Research On Mechanical Behavior Of Long-span Railway Composite Bridges Based On Stiffening Effect

Railway bridges bear large live load and have high stiffness requirements.The design of long span bridges is controlled by stiffness.To meet the stiffness requirements,the section size of main beam can be increased or composite structure can be adopted.Increasing the section size of main beam leads to the increase of structural dead weight,so when the span is large,the economy and feasibility of this method are poor.Therefore,composite bridges are often used for long-span railway bridges.Extradosed cable-stayed bridges and beam-arch bridges are two kinds of composite bridges commonly used in long-span railway bridges,it is of great value to know the stress characteristics of these two composite bridges under the action of dead load,live load,creep and temperature.Therefore,in this paper,two practical projects were selected,and two continuous rigid frame bridges of the same span were taken as the reference bridges for comparison.The mechanical characteristics of the two composite bridges under load were analyzed in depth,the distribution of dead load and live load in different basic structures(beam,arch and cable)was summarized,and their stiffening effects were analyzed,the research results have some reference value for engineering design.The specific research work of this paper was as follows:1.The application and research status of the extradosed cable-stayed bridge and the beam-arch bridge were briefly described.Combined with the supporting project,the general characteristics of the two composite systems,such as materials usage,key and difficult points of construction were summarized.2.The finite element models of extradosed cable-stayed bridge and its reference bridge(continuous rigid frame bridge with the same span),beam-arch bridge and its reference bridge were established,and the whole construction process of the supporting project was analyzed,focusing on the internal forces of four bridges under the constant load at the maximum cantilever stage and the bridge completion stage.The results show that the cable of the extradosed cable-stayed bridge bears more dead load,while the arch rib of the beam-arch bridge bears less dead load.3.By means of influence line analysis and different loading methods,such as evenly distributed live load and the most unfavorable live load,etc,the variation of key indexes such as internal force and stress of composite bridges under live load were studied.The stiffening effects of extradosed cable-stayed bridges and beam-arch bridges on continuous rigid frame bridges were analyzed.The results show that under live load,the cables of the extradosed cable-stayed bridge only share a small amount of live load,and the stiffening effect on the main beam is limited,while the arch ribs of the beam-arch bridge bear a larger proportion of live load,and significantly improve the vertical stiffness of the main beam.4.The stress of suspension cables of the extradosed cable-stayed bridge and the beam-arch bridge under live load were calculated.The results show that the stress amplitude of the suspender is significantly larger than that of the cable.The influence of some design parameters on the mechanical properties of the structure was explored.5.When the span of the bridge increases,in order to improve the stiffness of the bridge,pier-girder rigid connection is often adopted,at this time,creep and temperature effects cannot be ignored.Therefore,the influence of concrete creep and temperature on the deformation and internal force of the bridge were also analyzed.The results show that the internal force of pier of rigid-frame bridges under temperature load is large,and the stiffening action of cables and arch ribs reduces the deformation of beam caused by concrete creep.