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