krantisirretrofitting-210405173053 (1).pptx

rretrofitting

1. Retrofit ting

2. Introduction 2 Seismic retrofitting is a collection mitigation technique for earthquake engineering. It is the modification of existing structures to make them more resistant to seismic activity, ground motion, or soil failure due to earthquake. It is of utmost important for historic monuments, areas prone to severe earthquakes and tall or expensive structures. The retrofitting techniques are also applicable for other natural hazards such as tropical cyclones, tornadoes and severe winds from thunderstorms. Retrofitting proves to be a better economic consideration and immediate shelter to problems rather than replacement of building.

3. Need of R etrofitting The two circumstances are: Earthquake damaged buildings, and Earthquake-vulnerable buildings(with no exposure to severe earthquakes) Nearly 5,00,000 earthquakes occur every year around world among which about 1,00,000 are felt and the rest occur nearly constantly almost anywhere. Large number of casualties occur in buildings due to earthquake. Reasons may be; Inadequate design Poor construction and maintenance Lack of resources Inadequate knowledge and awareness Inadequate safety implementation 3

4. Need of Retrofitting in Existing Earthquake Vulnerable Buildings 4 Buildings have been designed according to a seismic code , but the code has been upgraded in later years ; Buil d ings d e s ig n ed to m e et the m o de rn s e is m ic code s , but deficiencies exist in the design and/or construction; Essential b u il d ings m ust b e stren g thened li k e ho s pitals, histori c al monuments and architectural buildings; Important buildings whose services are assumed to be essential just after an earthquake like hospitals; Buildings, the use of which has changed through the years; Buildings that are expanded, renovated or rebuilt.

5. Basic Concept of Retrofitting 5 The aim is at :- Upgradation of lateral strength of the structure; Increase in the ductility of the structure Increase in strength and ductility

6. Earthquake Design Philosophy 6 Under 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; and Under strong but rare shaking , the main members may sustain severe (even irreparable) damage , but the building should not collapse .

7. Objectives of R etrofitting 7 Public safety : The main goal is to protect human life, ensuring that the structure will not collapse upon its occupants or passer-by and that the structure can be safely exited. Structure serviceability : The structure is to serve good serviceability while being subjected to vulnerable earthquakes. Structure functionality : Primary structure undamaged and the structure is undiminished in utility for its primary application. Structure unaffected : The level of retrofit is preferred for historic structures of high cultural significance. Therefore, the aim is to upgrade the lateral strength of the structure and to increase the ductility.

8. To R etrofit or not? Retrofitting should be adopted when the evaluation of the building indicates that the strength available before the damage was insufficient and restoration alone will not be adequate in future quakes. 6 S E I S MIC LOAD C A P A C ITY VE R S U S R I SK OF BUILDING COLLAPSE

9. Retrofitting T echniques 9

10. Adding shear wall 10 Used for non-ductile reinforced concrete frame buildings. The added elements can be either cast-in-place or precast concrete elements. New elements preferably be placed at the exterior of the building, however it may cause in the appearance. Increase the lateral strength, ductility and stiffness of the building substantially. SHEAR WALLS Global Retrofitting T echniques

11. Adding I nfill wall Masonry infills contribute significant lateral stiffness, strength, overall ductility and energy dissipation capacity. The structural load transfer m echanism is changed f ro m f r a m e action to predominant truss action. The frame columns now experience increased axial forces but with reduced bending moments and shear forces. When infills are non-uniformly placed in a building, cause soft storey e f fect, short- c ol u m n e f fect, torsion and out-of-plane collapse. Hence, seismic code tends to discourage such constructions in high seismic regions . BRICK M A SO N R Y INFILL W ALL R E T ROFITTING 11

12. Adding Steel Bracings 12 A n e f fect i ve so l ut i on w hen l a r ge open i ngs a r e 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

13. Adding Wing Wall or Buttress 13 To increase lateral strength, ductility and stiffness of structure. The wing wall are placed on the exterior side of an existing frame. QU A D R A E L EMENT R Y SCHOOL, C A N A DA

14. Wall Thickening Techniques 14 Incre a se t he thi c k n e ss b y add i ng br i c ks, concrete and steel reinforcement. It can bear more vertical and horizontal loads. Does not cause sudden failure of the wall. RET R O F IT T I N G BY AL U MIN I UM FL A T A T UJJAYANTA PALACE, AGARTALA, TRIPURA

15. Mass Reduction 15 In this process removing one or more storey of building as shown in the figure. Decrease the load at foundation. Increase the life and strength. MASS REDUCTION BY REMOVING ONE STOREY

16. Base Isolation 16 Isolation of superstructure from the foundation is known as base isolation. It is the most powerful tool for passive structural vibration control techniques. Isolates building from ground motion lesser seismic loads, hence lesser damage to the structure, minimal repair to the structure. Building can remain serviceable throughout construction. Does not involve major intrusion upon existing superstructure.

17. Concept of Base Isolation 17 Significantly Increase the Period of the Structure and the Damping so that the Response is Significantly Reduced. Fig: Spectral Response for a Typical Base Isolation System

18. Types of Base Isolations 21 Base isolation systems which uses Elastomeric Bearings Base isolation systems with Sliding System Fig: Elastomeric Isolators

19. Elastomeric Base Isolation Systems 22 This is the mostly widely used Base Isolator. The elastomer is made of either Natural Rubber or Neoprene . The structure is decoupled from the horizontal components of the earthquake ground motion A l a y e r with l o w hori z ont a l s t i f fness i s introduc e d between the structure and the foundation. Fig: Steel Reinforced Elastomeric Isolators

20. Sliding Base Isolation Systems It is the second basic type of isolators. This works by limiting the base shear across the isolator interface. 23 Fig: Metallic Roller Bearing

21. Spherical Sliding Base Isolators The s t ructure i s sup p ort e d b y bear i ng p a ds that h a ve curved surface and low friction. During an earthquake, the building is free to slide on the bearings. 24 Fig: Spherical Sliding Base Isolator

22. Friction Pendulum Bearing 25 These are specially designed base isolators which works on the principle of simple pendulum. It increases the natural time period of oscillation by causing the structure to slide along the concave inner surface through the frictional interface. It also possesses a re-centering capability. Fig: Cross-section of Friction Pendulum Bearing

23. Advantages of Base Isolation 30 Isolates Building from ground motion Lesser seismic loads, hence lesser damage to the structure. Minimal repair of superstructure. Building can remain serviceable throughout construction. Does not involve major intrusion upon existing superstructure .

24. Disadvantages of Base Isolation 31 Expensive Cannot be applied partially to structures unlike other retrofitting Challenging to implement in an efficient manner Allowance for building displacements Inefficient for high rise buildings Not suitable for buildings rested on soft soil.

25. Seismic Dampers 25 Seismic dampers are used in place of structural elements, like diagonal braces, for controlling damage in structures. It partly absorbs the seismic energy and reduces the motion of buildings. Types of mass dampers: V is c o u s da m pers (e n e r gy i s a b so r b e d b y si l i c o n -b a s e d f lu i d pa s s i ng b e tween p is t o n - cylinder arrangement), Friction dampers (energy is absorbed by surfaces with friction between them rubbing against each other), and Yielding dampers (energy is absorbed by metallic components that yield).

26. ENERGY DISSIPATION DEVICES WORLD’S LARGEST TUNED MASS DAMPER (SPHERE) IN TAIPEI 101 26

27. Jacketing (local retrofitting techniques) 27 Most popular method for strengthening of concrete building elements like as beams, columns and beam-column junctions. Purpose for jacketing: To increase concrete confinement To increase shear strength To increase flexural strength Materials to be used: Steel plates Steel reinforced concrete Fibre Reinforced Polymer (FRP wrap) Carbon fibre reinforced polymer (CFRP) Glass fibre reinforced polymer (GFRP ) FRP wraps increase the seismic energy absorption capacity of the structual elements.

28. Concrete jacketing involves addition of longitudinal bars, closely spaced ties, and a layer of concrete. The jacket increases both flexural strength and shear strength of concrete . The usual practice consists of first assembling the jacket reinforcement cages, arranging the formwork and then placing the concrete jacket. But it is cost effective.

29. Concrete jacketing Concrete jacketing disadvantages- 1.Cost effective Requires intensive labour Detailing of steel in the form of digital collars 4.Incrases Dimensions of structure 5.Incrases weight of structure

30. Steel J acketing Steel jacketing refers to encasing the member with steel plates and filling the gap with non-shrink grout. The jacket enhances both flexural strength and shear strength of concrete. Steel jacketing disadvantages – 1.Complicated working procedure. 2.Inner surface corrosion . 3. Heavy weight. 4.C/S increases.

31. Fiber Reinforced Polymer( FRP) Fiber reinforced polymer (FRP) is a composite material consisting of polymeric resin reinforced with high strength fibers. Composite materials are available in the form of sheets, pre-formed shapes and bars. The FRP sheets are thin, light and flexible enough to be inserted. The fibers can be of glass, carbon, aramid. Glass f ibe r s h a v e l o w er stiff n ess and c ost c ompa r ed to carbon fibers. They are suitable in low cost seismic retrofit applications.

32. Fiber Reinforced Polymer( FRP) The FRP composites are useful for repair, rehabilitation and retrofit of structures for the following reasons: facilitate f lexible, w hich drilling of c o nc r e t e or 1.The FRP sheets are light and installat i on . I t d oes n o t need masonry. The curing time required is less The sheets are thin and hence there is no marginal increase in the size of retrofitted member. 4. The material is chemically inert and has resistance against electro-chemical corrosion. 5. Th e r e is g ood fat i gue st r engt h , w hich is su i tabl e f or fluctuating loads.

33. External P late B onding Steel plates are attached to the surface of damaged members forming a three phase steel composite system Acts as supplement to existing reinforcement. Attachment of steel to concrete Adhesive connecting mechanism (glue). Bolting connecting mechanism. Advantages Stress reduction due to the external steel plate. Enhances load bearing capacity. decreases chances of cracks and deflection.

34. Disadvantages Increase in dead load. High installation cost due to heavy weight of steel plate. If there is any indications of corrosion in the reinforcement this technique cannot be used. Susceptible to high level of premature de-bonding. Chances of corrosion is high. Bonding between concrete and steel plate. Reaction between epoxy adhesive and concrete.

35. EXTERNAL POST TENSIONING. High strength steel strands or pre-stressing tendons are used. Tendons are pulled and connected to anchor points on member. Very much suitable for retrofitting of bridges. Advantages Ability to restress, destress and exchange any external pre- stressing cable. Crack free members. Reduce deflection. High fatigue and impact resistance. Immediate enhancing of load bearing capacity.

36. Disadvantages Usually requires a greater section depth. Exposed to environmental influences. Handling of the tensioning devices may be more difficult. High cost. Prone to corrosion. Skilled person is needed for post tensioning

37. RETROFIT T ING A T J U N CTION BY ST E EL PL A TE 38

38. 19 RETROFITTING BY STEEL REINFORCED CONCRETE BEAM JACKETING COLUMN JACKETING

39. RETROFIT T ING OF COLUMN BY C FRP WR A P 40

40. GFRP WRAP AND WRAPPED STRUCTURE 41