Ground motion intensity measures for collapse capacity prediction: Choice of optimal spectral period and effect of spectral shape

Previous research has shown that for extreme ground motions in California, the spectral shape is much different than that of a uniform hazard spectrum or a code design spectrum; and that differences in spectral shape significantly impact results of nonlinear dynamic analyses. This paper builds on this previous research and looks more closely at the question of selecting and scaling ground motions for nonlinear dynamic collapse analyses. Based on dynamic collapse simulation of single-degree-of-freedom models subjected to 70 ground motions, this work confirms the significant effects that spectral shape has on collapse capacity estimates. However, for collapse analyses, we find that the use of a more appropriate ground motion intensity measure can reduce how sensitive the collapse capacity estimates are to the spectral shape of the ground motions. A common ground motion intensity measure is the spectral acceleration at the building first mode period. However, when first-mode dominated ductile structures behave nonlinearly and are near collapse, the “effective period ” of the structure may be much larger than the fundamental period. For these types of structures, this paper shows that the spectral acceleration at a period larger than the fundamental period is a more appropriate intensity measure for use in collapse simulation. This paper proposes an equation that predicts this optimal period as a function of the building properties: fundamental period, ductility, and the negative stiffness after reaching the ductility capacity. For ductile structures, this optimal extended period is approximately twice the fundamental period of the building