Determining the effective system damping of highway bridges.
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2009-06-01
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Abstract:This project investigates four methods for modeling modal damping ratios of short-span and isolated
concrete bridges subjected to strong ground motion, which can be used for bridge seismic analysis
and design based on the response spectrum method. The seismic demand computation of highway
bridges relies mainly on the design spectrum method, which requires effective modal damping.
However, high damping components, such as embankments of short-span bridges under strong
ground motion and isolation bearings make bridges non-proportionally damped systems for which
modal damping cannot be calculated using the conventional modal analysis. In this project four
methods are investigated for estimating the effective system modal damping, including complex
modal analysis (CMA), neglecting off-diagonal elements in damping matrix method (NODE),
composite damping rule (CDR), and optimization in time domain and frequency domain (OPT) and
applied to a short-span bridge and an isolated bridge.
The results show that among the four damping estimating methods, the NODE method is the most
efficient and the conventional assumption of 5% modal damping ratio is too conservative for short-span bridges when energy dissipation is significant at the bridge boundaries. From the analysis of
isolated bridge case, the effective system damping is very close to the damping ratio of isolation
bearing.
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