Fatigue Problems At Longitudinal Ribs Of Orthotropic Steel Decks And Its Improving Measures

Although the orthotropic steel deck(OSD)has obvious advantages,its fatigue problem is prominent,which seriously affects the serviceability and safety of the bridge deck.The fatigue details at the rib-to-diaphragm weld seams are characterized by the structural complexity,large number of cracks,and difficulty to guarantee welding quality.This kind of welding joints is one of the positions where the fatigue cracks are most likely to occur.In this paper,taking the fatigue details here as the research object,the analysis is carried out to reduce the stress level caused by the second system effects.A new type of structure is proposed to brace the longitudinal ribs.The equivalent stress method was used to study and evaluate the fatigue performance of the new OSD.The main research work in the present thesis are summarized as follows:(1)The thesis reviewed the development history of OSD.The mechanical characteristics and common diseases of OSD were summarized and the existing solutions to the diseases were introduced.(2)The thesis presented the typical fatigue failure forms of the rib-to-diaphragm joints.The theoretical fundamentals,the fatigue analysis process and the application reasons were introduced.(3)A traditional orthotropic steel bridge deck finite element model was established to study the effects of a few factors on the equivalent structural stress at the rib-to-diaphragm joints,such as the roof plate thickness,the thickness of the longitudinal rib web,as well as the thickness and spacing of the diaphragms.The research results show that increasing the thickness of the roof plate,the longitudinal rib web and the diaphragm can effectively improve the fatigue performance of the rib-to-diaphragm joints.The equivalent structural stress can be reduced,but it is not sensitive to changes in diaphragm spacing.(4)Based on the fatigue cracking mechanism and cause analysis of the connection between the longitudinal rib and the diaphragm,a support structure between the bottom of the longitudinal rib and the diaphragm is proposed.A finite element model was established and the fatigue life was predicted for the stress concentrated zone according to the equivalent structural stress method.The new OSD was compared with the traditional one and the OSD with large longitudinal rib and UHPC.The results exhibited the improvement measures of setting the bracing structure have a great effect on reducing the fatigue stress of the longitudinal ribs caused by the second system.(5)The bracing structure was applied to the OSD with large longitudinal rib and UHPC pavement and establish the finite element model to focus on the details of the determined fatigue control parts.Based on the equivalent structural stress method,the evaluation and comparison are carried out.The stress level of the composite bridge deck is greatly reduced,and the fatigue life reaches an infinite fatigue life,which is a very effective improved structure.