Common forms of space grids

In space grid structures, where two plane grids are separated by inclined members, the top and bottom grids do not necessarily have to have the same pattern or orientation.

The common forms of double-layer grids are:

• square on square — where the top grid is directly above the bottom grid and the web members connect the layers in the plane of the grid lines (see Figure 8.1(a))

• square on square offset — where the bottom grid is offset by half a grid square relative to the upper grid, with web members connecting the intersection points on the top and bottom grids (see Figure 8.1(b))

• square on diagonal square — where the lower grid is set at 45° to the lines of support and is usually larger than the top grid and, again, with web members connecting the intersection points on the top and bottom grids (see Figure 8.1(c)). An

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(b) Square on square offset

(a) Square on square

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(c) Square on diagonal

(c) Square on diagonal

(d) Triangle on triangle offset

- Top chord members

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\ v / "—" V " / "—" \ / (e) Triangle on hexagon

(d) Triangle on triangle offset

\ v / "—" V " / "—" \ / (e) Triangle on hexagon

8.1 Different forms of double-layer grids alternative version of this grid is diagonal on square where the upper grid is at 45° to the lines of support and the lower grid is parallel to the supports

• triangle on triangle offset — where both grids are triangular but the lower grid intersections occur below the centroids of alternate triangles in the upper grid, with web members connecting the intersection points on the top and bottom grids (see Figure 8.1(d))

• triangle on hexagon — where the upper grid is triangular and the lower, more open, grid is hexagonal due to the removal of some joints and web elements from the grid type described in the bullet point above (see Figure 8.1(e)).

The choice of grid configuration and depth will affect the economy of the space grid, as the node joints are usually the most expensive components. Therefore, the more node points in a given plan area, the higher the cost is likely to be. Increasing the grid-module size reduces the number of nodes for a given plan area, but there may be adverse consequences, as follows:

the depth between the two grids may have to be increased to accommodate the bracing members at a sensible angle of 30 to 40°

when the longer members are subject to compressive forces, they will almost certainly be larger in cross-section for buckling reasons and, consequently, are heavier and more costly.

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