Ground Motions by 1Le6cE

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									    Geotechnical Earthquake Engineering:



            Ground Motions



                 Steve Kramer


Department of Civil and Environmental Engineering
            University of Washington
                   Seattle, WA
Ground Motions

  PSHA (or DSHA) provides                   For linear structural analysis,
  uniform hazard spectrum                   response spectrum is all you
  Codes generally produce design            need – no need for individual
  spectrum                                  ground motions



  For nonlinear analyses time histories are required
      Nonlinear structural analyses
      Nonlinear geotechnical analyses            All are becoming
                                                  more common
      Nonlinear SSI analyses



         Goal is to identify / create one or more motions that have
      amplitudes, frequency contents, and durations that are consistent
            with the ground shaking hazard at the site of interest.
Ground Motions

  Sources:   PEER NGA Database (http://peer.berkeley.edu/nga/)
Ground Motions

  Sources:   PEER NGA Database (http://peer.berkeley.edu/nga/search.html)




                                                     Can search for records
                                                     with characteristics
                                                     similar to those
                                                     controlling hazard at site
                                                     of interest
Ground Motions

  Sources:   COSMOS Database
             http://db.cosmos-eq.org/scripts/search.plx




                 Can search for records
             with characteristics similar
             to those controlling hazard
                       at site of interest
Ground Motions

  Problem: Find ground motion(s) that “match” target spectrum

      Two common approaches:
          Simulation – single spectrum-compatible ground motion
          Scaling – suite of motions with matching ensemble average

      Required information:                          Sa

           Target spectrum
               Uniform hazard spectrum (UHS)
               Code spectrum
                                                            To           T
           Fundamental period of structure
           Intent of analyses for which motions are to be used
               Mean or median response
               Mean / median and indication of variability of response
Ground Motion Scaling

  Simulation

     Alter characteristics of motion to “match” target spectrum
     Two common approaches:
          Time domain – wavelets (actually, time and frequency domains)
               Example: RSPMATCH (Norm Abrahamson)
          Frequency domain – Fourier analysis
               Example: RASCAL (Walt Silva)
     Both approaches start with some initial ground motion
          Important that initial motion has “correct” duration


        Spectrum-compatible motions are useful for determining
        the mean or median response of a system. They do not
        provide direct insight into the variability of that response.
Ground Motion Scaling

  Simulation

      Example:




                 Spectrum
                   after                     Initial motion
                 matching                      too strong




                            Initial motion
                                                              After Norm
                                 too weak                     Abrahamson
                                                              COSMOS workshop
                                                              presentation
Ground Motion Scaling

  Simulation

      Example:




                        Original




                        Modified
                                   After Norm
                                   Abrahamson
                                   COSMOS workshop
                                   presentation
Ground Motion Scaling

  Simulation

      Example:




                        Original




                        Modified
                                   After Norm
                                   Abrahamson
                                   COSMOS workshop
                                   presentation
Ground Motion Scaling

  Simulation

      Example:




                        Original




                        Modified
                                   After Norm
                                   Abrahamson
                                   COSMOS workshop
                                   presentation
Ground Motion Scaling

  Simulation

      Example:




                        After Norm
                        Abrahamson
                        COSMOS workshop
                        presentation
Ground Motion Scaling

  Simulation

      Example:
  Note: In areas where seismic hazards come from
  multiple sources, different parts of UHS may be
  controlled by different sources – single motion producing
  entire UHS may not be physically possible. In that case,
  use of spectrum-compatible motion may be quite
  conservative.




                                                  After Norm
                                                  Abrahamson
                                                  COSMOS workshop
                                                  presentation
Ground Motion Scaling

  Scaling

      Alternatively, we can identify and scale actual recorded motions for
      (ensemble average) consistency with a target spectrum
      Use deaggregation to find representative (mean / modal) values of:
            • Magnitude            Candidate motions should be
            • Distance             consistent with these
            • Style of faulting    characteristics

      Select consistent motions from database (e.g. PEER NGA database)
      based on seismological properties
            • Similar magnitude (within +/- 0.5 provides reasonable duration)
            • Similar distance range
            • Similar spectral shape (or epsilon)
            • Same style of faulting
Ground Motion Scaling

  Scaling

     Scale motions by constant factor to “match” target spectrum
     What constitutes a match?
            Match is in average sense – average of suite of motions
            May be defined in terms of SRSS spectra (multi-directional components)
            Usually need to exceed target over significant period range
                For structures, typically 0.2To – 1.5To



    Lower periods (higher frequencies)
      covers higher mode response

                       Higher periods (lower frequencies)
                       covers damage-induced softening
Ground Motion Scaling

  Scaling

      Select large suite of ground motions (50 – 100 or so)
      Use deaggregation to find representative (mean / modal) values of:
            • Magnitude
            • Distance
            • Style of faulting
      Select consistent motions from database (e.g. PEER NGA database)
      based on seismological properties
            • Similar magnitude (within +/- 0.5 provides reasonable duration)
            • Similar distance range
            • Similar spectral shape (or epsilon)
            • Same style of faulting                           Significant
                                                              period range
Ground Motion Scaling
   Epsilon




                           To                  To



                                     Scaled negative e
   Be careful – look for             motion is too strong
local peaks and valleys
  in candidate motions               Scaled positive e
      prior to scaling               motion is too weak


                                To
Ground Motion Scaling
  Epsilon

    Baker (2007) took 382 representative ground motions
    Computed e values for each, chose 20 highest and 20 lowest
    Computed spectra after scaling to same Sa(T=0.8)
Hazard Analysis and Ground Motions

   Summary
      Design levels of ground motion determined by seismic hazard analysis
          DSHA – deterministic
          PSHA – probabilistic
          Attenuation behavior is critical
      Prediction of response spectrum may be sufficient
      Ground motions may be required
          Synthetic motions – describe mean/median level of shaking
          Scaled motions
              Reflect actual earthquake characteristics
              Suite of motion required – can account for record-to-record
              variability

     Results of site response analyses will be sensitive to ground
      motion inputs – need to pay careful attention to this issue

								
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