Torsion box shelves

**Andsally** · 16th January 2010, 07:22 AM

Hello there

I am designing a two built-in bookcases, using torsion-box shelves that will be permanently fixed to sides and rear.

The shelf span will be 1040mm (3ft 5ins) and the depth will be about 220mm (9ins). Each shelf will be filled; the typical load on each would be between 25kg (55lbs) to 55kg (120lb).

I plan to make the shelves with ply top and bottom, and to use redwood pine between. To cover front shelf edges, I will use a decorative moulding. I am happy to end up with shelves of up to 50mm thickness - as long as I have no bowing.

Question: which would be the strongest way of constructing the torsion box shelves? 18mm ply top, 6mm ply bottom, with 25mm battening between? Or 12mm top and bottom?

I did read somewhere that torsion box shelves are almost as strong as their counterparts but am unsure about the combinations of top bottom and filling.

Any suggestions much appreciated!

Thanks

**George_N** · 16th January 2010, 09:03 AM

There is a tutorial on making torsion box shelves here...Torsion Box Shelves - The Woodworkers Institute. I'd have thought 6 mm ply skins would be adequate for the size of shelves you are planning.

George

**Andsally** · 18th January 2010, 12:23 AM

Thanks for that, George. I have already seen the page for which you gave a link. Seeing this page made me wonder if 18mm skins were a tad excessive. If I was to suggest 12mm top skin, 6mm top skin with 28x18mm battons, would you be able to say with more certainty that they would be more than adequate?

Thanks,
Andrew

**George_N** · 19th January 2010, 09:31 AM

A lot of the strength of a torsion box comes from a good glue joint between the skins and the core. I'd be inclined to make the core as a grid with a couple of full length strips as well as cross pieces, as in the pic in post #4 here "http://www.phoenixwood.ca/forum/index.php?showtopic=1280". 6 mm skins will make it strong enough to stand on without breaking.

**thebicyclist** · 19th January 2010, 09:52 PM

Most of the strength in a torsion box comes from 1) a good gllue join between the base and the box and 2) (and more importantly) the depth of the inner frame, a 50mm deep torsion box with 18mm base and top will not be as stiff as a 50mm torsion box with a 6mm top and base.

IIRC the stiffness of a beam is proportional of the cube of the depth.

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