How to build a rigid metal frame to resist twist?

[BigBlue]

If I needed to make a frame out of metal which would be used to mount boards for a bench/table top, what would be the best material to make it out of to resist twisting (i.e. where one or opposite corners want to twist)? Angle, rect tube, round tube, something else? And what should the cross-member design look like for best resistance to said twisting? The outer dimensions are 7' x 3.5'.

I'm thinking something like 2x4 1/4" wall rectangular tube for the perimeter and 2.5" or 3" angle iron for cross-members. But is a ladder style of cross-member best or would some type of X be better?

The reason I'm looking into this is that I have 3" thick x 15" maple beams I'm going to use for the top and they all have a twist to them. No, I can't plane the twist out of them. Yes, they are dry like this. Rigidity is the main concern. Cost isn't the primary factor. I've tried a wood frame but the top beams pull up opposite corners about 1/2". 120# of weight on each corner flattens it out nice but that's not a solution. It's been 30 years since I took mechanical engineering classes in college and I don't recall enough to figure it out.

 

[A-one]

Angle iron will twist, but anything would work with proper bracing.

 

[BigBlue]

I think I've settled on building a torsion box.

 

[California]

I've seen articles on steam bending for furniture, stair rails etc. I wonder if you could steam a tray under that tabletop, within some sort of enclosure, and draw the planks down flat.

Obviously this concept would need some research to make it practical.

 

[BigBlue]

I can get them flat with pressure on them. I have actually put relief cuts in the planks for this. The problem is that the perimeter frame isn't rigid enough so when the weight is removed the planks twist the frame/legs. I think the torsion box will solve my issues. I'm not sure if steam would be permanent and I don't have the ability. These planks about 17" x 96" x 3" thick. Tough to fit them into something to steam them.

Rob

 

[CA_Bgrwldr]

What is the twist with both ends free, 1/2", 1"?

With the relief cuts, do both ends still pull up 1/2", or just one end?

Is it a gradual twist through the length of the boards or sharp?

If it were me, I would glue the planks together making it a solid bench top, using 1" dowels along the length, and possibly a channel cut in each end with an insert the length of the table glued in at each end. This would use the planks themselves to remove/hold the twist of each plank, reducing the force being applied the table frame rather than just the frame. While this is a small version, this is what the ends of the table would look like with the channel cut and would insert glued in.


From there, you could likely use the frame you have now, or build a new one using 3x3x.125 angle iron for the perimeter, maybe ,250 for the ends, then use 2" C-channel or angel iron for the cross braces spaced about 16" apart, and 45 degree braces down to the legs, splitting each end into 3rds. For each end, if you don't want exposed bolts, I would bolt the ends down both vertically and horizontally.

Another option, if the twist is gradual, would be to run the planks horizontally to the length of the table, this would reduce the amount of force being applied to the ends when bolted down.

 

[BigBlue]

The twist free is between 3/8" and 3/4" (varies on each of the 3 planks). I did relief cuts about 2.25" deep about every 3". The planks are 3" thick. Both ends pull up. With the relief cuts I can pull them down with moderate pressure. Gradual twist on the full length.

I have now built a torsion box out of 3/4" MDF within the perimeter frame. Haven't gotten the tops back on to test. Crossing my fingers. When I lift a corner of the table (sans tops) right now it doesn't twist much. Maybe lifts one leg less than 1/4". Before it would lift opposite corners 3/8" each.

The idea of using an insert though all three planks might be a good plan B (well, plan F at this point). The theme of the room the table goes in is industrial looking so raw metal is used in other places.

Thanks for the tips!

Rob

 

[BigBlue]

The torsion box out of 3/4" MDF resolved my issues. Happy days....

 

[BigBlue]

Figured I should follow this up now that the project is completed. Here's the torsion box I built inside the table top frame (minus the final piece of MDF on top to sandwich things). And the finished table. The combination of relief cuts in the maple slabs (about 2.25" deep into the 3" slab, every 3" or so) and the torsion box managed to control the twist in the slabs just fine. No more lift on opposite table legs. Didn't end up needing a metal frame once I learned about the torsion box concept.

Yeah, it's not really a metal project but I did weld up some T accent plates for the ends, so it isn't entirely off topic. And I would have welded up a steel frame if I wouldn't have learned about torsion boxes and MDF.

Rob

 

[CA_Bgrwldr]

Nice work, when you posted, I thought you were just making a work table.

With what you were going to use the table for, not making a metal frame, was the right choice, I think the metal would have looked
out of place with the rough sawn lumber used.

 

[BigBlue]

Actually, bare metal is part of the theme of the room (industrial/warehouse look). If you notice on the table there are metal plates as accent pieces which cover where some structural bolts were used (versus a more traditional wood dowel plug setup). So using a frame made of metal would have been OK. Additionally, a metal frame that I hid inside the top perimeter wood frame (where the MDF torsion box ended up) would also have worked and hid the metal. But MDF was a lot quicker to work with to make the torsion box and cheaper so I went with that.

My original post was intentionally vague because I figured stating that it was going to be for a dining/bar table would have limited the creativity in responses.

Thanks for the compliment!

Rob

 

[CADplans]

Hmmmmmmm,,
"Actually, bare metal is part of the theme of the room (industrial/warehouse look)."
is actually the look I ended up with in my home shop,, except, I went a little more,, shall we say, extreme??



The only nails in the building are in the floor that you see here, and around the one door, and a coupe windows..