This paper describes an application of modal analysis to predicting the effect of tuned mass dampers which are attached to a bridge. To measure the frequency response functions of a bridge using step relaxation technique, we propose a new practical method which does not require any complex signal processing. Once the frequency response functions of the bridge are measured by step relaxation, the behavior of the bridge to which tuned mass dampers are attached is predicted by impedance method. Several examples of test bridges with various tuned mass dampers are presented in this paper which demonstrate the accuracy and suitability of these methods.