Rigid body stiffness matrix for identification of inertia properties from output-only data

•A new method is proposed for scaling the rigid body mode shapes using stiffness matrix.•The method does not need adding extra masses to the structure and repeating the test.•All of the rigid body modes of the structure are extracted using the output-only data.

Identification of inertia properties (mass, location of the center of mass and inertia tensor) is essential for designing of engineering structures. Using modal testing is a possibility for estimation of the inertia properties in which they can be identified using the orthogonality property of mass-normalized rigid body mode shapes. However, identification of rigid body mode shapes using modal testing is not always possible, because it is not possible to excite the structure at all degrees of freedom. In this paper, output-only modal analysis in which the structure can be excited in different directions is used to identify the rigid body modes of the structure. It is shown that all of the rigid body modes of the structure can be extracted using the data extracted from output-only modal analysis. As the obtained rigid body mode shapes from output-only modal analysis are not scaled, a new method is proposed for scaling them using rigid body stiffness matrix. The inertia properties of the structure are obtained from the scaled mode shapes. The accuracy of the proposed method is studied using a numerical case study of a steel structure as well as an experimental case study of a steel frame.