Seismic Input and Soil-Structure Interaction

(Ch. 5 of TBI report, PEER 2010/05)

TBI Committee MembersY. Bozorgnia C.B. CrouseJ.P. Stewart

October 8, 2010

Outline1. Seismic Hazard Analysis

Probabilistic Deterministic Site-Response Analysis

2. Soil-Foundation-Structure Interaction Kinematic Inertial Input Motion Specification

3. Ground Motion Selection and Scaling Identification of Controlling Seismic Sources Ground Motion Selection Accelerogram Modification

Two SHA Approaches

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

Two SHA Approaches (cont.)2. Site-Specific (Preferred)

Probabilistic Deterministic

Probabilistic Seismic Hazard Analysis (PSHA)

Source models Eqk locations M range Recurrence

Probabilistic Seismic Hazard Analysis (PSHA)

Source models

Ground motion prediction equations (GMPEs):

mSa, sSa | (M, r, S, …)

PSHA Output: Ground-Motion Hazard Curves

Uniform Hazard Spectrum

Recommendations for PSHA For experienced PSHA users only

Use QA-checked software

Account for alternate seismic source parameters and GMPEs (epistemic uncertainty)

Logic Tree

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)

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

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

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 >1.5Sa

median (ASCE 7-10)

Site Response AnalysisASCE 7-05; Ch.21

Site-Specific Ground Motion

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PSHA/DSHA – Vs30

PSHA/DSHA – Ref. Vs30

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:

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

2. Soil-Foundation-Structure Interaction (SFSI)

SFSI for MCE 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

3. Ground Motion Selection and Modification Identify controlling earthquakes

Select representative ground motions

Modify accelerograms to match target spectrum

Identify Controlling Earthquakes Specify natural period band – SE decision Deaggregation Plots

T = 1 sec T = 5 sec

M1 – R1 M2 – R2

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

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

Near Fault Effects

Select a(t) for both cases

Transform FN & FP a(t) into X & Y a(t)

Fault

Simulated Ground Motions (e.g., ShakeOut)Sa (T = 3 sec, 5 = 5%)

gGraves et al. (2008)

Simulated Ground Motions (e.g., ShakeOut)

Can produce realistic-appearing wave forms

Need for calibration

Most broadband methods are inadequately validated or have biases

Issues with Ground Motion Modification

Target Sa Site-specific Sa Conditional mean Sa (CMS)

Modification procedures constant scaling spectral matching

Target Sa

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

Accelerogram Modification Constant Scaling

Spectral Matching

Accelerogram Modification Constant Scaling

Spectral Matching

Spectral Matching

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