A Seminar on SEISMIC RETROFITTING OF COLUMNS & FOUNDATIONS

CONTENTS:Introduction Basic concepts of retrofitting2. Need for retrofitting3. Building deficienciesPlan irregularitiesVertical irregularities4. Steps taken prior to retrofitting 5. Classification of seismic retrofitting techniquesSeismic retrofitting for Columns & Foundations ConclusionsReferences

INTRODUCTION: DEFINITION: Methods of increasing resistant capacity of structure by various techniques called retrofitting. Seismic retrofitting is the modification of existing structures to make them more resistant to the seismic activity, ground motion, or soil failure due to earthquakes.

The retrofit techniques are also applicable for other natural hazards such as tropical cyclones, tornadoes & severe winds from thunderstorms. Structure can be Earthquake damaged Earthquake vulnerable

Up gradation of lateral strength of the structure.Increase the ductility of structure.Increase the strength & ductility.

BASIC CONCEPT OF RETROFITTINGNEED FOR RETROFITTING

Makes building safer & less prone to damage.To ensure the safety & security of the building, employees, structural functionality, machinery & inventory.Essential to reduce hazard & losses from non structural elements.Predominantly concerned with structural improvements to reduce seismic hazard.Demands on existing structures are increasing.

BUILDING DEFICIENCIES:

Plan irregularitiesTorsional irregularity due to plan symmetry & eccentric mass from water tank.Re-entrant corners in U-,L-, & T- shaped buildings.Diaphragm discontinuity due to openings Out-of-plane offset for floating columns along perimeter

Vertical irregularitiesStiffness irregularity, extreme soft storey due to open ground storey.Mass irregularity.Vertical geometric irregularity In-plane discontinuity for floating columns along perimeter.

STEPS TAKEN PRIOR TO RETROFITTING:ScreeningEvaluationretrofitRETROFIT OF STRUCTURES USING INNOVATIVE MATERIALS:High performance concreteHigh performance steelFiber reinforced plastic

CLASSIFICATION OF RETROFITTING TECHNIQUES:GlobalAdding shear wallAdding infill wallAdding bracingAdding wing wall Wall thickeningMass reduction Base isolation Mass dampersLocalJacketing of beamsJacketing of columnsJacketing of beam-columns jointsStrengthening of individual footing

RETROFITTING OF RC STRUCTURES:Strengthening of existing members Reinforced concrete jacketing Steel plate bonding & steel plate caging: FRPPlate bonding & jacketing

b) adding a new memberShear walls FramesBracingsButtresses

RETROFITTING FOR THE COLUMNS: Most popular method of strengthening of columns.Types 1.Steel jacket 2.Reinforced concrete jacket 3. FRPC jacketPurpose : To increase the concrete confinement To increase the shear strength & flexural strengthJACKETING:

FRP wrappingIn this method fiber should be located horizontally perpendicular to the axis of column.It increases the strength of concrete by confinement & also improves ductility.Main drawback is high cost & inadequate fire resistance

ADDING NEW SHEAR WALLS:Frequently used for Retrofitting of non ductile Reinforced concrete frame Buildings

Added members are may be cast-in-place or pre cast concrete elements.

ADDING OF STEEL BRACINGS:An effective solutions when large openings are required.Potential advantages for the following reasons -Higher strength & stiffness -Openings for the natural light -adds less weight to existing structure

RETROFITTING OF FOUNDATION:Load from the superstructure get transmitted to the soil through foundation.Seismic retrofit includes the strengthening of inadequate foundations or supplementing with new foundation.Before retrofitting to be carried out we need to study the location of foundation, dimension, geometry, quality & bearing capacity & degree of damage to the foundation.Depending upon the evaluation of performance different methods are to be adopted.

EARTHQUAKE HAZARDS ON FOUNDATIONS:Ground sliding

Ground tilting

Differential settlement

Liquefaction

Important methods used for retrofitting of foundations of buildings are: Grouting:Grout is highly pressurized into loose ground & compresses the surrounding soil in the lateral direction.After the some time grout is solidified & increase the shear strength of underlying soil to mitigate liquefaction.Cement grout is commonly used because of its excellent durability.To restore the differential settlement impermeable grout is injected into ground on the depressed side which increases volume causing the ground to rise.

DRILLING OF MICRIPILES:Micro piles are of diameter 300mm or less can be drilled through the existing foundations.After the hole is drilled, it is grouted with the cement slurry & reinforcement bars are inserted.The piles extend much beyond the foundation level and transfer the loads to the deeper level over a wider width. UNDER PINNING WITH PILES:It is used when strata below the foundation is very weak & increasing area of footing is not sufficient.Additional piles are symmetrically placed around the edge of existing foundation to ensure the effective load transfer.After the construction of piles the size of footing is increased & dowel bars are provided between existing & new concrete.

MICROPILES UNDER PINNING WITH PILES

CONCLUSIONS:It is an effective method of reducing the risks for existing seismically deficient structures.Retrofitting techniques are also applicable for the other natural hazards.Gives the safety to the occupants.Cost effective.It is important to obtain accurate as-built information & analytical data to perform the seismic evaluation of existing structure & to select the appropriate retrofitting strategy based on the performance, economy & constructability.

Prevention is better than cure

REFERENCES:

Seismic retrofitting of RC buildings vol. 7 Issue 2 (may-june 2013), pp, 49-62ISET Journal of Earthquake technology, paper no.454, vol.42 no 2-3 (june- sept 2005.pp 21-46.Hand book on seismic retrofit of buildings, central public works dept. & Indian building congress in association with Indian institute of technology, Madras.Elnashai A.S & Salama, A.I(1992) Selective repair & retrofitting techniques for RC Structures in Seismic Regions, Research report ESEE/ 92.2, Engineering seismology & earthquake Engineering section, Imperial college, London.

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