Abstract
It is known that damping ratios of cable stayed bridges evaluated from forced excitation tests are generally very small. Because response amplitudes developed during forced excitation tests are very small due to limited excitation force, it is considered that the small damping ratios determined from forced excitation tests are attributed to the small oscillation amplitudes. However few clarifications have been conducted for damping ratios of cable stayed bridges during actual significant earthquakes. This paper analyzed damping ratios of a PC cable stayed bridge (Tokachi Bridge) based on measured response during the 2003 Tokachi-Oki earthquake (M8.0). It was found from the analysis that the damping ratios which provide the best correlation to the measured response were 10-6%. It was also found that the damping ratios have component and time dependency.
