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**Soil-Structure Interaction**

ECIV 724A Fall 2004

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**SSI – Problem Definition**

Earthquake Analysis Structures supported by rigid foundations Earthquakes=>Specified motion of base Rigid Base Analysis Tall Buildings Acceptable Light & Flexible Firm Foundations Methods focus on modeling of structure Displacements wrt fixed base Finite Element Methods Nuclear Power Plants Wrong Assumption Massive & Stiff Soft Soils Interaction with supporting soils becomes important

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**SSI – Problem Definition**

Machine Foundation Seismic Excitation Parameters Local Soil Conditions Peak Acceleration Frequency Content of Motion Proximity to Fault Travel Path etc Inertial Interaction Inertial forces in structure are transmitted to flexible soil Kinematic Interaction Stiffer foundation cannot conform to the distortions of soil TOTAL=INERTIAL + KINEMATIC

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SSI Effects Posin( w t) Half Space 2b H

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SSI Effects

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**Cross Interaction Effects**

3. …Reach Receiver… 1. Moment is applied 2. Waves Propagate… 4. …and life goes on…

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**SSI Effects Alter the Natural Frequency of the Structure Add Damping**

Through the Soil Interaction Effects Traveling Wave Effects

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**Methods of Analysis Objective:**

Given the earthquake ground motions that would occur on the surface of the ground in the absence of the structure (control or design motions), find the dynamic response of the structure.

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Methods of Analysis Methods Complete Idealized Direct MultiStep

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**Complete Interaction Analysis**

High Degree of Complexity Account for the variation of soil properties with depth. Consider the material nonlinear behavior of the soil Consider the 3-D nature of the problem Consider the nature of the wave propagation which produced the ground motion Consider possible interaction with adjacent structures.

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**Idealized Interaction Analysis**

Idealization Horizontal Layers Simplified Wave Mechanisms etc

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**Idealized Interaction Analysis**

Preliminary description of free field motion before any structure has been built The definition of the motion itself the control motion in terms of response spectra, acceleration records etc The location of the control motion free surface, soil-rock interface The generation mechanism at the control point vertically or obliquely incident SH or SV waves, Rayleigh waves, etc.

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**Idealized Analysis MultiStep Methods Direct Methods**

Idealized Interaction Analysis Tools: FEM, BEM, FDE, Analytical solutions MultiStep Methods Evaluation of Dynamic Response in Several Steps SUPERPOSITION Two-Step Kinematic+Inertia Interaction Three-Step Rigid Foundations Lumped Parameter Models Substructure Division to Subsystems Equilibrium & Compatibility Direct Methods Evaluation of Dynamic Response in a Single Step True Nonlinear Solutions

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**Finite Element Method (FEM)**

Governing Equation Solution Techniques Modal Analysis Direct Integration Fourier Analysis - Complex Response

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**FEM Solution Techniques**

Selection Criteria Cost and Feasibility Paramount Consideration Accuracy Differences - Handling of Damping - Ability to Handle High Frequency Components of Motion

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**FEM - Modal Analysis Damping is neglected during early stages**

Actual displacements are damped Damping is considered in arbitrary manner Structural Dynamics: First few modes need to be evaluated (<20) SSI: Acceleration response spectra over a large frequency range and large number of modes need to be considered (>150) Not recommended for Direct SSI - Stiff Massive Structure Soft Soil OK for Substructure

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**FEM - Direct Integration**

Time Marching Schemes Newmark’s Methods, WilsonJ Methods, Bathe and Wilson Cubic Inertia Method Small Time Step for Accuracy Stability and Convergence Choice of Damping Matrix Frequency Dependent Damping Ratio - filters out high frequency components Proportional Damping Good Choice if True Dynamic Nonlinear Analysis is feasible

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**FEM - Complex Response Fourier Transformation - Transfer Functions**

Transfer Functions Independent of External Excitation Control of Accuracy Efficient Only Linear or Pseudo non-linear analysis

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**FEM - Geometric Modeling**

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FEM Modeling Max Element Size Governed by Highest frequency which must be transmitted correctly within the element

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**FEM Modeling of Infinite Space**

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**FEM Modeling of Infinite Space**

Modeling Introduces Artificial Boundaries that Reflect Waves

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**FEM Modeling of Infinite Soil**

Absorbing Boundaries Viscous Boundary Variable Depth Method Damping proportional to Wave Velocities Radiating Boundaries (Hyperelements) Satisfy Boundary Conditions at Infinity Eigenvalue Analysis Frequency Domain Analysis

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**SSI – FEM Methods FEM Advantages Non-Linear Analysis Well Established**

Shortcomings Finite Domains Volume Discretizations

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**Boundary Element Methods**

Governing Equation Small Displacement Field Homogeneous Isotropic Elastic

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**Boundary Element Method**

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**Boundary Element Method**

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**BEM – Methods BEM Advantages Infinite Media Surface Discretization**

Shortcomings Non-symmetric matrices Not Efficient for Nonlinear

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**eliminate disadvantages of each method and retain advantages**

SSI Methods Combined BEM-FEM eliminate disadvantages of each method and retain advantages Approach FEM Approach BEM Approach Staggered Solutions

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Governing Equations

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**FEM Method Time Marching Scheme**

Governing Equation Discrete Form in Time

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**FEM-BEM Coupling Staggered Solutions**

Can be Solved in a Staggered Approach...

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**FEM-BEM Coupling Staggered Solutions**

Compatibility of Displacements at Interface BEM Solver FEM Equilibrium of Forces External Excitation At Every Time Step...

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**FEM-BEM Coupling Advantages**

Independent Solutions for BEM and FEM Independent Time Step Selection Smaller Systems of Equations BEM System of Reduced Size In the Absence of Incidence Displacement Field in Soil, BEM does not require Solution.

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**Lumped Parameter Models for SSI**

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**Lumped Parameter Foundation Models**

Reissner (1936) Analytic Solutions to Vertical Vibration of Circular Footing Due to Harmonic Excitation Assumptions: Elastic ½-space Material G,v,r Uniform Vertical Pressure Formed Basis of Almost All Analytical Studies

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**Lumped Parameter Foundation Models**

Quinlan and Sung Assumed Different Pressure Distributions Richart & Whitman Effects of Poisson’ Bycroft (1956) Displacement Functions Hsieh K and C in terms of Soil and Foundation Parameters

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**Lumped Parameter Foundation Models**

Lysmer Analog Constant Lumped Parameters Richart Hall & Wood(1970) Gazetas (1983) Wolf (1988)

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**Lumped Parameter Foundation Models**

Representative Lumped Parameter Values - Square

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**Lumped Parameter Foundation Models**

Representative Lumped Parameter Values Circular

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**Lumped Parameter Foundation Models**

Stehmeyer and Rizos (2003) The Real System Equivalent SDOF System Properties k, and c are known to be frequency (w) dependent

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**Lumped Parameter Foundation Models**

wn = 3.3 x = 0.975

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SSI Effects Posin( w t) Half Space 2b H

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SSI Effects

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SSI Effects Based on the Simplified Lumped Parameter Models it can be shown that Longer Period of Foundation-Structure System

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**SSI Effects – Cross Interaction**

Receiver Foundation Source Foundation

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**SSI Effects – Cross Interaction**

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**SSI Effects – Cross Interaction**

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**Traveling Wave Effects**

After Betti et al.

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**Traveling Wave Effects**

After Betti et al.

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**Traveling Wave Effects**

After Betti et al.

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**Traveling Wave Effects**

After Betti et al.

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SH-Waves After Betti et al.

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P-Waves After Betti et al.

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SV-Waves After Betti et al.

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Rayleigh Waves After Betti et al.

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**Traveling Wave Effects**

Inertia Effects were Not Important but yet SSI significantly affects the response Asynchronous Motion Excite Antisymmetric Vibration Modes SSI effects cannot be ignored After Betti et al.

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