seismic retrofitting strategies of reinforced concrete ... · economic considerations and immediate...

Seismic Retrofitting Methods of Reinforced Concrete Buildings

Pankaj Agarwal, Ph. D

Department of Earthquake EngineeringIndian Institute of Technology Roorkee

Roorkee - 247667

Why Retrofitting?

• Extensive damage to innumerable RC buildings ofvarying degree, causes irreparable loss of life

• As a result frightened occupants may refuse to enter thebuilding unless assured of the safety of building fromfuture earthquakes

• It has been observed that majority of such earthquakedamaged buildings may be safely reused if they areconverted into seismically resistant structures byemploying retrofitting measures

Why Retrofitting?

• This proves to be a better option catering to theeconomic considerations and immediate shelterproblems rather than replacement of buildings

• Moreover, retrofitting of buildings is generally moreeconomical as compared to demolition andreconstruction even in the case of severe structuraldamage

Therefore, seismic retrofitting of buildingstructures is one of the most importantaspects for mitigating seismic hazardsespecially in earthquake prone countries

Definition

Terms are associated to retrofitting with a marginaldifference like

Repair, Strengthening, Remolding, Rehabilitation,

Reconstruction, Re-engineering etc.

The most common definitions of Retrofitting are

To upgrade the earthquake resistance up to the level of the presentday codes by appropriate techniques (IS 13935: 1993)

Increasing the seismic resistance of a damaged building is calledretrofitting (Tomazevic, 1999)

It is an upgrading of certain building system, such as mechanical,electrical, or structural, to improve performance, function, orappearance (Newman, 2001)

Need of Retrofitting

• The buildings have been designed according to a seismiccode, but the code has been upgraded in later years

• Buildings designed to meet the modern seismic codes,but deficiencies exist in the design and /or construction

• Essential buildings must be strengthened like hospitals,historical monuments and architectural buildings

• Important buildings whose service is assumed to beessential even just after an earthquake

• Buildings, the use of which has changed through theyears

• Buildings that are expanded renovated or rebuilt

Problems Associated with Retrofitting

• To obtain sufficient records of buildings

- architectural and structural drawings- structural design calculations- material properties,- details of foundation and geo-technical reports

- records of at least natural period of the buildings etc

• Retrofitting and issues of their structural safety

• Guidelines or codes of practice on retrofitting

• Methods of seismic assessment of existing buildings

Concepts of Retrofitting

• Up gradation of the lateral strength of the structure

• Increase in the ductility of structure

• Increase in strength and ductility

Consideration in Retrofitting of Structures

• Retrofitting principally depends on the horizontal andvertical load resisting system of the structure and thetype of materials used for parent construction

• It also relies on the technology that is feasible andeconomical

• The understanding of mode of failure, structuralbehaviour and weak and strong design aspects exerciseconsiderable influence on selection of retrofittingmethods

• Usually, retrofitting method is aimed at increasing thelateral resistance of the structure

Consideration in Retrofitting of Structures

• To predict initial and final stiffness of the retrofittedstructure need clarification and quantification

• Avoid an irregular stiffness distribution in theretrofitted structure

• It is suggested that the design of retrofitted schemesshould be based on drift control rather than on strengthconsideration alone

• The use of three-dimensional analysis is recommendedto identify and locate the potential weakness of theretrofitted building

Source of Weakness in RC Frame Building

• Discontinuous load path/ interrupted load path/irregularload path

• Lack of deformation compatibility of structural members

• Quality of workmanship and poor quality of material

Structural Damage due to Discontinuous Load Path

• Every structure must have two load resisting systems vertical loadresisting system for transferring the vertical load to ground (b)horizontal load resisting system for transferring the horizontal loadto vertical load system

• It is imperative that the seismic forces should be properly collectedby the horizontal framing system and transferred into vertical lateralresisting system

• Any discontinuity/irregularity in this load path or load transfer maycause one of the major contributions to structural damage duringstrong earthquakes

• In addition it must be ensured that each member both of horizontalor vertical load resisting system must be strong enough and not failduring an earthquake

• All the structural and non-structural elements must have sufficientstrength and ductility and should be well connected to the structuralsystem so that the load path must be complete and sufficientlystrong

Structural Damage due to Lack of Deformation

• Due to limited amount of ductility and the inability toredistribute load

• The most common regions of failure in an existingreinforced concrete frame are


Columns

• In reinforced concrete columns several interactionmechanism influences its lateral load behaviour.

• The main actions that concern are associated with axial,flexure, shear, and bond

Action of concern force and its mode of failure in column


Beams

• In reinforced concrete beams, the major problems exist at theright end, considering seismic forces left to right

• A brittle shear failure could occur due to superposing of shearforces caused by vertical loading and seismic loading

Behaviour of beams for vertical and seismic loading

Quality of Workmanship and Materials

• There are numerous instances where faulty constructionpractices and lack of quality control have contributed tothe damage

• The faulty construction practices may be like, lack ofamount and detailing of reinforcement as per requirementof code particularly when the end of lateral reinforcementis not bent by 135 degrees as the code specified

• Many buildings have been damaged due to poor qualitycontrol of design material strength as specified, spallingof concrete by the corrosion of embedded reinforcing bars,porous concrete, age of concrete, proper maintenance etc.

Classification of Retrofitting Techniques

There are two ways to enhance the seismiccapacity of existing structures

• Structural-level approach of retrofitting

Global modifications to the structural system

• Member level approach or local retrofitting

Increase of the ductility of components with adequatecapacities to satisfy their specific limit states

Classification of Retrofitting Techniques

Retrofitting Strategies for RC Buildings

Structural Level (or Global) Retrofit Methods

Conventional Methods

• Adding New Shear Walls into/onto the ExistingFrames

• Adding Steel Bracing into/onto the Existing Frame

• Adding Infill Walls into/onto the Existing Frames

Non-Conventional Approach

• Seismic Base Isolation/• Supplemental Damping Devices

Retrofitting Strategies for RC Buildings

Member Level (or Local) Retrofit Methods

Jacketing/Confinement

Columns / Beam / Beam-Column Joint / Slab/ Foundations

Adding New Shear Walls

One of the most common methods to increase the lateral strengthof the reinforced concrete buildings - Added elements may becast in place or pre-cast

Increasing strength with shear walls: (a) adding techniques (b) infilling techniques

Technical Considerations

• Determining the adequacy of existing floor and roof to carry theseismic forces

• Transfer of diaphragm shear into the new shear walls with dowels

• Increase in the weight and concentration of shear by the addition ofwall, which may affect the foundations


Constructional Considerations

• To find locations which may align to the full height of the building tominimize torsion

• It is often desirable to locate walls adjacent to the beam betweencolumns so that only minimum slab demolition is required withconnections made to beam at the sides and /of columns

• The reinforcement has to pass through holes in slabs and around thebeams to avoid interference

• Wall thickness also varies from 15 to 25cm (6 to 10 inch) and isnormally placed externally

Limitations

• Increase in lateral resistance but it is concentrated at a few places

• Increased overturning moment at foundation causes very highuplifting that needs either new foundations or strengthening of theexisting foundations

• Increased dead load of the structure

• Excessive destruction at each floor level results in functionaldisability of the buildings

• Possibilities of adequate attachment between the new walls and theexisting structure

• Closing of formerly open spaces can have major negative impact onthe interior of the building uses or exterior appearance


Adding Steel BracingSteel bracing has a potential advantage over other schemes for the

following reasons

• Higher strength and stiffness

• Opening for natural light can be made easily

• Amount of work is less since foundation cost may be minimized

• The bracing system adds much less weight to the existing structure

• Most of the retrofitting work can be performed with prefabricatedelements and disturbance to the occupants may be minimized

Technical Considerations

• It has performed well-exhibited linear behavior even up to twice thedesign code force

• The effective slenderness ratio of brace should be kept relatively lowso that braces are effective in compression as well as tension,suggested l/r ratio are 80 to 60 or even lower

• Collector’s members are recommended for transferring forcesbetween the frame and bracing system

• Careful consideration of connections of strengthening elements to theexisting structures and to the foundations have to be consciouslydesigned to ensure proper shear transfer

• Local reinforcement to the columns may be needed to bear theincreased load generated on them

• The epoxies threaded rods have proved to be quite effective inconnecting the bracing system to the concrete frame and intransferring the forces

Adding Steel Bracing

Limitations

• Lack of information about the seismic behavior of the addedbracing;

• Steel bracing system may be sensitive to construction errors oromissions, which cause reduction in member capacity at a section

• A moderate to high level of skilled labour is necessary forconstruction, due to the need for member fit-up adjustment andwelding

• Close quality control particularly with respect to welding isessential

Adding Steel Bracing

Adding Infill Walls

• It is an effective and economical method for improvingstrength and reducing drift of existing frames

• Relatively strong masonry infill may result in a failure of thecolumns of existing frame

Column lap splices subjected to large axial force due to frame wall action

Technical Consideration

• Mode 1: Weak columns, strong beams and strong infill - failure occursin the columns followed by crushing of infill in the compressivecorners

• Mode 2: Strong columns, weak beams and strong infill - failure occursin the beam again followed by crushing of infill

• Mode 3: Strong columns, strong beams and weak infill - failure occurswhen corner crushing extends diagonally followed by frame jointfailure.

Limitations

• The benefit of retrofitting by infill walls is often limited by failure ofsplices in existing columns, which act as boundary elements for newinfill walls

Adding Infill Walls

Non-Conventional Approach

Seismic Base Isolation

It is a powerful and relatively cheaper method of seismic rehabilitation of buildings

Advantages

• Better protection against earthquake due to thedecreasing of shears

• Superstructure will need no reinforcement

• Foundation system will not need any reinforcement toresist the overturning moments

• Least interrupting the building activities, since the workis carried out in the basement

• Least temporary work is required


Process of seismic retrofitting by base isolation in mid storey isolation


• A typical base isolation system is evolved by the use of rubberbearing located at the base of the building, most often justbelow the first floor, under columns or shear walls

• Rubber bearing consists of laminated layers of rubber andsteel plates strongly bound together during the vulcanizingprocess of rubber

• They are designed with a vertical stiffness, which is usually300 to 1000 times higher than the horizontal stiffness

• Such a system increases the first natural period in both thehorizontal directions in between the range of 1 to 2.5 secondsand the response acceleration decreases accordingly

• Damping is usually comprised between 5% to 10% critical, butcan jump to as high as 20% with the addition of damper

Supplemental Damping Devices

• Use of supplemental damping may be an effective method to resistseismic force

• The most commonly used approaches to add supplementaldampers to a structure are installing of viscous damper or visco-elastic damper, frictional damper, and hysteretic dampers ascomponents of braced frames

Supplemental Damping Devices

Member Level (or Local) Retrofit Methods

Jacketing/ Confinement

• Jacketing is the most oftenly used and one of the most popularmethods for strengthening

• Most common types of jackets are steel jacket, reinforced concretejacket, fiber reinforced polymer composite jacket, jacket with hightension materials like carbon fiber, glass fiber etc.

• The main purposes of jacketing are: (i) to increase concreteconfinement by transverse fiber/ reinforcement, especially for circularcross-sectional columns, (ii) to increase shear strength

• Transverse fiber should be wrapped all around the entirecircumference of the members possessing close loops sufficientlyoverlapped or welded in order to increase concrete confinement andshear strength

Jackets

• Generally two type of jackets – circular / rectangular

• Circular cross-section will get better confinement than rectangularcross-section

• Where square or rectangular cross-sections are to be jacketed,circular/oval/elliptical jackets are most oftenly used and the spacebetween the jacket and column is filled with concrete

• Such types of multi-shaped jackets provide a high degree ofconfinement by virtue of their shape to the splice region proving tobe more effective

• Rectangular jackets typically lack the flexural stiffness needed tofully confine the concrete

• However, circular and oval jackets may be less desirable due to (i)need of large space, (ii) where an oval or elliptical jacket hassufficient stiffness to confine the concrete along the longdimension of the cross-section is open to question

Jacket

Various shapes of retrofitting jackets

Jacketing of Columns

Reinforced Concrete Jacketing

Steel Jacketing


Strap Jacketing

A narrow gap should be provided to prevent any possible increase in flexure capacity


• Jacketing of columns consists of added concrete withlongitudinal and transverse reinforcement around the existingcolumns

• This type of strengthening improves the axial and shearstrength of columns while the flexural strength of column andstrength of the beam-column joints remain the same

• Jacketing of columns is not successful for improving theductility

• A major advantage is that it improves the lateral load capacityof the building in a reasonably uniform and distributed wayand hence avoiding the concentration of stiffness as in thecase of shear walls

• Major strengthening of foundations may be avoided

Jacketing of Beams

• Jacketing of beams is recommended for several purposes as itgives continuity to the columns and increases the strength andstiffness of the structure

• In jacketing of a beam its flexural resistance must be carefullycomputed to avoid the creation of a strong beam-weak columnsystem

• In the retrofitted structure, there is a strong possibility ofchange of mode of failure and redistribution of forces as aresult of jacketing of column, which may consequently causebeam hinging

• The location of the beam critical section and the participationof the existing reinforcement should be taken intoconsideration

• Jacketing of beam may be carried out under different ways themost common are one-sided jackets or 3 and 4-sided jackets

• The beam should be jacketed through its whole length

Beam Jacketing

Four sided jacketing

Three sided jacketing

Beam Jacketing

Continuity of longitudinal steel in jacketed beams

Jacketing of Beam – Column Joint

• Due to lack of space in the joint region it is difficultenough to provide an adequate confinement

• Alcocer, 1992 has assessed experimentally the behavior ofseveral beam columns sub assemblages, where the jointis confined with a steel cage

• Test results have indicated that jacketing has beeneffective in rehabilitating the joint, with improving thestrength, stiffness and energy dissipation characteristicsof the existing joint

Beam – Column Joint Jacketing

Steel cage assembled in the joint

Comparative Analysis of Methods of Retrofitting

Typical load displacement relationship for different retrofitting techniques

Feasibility Study of Strengthened One-storey Frame

Strengthened Schemes

Construction Cost Structural Capacity

Workability Weight Stiffness Strength Ductility

Infilled concrete wall

Much work Heavy(1.000

Cheap(1.00)

High(1.00)

High(1.00)

Low(1.00)

Infilled concrete

block wall

Easy work Heavy(1.000

Slightlyexpansive

(1.61)

Low(0.30)

Low(0.30)

Low(1.13)

Compressionbrace

Simple connection,easy work

Light(0.39)

Slightlyexpansive

(1.47)

Low(0.27)

Low(0.63)

High(1.70)

Tension Brace Easy Work,

accuracy needed

Light(0.44)

Expansive(2.93)

Low(0.24)

Low(0.67)

High(1.70)

Comparison of Alternative Retrofit SchemesPartial list of retrofit

schemesBase Isolation Braced

FramesExternal

Shear wallsJacketing Do nothing

Seismic Risks @ MCE

Life safety - Injury Minor Moderate Moderate Moderate Extensive

Life loss Not Expected Not Expected Not Expected Not Expected Some

Equipment damage Minor Moderate Moderate Moderate Extensive

Business Interruption Hours - Days Weeks Weeks Weeks-Months

Monthsor Relocation

Construction

Business Impact Low Medium Medium High --

Architectural Impact Low – Mod. Low – Mod. High Low --

Schedule (Years) 3 1.75 2 1.5 --

Project Cost (Ratios) 2.2 1.0 1.2 1.0 --

Impact of Eng. Uncertainties

Ground Motion High Medium Medium Low --

Design and analysis Low Low Low Low --

Constructibility Medium Low Low Medium --

History of performance in Earthquakes

Some Moderate Extensive Some Extensive

seismic retrofitting strategies of reinforced concrete ... · economic considerations and immediate...

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