Successfully reported this slideshow.

CE 72.52 - Lecture 8a - Retrofitting of RC Members

37

Share

1 of 56
1 of 56

More Related Content

Similar to CE 72.52 - Lecture 8a - Retrofitting of RC Members

Related Books

Free with a 14 day trial from Scribd

See all

CE 72.52 - Lecture 8a - Retrofitting of RC Members

  1. 1. Dr. Pramin Norachan Manager, Structural Engineering Unit AIT Consulting Advanced Concrete Structures
  2. 2. 2. Global Retrofitting Presentation Outline 1. Introduction 3. Local Retrofitting 4. Example 1: Five Story RC Frame- Infill Building
  3. 3. Advanced Concrete Structures, Dr. Pramin Norachan 4
  4. 4. Advanced Concrete Structures, Dr. Pramin Norachan 5 (Global Retrofitting) (Local Retrofitting)
  5. 5. Advanced Concrete Structures, Dr. Pramin Norachan 6 The design philosophy for strengthening can be divided into two approaches. The first approach is the system strengthening (global strengthening) which new elements are added to a building to enhance its global stiffness. With an increase in the stiffness, the natural period of vibration of the building is to decrease. This will result in a decrease in the amount of horizontal displacement that must be achieved by the building to resist earthquakes. Moreover, addition of new members to the building shall mostly increase the horizontal load capacity of the building as well. Therefore, the increased capacity will require greater ground motions to allow the building to develop a yielding behavior. Thus, it can be said that the system strengthening does not only prevent collapsing but also delays structural damages.
  6. 6. Advanced Concrete Structures, Dr. Pramin Norachan 7 The second approach is element strengthening (local strengthening) which is a method based on the insufficient capacities of members due to the sustained damages without undertaking major changes in the load-deformation relationship of the building. It should also be noted that there will be no significant changes in the displacement demand after the member strengthening.
  7. 7. Advanced Concrete Structures, Dr. Pramin Norachan 9 Among the global strengthening methods, addition of RC shear walls is the most popular one. The installation of RC shear walls greatly improves lateral load capacity and stiffness of the structure. In the strengthening method with shear walls, the existing partition walls in the building are removed and high strength reinforced concrete shear walls are built instead.
  8. 8. Advanced Concrete Structures, Dr. Pramin Norachan 10 Shear walls have to be constructed from the foundation level and there may not need to strengthen other components. The shear walls can resist majority of the earthquake loads and limits the displacement behavior of the building.
  9. 9. Advanced Concrete Structures, Dr. Pramin Norachan 11 Under enormous cyclic forces during a seismic effect, Buckling Restrained Brace (BRB) which is system based strengthening techniques (global strengthening) devices can be used to increase the resistance of frame structures by providing energy dissipation and introducing nonlinear behavior.
  10. 10. Advanced Concrete Structures, Dr. Pramin Norachan 12 Testing and evaluating are required for designing and ensuring quality control. The structures are susceptible to collapse or large lateral displacements due to earthquake ground motions and require special attention to limit the displacement. This displacement can be brought into limit by providing BRB in the structure.
  11. 11. Advanced Concrete Structures, Dr. Pramin Norachan 13
  12. 12. Advanced Concrete Structures, Dr. Pramin Norachan 14
  13. 13. Advanced Concrete Structures, Dr. Pramin Norachan 15 Passive control systems reduce structural vibration and associated forces through energy dissipation devices that do not require external power. These devices utilize the motion of the structure to develop counteracting control forces and absorb a portion of the input seismic energy. Active control systems, however, enhance structural response through control forces developed by force delivery devices that rely on external power to operate. The actuator forces are controlled by real time controllers that process the information obtained from sensors within the structure. Semi-active control systems combine passive and active control devices and are sometimes used to optimize the structural performance with minimal external power requirements.
  14. 14. Advanced Concrete Structures, Dr. Pramin Norachan 16 The seismic base isolation technology involves placing flexible isolation systems. between the foundation and the superstructure. By means of their flexibility and energy absorption capability, the isolation systems reflect and absorb part of the earthquake input energy before this energy is fully transmitted to the superstructure, reducing the energy dissipation demand on the superstructure.
  15. 15. Advanced Concrete Structures, Dr. Pramin Norachan 17 Base isolation causes the natural period of the structure to increase and results in increased displacements across the isolation level and reduced accelerations and displacements in the superstructure during an earthquake. Base isolation is fundamentally concerned to reduce the horizontal seismic forces.
  16. 16. Advanced Concrete Structures, Dr. Pramin Norachan 18 A typical base isolation system is evolved by the use of rubber bearing located at the base of the building. Rubber bearing consist of laminated layers of rubber and steel plates. The main advantage are good protection against earthquake due to decrease shear. Superstructure will need no reinforcement.
  17. 17. Advanced Concrete Structures, Dr. Pramin Norachan 20 There are several options for the jacketing of concrete members which are element based strengthening techniques (local strengthening). Usually, the exiting member is wrapped with a jacket of concrete reinforced with longitudinal steel bars and ties, or with weld wire fabric. Based on this method, axial strength, bending strength, and stiffness of the original column are increased. Reinforcement concrete jacketing can be used as a repair of strengthening scheme. If there is damage in some of the existing members, they should be repaired before jacketing. There are several options for the jacketing of concrete members which are element based strengthening techniques (local strengthening). Usually, the exiting member is wrapped with a jacket of concrete reinforced with longitudinal steel bars and ties, or with weld wire fabric. Based on this method, axial strength, bending strength, and stiffness of the original column are increased.
  18. 18. Advanced Concrete Structures, Dr. Pramin Norachan 21 There are several options for the jacketing of concrete members which are element based strengthening techniques (local strengthening). Usually, the exiting member is wrapped with a jacket of concrete reinforced with longitudinal steel bars and ties, or with weld wire fabric. Based on this method, axial strength, bending strength, and stiffness of the original column are increased. Reinforcement concrete jacketing can be used as a repair of strengthening scheme. If there is damage in some of the existing members, they should be repaired before jacketing. Reinforcement concrete jacketing can be used as a repair of strengthening scheme. If there is damage in some of the existing members, they should be repaired before jacketing.
  19. 19. Advanced Concrete Structures, Dr. Pramin Norachan 22 There are several options for the jacketing of concrete members which are element based strengthening techniques (local strengthening). Usually, the exiting member is wrapped with a jacket of concrete reinforced with longitudinal steel bars and ties, or with weld wire fabric. Based on this method, axial strength, bending strength, and stiffness of the original column are increased. Reinforcement concrete jacketing can be used as a repair of strengthening scheme. If there is damage in some of the existing members, they should be repaired before jacketing. Jacketing of beams is recommended for several purposes as it gives continuity to the columns and increases the strength and stiffness of the structure. While jacketing a beam, its flexural resistance must be carefully computed to avoid the creation of a strong beam-weak column system.
  20. 20. Advanced Concrete Structures, Dr. Pramin Norachan 23 There are several options for the jacketing of concrete members which are element based strengthening techniques (local strengthening). Usually, the exiting member is wrapped with a jacket of concrete reinforced with longitudinal steel bars and ties, or with weld wire fabric. Based on this method, axial strength, bending strength, and stiffness of the original column are increased. Reinforcement concrete jacketing can be used as a repair of strengthening scheme. If there is damage in some of the existing members, they should be repaired before jacketing. Jacketing of beam may be carried out under different ways, the most common are one- sided jackets or 3- and 4-sided jackets. The beam should be jacketed through its whole length. The reinforcement has also been added to increase beam flexural capacity moderately and to produce high joint shear stresses.
  21. 21. Advanced Concrete Structures, Dr. Pramin Norachan 24 There are several options for the jacketing of concrete members which are element based strengthening techniques (local strengthening). Usually, the exiting member is wrapped with a jacket of concrete reinforced with longitudinal steel bars and ties, or with weld wire fabric. Based on this method, axial strength, bending strength, and stiffness of the original column are increased. Reinforcement concrete jacketing can be used as a repair of strengthening scheme. If there is damage in some of the existing members, they should be repaired before jacketing.
  22. 22. Advanced Concrete Structures, Dr. Pramin Norachan 25 The steel jacket retrofit has been used as a method to enhance the shear strength and ductility of square reinforced concrete (RC) columns in existing buildings Local strengthening of columns has been frequently accomplished by jacketing with steel plates.
  23. 23. Advanced Concrete Structures, Dr. Pramin Norachan 26 FRP composite materials have experienced a continuous increase of use in structural strengthening and repair applications around the world in the last fifteen years. In general, applications that allow complete wrapping of the member with FRP have proven to be effective.
  24. 24. Advanced Concrete Structures, Dr. Pramin Norachan 27 Wrapping of columns to increase their load and deformation capacity is the most effective and most commonly used method of retrofitting with composites. However, certain performance and failure mode issues regarding different wrapping configuration and fiber orientations.
  25. 25. Advanced Concrete Structures, Dr. Pramin Norachan 28 Influence of shear strengthening and anchorage on FRP strengthened beam behavior under cyclic loading by using FRP plates in various configurations. It can be seen from this figure that flexural strengthening of beams without proper attention to brittle shear and debonding failure modes not only renders the strengthening application ineffective, but also harms the member by decreasing its ductility.
  26. 26. Advanced Concrete Structures, Dr. Pramin Norachan 30 Seismic performance review of an existing 5-story RC frame-Infill wall school building and comparison of various retrofit options is presented here. Rastriya Higher Secondary School Building, Nepal
  27. 27. Advanced Concrete Structures, Dr. Pramin Norachan 31 Stage I: Collecting As-built Building Information The architectural and structural drawings of the building are provided by the client. However, it is understood that the drawings for the extension part of the building are not available. On-site measurements and investigation are carried out to collect the as- built information of extension part. Stage II: Performance Based Evaluation for the Existing Building Performance based evaluation is carried out to check the seismic performance of the existing building using the as-built information from the previous stage. Stage III: Performance Based Evaluation for the Strengthened Buildings Performance based evaluation is carried out to check the seismic performance of the strengthened buildings based on common strengthening techniques used by practical engineers.
  28. 28. Advanced Concrete Structures, Dr. Pramin Norachan 32 MCE level response spectrum is estimated by increasing the spectra values of DBE level response spectrum by 2.0 times. Modal pushover analysis (MPA) is conducted to determine the inelastic response of the building. Response spectrum for seismic zone V (Z = 0.36) based on type III of subsoil (soft soil), specified in IS 1893:2002, approximately equivalent to the response spectrum with 1.1 of seismic zone factor, mentioned in NBC 105:1994. 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 SpectralAcceleration,Sa(g) Natural Period(s) ResponseSpectra DBE MCE T1T2T3T4T5T6 MCE level response spectrum is estimated by increasing the spectra values of DBE level response spectrum by 2.0 times. Modal pushover analysis (MPA) is conducted to determine the inelastic response of the building.
  29. 29. Advanced Concrete Structures, Dr. Pramin Norachan 33 Finite Element Model (SAP2000) Nonlinear Components Columns : - Fiber hinge - PMM hinge Girders : - Moment hinge Shear Walls: - Layered Shell
  30. 30. Advanced Concrete Structures, Dr. Pramin Norachan 34 Modal analysis is performed in order to determine the vibration modes of a building. The first fundamental mode is coupled both X and Y direction (torsion) due to unsymmetrical configuration of the building as expected. It is found that more than 90% of total mass is participating to 6 modes. Mode Natural Period (s) UX UY RZ 1 0.90 2.0% 63.8% 20.9% 2 0.86 9.4% 19.5% 45.4% 3 0.67 76.6% 0.1% 14.7% 4 0.32 0.4% 4.7% 0.5% 5 0.30 0.3% 6.6% 9.3% 6 0.23 9.2% 0.0% 0.9% Total 97.8% 94.7% 91.7%
  31. 31. Advanced Concrete Structures, Dr. Pramin Norachan 35 In terms of lateral stiffness, the building is flexible is Y-direction since the building dimension in Y-direction is smaller than that in X-direction.
  32. 32. Advanced Concrete Structures, Dr. Pramin Norachan 36 For repairing and strengthening of the existing building, three common strengthening techniques used by practical engineers, column jacketing, adding steel braces and adding new shear walls were used to improve the seismic performance of the existing building. Then, the efficiency of each strengthening method was investigated on the basis of member strength and deformation.
  33. 33. Advanced Concrete Structures, Dr. Pramin Norachan 37 There are several options for the jacketing of concrete members which are element based strengthening techniques (local strengthening). Usually, the exiting member is wrapped with a jacket of concrete reinforced with longitudinal steel bars and ties, or with weld wire fabric. Based on this method, axial strength, bending strength, and stiffness of the original column are increased.
  34. 34. Advanced Concrete Structures, Dr. Pramin Norachan 38 Reinforcement concrete jacketing can be used as a repair of strengthening scheme. If there is damage in some of the existing members, they should be repaired before jacketing. The details of concrete jacketing with longitudinal steel bars are illustrated in the following figure.
  35. 35. Advanced Concrete Structures, Dr. Pramin Norachan 39 Among the global strengthening methods, addition of RC shear walls is the most popular one. Many researchers have focused on the addition of RC shear walls and found that the installation of RC shear walls greatly improves lateral load capacity and stiffness of the structure. In the strengthening method with shear walls, the existing partition walls in the building are removed and high strength reinforced concrete shear walls are built instead.
  36. 36. Advanced Concrete Structures, Dr. Pramin Norachan 40 In this method, shear walls have to be constructed the foundation level and there may not need to strengthen other components. The shear walls bear majority of the earthquake loads and limits the displacement behavior of the building.
  37. 37. Advanced Concrete Structures, Dr. Pramin Norachan 41 Under enormous cyclic forces during a seismic effect, Buckling Restrained Brace (BRB) which is system based strengthening techniques (global strengthening) devices can be used to increase the resistance of frame structures by providing energy dissipation and introducing nonlinear behavior. Testing and evaluating are required for designing and ensuring quality control.
  38. 38. Advanced Concrete Structures, Dr. Pramin Norachan 42 The structures are susceptible to collapse or large lateral displacements due to earthquake ground motions and require special attention to limit the displacement. This displacement can be brought into limit by providing BRB in the structure.
  39. 39. Advanced Concrete Structures, Dr. Pramin Norachan 43 The existing building is increased stiffness by column jacketing, adding shear walls and BRB which is the main reason in reducing time period when compared with that of the existing building. Shear wall and BRB can contribute more stiffness than column jacketing. Mode Natural Period (s) Original Jacketing SW BRB 1 0.90 0.76 0.43 0.52 2 0.85 0.72 0.39 0.50 3 0.66 0.54 0.30 0.41 4 0.32 0.25 0.17 0.18 5 0.30 0.24 0.15 0.18 6 0.22 0.18 0.15 0.16
  40. 40. Advanced Concrete Structures, Dr. Pramin Norachan 44 The pushover curves for different buildings in Y direction (weak direction) which represent the relationship between base shear and roof displacement are plotted in the following figure. 0 1000 2000 3000 4000 5000 0.00 0.05 0.10 0.15 0.20 0.25 BaseShear(KN) Roof Displacement (m) Pushover Curves for Different Buildings Existing Column Jacketing SW BRB The results show all strengthening methods increased the building base shear, while they reduced the maximum roof displacement. The shear walls were more effective than other strengthening methods in this purpose. In the term of ductility, the results show that column jacketing technique caused the highest ductility, while shear walls significantly reduced the ductility.
  41. 41. Advanced Concrete Structures, Dr. Pramin Norachan 45 Base shear resulting from modal pushover analysis (MPA) at MCE levels of different strengthened buildings are summarized in the following figures. 28.4 15.0 45.0 26.5 48.5 40.340.9 34.4 0 10 20 30 40 50 60 X Y BaseShear(%) Along Direction Base Shear Percentage in Total Weight of Building MPA-Existing MPA-Jacketing MPA-SW MPA-BRB Results showed that base shear of all strengthened buildings are increased approximately 1.5 times in X-direction and 2 times in Y-direction, respectively. Column jacketing, adding shear walls and BRB cause the increase in the structural base shear because they contribute more stiffness to the existing building.
  42. 42. Advanced Concrete Structures, Dr. Pramin Norachan 46 1 2 3 4 5 6 -6000 -4000 -2000 0 2000 4000 6000 StoryLevel Shear Force (KN) Story Shear in X-Direction Existing Jacketing SW BRB 1 2 3 4 5 6 -6000 -4000 -2000 0 2000 4000 6000 StoryLevel Shear Force (KN) Story Shear in Y-Direction Existing Jacketing SW BRB
  43. 43. Advanced Concrete Structures, Dr. Pramin Norachan 47 0 1 2 3 4 5 -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 StoryLevel Displacement (m) MPA - MCE - Story Displacement (X) Original Jacketi ng SW BRB Limit (H/200) 0 1 2 3 4 5 -0.30 -0.20 -0.10 0.00 0.10 0.20 0.30 StoryLevel Displacement (m) MPA - MCE - Story Displacement (Y) Original Jacketing SW BRB Limit (H/200) The story displacement and story drift for the buildings with shear walls (SW) and BRB are significantly decreased.
  44. 44. Advanced Concrete Structures, Dr. Pramin Norachan 48 Based on the results, the shear capacity of many girders seems to be adequate to resist the probable shear demand with approximately 90% of D/C smaller than one. Even thought the seismic performance existing structure is improved by using column jacketing, adding shear wall or BRB, it cannot avoid shear failure in girders because girders are primary structure which are used to transfer loads to vertical members. Thus, it is recommended that only some girders about 10% need to be strengthened to resist shear demand. The strengthening method can perform by using concrete girder jacketing or CFRP.
  45. 45. Advanced Concrete Structures, Dr. Pramin Norachan 49 In terms of shear capacity, for the building with column jacketing is the most effective method to resist shear demand under MCE earthquake level, while many columns of the other strengthened buildings seem to be insufficient to resist the shear demand. Therefore, it is recommended that the retrofit for shear capacity of columns have to be provided for only few columns for the column jacketing building, while most columns of the other strengthened buildings need to be strengthened.
  46. 46. Advanced Concrete Structures, Dr. Pramin Norachan 50 The axial-flexural interaction capacity of buildings with column jacketing, shear wall or BRB seem to be improved. However, some columns need to be retrofitted to increase the capacity to resist these biaxial demand forces.
  47. 47. Advanced Concrete Structures, Dr. Pramin Norachan 51 Girders: Flexural deformation of all girders generally acceptable for both DBE and MCE level earthquakes, while few girders seem to be inadequate to resist the demand forces at MCE level. However, the retrofitting for girder flexure is negligible. For girder shear capacity, only 10% of girders need to be strengthened to resist shear demand.
  48. 48. Advanced Concrete Structures, Dr. Pramin Norachan 52 Columns: For shear capacity, most columns are insufficient to resist the shear demand for both earthquake levels. Thus, it is recommended almost all columns need to be strengthened to increase shear capacities.
  49. 49. Advanced Concrete Structures, Dr. Pramin Norachan 53 Columns: Moreover, in terms of axial-flexural interaction capacity, approximately 50% of columns seem to be overstressed under earthquakes at MCE level. Therefore, these columns need to be strengthened to resist the biaxial demand force.
  50. 50. Advanced Concrete Structures, Dr. Pramin Norachan 54 For strengthening of the existing building, three common strengthening techniques were used to improve the seismic performances of the existing building. • Column jacketing (local strengthening approach) • Adding new shear walls, SW (global strengthening approach) • Adding bucking restrained braces, BRB (global strengthening approach) With increase in the stiffness for all three strengthening methods, results show that the natural period of vibration of the building is reduced, while the base shear is increased. The story displacement and story drift for the buildings with shear walls (SW) and BRB are significantly decreased.
  51. 51. Advanced Concrete Structures, Dr. Pramin Norachan 55 Shear and axial-flexural interaction capacities of columns seem to be reduced because new shear walls and BRB can help to resist some shear demand. However, local strengthening is still required for both girders and columns. However, in the case of the column jacketing, shears capacities of the girders and tension, compression, shear and axial-flexural interaction capacities of columns seem to be adequate to resist demand forces under MCE earthquake level. In conclusion, the results indicate that displacement at roof and story drift are within the limitation. Therefore, adding new shear walls and BRB may not be necessary because the local strengthening is still required for both girders and columns. Moreover, in general, adding new shear walls are more expensive since additional foundation should be provided in this method. Therefore, based on these results, it might be concluded that the column jacketing is the most effective and the most economic strengthening method for this building. However, there are still 16% of girders that need to be strengthened to increase their shear capacity.
  52. 52. Thank you for your attention

×