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Space Grid Structures
2.10 Square on square grid configuration (Drawing by Graeme Barker © John Chilton)
2.12 Square on diagonal square grid configuration (Drawing by Graeme Barker © John Chilton)
2.11 Square on square offset grid configuration (Drawing by Graeme Barker © John Chilton)
2.13 Triangle on triangle offset grid configuration (Drawing by Graeme Barker © John Chilton)
Double-layer space grid structures, where two plane grids are separated by web members, do not necessarily have to have the same pattern or orientation in the upper and lower grids. In practice, for reasons of cost and facility of connection of web members, the number of common configurations is usually quite limited. In the description of the more common forms of double-layer grid configurations that follows, the reference to square grids also encompasses rectangular grids with different spacing in each direction, as well as the possibility of rhombic grids:
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Square on square: where the upper chord grid is directly above the lower chord grid and the web members connect the layers in the vertical plane between the upper and lower grids. When viewed in plan only the top square grid is seen (see Figure 2.10).
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Square on square offset: where the top chord grid is offset, usually by half a grid square in both directions, relative to the lower chord grid. In this configuration the web members connect the intersection points in the upper grid with the adjacent intersections in the lower grid and a continuum of tetrahedral and half-octahedral cells is generated (see Figure 2.11). This is the most commonly used configuration. Square on diagonal square: where the lower chord grid is set at 45° to, and is usually at a greater spacing than, the top chord grid. Again the web members connect the intersection points on the top and bottom grids (see Figure 2.12). A further alternative version of this grid configuration 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.