Operational modal analysis based linear system identification of systems with elasto-perfectly-plastic nonlinear behaviour

Operational Modal Analysis (OMA) is widely used to identify the modal properties of systems under ambient vibrations or normal operating conditions. The fundamental assumption of existing OMA methods is that the system is linear and time-invariant. Civil engineering structures, however, often violate this assumption, especially, when their dynamic behaviour is influenced by various sources of nonlinearity such as; the hysteretic behaviour of structural elements, friction between sliding surfaces and softening of the soil-foundation system. Along these lines, this paper assesses the linear frequency and damping estimation performance of two correlation driven OMA methods applied to a nonlinear system with bilinear (i.e., elasto-perfectly-plastic) behaviour. This assessment of the estimation performance compares the OMA based estimates to theoretical values of equivalent linear frequency and viscous damping and discusses the discrepancies. It is found that the OMA methods provide reliable results for the investigated simulation system with nonlinear damping and stiffness, though, with a bias in the related frequency and damping estimates arising in the correlation function estimation. The bias is, however, relatively small.