Soil Structure Interaction and Seismic Effects

Mentor :Professor Chandan MahantaDepartment Of Civil engineeringIIT GUWAHATI

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 Methods2) Simple Methods

SSI in Seismic Codes SUMMARY

Soil type A(unweatheredintrusive igneous rock)

Vs > 1500 m/sec Occurs infrequently in the bay area.

Soil type B(volcanic, most Mesozoic bedrock, and some FranciscanBedrock)

1500 m/sec > Vs > 750 m/sec Both soil type A & B do not contribute greatly to shaking amplification.

Types of soil and their interaction with seismic waves

Soil Type C(Quaternary sands, sandstones andmudstones, some Upper Tertiary)

750 m/sec > Vs > 350 m/sec It is in mid range neither very high shaking nor very low

Soil Type D (Quaternary muds, sands, gravels, silts and mud)

350 m/sec > Vs > 200 m/secSignificant amplification of shaking by these soils is generally expected.

Soil Type E(water-saturated mud and artificial fill)

200 m/sec > VsThe strongest amplification of shaking due is expected for this soil type.

Types of soil and their interaction with seismic waves

Effect Of S and P Waves on Engineering Structures

Soil liquefaction describes a phenomenon whereby a saturated or partially saturated soil substantially loses strength and stiffness in response to an earthquake .

Soil Liquefaction

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 )

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).

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.

Soil structure interaction

In this case neither the structural displacements nor the ground displacements are independent from each other

Soil-structure interaction can be broadly divided into two phenomena:A. Kinematic interactionB. 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

Dynamics of Soil-Structure Interaction

The second effect considering the existence of soft soil under the foundation of the structure is denoted as inertial interaction

Inertial Interaction

Dynamics of Soil-Structure 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.

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 .

Dynamics of Soil-Structure Interaction

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.

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.

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

Severe damages in past due to SSI

Dramatic collapse of Hanshin Expressway in 1995 Kobe Earthquake

Observations from recent earthquakes have shown that the response of the foundation and soil can greatly influence the overall structural response.

Severe damages in past due to SSI

Damage of Yashinsky cites Loma Prieta Earthquake in 1989

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.

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.

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.

SSI in Seismic CodesBeneficial 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 apronounced 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

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.

An IllustrationIn 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.

Girder Pounding

Prevention from girder pounding

By :2nd Year B.TechCivil Engineering Department

Amit Kumar Meena 120104006Amit Sathi 120104007Ankit Kumar 120104008Apurbajyoti Biswasi 120104009Arpan Banerjee 120104010