U Ribbed Flat Steel Box Girder Welding Defects And Reinforcement Technology Research

The Severn Bridge is the first bridge in the word to be built with an orthotropic deck steel box girder of aerodynamic cross-section and inclined hangers in1966. Using this section mostly idea is to reduce aerodynamic instability. The flat steel box girder replace with the truss bridge is thought to be the same important of a wing aircraft to improve to a biplane in aircraft industry. Since1966, including France, Denmark, Japan, China and many other countries use orthotropic deck steel box girder as the main forms of stiffening beam of span cable bridges because of the flat steel box is light weight, stiffness and easy maintenance.In recent years, some built flat steel box girder bridge in using is founded some damage because of the structure, construction and manufacturing-related issues, and with the growth of service time, the damage is more and more serious. The reason is that the structure of flat steel box girder is complex, it’s cross section is not irregular, and the loads on it is not a single one. At this stage, manufacturing technology of flat steel box girder is welding with thin plate, no matter its closed section or slats. As the result, there contains hundreds of weld in different of groove, welding methods and fusion shapes in the same section. One of the most serious blemish of weld is that it can produce large residual stress, there for, before the force put on flat steel box girder, there’s already exited the stress and deformation fields generated by temperature, gravity effect, especially the welding process. Coupled with the role of vehicle load, the flat steel box girder is easy to appear cracks.TOHN.W.Fisher has counted and analysis the structural faults and cracks of149steel bridges in U.S. Among them, causing plane deformation and structural fatigue because of the unreasonable of design structure is45.7%; because of the various defects including initial missed crack, penetration, weld strength constraints and so on is49.7; and because of the other reasons like erosion transportation, installation is4.6%.Therefore, the resulting stress and defects generated by weld is great influence on the steel bridge, including flat steel girder bridge. So, to study the generation process and distribution of weld residual stress is important to understanding the iMPact on the damage of the flat steel box. In this paper, we use a real flat steel box girder bridge as the research object, with FEM and ANSYS to study in the following problems:1. Introduce the causes and control methods of welding residual stress.2. Use thermal-structural couple methods to analysis of the welding residual stress and temperature in the junction of U rid and decks of a flat steel box. Temperature analysis showed that there’s a stable temperature field in the welds. Stress analysis showed that the stress in the weld poor is very low, in front of the poor, the stress is very big compression stress, and after the poor, the stress is tensile stress. The stress in weld is compressive stress first, then a tensile stress with the movement of heat, and the stress is bigger than the yield limit at that temperature. The high stress is concentrated in the weld and it’s affect zone, no matter the residual in horizontal or vertical direction shows a stable area, in this area, the longitudinal and transverse residual stresses are tensile stress. In the end of the weld, there is a transition zone, in this area, the longitudinal residual stress decreases, and the tensile transverse residual stress become to a big compressive stress. While, different position along the thickness direction, the residual stress is not the same.3. Use the sub-model method, we analyzed the overall structure of bridge with shell model first, then analyzed the distribution of the standard flat steel box with submodel method in different cases. The result showed that, no matter the crack is exited, the decks and U-stiffener is compressed if without taking into account residual stress.4. With the analysis results and the actual situation of a flat steel box, study the reason of the cracks, then I gave some advises on the reinforcement and reform the structures.Through analysis of these issues, some conclusions and data is obtained, we hope that them can give some help in the design, manufacture, operation, maintenance and reinforcement of the flat steel box.