Study On Damping Model For Seismic Response Analysis Of Suspension Bridges

Damping is one of the most important dynamic characteristics in a vibration system and have a great influence on the seismic response analyses of structures.Although various damping models have been proposed,only a few of them have been used for the dynamic anlayses of suspension bridges.The constant damping is usually used in the spectrum analyse and time-history analysis based on the mode-superposition method,while the rayleigh damping model is often for nonlinear seismic analysis.This thesis aims to propose an appropriate method for determining the reyleigh damping coffcient used for the seismic anlyses of suspension bridges.A non-proportional damping model is also proposed for suspension bridges.The following aspects are addressed:(1)The characteristics of damping in suspension bridges and some popular damping models are summarized and analyzed.Specially,the state-of-the-art of the Rayleigh damping model are brifly reviewed.(2)The finite element model of a three-tower suspension bridge is developed.The structural vibration modes and modal particaipation factor are calculated by modal analyse.Based on this long-span bridge and one group of accelerations of ground motions,the number of vibration modes in mode-superposition method is determined.The effect of geometric nonlinearity in time-integration method is also measured.The result shows that,for most of responses,geometric nonlinearity has little influence on the seismic response.However,for a few responses,geometric nonlinearity can have significance influence.(3)Based on the response spectrum SRSS method,a new parameter is introduced to measure the effects of vibration modes on the seismic response of suspension bridges.This parameter is calculated for some key response,and it is shown that,for most of respnses,only a few modes are of significant importance.The seismic responses of the suspension bridge to a three-dimensional uniform excitation show that the difference between the anlysis with those significant modes and that with all the modes are very small.(4)By equating the seismic responses of the structue with modal damping ratio and Rayleigh damping,a new approach to deciding Rayleigh damping coffcient for a single response is developed and used for the responses of the critical part of the structures.As an example,this paper treated the constant damping as prospective modal damping ratio.The three-dimensional uniform excitation with Rayleigh damping and constant damping are compared.The result shows that this method had a good applicability.(5)A kind of non-classcial-damping in that damping matrix is directly assembled by Rayleigh damping matrices for steel and concret components.Analysis of three-tower suspension bridge shows that the non-proportional damping significantly affects the seismic response of suspension bridges.Based on this research,it is known that the equivalent damping ratio can also be based on the Modal Strain Energy(MSE).Selecting a proper value of damping ratio of concret can lead to better results.