The primary aim of the paper is to present a simple method for determining the changes in stiffness of a composite beam. The experimental model was made from wood and plaster boards. It was simply supported beam with a length of 4 m. Accelerations were measured at 24 points along the beam. In either case data for intact and damaged models were acquired. The identification of the damage was done using a code developed by the authors. The vertical mode-shapes were analyzed, so changes in the stiffness could be detected.