19603297 Retrofit PPT DCR

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Seismic Rehabilitation of Structures

Durgesh C. RaiAssistant Professor

Department of Earthquake Engineering

University of RoorkeeRoorkee 247 667


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Rehabilitation Strategies & Measures

Structural

Enhancements

Seismic

Rehabilitation

Recover OriginalPerformance

Upgrade original

performance

Reduce seismic

response

Repair damage and deterioration

Stiffen existing structure

Strengthen existing structure

Reduce irregularity and

Using supplemental damping devices

Reduce masses

Isolate existing structure


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Repair Methods

Cosmetic repairs only improve the visualappearance of component damage and

may restore non-structural properties(weather protection) but any structuralbenefit is negligible.

Structural repairs intends to restore

structural properties.

Repairs

Cosmetic Repairs

Structural Repair

Surface Coating,

Repointing

Crack injection with epoxy

Crack inj ection with grout

Spall repair

Rebar replacement

Wall replacement


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Repair Methods

Cosmetic repairs only improve the visualappearance of component damage and

may restore non-structural properties(weather protection) but any structuralbenefit is negligible.

Structural repairs intends to restore

structural properties.

Repairs

Cosmetic Repairs

Structural Repair

Surface Coating,

Repointing

Crack injection with epoxy

Crack inj ection with grout

Spall repair

Rebar replacement

Wall replacement


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Seismic Strengthening

Se

ism

ic

Str

eng

the

nin

g

Increase strength

Increase strength& ductility

Backupstructure

Infillexistingframes

Brace

existingframes

Install

shearwalls

Jacket

existingmembers

Increase ductility

Peripheral framesButtresses

Cast-in-situ concretePrecast concrete panelsBrick/block infills

Comp. /tens. BracesComp. And tens. BracesSteel or concrete

Cast-in-situ concrete

Precast concrete panel

Steel encasementSteel straps

Concrete or mortarCarbon fibre


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Choice of a Seismic Strengthening Scheme

The strengthening solution

must correct known seismic deficiencies of the system

must be structurally compatible with the existing system

must be functionally and aesthetically compatible must meet the expected performance goal such as life-

safety or limited damage.

must minimize the disruption to occupants

must be cost-effective and use available materials andequipment


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Adding New Shear Walls

Applications

For strengthening RC frames, especially open storeys Complete shear walls with boundary elements and

foundation

Advantages

Adds significant strength and stiffness to framedstructures

Disadvantages Add considerable mass to the structure New footings are required and can be a major problem

on soft soils and in pile-supported structures

Design Guidelines Locate so that they align full height of the building,

minimize torsion and can be easily tied with existingframe

Maximize the dead weight that wall can mobilize toresist overturning uplift.

FEMA-172


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Adding New Shear Walls

Design Guidelines

It is economical to locate shear walls alongexisting framing lines in order to provideboundary members, collectors and dead load tohelp resist overturning forces.

On the interior the shear wall continues throughthe slab and it should be cast in 2 pours 48 hoursapart to avoid sagging away of concrete from theunderside of the concrete slab.

The initial pour is stopped at 450 mm from theslab soffit to allow enough space to form shearkeys and prepare the surface for next pour up tothe top of the slab.

Functional consideration dictate the location asthey break up the interior space

FEMA-172


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Adding Infill Walls

Applications

For strengthening RC frames, especially open

storeys Most applicable for upto 5 storeyed buildings

Advantages

Adds significant strength and stiffness to framedstructures

Disadvantages

Add considerable mass to the structure and neednew footings between existing spread footings

Existing columns may become weak link

Design Guidelines Locate so that they align full height of the

building, minimize torsion and can be easily tiedwith existing frame

Maximize the dead weight that wall can mobilizeto resist overturning uplift.

Insure concrete/mortar is placed tight to overheadbeam else column shearing my result.


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Filling Openings

Applications

For URM buildings significantly weak in in-plane

shear strength due to openings Most applicable for upto 5 storeyed buildings

Advantages

Adds significant strength and stiffness

Disadvantages Add considerable mass to the structure and neednew footings between existing spread footingsover the increased shear wall

Design Guidelines

Fill in openings with RC or masonry The technique is very economical if no foundation

enhancement is required.

Concrete overlay (shotcrete) on the entire wall maybe necessary after filling the opening

FEMA-172


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Adding Shotcrete to Existing Masonry

Applications

Ideal for URM when masonry is not strong or itsin-plane shear strength is weakened by large

openingsAdvantages

Comparable stif fness to existing URM walls With epoxied dowels at about 600 mm each way,

shotcrete and URM will work compositelyenhancing its out-of-plane stability as well

Disadvantages

Messy with rebound on the inside face andtransferring through floor system is difficult and

may require review of foundation detailsDesign Guidelines

Provide enough shotcrete so that failure ofunreinforced section can be prevented

Design shotcrete (thickness and reinforcement) forshear demand ignoring masonry contribution

FEMA-172


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Adding Jackets to RC Frame Members

Applications

For strengthening non-ductile RC frame memberswhere functional use prohibits new shear walls

Advantages

Minimum loss to floor area Wide variety of choices for jacketing materials

Disadvantages Easy procedure for columns, but cumbersome for

beams and joints

No significant increase in building stiffness

Design Issues Correcting one deficiency may cause other

components vulnerable

A narrow gap at the end column jacket ensures

undesired increase of shear forces resulting fromincrease flexural capacity.Hagio et al 2000

JBDPA, 1990


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Adding Jackets to RC Beams

Design Issues

Flexural capacity of frame is increasedwith jacket and long. And transverse

reinforcement

Beam jackets provide confinement,enhance shear capacity and provide formissing long. Bars

Difficult to jacket the top of beam andslab may have to be drilled

FEMA-172


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Adding Jackets to RC Columns

Design Issues

Flexural capacity offrame is increasedwith jacket and long.And transversereinforcement

Column jacketsprovide confinementand can remedy shortlap splices of existingcolumn

reinforcement.

FEMA-172


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Adding Wing (Side) Walls

For strengthening columns of non-ductileRC frames

Characteristics similar to new shear wall

JBDPA, 1990

Roach & Jirsa, 1986


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Adding Buttresses

Applications

For strengthening non-ductile RC and URM

structures weak in shear strengthAdvantages

Exterior work results in minimal disruption tofunctional use

Disadvantages

Need large vacant space adjacent to building Significantly affect the aesthetics Large resistance from the piles or foundation

of the buttress as it will not be able to

mobilize the dead weightDesign Issues

A load path to transfer shear forces from thebuilding to buttress is required such ascollectors on the interior of the building

Capacity required to resist overturning forcesis small for buttresses away from the building

FEMA-172


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Adding Braces

Applications

For strengthening almost all types of RC, URM and

steel structuresAdvantages

Lightweight causing minimum influence onfoundation and structures mass

Many configurations possible which can allow foropenings, passages, services, etc.

Disadvantages

Steel bracing is usually less stiff than masonry orconcrete buildings, therefore, they have to cracksignificantly before steel braces are effective

Design Issues

Place braces where significant dead weight can bemobilized to overcome overturning forces

Bracing bays will require columns as well horizontal

members as collectors to form complete truss Avoid tension only braces

FEMA-172


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Concentric Braced Frames

Bracing Configuration

CBFs are most efficient system for resisting lateral loadsas they provide complete truss action

Many configuration to choose from Popular chevron bracing impose large flexural demand

on floor beams after buckling of the compression brace.

K bracing is not suitable for resisting seismic loadsbecause buckled braces cause column to deformhorizontally leading to buckling and collapse.

Effects of Brace Buckling

Rapid loss of strength and tension brace overload

Excessive rotation of brace ends and local connection

failure Local or torsional buckling at near mid span

Out-of-plane deformation (bowing)

Non-symmetrical deformation induce large torsionalresponse

Energy dissipation is deficient

Steel Structures


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CBFs and Connections

Design Objectives

Hysteretic behaviour of CBFs is characterized withseverely pinched loops. However, reasonable stabledeformation can be achieved to protect against brittlefailures.

Braces

Stockier braces dissipate more energy than slender

ones. Use Kl/r less than 1900/ fy Use compact sections to avoid local instability

Brace Connections

Connection should be adequate against out-of-plane

failure of gusset plate and brittle fracture Gusset Plate is most critical component of connection:

Enough strength when brace buckles in plane ofthe frame

Provide for formation of hinge line if bracebuckles out-of-plane

Steel Structures


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Underpinning the Footing

Applications

Increase bearing capacity of the footing

Advantages Most effective procedure for excessive soil

pressure due to overturning forces

Many configurations possible which canallow for openings, passages, services, etc.

Disadvantages

Expensive and disruptive Cost effective to change strengthening

scheme so that foundation strengthening is

not requiredDesign Issues

The new footing is constructed in staggeredincrements each increment should bepreloaded by jacking prior to transfer of load

from the existing footing

FEMA-172


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Adding Drilled Piers

Applications

Increase vertical capacity of footing when

soil bearing pressure and uplift is excessiveAdvantages

Most effective procedure for excessive soilpressure due to overturning forces

Disadvantages Expensive and disruptive

Design Issues

RC piers should be cast-in-situ in uncased

holes so as to develop both tension orcompression else use under-rimmed piles

Each RC pier extend above the existingfooting and connected by RC beam throughthe existing wall FEMA-172


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Upgrading Pile Foundation

Applications

For excessive tensile and compressive loads

due to lateral and gravity loadsDisadvantages

Expensive and disruptive

Design Issues

Large footing overlay will be required tocreate new pile cap so that forces can betransferred to new piles

FEMA-172


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Efficacy of Shear Enhancements

Qualitatative indication of improvement in strength and ductility

Sugano 1989

Compared with original bare frame, cast-in-situ wall provides higher strengthand the framed steel brace contributes to both strength and ductility


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Efficacy of Column Enhancements

Any jacketing technique significantly increased strength and ductility

Jacketing without end gaps resulted in decrease of strength after a higher peak

Sugano 1996

Qualitatative indication of improvement in strength and ductility


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Selected References

1. CEB (1995). Fastenings for Seismic Retrofitting: State-of-the-Report, ComiteEuro-

International Du Beton, Thomas Telford, London

2. BSSC(1992). NEHRP handbook for Seismic Rehabilitation of Existing Buildings,

FEMA-172, Building Seismic Safety Council, Washington, D.C.

3. FEMA 308 (1999). Repair of Earthquake Damaged Concrete and Masonry Wall

Buildings. Applied Technology Council, Redwood City, CA.

4. BIS (1993). IS:13935-1993 Repair and Seismic Strengthening of Buildings-

Guidelines, Bureau of Inidan Standards, New Delhi

5. Sugano S. (1996). State-of-the-Art in Techniques for Rehabilitation of Buildings,

11 WCEE, Acapulco, Mexico, Paper no. 2179 on CD-ROM, Elsevier.

6. Wyllie, L.A.(1996). Strengthening Strategies for Improved Seismic Performance,

11 WCEE, Acapulco, Mexico, Paper no. 1424 on CD-ROM, Elsevier.

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19603297 Retrofit PPT DCR