Laminating steel to wood?

[Jamie Buxton]

I'm looking at making a thin shelf, about 16" by 60" by 1". I'm thinking about a wood and steel torsion box. The strength of a torsion box comes from the skin. Steel is much stronger in tension than wood, so a steel-skinned torsion box should be stiffer than a wood-skinned one. But I'm wondering about how to laminate steel to wood.

Here's what I'm thinking... The core would be 3/4" plywood. Using epoxy and a vacuum bag, glue .1"-thick steel to both faces. The epoxy would be West Systems 105. Then again using epoxy and the vacuum press, laminate shop-sawn wood veneer on to the steel.

Anybody done anything like this?

[Richard Coers]

The strength of a torsion box comes from the height of the internal grid inside the skins. If it came completely from the skin, you wouldn't need a torsion box, and what you are proposing is not really a torsion box but a lamination.

[Curt Harms]

I presume the 3/4" plywood would be on edge forming the 'egg crate' center. How much weight will this shelf be expected to hold? I built an assembly table using 3/8" ply on the top, 1 X 3 for the center and 1/4" ply for the bottom. The top is in compression and the bottom is in tension so they don't typically have to be that heavy. I wanted a table that wasn't too heavy so as to be easy to move. I got what I expected. This is free standing so I needed to figure out a way to attach legs which was the trickiest part.

[andy bessette]

I love and use WEST epoxy for all my wood gluing. But I would prefer to use 3M 5200 for the wood-to-metal bonds you are proposing.

[John TenEyck]

Unless there is some design limitation on thickness you will get all the strength you need by making the core higher, as Richard said. If you really need it to be that thin and want to pursue the steel to wood approach, then I'd look at G/Flex epoxy.

John

[Lee Schierer]

I made some risers for our local church that were 22 x 48, using a 1 x 1-1/2 frame and 5 mm plywood for the surface layer. I filled the void with 1" thick solid foam insulation. I glued the plywood to the foam with polyurethane glue. When they were finished, I weight tested the first one by placing 35 pounds on the center of the 4 foot span. The dial indicator under it deflected less that 0.01 inches. I am confident that you could double that weight with no issues.

[Jim Becker]

Use the steel for skins if that's the look you want, but I agree with the others, the strength comes from the web and the big advantage of a torsion box is that you can get significant strength while maintaining a much lower weight than anything "solid". It would be disingenuous to say that the steel would not add strength because it would; however, a 1/4" plywood or even solid wood skin on a well designed torsion box construction will hold a large amount of weight. Any weakness is "point weakness" where pressure is focused on a small spot between the web components which could allow a hole to form. But it will not break the torsion box every easily.

[Wes Grass]

You could use unidirectional carbon fiber instead. The problem there being, epoxies tend to creep and it may sag a bit after time.

It'd be a whole lot lighter. Your steel skins are 27 lbs each. Not sure you need both faces to begin with, just the bottom in tension.

Cost of 12oz (pretty heavy I think) is about $50 for 2 yds of 50" wide cloth. Might be more for tape, but it would be easier to handle.

[Mark Gibney]

My first reaction is no! don't use metal. It will only add expense and weight. The only notable strength it will add is if you decide to hit the surface hard with a hammer.
I've used rigid foam and honecomb sheets for torsion box shelves, with plywood skins, and they have been perfectly fine for purpose.

My second reaction is that maybe the OP, Jamie, has made many torsion boxes and thru experience has decided that steel skins might really help, so I'm interested in hearing more about this.

Also - Wes says epoxies tend to creep. I always thought then didn't creep in any way and that was one of their attractions. If you hit epoxy with a chisel it shatters.

[Maurice Mcmurry]

I watched this video the other day and found parts of it helpful. I use steel flat bar and square steel tubing to strengthen long shelves. For a 1 inch thick shelf I dado 1/2 of the profile of the bar or tube into two pieces of 1/2 inch ply or MDF and glue the sandwich, with steel in the dados, together with titebond. 1/4 X 3/4 bar or 1/2 X 1/2 tubing work well. Image is a mock up of a 3/8 tube in 1/2 ply.

DSCN1870.jpg

[andy bessette]

...the strength comes from the web...

Just to clarify, it is the extreme fibers in a beam that do the preponderance of the work. ie in an I-beam, it is the flanges, one in tension and one in compression, that are doing the work. The web mostly just holds the flanges together/apart. In the shelf, for example, it is the upper and lower skins, one in tension and one in compression, that are doing the work. Thickening the shelf makes it stiffer, not because the "web" is thicker, but because it separates the skins by a greater distance.

[Alex Zeller]

Only one surface is under tension. The other surface is being compressed. It's the compressed surface that needs to be well bonded to the core. Personally if you need more strength than a 1" board would provide I think I would reverse your thinking. Use a couple rectangle thin walled steel tube for the core with a wood veneer.

[Wes Grass]

Re creep in epoxy, we were building structures (wings, specifically) with carbon skins inside and out. And 3mm PU foam between them. Sitting indoors they'd sag visibly under the weight of batteries and motors.

Tests were done with various epoxies, and some were better with post curing. 'Real' aircraft grade epoxy was worthless without post curing. Goal being lowest cost, epoxy/process. I wasn't able to find the test results on the servers before I left ... for my own knowledge, not to be shared of course. Bummer.

I dont remember MAS being significantly different from West, FWIW. Both good for use with wood AFAIK. Both boat building materials...

[Richard Coers]

I presume the 3/4" plywood would be on edge forming the 'egg crate' center. How much weight will this shelf be expected to hold? I built an assembly table using 3/8" ply on the top, 1 X 3 for the center and 1/4" ply for the bottom. The top is in compression and the bottom is in tension so they don't typically have to be that heavy. I wanted a table that wasn't too heavy so as to be easy to move. I got what I expected. This is free standing so I needed to figure out a way to attach legs which was the trickiest part.

If he uses the plywood as a grid and uses the vacuum bag as he mentions, that will be really exciting under vacuum! I want to see a video on how much the steel bends

[Jamie Buxton]

Thank you all for your thoughts. But the customers have changed their minds, so this extreme shelf will not be necessary. I'll get to use more ordinary construction approaches.