Report - PEER - University of California, Berkeley

2.2.4 Spectral Modification FactorsIn all seismic design codes, the elastic response spectrum of acceleration is reducedby a factor that accounts for, amongst several other features, the dissipation of seismicenergy through inelastic deformations in the structure. The reduction factors aredefined in terms of the structural system and the construction material; some codes,amongst them EC8, also define the factors as a function of period.Figure 2. Design acceleration spectra (for ductility demand of 4 in reinforcedconcrete) as function of the significant duration (5–95% Arias intensity) asproposed by Chai and Fajfar (2000).In all current codes, however, the reduction of spectral ordinates is independent of thenature of the expected ground motion. Chai and Fajfar (2000) propose a procedure forgenerating inelastic acceleration spectra that accounts for the increased number ofcycles into the inelastic range experienced by yielding structures subjected to largemagnitude earthquakes producing ground motions of long duration (Figure 2).2.3 Acceleration Time HistoriesThere are cases when accelerograms are required for dynamic analysis; if one acceptsthe premise that records should be selected from magnitude-distance pairs consistentwith the seismic hazard, the guidelines provided in current codes are generallydeficient in this respect because dominant magnitude-distance scenarios are notidentified (Bommer and Ruggeri, 2002). Shome et al. (1998) assert that providedrecords are scaled to the correct spectral acceleration at the fundamental period of the474