| With the rapid development of economy, the steel structure in building structure in the field got rapid application.Because welding connection make the structure simple and have high rigidity,it is widely used in steel structure engineering.However the welding defects exists in the welding process, have a bad influence on the security of steel frame structure. Especially under the dynamic load, welding defects are easy to expand and develop, even cause the destruction of welded joint.Welding defect may affect the structural dynamic characteristics, but there are no literatures which have developed the systematic study.It is hard to research the welding defects in the experimental conditions, so we can not build the quantitative relationships between the the fatigue strength and the dimensions of welding defects. It is unable to verify the theory, and makes a lot of research work to be carried only through the approximate simulation method to achieve.Modal is the structural natural vibration characteristic, each mode has the specific natural frequency, damping ratio and mode of vibration.Based on finite element method computational modal analysis can identify the structural dynamic characteristics systematically. Welding defects of welding structure would has a certain impact on the dynamic characteristics. This effect may be associated with the defect distribution and location and other factors.This paper mainly discusses the effect of geometrical welding defects of steel frame on its dynamic characteristic influence. The dimensions of geometrical welding defects are controlled by welding rate, which is respectively seted to14.7%,26.7%,50.7%,65.3%,100%.The corresponding welding defect rates are85.3%,73.3%,49.3%,34.7%, relative to the full welding. Finite element analysis is completed by ANSYS software. We analyzed in the frequency and mode vibration of steel frame after change the welding rate.Based on this study we research other factors such as distribution and location of weld,frame number. Modeling method and connection mode is considered in the analysis. We build about40frame specimens in tolal, Divided into4series.The main work of this paper can be summarized as follows:1. This paper introduces the classification and the causes of the welding defects in the steel structure, gives the calculation formula of defect equivalent.It introduces the composition of welded joint, summarizes the research progress of welding defect.2. This paper summarizes the basic theories and main method of the dynamic characteristic calculation, discusses basis theoretical of the relationship between the dynamic characteristics and welding defects.Generalized dynamic stiffness theory, analysis relationship between dynamic stiffness between the elastic modulus of the material, inertia moment of the beam and the span of the beam. Given stress distribution of the area around the weld defects, and the method for calculating the size of welding defects.Give the reasonable assumption for weld defects. Summarizing the classifications of beam-column. 3. This paper illustrates the modal analysis process of ANS YS software, and the basis theory of modal analysis. ANSYS modal analysis is a geometrical linear analysis, only geometrical linear behavior is effective, the process of calculating the nonlinear problem will be ignored. Describe the ANSYS contact analysis, and summarizes the connection modes of beam-column.Discussion on ANSYS simulation analysis method, summarizes the advantages and disadvantages of the different modeling methods and parameter settings.4. The calculation results show that: BEAM4unit model cannot build up relationship between the welding defect rate and the frequency. But SOLID45unit mode can simulate the test mode and get the reasonable result. The change of the dynamic properties are not in proportion to the weld defect rate, when the welding defect rate is about85%, influence is the greatest. With the welding defect rate increases, the first10order natural frequencies are increased. In the first10order frequency, the influence of weld defects on third order and fifth order frequency is bigger,the influence on second order is smaller. Due to vibration modes with different welding rate are basically the same, and the maximum displacement patterns, so we cannot establish the relation between welding defects and vibration modes5.The comparison with related experiments are concluded: the results in ANSYS calculated and test are basically consistent, proved the feasibility of ANSYS numerical simulation method. But the vibration modes in ANSYS are very different from the test result, because of the limitations of test instrument measurement.6.The frequency of mode when weld in the middle of beam is littler than that in the end of beam.When the stiffness of beam increases, vibration frequency is increased. So the stiffness of mode that weld located at the middle of beam is littler than located at the end of beam. The effect that weld location on the frequency is increased in a certain range, it begins to reduce when exceeding this range. In general, the effect of the welding defects on basic frequency of steel frame is little, and especially smaller on fundamental frequency. |
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