertical Geometric Irregularity

Likely Damage









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Adjacent Buildings

Closely spacedstructures with differentvibration properties willpound against each

other

Especially damaging iffloors not aligned

Mexico City, 1985 Michoacan,

Mexico Eq.


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Related to Adjacent Buildings

Likely Damage








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PLAN CONFIGURATION PROBLEMS

Torsion Irregularities

Torsion irregularity shall be considered when floor diaphragms are rigidin their own place in relation to the vertical structural elements that

resist the lateral forces.

Torsion irregularity is considered to exist when the maximum storeydrift, computed with design eccentricity, at one end of the structuretransverse to an axis is more then 1.2 times the average of the storey

drifts at the two ends of the structure.



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Torsion

Occurs when center of mass and center of

rigidity are not close together

Stiff

wall

Plan view

Center of

mass

Center of

rigidity

>0.2L

L

Stiff

wall

Plan view

Center of

mass

Center of

rigidity

>0.2L

L



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Significant torsion will be taken as the condition where the distance

between storeys centre of rigidity and storeys centre of mass is

greater than 20% of the width of structure in either major plan

dimension.

Torsion or excessive lateral deflection is generated in asymmetricalbuildings or eccentric and asymmetrical layout of the bracing system

that may result in permanent set or even partial collapse.

Torsion is mostly effectively resisted at point farthest away from centreof twist, such as at the corners and perimeters of the building.


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Damage Due to Torsion

Photo courtesy Japan-Hong Kong Reconnaissance Team, U. of Kyoto

Dali, 1999 Chi-Chi, Taiwan Eq.


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Related to Torsion

Likely Damage









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Re-entrant Corners

The re-entrant, lack of continuity or inside corner is the commoncharacteristic of over all building configuration that in plan assume the

shape of an I, T, H, + or combination of these shapes occurs due tolack of tensile capacity and force concentration.



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(a) They produce variations of rigidity and hence differentialmotions between different parts of the building, resulting in a local

stress concentration at the notch of the re-entrant corner.

(b) They produce Torsion.

The magnitude of the induced forces will depend on mass ofbuildings, structural system, length of the wings, their aspect ratios,

height of the wings and their height/depth ratio.

To avoid this type of damage, either provide a separation joint

between two wings of the buildings or tie the buildings togetherstrongly in the area of stress concentration and locate resistance

elements to increase the tensile capacity at re-entrant corner.

Two types of problems may occur:


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Non-Parallel Systems In such a system load resisting elements are not parallel or

symmetric about the major orthogonal axis of the lateral-force

resisting system.



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Result in a high probability of torsional forces under a groundmotion, as centre of mass and centre of resistance does notcoincide.

Problems aggravates in triangular or wedge shaped buildings.

The narrow portion of the building will tend to be more flexible thanthe wider ones.

Special care is needed in performing analysis and modeling forcomputer programmes.


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Diaphragm Discontinuity

Diaphragm is a horizontal resistance element that transfer forcebetween vertical resistance elements.

Diaphragm discontinuity, therefore, lead to abrupt variation in stiffness,

including those having cut-outs or open areas greater than 50% of thegross enclosed diaphragm area, or change in effective diaphragm

stiffness of more than 50% from one storey to the next.

The diaphragm acts as a horizontal beam, and its edges acts as

flanges, cuts in flanges seriously weakens the load carrying capacity.

SUMMING UP EFFECTS OF STRUTURAL


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SUMMING UP EFFECTS OF STRUTURAL

IRREGULARITIES Multi-storied reinforced concrete buildings with vertical irregularities like

soft storey, mass irregularities; floating box construction should be

designed on the basis of Dynamic Analysisand In- elastic Design.

The proper effect of these irregularities can be accounted for by 3-Dmathematical modeling of the building and performing dynamic analysis,

where ductility provisions have to be given more emphasis.

It is always better to take extra care during structural planning andeliminating these irregularities.


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The torsional effects in a building can be minimized by properlocation of vertical resisting system and mass distribution.

Shear walls should be employed for increasing stiffness where evernecessary, and these should be intelligent and uniformly located and

distributed in both principal directions.

Reference to case studies and lesson learnt during pastearthquakes in areas having similar construction should always be

looked and guidance taken from recommendation provided.


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Captive Columns


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Captive Columns

Occur when architecturalelements brace a columnover part of its height

Deformation concentrated

over a portion of height,rather than full height asdesigned

Column fails in shearPhotocou

rtesyPacificEarthquakeEngineeringResearchCenter

1999 Koaeli,

Turkey Eq.

Column

Cladding restrains

column


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Column




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Column




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Column




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Column




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Column




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Damage Due to Captive Columns

2001 Arequipa, Peru Eq.

Photos courtesy Eduardo Fierro, EERI Reconnaissance Team Member


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Related to Captive Columns

Likely Damage




High likelihood of major damage / closureZone 2b Low likelihood of gross loss of life




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Mezzanines

Mezzanines often lack a lateral force-resisting

system because they are suspended from the

story above

Unbraced mezzanines can collapse

Mezzanines should be braced in both

directions


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Related to Mezzanines

Likely Damage








i i f


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Deterioration of Concrete

Spalling or rust stains indicate

that rebar is corroding

Deterioration in concrete and

rebar can significantly reduce

member strength

Related to Deterioration of Concrete


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Related to Deterioration of Concrete

Likely Damage

Zone 4 Medium likelihood of isolated loss of life


Zone 3 Low likelihood of isolated loss of life


Zone 2b Medium likelihood of major damage / closure



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Deterioration of Masonry Units

Deterioratedmasonry weakens

infill walls

Walls may failmore readily

Photo courtesy Thomas Tobin

Delhi


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Related to Deterioration of Masonry Units

Likely Damage

Zone 4 High likelihood of isolated loss of life


Zone 3 High likelihood of isolated loss of lifeHigh likelihood of major damage / closure

Zone 2b Medium likelihood of isolated loss of life


Zones 2a and 1 Low likelihood of isolated loss of life

Low likelihood of major damage / closure

Overall Quality


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Overall Quality

Poor quality ofmaterials and/orshoddyconstruction

Causes structureto be weaker thandesigned

Can hasten

deteriorationPhoto by David Mar

Karachi

Related to Overall Quality


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Related to Overall Quality

Likely Damage

Zone 4 Medium likelihood of isolated loss of life


Zone 3 Low likelihood of isolated loss of life


Zone 2b Medium likelihood of major damage / closure


Proportions


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Proportions

Slender infill walls areprone to out-of-plane

failure

Thicker walls can resist

out-of-plane forces

with good confinementand arching action

PhotobyDavidMar

Related to Proportions


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Related to Proportions

Likely Damage





Zone 2b Medium likelihood of isolated loss of life


Zones 2a and 1 Low likelihood of isolated loss of life

Low likelihood of major damage / closure


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Seismic Evaluation of ExistingBuildings


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Source: Adaptive Conceptual Frame work for Seismic Vulnerability Assessment of RC

Buildings in Pakistan, Haroon M., Rafeeqi S.F.A., and Lodi S.H., COMPDYN

2009, Greece, June 2009)

Flow diagram for the interpretation of Rapid Visual Screening scoring results

Introduction


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Introduction

Defined 3-Tier Procedure as per ASCE 31-03

(now being modified for Pakistan)

Tier 1 - Screening Phase

Checklist statements

Potential deficiencies

Tier 2 - Evaluation PhaseLinear analysis

Weak links

Tier 3 - Detailed Evaluation Phase

Nonlinear analysis

Failure mechanism

Evaluation Process


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Evaluation Process

AB C

CB

A


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CA

Building Configurations: Problems and Solutions


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Building Configurations: Problems and Solutions

(Arnold and Elsesser, 1980)

Architectural problems Structural problems Remedial measur es

Extreme height/depth ratio High overturning forces, large driftcausing non-structural damage,foundation stability

Revise proportion or special

structural system

Extreme plan area Built-up large diaphragm forces Subdivide building by

seismic joints

Extreme length depth ratio Built-up of large lateral forces in

perimeter, large differences in

resistance of two axes Experience

greater variations in ground

movement and soil conditions

Subdivide building by

seismic joints


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Variation in perimeter

strength-stiffness

Torsion caused by extreme varia-

tion in strength and stiffness

Add frames and disconnect

walls, or use frames and

lightweight walls

False symmetry Torsion caused by stiff asymmetriccore

Disconnect core, or useframe with non-structural

core walls

Re-entrant corners Torsion, stress concentrations at the

notches

Separate walls, uniform

box, centre box,

architectural relief,diagonal reinforcement


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Mass eccentricities Torsion, stress concentrations Reprogram, or add resistancearound mass to balance resist-ance and mass

Vertical setbacks andreverse setbacks

Stress concentration at notch, diffe-rent periods for different parts of

building, high diaphragm forces totransfer at setback

Special structural systems,careful dynamic analysis

Soft storey frame Causes abrupt changes of stiffness atpoint of discontinuity

Add bracing, add columns,braced

Variation in column

stiffness

Causes abrupt changes of stiffness,

much higher forces in stiffer columns

Redesign structural system to

balance stiffness

Architectural problems Structural problems Remedial measures


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Discontinuous shear wall Results in discontinuities in loadpath and stress concentrator, formost heavily loaded elements

Primary concern over thestrength of lower level columnsand connecting beams thatsupport the load ofdiscontinuous frame

Weak column-strong beam Column failure occurs beforebeam, short column must try and

accommodate storey heightdisplacement

Add full walls to reduce columnforces, or detach spandrels

from columns, or use lightweight curtain wall with frame

Modification of primarystructure

Most serious when masonry in-fill modifies structural concept,creation of short, stiff columnsresult in stress concentration

Detach in-fill, or use lightweightmaterials

Architectural problems Structural problems Remedial measures


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Building separation

(Pounding)

Possibility of pounding dependent

on building period, height, drift,

distance

Ensure adequate separation,

assuming opposite building

vibrations

Coupled Incompatible deformationbetween walls and links

Design adequate link

Random Openings maximum force transfer Careful designing, adequate

space for reinforcing design


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Seism ic Retrof it of Bu i ld ings

Distinction Between Terms:


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RETROFIT

REPAIR REHABILITATION

All three terms refer to modification carried out on a building,

however, in different context.

REPAIR is loosely used to describe any intervention but in context ofstrengthening for seismic forces is used to refer minor interventions that are

non-structural in nature.

REHABILITATION aims to regain the original strength or other structural

requirements of a building.

RETROFIT aims to strengthen a building to satisfy the requirement of thecurrent codes for seismic design i.e, strength, stiffness, ductility, stability and

integrity.


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Target base shear versus roof displacement curves

RETROFIT VS REHABILITATION

Source: Hand Book on Seismic Retrofit of Buildings, Central Public Works Department, Indian Building

Congress, IIT, Madras

()

FACTORS TO BE CONSIDERED FOR


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RETROFIT

Because of the vast variety of existing structures, thedevelopment of general rules of real use is difficult and to a

large extent each structure must be approached as a

strengthening problem on its own merit. Some of the factors

which need consideration are as follows:

1. The form of the structure and non-structure and the need for

change, e.g. to create symmetry.

2. The material used in the existing construction.

3. The permissible visual and functional effect of the strengthening.

4. The desired further design life.

5. The desired seismic resistance.


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6. The acceptable damage to the existing fabric in the design event.

7. The parts requiring strengthening and the problems of access

thereto e.g., piles.

8. The degree to which the ductile failure modes are required.

9. The extent to which other components are to be upgraded as well as

the strength.

10. Continuance of normal function during strengthening works

11. Whether Global or Local retrofit need.

12. Costs.

LEVEL OF RESISTANCE?


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LEVEL OF RESISTANCE?

In California the level of resistance aimed for is based on theconcept of an AcceptableRisk

The objectives are:

1. to resist minor earthquake without damage

2. to resist moderate earthquakes without significant structural

damage, but with some non-structural damage

3. to resist major or severe earthquakes without major failures of the

structural frame work of the building or its components membersand equipment and to maintain life safety


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Probabilistic Seismic Hazard Levels


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Probabilistic Seismic Hazard Levels

SERVICEABILITY EARTHQUAKES (Approx Return Period 75years) 50% probability of exceedance in 50 years.

DESIGN BASIS EARTHQUAKES (Approx Return Period 500

years) 10% probability of exceedance in 50 years.

MAXIMUM CONSIDERED EARTHQUAKE (Approx Return

Period 2500 years) 2% probability of ecxeedance in 50 years.

Keep in mind that lower level of EQ are more likely to occur.

WHAT SHOULD BE RETROFITTED?


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As resources will always be limited for retrofitting, a strategy

of assigning priorities to what should be retrofitted first islikely to be adopted, e.g. :

1) Post-earthquake emergency facilities.

2) Life lines.

3) URM buildings.

4) Buildings which are cheap to retrofit.

5) Vulnerable buildings containing many people.

6) Cultural heritage property.

7) Historic building.

8) Public buildings

9) Other property

PRINCIPAL WEAKNESSESS IN


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BUILDING

For Pre-EQ-Code era buildings the following list of principalweaknesses in RC buildings has been given by Coburn and

Spence (2002):

a) Insufficient lateral load resistanceas a result of designing for

two small a lateral load.

b) In adequate ductility caused by insufficient confinement of

longitudinal reinforcement especially at beam-column or slab-

column junction.

c) A tendency to local over stressing due to complex andirregular geometry in plan and in elevation.



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BUILDINGd) Interaction between structure and non-structural walls resulting

in unintended torsional forces andstress concentrations.

e) Soft ground floordue to lack of shear walls.

f) High flexibility combined with insufficient spacing between

buildings resulting in risk of neighbouring structures pounding

each other during shaking.

g) Poor quality materials or work in the construction.

To Retrofit or Not?


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Seismic load capacity versus risk of building collapse



Extent of Seismic Retrofit?


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Extent of Seismic Retrofit?

Depends on:

Importance of building

Expected Remaining Life

Construction Quality

Level of Intervention

RETROFIT STRATEGY REFERS TO


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RETROFIT STRATEGY REFERS TO

Option of increasing

Lateral Strength

Lateral Stiffness

Ductility

Integrity

Either at: Local level i.e Member level

Global level i.e Building as a whole

LATERAL STRENGTH


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Base shear versus roof displacement curves to illustrate increase in

lateral strength

LATERAL STIFFNESS


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Base shear versus roof displacement curves to illustrate increase in

lateral stiffness

DUCTILITY


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Base shear versus roof displacement curves to illustrate difference

in ductility

CONCEPT OF SEISMIC RETROFITS


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Source: Standards for Seismic Evaluation of Existing Reinforced Concrete Buildings, 2001,

Building Research Institute, The Japan Building Disaster Prevention Association

GOALS OF SEISMIC RETROFIT


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1) To increase the lateral strength and stiffness of the building.

2) To increase the ductility in the behaviour of the building. This

aims to avoid the brittle modes of failure.

3) To increase the integral action of the members and provide

uninterrupted load path in a building.

4) To eliminate or reduce the effects of irregularities.

5) To enhance redundancy in the lateral load resisting system.

This aims to eliminate the possibility of progressive collapse.

6) To ensure adequate stability against over turning and sliding.

7) To reduce damage in non-structural components for life-line

and important buildings.

OBJECTIVE OF SEISMIC RETROFIT


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OBJECTIVE OF SEISMIC RETROFIT

The objective of seismic retrofit are quantitativeexpression to achieve the goals of retrofit. The objectives

need to be defined before designing for retrofit.

Of course for a non-engineered building, the objective

may not be quantifiable. The implicit objective is toprovide adequate lateral strength by strategies that have

been tested or proved to be effective in past earthquakes.

For an engineeredbuilding the objectives are based onmeasurement of relevant quantities. The minimum

objective should be to ensure that a retrofitted buildingdoes not collapse during a severe earthquake.

GLOBAL AND LOCAL RETROFIT METHODS


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Source: Earthquake Resistant Design of Structures, Pankaj Agarwal and Manish

Shrikhande


Retrofitting technique

Global Local

Adding shear wall

Adding infill wall

Adding bracing

Adding wing wall/

buttresses

Wall thickening

Mass reduction

Supplemental damping

and base isolation

Jacketing of beams

Jacketing of columns

Jacketing of beam-column joints

Strengthening

individual footing


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CLASSIFICATION OF SEISMIC UPGRADING METHODS (Contd.)

Source: Standards for Seismic Evaluation of Existing Reinforced Concrete Buildings, 2001,

Building Research Institute, The Japan Building Disaster Prevention Association


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CLASSIFICATION OF SEISMIC UPGRADING METHODS (Contd.)

Source: Standards for Seismic Evaluation of Existing Reinforced Concrete Buildings, 2001, Building

Research Institute, The Japan Building Disaster Prevention Association


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CLASSIFICATION OF SEISMIC UPGRADING METHODS

Source: Standards for Seismic Evaluation of Existing Reinforced Concrete

Buildings, 2001, Building Research Institute, The Japan Building Disaster Prevention

Association


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GLOBAL AND LOCAL RETROFITOF

RURAL AND MASONRYBUILDINGS

GLOBAL RETROFIT OF RURAL AND

MASONARY BUILDING


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MASONARY BUILDING

The walls in a masonry building should be provided with reinforcedconcrete bands at plinth, sill, lintel and roof levels. Vertical steelbars should be provide at the edges of wall segments, ensuring that

they are well anchored to the foundation and roof and that the

whole structure should behave as one box.

Long walls are weak against the forces that act in a directionperpendicular to their length. To prevent possible collapse,

adequate cross walls or buttresses need to be provided, with proper

bonding at the junctions.

The floor slabs and roof need to be properly connected to thesupporting walls for effective transfer of seismic forces.

The bandage strengthening techniques can be adopted to retrofit amasonry building. For huts especially with heavy roof adequate

braces should be provided.

GLOBAL RETROFIT OF RURAL AND

MASONARY BUILDING


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MASONARY BUILDING

The openings for doors and windows should be rearranged asstaggered and should have sufficient distances betweenthemselves and the corners.

The corners should be well connected and the bricks should beplaced staggered with reinforcement at the corner as well as

connection with internal walls. This may be done by tying angle ironfrom outside.

Containment reinforcement should be provided with horizontal andvertical reinforcement with cross bars after 3 or 4 blocks, inserted in

the mortar.

At plinth level slab on grade may be provided to transmit horizontalload to foundation.

Columns in verandah should be tied by beams at roof level andS.O.G at plinth level.


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Source: Hand Book on Seismic Retrofit of Buildings, Central Public Work Department,


Introduction of crosswalls


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Connection of a new wall with an existing wall


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Source: Hand Book on Seismic Retrofit of Buildings, Central Public Works

Department, Indian Building Congress, IIT, Madras

Strengthening of long walls by buttresses


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Introduction of braces in a hut

Bracing without door opening (b) Bracing with door opening


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Bandage strengthening technique


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Containment reinforcement for strengthening a masonry wall


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Strengthening of wall


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Anchoring of walls


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Insertion of dowel

bars at comers and T-

junctions (dimensions

in mm)


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Source: Hand Book on Seismic Retrofit of Buildings, Central Public WorksDepartment, Indian Building Congress, IIT, Madras

Stitching of

perpendicular

walls


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Strengthening of pillars by concrete jacketing

Seismic Vulnerabili ty of Critical Buil dings and I nfr astructur e, (06th-08thJuly 2011)

Prof . Dr. S.F.A. Rafeeqi


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Strengthening of wall with steel plates


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Source: Hand Book on Seismic Retrofit of Buildings, Central Public Works Department, Indian

Building Congress, IIT, Madras

Strengthening of wall with

Internal reinforcement

RETROFIT OF REINGORCED CONCRETE BUILDINGS


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FACTORS FOR CONSIDERATION

For proper transfer of loads, the foundations must be stronger thanthe columns and the columns must be stronger than the beams.

Columns at open ground floor needs particular attention.

The beams should have adequate top and bottom reinforcement

which should be well anchored at the beam-column joints.

Shear walls should be well connected to the frames of the building.

Ensure that the stair cares are well framed to avoid collapse duringearthquakes.

Additional floors not accounted for in design should be knocked off. Walls between columns creating short or captive columns should be

removed.

GLOBAL RETROFIT STRATEGIES


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GLOBAL RETROFIT STRATEGIES

Addition of additional in fill walls

Addition of shear walls or braces

Removal of additional dead loads

Reduction of irregularities

Addition of wing walls

Addition of buttress walls


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Addition of a masonry infill wall


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Addition of a shear wall (Jain, 2001)


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Addition of buttress walls


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Addition of wing walls


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Types of connection of braces to an RC frame

Table: Comparative evaluation of the global retrofit strategies Cont.


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Table: Comparative evaluation of the global retrofit strategies


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Table: Comparative evaluation of the global retrofit strategies

LOCAL RETROFIT STRATEGIES


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LOCAL RETROFIT STRATEGIES

Concrete Jacketing

Steel Jacketing

FRP Sheet Wrapping

External Unbonded Prestressing Tenders

External Unbonded Ordinary Reinforcement

Increasing thickness of walls


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Concrete jacketing of a column


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Concrete jacketing of beams


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Source: Hand Book on Seismic Retrofit of Buildings, Central Public

Works Department, Indian Building Congress, IIT, Madras

Steel jacketing of

columns


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Use of steel sheets in beams


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Schemes for concrete jacketing of columns


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Seismic Vulnerabili ty of Critical Buil dings and I nfr astructur e, (06th-08thJuly 2011)

Prof . Dr. S.F.A. Rafeeqi


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Strengthening a wall

using concrete


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Source: Flexural Behaviour of Reinforced Concrete Beams Strengthened by External

Unbonded Reinforcement, S.F.A.Rafeeqi

Retro-fitting of external Unbonded Reinforcement for Strengthening of

Reinforced Concrete Beams in Flexural


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Source: Reinforced Concrete Beams under Repair, S.F.A. Rafeeqi, Proceedings of SEC 2001 India.

Schematic illustration of strengthening technique with externalunbonded ordinary reinforcement


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External tendons in a box-girder bridge in France

Source: External Pre-stressing a state of the art, Bruggeling, A.S.G, SP-120


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STEEL PLATE BONDING

(Provision of anchor plates to ensure ductile flexural failure)

Source: Repair and Strengthening of Concrete with Adhesive Bonded Plates, R. Narayan

Swamy and Robert Gaul, American Concrete Institute, Detroit, 1997.


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FERROCEMENT LAMINATES

(Typical Attachments of Woven Wire Mesh Layers)


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FRP WRAPS

(Wrapping of column using FRP composites)


Indian Building Congress IIT Madras


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FRP WRAPS

(Wrapping of beam-column joints (Mukherjee and Joshi, 2002))




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Underpinning of reinforced concrete footing with piles


Building Congress IIT Madras

Table: Comparative evaluation of the local retrofit strategies, Cont.


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Department Indian Building Congress IIT Madras

Table: Comparative evaluation of the local retrofit strategies


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RETROFIT EXAMPLE

STRUCTURAL RETROFITTING OF GGPS

NAYASHER # 3


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INTRODUCTION School buildings are vital for a society.

They are places of learning.

They also play a significant role in the reliefoperation in post-disaster situations.

These buildings can be used as temporaryshelters.

Assessment of seismic vulnerability of schoolbuildings is essential to ensure safety of childrenand teachers.

DESCRIPTION OF SCHOOL

School consists of 4 classrooms


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School consists of 4 classrooms.

These have been divided into 3 blocks. Load resisting system consists of cavity walls of stone

rubble masonry.

These provide resistance against gravity loads.

Roofing system consists is of timber trusses andcorrugated metal sheeting.

Construction inherently lacks resistance to lateral loads.

PLAN OF SCHOOL


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VIEW OF SCHOOL

A view of school is shown in figure below


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A view of school is shown in figure below.

TYPICAL SECTION OF WALL

Typical wall section of the construction in the area is


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Typical wall section of the construction in the area is

shown in figure below.

STRUCTURAL SYSTEM

Structural system consists of load bearing walls to resist both

gravity and lateral loads


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gravity and lateral loads

No connection between diaphragm and walls

Diaphragm is flexible and is unable to transfer shear to the walls

Good system for in-plane lateral forces but weak against out-of-

plane lateral loads. Mortar provide weak planes as it is weak in tension.

Failure of mortar joint could result in the toppling of wall.

Itsa problem of stability than strength.

EXISTING CONDITION

N j d i t i th b ildi


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No major damage is apparent in the buildings.

Mortar cracks can be seen between stone blocks.

Cracks are also visible at the corners of adjacent walls and door andwindow openings.

These cracks are typical of this type of construction.

Retrofitting of school buildings can be carried out to increase lateral loadresistance against out of plane forces.

RETROFITTING SCHEME

There are four objectives of the retrofitting scheme.


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To increase lateral load resistance of individual walls against out-of-plane forces locally.

To form a closed box action between the four walls to enable them toact as monolithic walls to increase their resistance globally.

To strength weak areas within the walls such as openings for doors andwindows

To tie individual and isolated members together such as stone masonrycolumns in the veranda.

RETROFITTING SCHEME


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These objectives have been realised by four different schemes.

Jacketing using MS steel strips, plates and angle iron.

Shotcreting of surfaces.

Provision of plinth beams to tie isolated columns.

Provision of Slab-on-Grade to increase the global stiffness of the system.

ANALYTICAL MODELLING

A mathematical model of the school was developed.


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Finite Element Program SAP was employed.

Individual blocks were modelled as solid brick elements.

Mortar was modelled using non-linear springs.

Non-linear properties of mortar were defined using existing constitutivemodels.

Geometric non-linearities were also taken into account.

Ground shakings were simulated using El-Centro ground accelerations

PROGRESS IN RETROFITING WORK


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THANK YOU

deficiencies and vulnerability assessment of buildings and retrofit

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