70%-Damped Spectral Acceleration as a Ground Motion Intensity Measure for Predicting Highly Nonlinear Response of Structures

We investigate 70%-damped spectral acceleration, Sa^(70%)(T), as a ground motion intensity measure for predicting maximum interstory drift ratios of 0.03, 0.06, and 0.1 as well as collapse. We perform incremental dynamic analysis with 50 ground motions on 22 steel moment frame building models with heights of 3, 9, and 20 stories. We find that if T₁ ≤ T ≤ 2T₁, Sa^(70%)(T) is efficient and usually sufficient for the considered levels of highly nonlinear response. Sa^(70%)(1.5T₁) is generally an efficient choice. We find that Sa^(70%)(T) is similar to average spectral acceleration, Sa_(avg), in many ways, as both intensity measures emphasize a wide range of periods in a ground motion when compared to Sa^(5%)(T₁). Sa^(70%)(T) is equivalent to the peak of a ground motion's low-pass filtered acceleration, and this interpretation may be useful for estimating the potential of a ground motion to elicit a highly nonlinear response.