Stochastic Seismic Response Of A Three-tower Long-span Suspension Bridge

In recent years,the three-tower suspension bridges have been widely built in China,because of its greater capacity of spanning and economicality than those of traditional two-tower suspension bridges.Due to the lack of effective longitudinal restraints on the middle tower,the three-tower suspension bridges have different dynamic characteristics than those of the two-tower suspension bridges.The time-history analysis method has been a main computational method for the seismic response of long-span suspension bridges up to now.Obviously,the seismic excitations are typical stochastic process.Therefore,the timehistory analysis method,as the deterministic one,is lack of statistical significance.This thesis uses the random vibration method to analyze the stochastic seismic response of the three-tower suspension bridge under three one-dimensional uniform excitations,three-dimensional uniform excitations,and three-dimensions non-uniform excitations.This analysis is based on the accuracy and appropriateness of models and parameters of the auto-power spectral density and spatial variability.The following aspects are addressed:(1)The dynamic characteristics of the three-tower suspension bridge are studied by the finite element method.As with most of long-span cable-supported bridges,major vibration modes of the three-tower suspension bridges have the charteristic of low frequency and long period.(2)The influences of the number of modes selected to obtain the seismic response of the three-tower suspension bridge are discussed in this paper.(3)The stochastic seismic responses of the three-tower suspension bridge are analyzed under three single dimension uniform excitations and three-dimensions uniform excitations.The directions of excitations have the different sensitivity on the seismic responses of the different responses of the components in the bridge.And the coherence of the excitations and directions has the obvious influence on the response.(4)The reasonable auto-power spectral density model and physically significant parameters are choosed in the analysis.The suitable models for spatial variability,which includes the partial coherence effect,the traveling wave effect and the local-site effect,are also selected.The influences of these effects which have different influence levels on the responses of the three-tower suspension bridge are discussed.Generally,the traveling wave effect has the immediate and large impact on the stochastic seismic responses of the bridge.But the local-site effect and the patial coherence effect has the smaller influence than that of the traveling wave effect.