Torsion-box construction

[Greg Wandless]

I want to make several torsion-box shelves for a book room. I have found a couple of articles on torsion-box construction and understand the basics. Can someone out there give me some insight as to what spacing I should use for the core material? Are there rules-of-thumb for spacing and thickness of the core stock?

Thanks,
Greg

[Todd Burch]

Greg, I've never seen anything published like what you are asking for, but it probably exists in some wood engineering college textbook somewhere.

What I have done in the past is basicaly just "wing it", working with the materials I had and the requirements of the job.

The longest shelf I made was 8' long, totally out of 1/4" BC plywood (voids and all). The inner "box" material was 2" tall and the perpendiculars were about 10" to 12" or so apart, staggered every row so that along every long "rung" had a joint every 5" to 6" (if this doesn't make sense, let me know and I'll draw a picture). (I've seen a lot of cores made in a grid... not sure which is better.) I put it all together very fast with brads and poly glue. If I were being real fussy next time, I would clamp instead of brad, as some of the brads pulled out as the poly glue expanded.

After curing, and before I even put the 1/4" skins on, I set one end up on a sawhorse and left the other on the ground, making a ramp, and stepped up on it. I bounced my weight up and down (~210 pounds - AT THE TIME!), and while there was some deflection with just my weight, and a little bit more with bouncing (not jumping), I was satisfied that it would do the job I needed once I put the skins on.

Todd.