In the present paper, a new approach to combine load conditions expressed by power spectral density functions and to synthesize them into an equivalent one was presented. This method is based on the concept of fatigue damage spectrum and on the system dynamics. It was developed to design or verify operative durability tests (i.e., tracks), able to test payloads transported by vehicles, which could be alternative to laboratory ones, defined by the norm. By analyzing the acceleration spectrum of the norm and the acceleration measurements (all expressed in terms of power spectral density functions) acquired on the designed tracks during an experimental activity conducted on a wheeled transport vehicle, it was possible to verify the goodness of the proposed approach.