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  1. Seismic Site Response Analysis Soil-Structure Interaction:Basic Concepts Steve Kramer University of Washington Steve Kramer University of Washington EERI Technical Seminar Series Impact of Soil-Structure Interaction on Response of Structures Seminar 2: Practical Applications to Deep Foundations

  2. Outline • Soil-structure interaction basics • Kinematic interaction • Inertial interaction • Analysis of soil-structure interaction • Effects of soil-structure interaction • Soil-pile-structure interaction basics • Deep foundations • Single pile/shaft behavior • Analysis of single piles • Forms of loading • Pile groups • Dynamic response • Methods of analysis • Conclusions

  3. Soil Soil-Structure Interaction How does the presence of soil affect the response of a structure? Rock Rock Does the structure founded on rock respond differently than when founded on soil?

  4. Foundation input motion Free-field motion Soil-Structure Interaction How does the presence of a structure affect the response of the soil? Soil Rock How does the motion at the base of the structure differ from the free-field motion?

  5. Soil-Structure Interaction Soil-Structure Interaction In reality, the response of the soil affects the response of the structure, and the response of the structure affects the response of the soil Two components: Kinematic interaction Presence of stiff foundation elements on or in soil cause foundation motions to deviate from free-field motions. Inertial interaction Inertial response of structure causes base shear and moments which cause displacements of foundation relative to free-field.

  6. Soil-Structure Interaction • Kinematic SSI has three primary causes: • Base slab averaging – results from stiffness of foundation • Embedment – variation of ground motion with depth • Wave scattering – scattering off corners and edges Base slab averaging – stiffness of foundation prevents it from matching free-field deformations. Exists even for massless foundation.

  7. Outline • Soil-structure interaction basics • Kinematic interaction • Inertial interaction • Analysis of soil-structure interaction • Effects of soil-structure interaction • Soil-pile-structure interaction basics • Deep foundations • Single pile/shaft behavior • Analysis of single piles • Forms of loading • Pile groups • Dynamic response • Methods of analysis • Conclusions

  8. Soil-Structure Interaction • Kinematic SSI has three primary causes: • Base slab averaging – results from stiffness of foundation • Embedment – variation of ground motion with depth • Wave scattering – scattering off corners and edges • Flexible pile • Deformation matches free-field deformation • Surface motion = free-field motion • No rotation at surface

  9. Soil-Structure Interaction • Kinematic SSI has three primary causes: • Base slab averaging – results from stiffness of foundation • Embedment – variation of ground motion with depth • Wave scattering – scattering off corners and edges • Rigid pile • Deformation different than free-field deformation – can translate and rotate • Surface motion = free-field motion • Rotation and displacement at surface

  10. Soil-Structure Interaction • Kinematic SSI has three primary causes: • Base slab averaging – results from stiffness of foundation • Embedment – reduction of ground motion with depth • Wave scattering – scattering off corners and edges Ground motion amplitude decreases with depth

  11. Soil-Structure Interaction • Kinematic SSI has three primary causes: • Base slab averaging – results from stiffness of foundation • Embedment – reduction of ground motion with depth • Wave scattering – scattering off corners and edges Vertically propagating shear waves can cause rocking as well as translation

  12. Soil-Structure Interaction • Kinematic SSI has three primary causes: • Base slab averaging – results from stiffness of foundation • Embedment – reduction of ground motion with depth • Wave scattering – scattering off corners and edges Wave scattering reduces amplitude of high frequency components

  13. Outline • Soil-structure interaction basics • Kinematic interaction • Inertial interaction • Analysis of soil-structure interaction • Effects of soil-structure interaction • Soil-pile-structure interaction basics • Deep foundations • Single pile/shaft behavior • Analysis of single piles • Forms of loading • Pile groups • Dynamic response • Methods of analysis • Conclusions

  14. Soil-Structure Interaction • Inertial SSI results from compliance of soil • Soil is not rigid – will deform due to loads from structure Translation in three directions

  15. Soil-Structure Interaction • Inertial SSI results from compliance of soil • Soil is not rigid – will deform due to loads from structure Rotation about three axes 6 x 6 stiffness matrix to describe foundation compliance

  16. 3 x 3 stiffness matrix to describe foundation compliance Soil-Structure Interaction • Inertial SSI results from compliance of soil • Soil is not rigid – will deform due to loads from structure Displacement in two directions Rocking about one axis

  17. Soil-Structure Interaction • Inertial SSI results from compliance of soil • Soil is not rigid – will deform due to loads from structure • Deformations resulting from structural forces will propagate away from structure Energy “removed” from structure – radiation damping

  18. Outline • Soil-structure interaction basics • Kinematic interaction • Inertial interaction • Analysis of soil-structure interaction • Effects of soil-structure interaction • Soil-pile-structure interaction basics • Deep foundations • Single pile/shaft behavior • Analysis of single piles • Forms of loading • Pile groups • Dynamic response • Methods of analysis • Conclusions

  19. Soil-Structure Interaction • Analysis of soil-structure interaction • Two approaches • Direct approach – model soil and structure together Requires detailed model of structure and soil in one computer program Can handle nonlinear soil and structural response

  20. Soil-Structure Interaction • Analysis of soil-structure interaction • Two approaches • Direct approach – model soil and structure together • Substructure approach – model separately and combine Kinematic SSI Inertial SSI Can use different codes for soil and structural response Superposition requires linearity

  21. Soil-Structure Interaction Analysis of kinematic soil-structure interaction

  22. qFIM uFIM Compute foundation input motions, uFIM and qFIM Model foundation as massless but with actual stiffness Soil-Structure Interaction Analysis of kinematic soil-structure interaction Influenced by stiffness and geometry of soil and foundation

  23. Qv M Qh kh kq cq ch cv kv Kv = kv + icvw Soil-Structure Interaction Impedance function – foundation stiffness and damping • 6 x 6 matrix of complex impedance coefficients • 3 translational coefficients • 3 rotational coefficients • Cross-coupling (off-diagonal) coefficients

  24. Soil-Structure Interaction Analysis of structure on compliant base subjected to FIM Based on principle of superposition – assumed linearity Frequently performed using equivalent linear approach kh uFIM kq cq ch cv kv qFIM

  25. Outline • Soil-structure interaction basics • Kinematic interaction • Inertial interaction • Analysis of soil-structure interaction • Effects of soil-structure interaction • Soil-pile-structure interaction basics • Deep foundations • Single pile/shaft behavior • Analysis of single piles • Forms of loading • Pile groups • Dynamic response • Methods of analysis • Conclusions

  26. Base of structure can translate and rotate Soil-Structure Interaction • Effects of soil-structure interaction • Consider simplified model • SDOF system on compliant base • Horizontal translation • Rocking

  27. Displacement due to distortion of structure Displacement due to rocking Displacement of ground Displacement due to horizontal translation Soil-Structure Interaction • Effects of soil-structure interaction • Consider simplified model • SDOF system on compliant base • Horizontal translation • Rocking

  28. Soil-Structure Interaction • Effects of soil-structure interaction • Consider simplified model

  29. Stiff soil Flexible structure Soft soil Stiff structure Soil-Structure Interaction Structure on circular footing of radius, r Period lengthening is negligible for a soft structure on stiff soil – it’s effects increase with increasing structure/soil relative stiffness. SSI effects are small for flexible structure on stiff site, but significant for stiff structure on soft soil. ~ T/T h/(VsT)

  30. Stiff soil Flexible structure Soft soil Stiff structure Soil-Structure Interaction Radiation damping is negligible for a soft structure on stiff soil – it’s effects increase with increasing structure/soil relative stiffness. Relative importance of radiation damping decreases with increasing h/r (increasing rocking response). SSI effects are small for flexible structure on stiff site, but significant for stiff structure on soft soil. h/(VsT)

  31. SSI can decrease structural deformations, loads Soil-Structure Interaction • Effects of soil-structure interaction • Consider simplified model • With increasing foundation flexibility, • Period lengthens • Damping increases

  32. SSI can increase total displacements Soil-Structure Interaction • Effects of soil-structure interaction • Consider simplified model • With increasing foundation flexibility, • Period lengthens • Damping increases

  33. Soil-Structure Interaction Basics • Summary • SSI is not significant for cases of flexible structures on stiff soil deposits • SSI can be quite significant for stiff structures founded on soft soils • Fundamental period of soil-structure system is longer than that of fixed-base structure • Effective damping of soil-structure system is higher than damping of structure alone • Total displacements can be increased by SSI – can be important for closely-spaced tall structures • Neglecting SSI is equivalent to assuming the structure is supported on rigid materials

  34. Outline • Soil-structure interaction basics • Kinematic interaction • Inertial interaction • Analysis of soil-structure interaction • Effects of soil-structure interaction • Soil-pile-structure interaction basics • Deep foundations • Single pile/shaft behavior • Analysis of single piles • Forms of loading • Pile groups • Dynamic response • Methods of analysis • Conclusions

  35. Deep Foundations Why? Poor soil conditions Soft, weak, compressible

  36. Deep Foundations Why? Poor soil conditions Soft, weak, compressible

  37. Deep Foundations Why? Poor soil conditions Soft, weak, compressible

  38. Deep Foundations Why? Poor soil conditions Soft, weak, compressible

  39. Deep Foundations Pile Foundations All deep foundation photos courtesy of Geo-Photo album (Ross Boulanger and Mike Duncan)

  40. Deep Foundations Pile Foundations Note excavation outside of forms

  41. Deep Foundations Pile Foundations Note excavation outside of forms

  42. Deep Foundations Drilled Shaft Foundations

  43. Deep Foundations Drilled Shaft Foundations

  44. Deep Foundations Drilled Shaft Foundations

  45. Deep Foundations Drilled Shaft Foundations

  46. Outline • Soil-structure interaction basics • Kinematic interaction • Inertial interaction • Analysis of soil-structure interaction • Effects of soil-structure interaction • Soil-pile-structure interaction basics • Deep foundations • Single pile/shaft behavior • Analysis of single piles • Forms of loading • Pile groups • Dynamic response • Methods of analysis • Conclusions

  47. High in clays (adhesion) Low in sands (friction) Low in clays (cohesive) High in sands (frictional) Deep Foundations Single pile/shaft behavior – vertical loading Applied load Skin resistance Tip resistance

  48. High in clays Low in sands Zero* in clays Zero in sands Possible momentary suction Tip resistance Deep Foundations Single pile/shaft behavior – vertical loading Applied load Skin resistance

  49. Deep Foundations Single pile/shaft behavior – vertical loading Applied load How do we measure vertical load resistance? Skin resistance Tip resistance

  50. Deep Foundations Pile load test