Report - PEER - University of California, Berkeley

earth science and engineering seismology communities to improve the accuracy ofconventional IMs and to investigate alternative seismic intensity measures (potentiallyvector IMs) that correlate best with earthquake-induced damage (e.g., Stewart et al.2001, Baker and Cornell 2004, Cornell 2004).One of the important questions in choosing an IM relates to how well itrepresents the damaging effects of earthquake ground motions on structures. Toillustrate this issue, consider the results from multiple inelastic time history analyses,shown in Figure 2, which were conducted as part of a trial application of the PBEEmethodology for a non-ductile reinforced concrete building (Krawinkler 2004). Eachcurve in this figure represents a so-called Incremented Dynamic Analysis (IDA, seeVamvatsikos and Cornell 2002), where each point on the curve corresponds to thepeak response (in this case the maximum interstory drift ratio) obtained through aninelastic time history analyses for an input ground motion that has been scaled to aspecified spectral acceleration (Sa), defined at a period equal to the elastic first-modeperiod of the structure, Sa T1 . Results are shown for fifteen different ground motionsscaled up to hazard intensities with a 2% mean annual frequency of exceedence in 50years (2/50). The solid dots at the end of each curve correspond to the spectralintensity and drift ratio where collapse was detected — often below the 2/50 level forthis seismically deficient existing building.The large scatter in response is due so-called record-to-record variability,resulting from the fact that IM = Sa T1 does not fully capture all the “damagingfeatures” of the earthquake records. As described later, the proposed PBEEframework can account for this variability; though, it would be advantageous toidentify alternative IMs that would reduce the variability and capture significantSa/g10.90.80.70.60.50.40.350/500.20.12/5010/50Median84%16%00 0.01 0.02 0.03 0.04 0.05Max IDRFigure 2. Incremental dynamic analysis (IDA) ofnon-ductile 7-story RC building subjected to 15scaled ground motions (Krawinkler 2004).features of the seismichazard at the site. Somepromising examples ofimproved IMs include pairsof spectral ordinates andinelastic spectraldisplacement, among others(e.g., Cornell 2004, Baker etal. 2004, Cordova et al.2001). Apart from reducingvariability, to the extent thatimproved IM capture betterthe damaging features of arecord and the site hazard,criteria for selecting inputground motions for inelastictime-history analysesbecome less important.18