Influence Of Bi-directional Seismic Action On Vulnerability Of RC Continuous Beam Bridge

This study mainly researches the influence of bi-directional horizontal ground motion and incident angle of ground motion on the fragility of reinforced concrete continuous box girder viaduct.The nonlinear dynamic time history analysis of bridge is carried out with bi-directional horizontal seismic action as input.The fragility surface of bridge is established based on bi-directional seismic intensity parameters.The difference of fragility of bridge structure under uni-directional and bi-directional input is compared,and the influence law of transverse seismic intensity on overall fragility of bridge under bi-directional input is analyzed.The nonlinear dynamic time history analysis of bridge under bi-directional ground motion is carried out with different incident angles.The fragility curves of pier and bearing are established based on the intensity parameters and incident angles of ground motion,and the influence rules of the incident angle of ground motion on the fragility of bridge are analyzed.The main research contents and conclusions are as follows:（1）Select the appropriate ground motion parameters.Through the regression analysis of the acceleration response spectrum value Sa（T₁）,peak acceleration PGA,peak velocity PGV and peak displacement PGD corresponding to the seismic response and natural vibration period of the bridge,the results show that the discreteness of the relationship between the seismic response and Sa（T₁）is relatively small,and the goodness of fit is the best,Therefore,Sa（T₁）as the input parameter of fragility analysis can increase the reliability of the analysis results.（2）The effects of uni-directional and bi-directional horizontal ground motions on seismic fragility of RC continuous girder bridges are compared.According to the results of seismic analysis,the probabilistic seismic demand model and fragility surface of pier and bearing based on bi-directional input acceleration response spectrum are established.The results show that the exceedance probability of pier in slight damage increases by 76%,and the state in medium damage state increases by70%,and the state in severe damage increases by 40%.The exceeding probability of complete failure state increases by 32%;The exceeding probability of basin type fixed bearing increases by 17%and the basin type sliding bearing increases by 35%.It can be seen that the failure probability of bridge pier and bearing under bi-directional seismic input is significantly higher than that under uni-directional seismic input.And with the increase of the transverse input seismic intensity,the exceeding probability of each failure state of the structure also has a significant increase trend.Therefore,in order to evaluate the damage level of bridge caused by earthquake more accurately and truly,it is necessary to consider the damage of bridge caused by bi-directional or three-dimensional earthquake.（3）Based on the bi-directional ground motion input,the nonlinear time history analysis of the bridge was carried out from the seismic incident angles of 0°,15°,30°,45°,60°,75°and 90°respectively,and the probabilistic seismic demand model and the fragility surface of the bridge are established based on the acceleration response spectrum and input angles.The results show that the exceedance probability of the pier increases by 2%in the case of slight and moderate damage,10%in the case of severe damage,and 12%in the case of complete damage;The exceeding probability of basin type fixed bearing increases by 2%and that of basin type sliding bearing increases by 7%.That is to say,when the acceleration response spectrum corresponding to the basic period of the bridge is taken as the fragility input parameter,the influence of the ground motion input angle on the fragility of the bearing of the regular symmetrical reinforced concrete continuous beam bridge is relatively small.The input angle of ground motion has little influence on the fragility of pier in the state of slight damage and medium damage,and relatively great influence on the fragility of severe damage and collapse,but it is also not significant.