retrofit ppt dcr
TRANSCRIPT
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.