The node type of the space frame

In the space frame structure, the nodes play the role of connecting the intersecting bars and transferring the roof load and crane load. The space frame structure belongs to the space rod system, and there are at least 6 rods intersecting on a node, and as many as 13 rods. This adds a certain difficulty to the node design. The number of nodes of the space frame is large, and the amount of steel used in the nodes accounts for 1/5~1/4 of the steel used in the whole space frame, and the reasonable design node to the safety of the space frame, production and installation, project progress, steel indicators and project cost has a direct relationship. Node design is one of the important links in the design of space frame.

1. The node structure of the space frame should meet the following points:

(1) the force is reasonable, the force transmission is clear, the node structure must be consistent with the calculation assumptions used as far as possible, so that the node is safe and reliable.

(2) to ensure that the intersection of bars in a point, no additional bending moment.

(3) Simple construction, simple production, easy installation.

(4) The amount of steel used is small, and the cost is low.

2. There are many forms of nodes in the mesh frame, divided by the node connection method:

(1) welding connection node: it is divided into butt weld connection and fillet weld connection.

(2) Bolt connection node: It is divided into tension high-strength bolt connection and friction high-strength bolt connection.

3. According to the structure of the nodes are mainly divided into

(1) Crossed steel plate node. It is developed from the basis of plane truss node, the rod is composed of angle steel, and the connection between the rod and the node plate can be made by corner weld, or by high strength bolt.

(2) Welded hollow ball node. It is made of two hot pressed into hemispheres and then butt welded into hollow spheres. The rod is welded on the sphere, and the connection weld between the rod and the sphere can be butt weld or fillet weld, and the rod is generally composed of steel pipe.

(3) Bolt ball node. It is through the bolt, sleeve and other parts of the rod and solid ball connected, the rod is generally composed of steel pipe.

(4) Direct convergence node. It is the web bar (branch) end of the grid by mechanical processing into a coherent surface, directly welded to the wall of the chord bar (main tube), or a directional chord bar can be welded to the wall of another chord bar. This node avoids the use of any connection, saving the amount of steel used in the node, but requires high precision of assembly, and the rod is composed of steel pipe or square tube. After years of engineering practice, the most commonly used node forms in China are welded hollow ball nodes and bolted ball nodes.

The welded hollow ball node is made of two round steel plates by hot pressing or cold pressing into two hemispherical shells, and then butt welding. It is suitable for all kinds of grids using steel pipe rods. The advantage is that the spheres are directionless and can be connected with the bars in any direction, and the steel pipe is cut off perpendicular to its own axis, and the welded hollow sphere nodes are naturally aligned with the welded hollow sphere nodes without eccentricity, and the nodes are clearly stressed.

The welded hollow ball node is divided into two types: ribbed and non-ribbed. The diameter of the welded hollow ball is firstly decided by the structure, and the gap between the bars connected to the node should be not less than 20mm. at the same time, in order to reduce the specification of the welded hollow ball node, one or two kinds of welded hollow balls should be used in a grid. The wall thickness of the ball can be taken as 1/30 to 1/45 of the diameter, while requiring the wall thickness of the ball and the maximum wall thickness of the connected steel pipe ratio of 1.2 to 2, if the ball wall is thin, it will be locally destabilized and damaged. Tensile may be damaged along the steel pipe and ball wall connection weld, if the ball wall is too thin, may also make the ball wall punching damage. In order to improve the bearing capacity of the welded hollow ball node, when the outer diameter of the welded hollow ball is equal to or greater than 300mm, and the internal force of the bar is large, can be set in the ball stiffening ribs, and make the rib is located in the plane of the main tension rod. The stiffening rib is a circle, and the welded hollow ball wall welded together, the thickness of the rib should not be less than the thickness of the ball wall. The disadvantage of welded hollow ball nodes is that stamping and welding is labor-intensive, and the hemispheres should be cut into round plates, which is a low material utilization rate and thus consumes more steel, and the amount of steel used for welded hollow ball nodes generally accounts for about 20% to 25% of the entire grid structure. At the same time, the length of the rod is required to be very precise, and this node has been gradually replaced by the bolted ball node in recent years.

Bolt ball node is in a solid steel cast ball surface, milling out a number of small square or triangular planes, these small planes are perpendicular to the radius of the bolt ball, becoming a diamond-shaped multi-faceted sphere, in each small plane drilling and buckle, and then bolted to the steel pipe and the sphere. The bolt ball node is lighter in weight, lighter in shape, and more beautiful because of the reduced size of the bar ends. Each bolt ball node can be connected to up to 18 rods. It can be disassembled and assembled pole by pole at height without scaffolding. Bolt ball nodes are suitable for all kinds of mesh structures, especially for the mesh structure composed of quadrangular cone and triangular cone, and their steel consumption accounts for about 1/10 of the steel consumption of the mesh, which is about 50% less than the steel consumption of welded ball nodes. However, the disadvantage of bolt ball nodes is that the processing of the sphere is complicated and the price is high, and the end of the steel pipe should be opened (for putting in the bolt), which is not conducive to rust prevention and should be galvanized inside and outside, and the bolt ball nodes are hinged.