The grid (lattice shell) structure, as a high-order statically indeterminate space rod structure, has good mechanical properties (theoretically the rods are only acted upon by axial force), high stiffness and integrity. It has good seismic performance, strong bearing capacity, is less affected by uneven settlement of the support, and has strong adaptability. The increasingly perfect calculation theory and the rapid development of computer technology have made it possible to analyze and design any extremely complex three-dimensional structure. Therefore, Grid structures are widely used in industrial and civil construction fields. However, if the support structure, bearing type and boundary conditions of the grid structure are unreasonably designed, it will have a significant impact on the safety and economy of the grid structure.
1. Support structure and support method
In many projects currently, the grid (lattice shell) is generally designed by professional steel structure companies based on pre-assumed boundary constraints. Then the bearing reaction force they calculated is used as an external load to act on the lower support structure. The grid frame (lattice shell) and the lower support structure are calculated separately. Although the displacement of the grid support relative to the lower structure can be simulated by the elastic constraint method, the deformation of the support itself such as the subsidence of the support caused by the deformation of the lower support structure It is difficult to estimate the position accurately, and the calculated internal force of the structure will be quite different from the actual situation in some cases, which may leave potential safety hazards for the project. The substructure may be a column, a beam, or other structural forms. Not only is the stiffness limited, but the stiffness of the specific project may vary greatly. Under this assumption, the calculated internal force of the rod and the reaction of the support The force and the internal force of the lower structure are definitely different from the results calculated by the mechanical model using the actual stiffness of the grid support and the upper and lower structures working at the same time. In addition, separate calculations also separate the collaborative work of the upper and lower structures, making the period and displacement calculations of the upper and lower structures inaccurate.
Usually, the support of the grid can be divided into three methods: peripheral support, point support, and a mixture of point support and peripheral support. Peripheral support is to place the peripheral nodes of the grid on beams or columns, and point support. The grid supports are placed on independent beams or columns at large intervals, and the columns have no connection with other structures. When the grid (lattice shell) is placed on a beam or column, it can be considered that the vertical stiffness 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 grid (lattice shell) support is zero. , The horizontal deformation of the grid (grid shell) support should be considered for the same work of the substructure. The lower support structure should be used as the elastic constraint of the grid (lattice shell) structure in the radial direction of the peripheral supporting grid (lattice shell) supports, while the boundary conditions of the point-supporting grid (lattice shell) supports should consider the horizontal X and Elastic constraints in both Y directions. The equivalent spring stiffness calculations of the support structure are as follows:
1) The equivalent spring stiffness of the support column in the horizontal displacement direction of the support column is: Kc=3EcIc/H3c where Hc: column height; Ic: Column section moment of inertia.
2) The equivalent spring stiffness of the simply supported beam supported by the length L at both ends and the grid support located at a distance from the beam end is: Kb=3EbIbL/a2 (L-a) 2 In the formula, a: the distance between the action point and the beam end; L: the length of the beam; Ib: the moment of inertia of the beam section.
3) The rubber pad bearing is supported by a rubber pad with a height of Hp. The equivalent spring stiffness of the bearing is: Kp=GpAp/Hp where Ap: rubber pad area; Hp: rubber pad height. In actual projects, rubber pad elastic supports are often added to the top of beams or columns, especially in large-span grids. Rubber pad supports are used to meet the deformation requirements of temperature stress, which requires consideration of the elastic stiffness of the beam or column. With the superposition of the elastic stiffness of the rubber pad, when K1 and K2 are superimposed, the superimposed stiffness K obtained by the superposition of displacements is: 1/K=1/K1 1/K2; K=1/(1/K1 1/K2).
2. Bearing (bearing node)
The connection area between the structure and the foundation is simplified into a bearing, which is divided into five types according to its stress characteristics: movable hinge bearing (roller bearing), fixed hinge bearing , directional bearings (sliding bearings), fixed (end) bearings and elastic (spring) bearings. The elastic support produces corresponding displacement while providing reaction force. The ratio of reaction force to displacement remains unchanged, which is called the stiffness coefficient of the elastic support.
Elastic supports provide both movement and rotation constraints. When the stiffness of the support is similar to the stiffness of the structure, it should be simplified to an elastic support. When a certain part of the structure is subjected to a load (such as studying structural stability problems), its adjacent parts can be regarded as the elastic support of the part. The stiffness of the support depends on the stiffness of the adjacent parts (such as the cable-stayed cables of a cable-stayed bridge). Simplified as a spring support). When the stiffness of the support is much greater or less than the stiffness of the part, the elastic support transforms into the first four ideal supports.
The grid structure is generally supported on lower support structures such as column tops or ring beams. The support nodes refer to the grid nodes located on the support structure. It must not only connect the rods that meet at the support of the grid, but also support the entire grid and transfer the load acting on the grid to the lower support structure. Therefore, the support node is the link between the grid structure and the lower support structure, and is also an important part of the entire structure. A reasonable support node must have clear force bearing, simple force transmission, safety and reliability. At the same time, it should also have a simple and reasonable structure, simple and convenient production, and good economy.
The support nodes of the grid structure should be able to ensure safe and reliable transmission of support reaction forces, so they must have sufficient strength and stiffness. Under the action of vertical load, the support nodes are generally under compression. However, in some tilted grids, local support nodes may bear tension and sometimes horizontal forces. The supports should be designed so that The structure of the seat nodes is adapted to their stress characteristics. At the same time, the structure of the support nodes should also conform to the calculation assumptions as much as possible and fully reflect the design intention. Since the grid structure is a high-order superstatically indeterminate member system, the constraint conditions of the support nodes have a greater impact on the node displacements of the grid and the internal forces of the members; the difference in constraint conditions between construction and design will directly lead to the internal forces of the members. Changes in the reaction force of the support and the support sometimes cause the internal force of the rod to change. Therefore, sufficient attention should be paid to the design of the support nodes of the grid structure. Whether the grid structure design is safe and economical, the most critical factor first lies in whether the selected support structure, support type and boundary conditions are reasonable. For this reason, in the specific design, we try to avoid analyzing the upper grid structure and the lower support system separately. , design, especially when the displacement of the grid support relative to the substructure is difficult to simulate through the elastic constraint method, the support structure and the upper grid should be integrated into the overall modeling and calculation analysis to make the calculated results more accurate. Be realistic.
I believe that after the above introduction, everyone has a certain understanding of the key points of the support design of the grid structure. Welcome to log in to Zhongda Consulting for more relevant information.
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