Estimation of Damping for a Double-Layer Grid Using Input-Output and Output-Only Modal Identification Techniques

In large civil engineering structures, the output-only modal identification is the most applicable technique for estimating the modal parameters such as damping. However, due to no measurement and control of excitation force, the identified parameters obtained by output-only technique have more uncertainty than those derived from the input-output technique. Given the different nature and uncertainties of the two modal identification techniques, in the present study, the damping related to the first 12 modes of a double-layer grid developed from the ball joint system were identified via the two techniques and compared with each other. For this purpose, a double-layer grid was constructed by pipes and balls with free-free boundary conditions provided for both input-output and output-only experiments. Exciting the grid, its acceleration response was measured at appropriate degrees of freedom. Then, by using these data and performing modal analysis, involving four different methods of input-output and five different methods of output-only, the natural frequencies and damping ratios of the desired modes were extracted. The results indicated that despite the good agreement between the modal damping of the grid, as identified by different methods of input-output together and by different methods of output-only together, the results of input-output and output-only methods were different with each other. The damping values through the input-output modal identification methods were on average 65% higher than the corresponding values of the output-only modal identification methods.