Elastoplastic Analysis Of Long Span Continuous Girder Bridge In Mountain Area Under Earthquake Action

With the development of ductile seismic system,it is necessary to carry out elasticplastic seismic analysis of structures.At present,scholars at home and abroad mainly focus on the theory of elastic-plastic analysis and finite element numerical model.The elastic-plastic analysis of bridge structures is a major research hotspot in the engineering field.When the strong earthquake comes,the bridge structure will have a large seismic response in both vertical and horizontal directions.The large displacement may cause the collision effect between the main beam and the abutment or stop,and eventually cause the bearing damage,beam falling and other damage.The large internal force may cause the pier bending or shear damage,and then cause the collapse of the superstructure.The response characteristics of long-span continuous girder bridge under earthquake action are obviously different from those of common small and medium span simply supported girder bridges.For various reasons,it is necessary to study the elastic-plastic seismic behavior of long-span continuous beams.Based on a continuous beam bridge in southwest mountain area,this paper studies the seismic response and seismic performance of the bridge structure under normal earthquake and near fault earthquake.The main work is as follows:1.Based on a large number of documents,the types and calculation principles of nonlinear elements are systematically summarized.The nonlinear characteristics of the bearing,the collision effect between the main beam and the abutment,the pile-soil interaction effect and the elastic-plastic reinforced concrete block are simulated in Midas civil software.The refined analysis model is established and the modal analysis of the structure is carried out.2.According to the latest seismic code of bridges in China,three seismic waves with high fitting degree with target spectrum are selected,and a vertical wave is synthesized manually.The nonlinear dynamic time history analysis of the structure is carried out under four seismic input modes.The results show that the displacement response of transverse and longitudinal bridge has no coupling relationship with vertical displacement response;when lateral earthquake acts,it has an effect on the longitudinal and horizontal internal forces,But the longitudinal earthquake only responds to the longitudinal internal force.3.Using IDA method,the IDA curve of curvature parameters in plastic hinge area of pier bottom is established,and the rotation and development law of plastic hinge are studied,The results show that: With the increase of PGA,the curvature ductility value of the pier column increases,the yield degree of the pier column increases,and the moment curvature curve in the plastic zone becomes fuller and fuller.After the pier enters the medium failure stage,the structural stiffness decreases sharply,and the structural deformation and internal force change more and more greatly.4.According to the structural characteristics of continuous beam bridge,The influence of pile-soil interaction effect,abutment impact stiffness,pier stiffness,gap between block and girder,and block strength on the seismic response of the structure are analyzed,And the change law of structural response caused by it,The results show that:(1)When there is seismic input along the bridge,whether the pile-soil interaction is considered has a great influence on the bending moment,shear force and axial force at the bottom of 2#pier.After considering the pile-soil structure,the maximum bending moment changes by 7.77%,the maximum shear force changes by 3.92%,and the maximum axial force changes by 0.72%;when there is seismic input across the bridge,the bending moment,shear force and axial force at the bottom of 1# and 2# pier of the two models are slightly different,so the pile-soil interaction is considered The maximum bending moment,shear force and axial force are changed by 2.02%,2.38% and 1.52%,respectively.(2)With the increase of the impact stiffness,the response values gradually decrease,and the change trend from the top to the bottom of the pier is the same;when the impact stiffness is small,the response values change greatly with the stiffness,but when the impact stiffness is large,the response values almost coincide,and the change value is very small.The collision effect between the beam end and the abutment can reduce the seismic pressure in the plastic hinge area of the pier.(3)With the decrease of pier stiffness,the transverse displacement of pier top gradually increases,the bending moment gradually decreases,the shear force first decreases and then increases,the longitudinal internal force gradually decreases,and the pier top displacement gradually increases.The impact effect of pier stiffness on the side near the fixed pier is greater than that on the side near the movable pier.(4)With the increase of the strength of the block,the internal force and curvature of the pier bottom show a fluctuating growth,with the maximum internal force increasing by 27.5% and the maximum curvature increasing by 65.79%.The overall deformation of the main pier bearing first "fast" and then "slow" decreasing trend;with the increase of the gap between the blocks,the internal force and curvature of the pier bottom first increase and then decrease,and then tend to be stable,and the deformation of the 1# pier and 2# pier top bearing shows an overall increasing trend.The influence of the strength and clearance of the block on the abutment bearing is weak.5.Twelve groups of near fault ground motions with different pulse characteristics are selected as the input.The effects of different pulse periods,types of ground motions and AP/VP parameters on the seismic response of continuous girder bridges are analyzed,and the variation of structural response caused by them is analyzed.The impact of pulse effect on seismic performance is evaluated.The results show that:(1)The effects of long,medium and short pulse periods on the structural response are different.Different pulse periods have significant effects on the moment,displacement and shear response,The response values of bending moment,displacement and shear force are obviously the largest under medium period earthquake,which indicates that the impact of pulse period on the structure should be considered in the seismic design of near fault bridges.For example,for medium period earthquake,the diameter of original stirrups and the spacing of stirrups can be increased in the plastic hinge area of piers,so as to improve the ultimate displacement and curvature of piers.(2)Compared with the near fault non pulse effect seismic wave,the near fault pulse effect seismic wave significantly increases the longitudinal and transverse internal force and deformation of 1 pier,the transverse internal force and deformation of 2 pier,and the elastic-plastic behavior of pier.The response value of near fault earthquake without impulse effect is basically the same as that of far field earthquake.(3)The results show that the variation law of the longitudinal seismic response of the fixed pier,the transverse seismic response of the fixed pier and the transverse seismic response of the active pier are generally similar,and they all show a downward trend with the increase of Ap/Vp value,and the calculation results are more discrete,while the variation law of the longitudinal seismic response of the active pier is not obvious,the change is gentle,and the calculation results are relatively concentrated.Ap/Vp can be used as a supplementary index for seismic design.(4)Compared with the non pulse earthquake,the minimum demand ratio of bending capacity is 0.613,the minimum demand ratio of shear capacity is 0.584,and the minimum demand ratio of deformation capacity is 1.257.In the seismic design of near fault longspan continuous girder bridge,the impulse effect of near fault is an important factor to reduce the seismic capacity.