Seismic Input and Soil-Structure Interaction (Ch. 5 of TBI report, PEER 2010/05)

1. Seismic Input and Soil-Structure Interaction(Ch. 5 of TBI report, PEER 2010/05) TBI Committee Members Y. Bozorgnia C.B. Crouse J.P. Stewart October 8, 2010

2. Outline Seismic Hazard Analysis Probabilistic Deterministic Site-Response Analysis Soil-Foundation-Structure Interaction Kinematic Inertial Input Motion Specification Ground Motion Selection and Scaling Identification of Controlling Seismic Sources Ground Motion Selection Accelerogram Modification

3. Two SHA Approaches

4. Recommendation Use General Procedure if geotechnical engineer is inexperienced or unqualified to perform site-specific probabilistic and deterministic SHA.

5. Two SHA Approaches (cont.) • Site-Specific (Preferred) • Probabilistic • Deterministic

6. Probabilistic Seismic Hazard Analysis (PSHA) • Source models • Eqk locations • M range • Recurrence

7. Probabilistic Seismic Hazard Analysis (PSHA) • Source models • Ground motion prediction equations (GMPEs): • mSa, sSa | (M, r, S, …)

8. PSHA Output: Ground-Motion Hazard Curves

9. Uniform Hazard Spectrum

10. Recommendations for PSHA • For experienced PSHA users only • Use QA-checked software • Account for alternate seismic source parameters and GMPEs (epistemic uncertainty)

11. Logic Tree

12. GMPEs Recommended for Shallow Crustal Western U.S. Earthquakes NGA GMPEs (2008) • Abrahamson & Sliva • Boore & Atkinson • Campbell & Bozorgnia • Chiou & Youngs • Idriss • See EERI Spectra Journal (Feb. 2008, v. 24, no. 1)

13. Empirical GMPEs Recommended for Subduction Earthquakes • Atkinson & Boore (2003) – Site Class B, C, D • Crouse (1991) – Soil • Youngs et al. (1997) Soil and Rock • Zhao et al. (2006) Soil Classes I – IV and Hard Rock

14. Deterministic MCE Calculation • Req’d per ASCE 7 Ch 21 • Provides “cap” near major faults • Arbitrary decisions regarding: • Ruptured fault segment (closest) • Magnitude (use average of Mmax from logic tree) • Use same GMPEs & wts from PSHA • Different sources may be most critical at short and long periods

15. Site-Specific Deterministic MethodASCE 7, Sect. 21.2.2 • Find Fault à largest median Sa • Compute 1.5 x median Sa (ASCE 7-05) • Compute Sa84th >1.5Samedian (ASCE 7-10)

16. Site Response Analysis ASCE 7-05; Ch.21Site-Specific Ground Motion PSHA/DSHA – Vs30 PSHA/DSHA – Ref. Vs30 `

17. Recommendations • SRA not needed in absence of pronounced impedance contrast (often the case for stiff soil sites) • Site effect can be accounted for in such cases through GMPE site terms SRA advisable/required for:

18. Recommendations • SRA produces amplification factors, AF(T)= Sa,soil/Sa,rock • Typically applied as deterministic modification of UHS (Hybrid proc.): Sa,soil=AF(Sa,rock)UHS • Can avoid with modification of site term in hazard integral (OpenSHA) Unconservative bias

19. 2. Soil-Foundation-Structure Interaction (SFSI)

20. Linear springs and dashpots model soil-foundation interaction Input motion same at all points along foundation Input can be reduced for kinematic effects See FEMA 440 & ASCE 41-06 for details SFSI for MCE

21. 3. Ground Motion Selection and Modification • Identify controlling earthquakes • Select representative ground motions • Modify accelerograms to match target spectrum

22. T = 1 sec T = 5 sec M1 – R1 M2 – R2 Identify Controlling Earthquakes • Specify natural period band – SE decision • Deaggregation Plots

23. Issues with Ground Motion Selection • Number of ground motion sets • Multiple controlling earthquakes • Near-fault effects • Effects poorly represented in ground motion database: • Basin Effects • M > ~ 8, long-duration motion Use of simulations

24. Number of Accelerograms - N • No less than three (use maximum responses) • Use average responses if 7 or more motions used • More needed if multiple controlling earthquakes

25. Near Fault Effects Select a(t) for both cases

26. Fault Transform FN & FP a(t) into X & Y a(t)

27. Simulated Ground Motions (e.g., ShakeOut) Sa (T = 3 sec, 5 = 5%) g Graves et al. (2008)

28. Simulated Ground Motions (e.g., ShakeOut) • Can produce realistic-appearing wave forms • Need for calibration • Most broadband methods are inadequately validated or have biases

29. Issues with Ground Motion Modification • Target Sa • Site-specific Sa • Conditional mean Sa (CMS) • Modification procedures • constant scaling • spectral matching

30. Target Sa • UHS encompasses many events • Not achievable in a given event • Scenerio spectra (CMS) more realistic; need > 1

31. Accelerogram Modification • Constant Scaling • Spectral Matching

32. Accelerogram Modification • Constant Scaling • Spectral Matching

33. Spectral Matching

34. Selection and Scaling Recommendations • N > 7 (N limited by $ and time) • Use hazard deaggregations®controlling EQs • CMS – use several ®different Sa shapes • Scaling (constant or spectral matching) SE’s decision • Simulated accelerograms(M > ~ 8) - ADV: long duration and basin effects - DISADV: verification issues, access to quality simulations • Peer Review – Important