Further Development of Site Response in

NGA Models

PEER Lifelines ProgramNGA-West2 Project

Topic #8

Working Group Meeting

Meeting #2 October 26, 2010

Agenda

• Review project objectives and status (Stewart)• Status of NGA-W2 database (Bozorgnia, Silva)• Update on comparison of NEHRP and NGA site

factors (Seyhan)• Project scope (Stewart and group)

– Scope as presented in proposal– Open discussion of research priorities

• Sub-contract procedures (Seyhan)

Project Objectives and Status

• NEHRP site factors– Wk motion: empirical AFs, reference site

approach– Nonlinearity from simulations

• NGA site factors– Empirical AFs, non-reference site approach– Simulations

• NEHRP and NGA site factors inconsistent

Project Objectives and Status

• “The principal objective of this project is to identify and better understand those discrepancies for the IMs of interest in the NGA-West2 project and propose new site factors for application in NEHRP that will resolve the differences.”

Project Objectives and Status

• “Additional, related objectives concern enhancing the site database developed in the original NGA project to incorporate the results of additional site characterization efforts, and to check the performance of the existing NGA site factors through analyses of residuals. Those residuals analysis efforts will emphasize, but not be limited to, data collected since the original NGA project (approximately 1500 recordings) .”

Status of NGA W2 Database

Yousef BozorgniaWalt Silva

Emel SeyhanUniversity of California, Los Angeles

Comparison of NEHRP Site Amplification Factors and the

NGA Relationships

Outline

• Introduction• Analysis Procedure

– Data Used for Site Classifications– Amplification within Vs30 Categories

– Variation of Amplification with Vs30

• Conclusions

NGA-NEHRP Comparisons

• In natural log units, site term = Fx(Vs30, Ax)– Fx=amplification relative to Vs30=x site condition

– Ax=ground motion amplitude for reference site condition of Vs30=x

• Use Vs30=157, 275, 425, 760, and 1047 m/s• Evaluate F at T=0.3 and 1.2 sec.

Input Parameters– AS: VS30, Median PGA1100

– BA: VS30, Median PGA760

– CB: VS30, Median PGA1100

• Take A = PGA– CY: VS30, Median + i (Sa)1130

• Take A= Sa(0.3)=2.2*PGAr and Sa(1.0)=1.0*PGAr

Adopt reference condition of 760 m/sF760(Vs30, Ax)=Fx(Vs30, Ax)-Fx(760, Ax)

xxxsxa

xa

xa

Vsa

a

Vsa AFAVFS

S

S

S

S

S,760,exp 30

760

30

760

30

Histogram of Vs30 for Strong Motion Sites

Class C and D

Short - Period T=0.1, 0.3, 0.5 sec

* Vs30=1074 m/s is typo. It is 1047 m/s.

Short - Period T=0.1, 0.3, 0.5 sec

* Vs30=1074 m/s is typo. It is 1047 m/s.

Mid - Period T=0.4, 1.2, 2.0 sec

* Vs30=1074 m/s is typo. It is 1047 m/s.

Mid - Period T=0.4, 1.2, 2.0 sec

* Vs30=1074 m/s is typo. It is 1047 m/s.

• Averaged across corresponding period ranges – 0.1-0.5 sec for

Fa;– 0.4-2.0 sec for Fv

• Used Vs30 values– 275 and 425 m/s

Differences:• NEHRP Fv high

– esp. Class C to E• NEHRP nonlinearity

stronger– Class C to D

• NEHRP Fa and Fv high for rock– Class B

Variation of Amplification with Vs30

Vs30(m/s)=[157,275,425,760,1047]ASBACBCYNEHRP Site Factors

Concluding Remarks

• Amplification at short and long period ranges exhibit a decreasing trend with increasing velocities

• Nonlinearity of amplification factors vary with Vs30, especially for Vs30<180 m/s and relatively small for Vs30~275 m/s.

• For amplification variation with Vs30 NEHRP mid period Fv are high, especially for Class C to E.

• Bias in weak motion amplification for Fv• NEHRP nonlinearity stronger for Class C to D.• NEHRP Fa and Fv high for rock associated with Class B.

Project Scope

• Task 1: Direct site factors comparison – complete?

• Task 2: Database development– Main NGA-W2 database from PEA– Review and possibly add additional parameters & data– Possible use of virtual geotechnical data center (?)

Project Scope

• Task 3: Data analysis– Non-Reference Site Approach

• Residuals analysis (new and original NGA data) relative to median for Vs30=760 m/s.

• Check against basin depth and other parameters (e.g., f0, etc.)• Look at standard deviations• Residuals for motions computed with NEHRP factors

– Reference Site Approach• Identify soil/rock pairs (need to develop criteria for selection)• Use GIS database to help in identifying station pairs? • Compare to site factors from non-reference site approach

– Emphasize soft soil sites in above work