2. Introduction
Advantages
Types of interaction
Degree of influence of SSI
General method of analysis
Analysis w.r.t software
References
3. 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.
Traditional Structural Engineering methods
disregard SSI effects.
SSI effects become prominent and must be
regarded for structures where P-δ effects play a
significant role.
5. V → relative foundation/free-field
displacement (uf)
6. M → relative foundation/free-field rotation
(θf)
7. Considering soil-structure interaction makes a
structure more flexible and thus increases the
natural period of the structure as compared to
the corresponding rigidly supported structure.
Considering the SSI effect increases the
effective damping ratio of the system
(Superstructure + Foundation + Soil ).
9. 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.
10.
11. Incoherent ground motions → base slab
averaging.
Ground motion reductions with depth.
12. Loads from superstructure inertia.
Deformations applied by soil.
13. 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.
15. 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 st .
The nodes along the interior foundation
medium/soil are denoted by s .
17. The good representation of damping matrix is
difficult.
If the superstructure is modeled as 3D system,
the problem size becomes very large and the
modeling of soil/foundation – structure
interface becomes complex.
18. 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.
20. In case of soil/foundation medium
modeling of some structures, a portion of the
soil may be included in the superstructure as
shown in Fig.
21. Three-dimensional finite difference models
were developed with the software FLAC3D.
This analysis is done in highly compressible
very soft clay.
This type of soil is present in Mexico City
valley.