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

Outline

1.  Seismic Hazard Analysis   Probabilistic   Deterministic   Site-Response Analysis

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

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

Two SHA Approaches

Recommendation

Use 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):

µSa, σSa | (M, r, S, …)

Probabilistic Seismic Hazard Analysis (PSHA)

  Source models   Ground motion prediction equations

(GMPEs)

Log Accel.

P depends on no. of standard deviations A is above/below am am

a

PSHA Output: Ground-Motion Hazard Curves

Uniform Hazard Spectrum

Deaggregation Plot for L.A.

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

Ratio of 1.0 s SA 2007 and 2002 National Seismic Hazard Maps for WUS 2% Probability in 50 Years

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

1992 Landers, CA M 7.3 Earthquake

Sommerville et al. (1997)

Lucerne Record, 1992 Landers Earthquake

Sommerville et al. (1997)

Fault Normal (FN) Fault Parallel (FP)

Lucerne Response Spectra

Sommerville et al. (1997)

FN

FP

Deterministic SHA

  Required per Ch 21 of ASCE 7   Usually governs MCE for sites near

active faults   Recommendations

  Use same GMPEs, weights and parameter values used in PSHA

  Maximum Magnitude   May want use higher Mmax than weighted average

Mmax from logic tree

Site-Specific Deterministic Method ASCE 7, Sect. 21.2.2

  Find Fault largest median Sa

  Compute 1.5 x median Sa (ASCE 7-05)

  Compute Sa84th >1.5Sa

median (ASCE 7-10)

Fault Map

Median Deterministic Response Spectra H Comp.

0

0.5

1

0 0.5 1 1.5 2 2.5 3 3.5 4

T (sec)

Elysian Park Thrust Newport Inglewood San Andreas

S a

M 7.8 San Andreas Earthquake Simulations

Graves et al. (2008)

Site Response Analysis ASCE 7-05; Ch.21

Site-Specific Ground Motion

`

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

(may not be necessary)

Site Class F

SRA (cont.)

input motion

SRA (cont.)

input motion

Building Input Motion

Seismic Wavelengths

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

Basement Wall – Soil Interaction

flexible wall

pt. x

floor

Wall Reponses at Pt. x

Pt. x

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%)

g Graves 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

Basin Effects

  Amplify long period motions   Increase duration

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

CMS – e Parameter

M > ~ 8, Long Duration Motion

  San Andreas fault M ~ 8

  Cascadia and S. Alaska subduction zone M 9+

Velocity Records for M 7.8 San Andreas Event

Graves et al. (2008)

km

Accelerogram Modification

  Constant Scaling

  Spectral Matching

Accelerogram Modification

  Constant Scaling

  Spectral Matching

Constant Scaling Method

0

1

2

3

0 1 2 3 4 5 T (sec)

EQ-IV x 1.3 1940 Imperial Valley, El Centro (2.00) 1971 San Fernando, 8244 Orion Blvd. (1.74) 1979 Imperial Valley, El Centro Diff Array (1.74) 1989 Loma Prieta, Saratoga Aloha Ave. (1.88) 1992 Landers, Yermo Fire Station (2.00) 1994 Northridge, Sylmar Hospital (1.10) 1999 Duzce, Turkey, Duzce Station (1.36)

Sa (g)

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