Identification of inertia properties from the results of output-only modal analysis

Accurate knowledge of the rigid body properties of a structure including the mass, the location of mass center and the moments of inertia is important in machine design, vibration analysis, optimization and modeling of mechanical systems. However, estimation of these properties through theoretical methods is difficult when the structure has a complicated shape. In practice, the inertia properties can be estimated using the conventional modal testing methods by extracting the rigid body modes when the structure is tested in free-free boundary condition. However, all the rigid body modes are not always detectable, due to this fact that the structure is not excited at all degrees of freedom. In order to obtain all of the rigid body modes, many activities have been conducted for selecting the type and location of excitation without much success. In operational modal analysis (OMA), the structure can be excited at any arbitrary point and in different directions. In this paper, a new approach is introduced for estimation of the inertia properties from OMA. The data from OMA are adequate to extract all the rigid body modes of structure. A modal method is used for estimating the inertia properties from the rigid body modes extracted from OMA. The suggested approach is applied to a numerical model of a two-dimensional steel beam as well as a numerical model of a 3D frame and the accuracy of results is evaluated. It is shown that OMA can provide enough data to extract the inertia properties. A real beam is also tested in order to evaluate the performance of the method in practice, needless of a complicated procedure as for conventional methods.