Spectral shape metrics and structural collapse potential
This paper examines various parameters that provide a measure of spectral shape and studies how they relateto the potential of ground motion records to cause the collapse of a given structure. It is shown that whenmeasuring the ground motion intensity by the spectral acceleration at the ﬁrst-mode period of the structure,Sa(T1), records causing collapse at low ground motion intensities typically have signiﬁcantly differentspectral shapes than those that do not cause collapse until much higher ground motion intensities. A spectralshape typical of damaging records is identiﬁed, and a metric for quantifying the spectral shape of a recordcalled SaRatio is proposed and evaluated. SaRatio is deﬁned as the ratio between Sa ( T1) and the averagespectral value over a period range. The ability of SaRatio to predict the collapse intensity, i.e. the minimumintensity at which a giv en ground motion causes the collapse of a given structure, is compared to otherrecently proposed spectral shape metrics including epsilon (ε), eta (η) and Np. The results demonstrate thatSaRatio is typically a much better predictor of collapse intensity than other spectral shape metrics.