Seismic Vulnerability And Risk Analysis Of The Offshore Bridge

The bridge is an important part of the lifeline engineering,and what is more,the offshorebridge is playing an important role in the politics,economics and culture,once destroyed in the earthquake will cause great economic losses,so it is necessary to study seismic risk analysis of offshore bridges.In view of this,an offshore bridge exposed to the ocean tides environment in China was taken as an example,this paper has systematically conduced research on seismic risk,structural vulnerability and seismic risk probability by performance-based probabilistic analysis method,and has established the theoretical framework of seismic risk analysis considering chloride ion erosion of the offshore bridge.The concrete contents are as follows:(1)Establish the whole bridge finite element model.Based on the chloride ion erosion model,15 groups of parameter samples were formed by uniform sampling considering the uncertainty of the structure,then whole bridge finite element model of bridge was established taking into account the chloride ion erosion by using Open Sees platform.Moreover,15 appropriate ground motions selected from PEER database suited to sites condition were adjusted to assign reasonably to form the 300 groups of bridge-ground motion samples.(2)Probabilistic seismic demand analysis of bridge components.Considering three kinds of component(abutment,rubber bearing,pier)failure impact on bridge system vulnerability,this paper selected the maximum displacement of abutment,the maximum displacement of rubber bearing and the maximum displacement ductility ratio of pier as the seismic demand parameters,then the probabilistic seismic demand analysis of each component was studied to obtain probabilistic demand models for the seismic response of each component.(3)Seismic fragility analysis of bridge structure.By analyzing the seismic demand and capacity of each component,we firstly established the seismic vulnerability curves of them.Then,first-order reliability theory was implemented to give upper and lower bounds of the fragility curves of bridge system under different damage states.Afterwards,considering the joint effect of seismic demand of each component,the true fragility curve of bridge system under different damage states was obtained based on Monte Carlo sampling.The result indicated that failure probability of bridge system was much larger than that of any single bridge component.The actual structural seismic performance can be reflected more accurately by using the system fragility curve.(4)Seismic hazard analysis of bridge site.On the basis of the seismic ground motion parameter zonation map of China,the seismic hazard curves were conducted within 50 yearsby using I and PGA as a parameter of ground motion intensity.At the same time,the relation between the seismic hazard curves of different years was deduced.According to conversion relation between I and PGA,we got the seismic hazard curve of intensity transformation.The result shows that the gap was large in a certain range,in order to achieve the most reasonable prediction effect,we should select reasonable parameter and corresponding extreme value distribution model of different ground motion parameters.(5)Seismic risk analysis of bridge.The seismic risk for example bridge was analyzed based on seismic fragility curves and seismic hazard curves,the seismic risk probability under different damage states was obtained to assess the overall seismic performance of the example bridge.