Abstract: Structural damping ratios are crucial for the wind resistance design and vibration control of bridges.The logarithmic decrement method,a classic approach for damping ratio identification,is widely used;however,its accuracy can be significantly affected by noise and the choice of decay periods.To enhance the applicability of the logarithmic decrement method in engineering practice,this paper first establishes a theoretical model for uncertainty quantification of the method,deriving theoretical formulas for the optimal number of decay periods and a constant optimal amplitude ratio.Next,the effectiveness and robustness of the proposed method are verified through a single-degree-of-freedom numerical example.Finally,the method is combined with the analytical mode decomposition method and applied to identify the damping ratio of a long-span steel-concrete composite girder cable-stayed bridge.The results indicate that the optimal number of decay periods corresponds to the point where the maximum amplitude decays to 0.3299 times its original value,minimizing the uncertainty in damping ratio identification.Due to amplitude dependency,the identified damping ratios for the long-span steel-concrete composite girder cable-stayed bridge are relatively dispersed but generally around 1%,aligning closely with the recommended values in wind-resistant design specification for highway bridges.The findings of this study provide valuable guidance for the engineering application of the logarithmic decrement method.