Further Development of Site Response in
NGA Models
PEER Lifelines Program
NGA-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 Bozorgnia
Walt Silva
Comparison of NEHRP Site
Amplification Factors and the
NGA Relationships
Emel Seyhan
University of California, Los Angeles
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/s
F760(Vs30, Ax)=Fx(Vs30, Ax)-Fx(760, Ax)
S a Vs30 Sa Vs30 Sa x


 exp Fx Vs 30 , Ax   Fx 760, Ax 
Sa 760 Sa x Sa 760
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
Variation of Amplification
with Vs30
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
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