Assessment of the ball joint behavior under combine loading

Space grids are highly modular structures assembled from components that are almost exclusively factory fabricated. The components therefore, are usually produced with high dimensional accuracy, with a high quality of surface finish and they are generally easily transportable, requiring little further work except assembly on site. Because of their modular nature, space grids may be extended without difficulty and even taken down and reassembled elsewhere. One of the most popular types of connectors that are widely used in the construction of double layer grids is the MERO system. The Mero KK space truss system, the first commercially available, is still considered to be one of the most elegant solutions for the construction of space grid structures. The elegance and simplicity of the Mero system means that it is not only used in buildings but also for shop displays and exhibition stands using lightweight materials. Circular tube members are connected to cast ‘ball’ joints at the nodes by a single concealed bolt for each tube. A double layer grid is combination of prefabricated tetrahedral, octahedral or skeleton pyramids or inverted pyramids having triangular, square or hexagonal basis with top and bottom members normally not lying in the same vertical plane. The connector is an extremely important part of a grid design. The type of connector depends primarily on the connecting technique, whether it is bolting, welding, or applying special mechanical connectors. It is also affected by the shape of the members. This system is multidirectional system allowing up to fourteen tubular members together at various angles. The system consists of tubular elements that are connected together by means of a MERO connector. The ball is located at the intersection of the longitudinal axes of tubular elements. The longitudinal axis of tabular element and all the constituent parts of its end connectors are along together. This axis is referred to as axis of member. The MERO system had only one type of standard joint, a sphere with 18 threaded holes and machined bearing surfaces at angles of 45, 60 and 90° to each other. A model of MERO connector is presented in this paper for double layer grid structure. The internal forces in the members of double layer grid are found using SAP software. These forces are applied as pressures on the MERO connector. The deflection and rotation patterns of the connector are studied under different loading conditions using the ABAQUS software. The forces to be applied on the connector are calculated using of displacementg control.

In order to take into account the connector effects in structural analysis, their behavior under combined load should be predicted. In double-layer grids that are an important family of space structures, the main internal forces are axial forces. In the present study, to determine the force-displacement relationship of MERO jointing system, some tensile, compressive and bending simulation tests were carried out on a connector of this type using 3D finite element method. The obtained force-displacement and moment- rotation relationship were used under different load. The results of the finite element simulation with experimental results have a good match. It was also found that in both compressive and tensile loading, the ball came to the plastic stage. Compressive axial force increases the flexural stiffness of the connection and with increasing compressive force decreases the elastic bending moment.