In the paper presented are results of experimental and theoretical investigations of dynamic properties of test rig support structure incorporating a three-support rotor, bearings and foundation. Experimental modal analysis of the object was performed by means of the impact test with use of SCADAS III frontend and CADA-X software. Theoretical modal analysis of the object was performed by means of numerical analysis with use of ABAQUS system. In the paper presented have been preliminary activities for the test, its course, the results of conducted investigations. The comparison of results obtained by two methods and expected further scope of research have been presented.