1.
Soil Structure Interaction and
Seismic Effects
Mentor :
Professor Chandan Mahanta
Department Of Civil engineering
IIT GUWAHATI
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.
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.
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.
Effect Of S and P Waves on Engineering
Structures
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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