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Wood over steel in a brutal climate

11/8/14       
Craig Caughlan Member

Website: http://click-design.com.mx

I’ve a project I’d appreciate comments/ideas. It’s a prototype; a tree branch 12’ long that will support 930 lbs. It’s essentially an art sculpture that in its final form will be a tree with three branches with ‘leaves’ at the end of the branches.

The core structure is a hollow steel beam 4” x 12” that tapers at 6’ to 4” x 3” the remaining 6’. It is to be covered in wood. Wood choices are Mexican tropical woods, each have interlocked grain.

The environment is in a semi-arid steppe climate, where the temperature during the day can swing from 50 – 105 degrees F, and RH can swing from 32 – 90%.

I’m working from the architect’s specs on the steel beam, the size of which was calculated by a structural engineer. Ultimately, they don’t care for the overall size of the ‘branch’ as they think it’s out of proportion. I’ve played with the ideas of carbon fiber, which I have some limited experience with, and considered a core structure of aluminum constructed with internal ribs as in an airplane wing. For the moment though, the architect wants a prototype on the above steel beam.

My main concern are the swings in temperature and RH. My idea is as follows:
1. Attach 5/8” birch ply to the steel with rivets;
2. Attach 1/4” veneer using West System epoxy. The veneer would have 1/8” gaps.
3. The finish would be a WB impregnated finish as Milesi, Klima or Solrac.

All comments appreciated.

Click the link below to download the file included with this post.

Profile_2.pdf

11/9/14       #2: Wood over steel in a brutal climate ...
Adam

Steel and wood don't mix well. Your current "design" would fail quickly. No movement is allowed because of the rivets.

The steel expands with the heat and the wood shrinks as the heat lowers the moisture.

I would stay away from the steel. What is wrong with making it out of wood? 930 lbs is not that much weight. How is it fastened to the ground? It would be helpful if you gave us a sketch or drawing.

11/9/14       #3: Wood over steel in a brutal climate ...
Craig Caughlan Member

Adam-
Attached is a rendering of the entire tree. I made a mistake on the weight - the total of the hexagonal sections is 2,250 lbs, plus the weight of the two secondary branches. As the architect has it, there would be a steel beam protruding from the trunk that the main branch would slip over.

We've also considered solid wood, laminated, although I'm not sure how the joints would be done. For the moment, the architect wants a prototype of their design.

I've doubts too on the efficacy of how I would cover the steel, which is why I'm throwing it out to WoodWeb. My thinking on the birch ply is that with all the plies it would restrict the movement from the steel heating and expanding. I'd thought of using another type of fastener in an elongated hole, but then I think it would introduce too much 'slop'.

The idea is that this prototype would be placed in the environment for six months to see how it behaves. I'd like to try and get it right the first time. I've told the architect of my doubts.

Click the link below to download the file included with this post.

141009_Mercado.pdf

11/10/14       #4: Wood over steel in a brutal climate ...
Larry

Seems like doing it in just steel would be the best bet. If wood is really required how about making it obvious that it is applied over a steel frame by allowing the steel to show @ the corners (1"+-) and allowing lots of room for the wood to move. Provide large headed stainless steel, through fasteners in slots. This solution makes the construction an integral part of the design. If he doesn't want to stoop to the level of us woodworker's! design ideas, tell him it came from an architect, which I am. But I'm mainly a woodworking business man these days.

11/10/14       #5: Wood over steel in a brutal climate ...
Adam

"My thinking on the birch ply is that with all the plies it would restrict the movement from the steel heating and expanding."

This line of thinking is fatally flawed. The steel may move a bit which you will never be able to control. The bigger problem is the steel is more or less stable and the wood grows and shrinks.

If they are hell bent on making a fake tree out of wood, then its time to forget about the steel. Start thinking of carbon reinforced wood beams. Build up box beams out of western red cedar and skin them with biaxial carbon fiber(45/45 fabric) and epoxy. Apply the 1/4" veneers on top of the carbon. We use cedar as core under glass & carbon in boatbuilding all the time.

Anyone(your engineer) who says their modulus of elasticity(flexibility) are too different fails to understand that the carbon is so stiff that it prevents the wood from ever flexing. They make kayaks with carbon and kevlar which take a huge amount of twisting force. The epoxy resin on every surface stabilizes the wood so it is no longer effected by the humidity.

Another problem to consider is the UV attacking the wood.

Carbon fiber is expensive and you need some experience with it to make it worth your time and money.

Very interesting project.

11/10/14       #6: Wood over steel in a brutal climate ...
Craig Caughlan Member

"My thinking on the birch ply is that with all the plies it would restrict the movement from the steel heating and expanding."

Yeah, that isn't what I meant to say. I know the steel is ± stable. My thoughts were that the birch ply would provide a barrier and stable base for the veneer.

My intuition tells me that CF is the way to go. I've limited experience in laminating plys; 3 x 2mm for a final 1/4" and the result is incredibly strong - more than steel in the bending moment.

I messed up with the weight; we're back to 930 lbs at the 12' end.

Questions:
1. Would a Prepeg Twill weave do?
2. Do you think only one layer of CF between the box beam and the veneer is sufficient, or would I laminate the box beam with CF?

It would appear that this is something that could be bagged pretty easily. at least for the 'branches'.

11/10/14       #7: Wood over steel in a brutal climate ...
Adam

You might be able to build the entire thing out of cedar box beam, then skin it with layers of biax carbon and epoxy(pre preg is only good for sticking to itself not wood). The carbon wrapping the joints wood make it very stiff. After its all cured. Trim the outside with the nice looking stuff.

I have no idea how many layers of carbon. It will be surprisingly stiff over the cedar.

11/10/14       #8: Wood over steel in a brutal climate ...
mark

Really, Larry's got the only good suggestions here. Look at the rendering: it's artistically mechanical, and overtly geometric. Make the structure out of coreten, make the honeycomb out of aluminum, and make the octogonal panels out of ply, plastic, glass, fabric, whatever.
Or, come on: just plant a tree if the point is just mimicry.

11/12/14       #9: Wood over steel in a brutal climate ...
lou

Make it our of steel and have an artist faux finish the wood grain.

11/13/14       #10: Wood over steel in a brutal climate ...
Brad Member

Maybe I'm missing something here... If you slotted the holes where plywood meets steel to allow movement between steel and wood, AND built is so that no plane is dependent on the other, why would this not work? No you would not have a tight joint between surfaces, but you mention a 1/8 gap anyways.

11/14/14       #11: Wood over steel in a brutal climate ...
Craig Caughlan

That was one of my thoughts and the reason the corners on the veneer (in my first drawing) are half lap and not mitre.

We've told the architects we couldn't guarantee, so we're exploring solid, laminated wood, and possibly carbon fiber. Thanks for the input everyone.

11/14/14       #12: Wood over steel in a brutal climate ...
John Costin

This reminds me of some classic jokes, but here goes: To me the biggest problem here is the use of 1/4" "veneer" glued to the plywood (or whatever substrate is used).

1/4" thick wood is lumber in my world, not veneer. By this I mean it behaves more like lumber, because at the glue line the forces from wood movement are ten times greater than the forces from a true veneer face (at around 1/40" thick, which is on the thick side for plywood anyway).

I can't evaluate the ideas about how to make the plywood skins stable because I don't know anything about carbon fiber. But when the 1/4" lumber faces start to move they could shear the glueline no matter what type of adhesive you use and no matter what gaps you use.

(FWIW, aesthetically I like it!)

My $0.02.

John Costin
Veneer Services Unlimited
vsu@gwi.net
207-985-7221
Custom Veneered Components for the Woodworking Industry


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