SEAONC Seismology Subcommittee on Ground Motions

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SEAONCSeismology Subcommittee on Ground Motions

• Kickoff Meeting, April 21, 2009

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Rough Agenda

• Current code provisions for site-specific ground
  motions (10mins)
• OSHPD CANs and DSA Bulletins modifying these
  provisions (10mins)
• Implementation of modifications into new code
  editions (5mins)
• Aim of the subcommittee (5-10mins)
• Discussion of subcommittee goals

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Code Provisions (Pre-CAN)

• ASCE 7-05 Chapters 11 and 21
• 2007 CBC Chapter 16 and 16A
• Application in ASCE 7-05 Chapter 12 and CBC
  1614A

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Site-Specific Procedures

• Probabilistic Seismic Hazard Analysis (PSHA)
• Derive 2475 year return period ground motion
• Use site lat/long and attenuation relationships
• Next Generation Attenuation (NGA) Relationships
  (2008) or
• Previous Attenuation Relationships (1997 through
  2003)
• Deterministic Seismic Hazard Analysis (DSHA)
• Ground Motions for Individual Faults
• Calculated based on Characteristic Magnitude of
  the fault and the distance to the site
• Uses same Attenuation Relations as PSHA

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PSHA, DSHA, and Statistics

• PSHA Provides a Probability of Exceedance in
  Return Period
• Can be calculated for confidence intervals
• Confidence associated with Attenuation
  Relationships
• Median, Mean s, Mean 2s, etc.
• DSHA - No Return Period
• For analysis purposes, use the worst case
  earthquake, or the largest magnitude at the
  closest distance (and the greatest PGA)
• Median, Mean s, Mean 2s, etc.

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PSHA Probability of Exceedance
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Derive Site-Specific Design Response Spectrum

• Objective 2/3 of the Site-Specific Maximum
  Considered Earthquake
• Compares Probabilistic MCE with Deterministic MCE
• Probabilistic MCE 2/50 year (2475) event
• Deterministic MCE
• greater of 1.5xDSHA and
• Deterministic Lower Bound ASCE 7-05 21.2.2

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Graphically 2475 Spectrum
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Graphically Deterministic Spectrum

• Deterministic spectra shown are for the 84th
  ile, following the provisions of the OSHPD CANs,
  discussed later. Current Code specifies use of
  150 of the Median Deterministic Spectrum

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Graphically Deterministic and Probabilistic
Comparison(Take the lower of the two.)
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Graphically Comparison of Probabilistic and
Deterministic MCEsResult of comparison must fall
above 80 of Simplified Method Spectrum from ASCE
7 11
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SDS and SD1

• SDS Short Period Acceleration Parameter
• SD1 Acceleration Parameter at 1-second Period
• Code Specifies How to Calculate These
• Base Shear Equations (ASCE 7-05 Chapter 12) Use
  These

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OSHPD CAN and DSA Bu.

• Apply Rotation Requirement to Motions Calculated
  with NGAs Maximum Rotated
• DSHA To Use Means Instead of 1.5x
• Older Relations Not Mixed with NGAs
• Modify SD1 Calculation for use in ASCE Chapter 12