عنوان مقاله [English]
Due to the some uncertainties during manufacture and assembly of double layer grids with ball joint system, the behavior of this jointing system in the structure is different from the behavior of individual and discrete joints. In the present work, the behavior of a ball joint system has been determined in true conditions of its operation in a double layer grid by the general framework of inverse problem. A full scale double layer grid was constructed using ball joint system. A series of modal testing were performed on the double layer grid in free-free support conditions and its FRFs (Frequency Response Functions) were measured in appropriate degrees of freedom. Using the measured FRFs, natural frequencies of the eight vibration modes of the grid were extracted. A beam element was used at each end of members in the finite element model of the grid in order to simulate the ball joint behavior. Through modal analysis of the finite element model, natural frequencies of the eight vibration modes of the grid were calculated for different section properties of the joint beam element. With the finite element model updating of the grid through reduction of difference between its experimental and analytical natural frequencies, section properties of the beam element representing behavior of the ball joint system in the model were obtained. Whilst the updated model resulted in a very good approximation of the natural frequencies of the double layer grid, a flexible behavior in different degrees of freedom was obtained for the ball joint system. Also, the analytical frequency response functions that were calculated using the obtained section properties for the joint beam element, had a good correlation with the experimental ones.