The support is essential for the space frame structure

The space frame structure is a kind of high-order super statically indeterminate spatial structure. According to the structure of the space frame itself, it can be divided into: single-layer space frame structure, double-layer space frame structure, and three-layer space frame structure. Single-layer spaces and three-tier space frames are respectively suitable for small spans and very large spans. According to the different composition methods, the space frames can be divided into four categories: triangular pyramid system space frames, quadrangular pyramid system space frames, Hexagonal cone system space frame, cross truss system space frame. The space frame structure has good mechanical performance, strong bearing capacity, high rigidity, good seismic resistance and overall performance, less affected by the uneven settlement of the support, and strong adaptability. The gradual improvement of calculation theory and the rapid development of computer technology make it difficult. However, the analysis and design of extremely complex three-dimensional structures becomes possible, so space frame structures are widely used in civil and industrial buildings.

Unreasonable design of the support structure, support type and boundary conditions of the space frame structure will have a very important impact on the safety and economy of the space frame structure. 1. Support structure and support method Now in many projects, the space frame structure is designed by a professional company according to the pre-assumed boundary constraints, and then the support reaction force calculated by them is applied to the lower supporting structure as an applied load. Calculate the space frame and the lower support structure separately. Although the displacement of the space frame support relative to the lower structure can be simulated by the elastic restraint method, it is difficult to estimate the displacement of the support itself such as the support subsidence caused by the deformation of the lower support structure. In some cases, the calculated internal force of the structure may be quite different from the actual situation, which may leave a safety hazard to the project. The substructure may be a column, a beam, or other structural forms. Not only is the rigidity limited, but the rigidity of the specific engineering may vary greatly. Under this assumption, the calculated internal force of the rod and the support reaction, the force and the internal force of the substructure are definitely different from the results calculated by the mechanical model that uses the rigidity of the space frame support as the actual stiffness and the upper and lower structures work together. Separate calculations also split the cooperative work of the upper and lower structures, making the period and displacement calculations of the upper and lower structures inaccurate. General space frame support can be divided into three methods: peripheral support, point support, and mixed use of point support and peripheral support. Peripheral support is to place the peripheral nodes of the space frame on beams or columns, and point support is to support the space frame. It is placed on independent beams or columns with a large spacing, and the columns have no connection with other structures. When the space frame is placed on a beam or column, it can be considered that the vertical rigidity of the beam and column is very large, and the vertical deformation of the beam and the axial deformation of the column are ignored. Therefore, the vertical displacement of the space frame support is zero, and the horizontal deformation of the space frame support should be Consider working together with substructures. In the radial direction of the peripheral support space frame support, the lower support structure should be used as the elastic constraint of the space frame structure, and the boundary conditions of the point support space frame support should consider the elastic constraints in the horizontal X and Y directions. There are several calculations for the equivalent spring stiffness of the supporting structure: 1) Support column support 　　 The equivalent spring stiffness of the column in the horizontal displacement direction is: Kc=3EcIc/H3c 　　 In the formula, Hc: column height; Ic: column section moment of inertia. 2) Simply supported beam support at both ends 　　 The equivalent spring stiffness of a simply supported beam whose length is L and the space frame support is located at a distance from the beam end is: Kb=3EbIbL/a2 (L-a) 2 　　 In the formula, a: the distance between the point of application and the end of the beam; L: the length of the beam; Ib: the moment of inertia of the beam section. 3) Rubber pad support 　　 The equivalent spring stiffness of the support supported by the rubber pad with a height of Hp is: 　　Kp=GpAp/Hp In the formula, Ap: rubber pad area; Hp: rubber pad height. In actual engineering, it is often to add elastic supports with rubber pads on the top of beams or columns, especially in large-span space frames, to meet the deformation requirements of temperature stress through rubber pads, which requires consideration of the elastic stiffness of beams or columns. Superimposed with the elastic stiffness of the rubber pad, when K1 and K2 are superimposed, the superimposed stiffness K is obtained by superimposing the displacement: 1/K=1/K1+1/K2; there is K=1/(1/K1+1/K2 ). 2. Support (support node) The connection area between the structure and the foundation is simplified as a support, which is divided into five types according to its force characteristics: living hinge support (roller support), fixed hinge support, directional support (sliding support), fixed (end) Support and elastic (spring) support. The elastic support produces a corresponding displacement while providing the reaction force, and the ratio of the reaction force to the displacement remains unchanged, which is called the stiffness coefficient of the elastic support. The elastic support can provide both movement restriction and rotation restriction. When the rigidity of the support is similar to that of the structure, it should be simplified as an elastic support. When a certain part of the structure is under load (such as studying structural stability), its adjacent part can be regarded as the elastic support of this part, and the stiffness of the support depends on the rigidity of the adjacent part (such as the cable-stayed bridge Simplified as a spring support). When the rigidity of the support is much larger or much smaller than the rigidity of this part, the elastic support transforms into the previous four ideal supports. The space frame structure is generally supported on a lower supporting structure such as column tops or ring beams, and the support node refers to the space frame node on the supporting structure. It does not only connects the rods that meet at the support of the space frame, but also supports the entire space frame, and transfers the load acting on the space frame to the lower supporting structure. The support node is the link between the space frame structure and the lower supporting structure, and is also an important part of the entire structure. A reasonable bearing node must be clear in force, simple in force transmission, safe and reliable. At the same time, it should have a simple and reasonable structure, simple and convenient production, and good economy. The support node of the space frame structure should be able to ensure the safe and reliable transmission of the supporting reaction force, and must have sufficient strength and rigidity. Under the action of vertical load, the supporting nodes are generally under compression. However, in some diagonally placed frame frames, the local supporting nodes may bear the action of tensile force, and sometimes they may also bear the action of horizontal force. The support should be designed. The structure of the seat node adapts to their force characteristics. At the same time, the structure of the bearing nodes should also conform to the calculation assumptions as much as possible, and fully reflect the design intent. Because the space frame structure is a high-order statically indeterminate member system, the constraint conditions of the support nodes have a greater impact on the node displacement of the space frame and the internal force of the members; the difference between the constraint conditions between the structure and the design will directly lead to the internal force of the members. The change of the reaction force with the support sometimes causes the internal force of the rod to change sign. Enough attention should be paid to the design of the support node of the space frame structure. Whether the space frame structure design is safe and economical, the key factor is whether the selected supporting structure, support type and boundary conditions are reasonable. Therefore, in the specific design, we try to avoid separate analysis of the upper space frame structure and the lower supporting system. Design, especially when the displacement of the space frame support relative to the lower structure is difficult to simulate by the elastic restraint method, the supporting structure and the upper space frame should be modeled and calculated together to make the calculated results more consistent actual.