Research On Mechanical Properties And Cable Force Optimization Of Long-span Double Deck Four Line Railway Steel Truss Cable-stayed Bridge

With the development of national high-speed railway network,cable-stayed bridge has become a reasonable choice for crossing rivers and valleys because of its crossing capacity.With the main girder span of the bridge entering the kilometer level,the double-layer four line railway steel truss cable-stayed bridge is more competitive with its advantages of reasonable stress,light shape and large rigidity.The bridge type of conventional cable-stayed bridge is generally symmetrical,but in the actual project,because of the terrain and other factors,the bridge type needs asymmetric design,because of the lack of relevant research,it will limit the wide application of railway cable-stayed bridge.In this paper,a double-layer four line railway steel truss cable-stayed bridge will be studied as the engineering background.Due to the geographical and navigation conditions,the bridge type needs asymmetric design,resulting in the difference in the height of the two main towers,the number of stay cables of the two towers is also different.Compared with the conventional cable-stayed bridge,the bridge type is complex,the mechanical properties are greatly different,and the optimization of the cable force is difficult.In this paper,it will be used The finite element software ANSYS carries on the modeling analysis research from these two aspects.The main research work and results are as follows:(1)Combined with the development and application of steel truss cable-stayed bridge at home and abroad,the research status of static and dynamic analysis and cable force optimization of steel truss cable-stayed bridge is reviewed.The significance and necessity of research on mechanical properties and cable force optimization of long-span double deck four line railway steel truss cable-stayed bridge are expounded.Chinese style(2)The three-dimensional space finite element model of the main tower,main beam and cable of the whole bridge is established by using the finite element software ANSYS.In the research and analysis of the static performance of the bridge,it is concluded that:①the axial force of the left main tower is greater than that of the right main tower,and the difference between the two axial forces is the largest at the position 1/3 from the top of the tower,up to 30000kn.The results show that there are obvious differences between the main tower of asymmetric four line railway steel truss cable-stayed bridge and symmetrical four line railway steel truss cable-stayed bridge.The axial force of the main tower at the larger span of the main girder is greater than that at the smaller span of the main girder.②the stress of the main truss on the main girder changes suddenly at the two main towers.The stress distribution is irregular and complex.The stress distribution of the lower main truss is uniform and relatively simple,but the overall stress of the lower main truss is greater than that of the upper main truss.③the overall stress distribution of the main girder is asymmetric,and the stress of the left girder is greater than that of the right girder.The results show that the stress distribution of the main girder of the asymmetric four line railway steel truss cable-stayed bridge is also asymmetric,and the beam section with larger span is more stressed.④the bending moment of the main beam is evenly distributed,and the larger positive bending moment is produced at the auxiliary pier and the main tower of the main beam,and the negative bending moment is produced at the middle of the mid span.⑤when the influence of geometric nonlinearity on the structure is taken into account in the static analysis of the bridge,the stress distribution of the structure is similar to that of the linear static analysis,but the stress of the structure increases.(3)In the study of the dynamic performance of the bridge,the modal calculation of the bridge is mainly carried out through ANSYS,and the following conclusions are drawn through the modal analysis:①the first-order torsional deformation of the beam occurs in the tenth order,and the ratio of the first-order torsional to the first-order flexural frequency of the main beam is relatively large,which indicates that although the structure adopts the truss main beam structure,the torsional rigidity of the main beam is still large,and the results are as follows It is proved that the anti wind stability of this type of bridge can meet the requirements;②the second-order vibration mode of the bridge tower is involved in the first 20 vibration modes,and the main tower is involved in 10 times of the vibration mode,and the bridge tower is involved in more vibration modes,the results show that the rigidity of the bridge tower of this type of bridge is small,and the rigidity of the bridge tower needs to be improved;③the independent vibration mode of the main beam vertical bending and horizontal bending The analysis shows that the displacement of the main beam of this type of bridge is the largest in the middle span under the static and dynamic action.(4)Based on the optimization module of ANSYS,the cable forces of asymmetric four line railway steel truss cable-stayed bridge are optimized.Through the comparative analysis of different optimization modes,the following conclusions are drawn:①the zero order optimization method and the first order optimization method can satisfy the optimization calculation of cable forces in engineering,and the first order optimization method is more suitable for the accurate optimization analysis of cable forces.②When using the first-order optimization method to calculate the optimal cable force,especially when the number of design variables is large(the number is more than 20),the results of cable force optimization by changing the value and state variables of design variables are more obvious than that by changing the objective function.③Through the optimization calculation,the optimal cable force is proved,which is substituted into the finite element model for mechanical calculation.Through the comparison of the structural response before and after,the optimization calculation of cable force can improve the mechanical performance of the whole bridge.Through the comparison of the total cable force and the mean square deviation of the cable force,it is concluded that the optimized cable consumption is relatively reduced and the cable force distribution among the cables is uniform.