under construction 12-1-10
A torsion box is complex structure designed to provide a "dead-flat", level surface. Components of structures designed to include the torsion-box components -- such as tops of tables -- are capable of bearing great weight and resist twisting and bending.
(Etymologically, the term "torsion box" traces back at least to 1827, as shown in this fragment, jpg below, on the left, from an 1827 Monthly Notices of the Royal Astronomical Society 1827, page 200
Strangely, though, nothing in the context of this Royal Astronomical Society text suggests what -- exactly -- a "torsion box" is. And, making the mystery even more mysterious, the numerous uses of "torsion box" in 19th century texts that you can dredge up with Google Print are equally undescriptive.
A torsion box is a grid of core material made to whatever thickness you want it to be, with sheet of manufactured sheet stock glued to each face. You will have encountered torsion boxes (hereafter abbreviated to T-box) on a regular basis, because nearly every flush door you walk past is made that way. This structural form offers furniture designers and makers some unique opportunities. One,it allows for the manipulation of the thickness of the parts of the piece. Two, it is very light for its size and dimensionally stable. Three, the inherent strength of the system can be used to develop furniture forms that are not possible using solid wood: you could liken this to wood engineering.
Source: Ian Kirby, "Torsion Box Fundamentals", Woodworker's Journal 31, no 3 May-June 2007, pages 34-39.
Torsion box tops are ideal for woodworker's assembly tables, because the perfectly flat surface is ideal for "developing designs, laying out projects, dry-fits, glue-ups, clamping and other elements of furniture assembly" (from David Marks' DIY website.
Furniture, as well, is often created where torsion boxes are part of the construction components.
(Click on this link to an image of a torsion box woodworker's table under construction. Here's an image of a coffee table , my friend, Charlie Belden, helped his son construct, using a torsion box top. Finally, check out this pdf from the geocities website on the details of constructing projects using torsion box designs -- see image from pdf on left.)
Today, with the ready availability of Medium Density Fiberboard (MDF), the inner core of a torsion box is a grid that's built with pieces of MDF, in Marks' case, "cut and stapled together". (Inner core torsion boxes can also be constructed with good quality Plywood.)
According to Marks, "MDF was selected over kiln-dried hardwood, softwood or plywood since MDF has a uniform density and is very stable". But, cautions Marks, "Whether you use MDF or another material", to ensure a consistent standard in the structure, "the important thing is to build the entire grid from only one material".
Sources: Ian Kirby, "Torsion Box Fundamentals", Woodworker's Journal 31, no 3 May-June 2007, pages 34-39; Bay Area Woodworkers' Newsletter June 2002; David Marks' DIY website.