[China Aluminum Network] spatial grid structure is a structural system in which discrete rods are integrated by nodes. Therefore, nodes are important parts of the structural system. Nodes not only connect components, but also transmit force flow. effect. Each node should be connected to at least three components to maintain stability, and it must be able to resist the torsional effects of unbalanced forces, but also to resist the residual stress generated in the structural system due to construction installation errors. In addition, the more axial components that meet at a node, the more morphological possibilities of the structural system. In the node system, the U.S. Unistrut system, the German Mero system, the Canadian Triodetic system, the American Teccor system, and the Japanese NS system are widely used. Due to its poor solderability, aluminum alloys mainly use mechanical connection nodes in the space grid structure. Bolt ball joints and Temcor's plate nodes are widely used. The former are mainly used for grids and double-layer lattice shells. For single-layer reticulated shells. In addition, Japan's bolt ball nodes and embedded nodes are also used in aluminum alloy single-layer reticulated shell structures.
4.1 Aluminum alloy single-layer lattice shell node
The existing mature aluminum alloy single-layer grid structure node system is a patent of the American Temcor company, but there is no reference to relevant foreign research literature. The node form is to meet several I-shaped section bars at the center, one aluminum alloy cover plate is arranged at the upper and lower flanges, and each bar member is connected with the aluminum alloy cover plate by the fastening bolts of the upper and lower flanges. The multiple node aluminum single-layer reticulated shells that have been completed and put into use in China have adopted this node system.
In the design of plate joints, the diameter of the disk, the amount of arching, the size and number of bolts (holes) and the thickness of the disk are the key parameters. The diameter of the disc must meet the requirements of non-interference of the intersecting bars. The amount of disc arching depends on the angle between the corresponding bar and the cutting plane of the joint. The finite element software ANSYS was applied to the analysis of the members, the gusset plates and the bolts of the plate joints: the bolts were mainly shear failure; the upper and lower gusset plates were mainly normal stress, and the closer to the edge the stress was smaller; Material damage does not generally occur. However, the number of bolts in the analysis model used is small, resulting in a single bolt with a large stress and becoming a weak link in the node structure. The variation of basic parameters such as the height, width, thickness of web, thickness of cover plate, and diameter of the bar section in the elastic range is analyzed. The bending stiffness of the joint around the bar is given by fitting. Calculation formula. However, the model does not consider the contact problem of the bolt connection. Only the stiffness formula of a certain characteristic parameter node is given, and the applicability is not strong. In order to analyze the stress state and failure mechanism of single-layer aluminum alloy shell-shell joints, the finite element analysis software ANSYS was used to simulate the typical nodes. The results show that the joint plate and the connection bolts have similar degrees of security and are in the plate node. The components with higher strength and stiffness are the weaker parts of the plate-type nodes. It is believed that the failure mechanism of the plate-type joints is firstly the failure of the members, followed by the buckling of the gusset plates, and later the destruction of the connecting bolts. Based on the finite element analysis of the plate nodes, the stress concentration occurs at the edge of the bolt hole at both the rod and the cover plate, and gradually decreases from the outer edge of the cover plate toward the center of the node. The stress at the contact surface between the rod and the cover plate is relatively large. However, the overall safety of the rod and the cover plate is high; the bending stiffness of the strong shaft around the rod member is calculated by the displacement of the key point, and the node is considered as a semi-rigid node. Since the bolts are not considered in the model, the interaction between the bolts and the bars and the gusset plate cannot be reflected, and the failure characteristics of the bolts cannot be studied. In order to verify the strength and rigidity of aluminum alloy plate joints, numerical simulations were carried out. The results show that under normal working conditions, the stress of the joint plate and the bolts are all small. The bending moment-angle curve of the joints is obtained and the stiffness of the joints is considered to meet the requirements. Follow assumptions.
In summary, it can be seen that there are still some deficiencies in the study of aluminum alloy single-layer reticulated shell plate nodes, including: 1) lack of experimental research, most studies are limited to numerical simulation, lack of experimental verification; 2) finite element The preload of the bolt was not considered in the analysis model. In some models, only the solid model of the coupling was not considered. 3) Most of the finite element analysis models ignore the in-plane bending moment and actually the surface of the slab node. The internal bending stiffness is much smaller than the out-of-plane bending stiffness, so the rationality of this simplified assumption is for further study. For other types of aluminum alloy single-layer reticulated shell nodes, domestic and foreign scholars have also conducted some research. An experimental study and numerical analysis of a single layer aluminum alloy monolithic shell cast aluminum alloy joint used for the connection of a box-section member are performed. The results show that the out-of-plane bending stiffness of the cast aluminum joint is significantly higher than that of the in-plane one. The bending stiffness indicates that the more unfavorable sections are weakened at the bolt holes, and the number of bolt holes should be properly reduced under the premise of ensuring the strength in the design. A simplified design formula for the bearing capacity of the cast aluminum joint is proposed. The damage of cast aluminum in this node is in the form of brittle failure. Although the literature suggests that appropriate safety factors should be considered in the design to avoid sudden failure of the node, the poor ductility of the cast aluminum still restricts the promotion of the node form. In order to analyze the rigidity of the embedded node of aluminum alloy single-layer spherical lattice shells, Sugizaki et al. conducted detailed experimental investigations, including the joint tension and compression test, the compression and strength of a single triangular mesh element, and the weak-axis bending test. This node is an out-of-plane rigid joint and an in-plane semi-rigid joint. Sugizaki et al. used NASTRAN software to perform finite element analysis on the nodes, and examined the stress of the deformed section of rods in the joint area and the part where the groove and the rod were in contact. In addition, the stiffness of the nodes was also quantitatively evaluated. Hiyama et al. performed experiments and numerical simulations on the aluminum alloy single-layer reticulated shell bolt ball joints, and proposed approximate calculation formulas for joint stiffness and bearing capacity. The results show that brittle fracture does not occur in weak areas, and the joint stiffness and bearing capacity vary depending on the connecting members. The cross section increases and increases.
4.2 Aluminum alloy double-layer net shell and grid frame node
The nodes of the aluminum double-layer reticulated shell structure of the FAST active reflective surface were studied. The embedded nodes connected by the high-strength bolts were selected to conduct the scaled model test. The results showed that the joint displacement was higher than the theoretical calculation value because the aluminum alloy was embedded The rigidity of the joints of the type node is weak, resulting in a large deformation of the structure, and this defect can be overcome by the node construction, but no specific improvement plan is proposed.
The most widely used grid structure is the bolt ball joint. In the construction, the cone head or Seal plate with bolts is first welded to the ends of the pipe fittings, and the long hexagonal sleeve is set on the screw that protrudes from the cone head or the cover plate. Connect the bolts to the sleeve with pins or fastening screws. When assembling, the long hexagonal sleeve is directly twisted, and the bolts or fastening screws are used to drive the bolts to rotate so that the bolts are screwed into the spheres until the bolt heads come into close contact with the sealing plate or the cone head. Each exchange rod member is connected in this manner. After the formation of the node. The strength of the bolt ball sphere can meet the requirement after the diameter meets the space geometric relation, but it will not be broken. However, the strength of the screw hole should be ensured to avoid premature destruction. It should be noted that the bolts of aluminum alloy bolt ball joints should be galvanized or stainless steel bolts used to prevent contact corrosion.
Experimental study was carried out on the bearing capacity of aluminum alloy bolt ball joints, including the tension bearing capacity of aluminum bolt ball internal thread, the tensile load bearing capacity of aluminum bolts, the tensile bearing capacity of aluminum sealing plates, and the welding tolerance of aluminum sealing plates and aluminum tubes. Force and so on. The internal thread test and the pull and pressure test of the rod were performed on the aluminum alloy bolt ball joint. Through experiments, the parameters of the bearing capacity of the joints and components were obtained, which provided reference for the design of the aluminum alloy grid, and a special program for designing the SFCAD aluminum alloy bolt ball node grid was also developed. Three types of joints of bolts and welded hybrid aluminum alloy grids in the Shanghai Botanical Garden were experimentally studied. The results show that the bearing capacity of each node is controlled by the strength of diagonal welded braces. By observing the relationship between the relative slippage and the axial force of the part of oblique bolt connection, and comparing with the theoretical analysis results, it is concluded that the relative force of the bolt makes the internal force of the web with large slippage at the connection point of the joint decrease.
The above-mentioned experimental researches on aluminum alloy bolt ball joints are limited to the mechanical performance aspects, and do not consider the rigidity of the joints and their impact on the overall performance of the structure, nor do they establish a fine node model for numerical simulation.
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