Investigations of Mode Shapes of Closely Spaced Modes from a Lattice Tower Identified Using Stochastic Subspace Identification

In this work, the complex mode shapes of closely spaced modes of a slender lattice tower structure identified with stochastic subspace identification (SSI) are investigated. The SSI delivers its results in the form of complex eigenvalues and eigenvectors. For a proportionally damped structure, the mode shape components of a single mode shape lie almost on a line in the complex plane, referred to as the mean phase. For closely spaced modes it is known, that the mode shape components are not necessarily on a line in the complex plane. The mean phase deviation as an identification quality criterion for the identification of mode shapes has less significance in connection with close modes, so that a different examination of these mode shapes is necessary. This study is performed using experimental data of a lattice tower under environmental variability. For the mode shapes of the closely spaced modes of the structure, two dominant phases are observed in the complex plane corresponding to the two measurement directions. The influence of the environmental conditions and different structural states of the structure on the corresponding mode shapes is investigated. A new approach in dealing with complex mode shapes of closely spaced modes was developed and allows a better interpretation of the identification results and can be useful in assessing the state of the structure.