Fatigue Properties Analysis Of Welded Joints Of U-ribs And Diaphragms On Steel Bridge Deck

Orthotropic steel bridge deck has been widely used in medium and large span bridge structures.However,structure of this kind of bridge deck is complicated and welding can easily cause stress concentration.Under the reciprocating action of local wheel loads,structure details are prone to fatigue.In this thesis,i focus my study on the rib to diaphragm connection which is most likely to be damaged by fatigue.The main work and conclusions are as follows:(1)The fatigue cracks at the joint of U-rib and diaphragm are analyzed.The analysis shows that the deformation that affects the fatigue performance of the detail includes out-of-plane deformation of the diaphragm,in-plane deformation of the diaphragm and distortion of the U-rib.(2)Based on the fatigue research project of Wuhan Qingshan Yangtze River Bridge,a finite element model is established to analyze the stress distribution and composition at the connection between the U-rib and the diaphragm.Loading positions are moved along the transverse and longitudinal directions.The analysis results show that the most unfavorable transverse position is directly above the lateral web of the U-rib and the stress of concerned detail is sensitive to transverse position of the wheel load.The diaphragm is mainly subjected to in-plane stress and the out-of-plane bending is very small.However,the U-rib web is simultaneously affected by in-plane deformation,out-of-plane deformation and the distortion.(3)Research shows that there are four kinds of common cracks at the connection of U-rib and diaphragm which are caused by concerned control stresses.Corresponding comparing and analyzing are done about these control stresses.The analysis results show that for crack C1,control stress is the tangential stress of opening free edge.For crack C2 and crack C3,control stresses are vertical tensile stress and longitudinal tensile stress respectively on the surface of U-rib web.The former is 2 ~ 3.9 times more than the latter numerically indicating that crack C2 appears more likely than crack C3.(4)Based on the stress composition of joints of U-ribs and diaphragms,structural details are optimized using common crack control stress as the evaluation criteria.Optimization measures include stiffening structures in U-ribs and the comparing of the fatigue performance of different open-cell types.The results show that the internal stiffener has the best structural optimization effect.Cutout shape 4 has the best fatigue performance among the six cutout shapes.The influence of the thickness of the top plate,the diaphragm,and the U-rib on fatigue performance of the joint between the U-rib and the diaphragm is also studied and a relatively better plate thickness is recommended.