Report - PEER - University of California, Berkeley

structure, the results of inelastic structural analyses are insensitive to the magnitudedistancepairs corresponding to the records. This assertion effectively circumvents theshortcoming in current code specifications of accelerograms, but the results of Shomeet al. (1998) are not conclusive regarding the influence of magnitude (due to the typeof structure and the damage measure employed), and therefore duration, on inelasticstructural demand and there is still a good case to be made for selecting records fromearthquakes of appropriate magnitude (e.g., Bommer and Acevedo, 2004).3. EARTHQUAKE ACTIONS FOR PBSDThe two most significant changes in the specification of earthquake actions for designfor PBSD as compared with current code-based design are the requirement to defineseveral different levels of ground shaking and to present the ground motion in termsof parameters that are more closely related to structural demand than maximum peaksof transient accelerations.3.1 Multiple Levels of Earthquake ShakingAll formulations for PBSD presented to date assume that the different design levelsconsidered will be determined from probabilistic seismic hazard assessment (PSHA)and a series of selected return periods (e.g., Stewart et al., 2002). Although PBSDmay ultimately aim to provide a check on performance over the full range of hazard,in most code applications this is likely to be approximated by a few pairs of loadlevels and performance targets. In the now famous matrix of design levels andperformance levels presented in Vision 2000 (SEAOC, 1995) — which renders theimportance factors defined in current codes redundant — the return periods specified(without explanation or commentary) for design ground motions are 43, 72, 475 and970 years, which correspond to exceedance probabilities of 69, 50, 10 and 5% duringan exposure period of 50 years. Clearly there remains a great deal of work to be doneto arrive at robust selections of design levels (the IBC 2000 design level alreadyexceeds the most severe loading case envisaged in the SEAOC proposals). Onepossible solution is the calibration of the pairs of design and performance levelsthrough iterative earthquake loss modeling facilitated by computationally efficientand mechanically-based techniques (Pinho, 2004).3.2 Format for Earthquake ActionsThe inadequacy of current forced-based approaches to seismic design is widelyacknowledged. Whilst it is possible that future codes may adopt energy-basedapproaches, these have not yet been formulated in a sufficiently simple fashion. At thesame time a very convincing case has been made for displacement-based approaches(Priestley, 2000) to form the basis of PBSD. Direct displacement-based design, usingthe substitute structure concept, leads to two new requirements in terms of seismic475