Bolt ball welded ball hybrid node of space frame structure

Welded hollow ball joints and bolted ball joints are the two most commonly used space frame joint forms in my country at present. These two joint forms are favored by the majority of engineers and technicians due to their respective advantages, especially the bolt ball joint. Due to the use of computer-aided design to draw the processing diagram, the angle of the bolt screw hole and the blanking length of the bar can be determined with high precision. To become a reality, the factory’s special processing equipment ensures the processing accuracy, thus ensuring the on-site installation. In addition, compared with welded balls, the on-site installation and construction are simpler, the technical level of the installers is not high, and the quality is easy to guarantee. Therefore, the application of bolted ball joints is on the rise.

However, the high-strength bolts used in the bolt ball , due to the restriction of the backward quenching process in our country, can only reach M64 in diameter, and can only withstand the internal force of 70t. Although M100 has been used in some projects, and M125 has been trial-produced High-strength bolts with large diameters, but the key to high-strength bolts with large diameters is to solve the problems of hardenability and delayed fracture. High C content is good for hardenability, but it is not good for toughness. Therefore, it needs to be solved by material modification and adjustment of tempering temperature. The cost is high, and the production technology is not mastered by ordinary space frame manufacturers. Many projects have to give up the bolt ball joint scheme just because the tensile force of a few rods is too large. Furthermore, if the space frame support bears a large tensile force, due to the large difference in material between the 45 steel of the bolt ball itself and the Q235 steel plate of the support gusset plate, it is difficult to ensure the welding quality, so that it cannot reliably transmit the tensile force, although some The unit has solved this welding problem for specific projects, but it must rely on new welding processes, which is difficult for general grid manufacturing enterprises. Based on the above reasons, foreign and domestic scholars put forward a new idea of using hybrid nodes, and successfully applied it in engineering practice, such as the Changsha hangar in my country.

1 The connection form of space frame structural members of mixed nodes

Bolted ball joints and welded ball joints have their own advantages and disadvantages, and the mixed joints can give full play to the advantages of the two joint forms, so that they can meet the requirements of the space frame structure in load, The special requirements in terms of span and form also provide greater possibilities for some small and medium-sized universal space frame manufacturers to compete in the market.

The so-called bolt ball welded ball mixed node means that in the same space frame, some nodes use bolt balls and some nodes use weld balls. If mixed joints are used in a certain space frame, under normal circumstances, the proportion of bolted ball joints in the total number of joints is still relatively large. Welding balls are used in special requirements, and the rods that meet these nodes need to be connected with the balls by welding, thus forming three forms of rod connection in the mixed node space frame: Form 1, both ends of the rods are bolted balls Node; Form 2, both ends of the bar are welded ball joints; Form 3, one end of the bar is a welded ball joint, and the other end is a bolted ball joint.

Due to the difference in the force and quality requirements of the rods, the connection forms of the rods corresponding to different situations are also different.

Situation 1: When the internal force of the member is small, does not exceed the tensile bearing capacity of M64 high-strength bolts, and both ends do not intersect with the nodes that use welded balls due to special needs, use Form 1.

Situation 2: When the internal force of the member is large and exceeds the tensile bearing capacity of M64 high-strength bolts, or although the internal force of the member is small and does not exceed the tensile bearing capacity of M64 high-strength bolts, the tensile force at both ends of the member exceeds M64 The tensile bearing capacity of high-strength bolts intersects, or one end of the rod intersects with a member whose tensile force exceeds the tensile bearing capacity of M64 high-strength bolts, and the other end intersects with a support node with a larger tensile force. In this case, Form 2 is adopted.

Situation 3: When the internal force of the member is small and does not exceed the tensile bearing capacity of the M64 high-strength bolt, but one end of the member intersects the member whose tensile force exceeds the tensile bearing capacity of the M64 high-strength bolt or intersects the support with a larger tensile force, This case uses form three.

2 Several issues that should be paid attention to in the application of mixed nodes

2.1 Scheme selection

  For small and medium-sized space frames, as long as the owner has no special requirements for the joint form, the internal force of most tension members will generally not exceed the tensile bearing capacity of M 64 high-strength bolts, and the space frame of bolt ball joints can be used in the scheme selection. During design calculation, if the internal force of some members is large or the tensile force of some supports is large, it can be treated as a mixed node space frame.

2.2 Structure Calculation

  Regarding the calculation length of space frame members of mixed nodes, because most of the members are bolt balls at both ends, it is recommended that all members in the entire space frame be used according to the calculation length of space frame members of bolt ball nodes, for those ends or for a rod connected with a welding ball at one end, the calculated length is too large for safety. After the design is completed according to the space frame of the bolted ball joint, due to the large force on some members or supports, the welded ball joint is used to form a mixed joint space frame, as long as the cross section of the member does not change, it does not need to be recalculated.

2.3 Construction

When one end of a rod is connected with a welding ball, due to the high rigidity of the welding connection, if the welding positioning is not correct, it will easily cause the bolt to be difficult to align with the hole, or although the bolt can be screwed in, the eccentricity of the rod will be affected due to the deviation from the axis of the bolt hole. Force, especially partial feather pressure, will have a bad effect on the mechanical performance. Therefore, it is suggested that for the rods of form three, the end connected to the bolt ball should be tightened first during construction, and then the welding end should be fixed by spot welding.