Effects of Strong Motion Processing Procedures on Time Histories, Elastic and Inelastic Spectra By Paolo Bazzurro, Brian Sjoberg, Nicolas Luco (AIR) Walter Silva, Robert Darragh (Pacific Engineering and Analysis) Presented at COSMOS INVITED WORKSHOP ON STRONG-MOTION RECORD PROCESSING Richmond, CA, May 26-27, 2004

© 2004 AIR Worldwide Corporation COSMOS WORKSHOP Motivation  To quantify on a statistical basis the effects on ground motion time histories and elastic and inelastic spectra of i. Causality of the filter ii. Filter order iii. Selection of the high-pass cut-off frequency iv. Preservation or removal of residual displacement offset  We considered  Butterworth filter only  Near-source (distance R  17km) ground motions from earthquakes with moment magnitude ranging from 6.5 to 7.6  Two components rotated fault-parallel and fault-normal

© 2004 AIR Worldwide Corporation COSMOS WORKSHOP Earthquakes and Recording Stations

© 2004 AIR Worldwide Corporation COSMOS WORKSHOP Processing Techniques Legend: Available Not Available * Parallel component only

© 2004 AIR Worldwide Corporation COSMOS WORKSHOP Response to Butterworth Filters of Different Orders

© 2004 AIR Worldwide Corporation COSMOS WORKSHOP Inelastic Displacement Response Spectra – What’s R?

© 2004 AIR Worldwide Corporation COSMOS WORKSHOP Effects of Causality of the Filter R=1 (Elastic) Bandwidth for cascade acausal filter Bandwidth upper bound for causal filter Bandwidth upper bound for acausal filter

© 2004 AIR Worldwide Corporation COSMOS WORKSHOP Effects of Causality of the Filter R=8 (Severely Inelastic)

© 2004 AIR Worldwide Corporation COSMOS WORKSHOP Effects of Filter Order: 4-pole vs. 5-pole Causal R=1 (Elastic) R=8 (Severely Inelastic)

© 2004 AIR Worldwide Corporation COSMOS WORKSHOP Effects of Filter Order: 4-pole vs. 5-pole Acausal R=1 (Elastic) R=8 (Severely Inelastic)

© 2004 AIR Worldwide Corporation COSMOS WORKSHOP Effects of Filter Order: 2p-2p vs. 2p-3p Acausal R=1 (Elastic) R=8 (Severely Inelastic)

© 2004 AIR Worldwide Corporation COSMOS WORKSHOP Effects of HP cut-off frequency: f HP vs. 1.5f HP pole Causal Filter R=1 (Elastic) R=8 (Severely Inelastic)

© 2004 AIR Worldwide Corporation COSMOS WORKSHOP Effects of HP cut-off frequency: f HP vs. 1.5f HP pole Acausal Filter R=1 (Elastic) R=8 (Severely Inelastic)

© 2004 AIR Worldwide Corporation COSMOS WORKSHOP Effects of HP cut-off frequency: f HP vs. 1.5f HP --- 2p-2p Cascade Acausal Filter R=1 (Elastic) R=8 (Severely Inelastic) ~5s1.5s

© 2004 AIR Worldwide Corporation COSMOS WORKSHOP Effects of HP cut-off frequency: f HP vs. 1.5f HP --- 2p-2p Cascade Acausal Filter R=1 (Elastic) R=8 (Severely Inelastic) 0.5s2.0s

© 2004 AIR Worldwide Corporation COSMOS WORKSHOP Effects of HP cut-off frequency: f HP vs. 1.5f HP --- 2p-2p Cascade Acausal Filter R=1 (Elastic) R=8 (Severely Inelastic) 1.0s3.0s

© 2004 AIR Worldwide Corporation COSMOS WORKSHOP Effects of Residual Displacement R=1 (Elastic) R=8 (Severely Inelastic) Notes: 1) normal components 2) 6 records only 3) Results for causal filters are similar

© 2004 AIR Worldwide Corporation COSMOS WORKSHOP Summary of Effects of Processing Techniques on Elastic and Inelastic Spectra  In general, effects are larger  for inelastic than for elastic spectra and  for longer periods at any given response level (i.e., R=1 through 8)  Causality of the filter does not systematically affect the amplitude of the spectra.  The statistical impact on spectra due to the filter order is negligible  Increasing the value of the HP cut-off frequency, f HP, generates inelastic spectra that are systematically lower at periods much lower than 1/ f HP regardless of the causality of the filter. This can be in part explained by the lengthening of the effective period of vibration of structures in the post-elastic regime.  Records with residual displacement offset preserved generate inelastic response spectra that are consistently higher than those caused by records with offset removed. (Phenomenon observed for 6 fault-normal records only).  NOTE: See paper for effects on spectra caused by applying a filter to simulated ground motion records

© 2004 AIR Worldwide Corporation COSMOS WORKSHOP Summary of Effects of Processing Techniques on Ground Motion TH’s  In general, effects are significant for PGD and, to a lesser extent, for PGV. The impact on other parameters (e.g., PGA, Arias Intensity, and duration) is negligible.  Causally filtered records have PGV and PGD values that are, on average, smaller (by 5-10% and 5-15%, respectively) than those of acausally filtered ones.  Acausally filtered records generally display a more prominent ramp of increasing displacement prior to the onset of strong-motion than causally filtered records. Peak-to-peak displacement amplitude, however, is similar for each processing technique.  The filter order does not affect the ground motion parameters considered here.  Increasing the value of the HP cut-off frequency, f HP, generates records with lower PGV and PGD values (5% and 15-20%, respectively), as expected.  The values of PGV and PGD are considerably larger (5-20% and 50-60%, respectively) in records with residual static offset preserved, as expected.