Report - PEER - University of California, Berkeley

hazard curves will often vary throughout a country, advantage can be taken of theapproximately linear relationship (over the range of return periods of usualengineering interest) between the logarithms of exceedance frequency and of theground-motion parameter to determine two coefficients that relate them; from maps ofthese two coefficients the value of the ground-motion parameter can be determinedfor any return period (Grases et al., 1992). This elegant approach would allow currentcode approaches such as that in IBC 2000 to cover all return periods of interest withjust four hazard maps: two for the short-period spectral acceleration and two for the1.0-second ordinate.For PBSD, assuming that it will involved displacement-based design, thedefinition of long-period spectral ordinates will require at least a third parameter to bemapped: one option is the corner period that defines the constant displacement plateaumapped in the 2003 NEHRP guidelines; another is that proposed by Bommer et al.(2000) of mapping PGA, PGV and PGD and then defining the corner periods from theratios of these parameters. Using the method of Grases et al. (1992) this leads to sixmaps, whereas if four return periods are to be considered — as recommended inVision 2000 — and mapped individually, then the code would include 12 separatemaps, which starts to be cumbersome (although this could be overcome by embeddingthe maps within a GIS provided on CD-ROM with the code). The situation becomesmore complicated if maps for other parameters, such as independently definedvertical motions and strong-motion duration, are also needed.4.2 Disaggregated Seismic HazardDisaggregation of seismic hazard (e.g., McGuire, 1995; Bazzurro and Cornell, 1999)allows the contributions made to the hazard at a site by different magnitude-distancepairs to be identified. In the USA there is the unique situation wherein the USGeological Survey operates a web site that allows users to enter the coordinates of thesite and obtain disaggregations of the hazard at selected response and return periods;since the hazard maps and UHS in the seismic design codes are very closely related tothe national hazard maps of the USGS, this effectively allows users of the code toidentify hazard-consistent scenarios. Such a facility offers many advantages, not leastof which is it enables selection of appropriate acceleration time-histories when theseare required for design (Bommer and Acevedo, 2004). The controlling magnitudedistancepairs identified for horizontal spectral ordinates at the structural period ofinterest and for the selected return period can be used to estimate other parameterssuch as the vertical spectral ordinates and the strong-motion duration. This approachis preferable to producing separate maps for these other parameters since thehorizontal and vertical spectral accelerations and the strong-motion duration, for agiven return period, will often be controlled by different earthquake scenarios. Thiswould imply that the motions were not compatible hence the use of multiple mapscould result in the elastic spectral ordinates controlled by one source of seismicitybeing reduced by a duration-related ductility or damping factor controlled by another.477