2 - Repair_ Restoration & Retro of Buildings RCC & Maosnry

8/8/2019 2 - Repair_ Restoration & Retro of Buildings RCC & Maosnry

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by

Dr. Anand S. Arya, FNA, FNAE

Padmashree awarded by the President of India

Professor Emeritus, Deptt. of Earthquake Engg., I.I.T. Roorkee

Former National Seismic Advisor GoI-UNDP, New Delhi

Islamabad Oct. 9, 2009, (2)


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SEISMIC RISK TO BUILDINGS


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EARTHQUAKE HAZARD ZONES 2002

Zone V MM IX or more IV MM VIII

III MM VII

Zone II MM VI or less

Area under the zones

V 12%

IV 18%

III ~27%

Total damageable area

~ 57%


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OLD LIFELINE BUILDINGS

1. Non Engineered Building Construction

Buildings in field stone, fired brick, concrete blocks,

adobe or rammed earth, wood or a combination of

these traditional locally available materials.

2. Engineered Constructions including

buildings and infrastructure.

Reinforced concrete and steel buildings andstructures normally designed by Architects and

Engineers working together or Civil Engineers.


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REPAIR & RESTORATION

Need

Buildings Damaged in Grades G1,G2,G3- NeedRepair, Restoration & Retrofitting

Repair

Means the repairing ofnon- structural elements, suchas plumbing, electrical works and cosmetic touch up.

Restoration

Means action taken to restore the structure to itsoriginal strength. It is limited to damaged structuralelements. It includes repairs of wall cracks, stitchingof walls, and grouting of cracks in RCC elements, etc.


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SELECTION OF MATERIALS & TECHNIQUES

Most common cement & steel

Non-shrink grouts for cracks & internal

strength

Polymer concrete

Shotcrete

Epoxy Resins grouts/mortars

Mechanical Anchors Fibre Reinforced Plastics.


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STRATEGY FOR UPGRADING

THE STRENGTH

OFEXISTING CONSTRUCTIONS

BY

RETROFITTING


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NEED FOR RETROFITTING

Existing Weak Unsafe Buildings Damageabilitygrades G3 to G5

Buildings not designed to codes

Upgrading of code based seismic design forces

Upgrading of seismic zone

Deterioration of strength on aging of the structure

Modification of the existing structures affecting its

strength adversely Change in the use of the building increasing the

floor loads

All such buildings needs to be upgraded by

Retrofitting.


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SEISMIC STRENGTHENING/RETROFITTING

RetrofittingMeans action taken to upgrade the

seismic resistance of an existing buildingso that it

achieves intended seismic performance level.It

includes:- addition of new members, shear walls, bracings,

- reducing load,

- strengthening of structural elements and- increasing ductility of members, etc.


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STRENGTHENING OR RETROFITTING VS

RECONSTRUCTION

Replacement of damaged buildings or existing

unsafe buildings by reconstruction is, generally,

avoided due to a number of reasons, the mainones among them being:

Higher cost of re-building than that of

strengthening or retrofitting,

Preservation of historical architecture, and

Maintaining functional social and cultural

environment.


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Cost of Reconstruction and Retrofitting

Generally 2.5 to 3 times the initial extra expenditure

required on seismic resisting features. Repair and

seismicstrengthening of a damaged building mayeven be 4 to 6 times as expensive.

Hospital Blocks (4 Storey RCC Ordinary Frames)

Retrofitting 31% Total 60% of ReconstructionCost

Refurbishing 29%


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ASSESSMENT OF DEFICIENCIES

ANDRETROFITTING REQUIREMENTS

1. Rapid Visual Screening

Study of Drawings and Simple Calculation

Testing (NDT & Core Cutting)

DetailedAnalysis as per Current SeismicCode.


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MASONRY BUILDINGS


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Strengthening / Retrofitting

Details (IS: 13935 1993)


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Structural

Restoration

ofCracked

Masonry

Walls


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In stone buildings of

historic importance,

consisting of fully dressed

stone masonry in good

mortar, effective sewing of

perpendicular walls may

be done by drilling inclined

holes through them

inserting steel rods andinjecting cement grout.

Connection between

Existing Stone Walls

Fig.: Sewing Transverse Walls

with Inclined Bars


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Strengthening

with

Wire Mesh&

Mortar


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Strengthening

an

Arched

Openingin

Masonry Wall


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Reinforcing Around Openings

i) In Cat.D & EbuildingsMesh of gauge 10 with 8

wires in vertical direction

spaced at 25 mm in a belt

width of200 mm or meshof gauge 13 with wires @

25 mm in a belt width of

250 mm may be used.

ii) In Cat. C

buildingsMesh of gauge 13 with 10

wires in vertical direction

spaced at 25 mm in a belt

width of250 mm.


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Strengthening of Walls by Prestressing


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Overall arrangement of Seismic Belts

Seismic Belt Locations

i) Seismic belts are to be provided on all walls on boththe faces just above lintels of door and window openings

and below floor or roof.


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Vertical Seismic Band at Corner & Junctions


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Splint and Bandage Strengthening

Technique


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RoofModification

to

Reduce Thrustof

Walls


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Details

of

New RoofBracing


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RCC BUILDINGS


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DEFICIENCIES IN R.C. BUILDINGS -1

Majority of structures not designed for any lateral forces.

The column to beam junctions assumed as hinge connection.

RCC shear walls not used in multi-storey buildings.

Use of floating columns leading to sudden discontinuity instiffness.

Use of small width of columns (150 to 200 mm). Open parking floor acting as a soft storey.

Poor connection (inadequate rigid floor diaphragm action) ofstaircase and RCC lift well to the rest of structure.

Structural planning and design-deficient engineering practice- Use of long cantilevers,

- Beams cantilevering out from cantilever beams,


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DEFICIENCIES IN R.C. BUILDINGS -2 Columns designed for axial load only without any

moments. Ductile detailing missing.

No soil investigation for assessing the strata andbearing capacity.

Insufficient frames in one or both horizontal directions. Irregular/complex shapes of buildings.

Re-entrant corners unattended.

Use of inferior quality material and bad constructionquality.

Strength of concrete of damaged structures foundbelow codal value.


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BUILDING STRUCTURE SURVEY-1

Detailed structural layout with schedule and reinforcement

detailing(if drawings not available, then do as built drawingswith the help of NDT.

Load path and lateral load resisting system.

Relative size- slenderness ratio (height divided by least lateral

dimension) & aspect ratio (length divided by width, in plan). Weak storey- the strength of the lateral force resisting system in

any storey having less than 80% of the strength in an adjacent

storey.

Soft storey- the stiffness of the lateral force resisting system in

any storey having less than 70% of the stiffness in an adjacent

storey or less than 80% of the average stiffness of the three

storeys above.

Geometry- no change in horizontal dimension of the lateral

force resisting system of more than 30% in adjacent storey.


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BUILDING STRUCTURE SURVEY-2

ass- there shall be no change in effective mass more than50% from one storey to the next.

Torsion- distance between the storey centre of mass and storeycentre of stiffness not more than 20% of the building width ineither plan dimensions.

Diaphragm continuity- no sudden discontinuity, large cut outs in

the diaphragm (i.e. slab). A typical example a building with twowings connected only by staircase (H shape building).

Plan irregularities- the complicated plans shapes (L,T,H,U)where both projections of the structure beyond re-entrantcorner greater than 15% of its plan dimension.

Redundancy- structure should have large indeterminacy, ie,multi-storey moment resistant building with many columns andbeams.

Strong column-weak beam- the sum of the moment capacity ofthe columns shall, be 20% more than that of beams at the

frame joint.


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FOUNDATION

Liquefaction

- During the violent shaking there is possibility thatthe building may sink down, tilt or ultimately

collapse down where resting or water bearing non-

cohesive soils due to liquefaction.

Elastic Displacement

- During strong shaking large elastic displacements

may occur in soft soils affecting the piles,

underground pipelines, and culvert-type structures.- Separate column footings in soft soils could have

relative vertical and horizontal displacements.


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MINIMUM SEISMIC RESISTANCE

MSR as per Codes and Standards

Design Basis

once-in-life earthquake intensity should not result

in the total collapse of the building, which has

been the basis of IS: 1893 all through.


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AVAILABLE SEISMIC RESISTANCE

ASR- the earthquake force under which the first ofthe columns of any building storey will reach its

ultimate limit strength, when the remaining

structure remains in the undamaged state.


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Cases for Retrofitting Requirements

1984 Code: (i) If no ductility provided & K = 1.6 used, base

shear co-efficient is more than 2002 code for

R=3.0, hence no action needed.

(ii) If ductility as for IS: 4326-1976 or 13920-1993

used with K=1.0, no action needed.

(iii) If ductility not used, K=1.0 used-Ductility

detailing needed or check if found safe for

about 1.5 times the seismic coefficients in 1984

code*.

1975 Code: There was no K-factor in code but ductility asper IS: 4326 was needed same as for 1984code.

Action is needed design seismic coefficient

similar to (iii) of 1984

as above*.


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1970 Code & 1966 Code No ductility required &

design seismic coefficient is seen, 0.4 to

0.6 of the required value in IS: 2002.Buildings need vulnerability assessment &

retrofitting*.

1962 Code No ductility, design seismic coefficient is

seen 0.2 to 0.3 of the present 2002 values.

Buildings need vulnerability assessment &

retrofitting*.

* In the vulnerability assessment of RC frames, effect of

infill walls may be included on stiffness as well

strength.


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OBJECTIVES AND PRINCIPLES OF

INTERVENTIONMain Objectives

To protect the buildings

i. From collapse in a future strong earthquake,

ii. To keep damage at tolerable levels in earthquakes of

moderate intensity and

iii. To eliminate damage in small magnitude frequently occuringearthquakes.

Intervention

To be done by

i. The restoration of the damaged structural elements, and

ii. The increase of the seismic resistance of the structure up to

the desired value of the Minimum Seismic Resistance (MSR)


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FACTORS TO DETERMINE TYPE OF

INTERVENTION

Ref: UNIDO / UNDP ProcedureStrength of the structure

i) ASR more than 80% of MSR: Retrofitting taken as

not needed.

ii) Ratio ASR/MSR in the range 0.8 to 0.5: Needs to be

retrofitted for strength upgradation.

iii) ASR less than half of MSR- The safety of the

structure is unsatisfactory, retrofitting required forupgrading the strength as well as ductility.


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REMAINING USEFUL LIFE OF BUILDING

The retrofitting level can be determined throughprobabilistic relationship:

Rstr /MSR = (Trem/Tdes)0.5

, butu

0.7

in any case

Rstr= Seismic force for redesign of the building for

strengthening,

Trem = Remaining life, and

Tdes = Design Life of the Building.


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TYPE OF INTERVENTION- no intervention at all;

- restriction or down graded change of use of the building;

- local or global modification of damaged or undamaged

elements;

- possible upgrading of existing non-structural elements into

structural ones;

- modification of the structural elements aiming at stiffness

regularity, elimination of vulnerable elements, or a beneficial

change of the natural period of the structure by base isolation;

- storey or mass reduction;

- addition of new structural elements (e.g. bracings, infill walls);

- full replacement of inadequate or heavily damaged elements;

- addition of damping devices at appropriate parts of the

structure;

- partial demolition.


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DESIGN OF RETROFITTING

Conceptual design

i) Selection of techniques and materials, as well as

the type and configuration of the intervention;

ii) Preliminary estimation of dimensions of additional

structural parts;iii) Preliminary estimation of the modified stiffness of

the strengthened elements;

iv) Preliminary estimation of the appropriate behavior

factor in relation to the local and global ductility ofthe modified structural system.


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Reanalysis

i) Identification of the non-seismic loads and actions;

ii) Selection of the seismic coefficients and actions;

iii) Determination of the action effects taking into

account the modified stiffnesses, and possible

unfavorable redistribution of load effects due toheavy damage (I.e. column damage, deviations

from the vertical axis;)

iv) Implementation of the simplified modal response

spectrum method of analysis.


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SAFETY VERIFICATION

i) Selection of the behavior model of

restored/retrofitted element;

ii) Selection of material partial safety factors; (See IS:

456-2000)

iii) Calculation of design resistance;iv) Verification of the safety inequalities regarding

seismic and non-seismic loads and actions, for both

the ultimate limit state (ULS) and the serviceability

limit state (SLS).

Drawings, Quantities and Cost Estimates


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WHEN BEAM & BEAM

COLUMN

STRENGTHENING IS

NECESSARY

TYPICAL JACKETING SCHEME OF

COLUMN


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WHEN BEAM &BEAM COLUMN

JOINT NEED NOT

BE

STRENGTHENED

TYPICAL JACKETING SCHEME OF COLUMN

TYPICAL CONNECTION DETAIL OF NEW SHEAR


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TYPICAL CONNECTION DETAIL OF NEW SHEAR

WALL WITH EXISTING STRUCTURAL ELEMENTS


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TYPICAL JACKETING SCHEME WITH DOWEL BARS

STIFFENING / STRENGTHENING BY BRACING


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STIFFENING / STRENGTHENING BY BRACING

MEMBERS


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TYPICAL DETAIL OF STRENGTHENING OF

FOUNDATION


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CONCLUDING REMARKS- 1

The restoration of a seismically damaged building

is a much more difficult task than the originaldesign and construction.

Difficulties arise during inspection, design and the

execution of the intervention.

A basic factor for the successful outcome of the

whole operation is the correct diagnosis of the

causes of damage, the level of intervention

(restoration or retrofitting or both). Design of Restoration must aim at providing the

structure with the stiffness, strength and ductility

that it had before the earthquake.


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CONCLUDING REMARKS- 2

The design of the restoration must aim at providing the

structure with the strength, stiffness and ductility required by thecurrent Codes.

For the choice of the techniques, the economic conditions and

the feasibility of application of the chosen technique in every

particular case must be taken into account.

The outcome of the restoration/retrofitting depends to a large

degree on the quality control of the design and construction.

The restoration of the heavily damaged infills and connections

with the structural frame is very important to the structure.

Finally, it has to be stressed that the structural rehabilitation

must

have the proper combination of strength, stiffness and

ductility.


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Cooperation among SAARC Countries

1. Capacity Building of Architects & StructuralEngineers

Teachers of Architecture & Civil / StructuralEngineering Courses

2. Demonstration Retrofitting Project Hospitals

Schools

Administrative Buildings

Buildings for large GatheringBoth Masonry and RCC constructions


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THANK YOU

FOR YOUR ATTENTION

Documents

2 - Repair_ Restoration & Retro of Buildings RCC & Maosnry