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ASCE 7-16 / 2015 NEHRP Provisions Chapter 19: Soil-Structure Interaction
Robert Pekelnicky, PE, SE Principal, Degenkolb Engineers
February 11, 2015
Foundation Force-Deformation > Foundation Flexibility > Soil-Foundation Interface Yielding Foundation Damping > Radiation Damping > Soil Hysteretic Damping
Kinematic Interaction > Base Slab Averaging > Embedment
Soil-Structure Interaction
What We Actually Do Geotechnical Engineer
Spatially-INCOHERENT ground motions from the rock motion through the soil column
nonlinear-inelastic soil spring-dashpots at each soil lens
What We Actually Do USGS Maps
What We Actually Do Structural Engineer
Based on ATC-83 Report (NIST NIST GCR 12-917-21)
Direct inclusion of soil flexibility
Merger ASCE 41-13 Provisions with ASCE 7-10
Updated foundation damping equations
Update & clarify limitations on use
ASCE 7-16/ 2015 NEHRP Provisions
Foundation Flexibility - Example
Reinforced concrete shear wall building 4 Story + 1 Basement Site class D soil Oakland, California
Fixed Base Modal Period T = 0.39 sec Flexible Base Modal Period T = 0.49 sec 25% increase in period due to foundation flexibility
Foundation Flexibility - Example
0.000
0.200
0.400
0.600
0.800
1.000
1.200
1.400
1.600
1.800
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Sp
ectr
al A
ccel
erat
ion
, S
a (g
)
Period, T (sec)
Sa_MCER_(MaxDir)
Sa_DE_(MaxDir)
Foundation Flexibility - Example
Foundation Flexibility - Example
Currently no requirement for foundation to be stronger than superstructure Foundations and soil actions designed with same forces Soil actions typically ASD
Foundation Yielding
Currently no requirement for foundation to be stronger than superstructure Foundations and soil actions designed with same forces Soil actions typically ASD
Foundation Yielding
Introduced provisions to allow LRFD for soil actions Phi factors based on material (clay vs. sand) and level of testing
ASCE 7-16 => LRFD Soil Design
Code Design: 40’x18’ footing 14-#7 Long & #7@12” Trans Soil stronger than wall: 40’x24’ footing 16-#8 Long & #8@12” Trans Footing stronger than soil or wall 40’x18’ footing 14-#8 Long & #8@12” Trans
Foundation Yielding – Example
Foundation Damping
Waves from structure moving interfere with ground motion waves
Soil Hysteretic damping affects shaking building feels
Radiation damping & soil hysteretic damping addressed Modification of the design base-shear or response spectrum Explicit damping elements required for nonlinear analysis
Foundation Damping
“you get two for the dirt” – Henry Degenkolb
How much SSI is in the R-factor?
ASCE 7-10 limited to a 30% reduction
Foundation Damping & the R-Factor
ASCE 7-16 Foundation Damping
WBCCV
SSI
ss
−=∆
~
( )2~
effTT
βββο += f
Soil Damping
Effective Peak Acceleration, SDS /2.51
Site Class SDS /2.5
= 0 SDS /2.5 = 0.1
SDS /2.5 = 0.4
SDS /2.5 = 0.8
A 0.01 0.01 0.01 0.01
B 0.01 0.01 0.01 0.02
C 0.01 0.01 0.03 0.05
D 0.01 0.02 0.07 0.15
E 0.01 0.05 0.20 * F * * * *
+
+
−= 6.18.08.6
2
65.0
BL
BLGBK y ν
+
−= 8.02.3
1
3
BLGBK xx ν
( )( )
=
2/4 0aGBKBL
yyβ
s0 vT
Ba ~2π
=
( )
( )
20 0
2033
(4 / 3) /20.42.2
/
xxxxxx
L B a a
K aGB L B
ψβ
α
= − +
( ) 5.221
)1(2≤
−−
=ννψ
( )( )
( )
20
203
0.55 0.01 / 11.0
0.42.4/
xx
L B a
aL B
α
+ − = − − +
βr=[1/(T/Ty)2]βy+[1/(T/Txx)2]βxx (Eq. 19.3-5) Ty=2πM*Ky (Eq. 19.3-6) Txx=2πM*h*2αxxKxx (Eq. 19.3-7)
(Eq. 19.3-8)
(Eq. 19.3-9)
(Eq. 19.3-10)
(Eq. 19.3-11)
(Eq. 19.3-12)
(Eq. 19.3-13)
(Eq. 19.3-14)
Radiation Damping
Structural Damping, β = 5% Radiation Damping, βf = 11% Soil Damping, βf = 7% Foundation Damping Total Damping
Foundation Damping Example
Damping Base Shear Reduction Base Shear R-factor limit Revised base shear based on limit
Foundation Damping Example
Foundation Damping Example Property Fixed Base – No SSI Flexible Base – SSI
Wall Thickness 14 inches 12 inches
Horizontal Reinf. #5 @ 12” o.c. #4 @ 12” o.c.
Vertical Reinf. #5 @ 12” o.c. #4 @ 12” o.c.
Boundary Region 14 - #7 None
Footing Dimensions 40-ft x 14-ft 40-ft x 14-ft
Footing Long. Reinf. 14 - #7 T&B 12 - #7 T&B
Footing Trans Reinf. #7 @ 12” T&B #7 @ 14” T&B
Kinematic Interaction
> Spatial variability of input ground motion along the foundation Embedment Effects – Deamplification of ground motion with depth
(Vertical) Base-Slab Averaging – Wave incoherence across foundation (Horizontal)
> Affects high-frequency input > Multi-support excitation required in modeling
Base Slab Averaging
Low frequency components…
uFIM (ω) ≈ ug (ω) 2003 Off-Miyagi Eqk FF, s-wave window
Prof. Jonathan Stewart
High frequency components…
uFIM (ω) < ug (ω) 2003 Off-Miyagi Eqk FF, p-wave window
Prof. Jonathan Stewart
Base Slab Averaging
Embedment Effect
Low frequency Long wavelength λ = Vs/f uFIM ≈ ug θFIM ≈ 0
Prof. Jonathan Stewart
Embedment Effect
High frequency Short wavelength λ = Vs/f uFIM < ug θFIM > 0
Prof. Jonathan Stewart
Ground Motions – Free Field
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
0 0.5 1 1.5 2 2.5 3 3.5 4
pSa
(g)
Period (sec)
Scaled Target SpectrumChi-Chi, Taiwan :: TCU078 :: SRSSChi-Chi, Taiwan :: TCU120 :: SRSSDenali, Alaska :: TAPS Pump Station #10 :: SRSSImperial Valley-02 :: El Centro Array #9 :: SRSSKobe, Japan :: Nishi-Akashi :: SRSSKocaeli, Turkey :: Izmit :: SRSSKocaeli, Turkey :: Yarimca :: SRSSLanders :: Joshua Tree :: SRSSNorthridge-01 :: LA - Sepulveda VA Hospital :: SRSSNorthridge-01 :: Newhall - Fire Sta :: SRSSScaled Modified SRSS Average
Ground Motions – At-Depth Amp-Scaled
Concerns / Issues
• Base slab averaging reduction is unlimited at short periods
• Base slab averaging equation predicts negative values at large base dimensions
• Embedment & base slab averaging can reduce PGA to 0.3 second values to 0.22*Free Field
• Kinematic effects greatest at short periods, but no requirement to include foundation flexibility
ASCE 41-06
Combined BSA & Embedment
0 0.5 1 1.5 2 2.5 3 3.5 40
0.25
0.5
0.75
1
1.25
1.5
EmbedmentBSACombination
Embedment Reduction
Period (sec)
Red
uctio
n Fa
ctor
0.184
Look at Existing Data – Wadsworth Hospital
39
Ratio between basement and free-field record (average)
ASCE 41-06 BSA + Embedment Effects
Issues addressed
• Require soil flexibility modeled
• Provide overall kinematic limit of 50%
• Temper reductions for kinematic effects
• Limit base area for BSA
• Update equations for BSA
ASCE 41-13
Kinematic Soil-Structure Interaction
0
0.2
0.4
0.6
0.8
1
1.2
0 0.5 1 1.5 2 2.5 3 3.5 4
41-06
41-13
Soil-Foundation-Structure Interaction
Base Slab Averaging & Embedment Equations Updated
Base area limited based on testing limits, 260 feet.
0.75 factor reduces the reduction
Max reduction can only be 50%
( )[ ]21
202
0
2exp1175.025.0
Β−−×+= bsabsa bb
RRS( ) 1
1611112exp
11242
1
0200
20
0
100
80
604
02
0
>
−−
≤+++++=Β
bbb
b
bbbb
bbbsa
π
50.02cos75.025.0 ≥
×+=
se Tnv
eRRS π
Only can be used with nonlinear response history
Limited to 20% reduction from site specific
Limited to 30% reduction from USGS spectra
2015 NEHRP allowed to 40% with peer review
• 40% eliminated in ASCE 7-16
Must model soil flexibility
ASCE 7-16 Kinematic Interaction
Kinematic Interaction - Example
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
1.60
1.80
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Sa
(g)
Periods (s)
Sa_MCE
Sa_MCE (SSI 7-16)
Sa_MCE (2015 NEHRP)
Kinematic Interaction - Example
0
10
20
30
40
50
60
70
80
0.00 1.00 2.00 3.00 4.00 5.00
Sto
ry e
leva
tio
n,
ft
displacement, in
Displacement
Sa_MCER
Sa_MCE (SSI 7-16)
Sa_MCE (2015 NEHRP)
Kinematic Interaction - Example
0
10
20
30
40
50
60
70
80
0.00% 0.10% 0.20% 0.30% 0.40% 0.50% 0.60% 0.70%
Sto
ry e
leva
tion
, ft
displacement, in
Story Drift Ratio
Sa_MCERSa_MCE (SSI 7-16)Sa_MCE (2015 NEHRP)
ASCE 7-16 / 2015 NEHRP Provisions Chapter 19: Soil-Structure Interaction
Robert Pekelnicky, PE, SE Principal, Degenkolb Engineers
February 11, 2015
