Damping characteristics of a footbridge: Mysteries and truths

As a consequence of a paper presented by Michael Mistler at the VDI-Baudynamik-Tagung in Kassel, Germany, in April 2015, the authors checked the damping coefficients having been estimated for a footbridge in autumn 2014. Mistler stated that the critical damping ratio estimated from a halfpower bandwidth procedure to be dependent on frequency resolution for low frequency modes.

Based on the data presented here this statement can be confirmed. The dependency on frequency resolution was found to be due to the leakage phenomenon on the spectral density. This fact may have been known in the academic world but not in the world of engineers applying OMA in practice. In this paper it is presented how the leakage on the spectral density estimate is affecting the damping estimation through OMA based frequency domain identification.

Finally the paper compares the damping estimated in the time and frequency domain from ambient tests, with the damping estimated from the free decays. Unfortunately, bias error on damping values determined from analyses in the frequency domain is worst on low frequency modes usually being the most important ones when dealing with a resonance problem in practice.