Satbility of Structures

8/12/2019 Satbility of Structures

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STABILITY OF STRUCTURES

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PREAMBLE Indian buildings built over past two decades are

seismically deficient because of lack of awareness regarding seismic resisting measures .

Also seismic design is not normally practiced inmost of the buildings being built in India.Therefore, seismic vulnerability estimation is

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- . The difficulties faced in seismic vulnerability

estimation of a building are there is no reliableinformation/database available for existingbuilding stock, construction practices, in-situstrength of material and components of thebuilding


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The aim of evaluation is to assess the seismiccapacity of earth quake vulnerable buildings orearthquake damaged buildings for the future use.

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

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The methodologies available so far for theevaluation of existing buildings can be broadlydivided into two categories

qualitative methods analytical methods


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Methods forseismic evaluation

Qualitativemethods

Analyticalmethods

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Conditionassessment

Visualinspection

Non destructivetesting

C/D method Push overanalysis

Inelastic timehistory method


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

Condition Assessment means the collection of information about the structure

and its past performance characteristics to similar type of

I. Qualitative methods

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s ruc ure ur ng pas ear qua eData collection Collection of data is an important portion for the seismic

evaluation of any existing building. The informationrequired for the evaluated building can be divided asfollows.


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Building data

Architectural, structural and construction drawings Vulnerability parameters:

no. of stories, year of construction and total floor area Seismicity of the site.Construction data

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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 others Detailing concept: ductile detailing, special confinement

reinforcement Pounding- column distress, possibly local collapse -

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effects and concentration of damage at the re entrantcorners

Unsymmetrical buildings in elevation- abrupt change in

lateral resistance Vertical strength discontinuities Short columns


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Large tie spacing in columns lack of confinement of core shear failures.

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

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

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. 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

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

materials, strength and ductility of structural componentsand detailing of reinforcement.

Materials evaluation Structural detailing

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, , Limitation of sections Limitation of min. & max. flexural reinforcement Restriction of splices

Development length requirements Shear reinforcement requirement Special confining requirements

Column steel dowelled into the foundation


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

vulnerability based on visual screening

Visual inspection is the most widely used form of non destructive evaluation.

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survey in which an experienced screener visuallyexamines a building to identify features that affect

the seismic performance of the building, such asthe building type, seismicity, soil conditions andirregularities


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The procedure for visual inspection are described

in the following steps Perform a walk through visual inspection tobecome familiar with the structure

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Gather background documents and information onthe design , construction, maintenance, andoperation of the structure


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Identify the location of vertical structural elements columns or

walls Sketch the elevation with sufficient details dimensions, openings,

observed damage such as cracks, spalling, and exposed reinforcingbar, width of cracks

After performing a detailed visual inspection, the investigator mustbe able to distinguish between recent damage and pre existingdamage , ensuring that the observed damage may or may noit prove

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Limitations of Visual inspection method

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 testing

Visual inspection has the obvious limitation thatonly visible surface can be inspected.: internaldefects go unnoticed. So a visual inspection is

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Some of the non destructive testing methods for

condition assessment of structures are described

below.


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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 on

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t e eterm nat on o t e ept o penetrat on oprobes 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 velocity

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

Impact echo

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discontinuities within the thickness of a wall. Penetrating radar

It is used to detect the location of reinforcing bars,cracks, voids and other material discontinuities,verify thickness of concrete.


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II Analytical methodsCapacity/demand (C/D) method

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

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members to resist these forces and displacements. A (C/D) ratio less than one indicates member failure and

thus needs retrofitting.The main difficulty in using this method is that there is norelationship between member and structure ductility factor


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Push over analysis The push over analysis of a static non linear

analysis under permanent vertical loads andgradually increasing lateral loads.

The equivalent static lateral loads approximatelyrepresent earth quake induced forces.

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in a structure is obtained by the analysis that wouldindicate any premature failure.

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

This type of analysis enables weakness in the


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

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

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

This is the most rational method available for assessin

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building performance. There are computer programs available to perform this

type of analysis.

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


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Seismic evaluation of RCC columns

Possible damages which are frequently observed

after the earth quakes are mainly due to lack of confinement,

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, insufficient splices length, inadequate splicing at the same section ,

hook configurations, poor concrete quality etc.


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Seismic evaluation of RCC beams

There is little evidence that the buildings havecollapsed due to beam failure.Reasons for the possible damages are due to

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ac o ong tu na compress ve re n orcement, infrequent transverse reinforcement, bad anchorage of the bottom reinforcement into the

support, bottom steel termination at face of column .


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Seismic evaluation of RC beam column joints

Beam column joints are critical element in framestructures and are subjected to high shear and bondslip deformations under earthquake loading

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are due to inadequate reinforcement in beam column joint,

absence of confinement reinforcement, inappropriate location of bar splices in column etc.


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Seismic evaluation of RC slabs

Generally slab on beams performed well duringearth quakes and are not dangerous but cracks inslab creates serious aesthetics and functional

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Damage to slab often occurs due to irregularities such as large openings at

concentration of earth quake forces, close to widely spaced shear walls.


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SUMMARY

There are many buildings that have primary

structural system, which do not meet the currentseismic requirements and suffer extensive damageduring the earthquake.

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The methodology can be implemented to estimateseismic vulnerability of different types of existingbuilding stock in Indian Cities and further extended

to investigate the impact of mitigation measures onthe consequences of an earthquake.


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Based on the vulnerability analysis, risk in theform of casualties and economic losses can furtherbe estimated after collecting wardwise

demographic and census information for a city. Further, the vulnerability studies demands special

attention with reference to monumental buildin s

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lifelines like rail/road, water supply, electricsupply, sewage, communication, dams, hospitals &schools, vulnerable industries.

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

relief centers and relief dispersion on incidence of

Satbility of Structures

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