In order to investigate the influence of nonstationary frequency content variations, nonlinear response analysis for origin-oriented type hysteretic model is carried out and results are presented by applying two different S-I and S-II ensembles with varied frequency contents. The theoretical background of strong motion simulation is explained briefly depending on frequency parameters of stiff soil data. Based on models and applying Monte-Carlo's simulation mean values and standard deviations of displacement ductility demands for both ensembles are calculated. Required strength levels are investigated in probabilistic sense adopting light and heavy (but controlled) damage criteria. It is concluded that the nonlinear behaviour of the structure is greatly influenced by the nonstationary frequency content deviations as being shown that larger strength requirements for the S-I type strong ground motion ensemble than the S-II, although both ensembles have almost the same acceleration response spectra.