Analysis Of The Mechanical Behavior And Construction Influence Of The Oblique Nielsen System Beam-Arch Composite Bridge

This newly built Jing-Xiong Intercity Railway(42.65+132+42.65)m oblique cross Nielsen system continuous beam-arch combination bridge is taken as the engineering background of this paper.The static and dynamic characteristics and stability of the bridge and the tension of the suspension rod during the construction process are mainly researched in this paper.The influence of tension on structural stress is available as a reference for the design and construction of this type of bridge.The main research contents are as follows:(1)The stress characteristics of the Nielsen system arch bridge and the continuous beam-arch combination bridge are summarized,and the development status of the two is described.The related theories of the grid grid method are introduced,which provides theoretical support for the establishment of the bridge grid model.(2)The finite element calculation software Midas/Civil is used to establish the spatial grid model of the structure,the static characteristics of the structure are studied,and the displacement,internal force and stress distribution law of the main girder and arch ribs under different loads are analyzed.The results show that the distribution law of displacement and internal force is closely related to its own structure obliquely and antisymmetrically.Under the action of dead load,live load and various load combination effects,the vertical displacement of the main girder is larger than the side span,and the main girder is bent and deformed as a whole.The deformation of the two side spans is greater than that of the mid-span,and the deformation of the main girder is distributed in antisymmetrical distribution.The vertical displacement of the arch ribs gradually increases from the arch toe to the mid-span section,and the vertical displacements of the left and right arch ribs are distributed in anti-symmetric overall.The axial force of the main girder,the bending moment is distributed in anti-symmetric,the axial force is evenly distributed in the mid-span,and the negative bending moment appears at the position of the left and right arch feet.The axial force and bending moment of the arch rib are distributed in anti-symmetric as a whole.The tension and compression stress of the main beam and the arch rib are in the specification within the allowable range,and the structure is safe and reliable.(3)The dynamic characteristics of the structure are calculated and analyzed,and the influence of the stiffness of the arch rib,the stiffness of the transverse brace and the layout of the transverse brace on the dynamic characteristics of the bridge is discussed.The results show that the first-order mode of vibration is the lateral bending of the arch rib,and the out-of-plane mode of the structure appears before the in-plane mode.The change in the stiffness of the arch rib has a greater influence on the in-plane fundamental frequency than the out-of-plane fundamental frequency,and the structural mode shape no change.The influence of the change of cross brace stiffness on the bridge’s natural vibration characteristics is more complicated.The form of cross brace has a greater impact on the increase of the out-of-plane stiffness of the arch rib,and the number of cross braces has a greater impact on the out-of-plane stiffness of the arch rib.With the increase in the number of cross braces,the outer stiffness of the arch rib surface gradually increases.(4)The elastic stability of the structure was studied,the stability of the bridge under different loads was compared and analyzed,and the influence of the stiffness of the arch rib,the stiffness of the cross brace and the layout of the cross brace on the stability of the bridge was discussed.The results show that under different load combinations,the first five buckling modes of the structure are the same,and they are all instability outside the arch rib surface,indicating that the vertical stiffness of the bridge is greater than the lateral stiffness.The dead load affects the stability of the structure the largest,the live load has a great influence on the stability of the structure,and the wind load has no obvious influence on the stability of the structure.The stiffness of the arch rib has a great influence on the stability of the structure,and the stiffness of the cross brace has no obvious effect on the overall stability of the structure.The form and quantity have obvious influence on the stability of the structure.(5)The forward installation iterative method is used to solve the initial tension of the suspender during the construction process,and the stress change law of the main beam and arch rib during the tensioning process of the suspender is studied.The results show that the initial tension force of the boom determined by the forward-installation iteration method is reasonable and accurate,and the boom tension force that needs to be achieved after the completion of the two stretched booms in the design is achieved.During the boom tensioning process,the main girder the span stress changes more obviously.The span stress on both sides of the main girder remains basically unchanged.The stress changes at the two arch feet of the main girder are complex and there are stress mutations.During the tensioning process of the boom,the arch rib dumbbell-shaped steel pipe,upper-chord concrete,and lower-chord concrete are affected.The stress changes are different,and the stress changes at the upper and lower edges are inconsistent.From the beginning to the end of the suspender tension,the overall compression stress of the arch ribs increases,but they all meet the specification limit requirements.