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There are three main types: the first type is composed of plane truss system, which has four forms, namely, two-direction orthogonally straight laying grid, two-direction orthogonally inclined laying grid, two-direction oblique laying grid, and three-direction inclined laying grid. The second type is composed of quadrilateral pyramidal units, including five forms: positive quadrilateral grid, positive evacuated quadrilateral grid, oblique quadrilateral grid, checkerboard quadrilateral grid and star quadrilateral grid. The third type is composed of triangular pyramidal units. There are three types of triangular pyramidal nets, extractive tricone nets and honeycomb tricone nets. According to the form of shell surface, shell structure can be divided into cylindrical shell, spherical shell and hyperbolic parabolic shell. According to the materials used in the grid structure, there are steel grid, reinforced concrete grid and steel and reinforced concrete composite grid, of which steel grid is used more.