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 outputonly technique have more uncertainty than those derived from the inputoutput 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 doublelayer grid developed from the ball joint system were identified via the two techniques and compared with each other. for this purpose, a doublelayer grid was constructed by pipes and balls with freefree boundary conditions provided for both inputoutput and outputonly 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 inputoutput and five different methods of outputonly, 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 inputoutput together and by different methods of outputonly together, the results of inputoutput and outputonly methods were different with each other. the damping values through the inputoutput modal identification methods were on average 65% higher than the corresponding values of the outputonly modal identification methods.