Frequency Domain versus Time Domain Modal Identifications for Ambient Excited Structures

Ambient vibration test is one of the most applicable and reliable methods among vibration tests due to recently fast evolvement of measurement, sensor technologies and data processing, and economical benefit as well. Modal identification methods of the ambient vibration structures are output-only identification techniques which have commonly been branched into either the frequency domain or the time domain. Each the frequency domainbased or the time domain-based modal identification methods has its own advantage and limitation. The frequency domain-based modal identification methods or non-parametric identification ones have been widely used for the output-only system identification due to their advantages in estimating natural frequencies high frequency resolution and mode shapes, but they have uncertainty and troublesome in the damping estimation. Inversely, the time domain-based modal identification methods or parametric ones benefit in the damping estimation, but difficulty in parameters. This paper will present theoretical backgrounds of the most modern and recent modal identification methods in the frequency domain and in the time domain such as Frequency Domain Decomposition (FDD) and Stochastic Subspace Identification (SSI). Full-scale ambient measurements of 5-storey steel building have been carried out for the modal parameter estimation. Applicable evaluation of the frequency domain-based and the time domain-based modal identification methods will be also discussed.