Torsion boxes are sometimes called stressed skin panels because of how they use the skin to uniformly transfer stress throughout the structure. Torsion boxes are structural panels that are both rigid and light. Because none of the individual components are so large that their changes due to temperature and humidity can not be controlled by the other componenents in the sturcture, a torsion box is remarbably stable. A torsion box has the following components:
- An upper and lower panel skin. Typically something like luan plywood sold as underlaymnet or door skins.
- A lightweight filler, core grid or matrix bonded between the upper and lower skins. This core grid can be made of wood in a pigeon hole or eggcrate construction, foamboard or something like cardboard matrix used in hollow core doors.
- An outer frame of a solid material is typically used to protect both the lightweight skins and inner filler matrix. The frame also creates a place for attaching hardware.
North Carolina’s Sipping and Switching Society (S&SS) builds torsion boxes for their modules. S&SS has skeletonized the lower skin to allow switch machines and wiring to be tucked up inside the module. S&SS’s website contains both blue-print style module diagrams and a complete photo illistrated article on building the modules.
Eggcrate and grid frameworks appear similar to torsion boxes, but have significant differences. Eggcrates lack one of the outer skins, typically the lower one. This requires the core grid and remaining skin to be sigificantly heavier. Grid frameworks have neither an upper or lower skin are are essentially a heavy duty core grid. The absense of the skins mean the grid can “rack” and collapse unless somehow braced diagonally. Diagonal bracing can be done either by inserting braces diagonally across some of the grid cells or attaching at least a partial skin to part of the grid.
Additional Articles on Building Torsion Boxes
The Bay Area Woodworkers Accociation has an excellent article on the theory, building, and making flat of torsion boxes in their June 2002 newsletter. The pdf article is available here BAMA Torsion Box article.
Fine Wood Working has both a brief overview of torsion boxes and a comprehensive article for members available online.
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