Utilization of multi–axial shaking tables for the modal identification of structures

Abstract

A multi–axial shaking table is normally used for earthquake simulation and the dynamic qualification of structures. In most cases the same set–up can also be used for a modal identification of the tested structure. However, this requires the application of suitable modal identification methods. Here, the theory and the basic principles of modal identification by multi–axial base excitation are described. Starting with the relative kinematics and the fundamental dynamic relationships, the specifics of base excitation are explained. Equations for the structural responses and the dynamic forces at the interface are presented and discussed. A survey of identification approaches developed at DLR, Gottingen is given. A frequency– and a time–domain method are described in more detail and illustrated by their application to a laboratory structure. The frequency–domain method is best suited for a general and reliable modal identification, whereas the time–domain method is capable of analysing transient vibrations. Also, the frequency–domain identification of a larger structure is outlined.