The seismic collapse risk of a structure is largely influenced by the intensity and other characteristics of the earthquake ground motions. This study addresses the influence of the shape of the ground motion spectra and the ground motion duration on the structural collapse capacity, as determined by nonlinear response history analysis. The conditional spectrum is proposed as a more realistic characterization of spectral shape, compared to the commonly used uniform hazard spectrum. Ground motion duration is another important characteristic, which is quantified in terms of significant duration. The effects of spectral shape and duration on the estimated collapse capacity of a 5-story steel moment frame are demonstrated. Spectrally equivalent long and short duration record sets are employed to isolate the effects of duration. Preliminary findings indicate that cyclic strength and stiffness deterioration of components and accumulation of drift due to ratcheting can significantly reduce structural collapse capacity under long duration shaking.