Research On Influence Parameters Of The Seismic Response Of Composite Girde Cable-stayed Bridge With Two Unequal-height Pylons

The cable-stayed bridge with two unequal-height pylons is usually an adaptive bridge scheme because of the limitation of navigation and topography.When the piers,towers and beams of the cable-stayed bridge are consolidated together,the rigid fixity system is formed,which is beneficial to the advantages of cantilever construction.However,under the action of ground motion,the horizontal force of the upper structure of the bridge type is directly transmitted to the lower tower column to make it bend and shear,so it is necessary to pay attention to seismic design and strengthen structural measures.Therefore,based on the project of Chongqing Shuitu Jialing River Bridge,this paper studies the seismic response analysis under different influence parameters,mainly from the following aspects:(1)Aiming at the unsymmetrical supporting points of the unsymmetrical of cable-stayed bridge with two unequal-height pylons,the spatial finite element dynamic model is established,and the dynamic characteristics of the cable-stayed bridge with two unequal-height pylons based on this model are deeply analyzed.This paper mainly discusses the influence of parameters such as different connection forms of pylon and beam,different proportion of two unequal-height pylons,different height of the bridge deck,pile-soil effect and number of auxiliary piers on the dynamic characteristics of composite girde cable-stayed bridge with two unequal-height pylons.The results show that the rigid fixity system of pylon and girder is much greater than that of the floating system and semi-floating system;with the increase of the proportion of two unequal-height pylons,the higher pylon is more likely to have the same or opposite side bending of the left and right limbs of it in the low-order mode;with the increase of the height of the bridge deck,the overall rigidity of the structure increases,and the high-order mode of the vertical bending of the bridge deck is correspondingly delayed;considering the pile-soil effect,the overall rigidity of the structure decreases;setting the auxiliary piers is beneficial to improve the overall stiffness of the structure,but it is not helpful to set two auxiliary piers.(2)According to the standard acceleration response spectrum with the exceedance probability of 10% 50 years provided in report of the Seismic Safety Evaluation as its response spectrum input,the response spectrum analysis is carried out under the condition of longitudinal,transverse,vertical and two-dimensional seismic input of the cable-stayed bridge with two unequal-height pylons,and the similarities and differences between the structural response under the unidirectional input and the combined input are compared.The results show that under the action of one-way ground motion,the peak response of displacement and internal force in this direction is mainly caused,while the response of other two orthogonal directions is not obvious;the peak value of lateral displacement of P6 higher pylon is much smaller than P5 lower pylon under the action of lateral ground motion;the vertical displacement of the brigde deck in the span is obvious under the action of vertical ground motion alone;it is worth noting that the transverse bending moment of the rigid fixity sections of pylon and girder of P6 higher pylon and P5 lower pylon is large under the action of lateral ground motion,and vertical shear force changes abruptly along the direction of pylon height at the position of cross beam,and the shear failure mainly occurs at the lower pylon column of the two unequal-height pylons.After considering the vertical seismic action in the two-dimensional seismic input,the axial force and the vertical bending moment in the the mid-span of brigde deck increase obviously.(3)According to the two natural seismic waves suitable for the site provided in report of the Seismic Safety Evaluation as the two-dimensional ground motion input,the nonlinear elastic time history analysis is carried out under the uniform excitation,and the results are analyzed and compared with the response spectrum calculation results.The results show that the peak internal force response of the pylons bottom and the rigid fixity sections of pylon and girder is prominent in the rigid fixity system of pylon and girder,and there is some gaps between the standard response spectrum analysis and the time history analysis of natural seismic wave as input.(4)In this paper,a multi-point excitation is carried out to analyze the seismic response of cable-stayed bridge with two unequal-height pylons,which considers the traveling wave effect from positive and negative propagation directions.The results show that there is a sudden change in the peak response of displacement and internal force when the wave velocity is 500m/s and 1000m/s.In view of the asymmetry of the cable-stayed bridge with two unequal-height pylons,the structural response caused by the ground motion propagating from the lower pylon side to the higher pylon side is greater than the ground motion propagating from the higher pylon side to the lower pylon side.Considering the traveling wave effect is beneficial to the vertical displacement control of the cable-stayed bridge with two unequal-height pylons and the vertical displacement control of the brigde deck;but in terms of the internal force response,the internal force response of P5 higher pylon and P6 lower pylon is significantly larger than that under the uniform excitation.(5)By using the method of nonlinear time history analysis,the influence of the parameters such as such as different connection forms of pylon and beam,different proportion of two unequal-height pylons,different height of the bridge deck,pile-soil effect and number of auxiliary piers on the seismic performance of cable-stayed bridge with two unequal-height pylon is analyzed in detail.The results show that compared with the rigid fixity system of pylon and girder and semi-floating system,The peak value control of displacement and internal force of the floating system is the most reasonable,and the internal force response of the rigid fixity sections of the rigid fixity system of pylon and girder is prominent,which is the weak part of the seismic design;Compared with the other two models,the peak value control of displacement and internal force of the original design model(the ratio of two unequal-height pylons is4:3)is the best;the increase of girder height is beneficial to coordinate the internal force response of higher pylon and lower pylon,but not conducive to control the internal force response of girder;Considering the pile-soil effect,the bending moment and shear response at the bottom of higher pylon are obviously reduced,while the bending moment response at the bottom of lower pylon is obviously reduced and the shear response is obviously increased;it is necessary to add auxiliary piers to strengthen the overall stiffness of the structure,while setting two auxiliary piers is not conducive to control the vertical displacement of the side span of the lower pylon and the vertical displacement of the unequal-height pylons.