Time-Dependent Seismic Fragility Analysis Of High-speed Railway Continuous Beam Bridge Subjected To Near-fault Ground Motion

Seismic fault zones are widely distributed in China,so it is inevitable for high-speed railway to cross the fault or be located near the fault zone.In this paper,a high-speed railway continuous girder bridge is taken as the research object.Combined with the characteristics of near-fault ground motions at the bridge site,the influence of near-fault ground motion velocity pulse effect on the seismic response of high-speed railway bridge is studied through nonlinear dynamic time-history analysis.The seismic vulnerability analysis method is used to compare the seismic vulnerability of bridge components and systems under near-fault and far-field earthquakes.Considering the durability damage of high-speed railway bridges during long-term service,the time-varying vulnerability surfaces of bridge components and systems under near-fault earthquakes are established.The main research contents and conclusions of this paper are as follows:(1)According to the basic characteristics of near-fault ground motions,the near-fault and far-field seismic waves suitable for the bridge site and site conditions were selected to analyze the seismic response characteristics of high-speed railway bridges under the action of near-fault and far-field earthquakes.The results show that,compared with the far-field earthquakes,the energy of the near-fault pulsed ground motions is more concentrated,and the bearing displacement and the internal force and displacement of the bridge pier are significantly increased.(2)The seismic vulnerability of bridges under near-fault and far-field earthquakes is analyzed based on the incremental dynamic method by selecting suitable near-fault and far-field seismic waves and conducting amplitude modulation processing.Under the action of near-fault earthquakes,the exceedance probability of bridge members under the four damage states is significantly higher than that that of far-field earthquakes,and the maximum deviation is up to 21.6%.(3)Based on the incremental dynamic analysis method,the first-order and second-order boundary estimation methods are used to analyze the seismic vulnerability of the bridge system under near-fault and far-field earthquakes.The seismic vulnerability of the bridge system is higher than that of the components,and the maximum width of the upper and lower boundaries of the vulnerability curve of the bridge system calculated by the first-order boundary estimation method is 43.3%.The second order boundary estimation method takes into account the correlation of each component,and the maximum width of upper and lower bounds is 10.4%.Under the action of rare earthquakes with fortification intensity and near-fault earthquakes,the vulnerability of bridge system under four damage states calculated by second-order limit method is99.3%,92.7%,62.1% and 3.5%,which is increased by 3.4%,12.5%,23.0% and 2.4%respectively compared with far-field earthquakes.(4)The time-varying vulnerability of high-speed railway bridges under near-fault earthquake is studied by analyzing the deterioration law of material properties during the service period of bridges.The transcendence probability of bridge components and systems under different damage states increases with the prolongation of bridge service time.At the early stage of bridge service,the influence of chloride ion erosion on the structure is small,and the vulnerability curves of the bridge components and systems have little change from the ones when the bridge is completed.After reinforcement corrosion,the damage of bridge pier caused by chloride ion erosion is gradually aggravated.When the bridge reaches the designed service life,the system surpassing probability of the four damage states increases by 0.7%,4.8%,19.7% and 6.8%,respectively,compared with that of the completed bridge under the fortification intensity rare earthquake action.