VULNERABILITY ASSESSMENT ANDVULNERABILITY ASSESSMENT AND SEISMIC EVALUATION OF STRUCTURES

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INTRODUCTION• Indian buildings built over past two decades are• Indian buildings built over past two decades are

seismically deficient because of lack ofawareness regarding seismic resisting measures .g g g

• Also seismic design is not normally practiced inmost of the buildings being built in India.Th f i i l bilit ti ti iTherefore, seismic vulnerability estimation ispre-requisite for disaster mitigation management.

• The difficulties faced in seismic vulnerability• The difficulties faced in seismic vulnerabilityestimation of a building are there is no reliableinformation/database available for existinggbuilding stock, construction practices, in-situstrength of material and components of thebuilding

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building

Th i f l ti i t th i i• The aim of evaluation is to assess the seismiccapacity of earth quake vulnerable buildings orearthquake damaged buildings for the future use.earthquake damaged buildings for the future use.

• The evaluation may also prove helpful for degreeof intervention required in seismically deficientstructures

• The methodologies available so far for thel ti f i ti b ildi b b dlevaluation of existing buildings can be broadly

divided into two categories– qualitative methodsqualitative methods – analytical methods

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Methods for seismic evaluation

Qualitative methods

Analytical methodsmethods

Condition Visual Push over Inelastic time Condition assessment

Visual inspection C/D method Push over

analysis history method

Non destructive testing

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I Qualitative methods

Qualitative methods are based on the backgroundinformation available like architectural & structural dwg

I. Qualitative methods

information available like architectural & structural dwg.Condition Assessment

• means the collection of information about the structuremeans the collection of information about the structureand its past performance characteristics to similar type ofstructure during past earthquakeData collectionData collection

• Collection of data is an important portion for the seismicevaluation of any existing building. The informationy g grequired for the evaluated building can be divided asfollows.

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Building data• Architectural structural and construction drawings• Architectural, structural and construction drawings• Vulnerability parameters:

f t i f t ti d t t l flno. of stories, year of construction and total floor area• Seismicity of the site.C i dConstruction data• Identification of gravity load resisting system• Identification of lateral load resisting system• Maintenance, addition, alteration, or modification in

structure• Field surveys of the structures existing condition

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Structural data• Structural concept: vertical and horizontal irregularities,

torsional eccentricity, pounding, short column and othersD t ili t d til d t ili i l fi t• Detailing concept: ductile detailing, special confinement reinforcement

• Pounding- column distress possibly local collapsePounding column distress, possibly local collapse• Unsymmetrical buildings (U,T,L,V) in plan- torsional

effects and concentration of damage at the re entrant corners

• Unsymmetrical buildings in elevation- abrupt change in l t l i tlateral resistance

• Vertical strength discontinuities• Short columns

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• Short columns

• Large tie spacing in columns lack of confinement of core –h f ilshear failures.

• Insufficient column lengths – concrete to spall.• Locations of inadequate splices- brittle shear failureLocations of inadequate splices- brittle shear failure• Insufficient column strength for full moment hinge

capacity – brittle shear failure.• Lack of continuous beam reinforcement- hinge formation

during load reversals.• Inadequate reinforcing of beam column joints or location• Inadequate reinforcing of beam column joints or location

of beam bar splices at columns joint failures.• Improper bent up of longitudinal reinforcing in beams as

h i f t h f il d i l d lshear reinforcement – shear failure during load reversal• Foundation dowels that are insufficient to develop the

capacity of the column steel above local column distress.

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capac ty o t e co u stee above oca co u d st ess.

Seismic evaluation data• The criteria of evaluation of building will depend on

materials, strength and ductility of structural components d d ili f i fand detailing of reinforcement.

• Materials evaluationSt t l d t ili• Structural detailing– Flexural members, columns, foundations

• Limitation of sectionsLimitation of sections• Limitation of min. & max. flexural reinforcement• Restriction of splices• Development length requirements• Shear reinforcement requirement• Special confining requirements

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• Special confining requirements• Column steel dowelled into the foundation

Field evaluation /Visual inspection method• This is very quick way of assessing the building

vulnerability based on visual screeningy g• Visual inspection is the most widely used form of

non destructive evaluation.non destructive evaluation.• This methodology is referred to as a “sidewalk

survey” in which an experienced screener visuallysurvey in which an experienced screener visuallyexamines a building to identify features that affectthe seismic performance of the building such asthe seismic performance of the building, such asthe building type, seismicity, soil conditions andirregularities

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irregularities

The procedure for visual inspection are describedi h f ll iin the following steps

• Perform a walk through visual inspection tobecome familiar with the structure

• Gather background documents and information ongthe design , construction, maintenance, andoperation of the structurep

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• Identify the location of vertical structural elements – columns orwallswalls

• Sketch the elevation with sufficient details – dimensions, openings,observed damage such as cracks, spalling, and exposed reinforcingbar width of cracksbar, width of cracks

• After performing a detailed visual inspection, the investigator mustbe able to distinguish between recent damage and pre existingd i h h b d d idamage , ensuring that the observed damage may or may noit proveto be dangerous for the structure.

Limitations of Visual inspection method

A li bl f f d th t b i li d• Applicable for surface damage that can be visualized• No identification of inner damage- health monitoring of building,

change of frequency and mode shapes

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Non destructive testingNon destructive testing

• Visual inspection has the obvious limitation thatVisual inspection has the obvious limitation that only visible surface can be inspected.: internal defects go unnoticed So a visual inspection isdefects go unnoticed. So a visual inspection is usually supplemented by NDT methods

• Some of the non destructive testing methods for• Some of the non destructive testing methods for condition assessment of structures are described belowbelow.

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• Rebound hammer testIt is the most widely used non destructive devicefor quick surveys to assess the quality of concrete.

• Penetration resistance methodThis method is used to determine the quality andcompressive strength of in-situ concrete based onthe determination of the depth of penetration of

b i t th tprobes into the concrete.• Rebar locator

It is used to determine quality, location, size andcondition of reinforcing steel in concrete

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• Ultrasonic pulse velocityIt is used for determining the elastic constantsIt is used for determining the elastic constants(modulus of elasticity and Poissons ratio) and thedensity by conducting tests at various points on thedensity by conducting tests at various points on thestructure.

• Impact echoImpact echoImpact echo is a method for detectingdiscontinuities within the thickness of a walldiscontinuities within the thickness of a wall.

• Penetrating radarIt is used to detect the location of reinforcing barsIt is used to detect the location of reinforcing bars,cracks, voids and other material discontinuities,verify thickness of concrete

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verify thickness of concrete.

II Analytical methodsCapacity/demand (C/D) method

• The forces and displacements resulting from an elasticanalysis for design earth quake are called demandanalysis for design earth quake are called demand.

• These are compared with the capacity of differentmembers to resist these forces and displacements.members to resist these forces and displacements.

• A (C/D) ratio less than one indicates member failure andthus needs retrofitting.gThe main difficulty in using this method is that there is norelationship between member and structure ductility factor

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p y

Push over analysisTh h l i f i li• The push over analysis of a static non linearanalysis under permanent vertical loads andgradually increasing lateral loadsgradually increasing lateral loads.

• The equivalent static lateral loads approximatelyrepresent earth quake induced forcesrepresent earth quake induced forces.

• A plot of total base shear verses top displacementin a structure is obtained by the analysis that wouldin a structure is obtained by the analysis that wouldindicate any premature failure.

• The analysis is carried out upto failure thus it• The analysis is carried out upto failure, thus itenables determination of collapse load andductility capacity.

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ductility capacity.• This type of analysis enables weakness in the

structure to be identified.

Inelastic time-history analysis• A seismically deficient building will be subjected to• A seismically deficient building will be subjected to

inelastic action during design earthquake motion.• The inelastic time history analysis of the building underThe inelastic time history analysis of the building under

strong ground motion brings out the regions of weaknessand ductility demand in the structure.y

• This is the most rational method available for assessingbuilding performance.

• There are computer programs available to perform thistype of analysis.

• This methodology is used to ascertain deficiency and postelastic response under strong ground shaking.

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Seismic evaluation of RCC columnsSeismic evaluation of RCC columns

Possible damages which are frequently observedPossible damages which are frequently observed after the earth quakes are mainly due to

• lack of confinement,lack of confinement, • large tie spacing, • insufficient splices length• insufficient splices length, • inadequate splicing at the same section ,

h k fi ti• hook configurations, • poor concrete quality etc.

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Seismic evaluation of RCC beamsSeismic evaluation of RCC beams

• There is little evidence that the buildings haveThere is little evidence that the buildings have collapsed due to beam failure.Reasons for the possible damages are due toReasons for the possible damages are due to

• lack of longitudinal compressive reinforcement,• infrequent transverse reinforcement, • bad anchorage of the bottom reinforcement into the g

support, • bottom steel termination at face of column

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bottom steel termination at face of column.

Seismic evaluation of RC beam column jointsSeismic evaluation of RC beam column joints

• Beam column joints are critical element in frameBeam column joints are critical element in framestructures and are subjected to high shear and bondslip deformations under earthquake loadingslip deformations under earthquake loadingCommon causes of failure of beam column jointsare due toare due to

• inadequate reinforcement in beam column joint,• absence of confinement reinforcement,• inappropriate location of bar splices in column etc.

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pp p p

Seismic evaluation of RC slabsSeismic evaluation of RC slabs

• Generally slab on beams performed well duringGenerally slab on beams performed well duringearth quakes and are not dangerous but cracks inslab creates serious aesthetics and functionalslab creates serious aesthetics and functionalproblemsDamage to slab often occurs due toDamage to slab often occurs due to

• irregularities such as large openings ati f h k fconcentration of earth quake forces,

• close to widely spaced shear walls.

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SUMMARYSUMMARY

• There are many buildings that have primaryThere are many buildings that have primarystructural system, which do not meet the currentseismic requirements and suffer extensive damageq gduring the earthquake.

• The methodology can be implemented to estimate gy pseismic vulnerability of different types of existing building stock in Indian Cities and further extended

i i h i f i i ito investigate the impact of mitigation measures on the consequences of an earthquake.

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• Based on the vulnerability analysis, risk in they yform of casualties and economic losses can furtherbe estimated after collecting wardwisedemographic and census information for a city.

• Further, the vulnerability studies demands specialFurther, the vulnerability studies demands specialattention with reference to monumental buildings,lifelines like rail/road, water supply, electriclifelines like rail/road, water supply, electricsupply, sewage, communication, dams, hospitals &schools, vulnerable industries.sc oo s, vu e ab e dust es.

• Also there is a need to identify safe zones/domains/ structures and secure routes to work as a

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domains/ structures and secure routes to work as arelief centers and relief dispersion on incidence offuture disaster