Equivalent damping of short-span and isolated concrete bridges.

摘要

Traditionally, the design spectrum-based dynamic analysis of highway bridges assumes 5% damping for all modes of vibration. While this assumption may be reasonable for ordinary concrete bridges, it may not be applicable to short-span bridges with substantial influences from the damping of the soil at abutments/supports or bridges with supplemental damping elements such as those from seismic isolators.;The overall objective of this dissertation is to study the fundamental issue of damping in bridge structural systems involving significantly different damping components and to develop a more rational method for estimating the damping for a more economic and accurate prediction of the seismic demand of such bridges. Within the framework of the current response spectrum method, on which the design of highway bridges primarily relies, four damping estimation methods including the complex modal analysis method, neglecting off-diagonal elements method, optimization method, and composite damping rule method, are explored to compute the equivalent modal damping ratio of short-span bridges and isolated bridges.;From the application to a real short-span bridge utilizing earthquake data recorded at the bridge site, the equivalent damping ratio of the bridge was determined to be as large as 25%, which is much higher than the conventional damping ratio used. Meanwhile, the simulation with the 5% damping ratio produced nearly two times the demand of the measured data, which implies that the 5% value used in practice may be too low for the design of short-span bridges considering the strong ground motions which should be sustained.;The four damping estimating methods are also applied to an isolated bridge. By approximating the non-linear isolation bearings with equivalent viscoelastic elements, an equivalent linear analysis is carried out. The estimation of the seismic demand based on the response spectrum method using the equivalent damping computed by the four methods is verified by comparing the response with that from the non-linear analysis. A simple approximation to predict the equivalent damping of isolated bridges based on the ductility ratio of the isolation bearing is suggested.;Finally, the relation between equivalent damping and ground motion characteristics are investigated to determine whether the equivalent damping can be estimated based on ground motion intensity.