Seismic Vulnerability Analysis Of Continuous Rigid-frame Bridges Under Near-fault Ground Motions

Based on a large rigid frame bridge in the mountainous area of Yunnan Province,this thesis analyzes the influence of rectangular hollow,rectangular solid and circular solid pier sections on the seismic performance of the whole bridge.In this thesis,different site conditions,different fault spacing,different intensity of the earthquake are taken as an example.The research is carried out from the perspectives of natural vibration mode,response spectrum and time history.By comparing the seismic response and seismic vulnerability curve of the key nodes of the whole bridge,the seismic performance of three kinds of piers is given.The specific work of this thesis is as follows:1.Based on the general situation of the project,the finite element model of the long-span continuous rigid frame bridge is established,and the natural vibration modal analysis and acceleration response spectrum analysis of the model are carried out.Based on the acceleration response spectrum,the seismic responses of three bridge models are compared.The results show that the rectangular solid section can better control the maximum internal force of the whole bridge.The maximum axial force is reduced by 23.04%,the maximum bending moment is reduced by 9.44%,and the maximum shear force is reduced by 12.42%.The circular solid section can reduce the maximum displacement of the whole bridge by 10.86%in the longitudinal direction,12.51% in the transverse direction and 5.29% in the vertical direction.2.Taking two near fault ground motions and non near fault ground motions in Los Angeles Northridge earthquake as seismic excitation,the seismic time history analysis of three bridge finite element models is carried out.According to the results of seismic response time history analysis: from the overall response analysis of the bridge,there is no significant difference among the three kinds of piers under the action of non fault ground motion.However,when the structure is subjected to near fault ground motion,the rectangular hollow pier is the best one in this kind of near fault ground motion.Compared with the average value of the three piers,the axial force decreases by 57.81%,the bending moment decreases by28.92%,and the shear force decreases by 14.77%,according to the displacement evaluation,the rectangular solid pier performs better,with an average reduction of 8.73%;Compared with the other two kinds of piers,the rectangular hollow pier has the best performance,and its maximum value is reduced by 26% and 55% respectively;Based on the analysis of the relationship between the seismic response time history and the corresponding excitation time history,the difference between the displacement response and the moment response of the three piers under near fault ground motion is small,but the difference is significant.Under the action of velocity pulse,the bending moment of rectangular hollow pier is smaller,and the later bending moment amplitude decreases faster.3.Under the premise of considering the uncertainty of bridge structure and materials,using a variety of near fault and non near fault seismic actions as seismic samples,the maximum curvature of pier section is obtained based on finite element analysis,and the bridge damage state is quantified by dividing the curvature of pier,and then the bridge vulnerability curve is obtained.Through the analysis of vulnerability curve,it is found that the difference of seismic performance of bridges due to different pier forms is obvious,and this difference is particularly significant when bridges bear near fault ground motion.Under near fault ground motion,the seismic performance of bridges with rectangular hollow piers is the strongest,followed by rectangular solid piers,and the seismic performance of bridges with circular solid piers is the weakest.The probability of slight damage caused by small earthquake is 40.82%,44.71% and 60.33% respectively.The probability of moderate damage caused by medium earthquake is 33.53%,39.17% and 55.86% respectively.The probability of serious damage caused by large earthquake is 9.94%,15.69% and 28.64% respectively;The influence of pulse amplitude and angle on bridge damage is analyzed.It is found that the influence of pulse amplitude is significant.Even a small pulse(20cm / s)can make the probability of slight damage of bridge reach more than 50%,while a large pulse(100cm / s)can make the probability of serious damage reach more than 41.42%;In terms of angle,the closer the pulse is to the transverse direction,the greater the damage to the bridge.In view of the above comparison,it can be found that there is little difference in the seismic capacity of the three piers under the action of non fault ground motion;Under the action of near fault ground motion,the seismic capacity of rectangular hollow pier is obviously higher,and it has good resistance to velocity pulse.