Space frame design methods. Good space frame design has the advantages of light weight, high stiffness, excellent seismic performance, convenient factory production, convenient installation, and can withstand uneven settlement of the foundation; the grid structure has strong adaptability and is suitable for large, medium and small spans. The support is flexible. It can adopt peripheral support and three-side support, four-point or multi-point support, and mixed support of peripheral support and point support.
The grid structure is widely used in my country, not only in public buildings such as gymnasiums, theaters, canteens, restaurants, but also in industrial buildings such as industrial workshop roofs. The advantages of the grid structure are listed in detail. This article is based on the basic form and selection of the grid structure of the steel structure, and provides some reference and help for space frame design beginners.
The selection of the space frame design should be comprehensively considered in combination with the plane shape, requirements, load and span size of the building, support conditions and economy. Usually we divide it according to the span size: large spans above 60m; medium spans from 30m to 60m; small spans below 30m.
1. Plane truss system
- Two-way orthogonal positive placement grid
Structural features: The plane is geometrically variable. In order to increase its spatial rigidity and effectively transmit horizontal loads, additional inclined rods should be set along the upper (lower) chord plane of the support periphery of the grid. The peripheral support is close to a square plane, the force is uniform, and the internal force of the rod is not very different. As the side-length ratio increases, the unidirectional force characteristics are obvious. For the point-supported grid, the internal force of the members near the support and the mid-span chord of the main truss is large, and the internal force of other parts is small.
Applicable situation: Rectangular plane, peripheral support, side-to-length ratio less than 1.5, various spans can be used.
- Two-way orthogonal inclined grid
Structural features: Since the grid is of the same height, the short truss at the corners has greater rigidity and is aligned with it. The vertical long truss acts as a certain elastic support, thereby reducing the bending moment in the middle of the truss. The rigidity is larger than that of the two-way orthogonal grid. When the plane is rectangular, the force is more uniform. There is upward pulling force at the four corner supports of the grid frame.
Applicable situation: Rectangular plane, peripheral support, side-to-length ratio less than 1.5, various spans can be used.
- One-way zigzag grid
Structural features: Similar to the three-dimensional truss, but do not need to arrange the support system. Only the upper and lower chords along the span direction are in a one-way stress state. In order to strengthen its spatial rigidity, some upper chord members should be added around it.
Applicable situation: Rectangular plane, peripheral support, side-length ratio greater than 2.
- Three-way grid
Structural features: The basic unit is geometrically invariant. The space rigidity of the whole grid is greater than that of the two-way grid. It can evenly transmit the force to the support system, and the force performance is better. The number of members and nodes is large, and the node structure is complex (at most one node meets 13 members).
Applicable situation: It is suitable for circular or polygonal planes with regular grids around it, suitable for large-span projects.
2. Quadrangular pyramid system
- Positively placed quadrangular pyramid grid
Structural features: The space rigidity is larger than other quadrangular pyramid grids and two-way grids. The force is relatively uniform.
Applicable situation: It is suitable for the peripheral support grid with rectangular plane shape, the side-to-length ratio is less than 1.5, and the large-span project.
- Evacuated quadrangular pyramid grid
Structural features: The space rigidity is smaller than that of the square pyramid grid, and the internal force of the lower chord is increased.
Applicable situation: It is suitable for the peripheral support grid with a rectangular plane shape. And the side-to-length ratio is greater than 1.5.
- Checkerboard-shaped quadrangular pyramid grid
Structural features: The space rigidity is smaller than that of the square pyramid grid, and the internal force of the lower chord is increased. When the periphery is arranged in a full cone, the stiffness is better. This kind of grid frame has a short upper chord under compression and a long lower chord under tension, which can give full play to the function of the cross-section of the rod, and the force is reasonable.
Applicable situation: It is suitable for the peripheral support grid with a rectangular plane shape, the side-to-length ratio is greater than 1.5, and the medium and small spans.
- Inclined quadrangular pyramid grid
Structural features: The space rigidity is smaller than that of the quadrangular pyramid grid.
Applicable situation: It is suitable for the peripheral support grid with a rectangular plane shape. And the side-to-length ratio is greater than 1.5.
- Star-shaped quadrangular pyramid grid
Structural features: Its rigidity is poor, and it is not as good as the square pyramid grid. The vertical rod is compressed, and the internal force is equal to the upper chord node load.
Applicable situation: It is suitable for the peripheral support grid with a rectangular plane shape, the side-to-length ratio is less than or equal to 1.5, and the medium and small spans.
3. Three, triangular pyramid system
- Triangular cone grid
Structural features: The basic unit is a geometrically invariant system, with good overall torsional and bending stiffness, and relatively uniform stress.
Applicable situation: It is suitable for the peripheral supporting grid frame with circular and polygonal plane shape. The side-to-length ratio is greater than 1.5, and the large-span project.
- Evacuate the triangular cone grid
Structural features: The internal force of the lower chord is increased. The uniformity is slightly worse, while the rigidity is worse than that of the triangular pyramid grid.
Applicable situation: It is suitable for the peripheral support grid with the plane shape of circle and hexagon. And the side-to-length ratio is greater than 1.5.
- Honeycomb triangular pyramid grid
Structural features: This kind of grid frame has short upper chords under compression and long lower chords under tension. It can play the full role of the cross-section of the rods with reasonable stress. But the rigidity is lower than triangular pyramid grids.
Applicable situation: It is suitable for the peripheral support grid with the plane shape of circle and hexagon. And medium and small spans is required.
