Foundation Flexibility and Damping from Forced Vibration Testing of Field Structure

Lisa M. StarMichael J. GivensGeorge MylonakisJonathan P. Stewart7-11-2012

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What is Soil-Structure Interaction (SSI)?

SSI is the collection of effects that soil behavior has on a structure and structural behavior has on the soil

Two Important SSI effects:Altered Ground Motions (Kinematic

Interaction Effects)Foundation Stiffness and Damping

(Inertial Interaction Effects)

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What is SSI? – Inertial Effects

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What is SSI? – Inertial Effects

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What is SSI? – Inertial Effects

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What is SSI? – Inertial Effects

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What is SSI? – Inertial Effects

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The SSI Problem Small

Stiff/Strong Foundation

Small displacements protect frame from damage

High forces cause wall damage

Large

Flexible/Weak Foundation

Large displacements cause frame damage

Foundation yield and rocking protects wall

Small

Stiff/Strong Foundation

Small displacements protect frame from damage

High forces cause wall damage

Large

Flexible/Weak Foundation

Large displacements cause frame damage

Foundation yield and rocking protects wall

Foundation flexibility changes structure behavior and the failure mode

Conservative or unconservative?

from ATC 40

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UCLA SSI Test Structure2.13 m x 4.26

m foundation2.13 m tall

HSS 12”x12”x1/2” steel columns

0.25 m thick reinforced concrete upper deck

Removable bracing

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Field Test Sites - WLA

NEES@UCSB Wildlife Refuge Liquefaction Field Site

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Field Test Sites - GVDANEES@UCSB Garner Valley Field Site

Test structure-soil-structure interaction

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Foundation Stiffness and Damping

Adapted from Tileylioglu et al. (2011)

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Existing SSI ModelsFunction of G, , and foundation

dimensions

Baseline Knowledge Report, Stewart et al. 2010

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Foundation Impedance - Theory

Translation and Rotation:

2 2sh s ff f s ff ff

xf

F m u h u m u hk

u

*

2 2 2sh s ff f s ff ff ff f

yyf

hF m h u h u I m h u hk

*

*j

jj

Imaginary(k )( * k ) 2

j jk real k *( )

Adapted from Tileylioglu et al. (2011)

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Foundation Impedances

Longitudinal

Transverse

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WL - Foundation Impedance - Shaking in Longitudinal Direction

Compared to Pais and Kausel (1988)Model 1: Vs=108 m/sModel 2: Vs=98 m/s

Max Strain: 0.00045% G/Gmax: >0.985

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WL - Foundation Impedance - Shaking in Transverse Direction

Compared to Pais and Kausel (1988)

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GVDA - Foundation Impedance - Shaking in Transverse Direction

Compared to Pais and Kausel (1988)Model 3: Vs=207 m/s

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Large Amplitude Shaking in Longitudinal Direction

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Large Amplitude - Shaking in Longitudinal Direction

Compared to Pais and Kausel (1988)

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Large Amplitude - Shaking in Longitudinal Direction

Compared to Pais and Kausel (1988)

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Compared to Pais and Kausel (1988)

Large Amplitude - Shaking in Transverse Direction

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Summary and Conclusionso There were a very small number of field

scale tests available. The experiment provides high quality data that is freely available to other researchers.

o Soil-Foundation Impedances:oWe find that kx weakly decreases with f and kyy

strongly decreases with foWe find that is strongly dependant on f

o Nonlinear Soil-Foundation Impedance:oFoundation stiffness decreases as strain

increasesoDamping increases as strain increases

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AcknowledgementsNEES@UCLANEES@UCSBNEES@UtexasSalih Tileylioglu, Ph.D.NSF CMMI 0618804, PI Jack

MoehleNSF CMMI 0927178 (NEES

Operations, PI Julio Ramirez)

Thank You