farnaz bayat - University of Tehran, Institute of Geophysics, M.Sc. Student,
elham shabani - University of Tehran, Institute of Geophysics, Assistant professor

A common goal of dynamic structural analysis is to predict the response of a structure subjected to ground motions having a specified spectral acceleration(Sa) at a given period(Baker,2011). The prediction is often obtained by selecting ground motions that match a target response spectrum. The Uniform Hazard Spectrum (UHS) is an unsuitable target for this purpose, as it conservatively implies that large-amplitude spectral values will occur atall periods within a single ground motion. An alternative, termed a Conditional Mean Spectrum(CMS) provides the expected (i.e., mean) response spectrum, conditioned on occurrence of a target spectral acceleration value at the period of interest, on the other hand, the epsilon values are of engineering interest because of their impact on the (CMS). When target epsilon values which is takenresultant from seismic hazard disaggregation process are positive, the CMS calculation produces relatively lower response spectra than the UHS.but iIn the case of negative target epsilon, some of the relationships between the CMS and UHS would reverse.The case studies indicate that positive target ε values occur almost in active seismic regions at long return periods,and alsothe special relevantseismicity models and certain appliedground motion prediction equations have the most significant effect on ε values.and So that a combination of thesethe above mentionedcharacteristics parameterscreates the most likely situation for negative target ε tooccure.

ground motions; uUniform hazard spectrum(UHS); conditional mean spectrum(CMS); Disaggregation; negative epsilon; return periods; ground motion prediction equation