COSMOS Technical Session and

Transcription

1 Conclusions from Previous COSMOS Technical Session and It Introduction ti to Today s Td Topic Norm Abrahamson Nov 19,

2 2004 Meeting Discussion of the objective of time history analyses Average response Variability of response Best selection of time histories depends on objective of the engineering analysis 2

3 2005 Meeting Time histories for building code Requirements Intent Examples of time history selection and modification used in different projects Scaling vs matching No common basis for comparing methods 3

4 2006 Meetings Satisfying code requirements Selection Approaches Seismological Properties Similar Mag, Dist, Mech Goal: capture key unknown characteristics of ground motion that are important to the structural response Recording Properties Wider Mag, dist, mech Identify key characteristics of ground motion that are important to the structural response E g spectral shape, pulses, duration, E.g. spectral shape, pulses, duration, Select recordings that sample the key characteristics 4

5 2006 Meetings Selection Approaches Best approach for selection and modification of time histories depends on what you want to get from the analysis Average response Variability of response Selecting Good recordings or using spectrum compatible recordings leads to more stable estimate of the average response, but suppresses variability of the response 5

6 Selecting Good Recordings Selecting only based on seismological properties leads to large variability Choosing good records suppresses variability 6

7 Objectives for Use of Time Histories For fragility (PBEE) Need full variability Can t use spectrum compatible or subset of good records 7 is a small number to define variability For meeting code requirements Average response: reduce variability for more stable estimate of the average response Can use spectrum compatible or good records To learn something about structure What you learn is at the mercy of the selection of the records 7

8 2007 Meeting Summary Review of selection and modification methods used for performance-based engineering ATC-58 ATC-63 PEER GMSM Working Group results Testing of multiple methods for record selection and modification Test methods for 3 buildings 8

9 2007 Meeting Use of CMS for Buildings Leads to a realistic spectrum given a single spectral point on the UHS Avoids the enveloping characteristics of the UHS Important even for sites with a single source controlling hazard if epsilon is high Leads to more accurate estimate of the structural response 9

10 2008 Meeting Consider record selection for geotechnical problems, not just buildings Optimal small set of recordings may be different for geotechnical problems than for building response Conditional mean spectrum focused on building period What are key features of ground motions for geotechnical problems? What about projects with both geotechnical and structural parts? Conclusion Improved estimates of the response if record properties are considered, not just seismological properties 10

11 2009 Meeting Pending Code Changes Maximum direction spectrum Impacts on ground motion models Impacts on time history selection and modification Confusion in the application Early adoption for California Hospitals 11

12 Site-Specific Max Rotated Spectra Available scale factors to apply to GM models Beyer & Bommer Watson-Lamprey & Boore Huang et al Which models are applicable to site-specific specific studies? 12

13 Directivity Site-Specific Maximum Direction Spectra Two parts to the model Change in the geo-mean (forward vs backward) Systematic difference in horizontal components Covered by use of Max rotated spectrum Should the impact on geo-mean be included in site-specific studies? If yes, what about the deterministic case? 13

14 Time History Selection Variability of rotation angles for max direction component Significant variability from max rotated if using SN about 50% of records > 0.9 SaMax Small variability from max rotated if a best angle is selected independent of SN about 90-95% of records > 0.9 SaMax Should the selected time histories be rotated to a best (principal axes) angle based on minimizing the deviation from SaMax? Code says use FN 14

15 Time History Modification Criteria Check on SaMax or SRSS? For less than 5 km, SN> 1.0 Max direction Since SN is not always max direction, this can lead to SRSS much larger than 1.0 Max rotated. 15

16 Today s Topic CMS Lead to improved estimates of structural response risk (PEER GMSM project) Practical issues on computing CMS Issue of T 0 Multiple ground motion models Beyond CMS Conditional spectra Examples of use of CMS on projects Should CMS be used for engineering gprojects? 16