Study On Stability Of Flexible Arch Bridge With Long Span Railway Through Steel Truss Beam

Steel truss arch bridge has the advantages of beautiful appearance,strong spanning capacity and super high bearing capacity of the arch bridge.It occupies an important position in various forms of bridge structure and is favored by contemporary railway bridge designers.In recent years,a number of large-span steel truss flexible arch bridges have been constructed at home and abroad.The span-width ratio of such bridges is also decreasing,especially in railways where the width-span ratio of bridges may be less than 1/20,which also brings about the stability of the structure in the transverse direction.In this paper,the stability and stability of flexible arch bridges with steel truss girders are systematical y studied in conjunction with the main bridge project of Hongqili steam Bridge in Guangzhou.The research contents are as follows:(1)This thesis introduces the mechanical characteristics of steel truss arch bridge,summarizes the development of this type of bridge at home and abroad,and lists some famous bridges built in China in recent years.On the basis of summarizing the current research status of bridge stability,it points out the existing problems in the research,and puts forward the necessity of studying the stability of long-span steel truss flexible arch bridge.(2)According to the engineering situation of the Hongqili steam Bridge,the world's largest steel truss flexible arch bridge,ANSYS and MIDAS CIVIL finite element models of the steel truss flexible arch bridge are established,and the load conditions of the bridge in different stages are determined according to the different load combinations in the construction process and operation stage of the bridge.Through theoretical analysis,the bridge in the preclosure stage of the steel beam and the arch are pointed out.Instability may occur in key construction stages,such as rib horizontal assembly stage,before and after closure of main steel arch.(3)Based on the stability theory of bridges,the linear elastic analysis method,geometric nonlinearity analysis method and material and geometric double nonlinearity analysis method for solving bridge stability problems in finite element method are introduced,and the implementation steps of the three analysis methods in finite element software are briefly introduced.The linear elastic analysis method is used to study the first kind of stability of Hongqili steam Bridge under various load combinations in different construction and operation stages.The results show that with the development of bridge construction process,the stability of the structure has changed significantly,and the stability safety factor and instability form of bridges in different stages have obvious differences.The stability factor of steel truss bridge is the lowest in the pre-closure stage,while that of steel truss is the highest after closure,which indicates that temporary support has an obvious assistant effect on the stability of the structure;the stability safety factor of steel truss flexible arch bridge varies greatly under different load combinations in the operation stage,but the structure can better guarantee the stability;the train braking applied to the asymmetric transverse bridge The force and train load asymmetrical y distributed along the bridge direction have great influence on the first kind of stability of the bridge,while the transverse wind load and system temperature load have little effect on the elastic stability of the structure.(4)By introducing geometric and material nonlinearity into the finite element model of steel truss flexible arch bridge,the second stability problem of bridge is studied.On the basis of obtaining the ultimate bearing capacity of structure,the influence of different non-linear factors on the second stability of bridge is compared and analyzed.The load-displacement curve of structure and the variation of cross-section internal force of typical members are used in the process of increasing load.The failure path and failure mechanism of steel truss flexible arch bridge are studied.The results show that the difference between linear elastic method and geometric non-linear method is smal,while the structural safety factor calculated by double non-linear method is obviously smal er than the former two,and the load-displacement curve shows a strong non-linear relationship.Therefore,the geometric non-linearity of steel truss arch bridge has a strong non-linear effect on the structural stability.The material nonlinearity can not be neglected when calculating the bearing capacity of structures.Considering the initial defect,the safety factor of the bridge decreases by 5.82%.The vertical stiffness of the structure does not change obviously after applying the defect,but the transverse stiffness decreases to a certain extent.Moreover,the load-displacement curve of the flexible arch bridge with steel truss girder is more non-linear in the transverse direction,and the transverse displacement is larger when the structure fails.Under the ultimate load,the arch is calculated by geometric nonlinearity and linear elasticity methods.The results of the key positions of ribs and trusses are slightly different,but both of them are obviously different from the double nonlinearity.The difference between the middle span and the 3/4 span is the most obvious.The deviation of the results from other positions is smal.The displacement and axial force of the structure tend to increase after considering the double nonlinearity of the structure.All members of the steel truss arch bridge are in the elastic stage at the initial stage of loading,with the load increasing.When the load increases to a certain value,the bending moment of the upper chord member in the side span decreases and the axial force increases slowly.When the bending moment decreases to zero,the member enters the plastic state,the stiffness of the structure decreases,and the internal force redistributes.Then the lower chord member in the main span enters the plastic state,and the bending moment of the plastic member transfers to other members until the structural stiffness decreases to the point where the load cannot be borne.(5)Based on the established ANSYS finite element model,the influence of key parameters,such as live load distribution,initial defects,transverse load and temperature change,on the second kind of stability of steel truss flexible arch bridge is studied.The influence trend and mechanism of these external factors on the bearing capacity of bridge are quantitatively analyzed.The results show that different train load forms have influence on the structural bearing capacity.The ultimate bearing capacity of the bridge is the lowest under the double-track action of train spanning;the mode and magnitude of the defects will have a significant impact on the bearing capacity of the structure.The initial defects added through the instability mode have a greater impact on the bearing capacity of the structure,while the eccentricity mode has a smal er impact on the bearing capacity of the structure;the transverse wind load and the change of system temperature.The ultimate bearing capacity of flexible arch bridges with steel truss girders is less affected by the change of temperature.Only under the ultimate conditions can the safety factor of the structure be reduced by 0.291% under the ultimate wind load compared with the operation wind load.Considering the maximum temperature difference in the whole year,the safety factor of the structure is reduced by 2.3%.