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# Modelling and analysis of base isolated structures

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Finite Elemt softwar for structural calculation about: seismic isolator, concrete, wood, stell.

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Ing. Francesco Ambrosio

email salesdirector@soft.lab.it

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### Modelling and analysis of base isolated structures

1. 1. MODELLING AND ANALYSIS OF BASE ISOLATED STRUCTURES THROUGH IPERSPACE MAXD.M. 14/01/2008 (Italian Technical Construction Regulation) Phd Ing. Stefano Ciaramella Technical Consultant R&D Copyright  Soft.Lab srl 1
2. 2. The Software… Copyright  Soft.Lab srl http://www.soft.lab.it – salesdirector@soft.lab.it 2
3. 3. The Software… Copyright  Soft.Lab srl http://www.soft.lab.it - 3 salesdirector@soft.lab.it
4. 4. The Software… Copyright  Soft.Lab srl http://www.soft.lab.it - 4 salesdirector@soft.lab.it
5. 5. The Software… Copyright  Soft.Lab srl http://www.soft.lab.it - 5 salesdirector@soft.lab.it
6. 6. The Software… Copyright  Soft.Lab srl http://www.soft.lab.it - 6 salesdirector@soft.lab.it
7. 7. The Software… Copyright  Soft.Lab srl http://www.soft.lab.it - 7 salesdirector@soft.lab.it
8. 8. Seismic isolation The approach to the earthquake-resistant construction problem: CAPACITY  DEMAND where:  the demand depends on the seismic event, which generates inertial forces in the structure. These forces are equal to the product of the masses of the structure and the accelerations due to the vibration induced by the event itself.  the capacity depends on the strength and on the non-linear deformability of the structure. Seismic Isolation: is an alternative design approach that acts on demand drastically limiting the accelerations Copyright  Soft.Lab srl http://www.soft.lab.it - 8 salesdirector@soft.lab.it
9. 9. Seismic isolation strategya) increase of the fundamental period of the building to bring it in the field of lower responses to accelerationsb) limitation of the maximum horizontal force transmitted Model of a base isolated building a) Increase of the period (and dissipation) b) Limitation of the force (and dissipation) Copyright  Soft.Lab srl http://www.soft.lab.it - 9 salesdirector@soft.lab.it
10. 10. Seismic isolation system Isolation Interface Superstructure Substructure Copyright  Soft.Lab srl http://www.soft.lab.it - 10 salesdirector@soft.lab.it
11. 11. Benefits of seismic isolation  Economically acceptable and convenient structures  Drastic reduction of the story drift which allow to create structures that do not suffer damage for devastating earthquakes  High protection of structural content  The people in the building have a minor perception of the seismic event  Great savings for repairs after high intensity earthquakes  If the building has strategic importance the earthquakes does not cause the interruption of the service. Copyright  Soft.Lab srl http://www.soft.lab.it - 11 salesdirector@soft.lab.it
12. 12. System pre-dimensioning Definition of the characteristics of the isolating system:  Stiffness  Dissipative capacity Identification of the period-damping couple (Tis, esi). Compared to the configuration of fixed-based structure (FB), this approach determines a better balancing between a satisfactory reduction of the seismic effects and horizontal displacement of the superstructure. Case Configuration T  1 Structure (FB) 0.47 sec 5% T fb  C1  H 3/4  0.47sec 2 Structure (BI) 1.50 sec 10% 3 Structure (BI) 2.00 sec 10% fixed-based structure (FB) 4 Structure (BI) 2.50 sec 10% base-isolated structure (BI) 5 Structure (BI) 1.50 sec 15% 6 Structure (BI) 2.00 sec 15% 7 Structure (BI) 2.50 sec 15% Copyright  Soft.Lab srl http://www.soft.lab.it - 12 salesdirector@soft.lab.it
13. 13. System pre-dimensioning  Equivalent period of the isolating M iso Tis  2 system: K esi 2  Horizontal equivalent stiffness of the  2  isolating system: K esi     M iso  Tis   Resultant of horizontal forces applied to the isolated system: F  M iso Se Tis , esi  M S T ,    T  2  Displacement of the stiffness centre d dc  iso e is esi   is  Se Tis , esi  of the isolating system: Kesi  2  Copyright  Soft.Lab srl http://www.soft.lab.it - 13 salesdirector@soft.lab.it
14. 14. Palette WidgetProperty Widget Copyright  Soft.Lab srl http://www.soft.lab.it - 14 salesdirector@soft.lab.it
15. 15. Copyright  Soft.Lab srl http://www.soft.lab.it - 15salesdirector@soft.lab.it
16. 16. Copyright  Soft.Lab srl http://www.soft.lab.it - 16salesdirector@soft.lab.it
17. 17. Response Spectrums Period [sec]Copyright  Soft.Lab srl http://www.soft.lab.it - 17salesdirector@soft.lab.it
18. 18. Acceleration Displacement Response Spectrum Copyright  Soft.Lab srl http://www.soft.lab.it - 18 salesdirector@soft.lab.it
19. 19. Stiffness Centre Displacement 2  T  d dc    Se  T ,    2  T  ddc Case Configuration [sec] [%] [mm] 2 Structure (BI) 1.50 10% 156 3 Structure (BI) 2.00 10% 218 4 Structure (BI) 2.50 10% 280 5 Structure (BI) 1.50 15% 135 6 Structure (BI) 2.00 15% 189 7 Structure (BI) 2.50 15% 242 Copyright  Soft.Lab srl http://www.soft.lab.it - 19 salesdirector@soft.lab.it
20. 20. Copyright  Soft.Lab srl http://www.soft.lab.it - 20salesdirector@soft.lab.it
21. 21. Response Spectrums Elastic Spectrum Structure Project Spectrum Structure (FB) Period [sec] Copyright  Soft.Lab srl http://www.soft.lab.it - 21 salesdirector@soft.lab.it
22. 22. Shear force at the bottom of the superstructure F  M  Se  T ,   M  600 t T  Shear Force Case Configuration [sec] [%] [KN] 1 Structure (FB) 0.47 5% 1550 2 Structure (BI) 1.50 10% 1260 3 Structure (BI) 2.00 10% 960 4 Structure (BI) 2.50 10% 740 5 Structure (BI) 1.50 15% 1100 6 Structure (BI) 2.00 15% 770 7 Structure (BI) 2.50 15% 630 Copyright  Soft.Lab srl http://www.soft.lab.it - 22 salesdirector@soft.lab.it
23. 23. Horizontal stiffness 2  2  K esi     M iso  Tis  ki  Kesi / n of pillars M iso  790 t T Kesi ki Case Configuration [sec] [KN/m] [KN/m] 2-5 Structure (BI) 1.50 13861 770.0 3-6 Structure (BI) 2.00 7896 438.7 4-7 Structure (BI) 2.50 5053 280.7 Copyright  Soft.Lab srl http://www.soft.lab.it - 23 salesdirector@soft.lab.it
24. 24. Summary of the results T  ddc Shear Force Kesi ki Case Configuration [sec] [%] [mm] [KN] [KN/m] [KN/m] 1 Structure (FB) 0.47 5% - 1550 - - 2 Structure (BI) 1.50 10% 156 1260 13861 770.0 3 Structure (BI) 2.00 10% 218 960 7896 438.7 4 Structure (BI) 2.50 10% 280 740 5053 280.7 5 Structure (BI) 1.50 15% 135 1100 13861 770.0 6 Structure (BI) 2.00 15% 189 770 7896 438.7 7 Structure (BI) 2.50 15% 242 630 5053 280.7 Seismic Effects: 50% reduction compared to the FB configuration Copyright  Soft.Lab srl http://www.soft.lab.it - 24 salesdirector@soft.lab.it
25. 25. Palette WidgetProperty Widget Copyright  Soft.Lab srl http://www.soft.lab.it - 25 salesdirector@soft.lab.it
26. 26. d =189 mm + 30% =246 mmPreliminary Analysis Ko = 0.439 kN/mm Copyright  Soft.Lab srl http://www.soft.lab.it - 26 salesdirector@soft.lab.it
27. 27. Adding an isolating element to the program library1. Go to the section Isolatori(“Isolator”) in the widget Elementi 3. In the property widgete click on Nuovo (“New”). (“Proprietà”) through the section Generici, insert the vertical and horizontal stiffness taken from the catalogue. 2. Insert the code for the new isolator. Copyright  Soft.Lab srl http://www.soft.lab.it - 27 salesdirector@soft.lab.it
28. 28. Inserting isolators in the model of the structure 1. Selecting one or more pillars in the substructure. 2. Click on Crea (“Create”)  Isolatore sui selezionati (“selected isolators”) 3. Choose the isolator type, define its high and confirm (√) Copyright  Soft.Lab srl http://www.soft.lab.it - 28 salesdirector@soft.lab.it
29. 29. Structural analysis: fixed-based structure 2nd mode 1st mode 3rd mode Copyright  Soft.Lab srl http://www.soft.lab.it - 29 salesdirector@soft.lab.it
30. 30. Structural analysis: fixed-based structure Preliminary Analysis T = 0.47 sec Copyright  Soft.Lab srl http://www.soft.lab.it - 30 salesdirector@soft.lab.it
31. 31. Structural analysis: fixed-based structure Preliminary Analysis F = 155000 daN Copyright  Soft.Lab srl http://www.soft.lab.it - 31 salesdirector@soft.lab.it
32. 32. Structural analysis: base-isolated structure  Use of isolation devices “FIP INDUSTIALE” series SI-S 400/125  Reduction of the elastic spectrum for T  0,8 Tis = 1.6 sec  Assumes  = esi = 15% for T  0,8 Tis and  = 5% for T < 0,8 Tis Copyright  Soft.Lab srl http://www.soft.lab.it - 32 salesdirector@soft.lab.it
33. 33. Structural analysis: base-isolated structure For the ultimate limit state verification, the needed resistance of structural elements of the superstructure can be met by considering the seismic effects reduced by the factor of 1/q=0.6667, where q=1.5 is the structure factor. Copyright  Soft.Lab srl http://www.soft.lab.it - 33 salesdirector@soft.lab.it
34. 34. Structural analysis: base-isolated structure Preliminary Analysis T = 2.0 sec Copyright  Soft.Lab srl http://www.soft.lab.it - 34 salesdirector@soft.lab.it
35. 35. Structural analysis: base-isolated structure Preliminary Analysis F = 77000 daN Copyright  Soft.Lab srl http://www.soft.lab.it - 35 salesdirector@soft.lab.it
36. 36. The following figure shows the deformation of the structure due to a seismic event aligned with the x-axis.The isolator maximum horizontaldisplacement is d = 221 mm, not far fromour preliminary prediction (246 mm) andhowever under the limit of the isolator (250mm). Copyright  Soft.Lab srl http://www.soft.lab.it - 36 salesdirector@soft.lab.it
37. 37. Limit State Verification Ultimate Limit State Verification Damage Limit State Verification Copyright  Soft.Lab srl http://www.soft.lab.it - 37 salesdirector@soft.lab.it
38. 38. Ultimate Limit State VerificationThe superstructure and substructure should be designed with reference to constructiondetails related to the non seismic zone (Geometric and Reinforcement Limitations) Copyright  Soft.Lab srl http://www.soft.lab.it - 38 salesdirector@soft.lab.it
39. 39. Ultimate Limit State Verification Copyright  Soft.Lab srl http://www.soft.lab.it - 39 salesdirector@soft.lab.it
40. 40. Damage Limite State Verification For the superstructure, the verification must be carried out controlling that the story drift, obtained from the analysis, is under the 2/3 of the Damage Limite State limits of conventional structures. This verification is carried out by setting k(*h) = 0.005x2/3 = 0.00333333 into the “Impalcati” section of the property widget and finally checking the results. Copyright  Soft.Lab srl http://www.soft.lab.it - 40 salesdirector@soft.lab.it
41. 41. Further Verifications  However, it remains to be performed the verification for the parts involved in the non-dissipative function. These should remain in the elastic range even under the conditions of maximum stress, according to the rules relating to the materials they are made. For this verification, also a safety factor (≥1.5) have to be taken into account.  For the replacement of isolators, the lifting by hydraulic jacks could be required. Therefore it is necessary to evaluate the dimensions of the concrete squat above the isolation interface and calculate an additional bottom reinforcement.  In order to prevent or reduce traction in the seismic isolation devices, the vertical load design "V“, due to seismic actions, should be compressive or zero (V ≥ 0). In the case that V < 0, the modulus of the tensile stress should be minor both of 2G and 1 Mpa into the isolators (G is the shear modulus).  For further examinations regarding these issues, the reader can refer to the specific publications available. Copyright  Soft.Lab srl http://www.soft.lab.it – salesdirector@soft.lab.it 41