Abstract: Experimentally evaluated frequencies and modes of natural vibration reflect the actual state of an observed structure in the time of its experimental investigation. They can be used for creation of a modal model and using that model a dynamic response can be computed for certain dynamic loading. The paper deals with verification of convenience of modal model of a structure for calculation of its fatigue life. There are used results of experimental dynamic study of a RC beam. The beam was loaded and damaged by cyclic dynamic fatigue loading. During the dynamic loading the strain of steel reinforcement was measured using gauges placed on bars in the middle of the beam span and stresses of reinforcement were obtained from this measurement. There was measured a response of the beam loaded by random force. That response was measured using accelerometers in a chosen net of points on the beam but also using gauges placed on bars in the middle of the beam span. Frequencies and modes of natural vibration were evaluated from this response and from them a modal model was created. Stresses in reinforcement induced by used cyclic dynamic loading were calculated using this modal model with nonproportional damping and these stresses were compared with measured stresses.