Seismic Fragility Analysis Of Bridges Considering Scour Effect Based On Endurance Time Method

Scour can cause the loss of the soil around the bridge foundation and reduce the lateral support of the soil to the bridge foundation.Thus,the stiffness and dynamic characteristics of bridge structures will be changed and the seismic fragility will be affected as well.The highway and railway systems are under construction in the western mountainous region in China at the present and a lot of bridges will be built.However,the hydraulic gradient of rivers in this region is very large so that the scour effect is very common.In the meanwhile,the bridge site region also has great potential of earthquake hazards.Therefore,it is necessary to investigate the influence of scour on the seismic fragility of bridges.Moreover,with the development of “Ocean Strategy” of China,a large number of sea-crossing bridges has been built or under construction and the bridge site region often presents the characteristics of the deep water and rapid flow.Hence,the scour effect cannot be ignored and it is also worthwhile to study the influence of scour on the seismic fragility of sea-crossing bridges.In this paper,the seismic fragility analysis framework based on endurance time method was illustrated.The common two-span continuous girder bridge and a sea-crossing cable-stayed bridge were used as the research objects.The seismic fragility analyses of bridges considering scour effect were conducted by endurance time method and the influence of scour depths on the seismic fragility for key components of example bridges was discussed.The main research contents are listed as follows:(1)The seismic fragility analysis framework based on endurance time method(ETM)was illustrated.Two groups of endurance time acceleration functions(ETAFs)with endurance time of 25 s and 40 s were generated respectively based on the acceleration response spectrum in the Chinese seismic design code;the Open Sees software was used to create the finite element models of bridges without scour effect at first.ETM and incremental dynamic analysis(IDA)method were employed to calculate the seismic response of bridges respectively and the accuracy of ETM was verified through the comparison between the results from two methods.The result shows that the average results of ETM agreed well with the average results of IDA but ETM reduced the number of nonlinear time history analyses and improved the calculation efficiency compared with IDA;(2)As for the two-span continuous girder bridge,three different bridge foundations including the shaft foundation,two-column bent foundation and pile group foundation were considered.The dynamic p-y method was used to model the soil-structure interaction and the scour depth was modeled by removing the soil springs.The transversal and longitudinal fragility curves of the bridge column and pile foundation under different scour depths were calculated based on ETM and the influence of scour effect and substructure design on the seismic fragility of bridges was discussed.The result shows that the most adverse seismic responses of example bridges do not always occur under the maximum scour depth.For the bridge supported by the pile group foundation,structural seismic vulnerable components of the bridge can be transferred from the column to the pile with the increase of scour depths;(3)The same methods used for the two-span continuous girder bridge were adopted to model the soil-structure interaction and scour depths for the sea-crossing cable-stayed bridge.The Morison equation and linear potential theory were used to calculate the hydraulic added mass of the pile and pile cap respectively.The transversal and longitudinal seismic fragility curves were obtained based on ETM and the influence of scour effect on the seismic fragility of the tower,pile foundation,assistant pier,side pier and bearing was discussed.The result shows that the seismic fragility of the tower decreases with the increasing scour depth;the scour effect may increase the seismic fragilities of the pile foundation,assistant pier,side pier and bearing but the most adverse seismic responses of these components do not occur under the maximum scour depth.The influence of scour effect on the seismic fragility of the seacrossing cable-stayed bridge should be considered in the seismic design of the bridge.