retrofitting of buildings
TRANSCRIPT
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RETROFITTING OF
EARTHQUAKE AFFECTED
BUILDINGS
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WHAT IS RETROFITTING
IS 13935To upgrade the earthquake resistence up to the
level
of the level of the present day codes by approriate
techniques.
CEB 1995
Concepts including strengthening, repairing and
remoulding
Newman , 2001
It is an upgrading of certain building system, such
as mechanical, electrical, or structural, to improve
performance, function or appearance
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Seismic retrofitting is the modification of
existing structures to make them more
resistant
to seismic activity, ground motion, or soil
failure
due to earthquakes.
The retrofit techniques are also applicablefor
other natural hazards such as tropical
cyclones,
tornadoes, and severe winds from
thunderstorms.
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WHEN & FOR WHAT ???
CIRCUMSTANCES
(i ) earthquake damaged buildings( ii) earthquake vulnerable buildings.
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WHY RETROFITTING This proves to be a better option
cateringto the economic considerations and
immediate shelter problems rather than
replacement of buildings
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RETROFIT PERFORMANCE
OBJECTIVES With the development of Performance based earthquakeengineering (PBEE), several level of performance
objectives are
gradually recognized:
PUBLIC SAFETY ONLY. The goal is to protecthuman life, ensuring
that the structure will not collapse upon its occupants orpassers
by, and that the structure can be safely exited. Under severe
seismic conditions the structure may be a total economic
writeoff,
requiring teardown and replacement.
STRUCTURE SURVIVABILITY. The goal is thatthe structure, while
remaining safe for exit, may require extensive repair (but
not
replacement) before it is generally useful or considered safe
for
occupation. This is typically the lowest level of retrofit
applied to
bridges.
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RETROFIT PERFORMANCE
OBJECTIVES
STRUCTURE FUNCTIONALITY.Primary structure
undamaged and the structure is
undiminished in
utility for its primary application. A high
level ofretrofit, this ensures that any required
repairs are
only "cosmetic" for example, minor cracks
in
plaster , drywall and stucco. This is theminimum
acceptable level of retrofit for hospitals.
STRUCTURE UNAFFECTED. Thislevel of retrofit is
preferred for historic structures of highcultural
significance.
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NEED IN EARTHQUAKE VULNERABLE
BUILDINGS (a) the buildings have been designed according toa seismiccode, but the code has been upgraded in later years;
(b) buildings designed to meet the modernseismic codes,but deficiencies exist in the design or construction;
(c) essential buildings must be strengthened like
hospitalshistorical monuments and architectural buildings;
(d) important buildings whose service is assumedto beessential even just after an earthquake;
(e) buildings the use of which has changed
through the years; (f) buildings that are expanded, renovated orrebuilt.
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SEISMIC EVALUATION OF
BUILDINGS to assess the seismic capacity ofearthquakevulnerable buildings or earthquake
damaged
buildings for the future use.
helpful for degree of interventionrequired inseismically deficient structure
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Methodologies
(i) qualitative methods
(ii) analytical methods
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QUALITATIVE
METHODS based on the background informationavailable of the building and its
construction sitearchitectural and structural drawings
past performance of similar buildings undersevere earthquakes,visual inspection report,
some nondestructive test results.
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ANALYTICAL
METHOD
Based on the consideration of the capacity and ductility of
buildings on the basis of available drawings.
METHODS
Capacity/Demand(C /D) method,
Screening method,
Pushover analysis,
Nonlinear inelastic analysis etc.
Evaluation procedure should be very simple and immediate
based on synthetic information that can prove suitable for
risk evaluation on largepopulations.
Therefore, qualitative evaluation of the buildings is
generally
being carried out.
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COMPONENTS OF SEISMICEVALUATION
METHODOLOGY1.CONDITION ASSESSMENT basedon(i) data collection or information gatheringof
structures from architectural and structuraldrawings
(ii) performance characteristics of similartypeofbuildings in past earthquakes
(iii) rapid evaluation of strength ,drift,materials,structural components and structural details.
used basically for undamaged existingstructures.
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2. VISUAL INSPECTION/FIELDEVALUATION
based on observed distress and damage instructures.
Visual inspection is more useful fordamagedstructures however it may also be conducted
for
undamaged structures
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3. NONDESTRUCTIVEEVALUATION (NDE)
for quick estimation of materials strength,
determination of the extent of deterioration
to establish causes
used for preparation of drawing in case ofnon
availability.
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SOURCE OF WEAKNESS IN
RC FRAME
BUILDING (i) discontinuous load path/interruptedloadpath/irregular load path
(ii) lack of deformation compatibilityofstructural members
(iii) quality of workmanship and poorquality
of materials
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CONCRETE
RETROFITTING
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BASIC CONCEPTS at
(CEB, 1997): (a) up gradation of the lateral strengthof thestructure;
(b) increase in the ductility of structure;
(c) increase in strength and ductility.It is suggested that the cost of retrofitting
of a
structure should remain below 25% of the
replacement as major justification of
retrofitting
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CONSIDERATION IN
RETROFITTING OF
STRUCTURESMETHOD USED depends on
the horizontal and vertical loadresisting
system of the structure the type of materials used for parentconstruction.
On the technology that is feasible andeconomical.
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CONSIDERATION IN
RETROFITTING OF
STRUCTURES SELECTION OF RETROFITTINGMETHODS OFBUILDINGS ALSO DEPENDS ON: as derived from the earthquake damage surveys by
understanding of mode of failure,structural behavior
weak and strong design aspects
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SOURCE OF WEAKNESS IN
RC FRAME
BUILDING (i) discontinuous load path/interruptedloadpath/irregular load path
(ii) lack of deformation compatibilityofstructural members
(iii) quality of workmanship and poorquality
of materials
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STRUCTURAL DAMAGE
DUE TO
DISCONTINUOUS LOAD
PATH seismic forces should be properlycollected by
the horizontal framing system andproperly
transferred into vertical lateral resisting
system
discontinuity/irregularity in this load
path orload transfer may cause structural
damage
during strong earthquakes.
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STRUCTURAL DAMAGE
DUE TO LACK
OF DEFORMATION MAIN PROBLEMSlimited amount ofductility and
the inability to redistribute load in order tosafely withstand the deformations imposed
upon
in response to seismic loads.
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STRUCTURAL DAMAGE
DUE TO LACK
OF DEFORMATION
The most common regions of failure
in an existing reinforced concrete frame.
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Beams
In reinforced concrete beams, the majorproblems exist at the right end,
considering
seismic forces left to right
A brittle shear failure could occur due
tosuperposing of shear forces caused by
vertical loading and seismic loading.
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Beam
column joints in case of strong columnweak beambehaviour,the joint may be heavily stressed after beamyielding and diagonal cracking may be formed
in
the connection.
Wide flexural cracks may develop at thebeam endpartially attributable to the slip of beamreinforcement within the connection.
Such shear cracking may reduce thestiffnessof a building.
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QUALITY OF WORKMANSHIP
AND MATERIALS
faulty construction practiceslack of amount and detailing of reinforcement as perrequirement of code
the end of lateral reinforcement is not bent by 135 degree
lack of quality controlof design material strength as specified,
spalling of concrete by the corrosion of embeddedreinforcingbars,
porous concrete,
age of concrete,
Proper maintenance etc
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RETROFITTING
STRATEGIES FOR RCBUITDINGS Structural Level (or Global) RetrofitMethods Two approaches
(i) conventional methods based onincreasing the seismic resistance of existing
structure
(ii) nonconventional methods.basedonreduction of seismic demands.
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CONVENTIONAL
METHODS ADDING NEW SHEAR WALLSFrequently used for retrofitting of nonductilereinforced concrete frame buildings.
The added elements can be either castinplace orprecast concrete elements.
New elements preferably be placed at theexteriorof the building.
Not preferred in the interior of the structure
toavoid interior mouldings.
TECHNICAL CONSIDERATIONS:
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(a) determining the adequacy of existing floorandroof slabs to carry the seismic forces;
(b) transfer of diaphragm shear into the newshearwalls with dowels;
(c) adding new collector and drag members tothediaphragm;
(d) increase in the weight and concentration ofshear by the addition of wall which may affect the
foundations
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CONSTRUCTIONALCONSIDERATION
to find locations where walls can be added and welllocatedwhich may align to the full height of the building to
minimize
torsion
desirable to locate walls adjacent to the beambetween
columns so that only minimum slab demolition isrequired
with connections made to beam at the sides of
columns
The longitudinal reinforcement must be placed atthe ends of
the wall running continuously through the entireheight.
the reinforcement has to pass through holes in slabsandaround the beams to avoid interference.
Wall thickness also varies from 15 to 25 cm (6 to10 inch) and
is normally placed externally.
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LIMITATIONS:
(i) increase in lateral resistance but it is concentrated at afew
places ( ii) increased overturning moment at foundation causesvery highuplifting that needs either new foundations or strengthening
of the
existing foundations,
(iii) increased dead load of the structure
(iv) excessive destruction at each floor level results infunctionaldisability of the buildings
(v) possibilities of adequate attachment between the newwallsand the existing structure,
(vi) closing of formerly open spaces can have majornegativeimpact on the interior of the building or exterior
appearance.
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ADDING STEEL
BRACINGS an effective solution when large openings arerequired. Potential advantage over other schemes for thefollowing reasons:
higher strength and stiffness , can be proved,
opening for natural light can be made easily,
amount of work is less since foundation cost maybe
minimized,
the bracing system adds much less weight to theexisting
structure,most of the retrofitting work can be performed withprefabricated elements and disturbance to the
occupants
may be minimized.
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TECHNICAL CONSIDERATIONS
can be used for steel structures as well asconcrete structure. The effective slenderness ratio of bracekeptrelatively low so that braces are effective
in
compression as well as tension,
suggested ratio are 80 to 60 or evenlower Collector's members are recommendedfor
transferring forces between the frame andbracing system.
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CONSTRUCTIONAL CONSIDERATION
The available dead load of structure has to be
considered to determine the amount or number of bays ofbracingthat can be mobilized to resist overturning
uplift, as steel bracing is relatively light.Bracing bays
usually require vertical columns at ends to resist overturning forces to work vertically,aschords of a cantilever truss are arranged
horizontally at each floor level. It is to beconnected to
the horizontal diaphragms by collectors
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CASE STUDY: SEISMIC RETROFITTING OF RC
BUILDING BY STEEL
BRACING AND INFILL WALLS
Typical features of the building
Number of stories six stories consisting of a basement, groundlevel with five upper floors
Lateral load resisting systems reinforced concrete frames
Floor system waffle slab
Foundation system mat foundation with retaining walls
around the perimeter, friction piles were placed under the mat
foundation
Features of Damages in Mexico Earthquake, 1985
No significant damage during the earthquake. Only minor
damage to
nonstructural walls foundation performed well.
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Retrofitting Techniques Employed
Although there is no significant damage
retrofitting was done for future events and to
eliminate the damage in nonstructural elements.
Steel bracing in transverse direction.
Infilled masonry walls were reinforced to stiffen
the structure in the longitudinal direction.Expected Performance
Analysis was performed to verify that the
upgraded structure could resist the code loads
The bracing frames were designed in such a
way
that they would carry all the lateral loads while
the existing structure was considered to carry all
the vertical loads.