The input energy to a structure during an earthquake is an important measure of seismic
demand. The elastic input energy in a multi-degree-of-freedom system can be computed from a Fourier
Amplitude Spectrum (FAS) and the real part of the relative velocity transfer function of different modes.
One of the essential characteristics of the seismological method is that it distills what is known about the
various factors affecting ground motions into different functional forms and, for this reason, the modal
analysis-based method in the frequency domain is very efficient in the computation and evaluation of
earthquake input energy. The earthquake input energy reliability is dependent on ground motion variables.
In this paper, to compare the effect of strong ground motion variables, the contribution of these sources
of variability to the input energy's uncertainty is examined by using a stochastic analysis. The analytical
results show that earthquake source factors and soil condition variables are the main source of uncertainty
in the input energy spectra, while path variables, such as source-site distance, anelastic attenuation and
upper crust attenuation, have relatively little effect.
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