1、 Structural form

   Large span structures can be divided into two types: plane structure system and space structure system according to different geometry, combination mode, structural materials and mechanical characteristics.

   Plane structure system: beam structure (plane truss, space truss), plane rigid frame and arch structure.

   Spatial structure system: flat grid structure, reticulated shell structure, most of the suspension structure, cable-stayed structure, tension structure, etc

2、 Form of grid structure

   According to the number of layers of chords, the grid can be divided into two layers and three layers.When the span of space truss is large, the steel consumption of the three-layer grid decreases, but the increase of members and nodes is more complex.

1.The space truss is a spatial hinged truss structure, and the arrangement of members must ensure that there is no binding variability.The following conditions are satisfied: w = 3j-m-r ≤ 0

2.Common forms of double-layer grid structure:

     2.1 Plane truss grid: the upper and lower chords are completely corresponding to each other and are located in the same vertical plane with the web members, which are vertically compressed and inclined members are in tension.(two-way orthogonal space grid, two-way orthogonal inclined grid, three-way grid)

     2.2 Tetragonal pyramid system grid: it is composed of several inverted quadrangular pyramids according to certain rules.(upright quadrangular pyramid grid, upright evacuated quadrangular pyramid grid, checkerboard quadrangular pyramid grid, inclined quadrangular pyramid grid, star shaped pyramid grid)

     2.3 Triangular pyramid system grid: the basic element is the inverted triangular cone with regular triangle at the bottom of the cone.(triangular cone grid, evacuated triangular cone grid, honeycomb triangular cone grid)

3  Selection of grid structure: the selection of grid structure shall be determined by comprehensive analysis based on the plane shape of the project, building requirements, size of load and span, support condition and cost.According to the “code for design and construction of grid structure” (JGJ 7-91), large span is more than 60m; medium span is 30-60m; small span is less than 30m.

3.1 Support of grid structure: the support modes of grid structure include peripheral support, point support, combination of peripheral support and point support, two and three side support, etc

3.2 Grid height and grid size: the height of grid structure is related to roof load, span, plane shape, supporting conditions and equipment pipeline.

3.3 The disturbance requirements and roof drainage slope of the grid: the allowable disturbance shall not exceed the following values for roof – L2 / 250; for floor – L2 / 300. L2 is the short span of the grid.Roof drainage is generally 3% – 5%.
3、 Key points of calculation for grid structure

   The structural design of the grid structure meets the requirements of the industry standard “code for design and construction of grid structure” (JGJ 7-91).

1.Direct action and indirect action: the internal force and displacement of the grid structure under the load in the service stage shall be calculated, and the internal force and displacement caused by the indirect action such as earthquake action, temperature change, support settlement and construction and installation load shall be calculated according to the specific situation.

2.Internal force analysis method of space truss: according to the principle of static equivalence, the external load of the grid structure is concentrated on the node in the subordinate area.The internal force analysis of the structure is to ignore the influence of the joint stiffness, assuming that it is hinged and the member only bears axial force.The influence of bending moment should be taken into account when there is joint load.The internal force and displacement of grid structure can be calculated according to elastic stage.According to the type of space truss and span size, it is selected according to the regulations (space truss displacement method, cross beam system difference method, quasi sandwich plate method and pseudo bending moment method)
4、 Finite element method of spatial bar system

    The space truss finite element method is also called space truss displacement method. In the analysis, the members of space truss are taken as the basic element, and the node displacement is the basic unknown quantity.The element stiffness matrix is established according to the relationship between the internal force of the member and the joint displacement, and then the relationship between the node load and the joint displacement is established according to the equilibrium and change coordination conditions of each node, and the structural stiffness matrix and total stiffness equation are formed.

    The basic assumptions are as follows: 1. The joints of space truss are spatial hinged joints, and the members only bear axial force.2. The structural material is completely elastic, and the deformation of the grid is very small under the load, which conforms to the small deformation theory.

   The calculation steps of the finite element method for spatial member system are as follows:

   1.In order to reduce the bandwidth of the total stiffness matrix, the node number should follow the principle of minimum good or bad of adjacent nodes

   2.Calculate the length of member element and cosine of angle between member and global coordinate axis

   3.The cross-sectional area of each pole is preliminarily selected

   4.Establish the element stiffness matrix in local and global coordinate system

   5.The total stiffness matrix is set to reduce the capacity of the matrix

   6.Establish load array

   7.The boundary condition is introduced to deal with the total stiffness

   8.The total stiffness equation is solved to obtain the displacement value of each node

   9.Calculate the internal force of the member according to the node displacement

   10.Adjust the cross section of the member according to the internal force of the member and recalculate it no more than 4-5 times
5、 Design of space truss members

   Steel pipe, hot-rolled section steel and cold-formed thin-walled steel can be used for grid members

 
6、 Node design

    Common joint forms: 1. Welded hollow spherical joint; 2. Bolt ball joint; 3. Welded steel plate joint; 4. Welded steel pipe joint; 5 

    The node structure of grid structure shall meet the following requirements: 1. Reasonable stress and clear force transmission; 2. Ensure that the members meet at one point without additional bending moment; 3. Simple structure, convenient fabrication and installation, and low steel consumption; 4. Avoid the dead corners that are difficult to check, clean, paint and easy to accumulate moisture or dust; the pipe type interface shall be closed at both ends.
7、 Reticulated shell

    According to the number of layers, the latticed shells are divided into single-layer latticed shells and double-layer latticed shells.According to the shape of curved surface, there are spherical reticulated shell, cylindrical reticulated shell, hyperbolic shallow reticulated shell, twisted surface reticulated shell, single torsion reticulated shell, hyperbolic paraboloid reticulated shell and curved surface reticulated shell formed by cutting or combining.

    The support of reticulated shell structure must ensure the geometric invariance of the structure under any vertical and horizontal loads and the requirements of various calculation models for supporting conditions.

    The internal force, displacement and necessary stability calculation of reticulated shell structure under external load shall be carried out, and the internal force, displacement and stability under earthquake, temperature change, support settlement and construction and installation load shall be calculated according to the specific situation.It meets the requirements of technical code for reticulated shell structure JGJ 61.

   The reticulated shell is a quasi flexible high-order statically indeterminate structure. Its geometric nonlinearity is more obvious than that of the general structure, and its overall stability is also sensitive to the change of the structural geometry. The finite element method of geometric nonlinearity is mainly used in the calculation of reticulated shell.The joints of double-layer reticulated shells are usually hinged, and the joints of single-layer latticed shells should adopt rigid connections, otherwise the normal stiffness of the coplanar joints of the elements is zero, which is geometrically variable.
8、 Suspension structure

     According to the stress characteristics of cables, the suspension structures can be divided into single-layer suspension system, prestressed double-layer suspension system, prestressed saddle cable network, single-layer prestressed transverse stiffening cable system, prestressed cable arch and tension structure, suspended thin shell tensegrity structure, cable membrane structure and hybrid suspension structure.

    The general design principle of suspension structure: the internal force and displacement of external load and prestressing force are calculated respectively in the use and construction stages of suspension structure, and the internal force and displacement under the action of earthquake and temperature change are calculated according to the specific situation.