ASCE 7-16 / 2015 NEHRP Provisions Chapter 19: Soil-Structure Interaction Robert Pekelnicky, PE, SE Principal, Degenkolb Engineers February 11, 2015
Soil-Structure Interaction Foundation Force-Deformation > Foundation Flexibility > Soil-Foundation Interface Yielding Foundation Damping > Radiation Damping > Soil Hysteretic Damping Kinematic Interaction > Base Slab Averaging > Embedment
What We Should Do
What We Actually Do Geotechnical Engineer nonlinear-inelastic soil spring-dashpots at each soil lens Spatially-INCOHERENT ground motions from the rock motion through the soil column
What We Actually Do USGS Maps
What We Actually Do Structural Engineer
ASCE 7-16/ 2015 NEHRP Provisions 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
Foundation Flexibility
Foundation Flexibility - Example Reinforced concrete shear wall building 4 Story + 1 Basement Site class D soil Oakland, California
Foundation Flexibility - Example 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 1.800 1.600 1.400 Spectral Acceleration, Sa (g) 1.200 1.000 0.800 0.600 0.400 Sa_MCER_(MaxDir) Sa_DE_(MaxDir) 0.200 0.000 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 Period, T (sec)
Foundation Flexibility - Example
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 Currently no requirement for foundation to be stronger than superstructure Foundations and soil actions designed with same forces Soil actions typically ASD
ASCE 7-16 => LRFD Soil Design Introduced provisions to allow LRFD for soil actions Phi factors based on material (clay vs. sand) and level of testing
Foundation Yielding Example 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 Damping Waves from structure moving interfere with ground motion waves Soil Hysteretic damping affects shaking building feels
Foundation Damping Radiation damping & soil hysteretic damping addressed Modification of the design base-shear or response spectrum Explicit damping elements required for nonlinear analysis
Foundation Damping
Foundation Damping & the R-Factor you get two for the dirt Henry Degenkolb How much SSI is in the R-factor? ASCE 7-10 limited to a 30% reduction
ASCE 7-16 Foundation Damping V = C s ~ C B s SSI W β ο = β f + β ~ ( T T ) 2 eff
Soil Damping Effective Peak Acceleration, S DS /2.5 1 Site Class S DS /2.5 = 0 S DS /2.5 = 0.1 S DS /2.5 = 0.4 S DS /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 * * * *
Radiation Damping 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) 0.65 GB L L K y = 6.8 + 0.8 + 1. 6 2 ν B B 3 GB L K xx = 3.2 + 0. 8 1 ν B 4( L / B) a0 β y = ( ) K y GB 2 a 0 = 2πB ~ Tv β xx ψ = α xx s 2 (4 ψ / 3) ( L/ B) a0 a 0 = K 0.4 2α 2 xx xx 2.2 3 3 + a0 GB ( L/ B) 2(1 ν ) 2.5 ( 1 2ν ) 2 ( 0.55 + 0.01 ( L/ B) 1) a0 = 1.0 0.4 2 2.4 3 + a 0 ( L/ B) (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)
Foundation Damping Example Structural Damping, β = 5% Radiation Damping, β f = 11% Soil Damping, β f = 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 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 2003 Off-Miyagi Eqk FF, s-wave window u FIM (ω) u g (ω) Prof. Jonathan Stewart
Base Slab Averaging High frequency components 2003 Off-Miyagi Eqk FF, p-wave window u FIM (ω) < u g (ω) Prof. Jonathan Stewart
Embedment Effect Low frequency Long wavelength λ = V s /f u FIM u g θ FIM 0 Prof. Jonathan Stewart
Embedment Effect High frequency Short wavelength λ = V s /f u FIM < u g θ FIM > 0 Prof. Jonathan Stewart
Kinematic Interaction
Ground Motions Free Field psa (g) 4.5 4 3.5 3 2.5 2 Scaled Target Spectrum Chi-Chi, Taiwan :: TCU078 :: SRSS Chi-Chi, Taiwan :: TCU120 :: SRSS Denali, Alaska :: TAPS Pump Station #10 :: SRSS Imperial Valley-02 :: El Centro Array #9 :: SRSS Kobe, Japan :: Nishi-Akashi :: SRSS Kocaeli, Turkey :: Izmit :: SRSS Kocaeli, Turkey :: Yarimca :: SRSS Landers :: Joshua Tree :: SRSS Northridge-01 :: LA - Sepulveda VA Hospital :: SRSS Northridge-01 :: Newhall - Fire Sta :: SRSS Scaled Modified SRSS Average 1.5 1 0.5 0 0 0.5 1 1.5 2 2.5 3 3.5 4 Period (sec)
Ground Motions At-Depth Amp-Scaled
ASCE 41-06 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
Combined BSA & Embedment 1.5 Embedment Reduction 1.25 1 Reduction Factor 0.75 0.5 0.25 0.184 0 0 0.5 1 1.5 2 2.5 3 3.5 4 Embedment BSA Combination Period (sec)
Scatter on Observed Data
Look at Existing Data Wadsworth Hospital Ratio between basement and free-field record (average) ASCE 41-06 BSA + Embedment Effects 39
ASCE 41-13 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
Kinematic Soil-Structure Interaction 1.2 1 0.8 41-13 0.6 41-06 0.4 0.2 0 0 0.5 1 1.5 2 2.5 3 3.5 4
Soil-Foundation-Structure Interaction Base Slab Averaging & Embedment Equations Updated RRS bsa 1 2 [ 1 exp( 2b ) Β ] = 0.25 + 0.75 2 0 bsa b0 Base area limited based on testing limits, 260 feet. 1 2 Β bsa 1+ b = exp 2 0 + b b0 + 2 b0 b0 + + 4 12 2 16 b0 1 2 1 1 ( 2b ) 1 1 b > 1 0 4 0 6 π b 0 8 10 b 0 0 RRS e = 2πe 0.25 + 0.75 cos Tnvs 0.50 0.75 factor reduces the reduction Max reduction can only be 50%
ASCE 7-16 Kinematic Interaction 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
Kinematic Interaction - Example 1.80 1.60 1.40 1.20 Sa_MCE Sa_MCE (SSI 7-16) Sa_MCE (2015 NEHRP) Sa (g) 1.00 0.80 0.60 0.40 0.20 0.00 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 Periods (s)
Kinematic Interaction - Example 80 Displacement 70 60 Story elevation, ft 50 40 30 20 10 Sa_MCER Sa_MCE (SSI 7-16) Sa_MCE (2015 NEHRP) 0 0.00 1.00 2.00 3.00 4.00 5.00 displacement, in
Kinematic Interaction - Example 80 Story Drift Ratio 70 60 Story elevation, ft 50 40 30 20 10 Sa_MCER Sa_MCE (SSI 7-16) Sa_MCE (2015 NEHRP) 0 0.00% 0.10% 0.20% 0.30% 0.40% 0.50% 0.60% 0.70% displacement, in
ASCE 7-16 / 2015 NEHRP Provisions Chapter 19: Soil-Structure Interaction Robert Pekelnicky, PE, SE Principal, Degenkolb Engineers February 11, 2015