A Seminar on

SEISMIC RETROFITTING OF COLUMNS & FOUNDATIONS

CONTENTS: 1. 2. 2. 3.

Introduction Basic concepts of retrofitting Need for retrofitting Building deficiencies • Plan irregularities • Vertical irregularities

4. 5. 6. 7. 8.

Steps taken prior to retrofitting Classification of seismic retrofitting techniques Seismic retrofitting for Columns & Foundations Conclusions References

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

BASIC CONCEPT OF RETROFITTING Up gradation of lateral strength of the structure. Increase the ductility of structure. Increase the strength & ductility. NEED 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 irregularities 1. Torsional irregularity due to plan symmetry & eccentric mass from water tank. 2. Re-entrant corners in U-,L-, & T- shaped buildings. 3. Diaphragm discontinuity due to openings 4. Out-of-plane offset for floating columns along perimeter Vertical irregularities 1. Stiffness irregularity, extreme soft storey due to open ground storey. 2. Mass irregularity. 3. Vertical geometric irregularity 4. In-plane discontinuity for floating columns along perimeter.

STEPS TAKEN PRIOR TO RETROFITTING:  Screening  Evaluation  retrofit

RETROFIT OF STRUCTURES USING INNOVATIVE MATERIALS:   

High performance concrete High performance steel Fiber reinforced plastic

CLASSIFICATION OF RETROFITTING TECHNIQUES: Global 1. Adding shear wall 2. Adding infill wall 3. Adding bracing 4. Adding wing wall 5. Wall thickening 6. Mass reduction 7. Base isolation 8. Mass dampers

Local 1. Jacketing of beams 2. Jacketing of columns 3. Jacketing of beamcolumns ts 4. Strengthening of individual footing

RETROFITTING OF RC STRUCTURES: a) Strengthening of existing  Reinforced concrete jacketing  Steel plate bonding & steel plate caging: FRP  Plate bonding & jacketing b) adding a new member  Shear walls  Frames  Bracings  Buttresses

RETROFITTING FOR THE COLUMNS: JACKETING:  Most popular method of strengthening of columns.  Types 1.Steel jacket 2.Reinforced concrete jacket 3. FRPC jacket  Purpose : •

To increase the concrete confinement

•

To increase the shear strength & flexural strength

FRP wrapping  In 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 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

 Differential settlement

 Ground tilting

 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 PILES

UNDER PINNING WITH

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 (mayjune 2013), pp, 49-62



ISET 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 .