Fragility Analysis And Assessment For Bridge Supported On Pile-group Foundations Under Flood Condition

As an important hub in the traffic lifeline system,bridges play a vital role in the daily life of society.For river-crossing bridges,floods are one of main cause of bridge collapse.The frequency of flood is much higher than that of earthquake,which often results in large-scale river scouring and causes dramatic changes in the riverbed.Although scour does not form a direct load on the bridge structure,but it poses a potential threat to the safety of the bridge,which will make the bridge in the face of floods,debris flow,earthquakes and other disasters easier to damage.Therefore,it is very necessary to study the safety of bridge structure in flood condition.The fragility analysis is a kind of method to evaluate the safety performance of the structure.It is widely used in the seismic fragility analysis.However,appling this method to analysis and evaluate the fragility of the bridge under the flood disaster has little been aroused concern.Therefore,this paper will try to use the fragility analysis method to evaluate the safety of the bridge under the flood disaster,and scarry on a preliminary exploration on the fragility analysis and safety evaluation of typical pile group foundation bridges under flood disaster.This paper first reviews the research status of scholars at home and abroad in flood disaster,erosion and fragility,and then describes the basic theory of fragility analysis and the selection of structural damage index.Further an improved hybrid fragility analysis method for evaluating bridge safety performance under flood condition has been proposed.Subsequent content will conduct Taiwan Shuangyuan Bridge as an engineering example,and use Opensees platform to establish its lower structure model to do following studies.In the third chapter,the paper introduces the element algorithm in Opensees,the nonlinear finite element analysis theory and the key parameters and the problems concerned in the calculation process of Shuangyuan Bridge model,including the treatment of pile-soil interaction,the simplification of pile group effect,the application of flood loads and so on.Then the fourth chapter uses the random pushover analysis method and the point estimation method of multivariable random function statistical moments to get the probability distribution of flood capacity with different flood level and different scour depth,considering the uncertainty of the flood level and the scour depth of the pile foundation.The fifth chapter considers the uncertainty of flood level,the scour depth and the flood load and the first order reliability method in the finite element reliability analysis method is used to obtain the probability distribution of the structural flood demand in 96 cases.Chapter 6 combines the results of the previous two chapters and use the hybrid fragility analysis method proposed in Chapter 2 to fit and plot the fragility curves of four flood levels and three flood flow rates.This assessment of the bridge safety performance and analysis results can be used as an important reference for decision making process.The main conclusions obtained in this paper are:1)The results of this paper coincide with the accident case of Taiwan Shuangyuan Bridge which was destroyed by flood in 2009,and it is also proved feasible to use the fragility analysis method to the safety evaluation of the structure in flood condition.It can give qualitative judgment and quantitative parameter analysis according to the specific research object.The result can be used as an important reference for decision-making process,making a certain engineering significance.2)The fragility curves of different flood level but with the same flood flow velocity have similar statistical parameters and show similar trend,and thus we can conclude that before the flood level reach the bottom of superstructure,the height of the flood level can be drawn little influence on the safety performance of the structure resisting flood.3)With the increase of the scour depth,the probability of each damage degree of the structure will increase rapidly.The transcendence probability of each damage level is more sensitive to the velocity of the water flow,that is,with the increase of the flow velocity,the scour depth corresponding to the damage level will significantly reduced and corresponding to the same scour depth,the transcendence probability of a certain damage degree will significantly increase,which also means that the structure is more vulnerable to be damaged or even destroyed.4)The safety performance of the structure is closely related to the flood velocity and scour depth.For bridge management,the reinforcement of the lower structure and taking the relevant measures to mitigate the pile scour are necessary and also important means of improve safety performance of bridges in flood condition.