Seismic Response Analysis Of A Seismically Isolated Curved Continuous Beam Bridge

The curved girder bridge is an important part of beam-bridge,which is widely used in urban viaducts,overpasses and high-grade highway projects because of good adaptability to space and linear constraints.Due to geometric irregularity,the mass center of the curved girder bridge is no longer coincinding with the stiffness center,and the in-plane torsion may occur under earthquake action,leading to complex loading.At present,a preliminary system for research on curved bridge aseismic has been developed,and the corresponding standards and codes have been drafted and improved.However,there are still some projects need to be further investigated,for example,temporary conditions for seismic analysis of curved beam-bridge simplified as a straight bridge,the influence of central angle of the main beam,support position and the bearings parameters on the seismic response of isolated curved beam-bridge,determination of the most unfavorable input angle of ground motion,and the seismic isolation analysis of curved beam-bridge under long-period ground motion,etc.On the basis of previous studies,the main foucus and research results of this dissertation are presented in the following:(1)Simplifing the isolated curved bridge to a three-degree-of-freedom system,a simplified formula for the first three modes is developed based on Lagrange equation.Results show that the central angle has little effect on the mode of isolated curved beam-bridge,but the increase of the central angle may increase the distance between the mass center and stiffness center,and then increase the possibility of the main girder rotation.The difference in support position will affect the rotational stiffness of the bridge to a certain extent,which will change the rotational frequency.When ? is gradually increased in the range of 0.25~0.4,the first mode of the bridge is translational motion coupling with rotation and its frequency value gradually decreases.So special attention should be paid to the effect of long-period ground motion for commonly used continuous curved girder bridge with equal span.(2)The influence of the structural parameter,such as the yield force of the seismic isolation bearing,the pre-yield stiffness,the post-yield stiffness,and the central angle,on the seismic response of isolated curved bridge is studied.The results show that the central angle has little effect on the longitudinal displacement of the main girder,but it has significant effect on the lateral displacement of the main girder.With the increase of the central angle,the lateral displacement of the main girder at one end of the bridge increases and the lateral displacement of the girder at the other end of the bridge decreases.The main girder undergoes a certain degree of in-plane rotation which increases with the increase of the central angle.The existence of isolation bearings greatly reduces the seismic inertia force of the piers,which can even ensure that the piers always remain in elastic.But at the same time,it will also cause the problem of excessive displacement of the bearings.Increasing the yield force appropriately,increasing the pre-yield stiffness and decreasing the post-yield stiffness properly can improve the isolation effect and reduce the bearing displacement to some extent.(3)The influence of ground motion peak acceleration,PGA/PGV,and the earthquake input angle on seismic response of isolated curved girder bridge is studied.Results show that the increase of the ground motion peak acceleration will increase the seismic response of the bridge and cause the in-plane rotation so as to increase the lateral displacement at one end of the main girder.The ground motion with small PGA/PGV is relatively unfavorable,which will result in large bearing displacement and shearing force of the piers,but the change of PGA/PGV will not cause significant in-plane rotation of the main girder.In addition,the most unfavorable earthquake input angle is different for different critical parts of the bridge,and for the same key part,the most unfavorable input direction is different if the type of ground motion is different.That is to say,the definition of the most unfavorable input angle makes sense only after many factors such as the type of the ground motion,key part of the bridge,and evaluation criteria have been determined.It is unreasonable to define a certain angle in a general way that is the most unfavorable earthquake input direction of the curved beam-bridge.(4)Viscous dampers are installed on the sliding bearings at both ends of the isolated curved bridge,and the sensitivity analysis of damper's damping coefficient and velocity index is performed.Based on this,a comprehensive evaluation of damping effect is conducted by Analytic Hierarchy Process(AHP).Results show that the translational displacement and in-plane rotation of the main girder,bearing displacement,and the shearing force of the middle piers are all greatly reduced after viscous dampers' introduction.Although the inner force of the side piers will be increased,the piers are still in elastic because that the pier force transmitted from the sliding bearing is small,which indicates that the damping effect of viscous dampers for isolated curved bridge is obvious.