- Soil-structure interaction (SSI) describes how the response of soil influences the motion of a structure, and vice versa, rather than having independent displacements. - The three critical aspects of SSI are: 1) Inertia effects on base shear and moment, 2) How base shear relates to foundation/soil displacement, 3) How moment relates to foundation/soil rotation. - The degree of SSI influence depends on soil stiffness, structure properties like period and damping, and structure stiffness and mass. SSI is more important for flexible structures on soft soil.

1. Soil - Structure Interaction
SSI
-Thurumella Jayanth

2. Over View
● Introduction to SSI
● Three critical aspect of SSI
● Degree of influence of SSI
● Method of analysis
● Analysis (Flac 3D)

3. Introduction to SSI
● 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 SSI.
● In this case neither the structural displacements nor the
ground displacements are independent from each other.
● Traditional Structural Engineering methods disregard SSI
effects, which is acceptable only for Light structures on
relatively stiff soil.
Ex. low rise structures , simple rigid retaining walls.

4. Three critical aspect of SSI
● Inertia →base shear (V)
and moment (M)
● V →relative
foundation/free-field
displacement (uf)
● M →relative
foundation/free-field
rotation (θf)

5. ● uf, θf → foundation damping
● Radiation damping foundation
acts as wave source
Three critical aspect of SSI (Cont..)

6. Type of interaction
● Inertial soil structure interaction
● Kinematic interaction
● Foundation deformations
Three critical aspect of SSI (Cont..)

7. Inertial soil structure interaction
● Inertia from vibration of structure and
foundation Causes foundation
translation and rotation (uf and θf).
● Directly affects system flexibility and
mode shapes.
● Introduces foundation damping.

8. Kinematic interaction
● Incoherent ground motions → base
slab averaging.
● Ground motion reductions with
depth.

9. Foundation deformations
● Loads from superstructure
inertia.
● Deformations applied by soil.

10. Degree of influence of SSI
The degree of Influence of SSI on response of
structure depends on the following factors
● Stiffness of soil.
● Dynamic Characteristics of structure itself i.e.
Natural Period and damping factor.
● Stiffness and mass of structure.

11. Method of analysis
● Direct method
● Sub-Structure Method

12. Direct method
● In this soil, structure and foundation is modeled together using
finite element method (FEM).
● The ground motion is specified as free field motion and is applied
at all boundaries.
● The nodes along the soil-structure interface are denoted by
subscript f (foundation).
● The nodes of the structure are denoted by stright
● The nodes along the interior foundation medium/soil are
denoted by s .

13. Direct method

14. Sub-Structure Method
● It is computationally more efficient than the direct method as
most of the disadvantages of the direct method can be
removed.
● In this method the effective input motion is expressed in
terms of free-field motions of the soil layer initially.
● In continuation to this step, the soil/foundation medium and
the structure are represented as two independent
mathematical models or substructures as shown in Fig.

15. Sub-Structure Method

16. Analysis (Flac 3D)
● Three-dimensional finite difference models were
developed with the software FLAC3D.
● This analysis is done in highly compressible very
soft clay.

17. Thank You