TECHNICAL SEMINAR REPORTON

SEISMIC RETROFITTING TECHNIQUES

In Partial Fulfillment of theAcademic requirements for the award of Bachelor of Technology inCivil EngineeringSubmittedByD.CHANDANA VARMA (10J41A0168 )

MALLA REDDY ENGINEERING COLLGE (Afillated to JNTU, Hyderabad) Maisammaguda, Dhulapally ( Post Via Kompally) Secunderabad ,500100

SEISMIC RETROFITTING TECHNIQUES

AbstractMany existing structures located in seismic regions are inadequate based on thecurrent seismic design codes. In addition, a number of major earthquakes during recentyears have underscored the importance of mitigation to reduce seismic risk.

Seismic retrofitting of existing structures is one of the most effective methods of reducing this risk. In recent years, a significant amount of research has been devoted to the study of variousstrengthening techniques to enhance the seismic performance of RC structures. However, the seismic performance of the structure may not be improved by retrofitting orrehabilitation unless the engineer selects an appropriate intervention technique based onseismic evaluation of the structure. Therefore, the basic requirements of rehabilitationand investigations of various retrofit techniques should be considered before selectingretrofit schemes.

The purpose of this report is to present the various seismic retrofitting characteristics and technical aspects of the major intervention methods for reinforced concrete (RC) building structures.

1. INTRODUCTION

Seismic retrofittingis the modification of existingstructuresto make them more resistant toseismic activity, ground motion, orsoilfailure due toearthquakes. With better understanding of seismic demand on structures and with our recent experiences with large earthquakes near urban centers, the need of seismic retrofitting is well acknowledged.

The aftermath of an earthquake manifests great devastation due to unpredicted or slight. This damage to structures in its turn causes irreparable loss of life with a striking extensive damage to innumerable buildings of varying degree i.e. either full or part of casualties. As a result frightened occupants may refuse to enter the building unless assured of the safety of the building from future earthquakes. It has been observed that majority of such earthquake damaged buildings may be safely reused, if they are converted into seismic resistant structures by employing a few retrofitting measures. This proves to be a better option catering to the economic considerations and immediate shelter problems rather than replacement of buildings. Moreover it has often been seen that retrofitting of buildings is generally more economical as compared to demolition and reconstruction even in the case of severe structural damage. Therefore, seismic retrofitting of building structures is one of the most important aspects for mitigating seismic hazards especially in earthquake-prone countries. Various terms are associated to retrofitting with a marginal difference like repair, strengthening, retrofitting, remoulding, rehabilitation, reconstruction etc. but there is no consensus on them.The need of seismic retrofitting of buildings arises under two circumstances: (i) earthquake damaged buildings and (ii) earthquake-vulnerable buildings that have not yet experienced severe earthquakes. The problems faced by a structural engineer in retrofitting earthquake damage buildings are: (a) lack of standards for methods of retrofitting; (b) effectiveness of retrofitting techniques since there is a considerable dearth of experience and data on retrofitted structures: (c) absence of consensus on appropriate methods for the wide range of parameters like type of structures, condition of materials, type of damage, amount of damage, location of dam4e, significance of damage, condition under which a damaged element can be retrofitted etc. Buildings have been designed according to a seismic code, but the codes have been upgraded in the later years. Essential buildings must be strengthened like hospitals, historical monuments and architectural buildings. Buildings that are expanded, renovated or rebuilt.Theretrofittechniques outlined here are also applicable for other natural hazards such astropical cyclones,tornadoes, and severewindsfromthunderstorms. Whilst current practice of seismic retrofitting is predominantly concerned with structural improvements to reduce the seismic hazard of using the structures, it is similarly essential to reduce the hazards and losses from non-structural elements. It is also important to keep in mind that there is no such thing as an earthquake-proof structure, althoughseismic performancecan be greatly enhanced through proper initial design or subsequent modifications.A large number of existing buildings in India are severely deficient against earthquake forces and the number of such buildings is growing very rapidly. This has been highlighted in the past earthquake. Retrofitting of any existing building is a complex task and requires skill, retrofitting of RC buildings is particularly challenging due to complex behavior of the RC composite material. The behavior of the buildings during earthquake depends not only on the size of the members and amount of reinforcement, but to a great extent on the placing and detailing of the reinforcement. The construction practices in India result in severe construction defects, which make the task of retrofitting even more difficult. There are three sources of deficiencies in a building, which have to be accounted for by the retrofitting engineer: (i) inadequate design and detailing, (ii) degradation of material with time and use, and (iii) damage due to earthquake or other catastrophe. The retrofit engineer is expected to estimate the deficiency resulting from all the three sources, suggest a retrofit scheme to make up for the deficiencies and demonstrate that the retrofitted structure will be able to safety resist the future earthquake forces expected during the lifetime of the structure. This paper presents a brief review of the available methods and techniques for retrofitting of RC building.

2. PERFORMANCE OBJECTIVES

In the past, seismic retrofit was primarily applied to achieve public safety, with engineering solutions limited by economic and political considerations. However, with the development of Performance based earthquake engineering(PBEE), several levels of performance objectives are gradually recognised: Public safety only. The goal is to protect human life, ensuring that the structure will not collapse upon its occupants or passersby, and that the structure can be safely exited. Under severe seismic conditions the structure may be a total economic write-off, requiring tear-down and replacement. Structure survivability. The goal is that the 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. Structure functionality. Primary structure undamaged and the structure is undiminished in utility for its primary application. A high level of retrofit, this ensures that any required repairs are only "cosmetic" - for example, minor cracks inplaster,drywallandstucco. This is the minimum acceptable level of retrofit forhospitals. Structure unaffected. This level of retrofit is preferred for historic structures of high cultural significance.

3. EARTHQUAKE DAMAGE AND HERITAGE STRUCTURES

Typical earthquake damage to most older and historic buildings results from poor ductility--or flexibility--of the building and, specifically, poor structural connections between walls, floors, and foundations combined with the very heavy weight and mass of historic materials that are moved by seismic forces and must be resisted.

In buildings that have not been seismically upgraded, particularly unreinforced masonry buildings, parapets, chimneys, and gable ends may dislodge and fall to the ground during a moderate to severe earthquake. Walls, floors, roofs, skylights, porches, and stairs which rely on tied connections may simply fail. Interior structural supports may partially or totally collapse. Unreinforced masonry walls between openings often exhibit shear (or diagonal) cracking.

Upper stories may collapse onto under-reinforced lower floors with large perimeter openings or atriums. Unbraced infill material between structural or rigid frame supports may dislodge. Adjacent buildings with separate foundations may move differently in an earthquake creating damage between them. Poorly anchored wood frame buildings tend to slide off their foundations. Factors influencing damage in an earthquake

4. CONDITION OF EXISTING BUILDING

Much of the damage that occurs during an earthquake is directly related to the building's existing condition and maintenance history. Well-maintained buildings, even without added reinforcement, survive better than buildings weakened by lack of maintenance. The capacity of the structural system to resist earthquakes may be severely reduced if previous alterations or earthquakes have weakened structural connections or if materials have deteriorated from moisture, termite, or other damage. Furthermore, in unreinforced historic masonry buildings, deteriorated mortar joints can weaken entire walls. Maintenance, which reduces moisture penetration and erosion of materials, is therefore essential. Since damage can be cumulative, it is important to analyze the structural capacity of the building. Over time, structural members can become loose and pose a major liability. Unreinforced historic masonry buildings typically have a friction-fit connection between horizontal and vertical structural members, and the shaking caused by an earthquake pulls them apart. With insufficient bearing surface for beams, joists, and rafters against the load bearing walls or support columns, they fail. The resulting structural inadequacy may cause a partial or complete building collapse, depending on the severity of the earthquake and the internal wall configuration. Tying the building together by making a positive anchored or braced connection between walls, columns, and framing members, is key to the seismic retrofit of historic buildings. 5. SEISMIC RETROFITTING AND BUILDING CONSERVATION

Heritage structures can be divided mainly from earthquake point of view in to two main categories: 1. Undamaged or before earthquake heritage structures: The retrofitting needed for these of structures is mainly to increase their seismic strength through reinforcement of structural members. This type of approach is known as conservation approach. 2. Damaged or after earthquake heritage structures: For earthquake damaged heritage structure retrofitting techniques are adopted to restore and rehabilitate the damaged structure. This is done by strengthening walls, columns, and overall fabric of the structure without damaging the character of the building. In case of partial or full collapse of the portion of the building it is reconstructed by use of as much historic material is available as possible and after proper documentation old structure from available records. 6. RETROFITTING STRATEGIES FOR RC BUILDINGS

The need for retrofitting or strengthening of earthquake-damaged or earthquake-vulnerable buildings in India have been tremendously increased during recent years after the devastating Bhuj earthquake with an alarming awakening for sufficient preparedness in anticipation to face future earthquakes. Many professional engineers are accustomed to the designing of new buildings but they may find themselves not fully equipped to face the challenges posed at the time of strengthening the existing buildings with a view to improve their seismic performance. This section presents the most common devices for retrofitting of reinforced concrete buildings with technical details, constructional details and limitations.

7. METHODS FOR SEISMIC RETROFITTING OF STRUCTURES

Retrofitting of existing structures with insufficient seismic resistance accounts for amajor portion of the total cost of hazard mitigation. Thus, it is of critical importance thatthe structures that need seismic retrofitting are identified correctly, and an optimalretrofitting is conducted in a cost effective fashion. Once the decision is made, seismicretrofitting can be performed through several methods with various objectives such asincreasing the load, deformation, and/or energy dissipation capacity of the structure(FEMA, 2000). Conventional as well as emerging retrofit methods are briefly presentedin the following subsections.

7.1 CLASSIFICATION OF RETROFITTING TECHNIQUES

There are two ways to enhance the seismic capacity of existing structures.. The second is a member level approach of retrofitting or local retrofitting which deal, with an increase of the ductility of components with adequate capacities to satisfy their specific iirnit states. Based On the above concept the available techniques of retrofitting of reinforced concrete buildings may be classified as:

Retrofitting Techniques

LocalGlobal

Adding new shear wallJacketing of beams

Jacketing of columnsAdding steel bracing

Wall thickening

Base isolation

Generally structural level retrofittings are applied to the entire structural lateral lo4 resisting system is deemed to be deficient. Common approaches in this regard are employed increase stiffness and strength with limited Achieving desired ratio between to additional stiffening and strengthening is the art of seismic retrofitting. The most common modifications include the addition of structural loads, steel braces, infill walls, base isolate or supplemental energy dissipation devices.

The addition of new reinforced concrete shear wall is the most oftenly practised devicewhich has proved to be effective for controlling global lateral drifts and for reducing damage in frame members. Steel braces are used to make the existing buildings stiffen. Concentric eccentric bracing schemes may be used, in the selected bays of an RC frame contributing increase the lateral resistance of the structure infill wall MAW be employed for strengthen of reinforced concrete buildings. which has ,teen effective in the case of one to three so buildings that may be extended up to five stories. The lateral strength of existing columns be increased by adding wing walls {buttresses similar to infilling. These technique are not so popular because it may require a vacant site around the building and enough resistance from piles or foundation of the buttress CEB. 1997). At some occasions it might be easy to achieve the retrofitting objectives by means of global mass reduction. Mass reduction can be accomplished by removal of upper stories heavy cladding, partitions and stored good. The concept of seismic base isolation is based on decoupling of structure by introducing low horizontal stiffness bearing between the structure and the foundation. This is found to be efficient for seismic resistance and intervention is required only at foundation level. The supplemental devices such as addition of viscous damper, visco-elastic damper, frictional damper in diagonals of bays of frame substantially reduces the earthquake response by dissipation of energy.

Local retrofittings arc typically used either when the retrofit objectives are limited or treatment of the vulnerable components is needed. The most popular and frequently usedmethod in local retrofitting is jacketing or confinement by the jackets of reinforced concrete, method reinforced polymer (MP), carbon fibre etc. Jacketing around the existing members increases lateral load capacity of the structure in a uniformly distributed way with a minimal increase in loading on any single foundation and with no alternative in the basic geometry of increase the building.

7.2STRUCTURAL LEVEL (OR GLOBAL) RETROFIT METHODS

Two approaches are used for structure-level retrofitting: ti) conventional methods based on increasing the seismic resistance of existing structure, and (ii) non-conventional methods based on reduction of seismic demands.

Conventional methods

Conventional methods of retrofitting are used to enhance the seismic resistance of existing structures by eliminating or reducing the adverse effects of design or construction. The methods Include adding of shear wall, infill walls and steel braces.

7.3ADDING NEW SHEAR WALLS

One of the most common methods to increase the lateral strength of the reinforced concrete is to make a provision for additional shear walls. The technique ofinfilling/adding new shear walls is often taken as the best and simple solution for improving seismic performance. Therefore, it is frequently used for retrofitting of non-ductile reinforced concrete frame buildings. The added elements can be either cast-in-place or pre-cast concrete elements. New elements preferably be placed at the exterior of the building, however it may cause alteration in the appearance and window layouts. Placing of shear walk in the interior of the structure is not preferred in order to avoid interior mouldings,

Technical considerations:

The addition of new shear walls to existing frame has many technical considerations which may be summarized as (a) determining the adequacy of existing floor and roof slabs to carry the seismic forces; (b) transfer of diaphragm shear into the new shear walk with dowels; (c) adding new collector and drag members to the diaphragm; (d) increase in the weight and concentration of shear by the addition of wall which may affect the foundations.

Constructional considerations: The first consideration during construction is to find locatir7, where walls can be added and well located which may align to the full height of the building to minimize torsion (Wylie, 1996). It is often desirable to locate walls adjacent to the bear between columns so that only minimum slab demolition is required with connections male to beam at the sides of columns. The design of the shear wall may be similar to new construction. The longitudinal reinforcement must be placed at the ends of the wall running continuously through the entire height. In order to realize this end, the reinforcement has to pass through holes in slabs and around the beams to avoid interference. To achieve both conditions, boundary elements can be used. Although it would also be convenient to have continuous shear reinforcement but in its absence, the walls must be adequately connected to the beams, slabs and columns ensuring proper shear transfer through shear connectors. Wall thickness also varies from 15 to 25 cm (6 to 10 inch) and is normally placed externally. This retrofitting system is only adequate for concrete structures, which bring forth a big increase in the lateral capacity and stiffness. A reasonable structural ductility may be achieved if the wall is properly designed with a good detailing. The connection to the existing structure has to be carefully designed guarantee shear transfer.

Limitations:

The main limitations of this method are: (i) increase in lateral resistance but it is concentrated at a few places, (ii) increased overturning moment at foundation causes very high uplifting 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 result in functional disability of the buildings, (v) possibilities of adequate attachment between the new walls and the existing structure, (vi) closing of formerly open spaces can have major impact on the interior of the building or exterior appearance.

7.4ADDING STEEL BRACINGS

Another method of strengthening is the use of steel bracing, which also has similar advantages. The structural details of connection between bracing and column are shown I the fig. The installation of steel bracing members can be an effective solution when large openings are required. This scheme of the use of steel bracing has a potential advantage over other schemes following reasons:

higher strength and stiffness can be proved 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 prefabricated elements and disturbance to the occupants may be minimized.

Technical considerations:

The steel bracing system can be used for steel structures as well as concrete structures: several researchers have reported successful results while using steel bracing to upgrade the strength and stiffness of reinforced concrete structures. It has performed well linear behaviour even up to twice the design code force. The effective slenderness ratio should be kept relatively low so 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 forces between the frame and bracing system. Careful consideration of of strengthening elements to the existing structures and to the foundations have to be consciously designed to ensure proper shear transfer. Column shear failure is not specifically prevented; therefore close attention must be given to limit drifts of the strengthened frame. Local reinforcement to the columns may be needed to bear the increased load generated on them.

7.5BASE ISOLATIONBase isolation, also known asseismic base isolationorbase isolation system, is one of the most popular means of protecting a structure againstearthquakeforces.It is a collection of structural elements which should substantiallydecoupleasuperstructurefrom its substructure resting on a shaking ground thus protecting abuildingornon-building structure's integrity. Base isolation is one of the most powerful tools ofearthquake engineeringpertaining to the passive structuralvibration controltechnologies. It is meant to enable a building or non-building structure to survive a potentially devastatingseismicimpactthrough a proper initial design or subsequent modifications. In some cases, application of base isolation can raise both a structure'sseismic performanceand itsseismicsustainabilityconsiderably. Contrary to popular belief base isolation does not make a building earthquake proof.Base isolation systemconsists ofisolation unitswith or withoutisolation components, where:1. Isolation unitsare the basic elements of abase isolation systemwhich are intended to provide the aforementioneddecouplingeffect to a building or non-building structure.2. Isolation componentsare the connections betweenisolation unitsand their parts having no decoupling effect of their own.

Significantly Increase the Period of the Structure and the Damping so that the Response is Significantly Reduced

Base Isolation inBuildings

Isolator Components between the Foundation and Superstructure

Fig: An Isolation Interface is formed

How exactly does Base Isolation Work?

Most types of Isolators exhibit nonlinear behavior

Lengthening of the Structures Period and increased damping that result in a large scale decrease of the Seismic Response.

Force Displacement Relationship at the Isolation Interface

Response of Base Isolated Buildings versus Fixed Base Response

7.6JACKETING

Jacketingis the process whereby a section of an existing structural member is restored to original dimensions or increased in size by encasement using suitable materials. A steel reinforcement cage or composite material wrap can be constructed around the damaged section onto whichshotcreteor cast-in- place concrete is placed.

Jacketingis particularlyused for the repair of deteriorated columns, piers, and piles and may easily be employed in underwater applications. The method is applicable forprotecting concrete, steel, and timber sections against further deterioration and for strengthening. Permanent forms are preferred where protection against weathering, abrasion, and chemical pollution is desired.

Before applying jackets, all deteriorated concrete must be removed, cracks must be repaired, existing reinforcement must be cleaned, and surfaces must be prepared. The surface preparation improves the bond of the newly placed materials with the existing structure, which is difficult for underwater repairs. For underwater conditions, a plastic shell may be applied at the splash zone to help minimize abrasion. A drawback of jackets is that they occupy space that was earlier available for other uses.

Jacketing of colums

jacketing of beams

8. CONCLUSION

Seismic Retrofitting is a suitable technology for protection of a variety of structures.

It has matured in the recent years to a highly reliable technology.

But, the expertise needed is not available in the basic level.

The main challenge is to achieve a desired performance level at a minimum cost, which can be achieved through a detailed nonlinear analysis.

Optimization techniques are needed to know the most efficient retrofit for a particular structure.

Proper Design Codes are needed to be published as code of practice for professionals related to this field.

9. REFERENCES:1. http://theconstructor.org/earthquake/earthquakes-effects-reinforced-concrete-buildings/3790/2. http://theconstructor.org/concrete/retrofitting-of-rcc-structural-members/7363/3. http://theconstructor.org/structural-engg/base-isolation-method/498/4. http://www.iitk.ac.in/nicee/wcee/article/10_vol11_6639.pdf5. http://www.ncree.org.tw/itp2002/09_FundamentalsOfSeismicBaseIsolation.pdf6. http://oonegroup.com/html/images/Articles/Seismic%20Retrofit.pdf7. http://cpwd.gov.in/Units/FinalDraftHandbook_Apr2007.pdf8. http://web.mit.edu/istgroup/ist/documents/earthquake/Part5.pdf