Foundation Flexibility and Damping from
Forced Vibration Testing of Field Structure
Lisa M. Star
Michael J. Givens
George Mylonakis
Jonathan P. Stewart
7-11-2012
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)
2
What is SSI? – Inertial Effects
3
What is SSI? – Inertial Effects
4
What is SSI? – Inertial Effects
5
What is SSI? – Inertial Effects
6
What is SSI? – Inertial Effects
7
The SSI Problem
 Small
High forces
cause wall
damage
Foundation
yield and
rocking protects
wall
Small
displacements
protect frame
from damage
Stiff/Strong Foundation
 Large
Foundation flexibility
changes structure
behavior and the
failure mode
 Conservative or
unconservative?

Large
displacements
cause frame
damage
Flexible/Weak Foundation
from ATC 40
8
UCLA SSI Test Structure




2.13 m x 4.26 m
foundation
2.13 m tall HSS
12”x12”x1/2”
steel columns
0.25 m thick
reinforced
concrete upper
deck
Removable
bracing
9
Field Test Sites - WLA
NEES@UCSB Wildlife Refuge
Liquefaction Field Site
10
Field Test Sites - GVDA
NEES@UCSB
Garner Valley
Field Site
Test structure-soil-structure interaction
11
Foundation Stiffness and Damping
Adapted from Tileylioglu et
al. (2011)
12
Existing SSI Models

Function of G, , and foundation
dimensions
Baseline Knowledge Report, Stewart et al. 2010
13
Foundation Impedance - Theory
Translation and Rotation:
Fsh   m s  u f  h f  f  u s    m f  u f  h f  f
2
kx 
*
2
uf
hFsh   m s h  u f  h f  f  u s    I f  f   m f h f  u f  h f  f
2
k
*
yy


2
2

f
k j  re a l (k j )
*
Adapted from Tileylioglu et al. (2011)
*
j 
Im aginary ( k j )
(2* k j )
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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/s
Model 2:Vs=98 m/s
Max Strain: 0.00045%
G/Gmax: >0.985
16
WL - Foundation Impedance Shaking in Transverse Direction
Compared to
Pais and Kausel (1988)
17
GVDA - Foundation Impedance Shaking in Transverse Direction
Compared to
Pais and Kausel (1988)
Model 3:Vs=207 m/s
18
Large Amplitude Shaking in
Longitudinal Direction
19
Large
Amplitude Shaking in
Longitudinal
Direction
Compared to
Pais and Kausel (1988)
20
Large
Amplitude Shaking in
Longitudinal
Direction
Compared to
Pais and Kausel (1988)
21
Large
Amplitude Shaking in
Transverse
Direction
Compared to
Pais and Kausel (1988)
22
Summary and Conclusions
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:
o
o We find that kx weakly decreases with f and kyy
strongly decreases with f
o We find that  is strongly dependant on f
o
Nonlinear Soil-Foundation Impedance:
o Foundation stiffness decreases as strain increases
o Damping increases as strain increases
23
Acknowledgements
NEES@UCLA
 NEES@UCSB
 NEES@Utexas
 Salih Tileylioglu, Ph.D.
 NSF CMMI 0618804, PI Jack Moehle
 NSF CMMI 0927178 (NEES Operations,
PI Julio Ramirez)

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Thank You