Merging Strategies for Multi-Setup Operational Modal Analysis: Application to the Luiz I steel Arch Bridge

In Operational Modal Analysis (OMA) of large structures, it is often necessary to measure the Degrees Of Freedom (DOFs) of interest in different setups, which are processed separately, resulting in different modal parameter estimates for each of the setups. Subsequently, the DOFs that are common to the different setups are used to combine the different parts of the mode shapes, while the eigenfrequencies and damping ratios are averaged. This strategy is named the PoSER approach. If the number of setups is large, this approach is tiresome, especially if some modes of interest are not well excited and hence might be difficult to extract from the data. Therefore, there is an increasing interest towards processing all setups at once, which results in so-called ‘global’ modal parameter estimates. In this article, two strategies for achieving this goal, named the PoGER and PreGER approaches, are presented, both in the time and in the frequency domain. The PoSER as well as the global strategies are then used for the extraction of the modal parameters from data measured on the steel Luiz I arch bridge in Porto, Portugal, using both the SSI-cov/ref and the pLSCF system identification methods. From the comparison of the obtained results, it is concluded that the PoGER strategy is the most robust global approach.