Seismic ssi effects and liquification


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Seismic ssi effects and liquification

  1. 1. Soil Structure Interaction and Seismic Effects Mentor : Professor Chandan Mahanta Department Of Civil engineering IIT GUWAHATI
  2. 2. Presentation Outline  Types of soil and their Interaction with seismic waves  Soil Liquefaction  Remedial Measures  Effect of P and S waves on Engineering Structures  Soil Structure Interaction (SSI)  SSI : 1) Kinematic Interaction. 2) Inertial Interaction.  Effects Of SSI  Detrimental Effects of SSI  Severe Effects In the past  Method For Modelling SSI 1) Direct Methods 2) Simple Methods  SSI in Seismic Codes  SUMMARY
  3. 3. Types of soil and their interaction with seismic waves Soil type A (unweathered intrusive igneous rock) Vs > 1500 m/sec Occurs infrequently in the bay area. Soil type B (volcanic, most Mesozoic bedrock, and some Franciscan Bedrock) Soil Type C (Quaternary sands, sandstones and mudstones, some Upper Tertiary) 1500 m/sec > Vs > 750 m/sec 750 m/sec > Vs > 350 m/sec Both soil type A & B do not contribute greatly to shaking amplification. It is in mid range neither very high shaking nor very low
  4. 4. Types of soil and their interaction with seismic waves Soil Type D (Quaternary muds, sands, gravels, silts and mud) Soil Type E (water-saturated mud and artificial fill) 350 m/sec > Vs > 200 m/sec 200 m/sec > Vs Significant amplification of shaking by these soils is generally expected. The strongest amplification of shaking due is expected for this soil type.
  5. 5. Effect Of S and P Waves on Engineering Structures
  6. 6. Soil Liquefaction Soil liquefaction describes a phenomenon whereby a saturated or partially saturated soil substantially loses strength and stiffness in response to an earthquake .
  7. 7. Remedial Measures There are basically three possibilities to reduce liquefaction hazard when designing and constructing new buildings or other structures as bridges, tunnels and roads. These are as follows 1)Avoid liquefaction susceptible 2)Build liquefaction resistant structures 3)Improvement of soil (grouting )
  8. 8. Soil structure interaction The process in which the response of the soil influences the motion of the structure and the motion of the structure influences the response of the soil is termed as soil structure interaction (SSI).
  9. 9. Soil structure interaction In this case neither the structural displacements nor the ground displacements are independent from each other SSI effects become prominent and must be regarded for structures where P-δ effects play a significant role, structures with massive or deep seated foundations, slender tall structures and structures supported on a very soft soils with average shear velocity less than 100 m/s.
  10. 10. Dynamics of Soil-Structure Interaction Soil-structure interaction can be broadly divided into two phenomena: A. Kinematic interaction B. Inertial interaction Kinematic Interaction An Embedded Foundation into soil does not follow the free field motion ( Earthquake ground motion causes soil displacement known as free- feild motion), this instability of the foundation to match the free field motion causes the kinematic interaction
  11. 11. Dynamics of Soil-Structure Interaction Inertial Interaction The second effect considering the existence of soft soil under the foundation of the structure is denoted as inertial interaction Inertial forces induced by foundation motion during the earthquake can cause the compliant soil to deform which in turn affects the super-structure inertial forces.
  12. 12. Dynamics of Soil-Structure Interaction  At low level of ground shaking, kinematic effect is more dominant causing the lengthening of period and increase in radiation damping.  With the onset of stronger shaking, inertial interaction becomes predominant, causing excessive displacements and bending strains concentrated near the ground surface, resulting in pile damage near the ground level .
  13. 13. EFFECTS OF SSI The main effects of taking soil-structure interaction into consideration can be summarized as :  First, the seismic-input motion acting on the structure- soil system will change.  Second, the radiation of energy of the propagating waves away from the structure will result in an increase of the damping of the final dynamic system.
  14. 14. EFFECTS OF SSI  Third, the presence of the soil in the final dynamic model will make the system more flexible, decreasing the fundamental frequency to a value which will, in general, be significantly below that applicable for the fixed-base structure.
  15. 15. Detrimental effects of SSI  An Increase in the natural period of a structure due to SSI is not always beneficial as suggested by the simplified design spectrums  Soft Soil Sediments can significantly elongate the period of seismic waves. The increase in the natural period of a structure (due to SSI) may lead to resonance with this long period ground vibration  The ductility demand can increase significantly with the increase in the natural period of the structure due to SSI effect. The permanent deformation and failure of soil may further aggravate the seismic response of the structure
  16. 16. Severe damages in past due to SSI Observations from recent earthquakes have shown that the response of the foundation and soil can greatly influence the overall structural response. Dramatic collapse of Hanshin Expressway in 1995 Kobe Earthquake
  17. 17. Severe damages in past due to SSI seismic behavior of a structure is highly influenced not only by the response of the superstructure, but also by the response of the foundation and the ground as well. Damage of Yashinsky cites Loma Prieta Earthquake in 1989
  18. 18. Methods for Modelling SSI Modelling soil-structure interaction in dynamic analysis falls into two main categories namely 1) Multistep methods (substructure approach) 2) Direct methods.
  19. 19. Direct methods In this approach, the equations of motion are solved directly in their coupled form and in one step Simple methods Using frequency-independent spring stiffness and a damping coefficient to account for frequency dependency of interaction is the simplest way to consider the SSI effects.
  20. 20. SSI in Seismic Codes Beneficial effect of soil structure interaction and its complicated process of analysis is the main cause to ignore their existence in seismic codes. Eurocode 8 is probably the only exception in which SSI effect is respected. The important cases in which SSI has a pronounced effect need to be considered according to part five of Eurocode 8. Some cases are as follows  Structures with massive or deep-seated foundations, such as bridge piers, offshore caissons, and silos.  Slender tall structures, such as towers and chimneys.  Structures supported on very soft soils, with average shear wave velocity less than 100m/s, such as subsoil class S1
  21. 21. Summary of soil-structure interaction effects  SSI can induce detrimental effect on some moderately flexible structures  The response of soil-structure system is very sensitive to intensity of the input motion.  Seismic Performance stipulate that the response analysis should be conducted by taking into consideration a whole structural system including superstructure, foundation and ground.
  22. 22. An Illustration In the local site at adjacent bridge pier supports is normally not the same. As bridges are commonly build in river valley where the subsoil is soft which additionally amplify the incoming seismic waves and consequently each bridge segment will respond differently, even if the ground excitation is the same.
  23. 23. Girder Pounding
  24. 24. Prevention from girder pounding
  25. 25. By : 2nd Year B.Tech Civil Engineering Department Amit Kumar Meena Amit Sathi Ankit Kumar Apurbajyoti Biswasi Arpan Banerjee 120104006 120104007 120104008 120104009 120104010