Numerical Investigation on the Measurement Uncertainty in Operational Modal Analysis of a Civil Structure

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.