Report - PEER - University of California, Berkeley

actions: response spectral ordinates at long periods and spectral ordinates at multiplelevels of damping. The displacement spectra for higher damping values would usuallybe derived from the spectrum constructed for the nominal 5% of critical damping;recent work has indicated that the reduction of the spectral ordinates with increasingdamping increases with earthquake magnitude (Figure 3) and hence with the durationof the ground shaking (Bommer and Mendis, 2004).Figure 3. Variation of spectral scaling with damping for different magnitudesinferred for rock sites at 10 km from earthquakes of different magnitudes. Alsoshown are the scaling factors from EC8.The variation of the spectral scaling factors indicated in Figure 3 is appreciable atcertain response periods and this suggests that the use of scaling factors that areindependent of the nature of the ground motion to adjust the 5% damped spectrum forhigher levels of damping may be an oversimplification.4. EARTHQUAKE ACTIONS IN PBSD CODES4.1 Extrapolation of Current Code FormatsSomerville (1996) points out that seismic hazard curves are ideally suited to PBSD“because they specify the ground motions that are expected to occur for a range ofdifferent annual probabilities (or return periods) that correspond to differentperformance objectives”. If it were found that the hazard curves for a given groundmotionparameter were of the same shape at all sites in a country, then it would besufficient to provide 475-year maps of the selected ground-motion parameters and anequation to scale these amplitudes to other return periods. Since the shape of the476