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Seismic Fragility Analysis of Bridge Pile Foundation Considering Scour Effect

LIANG Fa-yun;LIU Bing;LI Jing-ru;Department of Geotechnical Engineering,Tongji University;Department of Mathematics,Tongji University;  
The erosion of soil by scour around a pile weakens the lateral support in bridge foundations and this phenomenon has an effect on the seismic vulnerability of bridge pile foundations.Therefore,it is important to study the seismic fragility of bridge piles in the presence of scour.We used SAP2000 software to establish a three-dimensional finite element bridge model in the presence of scour.Damage occurs more readily at bridge piers and pile foundations and these components may become plastic during earthquakes.As such,we simulated these components using an elasto-plastic connection unit.We simulated the mechanical behavior of the soil using nonlinear p-ysprings and modeled the loss of soil by removing the p-ysprings above the scour surface.We selected 80 acceleration time histories of seismic waves that conformed to site II,and then translated the acceleration time history of each seismic wave into the displacement time history.We then applied the displacement time history to the p-y springs and carried out a nonlinear time history analysis on the bridge model.We obtained the seismic response of the bridge and recordedthe maximum response of the bridge pile under each seismic wave.Using aprobabilistic seismic demand analysis approach,we developed an analytical seismic vulnerability model of the bridge pile at different scour depths.We used curvature ductility as a damage index,and according to Hwang's suggestion,defined different damage states of the pile foundation.We obtained the boundaries of damage states by applying moment-curvature analysis to the pile cross section,thus obtaining quantitative data of the different damage states of the pile foundation.On this basis,we assumed that lognormal distribution functions were suitable for developing fragility functions,and we used regression analysis to estimate the parameters in the seismic fragility functions.We established the seismic fragility curves of the pile under different scour depths,and then discussed the influence of scour depth on the probability of the pile damage.The results show that with increasing scour depth,the probability of seismic damage to bridge pile foundations increases.Therefore,special attention should be paid to the adverse influence of scour when designing earthquake-resistant bridge pile foundations.
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