Effect of Damping Modeling and Characteristics on Seismic Vulnerability Assessment of Multi-Frame Bridges

Rayleigh damping is a common method used to model energy dissipation in bridges, but improper damping modeling can develop spurious damping forces and lead to inaccurate results for dynamic analysis and seismic vulnerability assessment. The Rayleigh damping matrix contains mass-proportional and stiffness-proportional parts. To reduce or eliminate possible spurious forces, previous studies suggested using stiffness-proportional damping only and/or using the updated tangent stiffness matrix of the structure instead of the initial stiffness for the Rayleigh damping. The objective of this article is to study the effect of different damping modeling techniques and characteristics on the seismic response of multi-frame box-girder bridges and the resulting fragility curves for vulnerability assessment. Nonlinear time history analysis and incremental dynamic analysis are performed using 3D OpenSees bridge models to develop system and component fragility curves for different damping modeling scenarios. Different sources of uncertainties as related to earthquakes, structural geometries, and material properties are considered. The changes in fragility curves and median fragilities due to the different damping modeling scenarios are discussed. Confidence bounds for the developed fragility curves are presented as well to better understand the effect of damping on the developed fragility curves.