How to build a rigid metal frame to resist twist?

[BigBlue1]

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.

 

[bcp]

Channel sides and several round tubular crossmembers of near the same height should be very twist resistant. (think of the twist resistance of a driveshaft tube)

Add angle across the ends to fasten to the top boards.

Bruce

 

[CADplans]

I just painted this one today after finishing welding last week,,

I doubt that you could twist it,,

The frame is 1" solid round steel bar, and 1/4" X 2" steel angle,,
all welded with 1/8" 7018 rod.

I have built several, I put composite wood on them.

When you sit at this bench, it is close to sitting on a cast concrete bench.

The bench is a little heavy,,
so I added a wheel so that the table can be moved like a wheel barrow..

The round bars make the table very stiff in torsion,, which is how most picnic benches twist.

 

[CADplans]

Here is a bench I built, using the same concept,,

Again, the solid round steel bar is incredibly stiff,,

The bottom bar is 1.5" solid steel to add some weight.
It is like sitting on a concrete bench

The bench, and the seat to the picnic table are 22 inches high.
Most picnic tables are much lower,,

The far end of the bench has a built in cup holder,,
(the board was too long, I did not want to waist it!)

 

[CADplans]

Here is a park bench,, again no twist,, the trailer is 8' long,,
so you can see how long the bench is.

Did I mention that I have a LOT of 1" solid round steel!!?? (LOL!)

Again, the seating is 22" high (we have an all tall family)

 

[BigBlue1]

Thanks for the ideas. I'll try to review and understand.

One challenge I have is that the rectangular frame itself is all I have to work with for rigidity. The legs and end frame are made of wood and already built so I can't do any X-bracing on the ends/legs.

Rob

 

[dstig1]

Well i can answer the question you posed but it may not be the right question to ask for what you want to do...

If you want a table to resist torsional forces, you need something aptly called a torsion box. The easiest way to visualize this is to think of a frame made of 2x4's, with 2x4 cross pieces on the shorter dimension spaced as appropriate. Then you skin both sides of this box with plywood and glue and screw the snot out of it to the 2x4 frame. This is how you get the stiffest and lightest torsion-resistant "table". The glue is far more important than the screws.

Translate all of that to metal, probably 1/8"/11ga/14ga rect tubing of whatever size makes sense (you want the depth of the table to be as tall as possible, so that drives your tubing size). Weld up that frame then skin it, both sides, with something like 16 ga or even less and skip weld it to the frame all around. You would also want to weld the skins to the cross members, so some slots/holes in the skin to do plug welds would be good.

Is that what you need? Dunno, but that is the answer to the direct question you posed.
-Dave

 

[BigBlue1]

Interesting. Thanks. I hadn't considered boxing/skinning the frame to stiffen it. But that's why I asked. Yes, your idea may be feasible for me.

Thanks,
Rob

 

[BigBlue1]

I read up on the torsion box. I think that's the solution I've been seeking. And I think I may be able to revise what I have as my wood frame right now and turn it into a torsion box without building the metal version. For the price of a couple pieces of plywood and some PL400 I should be able to give it a shot.

Thanks!

 

[AnotherBueller]

Since this is going to be a structural assembly you might want to check out some of the boat building sites for what glue they use, or some of the web sites for home built aircraft on glues.