I have a rectangular frame that I want to keep from flattening into a parallelogram. The frame is 8' x 4' and the members are 4 7/8" high by 2 5/8" thick.

There isn't a particular load that will be pressing it into the shape per se, but I want to prevent this from happening through incidental forces, because this rectangular frame will have some load on its flat side.

I want to avoid having to make corner to corner crossbars because the size of the rectangle makes it difficult to do this and I am also using wood joinery without screws or brackets. I have considered adding supports across each corner.

Is there a better/cleaner way that does not involve any 45° miter of any kind? I feel like there must be something obvious I am missing.

  • have you considered and/or rejected gussets? – fred_dot_u Mar 29 at 21:36
  • @fred_dot_u I haven't rejected them entirely, but the frame is 8 feet by 4 feet, so they may not be ideal (would be pretty large (right?) – orokusaki Mar 29 at 21:48
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    with a frame that size, you are on the right track for bracing, but your restrictions are, um, restrictive. As a layman, I'd be shooting for a gusset minimum size of one foot (five toes) on the short side and two on the long side. Cable bracing almost certainly involves screws. What do you think about cord (tension) bracing on the hypotenuses of the imaginary gussets described in my previous sentence? – fred_dot_u Mar 30 at 0:34
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    I think you've selected your Best Answer here too quickly, after all there are numerous potential solutions and some can be used in concert, for a belt-and-braces approach (which might be advisable given the size you're dealing with). But we definitely need more detail than just "a frame 8' x 4 feet". What wood? What frame members? Corners mitred? If so case mitres or frame mitres? And by no means least, what is this for? – Graphus Mar 30 at 12:11
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    OK then, in that case I think there's little of concern except if/when it may be needed to move the frame 'empty' in the future. With that plywood panel in place, even unfixed, there's not much worry of this racking (because the frame basically has nowhere to go). But if the ply is screwed down then racking becomes literally impossible. If you did want to strengthen the frame for moving to a new house/apartment you could consider screwing on temporary gussets (nothing fancy, scraps of ply will do) on the bottom to reinforce it just for handling, then simply take them off once at the new home. – Graphus Mar 31 at 10:20

The option of using cabling or cording of some sort is best presented with an image, curiously enough, to scale:

corded frame support

The blue lines are cord, passed through holes drilled at the intersections of the frame. One could get away with a single continuous piece, but unless it's secured to the frame, you'll still have undesirable flexibility in the frame. Additional lines could be run, the placement doesn't have to match the drawing. This is presented as an option in the comments discussion, but could be a solution within the restrictions.

Wire cable or non-stretchy cord would be stronger. Non-stretchy cord is also known as Spectra and sometimes UHMWPE.

  • Thanks again Fred, one more quick question: are there any other kinds of geometric solutions to this problem that don't involve corner braces of any kind? – orokusaki Mar 30 at 1:05
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    corner braces are effective because they create triangles. A triangle resists deformation more than just about any other geometric shape. You could avoid the corners by making the entire top and side a triangle, that is running a diagonal from one corner to the other, but it's still a corner brace. – fred_dot_u Mar 30 at 19:03

In general, using a strong joint such as a half lap, or a mortise and tenon will be about as strong as if it were a single piece of wood. Think gate construction. Gates are different in that they have all the weight on one side, thus they need a diagonal brace to transfer some of the load. If your frame is supported on both sides, and is made with quality, tight fitting joinery, I wouldn't expect any racking at all.

  • That's good to hear. Thank you for your answer. – orokusaki Mar 31 at 17:34

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