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 di erent modes. One of the essential characteristics of the seismological method is that it distills what is known about the various factors a ecting ground motions into di erent functional forms and, for this reason, the modal analysis-based method in the frequency domain is very ecient 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 e ect 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 e ect.
