SEISMIC RETROFITTING TECHNIQUES OF RCC

SEISMIC RETROFITTING TECHNIQUES OF RCCBUIIDINGS PRESENTED BYK.PRATYUSHAY14MTSE808

Introduction

Earthquake creates great devastation in terms of life, money and failures of structures.We know that earthquake is a important field of study from long time.Seismic Retrofitting is a collection mitigation techniques for Earthquake Engineering.It is utmost importance for historic monuments, areas prone to severe earthquake and tall or expensive structures.

Seismic retrofittingDefinition It 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 applicable for other natural hazards such as cyclones, severe winds from thunderstorms. The seismic retrofitting gives improvements to an existing buildings and increasing the global capacity of the building.

When is Seismic Retrofitting Needed?The two circumstances areEarthquake damaged buildings, andEarthquake-vulnerable buildings(with no exposure to severe earthquakes)The performance objectives arepublic safetyStructure survivabilityStructure functionality

Need of Retrofitting in ExistingEarthquake Vulnerable BuildingsBuildings have been designed according to a seismic code, but the code has been upgraded in later years;Essential buildings must be strengthened like hospitals, historical monuments and architectural buildings;Important buildings whose services are assumed to be essential just after an earthquake like hospitals;Buildings that are expanded, renovated or rebuilt.Buildings, the use of which has changed through the years.

Problems faced by StructuralEngineers are :-Lack of standards for retrofitting methodsEffectiveness of each methods varies a lot depending upon parameters like type of structures, material condition, amount of damage , etc.Basic Concept of RetrofittingUp gradation of lateral strength of the structure;Increase in the ductility of the structureIncrease in strength and ductility

Earthquake Design PhilosophyUnder minor but frequent shaking, the main members of the building that carry vertical and horizontal forces should not be damaged; however building parts that do not carry load may sustain repairable damage;Under moderate but occasional shaking, the main members may sustain repairable damage, while the other parts of the building may be damaged such that they may even have to be replaced after the earthquake; andUnder strong but rare shaking, the main members may sustain severe (even irreparable) damage, but the building should not collapse.

FLOW OF RETROFITTING PROCESSDrafting of retrofitting planPerformance requirements of existing structureInspection of existing structurePerformance verification for existing structure if ok continue and use of the structure if not determination use of the structure through retrofitting if no restriction on use of structure if yes go for selection of retrofitting methodImplement retrofitting work and use of structure.

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Classification of Retrofitting Techniques

Some Conventional ApproachesAdding New Shear WallsFrequently used for retrofitting of non ductile reinforced concrete frame buildings.The added elements can be either cast in place or precast concrete elements.New elements preferably placed at the exterior of the building. Additional shear wall

Adding Steel BracingsAn effective solution when large openings are required.Potential advantages for the following reasons: higher strength and stiffness, opening for natural light, amount of work is less since foundation cost may be minimized adds much less weight to the existing structure

Adding Steel Bracings

Base Isolation (or Seismic Isolation)Isolation of superstructure from the foundation is known as base isolation.It is the most powerful tool for passive structural vibration control technique.

Base isolated structures

Jacketing (Local Retrofitting Technique)Most popular method for strengthening of building columnsTypes-1. Steel jacket, 2. Reinforced Concrete jacket, 3. Fiber Reinforced Polymer Composite(FRPC) jacket.Purpose for jacketing: To increase concrete confinement To increase shear strength To increase flexural strength

STEEL JACKETINGAddition of steel often applied in the form of plates or jackets.Local strengthening of columns has been frequently accomplished by jacketing with steel plates.Advantage of steel include that it does not add significant weight to the structure in comparison with concrete and it saves on construction time(no curing).The main disadvantage of this type are linked to construction issues steel can be labor intensive and it require heavy equipments to handle thousands of tons and as well as having a more difficult maintenance.

Column jacketing

REINFORCED CONCRETE JACKETINGReinforced concrete jacketing can be employed as a repair or strengthening scheme.There are two main purposes of jacketing of columns: (i) Increase in the shear capacity of columns (ii) To improve the column's flexural strengthUsing concrete sections the other methods developed are section enlargement shot Crete fiber reinforced cement composites

SHOT CRETELarger development in section enlargement is shot Crete.It is a concrete or mortar pneumatically projected at high velocities.Main advantages are reduction of construction time and cost.Main disadvantage is it require special attention and good quality product.These include placing thick sections in layers using of a blow man to help reduce rebound and requiring quality control.

Shot Crete technique

FIBER REINFORCED PLASTIC METHODFRP composites present significant advantages over a traditional confinement techniques .The cross section dimensions of the column do not increase, which permits compliance with architecture restraints.The mass of the column does not increase; which means the seismic behavior of the building remains unchanged.Due to light weight of FRP materials implies that the installation procedure is faster, easier, less dangerous for the operator.

Fiber-reinforced plastic bonding method:A construction method in which glass fibers, carbon fibers, aramid fibers or other continuous fiber materials (continuous fiber sheets, etc.) are bonded to the outside of the concrete section, bonding to the existing members to form a composite configuration, after which an organic or other material coating is applied on top, both to prevent the entry of carbon dioxide, chloride ions, moisture etc. and to provide the necessary performance improvement. This method is used for bridge decks and almost all other concrete members.

Fiber-reinforced plastic jacketing construction method:A method in which continuous fiber sheets or other fiber materials are placed continuously around the entire periphery of existing column members, etc. with insufficient load-carrying capacity, bonding to the existing members to form a composite configuration in order to achieve the required performance improvement. This method is used for bridge piers, etc.

DIFFERENT TYPES OF FIBERSGLASS FIBERS:glass fibers have been commonly used for 50 years in the aeronautical industry given their very high strength to weight ratio.They also used in wind turbine blades and in the field of naval engineering.E-glass or Electrical glass is the most widely used glass fiber because of its low cost of production.CARBON FIBERS:This is mainly used for old structures that may have been damaged by earthquakes, chemical environment etc.

ARMID OR KELVARThese fibers are less used as regards applications for civil engineering and retrofitting of structures.These fibers gives best strength to weight ratio.Main disadvantage of these are inadequate compressive strength. Column without wrapping

Column with GFRP/CFRP wrapping

Fiber reinforced plastic jacketing

SELECTION OF STRENGTHING METHODNow a days old existing buildings with less load carrying capacity are usually strengthen by the application of a suitable strengthen method.There is a possibility of selecting a strengthen method with out comparing for the most efficient method.The recommended strengthen method should be relatively more economical, efficient and time saving.There are numerous strengthen methods available, each of which has its own advantages and disadvantages.Ex:-steel jacket strengthening is not suitable in marine environment due to corrosion.But steel jacketing yielded a good result for the columns designed for gravity loads with out any earthquake design.

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