Numerical investigation on the measurement uncertainty in operational modal analysis of a civil structure

Abstract

This paper deals with the study of the uncertainty associated to operational modal analysis parameter estimation. The main aim is to analyse how different key factors such as frequency resolution and average number can affect the accuracy of the estimated eigenfrequencies, non-dimensional damping ratios and mode shapes of a structure. This is considered a starting point towards any attempt to exploit structural health monitoring methods based on modal parameter changes. Knowing the parameter value and the associated spread is needed in order to asses those changes that are unusual and possibly caused by damages. Modal parameters of a civil structure are naturally evolving due to the environmental and operational conditions, this natural spread is added to the one given by the identification method leading to the final uncertainty value. Knowing the spread associated to the applied identification technique is therefore fundamental to identify and possibly compensate the changes given by the environment and thus achieve a complete knowledge of the actual structural condition. It has been decided to apply the uncertainty estimation to a real case in order to have the possibility to compare the numerical results to a set of experimental data. The modal model of one of the G. Meazza stadium grandstands in Milan has been developed thanks to the great amount of data collected by the authors in the past. The Monte Carlo method has been applied to data coming from numerical simulations of the structural response to random excitation. The number and location of the simulated measurement point are the same as the ones actually existing on the structure. Starting from this simulated measurement set-up different conditions have been considered, changing the frequency resolution used in the identification and varying the base time record length and so the number of averages to obtain an estimation of structural free response. The different estimated uncertainties associated to the analysed conditions have been produced and compared to the results obtained with real operational data coming from the structure. The obtained results are useful to identify the spread given by the proposed identification method, and thus quantify the environmental parameter effects on the structure.