Fragility Analysis Of High-speed Railway Continuous Girder Bridges Subjected To Near-field Earthquakes

China is adjacent to the Pacific seismic belt in the East and the Eurasian seismic belt in the south.It has always been one of the countries with the strongest seismicity and the most serious earthquake disaster in the world.Nearly half of the land area is located in the area of high earthquake intensity,and many high-speed railway bridges are inevitably located in the near fault.The serious impact of near-field earthquake on civil engineering structures has attracted extensive attention.However,there is no theoretical system of structural seismic design and evaluation specifically for near-field earthquake in China,and the impact of velocity pulse and vertical ground motion is seldom considered in the existing research.In recent years,highspeed railways have developed rapidly in China,and the seismic safety of high-speed railway bridges under the near-field earthquakes has become a must-consider.Hence,Xinyi Bridge on the Lianxu high-speed railway line is used as the engineering background in this paper.Based on the Open Sees software,the seismic fragility of the bridge under near-field pulse-type earthquakes is performed.The research contents include four aspects: dynamic characteristics and seismic response analysis of high-speed railway continuous girder bridge,assessment of near-field pulse-type ground motion intensity parameters based on PCA method,analysis of the critical angle of seismic incidence based on fragility method,and analysis of the influence of vertical earthquakes on the fragility.The specific contents are as follows:1)Analysis of dynamic characteristics and seismic response of high-speed railway continuous girder bridge.First,a spatial finite element model of the bridge was established based on Open Sees.Then,the dynamic characteristics of the bridge were analyzed and verified.Finally,according to the site soil conditions,three far-field and three near-field pulse-type earthquakes are selected respectively,and the seismic responses under the two kinds of earthquakes,such as the longitudinal displacement of the girder at the mid-span,vertical displacement at the top of the pier,vertical shear at the bottom of the pier,longitudinal sliding bearing displacement and longitudinal fixed bearing displacement,are compared and analyzed,and the power spectral density of the longitudinal displacement response of the girder is analyzed.2)Assessment of near-field pulse-type earthquake intensity parameters based on PCA method.First,100 near-field pulse-type ground motions were selected,nonlinear dynamic timehistory analysis was performed.And the peak response values of bridge members were recorded.On the basis,the PCA(Principal Component Analysis)method was used to reduce the dimensionality of the seismic requirements of the bridge members,and the comprehensive seismic demand response of the bridge was calculated.Then,a probabilistic relationship between the ground motion intensity parameters and the comprehensive seismic requirements of the bridge was established through regression analysis,and 18 different ground motion intensity parameters were evaluated from three aspects of effectiveness,practicability and adequacy.3)Analysis of the critical angle of seismic incidence for high-speed railway continuous girder bridges based on fragility method.The traditional method and fragility analysis method were used to determine the critical angle of seismic incidence of the bridge,and the results obtained by the two methods were compared.Then,considering the difference in anti-seismic capacity of the bridge in all directions,the fragility curves in the critical angle of seismic incidence of the full bridge are compared with the fragility curves in the critical angle of seismic incidence of the longitudinal and transverse directions.4)Analysis of the influence of vertical earthquake on the fragility of high-speed railway continuous girder bridges.First,the vertical and horizontal peak acceleration ratios of 100 selected near-field pulse-type earthquakes were analyzed.Then,the fragility curves of the critical angle of seismic incidence corresponding to the longitudinal,transverse,and the critical cross-sectional directions of the fixed middle pier with different vertical and horizontal peak acceleration ratios are compared.