Dimensionless fragility curves for rocking response to near-fault excitations

This paper assesses the seismic fragility of single degree of freedom rocking structures within a probabilistic framework. The focus is on slender rigid structures that exhibit negative stiffness during rocking. The analysis considers ground motions with near-fault characteristics, either solely coherent pulses or synthetic ground motions that include, in addition, a stochastic high-frequency component. The study offers normalized fragility curves that estimate the overturning tendency, as well as the peak response rotation of a rocking structure. It shows that the use of bivariate intensity measures (IMs) can lead to superior fragility curves compared with conventional univariate IMs. Regardless, the study advocates the use of dimensionless-orientationless IMs that offer an approximately universal' description of rocking behavior/fragility, a normalized description almost indifferent to the amplitude and the predominant frequency of the excitation or the size and the slenderness of the rocking structure. Importantly, the analysis unveils hidden order in rocking response. There exists a critical peak ground acceleration, below and above which, peak rocking response scales differently. In particular, when the structure does not overturn, the peak rotation follows approximately a biplanar pattern with respect to the intensity and the predominant frequency of the excitation. Finally, the analysis verifies that rocking overturning depends primarily on the velocity characteristics of the ground motion. Copyright (c) 2015 John Wiley & Sons, Ltd.