Multi-Layer Grids
When a Double-Layer-Grid (DLG) is used for a very long span (for example more than 300 ft) with large applied loads, the structure deflection is the governing design parameter, and therefore the required depth becomes very large. If the module sizes are small, the diagonal members will have sharp angles. If the module sizes are increased the compression members become excessively long and large member sizes will be required. For such cases, a third layer at the half-way between the top and bottom layers can be added to create a multi-layer grid (MLG). This allows the module sizes to be reduced and the angle of the diagonal members to be within a reasonable range. Each layer can have a different configuration. The middle layer is usually lightly-stressed and therefore made from smaller size members than DLG would have required, however, the number of members and joints becomes large. This additional cost from the extra members and joints is usually less than the savings achieved from using smaller size components. MLG structures have been proposed to cover cities.
While the middle layer carries very small loads compared to the top and bottom layers it stabilizes the entire structure and reduces the deflection significantly.
The benefits of MLGs are:
- Reduction in deflection due to increase in depth.
- Reduction of the maximum member forces while using standard size members.
- Smaller size modules compared to the depth (decking may not need intermediate support members or ‘purlins’)
MLGs can also be used for:
- Providing additional stiffness (trussing)
- Edge supporting for large span open sides (hangers).
- Additional supports for large localized loads.
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