Incremental dynamic analysis (IDA) has recently emerged as an alternative approach to investigate inelastic limit states in performance-based seismic engineering. IDA requires a series of nonlinear -time history analyses at increasing intensity levels of the same ground motion. The intensity of the earthquake load is typically expressed in terms of the first mode sp ectral acceleration. In this study the validity of such an intensity measure is investigated. IDA is applied to an existing six-story steel moment-frame building. Two sets of ground motions representing far-fault and nearfault records are considered separately to study the influence of different seismic source characteristics on the outcomes of the IDA . The study reveals that first mode spectral acceleration, S a(T1,5%), is an inadequate and incomplete measure of seismic intensity in the context of gener ating IDA capacity curves. It is concluded that alternative intensity measures are needed to make IDA more meaningful in seismic assessment. A preliminary investigation utilizing spectral magnitudes from constantductility inelastic spectra as a measure of seismic intensity is reported in this paper.
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