Hilbert-Huang transform for damping ratio identification of structures with closely spaced modes of vibration

The Hilbert-Huang transform (HHT) method is a newly developed data analyzing method, which consists of mainly the empirical mode decomposition (EMD) and the Hilbert transform (HT). This paper explores the possibility of using the HHT method to identify damping ratios of structures with closely spaced modes of vibration. A 2DOF system with well separated modes of vibration is first analyzed to demonstrate the basic procedure for identifying modal damping ratios of the system using the HHT method. Then, a 36-story shear building with a 4-story light appendage on its top, subjected to an ambient ground motion, is analyzed using both the HHT method and the FFT method. The first four modes of vibration of the structure are closely spaced. The computed results show that the HHT is able to identify the four modal damping ratios but the FFT method gives poor identification results. The first four modal damping ratios identified by the HHT method are close to the theoretical values. The HHT method performed in the time-domain seems to be a very promising tool for system identification of large civil engineering structures.