| The Orthotropic steel deck(hereinafter referred to as OSD structure)has many advantages,such as modular prefabrication,accelerating construction speed,improving service life and reducing the self-weight of bridge superstructure,which has been widely concerned and applied by researchers and engineers all over the world.However,as OSD structures are put into use,their fatigue problems are also exposed.The main reasons include: local load problem is prominent;short influence line,repeated fatigue load;complex structure,easy to produce stress concentration;welding connection will produce welding residual stress.Scholars at home and abroad have conducted many years of research and achieved a lot of research results,but the fatigue problem has not been thoroughly solved,so this problem is further studied.Based on the prototype of an existing bridge,the fatigue control stress and stress composition at the joint of longitudinal rib and floorbeam(Rib-Floorbeam,hereinafter referred to as FB)are analyzed,and the corresponding ideas for structural optimization are worked out.Starting from the structure of OSD structure,the design parameters and construction details are adjusted and optimized to reduce fatigue stress and improve fatigue life.The main research work is as follows:(1)Establish the whole finite element model and sub-model,compare the results of finite element calculation under different modeling conditions,and select a more rapid and less influential model.Comparing the location of stress concentration in the calculation results with the location of fatigue failure in statistics,the research focuses on point A above U rib and point B on diaphragm.Through the analysis of stress composition in diaphragm and longitudinal rib under moving load conditions,it is found that the stress in diaphragm is mainly in-plane stress,while the stress in longitudinal rib is mainly out-of-plane stress.Drawing the stress influence surface of point A and point B,extracting the control stress and principal stress direction of these two points,it is found that the influence area of these two points is small and seriously affected by fatigue load,and the principal stress direction is perpendicular to the direction of fatigue crack.The principal stress at the arc opening near point B is extracted,and it is found that the stress concentration is serious in the area with smaller curve radius at the arc opening.(2)The finite element models of OSD structures with different openings are established based on the suggestions of national codes and referring to the arc-shaped openings adopted by actual bridges.The control stresses at points A and B are compared and analyzed.It is found that the fatigue performance of point B can be improved by increasing the radius of the arc opening.A new type of opening is proposed and verified to be effective in improving the fatigue performance of point A and point B.(3)According to the design method of diaphragm in steel bridge,the thickness of diaphragm designed according to stiffness is calculated,which is much smaller than the thickness of diaphragm used in actual bridge.Finite element analysis of OSD structures with different diaphragm thickness shows that increasing diaphragm thickness can effectively reduce the control stress at point A and B,and reducing diaphragm thickness can reduce the out-of-plane stress at point B.The idea of variable thickness diaphragm is put forward to increase the upper thickness of diaphragm and keep the lower thickness of diaphragm unchanged.The calculation shows that this method has a good effect on improving the fatigue performance of point B.(4)Referring to the setting method of longitudinal rib inner diaphragm in American code,the inner diaphragm is set on the basis of the existing model.It is found that the inner diaphragm has a good effect on reducing the control stress at point A.A new structure with double inner diaphragms is proposed,which has been proved to be effective in improving the fatigue performance of point A. |
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