Research On Safety Assessment Method For Box Girder Of Bridge And Gantry Crane Based On The Theory Of Structural Integrity

As indispensable mechanical equipments in the modern industrial production, hoisting machinery and conveying machinery have been widely used in the areas of material loading and unloading, handling, production, rescue and maintenance, etc. With the further development of China’s society and economy, the number of in-service cranes especially the aging cranes increases year by year. On the one hand, as the in-service cranes get older, structure performance will be degraded and its carrying capacity will be lower, so the crane reliability and safety cannot be guaranteed, and the structure damage often leads to greater property damage and personal injury; On the other hand, in the crane industry and national level, we still lack a unified security assessment standard for crane structure in our country. When the quality inspection department evaluates the crane scrap and maintenance, it could lead to safety accident if they miss some key assessment items. It will result in crane retired or scrapped in advance if the assessment is too conservative, which causes economic loss and waste of resources. Therefore, from the perspective of structural safety and economic benefits, establishing a scientific and reasonable safety assessment method and process suitable for the crane structure containing defects, has important practical significance and practical value in engineering.Box girder as a typical bearing structure of bridge and gantry crane, fatigue fracture is one of the main damage forms. Because of the influence of the factors such as material and welding, there are inevitably micro defects and micro cracks in the structure. Under the action of working load, these defects and cracks will be gradually expanded which will impact the structure intensity seriously and become a potential safety hazard. In the actual production crane fracture accident often happened due to the structure fatigue. Taking a 36 t container gantry crane as research object, based on the strength theory, fatigue damage, fracture theory and reliability theory, combined with the British standard BS7910 guide to methods for assessing the acceptability of flaws in metallic structures, by using theoretical analysis and finite element simulation, the research contents include the strength degradation mechanism of the crane box girder structure containing defects, fracture control parameters and fatigue life prediction, etc. Our goal is to explore a set of safety assessment method and process suitable for bridge and gantry crane. In this paper, the main work includes:(1)Analyze the crack defects type, initiation location and extension mechanism of the in-service box girder structure. Combined with the bridge and gantry crane metal structure working background, we summarize the performance characteristics and existing problems of the welded box girder structure, and its defect shape and size parameters of engineering characterization rules.(2) It carries out a fracture assessment for the cracks on the crane box girder structure using level 2 conventional fracture assessment method. It established the standard model of crack, determined evaluation parameters, made evaluation calculation, and drew FAD figure, which explored a set of fracture assessment processes and methods suitable for the steel structure of crane.(3) With one stage and two stages of the crack propagation rule, the critical crack size and crack propagation life are derived under random loading. Fatigue crack propagation results for different initial crack sizes, in different positions and under different standards were compared and analyzed, which explored a set of fatigue assessment processes and methods suitable for the steel structure of crane.(4) Combined with the actual crack propagation mechanism and the finite element result of container gantry crane, parameterized sub model of crane crack was established to make finite element analysis of the box girder containing defects. The finite element analysis was used to study the stress distribution condition of the lower flange of girder from the center crack to edge crack. The theoretical value and finite element value of the stress intensity factors at the crack tip were calculated and compared under the typical conditions. At the same time, the influence law of stress intensity factors is deeply studied, such as the crack size parameters, the main web thickness and the lower flange thickness, etc.