Seismic Rehabilitation of Structures

Durgesh C. RaiAssistant Professor

Department of Earthquake EngineeringUniversity of Roorkee

Roorkee 247 667

Rehabilitation Strategies & Measures

Structural Enhancements

SeismicRehabilitation

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

Repair Methods

Cosmetic repairs only improve the visual appearance of component damage and may restore non-structural properties (weather protection) but any structural benefit is negligible.

Structural repairs intends to restore structural properties.

Repairs

Cosmetic Repairs

Structural Repair

Surface Coating,

Repointing

Crack injection with epoxy

Crack injection with grout

Spall repair

Rebar replacement

Wall replacement

Repair Methods

Cosmetic repairs only improve the visual appearance of component damage and may restore non-structural properties (weather protection) but any structural benefit is negligible.

Structural repairs intends to restore structural properties.

Repairs

Cosmetic Repairs

Structural Repair

Surface Coating,

Repointing

Crack injection with epoxy

Crack injection with grout

Spall repair

Rebar replacement

Wall replacement

Seismic Strengthening

Seism

icSt

reng

then

ing

Increase strength

Increase strength & ductility

Backup structure

Infill existing frames

Brace existing frames

Install shear walls

Jacket existing members

Increase ductility

Peripheral framesButtresses

Cast-in-situ concretePrecast concrete panelsBrick/block infills

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

Cast-in-situ concretePrecast concrete panel

Steel encasementSteel strapsConcrete or mortarCarbon fibre

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 and equipment

Adding New Shear Walls

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

foundationAdvantages• Adds significant strength and stiffness to framed

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

on soft soils and in pile-supported structuresDesign Guidelines• Locate so that they align full height of the building,

minimize torsion and can be easily tied with existing frame

• Maximize the dead weight that wall can mobilize to resist overturning uplift.

FEMA-172

Adding New Shear Walls

Design Guidelines• It is economical to locate shear walls along

existing framing lines in order to provide boundary members, collectors and dead load to help resist overturning forces.

• On the interior the shear wall continues through the slab and it should be cast in 2 pours 48 hours apart to avoid sagging away of concrete from the underside of the concrete slab.

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

• Functional consideration dictate the location as they break up the interior space

FEMA-172

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 framed

structures Disadvantages• Add considerable mass to the structure and need

new 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 tied with existing frame

• Maximize the dead weight that wall can mobilize to resist overturning uplift.

• Insure concrete/mortar is placed tight to overhead beam else column shearing my result.

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 need

new footings between existing spread footings over 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 may

be necessary after filling the opening

FEMA-172

Adding Shotcrete to Existing Masonry

Applications• Ideal for URM when masonry is not strong or its

in-plane shear strength is weakened by large openings

Advantages• Comparable stiffness to existing URM walls• With epoxied dowels at about 600 mm each way,

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

Disadvantages• Messy with rebound on the inside face and

transferring through floor system is difficult and may require review of foundation details

Design Guidelines• Provide enough shotcrete so that failure of

unreinforced section can be prevented• Design shotcrete (thickness and reinforcement) for

shear demand ignoring masonry contribution

FEMA-172

Adding Jackets to RC Frame Members

Applications• For strengthening non-ductile RC frame members

where 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 from increase flexural capacity.

Hagio et al 2000

JBDPA, 1990

Adding Jackets to RC Beams

Design Issues• Flexural capacity of frame is increased

with jacket and long. And transverse reinforcement

• Beam jackets provide confinement, enhance shear capacity and provide for missing long. Bars

• Difficult to jacket the top of beam and slab may have to be drilled

FEMA-172

Adding Jackets to RC Columns

Design Issues• Flexural capacity of

frame is increased with jacket and long. And transverse reinforcement

• Column jackets provide confinement and can remedy short lap splices of existing column reinforcement.

FEMA-172

Adding Wing (Side) Walls

For strengthening columns of non-ductile RC framesCharacteristics similar to new shear wall

JBDPA, 1990

Roach & Jirsa, 1986

Adding Buttresses

Applications• For strengthening non-ductile RC and URM

structures weak in shear strengthAdvantages• Exterior work results in minimal disruption to

functional useDisadvantages• 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 weight

Design Issues• A load path to transfer shear forces from the

building to buttress is required such as collectors on the interior of the building

• Capacity required to resist overturning forces is small for buttresses away from the building

FEMA-172

Adding Braces

Applications• For strengthening almost all types of RC, URM and

steel structuresAdvantages• Lightweight causing minimum influence on

foundation and structures mass• Many configurations possible which can allow for

openings, passages, services, etc. Disadvantages• Steel bracing is usually less stiff than masonry or

concrete buildings, therefore, they have to crack significantly before steel braces are effective

Design Issues• Place braces where significant dead weight can be

mobilized 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

Concentric Braced Frames

Bracing Configuration• CBFs are most efficient system for resisting lateral loads

as 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 loads

because buckled braces cause column to deform horizontally 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 torsional

response• Energy dissipation is deficient

Steel Structures

CBFs and Connections

Design Objectives• Hysteretic behaviour of CBFs is characterized with

severely pinched loops. However, reasonable stable deformation can be achieved to protect against brittle failures.

Braces• Stockier braces dissipate more energy than slender

ones. Use Kl/r less than 1900/ fy

• Use compact sections to avoid local instabilityBrace 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 of

the frame • Provide for formation of hinge line if brace

buckles out-of-plane

Steel Structures

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 can

allow for openings, passages, services, etc. Disadvantages• Expensive and disruptive • Cost effective to change strengthening

scheme so that foundation strengthening is not required

Design Issues• The new footing is constructed in staggered

increments each increment should be preloaded by jacking prior to transfer of load from the existing footing

FEMA-172

Adding Drilled Piers

Applications• Increase vertical capacity of footing when

soil bearing pressure and uplift is excessive Advantages• Most effective procedure for excessive soil

pressure due to overturning forcesDisadvantages• Expensive and disruptive

Design Issues• RC piers should be cast-in-situ in uncased

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

• Each RC pier extend above the existing footing and connected by RC beam through the existing wall FEMA-172

Upgrading Pile Foundation

Applications• For excessive tensile and compressive loads

due to lateral and gravity loads

Disadvantages• Expensive and disruptive

Design Issues• Large footing overlay will be required to

create new pile cap so that forces can be transferred to new piles

FEMA-172

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 strength and the framed steel brace contributes to both strength and ductility

Efficacy of Column Enhancements

Any jacketing technique significantly increased strength and ductilityJacketing without end gaps resulted in decrease of strength after a higher peak

Sugano 1996

Qualitatative indication of improvement in strength and ductility

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.