Vibration of engineering structures results in time variable displacements functions. For realistic structures modes of vibration are determined by exciting frequency and the natural frequencies of the structure. Hence, the components of stress tensor are time function too. In the presented paper the general idea of method of cycle-counting based on the FFT analysis for engineering structures is discussed and analytically verified for the case vibrating beam excited by series pulse-type force with no damping. and V názvu článku je jazyková chyba/překlep - analitical - správně = analytical