Seismic Fragility And Traffic Capacity Analysis Of Continuous Beam Bridges With Seismic Isolation Or Common Bearings

In the past decades China has frequently faced earthquakes.As an essential part of the traffic lifeline,bridges are likely to cause traffic disruption due to damage or collapse,which directly obstructs the post-earthquake rescue work.Therefore,the seismic fragility research of bridges has attracted considerable attention.With regard to commonly used concrete continuous beam bridges,a rotation-ductility ratio has been proposed as the damage index of piers.Its feasibility was compared with some existing damage indices.Meanwhile,taking into account the effects of piles-soil interaction,the numerical models of a continuous beam bridge under different site conditions of III and II were established using seismic isolation bearings and common bearings respectively.Then quasi-static tests were carried out on the scale models of a real-world bridge pier.Meanwhile,dozens of seismic waves were chosen which fitted the site conditions and had different intensities.Then the seismic waves were input into the bridge numerical model for the purpose of nonlinear time-history analysis,by which means the seismic fragility curves of the continuous bridge were obtained.After that,a new formula for calculating the average damage levels was developed to incorporate with fragility curves for estimating the traffic capacity of the girder bridge after earthquake.The post-earthquake traffic capacity per hour was estimated under different ground motion intensities.Finally,a site isolation index was proposed based on the traffic capacity curve in order to analyze the influence of site conditions on the seismic traffic capacity of the bridge.The main contents and research fruits are described as follows.Firstly,the establishment of bridge seismic fragility curves,as well as damage indices of piers and bearings,has been comprehensively introduced.The index of rotation-ductility ratio was proposed based on the moment-rotation curves of piers.Four damage states were defined with respect to the index.Meanwhile,for the purpose of estimating the traffic capacity curves of a bridge after earthquake,a new formula for calculating the average damage level of the bridge was developed and then incorporated with fragility curves,exceedance probability and damage expectations of different damage levels.Secondly,the proposed index of rotation-ductility ratio was compared with two existing damage indices of displacement-ductility ratio and pier-top drift rate.The numerical models of a continuous beam bridge using seismic isolation bearings and common bearings were individually constructed taking into account two different boundary conditions of fixed pier bottoms and pile-soil interaction.Fifty real-world seismic waves having different intensities were adopted as excitations to the bridge models.Nonlinear time-history analysis was performed to obtain the seismic responses of the piers and the bearings,whose seismic fragility curves were then established on these responses.The average damage level formula was used to estimate the post-earthquake traffic capacity of the bridge.In this way,the rationality of rotation-ductility ratio,as well as the practicability of the traffic capacity formula,was verified.Thirdly,quasi-static tests were performed on three 1:8 scale models of a real-world continuous beam bridge pier.The different damage states of the rotation-ductility ratio index were quantified according to the hysteretic behaviors and the energy dissipation capacities of the piers.The test results were compared with those given by Open Sees.It was found that the seismic fragility curves based on the rotation-ductility ratios were closer to the reality.Lastly,a real-world continuous beam bridges on the site condition of II were chosen to construct its numerical models using seismic isolation bearings or common bearings.The seismic fragility curves of the bridge were established based on the previous pier tests and the rotation-ductility ratio index,respectively.The effectiveness of the isolation bearings was discussed by comparing the experimental results with the numerical ones.In addition,a site isolation index was proposed to compare the effects of different site conditions of Ⅱand Ⅲ on the seismic traffic capacity of the continuous beam bridges after earthquake.