2015 NEHRP Provisions Chapter 19: Soil-Structure Interaction
February 11, 2015 BSSC Colloquium
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
NIST & ATC-83 Project Team PUC Study Group CB Crouse Jon Stewart Jacobo Bielak David Bonneville John Gillengerten Jay Harris Bob Bachman
Acknowledgements
What We Should Do
What We Actually Do
Fixed-base modeling
ASD Foundation Design
Same “R” soil yielding as for superstructure yielding
Only accounts for radiation damping
ASCE 7-10
Reduce base shear, capped at 30%
Theoretically based accounting for soil flexibility
Theoretical soil damping relationships
ASCE 7-10
• Essentially unchanged from ATC 3-06
• Do not need to explicitly model the soil flexibility
• 2009 Provisions Commentary has guidance on how to incorporate explicit soil flexibility
• Equations cumbersome so not commonly used
ASCE 7-10
• Strength Foundation Design
• Provides Separate m-factor for the soil
• Requires foundation flexibility modeling for high performance
• Requires bounding soil properties
• Radiation Damping
• Base Slab Averaging
• Embedment Effects
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
Kinematic Interaction
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
Issues to address
• 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
Soil-Foundation-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
Issues to address
• Merger ASCE 41-13 Provisions with ASCE 7
• Direct inclusion of soil flexibility
• ATC-83 Report
• Update foundation damping equations
• Limitations on use
2014 NEHRP Provisions
“you get two for the dirt” – Henry Degenkolb
How much SSI is in the R-factor?
Currently limited to a 30% reduction
SSI & the R-Factor
Scatter on Observed Data too Much
Look at Existing Data – Wadsworth Hospital
20
Ratio between basement and free-field record (average)
ASCE 41-06 BSA + Embedment Effects
Short Period Issue?
Should SSI only be used with Site Specific Spectra?
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
0 0.5 1 1.5 2 2.5 3 3.5 4
Foundation Damping
• Directly model flexibility at soil-foundation interface
• Updated radiation damping equations based on ATC-83
• Added a simplified table for Soil Damping
• Permissible reduction decreases with R-factor
• 30% for R < 3 to only 10% for R > 6
• Use with NLRH only using explicit dashpots @ interface
2014 NEHRP Provisions
Foundation Damping
• Directly model flexibility at soil-foundation interface
• Updated radiation damping equations based on ATC-83 – Rectangular Foundation
• Added a simplified table for Soil Damping
• Permissible reduction decreases with R-factor
• 30% for R < 3 to only 10% for R > 6
• Use with NLRH only using explicit dashpots @ interface
2014 NEHRP Provisions
Foundation Damping
2014 NEHRP Provisions
WBCCV
SSI
ss
−=∆
~
( )2~effTT
βββο += f
Soil Damping
4
' 4 ss s
s
Dv T
β β
=
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=1TTy2βy+1TTxx2βxx (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)
2014 NEHRP Provisions Radiation Damping
Kinematic Effects
• Limited to 20% reduction from site specific
• Limited to 30% reduction from USGS spectra
• Limited to 40% with peer review
• 40% eliminated in ASCE 7-16
• Must model soil flexibility
2014 NEHRP Provisions
• Understanding interplay between kinematic, inertial, and structural yielding
• More examples from actual earthquakes validating SSI phenomena
• Better guidance on creating dashpots for NLRH models
Research Needs
Thank You Questions?
ASCE 7-16 Provisions Soil-Structure Interaction
State of the Standard & Research Needs
January 28, 2015 SCEC Workshop