Seismic Fragility Analysis Of Continuous Girder Bridges Under Scour Conditions

Earthquake and scour are two major natural disasters that threaten bridge safety.In recent years,global climate change on extreme weather?the new-built hydropower station?the increase in the illegal mining of sand and other reasons cause serious erosion exposure around pile foundations of bridges.Simultaneously,our country is an earthquake-prone country and most of the bridges are located in the earthquake zone.The river-crossing and sea-crossing bridges will experience multiple scour and seismic hazards.So,this paper studies the effect of scour on the seismic response of bridges.The main research work and conclusions are as follows:1.Four four-span continuous girder bridges were considered in the current study.The finite element models of the bridges have been carried out with OpenSEES software.Through seismic fragility analysis,the fragility of bridges could be obtained,in which different structural parameters of bridges,the uncertainty of ground motions and the change in scour depth were considered.2.Determine the earthquake demand parameter and develop the probabilistic seismic demand models using the probabilistic seismic demand analysis approach.Then the ultimate damage states were defined and the seismic fragility of bridges under different scour depths were analyzed to fit the scour-seismic fragility surfaces.The results showed that in the longitudinal and transverse directions,the failure probability of the bridge almost increased nonlinearly with ground motion intensity and the scour depth.However,the nonlinearity depended on the severity of damage state.Furthermore,the transverse vulnerability of bridges was greater than the longitudinal vulnerability of bridges under different damage states.3.This paper studied the effect of span and the height of pier on the conditional failure probability.The results showed that in longitudinal direction,the failure probability of the bridge increased with the height of pier.For the bridge with unequal height piers,its failure probability was between the high pier bridge and the low pier bridge.However,in the transverse direction,the change in scour depth would influence the effect of the height of pier on failure probability.No matter longitudinal direction or transverse direction,if there was no scour or scour depth was small,the failure probability decreased with the span.If the scour depth was large,the failure probability increases with the span.4.The risk probability of bridges almost increased with scour depth,but decreased under certain circumstances.It showed that the natural vibration period would effect the seismic response of structure,because the change in scour depth and the free field would influence the natural vibration period in seismic risk analysis.5.Calculate the joint failure probability of bridges based on the analysis of site seismic hazard and scour risk.Then determine the reliability index and the corresponding failure probability according to the reliability index of the ultimate limit state and the corresponding failure probability.Calculate the failure probability with different normalized scour depths.The normalized scour depth that satisfied the reliability index was the scour load modification factor,which was suggested to be used to consider the effect of scour on seismic design of bridges.