Retrofitting Of Buildings

8/7/2019 Retrofitting Of Buildings

1/39

RETROFITTING OF

EARTHQUAKE AFFECTED

BUILDINGS


2/39

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


3/39

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.


4/39

WHEN & FOR WHAT ???

CIRCUMSTANCES

(i ) earthquake damaged buildings( ii) earthquake vulnerable buildings.


5/39

WHY RETROFITTING This proves to be a better option

cateringto the economic considerations and

immediate shelter problems rather than

replacement of buildings


6/39

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.


7/39

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.


8/39

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.


9/39

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


10/39

Methodologies

(i) qualitative methods

(ii) analytical methods


11/39

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.


12/39

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.


13/39

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.


14/39

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


15/39

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.


16/39

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


17/39

CONCRETE

RETROFITTING


18/39

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


19/39

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.


20/39

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


21/39

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


22/39

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.


23/39

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.


24/39

STRUCTURAL DAMAGE

DUE TO LACK

OF DEFORMATION

The most common regions of failure

in an existing reinforced concrete frame.


25/39

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.


26/39

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.


27/39

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


28/39

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.


29/39

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:


30/39

(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


31/39

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.


32/39

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.


33/39

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.


34/39


35/39

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.


36/39

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


37/39

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.


38/39


39/39

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.

Documents

Retrofitting Of Buildings