| Concrete-filled steel tube (CFST) is a composite structure Constructed by filling concrete in thin-walled steel tube; the working principle is the confinement effect of the steel tube on the core concrete in trip-direction compression stressed condition, which helps to significantly improve the compressive strength and plastic deformation ability. Due to this reason, concrete-filled steel tube is suitable for structural elements under pressure, such as columns, arch ribs, etc. The development of concrete-filled steel tube arch bridge is rapid due to its high bearing capacity, strong spanning ability and convenient construction. At present, the quantity of the concrete-filled steel tube arch bridge in china is more than200seats, and they has made great economic benefits.The section patterns of the concrete-filled steel tube arch bridge vary a lot. Most of them are made of directly welding steel tubes. In this way, the fatigue problem of the joints becomes prominent. Therefore, the fatigue life of the joints must be paid attention to. As for the hollow pipe connector, that has had a lot of effective test research in the offshore platform. But researches on the connector of T concrete-filled steel tube are very slim at home and abroad. In view of the facts that concrete-filled steel tube arch bridge belongs to a new structure type, the case of fatigue damage or destruction has not yet turned up. The research on its fatigue theory has just started. Its system of fatigue design has not been built. And there is no special specification. So studying its fatigue property, establishing fatigue analysis method and identifying fatigue design principles seem to be a very urgent work to the bridge engineering.First, this dissertation introduces the theories which have universal significance in the fatigue problem. Two methodologies of fatigue problem are applied to the steel tube joints:S-N curve method and fracture mechanics method. Hot stress method of S-N curve method is one of the important methods analyzing steel tube welded joint fatigue strength. As to fracture mechanics method, the stress intensity factor K has an important significance. In this work, common joint models of concrete-filled steel tube bridges are built to calculate the value of foreign and minister transversal stress concentration factor which lead to the phase of its transversal distribution. Based on the same joint models, the stress intensity factor K of K concrete-filled steel tube joint can be calculated. After that, the corresponding algorithms are summarized briefly. In the meanwhile, the current situation about concrete-filled steel tube welded joints studying is evaluated. Thus all these works has laid a solid foundation for life prediction studying on concrete-filled steel tube arch bridges.Hot spot stress is the maximum structure stress mostly located in the weld toe of major and branch pipe. Fatigue cracks are common at position like that. In order to simulate numerically this kind of complex practical problems, the contact of steel tube and concrete in the concrete-filled steel tubes still should be considered when building the analysis model. The contact problem involves the stress concentration, boundary nonlinear and material or geometric nonlinear problems.In the loading process, as the change of time, the mutual position t of contact surface, the size of area and the contact state are changing. All these factors are to be determined in solving process. This makes the contact problem of the contact of steel tube become a kind of highly nonlinear behavior. For this kind of complicated behavior,0.1millimeter’s penetration error will lead to divergent computation or error. or error, this dissertation’s computations experience shows that when modeling, as well as keep the node in the steel pipe’s interior walls and on the concrete surface should be one-to-one correspondence. This can not only shorten the iteration time, but also guarantee the convergence of the results and less the calculation results is error.In this dissertation, models for K shape joints and three-dimensional joints are established, these models including joints between hollow pipes and joints of hollow pipes to CFST, mechanical characteristics of CFST are simulated in these models. At last, the value of the hot spot stress concentration factor of the type K node in the axial loads, and its distribution of the phase transverse line. Because of the string tube bear the larger axial pressure in concrete-filled steel tube arch bridge, the influences of the string tube axial pressure exerts on the stress concentration factor and distribution of the K shape joints of hollow pipe to CFST are analyzed in this dissertation.In the process of computing the node stress intensity factor, the dissertation compared the advantages and disadvantages of the displacement extrapolation method, J integral method, the whole domain virtual crack propagation dissertation suitable for steel pipe concrete node, In more detail, the methodology integrated the advantages of the singular and nonsingular unit displacement extrapolation method, adopted the singular unit on the crack tip, encrypted grid around the crack tip, and designed the reasonable transition grid. Wedge singular elements are realized by20node isoperimetric element SOLID95programmed by APDL macros. By suggest the hypothesizes about factors such as:the initial crack position, shape, size, the stress intensity factors of concrete-filled steel tube in different crack size (a/t, a/c) are computed successfully. After that some relevant suggestions can be offered about the crack tip meshing. The crack tip grid is better to be divided into five layers, and the element Angle is22.5°, continue encrypt out the crack tip element, guarantee set up a unit along the crack surface every10°, encrypt appropriately near the crack end, because the slope changes rapidly.Second, as the traffic infrastructure investment increases steadily, number of bridges damaged shows ascendant trend mainly due to fatigue problems. So it is very necessary to research the fatigue load spectrum of the road bridge. The research results is used in designing and evaluating the highway bridge’s fatigue. This will greatly reduce the risk of structural failure, and reduce the loss of public properties and casualties. In this dissertation, principle and design methods of the fatigue load spectrum of road bridge are introduced. Also the fatigue load spectrum of the road bridge and its current research situation at home and abroad are summarized. In the end, based on Yunnan provincial highway traffic survey, the practical fatigue load carrier frequency value spectrum suited for Highway Bridge’s fatigue problem is proposed. Drawn up the practical fatigue load spectrum that suited for analyzing the highway bridge’s fatigue according to Yunnan provincial highway traffic survey.At present, many domestic regions in China has set down their local fatigue load spectra. But research on this field in Yunnan province is still in blank. The fatigue load spectrum suited for Yunnan province on the basis of Yunnan provincial highway traffic survey. According to the survey, there are2003variously traffic stations in Yunnan province,2010. Among them, there are16continuous stations installed on the national highway——Keeping counting the traffic volume per hour continuously. These stations are representative and can qualitatively reflect the change rule of traffic volume in Yunnan province. According to the observations and the data in China automobile type manual, divided the vehicle into seven categories——V1～V7, according to statistics. The largest number is small van (VI), accounting for61.22%of the total number of vehicles At the same time. The second is medium-sized truck (V2) and large buses (V4). These two kinds of car are counted for about12%of the total number of vehicles. Besides, a small amount of large truck (V3), jumbo truck (V5), container (V6) and articulated vehicle (V7). As the types of vehicles is numerous, it is quite tedious to be used in fatigue analysis, drawn up the practical fatigue load spectrum using the suitable methods, ultimately practical fatigue load spectrum includes three kinds of model vehicle that can be used in fatigue analysis of the highway bridge’s in Yunnan province is concluded.Finally, further analysis on local hot spot stress of the key fatigue node, the basis of three-dimensional finite element mode of the concrete-filled steel tube arch bridge and its dynamic response exerted by vehicle’s fatigue load. In this dissertation, a three dimensional finite element model of three nodes from different parts are constructed. The load and displacement boundary conditions according to the actual working condition of bridge are applied on these models. And the hot spot stresses of the node are analyzed. The fatigue life the joints is the sum of the life of crack’s initiation and life of crack propagation. The life of crack’s initiation is calculated using hot stress method, and the life of crack propagation is calculated using fracture mechanics method. |
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