Structural system identification of plane frames based on frequency domain decomposition-natural excitation technique (FDD-NExT)

Nowadays, system identification of structures has become an important subject in the civil engineering community. The reason is that with a good system identification method, modal parameters such as natural frequencies, mode shapes, and damping ratios can be obtained accurately, leading to an excellent structural health monitoring system. The two systems identification techniques are categorized as frequency-based and time-based domain identifications. In addition, the identifications based on the output-only measurements are gaining more attention from the researchers recently. One of the advantages of this type of identification is that the modal parameters can be obtained without knowing the excitation forces. In other words, it is not necessary to know the input load of the structures. This paper considers output-only identification based on Frequency Domain Decomposition (FDD) and Natural Excitation Technique (NExT) as FDD-NExt. FDD is one of the methods that relies on the output-only measurements such as displacement, velocity, or acceleration, in order to obtain the natural frequencies and mode shapes of the structures. On the other hand, NExT has good ability for determining the damping ratios of structures. This paper aims to apply the FDD-NExT to obtain the modal parameters and the damping ratio of plane frame structures. Two plane frame structures are considered in this paper to show the ability of the proposed method to predict modal parameters and the damping ratio of the structures. The first structure is a one-story frame with 6 degrees of freedom, while the second structure is a four-story frame structure with 48 degrees of freedom. For comparison, Modal Assurance Criterion (MAC) has been used to determine the difference between the modal parameters obtained by FDD and finite element analysis. The results show a good correlation between those two methods.