ASCE 7-16 / 2015 NEHRP Provisions Chapter 19: Soil-Structure Interaction. Robert Pekelnicky, PE, SE Principal, Degenkolb Engineers February 11, 2015

Transcription

1 ASCE 7-16 / 2015 NEHRP Provisions Chapter 19: Soil-Structure Interaction Robert Pekelnicky, PE, SE Principal, Degenkolb Engineers February 11, 2015

2 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

3 What We Should Do

4 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

5 What We Actually Do USGS Maps

6 What We Actually Do Structural Engineer

7 ASCE 7-16/ 2015 NEHRP Provisions Based on ATC-83 Report (NIST NIST GCR ) Direct inclusion of soil flexibility Merger ASCE Provisions with ASCE 7-10 Updated foundation damping equations Update & clarify limitations on use

8 Foundation Flexibility

9 Foundation Flexibility - Example Reinforced concrete shear wall building 4 Story + 1 Basement Site class D soil Oakland, California

10

11 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

12 Foundation Flexibility - Example Spectral Acceleration, Sa (g) Sa_MCER_(MaxDir) Sa_DE_(MaxDir) Period, T (sec)

13 Foundation Flexibility - Example

14 Foundation Yielding Currently no requirement for foundation to be stronger than superstructure Foundations and soil actions designed with same forces Soil actions typically ASD

15 Foundation Yielding Currently no requirement for foundation to be stronger than superstructure Foundations and soil actions designed with same forces Soil actions typically ASD

16 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

17 Foundation Yielding Example Code Design: 40 x18 footing 14-#7 Long & Trans Soil stronger than wall: 40 x24 footing 16-#8 Long & Trans Footing stronger than soil or wall 40 x18 footing 14-#8 Long & Trans

18 Foundation Damping Waves from structure moving interfere with ground motion waves Soil Hysteretic damping affects shaking building feels

19 Foundation Damping Radiation damping & soil hysteretic damping addressed Modification of the design base-shear or response spectrum Explicit damping elements required for nonlinear analysis

20 Foundation Damping

21 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

22 ASCE 7-16 Foundation Damping V = C s ~ C B s SSI W β ο = β f + β ~ ( T T ) 2 eff

23 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 B C D E * F * * * *

24 Radiation Damping βr=[1/(t/ty)2]βy+[1/(t/txx)2]βxx (Eq ) Ty=2πM*Ky (Eq ) Txx=2πM*h*2αxxKxx (Eq ) 0.65 GB L L K y = ν B B 3 GB L K xx = 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 a0 GB ( L/ B) 2(1 ν ) 2.5 ( 1 2ν ) 2 ( ( L/ B) 1) a0 = a 0 ( L/ B) (Eq ) (Eq ) (Eq ) (Eq ) (Eq ) (Eq ) (Eq )

25 Foundation Damping Example Structural Damping, β = 5% Radiation Damping, β f = 11% Soil Damping, β f = 7% Foundation Damping Total Damping

26 Foundation Damping Example Damping Base Shear Reduction Base Shear R-factor limit Revised base shear based on limit

27 Foundation Damping Example Property Fixed Base No SSI Flexible Base SSI Wall Thickness 14 inches 12 inches Horizontal Reinf. 12 o.c. 12 o.c. Vertical Reinf. 12 o.c. 12 o.c. Boundary Region 14 - #7 None Footing Dimensions 40-ft x 14-ft 40-ft x 14-ft Footing Long. Reinf #7 T&B 12 - #7 T&B Footing Trans Reinf. 12 T&B 14 T&B

28 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

29 Base Slab Averaging Low frequency components 2003 Off-Miyagi Eqk FF, s-wave window u FIM (ω) u g (ω) Prof. Jonathan Stewart

30 Base Slab Averaging High frequency components 2003 Off-Miyagi Eqk FF, p-wave window u FIM (ω) < u g (ω) Prof. Jonathan Stewart

31 Embedment Effect Low frequency Long wavelength λ = V s /f u FIM u g θ FIM 0 Prof. Jonathan Stewart

32 Embedment Effect High frequency Short wavelength λ = V s /f u FIM < u g θ FIM > 0 Prof. Jonathan Stewart

33 Kinematic Interaction

34 Ground Motions Free Field psa (g) 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 Period (sec)

35 Ground Motions At-Depth Amp-Scaled

36 ASCE 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

37 Combined BSA & Embedment 1.5 Embedment Reduction Reduction Factor Embedment BSA Combination Period (sec)

38 Scatter on Observed Data

39 Look at Existing Data Wadsworth Hospital Ratio between basement and free-field record (average) ASCE BSA + Embedment Effects 39

40 ASCE 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

41 Kinematic Soil-Structure Interaction

42 Soil-Foundation-Structure Interaction Base Slab Averaging & Embedment Equations Updated RRS bsa 1 2 [ 1 exp( 2b ) Β ] = bsa b0 Base area limited based on testing limits, 260 feet. 1 2 Β bsa 1+ b = exp b b0 + 2 b0 b b ( 2b ) 1 1 b > π b b 0 0 RRS e = 2πe cos Tnvs factor reduces the reduction Max reduction can only be 50%

43 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

44 Kinematic Interaction - Example Sa_MCE Sa_MCE (SSI 7-16) Sa_MCE (2015 NEHRP) Sa (g) Periods (s)

45 Kinematic Interaction - Example 80 Displacement Story elevation, ft Sa_MCER Sa_MCE (SSI 7-16) Sa_MCE (2015 NEHRP) displacement, in

46 Kinematic Interaction - Example 80 Story Drift Ratio Story elevation, ft Sa_MCER Sa_MCE (SSI 7-16) Sa_MCE (2015 NEHRP) % 0.10% 0.20% 0.30% 0.40% 0.50% 0.60% 0.70% displacement, in

47 ASCE 7-16 / 2015 NEHRP Provisions Chapter 19: Soil-Structure Interaction Robert Pekelnicky, PE, SE Principal, Degenkolb Engineers February 11, 2015