EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
Practical Modeling Considerations
Impact of Foundation Modeling on the
Earthquake Responseof a
RC Shear Wall and MRF Building
Mark A. Moore S.E. and Emma Goodson P.E.
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
Overview
Case Study – Shallow FoundationFoundation Flexibility
Soil Stiffness, G and G0
“K” by Method 1 through Method 3 and moreSE / GE collaboration
Impact on Global and Local ResponsesSuggested Modeling Improvements
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
FEMA 440/ASCE 41
FEMA 440/ASCE 41
FEMA 440/ASCE 41
FEMA 356/ASCE 41
Overview con’t
Inertial effectsFoundation stiffness and strengthRadiation damping
Kinematic effectsBase slab averaging (x,y)Embedment (z)
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
Related documents
FEMA 356 (2000): Prestandard and Commentary for the Seismic Rehabilitation of Buildings [References herein are to this document]FEMA 440ASCE 41 + Supplement 1
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
FEMA 440 – Chapter 8:Procedures for Including Soil-Structure Interaction Effects
Acceleration Reponse Spectra
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0 0.5 1 1.5 2
Period (sec)
Spec
tral
Acc
eler
atio
n, S
a (g
)
BSE-2BSE-13/4 BSE-1SSI
Used for DBE
BSE 1 reduced forkinematics effectsand radiationdamping
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
Effects of Foundations on Performance
Foundation stiffness and strength affectvarious structural components differently.
Δ, small
Stiff and Strong Foundation
Smalldisplacementsprotect framefrom damage
High forcescause shearwall damage Δ, large
Flexible and Weak Foundation
Largedisplacementscause frame
damageFoundationyielding androcking protectsshear wall
Stiff and strong is not always favorable;nor is flexible and weak always conservative.
Foundation stiffness and strength affectvarious structural components differently.Foundation stiffness and strength affectvarious structural components differently.
Δ, smallΔ, smallΔ, small
Stiff and Strong Foundation
Smalldisplacementsprotect framefrom damage
High forcescause shearwall damage
High forcescause shearwall damage Δ, largeΔ, largeΔ, large
Flexible and Weak Foundation
Largedisplacementscause frame
damage
Largedisplacementscause frame
damageFoundationyielding androcking protectsshear wall
Stiff and strong is not always favorable;nor is flexible and weak always conservative.
Stiff and strong is not always favorable;nor is flexible and weak always conservative.
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
Case Study
1965 ConstructionReinforced Concrete 6 Stories Above Grade24’ by 24’ Bays (31,000 SF)Two-way Slab with Drop PanelsFull Basement with Shallow FoundationsSite Class DSeismic Design Category CAt ¾ BSE 1: S-3; N-D
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
Typical Floor Plan
Transverse Shear Walls
Longitudinal Shear Walls(Two coupled walls)
Perimeter Moment-Resisting Frame with Precast Infill
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
Two rigid foundations coupled by structural components
Longitudinal Wall Elevation
Ground Floor
Basement
General wall element modeling
One rigid foundation response
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
Displacement Compatibility
Basement SOG
Ground Floor
LBW supports slab and roof (not shown)
Nonlinear elastic bar – NSPInelastic bar + gap - NDP
General wallInelastic section
Inelastic frame element
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
Typical Atypical Condition
Beam
Column
Two-waySlab
Plan Elevation
Exterior Beam-Column Joint
Torsional deformation?
Precast betweenand connected to columns and beams
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
Displacement Compatibility
Summary of Component Actions to TrackShear of foundation coupling walls
Shear of LBW lintel
Torsion of perimeter beam-column joint
Slab strips parallel and orthogonal to wall
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
ASCE 41 Supplement 1
Proposed Figure 2-3Component Force versus Deformation Curves
Lateral deformation following loss oflateral strength capacity
b
Notes:1. Only secondary component actions permitted between points 2 and 4.2. The force, Q, after point 3 diminishes to approximately zero.
Type 1 Curve Type 2 Curve Type 3 Curve
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
Foundation Plan
Individual Footings Coupled toForm Krot for Wall Line
Type A(Vertical springBeneath col.)
Type D (3 springs)
Type C
Type B(2 vert. springsto capture Krot)
Foundation Plan – Vertical Spring ID
One storyshear wall
Postulate shear overstressdue to foundation flexibility
Plenum
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
Chapter 4:Effective Stiffness of “Foundation”
Initial Soil StiffnessInitial Shear Modulus, G, Derived By
1> Soil Shear Wave Velocity (Eq 4-4)2> Standard Penetration Test (Blow Count – N1/60) ) (Eq 4-5)
Effective Soil StiffnessEffective Stiffness, Go
Modulus Reduction Factors (Table 4-7)
Foundation StiffnessRelative Stiffness Between Soil and Structure (C4-1 & C4-2)Method 1 to Method 3 Proportional to Go
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
Quantitatively Define Soil Properties
Nominal “N” value used by GEInfluence depth – B or 4B?
Site Class “D”v= 600 ft/sec 1,200 ft/sec(over great depth)
G0 proportional to v2
Strain level dependent –10%/50yr or 2%/50yr (Go/G 0.80 & 0.66)
OriginalGround
22
280’
260’
47
64
21
65
Footing location
? How deep?
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
Other Means of Arriving at Soil Properties:Modulus of Subgrade Reaction and Units
In lieu of FEMA 356, the GE may provide othersoil property recommendations. The following may help the SE, but collaboration with GE is the bestanswer:
Soil with cohesive properties reacts independent of depth and may be recommended in terms of F/L3, which when multiplied by contact area (BxL) gives F/L.
Soil reliant on internal friction for strength reacts dependent on depth and may be recommended in terms of F/L4, which when multiplied by depth (h) and contact area (BxL) gives F/L.
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
Foundation Overturning Stiffness:
Model vertical foundation stiffness and couple those with explicit structure modeling. Determine Kvert by:
Method 1: Rigid isolated foundation 18’ by 18’coupled by explicit structure modeling
Method 2: Decoupled end and middle zones by considering footing as 64’ by 18’
Method 3: Unit subgrade modulusMethod 1 Revised: Determine Rotational stiffness
and convert to vertical stiffness
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
Foundation Plan
Individual Footings Coupled toForm Krot for Wall Line
Type A(Vertical springBeneath col.)
Type D (3 springs)
Type C
Type B(2 vert. springsto capture Krot)
Foundation Plan – Vertical Spring ID
One storyshear wall
Postulate shear overstressdue to foundation flexibility
Plenum
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
Foundation Partial Plan
Type D Foundation Elevation
Ground Floor
Basement
24’ 24’
Partial Plan
K vert K vert K vert
Considered as:3 No. 18’ by 18’ Pads
or64’ long by 18’ wide pad
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
Chapter 4, Method 1:Rigid Foundation Structure?
rigid test 4 k sν⋅
1
5
m_m 1
5
nn
sinm_m π⋅
2⎛⎜⎝
⎞⎟⎠
2sin
nn π⋅
2⎛⎜⎝
⎞⎟⎠
2⋅
π4 D f⋅m_m2
L1( )2nn2
B1( )2+
⎡⎢⎢⎣
⎤⎥⎥⎦
⋅ k sν+∑=
∑=
⋅:=
Use of elastic properties to determine relative rigidities… perhaps strengthwould be a better test? See C4-2 for equation, discussion and limitations.
whereFor a 3’-3” thick by 18’ square footingwith a point load, the structuralcomponent is considered rigid. Strength(shear and flexure) would likely deemotherwise.
D fE f t3×
12in 1 ν f−( )2×:=
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
Chapter 4, Method 1 Con’t:Gazetas’ Equation: 18’ by 18’ pad
Use isolated footing vertical stiffness (Figure 4-4)
Modify for embedment
β zi1
121
Di
Bi⋅ 2 2.6
Bi
Li⋅+
⎛⎜⎝
⎞⎟⎠
⋅+⎡⎢⎣
⎤⎥⎦
1 0.32di Bi Li+( )⋅
Bi Li⋅
⎡⎢⎣
⎤⎥⎦
23
⋅+
⎡⎢⎢⎢⎣
⎤⎥⎥⎥⎦
⋅:=
K z_suri
G Bi⋅
1 ν−1.55
Li
Bi
⎛⎜⎝
⎞⎟⎠
0.75
⋅ 0.8+⎡⎢⎢⎣
⎤⎥⎥⎦
⋅:=
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
Chapter 4, Method 1 Con’t:Gazetas’ Equation: 18’ by 18’ Pad
q ult 12 ksf=L 18 ft=B 18 ft=
Ultimate bearing pressure provided byGE and not discussed herein
For a unit ft2
For lower bound use 80% (discussed later) of the ultimate bearing pressure. A nominal tension force and stiffness, and strain hardening is assumed.
Sq Ftg Axial F-Defl
Defl
Forc
e FD1 Δ( )
Δ
.
K z 6378kipin
=K z
2 L⋅ B⋅9.8
ksfin
=
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
Chapter 4, Method 2:End Stiffening/Decoupling P and M For a 64’ by 18’ footing
K z6.83 G⋅ B⋅
1 ν−( ):=
Note: B or L/6 used in lieu of B/6. For High L/B ratios, this is judged as moreappropriate. For this case “B/6”is made equal to “B”.
Kz
2 L⋅ B⋅23
ksfin
=
Again, 1/2 is introduced to determinea lower bound stiffness.
End footing (zone)
K z_mid0.736.83
K z⋅:=
Middle footing (zone)
K z_mid
2 L⋅ B⋅2.5
ksfin
=
PlanL
B
B/6 B or L/6or
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
Chapter 4, Method 3:Unit Coef. Of Subgrade Reaction: 18’ by 18’ Pad
K z1.3 G⋅
B 1 ν−( )⋅B⋅ L⋅:= The multiplication of B and L converts
to force per footing versus deformation.
K z
2 L⋅ B⋅4.4
ksfin
=QED!
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
Chapter 4, Method 1 Revised:Gazetas’ Equation: 64’ by 18’ pad
K yy_suri
G Bi( )3⋅
1 ν−0.47
Li
Bi
⎛⎜⎝
⎞⎟⎠
2.4
⋅ 0.034+⎡⎢⎢⎣
⎤⎥⎥⎦
⋅:=
Modify for embedment
β yyi1 1.4
di
Li
⎛⎜⎝
⎞⎟⎠
0.6
⋅ 1.5 3.7di
Li
⎛⎜⎝
⎞⎟⎠
1.9 di
Di
⎛⎜⎝
⎞⎟⎠
0.6−
⋅+⎡⎢⎢⎣
⎤⎥⎥⎦
⋅+:=
Use isolated footing rotational stiffness (Figure 4-4)
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
Chapter 4, Method 1 Revised:Gazetas’ Equation: 64’ by 18’ pad
I 2 242( )⋅ ft2⋅:=
Performing a rivet-type analysis with a unit area for each pad:
The equivalent vertical spring for each pad is
K yy
I
2 L⋅ B⋅10.9
ksfinch
= Again, 1/2 is introduced to determinea lower bound stiffness.
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
Foundation Plan
Type A(Vertical springBeneath col.)
Type D (3 springs)
Type C
Foundation Plan – Type A, Method 2
One storyshear wall Plenum
Middle Zone(Strip Footing)
End Zone, Typical(Say “B/6” = 2 ftg’s)
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
Chapter 4, Method 2 Revised For Type A ftgEnd Stiffening/Decoupling P and M
KMethod2 9982kipin
= KMethod2_mid 1067kipin
= rKMethod2_mid
KMethod2:=
IM2 2 1 1082⋅ 1 842⋅+ r 602⋅+ r 362⋅+ r 122⋅+( )⋅ ft2⋅:=
IM2 KMethod2⋅ 5.536 1010×kip in⋅rad
=
KMethod2IM2
I⋅ 8091
kipin
=KMethod2
IM2
I⋅
2 L1⋅ B1⋅28.1
ksfin
=
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
Foundation Mechanism:
Soil Foundation Flexure Flexural-shear/Shear
==>
qult qult
< qult
qult
PSoil + 2 Vngoverns and equalsabout 80% of soil-governed capacity:0.8 * qult = 9.6 ksf
VnVn
P ~
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
Limit State StrengthShear Capacity - to one side and at d/2 from face of wall
Vsoil L1B1 12inch wall⋅−
2
d1
2−
⎛⎜⎝
⎞⎟⎠
⋅ qult⋅:= Vsoil 612 kip=
φVn 2 4500⋅lb
in2⋅ 0.8⋅ d1⋅ L1⋅:= φVn 464kip=
At 3 root f'c, soil governs.
For lower bound strength capacity, use a factor of
qult B1 2B1 12 inch⋅ wall⋅−
2
d1
2−
⎛⎜⎝
⎞⎟⎠
⋅−⎡⎢⎣
⎤⎥⎦
⋅ L1⋅⎡⎢⎣
⎤⎥⎦
2 φVn⋅+
B1 L1⋅ qult⋅0.83=
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
Lower Bound Stiffness SummaryEffective Shear Modulus (1200/645)2 = 3.5(GE rec. versus SE’s Guess)
Method 1 9.8 ksf/inchMethod 2
End Zone 23 ksf/inchMiddle Zone 2.5 ksf/inch
Method 3 4.4 ksf/inchMethod 1 Revised 11 ksf/inch
Range of variation:All methods : 23/4.4= 5.2Considering Method 1 and 3 only: 11/4.4 = 2.5
Worst case scenario ==> 3.5 x 5.2 x 1/2 = 9.1 x too stiff
Note: Even with this range, any springs are far better than fixed base!
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
Spectral Acceleration versus Displacement
0
0.05
0.1
0.15
0.2
0.25
0.3
0.0 1.0 2.0 3.0 4.0
Spectral Displacement (in)
Spec
tral
Acc
eler
atio
n (g
)
3/4 BSE-1
BSE-1
BSE-2
Fixed Base
Lower Bound
Global Response
Target displacement increase by 75 %
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
Foundation Response – Longitudinal Wall
Foundation "Tension" Spring - Longitudinal Wall
-1200
-1000
-800
-600
-400
-200
0 1 2 3 4 5
Spectral Displacement (in)
Axia
l For
ce (
kips
)
3/4 BSE-1BSE-1
BSE-2Lower Bound
Foundation Compression Spring Longitudinal Wall
-2500
-2000
-1500
-1000
-500
0 1 2 3 4 5
Spectral Displacement (in)
Axi
al F
orce
(ki
ps)
3/4 BSE-1
BSE-1
BSE-2
Lower Bound
Nonlinear Elastic Springs – Lower Bound Stiffness and Strength
Ground Floor
Basement
K vert K vert K vert
System not dominated by rocking
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
Local (Component) Response
Longitudinal WallsCoupling Beam
0.0
0.1
0.2
0.3
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Spectral Displacement (in)
Shea
r St
ress
(ks
i) Fixed BaseLower Bound3/4 BSE-1BSE-1BSE-2
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
Local (Component) Response
Longitudinal WallsCoupling Beam, Lower Bound
0.0
0.1
0.2
0.3
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Spectral Displacement (in)
Shea
r St
ress
(ks
i)
3/4 BSE-1BSE-1BSE-2Lower BoundLSCP
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
Local (Component) Response
Outrigger Wall - Above OpeningLBW
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Spectral Displacement (in)
Shea
r St
ress
(ks
i)
Fixed Base
Lower Bound
3/4 BSE-1
BSE-1
BSE-2
LS
CP
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
Beam
Column
Two-waySlab
Plan Elevation
Exterior Beam-Column Joint
Torsional deformation?
Precast betweenand connected to columns and beams
Beam-Column Joint Torsion
0.000
0.005
0.010
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Spectral Displacement (in)
Tors
ion
Def
orm
atio
n (r
adia
ns)
3/4 BSE-1 LB
BSE-1 LB
BSE-2 LB
Joint Rotation
3/4 BSE-1 FB
BSE-1 FB
BSE-2 FB
Local (Component) Response
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
Local (Component) ResponseSlab End Moment on Line D at Column Face
-10
100
210
320
430
540
0 1 2 3 4 5
Spectral Displacement (in)
Slab
End
Mom
ent
(kip
-ft)
3/4 BSE-1BSE-1BSE-2Fixed BaseLower Bound
Slab End Moment on Line D at Wall Face
-750
-600
-450
-300
-150
0
0 1 2 3 4 5
Spectral Displacement (in)
Slab
End
Mom
ent (k
ip-f
t)
3/4 BSE-1BSE-1BSE-2Fixed BaseLower Bound
Indicates significant slab stress demandbut no inelastic behavior for 24’ span.Short spans showed inelastic behavior.
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
Exterior Beam-Column Joint
Beam (E)
Col. (E)
Plan Elevation
Exterior Beam-Column Joint
Slab (E)
Col. (N)
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
Typical Floor Plan
New columns (Wall Piers)
Deleted columns as aresult of the analysis
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
Seismic Rehabilitation RecommendationsDo nothing to overstressed coupling beam that is between the longitudinal wallsProvide “catcher” to LBW at 5th floor and at other locationsDo nothing to beam-column joint in longitudinal direction:
• Deformation levels• Higher confidence in determining deformations due to inclusion of
SSI effectsSlabs proven to within acceptance limits.
Without modeling foundation flexibility and capturing the kinematics three dimensionally, wall piers may have been added in the longitudinal direction and the LBW deficiency may not have been identified.
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
Displacement-Based Design
More liberal evaluation techniques
ASCE 41 Supplement 1 will require more comprehensive modeling
Foundation flexibility and strength:Adds to total lateral deflectionChanges the distribution of inelastic displacement demand between components and may change strength hierarchy
EERI TechnicalSeminar Series
Impact of Soil-Structure Interaction on Response of StructuresSeminar 1: Practical Applications to Shallow Foundations
Ways to Improve SSI ModelingGE and SE collaboration
Consider displacement compatibility in 3D
Use Winkler models calibrated by testing
Use capacity spectrum for systems dominated by rocking
Identify and include uncertainty in the evaluation
Determine residual displacements - NDP