OK, I’m getting old, so let’s not rub it in. But what is the name of the type of structure sometimes made for table tops or outfeed tables where criss-crossed spines are sandwiched between two thin outer layers of plywood, making a flat, thin surface that is stronger than it looks?
I am making a queen size platform bed and the client wants to put the mattress on a solid surface, not slats, placed an inch below the surface of the outside platform. I thought I would make one of those sandwich things that would lay on stringers attached to the rails. I know FW had an article about making them, but since I can’t recall the name of the “thing”, I can’t look it up in the index.
So who will help this old, brain dead woodworker?
-Bob
Replies
I think you're referring to Torsion Boxes.
Chatobob,
I agree it is called a torsion box.
But I did a queen size bed a few years ago with 3/4" plywood spanning the entire width without any problem.
ASK
Bob,
As already mentioned, what you are looking for is a torsion box.
You can save yourself a lot of trouble by purchasing them pre-made: standard hollow core interior doors are torsion boxes and they conveniently come 80 inches long, the same length as a typical mattress. Two 30 inch doors side by side will give you the correct size for a typical queen sized mattress.
You should check the size of the mattress your client intends to use, there is no exact sizing for mattresses, some companies make them larger or smaller than the "standard" size.
John W.
I would agree, but with the caution of asking how the door is made. Cheap hollow core doors are wood on the faces and edges, but corrugated cardboard inside. They are not made for this use, and definitely not for anything more strenuous that sleeping.I cut this piece four times and it's still too short.
Most hollow core doors use a specially made cardboard honeycomb to create the grid structure inside of the panels, the cores are not simply made from corrugated cardboard. A custom made torsion box might be slightly stronger than a door of similar thickness but probably not enough to justify the cost, but that's the client's decision.
I have seen hollow core doors used, and abused, as bench tops and assaulted in attempts to break into a room, they are amazingly strong, I wouldn't have mentioned them if I didn't think they would be adequate for the job.
John W.
I specifically mentioned cheap hollow core doors. I don't know if there is a supplier near any of you, but in MKE, we have a place that sells a lot of factory second doors, along with salvaged doors and windows. Mostly minor dings. If there is, I think a good birch hollow core would be a good solution, at a better price than new. More durable than luan. I cut this piece four times and it's still too short.
Interesting post, John. Hollow core doors can be strong over the flat plane of the door, but what if an elbow or knee pushed thru the 1/8 inch luan skin?
The 1/8 inch skin doesn't worry me under a mattress.
I did use a torsion box with 1/4 inch skin for a desk, and something fell and broke through the surface. General opinion was that 1/4 baltic birch or 3/8 anything would have been fine, but I ended up replacing it with 1/2 inch ply, since that was the thinnest walnut ply I could get that was better than the original 1/4 inch stuff (that was closer to 3/16). ________________________Charlie Plesums Austin, Texashttp://www.plesums.com/wood
Uncle Dunk- torsion is a twisting motion and if all of the members of the box are glued, the whole box is going to be really stiff. Won't bend along its length, width or twist. Making it thicker adds to its resistance to deflection in all directions.I cut this piece four times and it's still too short.
I figured that, in this application, the mattress would protect the face of the door from being damaged, there is no reason why anyone would directly strike the door faces once the bed was assembled.
To reinforce the door faces and to tie the doors together, you could butt the doors against each other and then glue on 1/4 inch plywood crosswise, top and bottom, to make a 60" x 80" panel that would be lightweight, economical, and even stronger.
John W.
Edited 11/17/2004 11:11 am ET by JohnW
That's a good idea that has merit. I'll have to remember hollow core doors for the next bed project.
"there is no reason why anyone would directly strike the door faces once the bed was assembled"
I can think of one...
Viagra and poor aim :)John O'Connell - JKO Handcrafted Woodworking
The more things change ...
We trained hard, but it seemed that every time we were beginning to form up into teams, we would be reorganized. I was to learn later in life that we tend to meet any new situation by reorganizing; and a wonderful method it can be for creating the illusion of progress while producing confusion, inefficiency, and demoralization.
Petronious Arbiter, 210 BC
Torsion box is the right answer, and I won't enter the "is a door good enough" debate. But I will add a couple points.
On one bed I used a 3/4 inch plywood sheet under the box springs and mattress, since it was also the support for some suspended under bed storage. No problems.
On one queen size mattress or foundation, there was a note that all warranty was void if there was not a center post in the support. (I wondered why the larger bed frames always came with 5 legs). So I added a center leg to the 3/4 inch plywood sheet to avoid later arguments if the mattress set failed, even though it didn't seem necessary.
On a murphy bed I just made, 1/4 inch plywood was specified for the top of the torsion box foundation, and as I recall there were only 3 cross braces, plus the ends, for a queen size bed. If you firmly attach (glue) the plywood to the braces it adds a lot of strength to the overall structure.
________________________
Charlie Plesums Austin, Texas
http://www.plesums.com/wood
To all. Torsion box is correct.. Last year David Marks, a very well known wood artisan in Marin Countynorth of San Francisco Calif. Made one on D I Y Woodworks. It really looked easy to build and gave a dead flat surface to work on. Like a granite surface plate. Regards Woodmann
Anybody know why they are called torsion boxes? Every application I've ever seen has been loaded in bending rather than torsion.
I took a class from David earlier this year and the torsion box you mentioned is indeed strong and true.His shop is REALLY tiny (camera makes it look big ;>) ) and when I had to walk across it nothing bounced. It makes a great assembly table at about 20" off the floor._________________________________
Michael in San Jose
"In all affairs it's a healthy thing now and then to hang a question mark on the things you have long taken for granted." Bertrand Russell
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