Estimation of damping ratios of steel structures by Operational Modal Analysis method

• The modal damping ratios of steel structures were identified by OMA.
• The investigations were performed on a 3 storey steel building model.
• The natural frequencies and modal damping ratios were presented.
• The modal damping ratios were changed considerably.

In this study, it is aimed to compare the modal damping ratios attained by considering the measurement duration, frequency range, sampling rate, and the method used in modal parameter identification as variable parameters. The investigations were performed on a 3 storey steel building model. The measurements were taken from only top floor level of the building model by using twelve uni-axial seismic accelerometers. An impact hammer with rubber tip was used to vibrate the model by generating random impacts. The measurements were repeated by taking into account four different time intervals (5–10–30–60 min) using 0–12.5 Hz as the base frequency range. The collected signals were analyzed within 0–6.25 Hz frequency range considering the sampling rates as 512, 1024 and 2048. Both the natural frequencies and the modal damping ratios were obtained from each measurement by using Enhanced Frequency Domain Decomposition (EFDD) and Stochastic Subspace Identification (SSI) techniques. The natural frequencies and their corresponding modal damping ratios were presented and compared with each other for all cases. It was observed from the study that the change in the natural frequencies was very small while the modal damping ratios were changed considerably depending on the selected analysis options.