modeling of cyclic load-deformation behavior of footing
TRANSCRIPT
Modeling of Cyclic Load-Deformation Behavior of Footing-Soil Interface
Sivapalan Gajan
Advisor: Bruce Kutter
Geotech Seminar
University of California, Davis
11. 10. 2004
Overview
Brief introduction to the project
Centrifuge experiments
Modeling of footing-soil interface
Comparison of model simulations with experiments
Implementation of the model in OpenSEES
Shallow foundations supporting rocking shear wallsRocking of shear wall and foundation
Partial separation of footing (uplift) and soil yielding
Highly nonlinear bearing pressure distribution
Location of footing-soil contact area and the bearing pressure distribution
Base shear loading produces sliding of footing
Shear wall and frame structure (after ATC, 1997)
Forces and displacements acting on the footing-soil interface
s
uθ
M
V
H
s: vertical displacement (settlement)u: horizontal displacement (sliding)θ: rotationV: vertical loadH: horizontal loadM: moment
Purposes of Research
To study the effects of soil-foundation interaction in shallow foundations during cyclic and dynamic loading
To explore the nonlinear load-displacement behavior of shallow foundations under combined (V-H-M) loading
Vertical force – settlementHorizontal force – slidingMoment – rotation
To perform centrifuge experiments with influencing parameters systematically varied
To develop numerical models that can be used in design
Scope of Research
Physical ModelingCentrifuge experiments – model footings attached with shear wall structures tested on both sand and clay
Constitutive ModelingModeling of cyclic load-displacement behavior of footing-soil interface
Numerical ModelingImplementing the footing-soil interface model in OpenSEES finite element framework
Centrifuge experiments
7 series of centrifuge experiments including about 60 shear wall-footing models
(KRR01, 02, 03, SSG02, 03, 04, JMT01)
Parameters varied
Soil propertiesSoil type (sand and clay)Dr (80% and 60%)
Structure propertiesShear wall weight (FS = 2 to 10)Footing geometry (rectangular and square)Footing embedment (D = 0 to 3B)
Loading typesPure vertical loadingLateral slow cyclic loading
Controlling moment to shear ratio (one actuator)Controlling rotation and sliding (two actuators)
Dynamic base shaking
Slow cyclic lateral loading – one actuator
1.23 1.1
0.43
M=H.h
H
h/L = 1.8
M=H.h
V/Vmax
V/Vmax = 0.1 ~ 0.5
1.23 1.1
0.43
M=H.h
H
h/L = 1.8
1.23 1.1
0.43
M=H.h
H
h/L = 1.8
M=H.h
V/Vmax
V/Vmax = 0.1 ~ 0.5
M=H.h
V/Vmax
V/Vmax = 0.1 ~ 0.5
s
uθ
M
V
H
Slow cyclic lateral loading – two actuators
θ
u
θ
1h2h2d1h1d2hu
1h2h1d2d
−⋅−⋅
=
−−
=θ
11hu
12hu
1d2d
+⋅
θ
+⋅
θ
=
Slow cyclic lateral loading - Animation
Experimental results
0.08 0.06 0.04 0.02 0 0.02 0.04 0.06400
200
0
200
400
Rotation (rad)
Mom
ent,
M (k
N.m
)
20 0 20 40 60
50
0
50
Horizontal displacement, u (mm)
Hor
izon
tal l
oad,
H (k
N)
0.08 0.06 0.04 0.02 0 0.02 0.04
100
50
0
Rotation (rad)
Settl
emen
t, s (
mm
)
20 0 20 40 60
100
50
0
Horizontal displacement, u (mm)
Settl
emen
t, s (
mm
)
file "U:\Gajan\SSG02\20g-cyclicTests\test3\Test#3a.prt"=
Data from test SSG02, test#3a, FS = 6, embedment = 0.0m, load height = 4.9m, footing length = 2.84m
Modeling of footing-soil interface behaviorBeam on nonlinear Winkler foundation
Collaborative researchTara Hutchinson (UCI)
Macro modeling
Houlsby and Cassidy Cremer et. al
Contact-Element modelingConsiders foundation and surrounding soil as a single macro-elementConstitutive model that relates the forces (V, H, M) and displacements(s, u, θ) acting at the center of the footing
contact-element
Forces at soil-footing interface
O
V
H
)sin(F)cos(RV θ⋅+θ⋅= )sin(R)cos(FH θ⋅−θ⋅=
eR)sin(hcgVhHM ⋅=θ⋅⋅+⋅=
Vinitial positionO
O
displaced position
V
M
F
R
H
R
θ
Eccentricity animation – experiment
θ
θ+
θθ
⋅=θθ
θθ
⋅=θθ
θ⋅=θ
d)(de
d)(deR
d)(dM
d)(deR
d)(dM
)(eR)(M
pressurecontact
Contact lengthFSCurvature (θmax)
Pressure distributionSoil typeContact length
Rounding of soil surface beneath footing
-4 -2 0 2 4 6 8 10Horizontal Distance (inches)
-1.5-1
-0.50
0.51
1.5
Ver
tical
D
ista
nce
(inch
es)
KRR02, Station AE, West Footing
-10 -8 -6 -4 -2 0 2 4 6Horizontal Distance (inches)
-1-0.5
00.5
1
Ver
tical
D
ista
nce
(inch
es)
KRR02, Station CE, West Footing
-10 -8 -6 -4 -2 0 2 4 6Horizontal Distance (inches)
-1-0.5
00.5
1
Ver
tical
D
ista
nce
(inch
es)
KRR02, Station CE, East Footing
-4 -2 0 2 4 6 8 10Horizontal Distance (inches)-1.5
-1
-0.5
0
0.5
1
Ver
tical
D
ista
nce
(inch
es)
KRR02, Station AE, East Footing
θ1, degree of rounding θ2, degree of rounding
θ=(θ1+θ2)/2=5 degrees
θ=3.5 degrees
θ=4 degrees
θ=10 degrees
Rosebrook (2002)
Factors affecting moment-rotation behavior
Vertical factor of safetycontact length
Type of pressure distributionlocation of eccentricity
Vertical stiffness of soilMaximum rotation experienced by the soil
curvature of soil surfacecontact location
Moment-rotation model formulation
Internal variables
xi
i c db
pressure distribution
V
O
M
footing
soil_min
a
soil_max
∆i
Ri
θ
Footing locationCurrent soil surface location (soil_min)Maximum past settlement (soil_max)Current bearing pressureMaximum past pressure experienced
Method of computation
xi
i c db
pressure distribution
V
O
M
footing
soil_min
a
soil_max
∆i
Ri
1. The incremental rotation (∆θ) is applied to the contact element
2. The point of rotation of the rigid footing, for that ∆θ, is initially assumed (starts with the footing-soil_max contact point at the back side of the footing)
3. The new location of the footing is updated at every node.
Method of computation
xi
i c db
pressure distribution
V
O
M
footing
soil_min
a
soil_max
∆i
Ri
4. The new position of the soil_min surface is located using the rebounding ratio and footing position
5. The new position of the soil_min surface is located using the rebounding ratio and footing position
6. The contact nodes of the footing with soil_min and soil_max surfaces at both sides of the footing is found from the new locations of footing and soil surfaces (nodes a, b, c, and d in Fig.)
Method of computation
xi
i c db
pressure distribution
V
O
M
footing
soil_min
a
soil_max
∆i
Ri
7. The new bearing pressure distribution at every node that is in contact with the soil_max surface (between nodes b and c) is calculated first
[ ]VultkvsoilMaxsoilMaxRR 1incr,iincr,i1incr,iincr,i ⋅−+= −−
0.1R0.0 incr,i ≤≤
Method of computation
xi
i c db
pressure distribution
V
O
M
footing
soil_min
a
soil_max
∆i
Ri
8. The distribution of R along the contact length is integrated using equation 3 to get the total resisting vertical force, Vr
[ ]∑=
θ⋅∆⋅⋅=d
aiii )cos(R
LVultVr∫ ⋅θ⋅⋅= dx)cos()x(R
LVultVr or
9. For vertical equilibrium,
VVr =
Method of computation
xi
i c db
pressure distribution
V
O
M
footing
soil_min
a
soil_max
∆i
Ri
10. If abs(V - Vr) < = tolerance
[ ]∑=
⋅∆⋅⋅=d
aiiii xR
LVultM
Else
Go back to step 2 and change the point of rotation depending onV > Vr or V < Vr
Initial condition
Initially uniform pressure distribution
As M increases,
If (FS > 2){
trapezoidaluplift before yielduplift + yieldultimate
}Else{
trapezoidalyield before upliftyield + upliftultimate
}
Parameters:FSkv
Allotey and Naggar (2003)
Model simulationFS = 5.0M/H = 5.0 mRotation measured in the experiment is applied to the model
Input parameters
Simulation animation
Comparison with experiment
0.06 0.04 0.02 0 0.02 0.04 0.06500
250
0
250
500
Rotation (rad)
Mom
ent (
kN.m
)
0.06 0.04 0.02 0 0.02 0.04 0.0680
60
40
20
0
experimentcontact element model
Rotation (rad)Se
ttlem
ent (
mm
)
Critical contact length (Lc) and ultimate moment (Mu)
LC
L
emax VFooting base
Rounded soil
LC
L
emax VFooting base
Rounded soil
LC – minimum contact length required to maintain a vertical FS = 1
−=LLc1
2L.VMu
vFS1
LLc
=
Theoretical extreme behaviors
M
θ
S
θ
0
0
VL/2
Lθ/2
FS = inf.
FS = 1
0.05 0 0.05100
0
100
M50
T500.05 0 0.05
200
0
200
M1_1
T1_1
0.05 0 0.050.02
0
0.02
0.04
0.06
s50
T500.05 0 0.05
1.5
1
0.5
0
s1_1
T1_1
Shear – sliding modeling: coupling with V
1
0
p[i]
i_nodeFS1
qult]i[q]i[p
==
10 p[i]Vult
VFS1Fv ==
VultHFh = [ ]Fv1Fv
21Fh −⋅⋅=
Shear – sliding modeling: local coupling with V
10 p[i]Vult
VFS1Fv ==
VultHFh =
[ ]Fv1Fv21Fh −⋅⋅=
[ ]
−⋅⋅⋅= iii_node p1p21
811local_f
When (p = 0.0 or 1.0)f_local = 0.0
When (p = 0.5)f_local = 1.0
Shear – sliding modeling: global coupling with M
−
=din_d
dglobal_f
When (d d_in)f_global infinite
When (d 0)f_global 0
2
2
2
2
AB
Lh
AB
FhFm
dud
⋅=⋅=θ
1BFh
AFm
2
2
2
2=+
LVultMFm⋅
=
VultHFh =
du
dθ
Combining local and global coupling factors
[ ]
−⋅⋅⋅= iii_node p1p21
811local_f
Local coupling factor for every node is calculated based on their q/qult and summed up
[ ] ∑⋅=Nodes.no
ii_nodelocal local_fkhekh
Global coupling factor is calculated based on the location of force point in M-H space
−
=din_d
dglobal_f
Final shear stiffness is obtained by combing local and globalcoupling factors
[ ] [ ] global_fkhkh localglobal ⋅=
[ ] dukhdH global ⋅=
Settlement induced by sliding
Fv
contraction
1 < FS < 2
ds
1
FS > 2
0 0.5
dilation
dudu
ds
Fh
Associate flow rule doesn’t work for settlement induced by sliding
Results: shear - sliding
20 0 20 40 60
50
0
50
Sliding (mm)
Shea
r for
ce (k
N)
20 0 20 40 6080
60
40
20
0
experimentSliding (mm)
Settl
emen
t (m
m)
40 20 0 20 40 60
50
0
50
Sliding (mm)
Shea
r for
ce (k
N)
40 20 0 20 40 6080
60
40
20
0
contact element modelSliding (mm)
Settl
emen
t (m
m)
Results – shear - sliding
40 20 0 20 40 60
50
0
50
Sliding (mm)
Shea
r for
ce (k
N)
40 20 0 20 40 6080
60
40
20
0
experimentcontact element model
Sliding (mm)Se
ttlem
ent (
mm
)
Overall comparison
0.06 0.04 0.02 0 0.02 0.04 0.06500
250
0
250
500
Rotation (rad)
Mom
ent (
kN.m
)
40 20 0 20 40 60
50
0
50
Sliding (mm)
Shea
r for
ce (k
N)
0.06 0.04 0.02 0 0.02 0.04 0.0680
60
40
20
0
Rotation (rad)
Settl
emen
t (m
m)
40 20 0 20 40 6080
60
40
20
0
experimentcontact element model
Sliding (mm)
Settl
emen
t (m
m)
Model parametersFooting geometry
width, Blength, Lembedment, D
Soil strength parametersfriction angle, Фunit weight, γ
Soil stiffness parametersvertical stiffness, kvinitial shear stiffness, khrebounding ratio, Rv
Soil parameters can be specified as a function of depth (settlement)
Implementation in OpenSEES
Need to add a new materialfootingSection2d.hfootingSection2d.cpp
Axial force and moment and corresponding displacements are coupled by
zeroLengthSection - elementsectionForceDeformation – material
There are no materials/elements that couple three forces and displacements (for a 2D problem) in OpenSEES
Implementation in OpenSEESTo implement the footing-soil interface model in OpenSEES:
1. M-V coupling only
Material
FootingSection2d
SectionForceDeformation
ZeroLengthSection
Element
TaggedObject
DomainComponent
2. M-H-V coupling
new-SectionForceDeformation
Material
FootingSection2dnew-ZeroLengthSection
Element
TaggedObject
DomainComponent
Implementation in OpenSEES
V
O
H
Properties of the model
Model is simple and computationally fast
Coupled force-displacement relationships
Only 4 model parameters
No need for external mesh generation
Reproduces the mechanisms observed in the experiments
Acknowledgements
PEERKey RosebrookJustin Phalen
Jeremy ThomasRoss BoulangerBoris JeremicDan WilsonChad JusticeTom Coker