Operational Modal Analysis in presence of dynamic interaction among adjacent buildings - an assessment

Operational Modal Analysis relies upon the fundamental assumption that input is unmeasured and consists of a Gaussian white noise characterized by a flat spectrum in the bandwidth of interest. If this assumption is not fulfilled, the modal identification process may fail due to spurious frequencies appearing in response spectra. It is often possible to identify such non-structural frequencies, but they can also sometimes bias estimates. This is basically the case of spurious harmonics close to structural modes. About civil structures, spurious frequencies can be due to rotating machines but also to interaction with adjacent constructions, as discussed herein, where natural frequencies of a given structure are mixed together with those ones due to interaction with similar close structures; as a result, response spectra are characterized by several close peaks in a narrow bandwidth. An integrated use of output-only techniques is proposed to discriminate modal from spurious frequencies and applied to a real test case.