Report - PEER - University of California, Berkeley

The first area, hazard and ground motions, will be addressed not from theperspective of the seismologist from whom we engineers traditionally get thisinformation, but rather from the perspective of what PBEA objectives, needs andresources. The second subject, probabilistic assessments, will in the space availablebe limited to a rather formal overview of the issues and solutions, and an illustrativeexample.2. SEISMIC HAZARD AND GROUND MOTIONS2.1 Current Practice and PBEA ObjectiveAdvanced U. S. practice today would find an engineering seismologist responsible forproviding input to an engineer who has set out to do a nonlinear dynamic assessmentof a design. The seismologist would provide (1) a probabilistic seismic hazardanalysis (PSHA) (site-specific or downloaded from a USGS web site), (2) for one ormore mean annual frequency (or annual probability) levels, a uniform hazard(response) spectrum (UHS), and (3) for each such level, a suite of n accelerograms foruse in nonlinear dynamic analyses. Typically the seismograms have been selected toreflect the likely magnitudes, distances, and other earthquake parameters thought todominate the hazard at the site (perhaps in some particular frequency range); thischoice is guided by study of the “disaggregated” hazard. The seismograms might berecordings, “UHS spectrum-matched” recordings, or various forms of syntheticaccelerograms. The engineer will subsequently run time history analyses for each ofthe n accelerograms in the suite of accelerograms associate with annual frequency, p,and observe for each a variety of outputs. Consider, for example, one usefulparameter, MIDR. If the average of MIDR of the n records exceeds 7% (in a steelmoment resisting frame) he may conclude that frame failure is likely given that suchground motions; in fact he may conclude that the annual frequency of failure is aboutp, but few if any current structural norms would require him to state his conclusionsin such explicit terms.In contrast it is presumed here that PBSA will require that the engineer confirmin direct or indirect terms that the annual frequency of important limit states, denoted,C, such as global structural collapse or economic loss greater than 10% ofreplacement cost, are less than prescribed or recommended values. More generally, hewill seek the annual frequency, λ C , of one or more “limit state” events, C. To beconcrete we shall refer here to structural response limit states such as globalinstability collapse, MIDR greater than x%, etc.Given the PBSA objective of estimating λ C, we observe first that the problemnaturally subdivides itself into characterizing the seismicity surrounding the site andassessing the behavior of the structure given a particular earthquake event occurs, orin formal terms into λ(X) and P[C|X], in which λ(X) is the mean annual exceedancefrequency of earthquake events in the region with the vector X of parameters (such asmagnitude, distance from the site, faulting style, etc.), i.e., the mean annual frequency40