In seismic design a variety of methods can be used in order to assess the response of the structure. This methods are discussed with respect to their impact of the accuracy of the results and the possibility to verify the findings. When using harmonic base excitation usually the natural frequency of the structure is used as driving frequency. This might lead to unsafe results in case when damping or nonlinearities such as plastic actions of the structure are present. This paper focuses on a study on the appropriate driving frequency.